1、备份原sshd配置文件

cp /etc/ssh/sshd_config /etc/ssh/sshd_config.backup

2、修改sshd配置文件

可以使用winscp、nano、vim之类的软件编辑 /etc/ssh/sshd_config

查找Port 22，有可能 Port 22 是注释的(即前面有#号，有的话删掉 # 号)。

在 Port 22 下面添加一行 Port 3322 其中3322为你更改SSH后的端口。

这里建议先保留原来的22端口，待新的SSH端口测试正常工作后再删掉原Port 22 端口行。

修改完成后保存。

查看端口占用情况

1
lsof -i:端口号
1
2
3
4
5
6
7
8
9
lsof -i:8080：查看8080端口占用
lsof abc.txt：显示开启文件abc.txt的进程
lsof -c abc：显示abc进程现在打开的文件
lsof -c -p 1234：列出进程号为1234的进程所打开的文件
lsof -g gid：显示归属gid的进程情况
lsof +d /usr/local/：显示目录下被进程开启的文件
lsof +D /usr/local/：同上，但是会搜索目录下的目录，时间较长
lsof -d 4：显示使用fd为4的进程
lsof -i -U：显示所有打开的端口和UNIX domain文件
1
2
3
4
5
6
7
8
9
10
11
netstat
netstat -tunlp 用于显示 tcp，udp 的端口和进程等相关情况。

netstat 查看端口占用语法格式：

netstat -tunlp | grep 端口号
-t (tcp) 仅显示tcp相关选项
-u (udp)仅显示udp相关选项
-n 拒绝显示别名，能显示数字的全部转化为数字
-l 仅列出在Listen(监听)的服务状态
-p 显示建立相关链接的程序名

3、重启SSH服务器

重启SSH服务器命令：

1
systemctl restart sshd

如果没有systemctl的话可以执行：

1
/etc/init.d/sshd restart** 或 **/etc/init.d/ssh restart

如果没有报错的话就生效了，可以 ss -ntl 或 netstat -ntl 查看一下端口。

4、防火墙、安全组规则设置

iptables添加SSH新端口规则：

1
iptables -A INPUT -p tcp --dport 3322 -j ACCEPT       #3322替换为新的端口#

然后保存iptables规则

firewalld添加SSH新端口规则：

1
2
firewall-cmd --permanent --zone=public --add-port=3322/tcp      #3322替换为新的端口
firewall-cmd --reload       #重载firewalld#

阿里云之类的安全组规则添加SSH新端口规则：

阿里云之类的有安全组之类设置的云服务器一定要在安全组规则里将新端口添加到“入方向”的允许规则。

5、putty、xshell之类的SSH软件连接测试

添加上新的SSH端口后一定要通过putty、xshell之类的SSH软件连接测试一下，测试可以正常连接然后再编辑 /etc/ssh/sshd_config 将Port 22 这一行直接删除或这一行前面加 # 注释掉，然后再按前面的命令重启SSHd服务。

SSH隧道代理

最近pypi镜像搭建好了，实际上pip本身有一个–proxy的指令，能够制定一个代理，就能完成内网机器无法通过pip访问外网资源的问题. 通过代理就能完成基本的下载包的需求了，而且其他的一些命令如gem，curl等等大多数常见的命令都支持走代理模式。

而且大家总在说 “用nginx做反向代理”，那么到底什么代理，什么是反向代理呢?

正向代理

正向代理是客户端和服务器中间的服务器，为了从原始服务器取得内容，客户端向代理服务器发送一个请求并指定目标（原始服务器）， 然后代理服务器向原始服务器转发请求并将获得的内容返回给客户端。

举个例子，比如正常情况下我没法办上youtube，但是我有个aws的机器，它不受GFW的限制，能够访问youtube，我也能正常访问那个aws的机器， 那么我通过发送请求给aws的机器，让他转发我的请求给youtube，然后把youtube返回的数据给我，我就能通过aws的机器作为跳板访问GFW， 那个aws的机器也就是代理服务器的角色，并且这种方式就是正向代理。

总结一下，正向代理就是我想访问一台机器，但是被墙了访问不到，我需要一台机器作为跳板转发我的请求。

反向代理

于正向代理不同，反向代理更多的是为了保护原始服务器。 对于客户端而言，反向代理中的代理服务器就是原始服务器，客户端并不需要知道有这个代理的角色存在， 因此客户端也不需要一些额外的设置，比如正向代理中制定代理服务器是谁。

比如在原始服务器A上配置防火墙，使得只有服务器B能够访问A，并且通过B服务器转发A的数据实现于外界的通信。这样对于客户端，它只需要和B交互， 从而隐藏了服务器A，B服务器也就是反向代理服务器。由于有代理服务器的存在，对于后面的原始服务器来说，也就多了一层做负载均衡的服务器。

透明代理

还有一种代理方式叫做透明代理，比如我在公司的时候机器就不能上qq，这个就是透明代理，它在内网和外网之间捕捉用户的请求，过滤一部分请求。

SSH端口转发

有一种很简单的方式就能做到代理功能，那就是SSH本身提供的端口转发功能。 要想理解清楚SSH端口转发，首先必须记住这样几个原则：

  • SSH简单的理解就是2台机器之间安全的数据通道，它包括ssh的client和ssh的server2个角色，这样的一条通道也就是ssh隧道(ssh tunneling)
  • SSH 端口转发自然需要 SSH 连接，而 SSH 连接是有方向的，从 SSH Client 到 SSH Server
  • 我们的应用的请求也是有方向的，一般是客户端向服务器端发出请求
  • 一旦这2个方向相同，我们称为ssh的本地转发(-L)，不同则为远端转发(-R)
  • 命令一般是跑在ssh client的机器上的

本地转发

ssh本地转发命令为：

1
ssh -L <local port>:<remote host>:<remote port> <SSH hostname>

这条命令可以翻译成：从本地的端口发出请求，通过这台机器作为跳板转发请求到的端口。 是相对而言的，比如是127.0.0.1的话，就是本身。 所以一般如果是127.0.0.1的话，跳板机或者代理服务器就是目标服务器。

举个例子：

  1. A是一台在我家的机器macbook air，它可以访问taobao，也就是服务器C
  2. B是一台在公司的机器imac，由于在公司的内网，所以在家的A访问不到B
  3. C是taobao的服务器，公司不让上taobao，所以服务器B访问不了C

现在要想在让B服务器能访问C的80端口，由于防火墙这条路本身是走不通的，但是由于B能访问A，A能访问C，所以能把A作为代理服务器实现这一要求。

在服务器B和服务器A之间建立ssh隧道，在SSH端口转发中，由于服务器B能连接到服务器A，并且请求是从服务器B发出，所以B既是ssh的client，也是请求的客户端 所以此时应该在B上去运行ssh的本地转发命令: ssh -L 8080:HOST_C:80 HOST_A

远端转发

ssh远端转发的命令为：

1
ssh -R <local port>:<remote host>:<remote port> <SSH hostname>

由于本身B是可以访问A的，但是A访问不到B，现在要想服务器A访问到B，也就是在家能连上公司的机器。

服务器B能连上A，所以B应该是ssh的client，此时请求是由A发起的，所以A是请求客户端，方向不同，所以是远端转发。 因此在B上运行命令: ssh -R 2222:127.0.0.1:22 HOST_A ,这里127.0.0.1是因为通过A服务器转发的目的服务器就是A本身。 这样在A上的2222端口就映射到了B的22端口。

动态转发

不管是本地转发还是远端转发，都需要一个具体的应用服务器的地址和端口号，要想访问其他机器的内容就得绑定很多条这样的转发命令， 通过动态转发就能省去这一个应用服务器的信息。

1
ssh -D <local port> <SSH Server>

当我们在一个不安全的 WiFi 环境下上网，用 SSH 动态转发来保护我们的网页浏览等信息无疑是十分必要的。 比如在本机运行： sh -D 7001 <SSH Server> 这样就相当于通过创建了一个SOCKS代理。

我们可以直接使用localhost:7001 来作为正常的 SOCKS 代理来使用，直接在浏览器上设置即可。 在 SSH Client 端无法访问的网站现在也都可以正常浏览。 而这里需要值得注意的是，此时 SSH 所包护的范围只包括从浏览器端（SSH Client 端）到 SSH Server 端的连接，并不包含从 SSH Server 端 到目标网站的连接。 如果后半截连接的安全不能得到充分的保证的话，这种方式仍不是合适的解决方案。

这个时候还可以在本机将SOCKS代理转成HTTP代理。 比如在本地安装polipo，修改polipo.conf文件，把SOCKS代理填上127.0.0.1:7001，然后 export http_proxy="127.0.0.1:8123" && export https_proxy="127.0.0.1:8123" 就在本地的8123端口起了一个http代理。

参考了IBM的实战 SSH 端口转发

服务端添加心跳时间

1
2
3
GatewayPorts yes          # 开启反向隧道
ClientAliveInterval 60    # 保证服务不断开，每60秒自动发送一次请求
ClientAliveCountMax 36000 # 十个小时没有发送请求即自动断开

客户端配置心跳包时间

如果没有服务器端的权限，也可在客户端进行配置，这样这个客户端所发起的所有会话都会产生效果。

修改/etc/ssh/ssh_config文件，与服务器端配置类似，添加以下两个参数即可：

1
2
ServerAliveInterval  
ServerAliveCountMax

会话连接添加心跳时间

如果不希望或不能修改配置文件，也可以在每次建立SSH连接时通过-o参数指定当前会话配置：

1
2
3

ssh -o ServerAliveInterval=30 root@192.168.1.1

Debian OpenSSH默认配置文件 （翻译）

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179

# This is the sshd server system-wide configuration file.  See
# sshd_config(5) for more information.

# This sshd was compiled with PATH=/usr/local/bin:/usr/bin:/bin:/usr/games

# The strategy used for options in the default sshd_config shipped with
# OpenSSH is to specify options with their default value where
# possible, but leave them commented.  Uncommented options override the
# default value.

Include /etc/ssh/sshd_config.d/*.conf

# 连接端口 默认22:
#Port 22
# 指SSH用什么地址 参数:any、inet、inet6:
#AddressFamily any
# 主机名|域名|ipv4 : 
#ListenAddress 0.0.0.0
#ListenAddress ::

# 私钥地址
#HostKey /etc/ssh/ssh_host_rsa_key
#HostKey /etc/ssh/ssh_host_ecdsa_key
#HostKey /etc/ssh/ssh_host_ed25519_key

# Ciphers and keying
# 指定可以传输的最大数据量或在重新协商会话密钥之前收到，以字节为单位，可能有后缀"K"、"M"或"G"来指示分别为千字节、兆字节或千兆字节。默认介于"1G"和"4G"之间，具体取决于密码。该操作第二个值以秒为单位指定，可以使用以下任何一个"时间格式"部分中记录的单位:
#RekeyLimit default none

# Logging
# 记录消息值使用的设施代码,参数:DAEMON,USER,AUTH,LOCAL0,LOCAL1,LOCAL2,LOCAL3,LOCAL4,LOCAL5,LOCAL6,LOCAL7,默认AUTH:
#SyslogFacility AUTH
# 参数:QUIET,FATAL,ERROR,INFO,VERBOSE,DEBUG,DEBUG1,DEBUG2,DEBUG3.默认INFO,DEBUG与DEBUG1等效,DEBUG2,DEBUG3各自指定更高级的调试:
#LogLevel INFO

# Authentication:

# 用户未登录成功,指定服务端多久断开连接,默认120s即2m,0表示不断开:
#LoginGraceTime 2m

# 指定root用户是否可以登录,参数:yes,prohibit-password,forced-commands-only,no.注意的是prohibit-password,forced-commands-only都为yes可以root用户登录
# prohibit-password:关闭root用户密码和键盘交互验证
# forced-commands-only:root允许公钥认证登录,禁止其他认证方法:

#PermitRootLogin prohibit-password

# 用户登录之前检查文件模式以及用户文件和主目录的所有权。这通常是可取的，因为新手有时会意外地将其目录或文件设置为全局可写。默认值为 yes:
#StrictModes yes
#  允许的最大身份验证尝试次数。一旦失败次数达到该值的一半，就会记录更多失败。默认值为 6,合理设置辞职，可以防止攻击者穷举登录服务器:
#MaxAuthTries 6
# 每个网络连接允许的打开 shell、登录或子系统（例如 sftp）会话的最大数量。支持连接复用的客户端可以建立多个会话。将MaxSessions设置为 1 将有效禁用会话复用，而将其设置为 0 将阻止所有 shell、登录和子系统会话，同时仍允许转发。默认值为 10:
#MaxSessions 10

# 指定是否公钥认证:
#PubkeyAuthentication yes

# Expect .ssh/authorized_keys2 to be disregarded by default in future.
# 指定授权密钥文件路径,该指令中可以使用下列根据连接时的实际情况进行展开的符号： %% 表示'%'、%h 表示用户的主目录、%u 表示该用户的用户名:
#AuthorizedKeysFile	.ssh/authorized_keys .ssh/authorized_keys2

#AuthorizedPrincipalsFile none

#AuthorizedKeysCommand none
#AuthorizedKeysCommandUser nobody

# For this to work you will also need host keys in /etc/ssh/ssh_known_hosts
#HostbasedAuthentication no
# Change to yes if you don't trust ~/.ssh/known_hosts for

# HostbasedAuthentication
# IgnoreUserKnownHosts用来设置ssh在进行RhostsRSAAuthentication安全验证时是否忽略用户的"/$HOME/.ssh/known_hosts"文件:
#IgnoreUserKnownHosts no
# Don't read the user's ~/.rhosts and ~/.shosts files
#IgnoreRhosts yes

# To disable tunneled clear text passwords, change to no here!
# 用来设置是否开启密码验证机制，如果用密码登录系统，则设置yes:
#PasswordAuthentication yes
# 设置是否允许用口令为空的账号登录系统，设置no:
#PermitEmptyPasswords no

# Change to yes to enable challenge-response passwords (beware issues with
# some PAM modules and threads)
# 指定是否允许键盘交互式身份验证。支持login.conf(5)中所有身份验证样式。默认值为yes。此关键字的参数必须为yes或no。ChallengeResponseAuthentication是此功能的过时别名:
KbdInteractiveAuthentication no

# Kerberos options
# 指定了用户为PasswordAuthentication提供的密码是否通过Kerberos KDC进行验证。要使用此选项，服务器需要一个Kerberos servtab，以便验证KDC的身份。默认情况下为no:
#KerberosAuthentication no
# 如果通过Kerberos的身份验证失败，密码将通过任何其他本地机制（如/etc/passwd）进行验证。默认值为yes:
#KerberosOrLocalPasswd yes
# 指定是否在注销时自动销毁用户的票据缓存文件。默认值为 yes:
#KerberosTicketCleanup yes
# 如果AFS已启用并且用户具有Kerberos 5 TGT，则在访问用户主目录之前尝试获取AFS令牌。默认情况下为no:
#KerberosGetAFSToken no

# GSSAPI options
# 指定了用户为PasswordAuthentication提供的密码是否通过Kerberos KDC进行验证。要使用此选项，服务器需要一个Kerberos servtab，以便验证KDC的身份。默认情况下为no:
#GSSAPIAuthentication no
# 指定注销时是否自动销毁用户凭据缓存。默认为yes:
#GSSAPICleanupCredentials yes
# 确定客户端对GSS API接受者的身份验证是否严格。如果设置为yes，则客户端必须针对当前主机名上的主机服务进行身份验证。如果设置为no，则客户端可以针对存储在机器默认存储中的任何服务密钥进行身份验证。此功能旨在协助多 homed 机器上的操作。默认情况下是yes:
#GSSAPIStrictAcceptorCheck yes
#GSSAPIKeyExchange no

# Set this to 'yes' to enable PAM authentication, account processing,
# and session processing. If this is enabled, PAM authentication will
# be allowed through the KbdInteractiveAuthentication and
# PasswordAuthentication.  Depending on your PAM configuration,
# PAM authentication via KbdInteractiveAuthentication may bypass
# the setting of "PermitRootLogin prohibit-password".
# If you just want the PAM account and session checks to run without
# PAM authentication, then enable this but set PasswordAuthentication
# and KbdInteractiveAuthentication to 'no'.
UsePAM yes

# 指定了是否允许ssh-agent(1)转发。默认值为yes。注意，禁用代理转发并不能增强安全性，除非还拒绝了用户的shell访问权限，因为他们总是可以安装自己的转发器:
#AllowAgentForwarding yes
# 指定是否允许TCP转发。可用选项包括：yes（默认）或all以允许TCP转发，no以阻止所有TCP转发，local仅允许本地（从ssh(1)的角度）转发，或remote仅允许远程转发。请注意，禁用TCP转发并不一定能增强安全性，除非同时拒绝用户shell访问，因为用户总能自行安装转发工具:
#AllowTcpForwarding yes
# 指定是否允许远程主机连接到为客户端转发的端口。默认情况下，sshd(8)将远程端口转发绑定到回环地址，从而阻止其他远程主机连接到这些转发端口。使用GatewayPorts选项可以指示sshd允许远程端口转发绑定到非回环地址，从而允许其他主机进行连接。该论点可能主张远程端口转发仅对本地主机可用，即选择"no"；也可能要求远程端口转发绑定到通配符地址，即选择"yes"；或者允许客户端自选绑定地址，即选择"客户端指定"。默认设置为"no":
#GatewayPorts no
# 指定是否允许 X11 转发。参数 必须是 yes 或 no。默认值为 no。启用 X11 转发后，如果 sshd(8) 代理 显示配置为监听通配符地址（请参阅 X11UseLocalhost），则服务器和客户端显示可能会受到额外的暴露，尽管这不是默认值。此外，身份验证欺骗和身份验证数据验证和替换发生在客户端。使用 X11 转发的安全风险是，当 SSH 客户端请求转发时，客户端的 X11 显示服务器可能会受到攻击（请参阅 ssh_config(5) 中有关 ForwardX11 的警告）。系统管理员可能希望保护可能因不知情地请求 X11 转发而使自己暴露于攻击的客户端，这可能需要设置为 no。请注意，禁用 X11 转发并不会阻止用户转发 X11 流量，因为用户始终可以安装自己的转发器:
X11Forwarding yes
# 指定 sshd(8) 的 X11 转发可用的第一个显示编号。这可防止 sshd 干扰真实的 X11 服务器。默认值为 10:
#X11DisplayOffset 10
# 指定 sshd(8) 是否应将 X11 转发服务器绑定到环回地址或通配符地址。默认情况下，sshd 将转发服务器绑定到环回地址并将 DISPLAY 环境变量的主机名部分设置为 localhost。这可防止远程主机连接到代理显示。但是，某些较旧的 X11 客户端可能无法使用此配置。可以将 X11UseLocalhost 设置为 no，以指定转发服务器应绑定到通配符地址。参数必须是 yes 或 no。默认为 yes:
#X11UseLocalhost yes
# 指定是否允许 pty(4) 分配。默认为是:
#PermitTTY yes
# 指定当用户以交互方式登录时，sshd(8) 是否应打印 /etc/motd。（在某些系统上，它也由 shell、/etc/profile 或等效文档打印。）默认值为是:
PrintMotd no
# 指定 sshd(8) 是否应在用户以交互方式登录时打印上次用户登录的日期和时间。默认为是:
#PrintLastLog yes
# 指定系统是否应向另一端发送 TCP 保持活动消息。如果发送了这些消息，则连接中断或其中一台机器崩溃将会被正确注意到。但是，这意味着如果路由暂时中断，连接将会中断，有些人会觉得这很烦人。另一方面，如果不发送TCP 保持活动消息，会话可能会无限期地挂在服务器上，留下“幽灵”用户并消耗服务器资源。默认值为是（发送 TCP 保持活动消息），服务器将注意到网络中断或客户端主机崩溃。这避免了无限挂起的会话。要禁用 TCP 保持活动消息，应将值设置为否:
#TCPKeepAlive yes
# 指定 sshd(8) 是否处理 ~/.ssh/environment 和 ~/.ssh/authorized_keys 中的 environment= 选项。有效选项为 yes、no 或指定要接受哪些环境变量名称的模式列表（例如“LANG,LC_*”）。默认值为 no。启用环境处理可能使用户能够使用 LD_PRELOAD 等机制绕过某些配置中的访问限制:
#PermitUserEnvironment no
# 指定用户成功验证后是否启用压缩。参数必须是 yes、delayed（yes 的旧同义词）或 no。默认值为 yes:
#Compression delayed
# 设置超时间隔（以秒为单位），如果在此间隔后未从客户端收到任何数据，sshd(8) 将通过加密信道发送一条消息，以请求客户端做出响应。默认值为 0，表示这些消息将不会发送到客户端:
#ClientAliveInterval 0
# 设置在 sshd(8) 未从客户端收到任何消息的情况下可以发送的客户端活动消息数。如果在发送客户端活动消息时达到此阈值，sshd 将断开客户端连接，终止会话。需要注意的是，客户端活动消息的使用与 TCPKeepAlive 非常不同。客户端活动消息通过加密信道发送，因此不会被伪造。TCPKeepAlive 启用的 TCP keepalive 选项是可伪造的。当客户端或服务器依赖于了解连接何时无响应时，客户端活动机制非常有用。默认值为 3。如果 ClientAliveInterval 设置为 15，并且 ClientAliveCountMax 保留为默认值，则无响应的 SSH 客户端将在大约 45 秒后断开连接。将 ClientAliveCountMax 设置为零将禁用连接终止:
#ClientAliveCountMax 3
# 指定 sshd(8) 是否应查找远程主机名，并检查远程 IP 地址的解析主机名是否映射回相同的 IP 地址:
#UseDNS no
# 指定包含 SSH 守护进程的进程 ID 的文档，或 none 表示不写入。默认值为 /var/run/sshd.pid:
#PidFile /run/sshd.pid
# 指定与 SSH 守护进程的最大并发未认证连接数。其他连接将被丢弃，直到认证成功或连接的 LoginGraceTime 到期。默认值为 10:30:100。或者，可以通过指定三个冒号分隔的值 start:rate:full（例如“10:30:60”）来启用随机早期丢弃。如果当前有 start (10) 个未认证连接，sshd(8) 将拒绝概率为 rate/100 (30%) 的连接尝试。如果未认证连接数达到 full (60)，概率将随时间增加，并且所有连接尝试都将被拒绝:
#MaxStartups 10:30:100
# 指定是否允许 tun(4) 设备转发。参数必须是 yes、点对点（第 3 层）、以太网（第 2 层）或 no。指定 yes 表示允许点对点和以太网。默认值为 no:
#PermitTunnel no
# 指定在身份验证后 chroot(2) 到的目录的路径名。在会话启动时，sshd(8) 检查路径名的所有组件是否都是 root 拥有的目录，这些目录不能由组或其他人写入。chroot 后，sshd(8) 将工作目录更改为用户的主目录。ChrootDirectory 的参数接受“TOKENS”部分中描述的令牌。ChrootDirectory 必须包含支持用户会话所需的文档和目录。对于交互式会话，这至少需要一个 shell（通常是 sh(1)）和基本 /dev 节点，例如 null(4)、zero(4)、stdin(4)、stdout(4)、stderr(4) 和 tty(4) 设备。对于使用 SFTP 的文档传输会话，如果使用进程内sftp 服务器，则无需对环境进行额外的配置，但在某些操作系统上，使用日志记录的会话可能需要 chroot 目录中的 /dev/log（有关详细信息，请参阅 sftp-server(8)）。为了安全起见，防止目录层次结构被系统上的其他进程（尤其是 jail 之外的进程）修改非常重要。配置错误可能导致 sshd(8) 无法检测到的不安全环境。默认值为 none，表示不 chroot(2):
#ChrootDirectory none
# 可选地指定附加文本以附加到服务器在连接时发送的 SSH 协议横幅。默认值为"FreeBSD-20240318"。值 none 可用于禁用此功能:
#VersionAddendum none

# no default banner path
# 设置登录前的横幅,指定文件路径:
#Banner none

# Allow client to pass locale environment variables
AcceptEnv LANG LC_*

# override default of no subsystems
# 配置外部子系统（例如文件传输守护进程）。参数应该是子系统名称和 根据子系统请求执行的命令（带有可选参数）。命令sftp-server实现了 SFTP 文件传输子系统。或者，名称  internal-sftp  实现进程内SFTP 服务器。这可以简化使用ChrootDirectory的配置	 ，以在客户端上强制使用不同的文件系统根。默认情况下没有定义子系统:
Subsystem	sftp	/usr/lib/openssh/sftp-server

# Example of overriding settings on a per-user basis
#Match User anoncvs
# 指定是否允许 X11 转发。参数 必须是 yes 或 no。默认值为 no,启用 X11 转发后，如果 sshd(8) 代理 显示配置为监听通配符地址（请参阅 X11UseLocalhost），则服务器和客户端显示可能会受到额外的暴露，尽管这不是默认值。此外，身份验证欺骗和身份验证数据验证和替换发生在客户端。使用 X11 转发的安全风险是，当 SSH 客户端请求转发时，客户端的 X11 显示服务器可能会受到攻击（请参阅 ssh_config(5) 中有关 ForwardX11 的警告）。系统管理员可能希望保护可能因不知情地请求 X11 转发而使自己暴露于攻击的客户端，这可能需要设置为 no。请注意，禁用 X11 转发并不会阻止用户转发 X11 流量，因为用户始终可以安装自己的转发器:
#	X11Forwarding no
# 指定是否允许TCP转发。可用选项为yes（默认值）或all，允许TCP转发，no阻止所有TCP转发，local仅允许本地（从ssh（1）的视角）转发，或remote仅允许远程转发。请注意，禁用TCP转发并不能提高安全性，除非还拒绝用户访问shell，因为他们总是可以安装自己的转发器
#	AllowTcpForwarding no
# 指定是否允许 pty(4) 分配。默认为是。
#	PermitTTY no
#	ForceCommand cvs server

FreeBSD解释

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
888
889
890
891
892
893
894
895
896
897
898
899
900
901
902
903
904
905
906
907
908
909
910
911
912
913
914
915
916
917
918
919
920
921
922
923
924
925
926
927
928
929
930
931
932
933
934
935
936
937
938
939
940
941
942
943
944
945
946
947
948
949
950
951
952
953
954
955
956
957
958
959
960
961
962
963
964
965
966
967
968
969
970
971
972
973
974
975
976
977
978
979
980
981
982
983
984
985
986
987
988
989
990
991
992
993
994
995
996
997
998
999
1000
1001
1002
1003
1004
1005
1006
1007
1008
1009
1010
1011
1012
1013
1014
1015
1016
1017
1018
1019
1020
1021
1022
1023
1024
1025
1026
1027
1028
1029
1030
1031
1032
1033
1034
1035
1036
1037
1038
1039
1040
1041
1042
1043
1044
1045
1046
1047
1048
1049
1050
1051
1052
1053
1054
1055
1056
1057
1058
1059
1060
1061
1062
1063
1064
1065
1066
1067
1068
1069
1070
1071
1072
1073
1074
1075
1076
1077
1078
1079
1080
1081
1082
1083
1084
1085
1086
1087
1088
1089
1090
1091
1092
1093
1094
1095
1096
1097
1098
1099
1100
1101
1102
1103
1104
1105
1106
1107
1108
1109
1110
1111
1112
1113
1114
1115
1116
1117
1118
1119
1120
1121
1122
1123
1124
1125
1126
1127
1128
1129
1130
1131
1132
1133
1134
1135
1136
1137
1138
1139
1140
1141
1142
1143
1144
1145
1146
1147
1148
1149
1150
1151
1152
1153
1154
1155
1156
1157
1158
1159
1160
1161
1162
1163
1164
1165
1166
1167
1168
1169
1170
1171
1172
1173
1174
1175
1176
1177
1178
1179
1180
1181
1182
1183
1184
1185
1186
1187
1188
1189
1190
1191
1192
1193
1194
1195
1196
1197
1198
1199
1200
1201
1202
1203
1204
1205
1206
1207
1208
1209
1210
1211
1212
1213
1214
1215
1216
1217
1218
1219
1220
1221
1222
1223
1224
1225
1226
1227
1228
1229
1230
1231
1232
1233
1234
1235
1236
1237
1238
1239
1240
1241
1242
1243
1244
1245
1246
1247
1248
1249
1250
1251
1252
1253
1254
1255
1256
1257
1258
1259
1260
1261
1262
1263
1264
1265
1266
1267
1268
1269
1270
1271
1272
1273
1274
1275
1276
1277
1278
1279
1280
1281
1282
1283
1284
1285
1286
1287
1288
1289
1290
1291
1292
1293
1294
1295
1296
1297
1298
1299
1300
1301
1302
1303
1304
1305
1306
1307
1308
1309
1310
1311
1312
1313
1314
1315
1316
1317
1318
1319
1320
1321
1322

NAME
       sshd_config -- OpenSSH daemon configuration file

DESCRIPTION
       sshd(8) reads configuration data	from /etc/ssh/sshd_config (or the file
       specified  with -f on the command line).	 The file contains keyword-ar-
       gument pairs, one per line.  Unless noted otherwise, for	each  keyword,
       the  first  obtained  value  will be used.  Lines starting with `#' and
       empty lines are interpreted as comments.	 Arguments may	optionally  be
       enclosed	 in double quotes (") in order to represent arguments contain-
       ing spaces.

       The possible keywords and their meanings	are as follows (note that key-
       words are case-insensitive and arguments	are case-sensitive):

       AcceptEnv
	       Specifies what environment variables sent by the	client will be
	       copied into the session's environ(7).  See SendEnv  and	SetEnv
	       in ssh_config(5)	for how	to configure the client.  The TERM en-
	       vironment  variable  is always accepted whenever	the client re-
	       quests a	pseudo-terminal	as it is  required  by	the  protocol.
	       Variables are specified by name,	which may contain the wildcard
	       characters  `*' and `?'.	 Multiple environment variables	may be
	       separated by whitespace or spread across	multiple AcceptEnv di-
	       rectives.  Be warned that some environment variables  could  be
	       used  to	bypass restricted user environments.  For this reason,
	       care should be taken in the use of this directive.  The default
	       is not to accept	any environment	variables.

       AddressFamily
	       Specifies which address	family	should	be  used  by  sshd(8).
	       Valid arguments are any (the default), inet (use	IPv4 only), or
	       inet6 (use IPv6 only).

       AllowAgentForwarding
	       Specifies  whether  ssh-agent(1)	 forwarding is permitted.  The
	       default is yes.	Note that disabling agent forwarding does  not
	       improve	security unless	users are also denied shell access, as
	       they can	always install their own forwarders.

       AllowGroups
	       This keyword can	be followed by a list of group name  patterns,
	       separated  by  spaces.  If specified, login is allowed only for
	       users whose primary group or supplementary group	 list  matches
	       one  of	the patterns.  Only group names	are valid; a numerical
	       group ID	is not recognized.  By default,	login is  allowed  for
	       all  groups.  The allow/deny groups directives are processed in
	       the following order: DenyGroups,	AllowGroups.

	       See PATTERNS in ssh_config(5) for more information on patterns.
	       This keyword may	appear multiple	times in sshd_config with each
	       instance	appending to the list.

       AllowStreamLocalForwarding
	       Specifies whether StreamLocal (Unix-domain  socket)  forwarding
	       is  permitted.	The available options are yes (the default) or
	       all to allow StreamLocal	forwarding, no to prevent all  Stream-
	       Local forwarding, local to allow	local (from the	perspective of
	       ssh(1))	forwarding  only  or remote to allow remote forwarding
	       only.  Note that	disabling StreamLocal forwarding does not  im-
	       prove  security	unless	users are also denied shell access, as
	       they can	always install their own forwarders.

       AllowTcpForwarding
	       Specifies whether TCP forwarding	is permitted.	The  available
	       options	are  yes (the default) or all to allow TCP forwarding,
	       no to prevent all TCP forwarding, local to  allow  local	 (from
	       the  perspective	 of ssh(1)) forwarding only or remote to allow
	       remote forwarding only.	Note  that  disabling  TCP  forwarding
	       does  not  improve  security unless users are also denied shell
	       access, as they can always install their	own forwarders.

       AllowUsers
	       This keyword can	be followed by a list of user  name  patterns,
	       separated  by  spaces.  If specified, login is allowed only for
	       user names that match one of the	patterns.  Only	user names are
	       valid; a	numerical user ID is not recognized.  By default,  lo-
	       gin  is	allowed	 for all users.	 If the	pattern	takes the form
	       USER@HOST then USER and HOST are	separately checked,  restrict-
	       ing  logins  to	particular  users from particular hosts.  HOST
	       criteria	may additionally contain addresses to  match  in  CIDR
	       address/masklen	format.	  The  allow/deny users	directives are
	       processed in the	following order: DenyUsers, AllowUsers.

	       See PATTERNS in ssh_config(5) for more information on patterns.
	       This keyword may	appear multiple	times in sshd_config with each
	       instance	appending to the list.

       AuthenticationMethods
	       Specifies the authentication methods that must be  successfully
	       completed for a user to be granted access.  This	option must be
	       followed	by one or more lists of	comma-separated	authentication
	       method  names,  or by the single	string any to indicate the de-
	       fault behaviour of accepting any	single authentication  method.
	       If  the	default	 is overridden,	then successful	authentication
	       requires	completion of every method in at least	one  of	 these
	       lists.

	       For		   example,		   "publickey,password
	       publickey,keyboard-interactive" would require the user to  com-
	       plete public key	authentication,	followed by either password or
	       keyboard	 interactive  authentication.	Only  methods that are
	       next in one or more lists are offered at	 each  stage,  so  for
	       this  example  it  would	not be possible	to attempt password or
	       keyboard-interactive authentication before public key.

	       For keyboard interactive	authentication it is also possible  to
	       restrict	 authentication	 to  a	specific device	by appending a
	       colon followed by the device identifier bsdauth	or  pam.   de-
	       pending	  on   the   server   configuration.	For   example,
	       "keyboard-interactive:bsdauth" would restrict keyboard interac-
	       tive authentication to the bsdauth device.

	       If the publickey	method is listed more than once, sshd(8) veri-
	       fies that keys that have	been used successfully are not	reused
	       for	subsequent	authentications.      For     example,
	       "publickey,publickey" requires successful authentication	 using
	       two different public keys.

	       Note  that each authentication method listed should also	be ex-
	       plicitly	enabled	in the configuration.

	       The available authentication  methods  are:  "gssapi-with-mic",
	       "hostbased", "keyboard-interactive", "none" (used for access to
	       password-less  accounts	when PermitEmptyPasswords is enabled),
	       "password" and "publickey".

       AuthorizedKeysCommand
	       Specifies a program to be used to look  up  the	user's	public
	       keys.  The program must be owned	by root, not writable by group
	       or  others  and	specified  by  an absolute path.  Arguments to
	       AuthorizedKeysCommand  accept  the  tokens  described  in   the
	       "TOKENS"	section.  If no	arguments are specified	then the user-
	       name of the target user is used.

	       The  program  should  produce  on  standard output zero or more
	       lines  of  authorized_keys  output  (see	 "AUTHORIZED_KEYS"  in
	       sshd(8)).   AuthorizedKeysCommand  is  tried  after  the	 usual
	       AuthorizedKeysFile files	and will not be	executed if a matching
	       key is found there.  By default,	 no  AuthorizedKeysCommand  is
	       run.

       AuthorizedKeysCommandUser
	       Specifies     the     user     under    whose	account	   the
	       AuthorizedKeysCommand is	run.  It is recommended	to use a dedi-
	       cated user that has no other role on the	host than running  au-
	       thorized	 keys commands.	 If AuthorizedKeysCommand is specified
	       but AuthorizedKeysCommandUser is	not, then sshd(8) will	refuse
	       to start.

       AuthorizedKeysFile
	       Specifies  the file that	contains the public keys used for user
	       authentication.	The format is described	in the AUTHORIZED_KEYS
	       FILE    FORMAT	 section    of	  sshd(8).     Arguments    to
	       AuthorizedKeysFile  accept the tokens described in the "TOKENS"
	       section.	 After expansion, AuthorizedKeysFile is	taken to be an
	       absolute	path or	one relative to	 the  user's  home  directory.
	       Multiple	 files may be listed, separated	by whitespace.	Alter-
	       nately this option may be set to	none to	skip checking for user
	       keys  in	  files.    The	  default   is	 ".ssh/authorized_keys
	       .ssh/authorized_keys2".

       AuthorizedPrincipalsCommand
	       Specifies  a program to be used to generate the list of allowed
	       certificate principals as  per  AuthorizedPrincipalsFile.   The
	       program	must be	owned by root, not writable by group or	others
	       and   specified	 by   an   absolute   path.    Arguments    to
	       AuthorizedPrincipalsCommand  accept the tokens described	in the
	       "TOKENS"	section.  If no	arguments are specified	then the user-
	       name of the target user is used.

	       The program should produce on  standard	output	zero  or  more
	       lines	of   AuthorizedPrincipalsFile	output.	   If	either
	       AuthorizedPrincipalsCommand  or	 AuthorizedPrincipalsFile   is
	       specified,  then	certificates offered by	the client for authen-
	       tication	must contain a principal that is listed.  By  default,
	       no AuthorizedPrincipalsCommand is run.

       AuthorizedPrincipalsCommandUser
	       Specifies     the     user     under    whose	account	   the
	       AuthorizedPrincipalsCommand is run.  It is recommended to use a
	       dedicated user that has no other	role on	the host than  running
	       authorized principals commands.	If AuthorizedPrincipalsCommand
	       is  specified  but AuthorizedPrincipalsCommandUser is not, then
	       sshd(8) will refuse to start.

       AuthorizedPrincipalsFile
	       Specifies a file	that lists principal names that	 are  accepted
	       for certificate authentication.	When using certificates	signed
	       by  a  key  listed in TrustedUserCAKeys,	this file lists	names,
	       one of which must appear	in the certificate for it  to  be  ac-
	       cepted  for authentication.  Names are listed one per line pre-
	       ceded by	key options (as	 described  in	"AUTHORIZED_KEYS  FILE
	       FORMAT"	in  sshd(8)).	Empty lines and	comments starting with
	       `#' are ignored.

	       Arguments to AuthorizedPrincipalsFile  accept  the  tokens  de-
	       scribed	  in   the   "TOKENS"	section.    After   expansion,
	       AuthorizedPrincipalsFile	is taken to be an absolute path	or one
	       relative	to the user's home directory.  The  default  is	 none,
	       i.e.  not to use	a principals file - in this case, the username
	       of the user must	appear in a certificate's principals list  for
	       it to be	accepted.

	       Note  that AuthorizedPrincipalsFile is only used	when authenti-
	       cation proceeds using a CA listed in TrustedUserCAKeys  and  is
	       not   consulted	 for  certification  authorities  trusted  via
	       ~/.ssh/authorized_keys, though the principals= key  option  of-
	       fers a similar facility (see sshd(8) for	details).

       Banner  The  contents of	the specified file are sent to the remote user
	       before authentication is	allowed.  If the argument is none then
	       no banner is displayed.	By default, no banner is displayed.

       CASignatureAlgorithms
	       Specifies which algorithms are allowed for signing of  certifi-
	       cates by	certificate authorities	(CAs).	The default is:

		     ssh-ed25519,ecdsa-sha2-nistp256,
		     ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
		     sk-ssh-ed25519@openssh.com,
		     sk-ecdsa-sha2-nistp256@openssh.com,
		     rsa-sha2-512,rsa-sha2-256

	       If  the	specified  list	 begins	with a `+' character, then the
	       specified algorithms will be appended to	the  default  set  in-
	       stead  of  replacing them.  If the specified list begins	with a
	       `-' character, then the specified algorithms  (including	 wild-
	       cards)  will be removed from the	default	set instead of replac-
	       ing them.

	       Certificates signed using other algorithms will not be accepted
	       for public key or host-based authentication.

       ChannelTimeout
	       Specifies whether and how quickly sshd(8) should	close inactive
	       channels.    Timeouts   are   specified	 as   one   or	  more
	       "type=interval" pairs separated by whitespace, where the	"type"
	       must  be	a channel type name (as	described in the table below),
	       optionally containing wildcard characters.

	       The timeout value "interval" is specified in seconds or may use
	       any of the units	documented in the "TIME	FORMATS" section.  For
	       example,	"session:*=5m" would cause all sessions	 to  terminate
	       after five minutes of inactivity.  Specifying a zero value dis-
	       ables the inactivity timeout.

	       The available channel types include:

	       agent-connection
		       Open connections	to ssh-agent(1).

	       direct-tcpip, direct-streamlocal@openssh.com
		       Open TCP	or Unix	socket (respectively) connections that
		       have  been  established from a ssh(1) local forwarding,
		       i.e. LocalForward or DynamicForward.

	       forwarded-tcpip,	forwarded-streamlocal@openssh.com
		       Open TCP	or Unix	socket (respectively) connections that
		       have been established to	a sshd(8) listening on	behalf
		       of a ssh(1) remote forwarding, i.e. RemoteForward.

	       session:command
		       Command execution sessions.

	       session:shell
		       Interactive shell sessions.

	       session:subsystem:...
		       Subsystem  sessions,  e.g.  for sftp(1),	which could be
		       identified as session:subsystem:sftp.

	       x11-connection
		       Open X11	forwarding sessions.

	       Note that in all	the above cases, terminating an	inactive  ses-
	       sion does not guarantee to remove all resources associated with
	       the  session,  e.g.  shell processes or X11 clients relating to
	       the session may continue	to execute.

	       Moreover, terminating an	inactive channel or session  does  not
	       necessarily  close  the	SSH  connection, nor does it prevent a
	       client from requesting another channel of the  same  type.   In
	       particular,  expiring  an  inactive forwarding session does not
	       prevent another identical forwarding  from  being  subsequently
	       created.	  See  also UnusedConnectionTimeout, which may be used
	       in conjunction with this	option.

	       The default is not to expire channels of	any type for  inactiv-
	       ity.

       ChrootDirectory
	       Specifies the pathname of a directory to	chroot(2) to after au-
	       thentication.   At session startup sshd(8) checks that all com-
	       ponents of the pathname are root-owned  directories  which  are
	       not  writable  by  any  other user or group.  After the chroot,
	       sshd(8) changes the working directory to	the user's home	direc-
	       tory.  Arguments	to ChrootDirectory accept the tokens described
	       in the "TOKENS" section.

	       The ChrootDirectory must	contain	the necessary files and	direc-
	       tories to support the user's session.  For an interactive  ses-
	       sion this requires at least a shell, typically sh(1), and basic
	       /dev  nodes  such  as  null(4),	zero(4),  stdin(4), stdout(4),
	       stderr(4), and tty(4) devices.  For file	transfer sessions  us-
	       ing SFTP	no additional configuration of the environment is nec-
	       essary  if  the in-process sftp-server is used, though sessions
	       which use logging may require /dev/log inside the chroot	direc-
	       tory on some operating  systems	(see  sftp-server(8)  for  de-
	       tails).

	       For  safety,  it	is very	important that the directory hierarchy
	       be prevented from modification by other processes on the	system
	       (especially those outside the jail).  Misconfiguration can lead
	       to unsafe environments which sshd(8) cannot detect.

	       The default is none, indicating not to chroot(2).

       Ciphers
	       Specifies the ciphers allowed.  Multiple	ciphers	must be	comma-
	       separated.  If the specified list begins	with a `+'  character,
	       then  the specified ciphers will	be appended to the default set
	       instead of replacing them.  If the specified list begins	with a
	       `-' character, then the specified ciphers (including wildcards)
	       will be removed from the	default	set instead of replacing them.
	       If the specified	list begins with a  `^'	 character,  then  the
	       specified  ciphers  will	 be  placed at the head	of the default
	       set.

	       The supported ciphers are:

		     3des-cbc
		     aes128-cbc
		     aes192-cbc
		     aes256-cbc
		     aes128-ctr
		     aes192-ctr
		     aes256-ctr
		     aes128-gcm@openssh.com
		     aes256-gcm@openssh.com
		     chacha20-poly1305@openssh.com

	       The default is:

		     chacha20-poly1305@openssh.com,
		     aes128-ctr,aes192-ctr,aes256-ctr,
		     aes128-gcm@openssh.com,aes256-gcm@openssh.com

	       The list	of available ciphers may also be obtained  using  "ssh
	       -Q cipher".

       ClientAliveCountMax
	       Sets  the  number  of  client  alive messages which may be sent
	       without sshd(8) receiving any messages back  from  the  client.
	       If  this	 threshold  is reached while client alive messages are
	       being sent, sshd	will disconnect	the  client,  terminating  the
	       session.	  It is	important to note that the use of client alive
	       messages	is very	different from TCPKeepAlive.  The client alive
	       messages	are sent through the encrypted channel	and  therefore
	       will  not  be  spoofable.   The TCP keepalive option enabled by
	       TCPKeepAlive is spoofable.  The client alive mechanism is valu-
	       able when the client or server depend on	knowing	when a connec-
	       tion has	become unresponsive.

	       The default value is 3.	If ClientAliveInterval is set  to  15,
	       and  ClientAliveCountMax	 is  left at the default, unresponsive
	       SSH clients will	be disconnected	after  approximately  45  sec-
	       onds.   Setting	a zero ClientAliveCountMax disables connection
	       termination.

       ClientAliveInterval
	       Sets a timeout interval in seconds after	which if no  data  has
	       been  received  from  the  client,  sshd(8) will	send a message
	       through the encrypted channel to	request	a  response  from  the
	       client.	 The default is	0, indicating that these messages will
	       not be sent to the client.

       Compression
	       Specifies whether compression is	enabled	after the user has au-
	       thenticated successfully.  The argument must be yes, delayed (a
	       legacy synonym for yes) or no.  The default is yes.

       DenyGroups
	       This keyword can	be followed by a list of group name  patterns,
	       separated  by spaces.  Login is disallowed for users whose pri-
	       mary group or supplementary group list matches one of the  pat-
	       terns.  Only group names	are valid; a numerical group ID	is not
	       recognized.   By	default, login is allowed for all groups.  The
	       allow/deny groups directives are	processed in the following or-
	       der: DenyGroups,	AllowGroups.

	       See PATTERNS in ssh_config(5) for more information on patterns.
	       This keyword may	appear multiple	times in sshd_config with each
	       instance	appending to the list.

       DenyUsers
	       This keyword can	be followed by a list of user  name  patterns,
	       separated  by  spaces.  Login is	disallowed for user names that
	       match one of the	patterns.  Only	user names are valid; a	numer-
	       ical user ID is not recognized.	By default, login  is  allowed
	       for  all	 users.	  If the pattern takes the form	USER@HOST then
	       USER and	HOST are separately  checked,  restricting  logins  to
	       particular  users from particular hosts.	 HOST criteria may ad-
	       ditionally contain addresses to match in	 CIDR  address/masklen
	       format.	 The  allow/deny users directives are processed	in the
	       following order:	DenyUsers, AllowUsers.

	       See PATTERNS in ssh_config(5) for more information on patterns.
	       This keyword may	appear multiple	times in sshd_config with each
	       instance	appending to the list.

       DisableForwarding
	       Disables	all forwarding features, including X11,	 ssh-agent(1),
	       TCP  and	StreamLocal.  This option overrides all	other forward-
	       ing-related options and may simplify restricted configurations.

       ExposeAuthInfo
	       Writes a	temporary file containing  a  list  of	authentication
	       methods and public credentials (e.g. keys) used to authenticate
	       the user.  The location of the file is exposed to the user ses-
	       sion  through  the SSH_USER_AUTH	environment variable.  The de-
	       fault is	no.

       FingerprintHash
	       Specifies the hash algorithm  used  when	 logging  key  finger-
	       prints.	 Valid	options	 are:  md5 and sha256.	The default is
	       sha256.

       ForceCommand
	       Forces the execution of the command specified by	 ForceCommand,
	       ignoring	 any  command  supplied	by the client and ~/.ssh/rc if
	       present.	 The command is	invoked	 by  using  the	 user's	 login
	       shell  with  the	-c option.  This applies to shell, command, or
	       subsystem execution.  It	is most	useful inside a	 Match	block.
	       The  command  originally	supplied by the	client is available in
	       the SSH_ORIGINAL_COMMAND	environment  variable.	 Specifying  a
	       command	of  internal-sftp  will	force the use of an in-process
	       SFTP server that	requires  no  support  files  when  used  with
	       ChrootDirectory.	 The default is	none.

       GatewayPorts
	       Specifies  whether remote hosts are allowed to connect to ports
	       forwarded for the client.  By  default,	sshd(8)	 binds	remote
	       port  forwardings to the	loopback address.  This	prevents other
	       remote hosts from connecting to forwarded ports.	  GatewayPorts
	       can  be used to specify that sshd should	allow remote port for-
	       wardings	to bind	to non-loopback	addresses, thus	allowing other
	       hosts to	connect.  The argument may be no to force remote  port
	       forwardings  to	be  available  to  the local host only,	yes to
	       force remote port forwardings to	bind to	the wildcard  address,
	       or clientspecified to allow the client to select	the address to
	       which the forwarding is bound.  The default is no.

       GSSAPIAuthentication
	       Specifies  whether  user	 authentication	based on GSSAPI	is al-
	       lowed.  The default is no.

       GSSAPICleanupCredentials
	       Specifies whether to automatically destroy the  user's  creden-
	       tials cache on logout.  The default is yes.

       GSSAPIStrictAcceptorCheck
	       Determines  whether to be strict	about the identity of the GSS-
	       API acceptor a client authenticates against.   If  set  to  yes
	       then  the  client must authenticate against the host service on
	       the current hostname.  If set to	no then	the client may authen-
	       ticate against any service key stored in	the machine's  default
	       store.	This  facility is provided to assist with operation on
	       multi homed machines.  The default is yes.

       HostbasedAcceptedAlgorithms
	       Specifies the signature algorithms that will  be	 accepted  for
	       hostbased authentication	as a list of comma-separated patterns.
	       Alternately  if the specified list begins with a	`+' character,
	       then the	specified signature algorithms will be appended	to the
	       default set instead of replacing	them.  If the  specified  list
	       begins with a `-' character, then the specified signature algo-
	       rithms  (including  wildcards) will be removed from the default
	       set instead of replacing	them.  If the  specified  list	begins
	       with  a	`^' character, then the	specified signature algorithms
	       will be placed at the head of the default set.  The default for
	       this option is:

		  ssh-ed25519-cert-v01@openssh.com,
		  ecdsa-sha2-nistp256-cert-v01@openssh.com,
		  ecdsa-sha2-nistp384-cert-v01@openssh.com,
		  ecdsa-sha2-nistp521-cert-v01@openssh.com,
		  sk-ssh-ed25519-cert-v01@openssh.com,
		  sk-ecdsa-sha2-nistp256-cert-v01@openssh.com,
		  rsa-sha2-512-cert-v01@openssh.com,
		  rsa-sha2-256-cert-v01@openssh.com,
		  ssh-ed25519,
		  ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
		  sk-ssh-ed25519@openssh.com,
		  sk-ecdsa-sha2-nistp256@openssh.com,
		  rsa-sha2-512,rsa-sha2-256

	       The list	of available signature algorithms may also be obtained
	       using "ssh -Q HostbasedAcceptedAlgorithms".  This was  formerly
	       named HostbasedAcceptedKeyTypes.

       HostbasedAuthentication
	       Specifies whether rhosts	or /etc/hosts.equiv authentication to-
	       gether with successful public key client	host authentication is
	       allowed (host-based authentication).  The default is no.

       HostbasedUsesNameFromPacketOnly
	       Specifies  whether  or not the server will attempt to perform a
	       reverse name lookup when	matching the name  in  the  ~/.shosts,
	       ~/.rhosts,      and	/etc/hosts.equiv      files	during
	       HostbasedAuthentication.	 A setting of yes means	 that  sshd(8)
	       uses  the name supplied by the client rather than attempting to
	       resolve the name	from the TCP connection	itself.	  The  default
	       is no.

       HostCertificate
	       Specifies  a  file  containing  a public	host certificate.  The
	       certificate's public key	must match a private host key  already
	       specified  by HostKey.  The default behaviour of	sshd(8)	is not
	       to load any certificates.

       HostKey
	       Specifies a file	containing a private host  key	used  by  SSH.
	       The	 defaults	are	  /etc/ssh/ssh_host_ecdsa_key,
	       /etc/ssh/ssh_host_ed25519_key and /etc/ssh/ssh_host_rsa_key.

	       Note  that  sshd(8)  will  refuse  to  use  a  file  if	it  is
	       group/world-accessible  and  that  the HostKeyAlgorithms	option
	       restricts which of the keys are actually	used by	sshd(8).

	       It is possible to have multiple host key	 files.	  It  is  also
	       possible	 to  specify  public  host key files instead.  In this
	       case operations on the private key  will	 be  delegated	to  an
	       ssh-agent(1).

       HostKeyAgent
	       Identifies  the	UNIX-domain socket used	to communicate with an
	       agent that has access to	the private host keys.	If the	string
	       "SSH_AUTH_SOCK"	is  specified, the location of the socket will
	       be read from the	SSH_AUTH_SOCK environment variable.

       HostKeyAlgorithms
	       Specifies the host key signature	algorithms that	the server of-
	       fers.  The default for this option is:

		  ssh-ed25519-cert-v01@openssh.com,
		  ecdsa-sha2-nistp256-cert-v01@openssh.com,
		  ecdsa-sha2-nistp384-cert-v01@openssh.com,
		  ecdsa-sha2-nistp521-cert-v01@openssh.com,
		  sk-ssh-ed25519-cert-v01@openssh.com,
		  sk-ecdsa-sha2-nistp256-cert-v01@openssh.com,
		  rsa-sha2-512-cert-v01@openssh.com,
		  rsa-sha2-256-cert-v01@openssh.com,
		  ssh-ed25519,
		  ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
		  sk-ssh-ed25519@openssh.com,
		  sk-ecdsa-sha2-nistp256@openssh.com,
		  rsa-sha2-512,rsa-sha2-256

	       The list	of available signature algorithms may also be obtained
	       using "ssh -Q HostKeyAlgorithms".

       IgnoreRhosts
	       Specifies whether to ignore per-user .rhosts and	.shosts	 files
	       during	   HostbasedAuthentication.	  The	   system-wide
	       /etc/hosts.equiv	and /etc/ssh/shosts.equiv are still  used  re-
	       gardless	of this	setting.

	       Accepted	 values	 are  yes (the default)	to ignore all per-user
	       files, shosts-only to allow the use of .shosts  but  to	ignore
	       .rhosts or no to	allow both .shosts and rhosts.

       IgnoreUserKnownHosts
	       Specifies    whether   sshd(8)	should	 ignore	  the	user's
	       ~/.ssh/known_hosts during HostbasedAuthentication and use  only
	       the system-wide known hosts file	/etc/ssh/ssh_known_hosts.  The
	       default is "no".

       Include
	       Include	the  specified	configuration file(s).	Multiple path-
	       names may be specified and each pathname	 may  contain  glob(7)
	       wildcards that will be expanded and processed in	lexical	order.
	       Files without absolute paths are	assumed	to be in /etc/ssh.  An
	       Include	directive  may	appear inside a	Match block to perform
	       conditional inclusion.

       IPQoS   Specifies the IPv4 type-of-service or DSCP class	for  the  con-
	       nection.	  Accepted  values  are	 af11, af12, af13, af21, af22,
	       af23, af31, af32, af33, af41, af42, af43, cs0, cs1,  cs2,  cs3,
	       cs4,  cs5, cs6, cs7, ef,	le, lowdelay, throughput, reliability,
	       a numeric value,	or none	to use the operating  system  default.
	       This  option may	take one or two	arguments, separated by	white-
	       space.  If one argument is specified, it	is used	as the	packet
	       class  unconditionally.	If two values are specified, the first
	       is automatically	selected for interactive sessions and the sec-
	       ond for non-interactive sessions.  The default is af21 (Low-La-
	       tency Data) for interactive sessions and	cs1 (Lower Effort) for
	       non-interactive sessions.

       KbdInteractiveAuthentication
	       Specifies whether to allow keyboard-interactive authentication.
	       All authentication styles  from	login.conf(5)  are  supported.
	       The  default  is	yes.  The argument to this keyword must	be yes
	       or no.  ChallengeResponseAuthentication is a  deprecated	 alias
	       for this.

       KerberosAuthentication
	       Specifies  whether  the	password  provided  by	the  user  for
	       PasswordAuthentication will be validated	through	 the  Kerberos
	       KDC.   To  use this option, the server needs a Kerberos servtab
	       which allows the	verification of	the KDC's identity.   The  de-
	       fault is	no.

       KerberosGetAFSToken
	       If  AFS is active and the user has a Kerberos 5 TGT, attempt to
	       acquire an AFS token before accessing the  user's  home	direc-
	       tory.  The default is no.

       KerberosOrLocalPasswd
	       If  password  authentication  through  Kerberos	fails then the
	       password	will be	validated via any additional  local  mechanism
	       such as /etc/passwd.  The default is yes.

       KerberosTicketCleanup
	       Specifies  whether  to  automatically destroy the user's	ticket
	       cache file on logout.  The default is yes.

       KexAlgorithms
	       Specifies the available KEX (Key	Exchange) algorithms.	Multi-
	       ple  algorithms	must  be  comma-separated.  Alternately	if the
	       specified list begins with a `+'	character, then	the  specified
	       algorithms  will	 be appended to	the default set	instead	of re-
	       placing them.  If the specified list begins with	a `-'  charac-
	       ter,  then  the specified algorithms (including wildcards) will
	       be removed from the default set instead of replacing them.   If
	       the specified list begins with a	`^' character, then the	speci-
	       fied  algorithms	will be	placed at the head of the default set.
	       The supported algorithms	are:

		     curve25519-sha256
		     curve25519-sha256@libssh.org
		     diffie-hellman-group1-sha1
		     diffie-hellman-group14-sha1
		     diffie-hellman-group14-sha256
		     diffie-hellman-group16-sha512
		     diffie-hellman-group18-sha512
		     diffie-hellman-group-exchange-sha1
		     diffie-hellman-group-exchange-sha256
		     ecdh-sha2-nistp256
		     ecdh-sha2-nistp384
		     ecdh-sha2-nistp521
		     sntrup761x25519-sha512@openssh.com

	       The default is:

		     sntrup761x25519-sha512@openssh.com,
		     curve25519-sha256,curve25519-sha256@libssh.org,
		     ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,
		     diffie-hellman-group-exchange-sha256,
		     diffie-hellman-group16-sha512,diffie-hellman-group18-sha512,
		     diffie-hellman-group14-sha256

	       The list	of available key exchange algorithms may also  be  ob-
	       tained using "ssh -Q KexAlgorithms".

       ListenAddress
	       Specifies  the  local  addresses	sshd(8)	should listen on.  The
	       following forms may be used:

		     ListenAddress hostname|address [rdomain domain]
		     ListenAddress hostname:port [rdomain domain]
		     ListenAddress IPv4_address:port [rdomain domain]
		     ListenAddress [hostname|address]:port [rdomain domain]

	       The optional rdomain qualifier requests sshd(8)	listen	in  an
	       explicit	 routing  domain.  If port is not specified, sshd will
	       listen on the address and all Port options specified.  The  de-
	       fault  is  to  listen on	all local addresses on the current de-
	       fault routing domain.  Multiple ListenAddress options are  per-
	       mitted.	  For	more   information  on	routing	 domains,  see
	       rdomain(4).

       LoginGraceTime
	       The server disconnects after this time if the user has not suc-
	       cessfully logged	in.  If	the value  is  0,  there  is  no  time
	       limit.  The default is 120 seconds.

       LogLevel
	       Gives  the  verbosity  level that is used when logging messages
	       from sshd(8).  The possible values are:	QUIET,	FATAL,	ERROR,
	       INFO,  VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.  The default
	       is INFO.	 DEBUG and DEBUG1 are equivalent.  DEBUG2  and	DEBUG3
	       each specify higher levels of debugging output.	Logging	with a
	       DEBUG  level  violates  the  privacy of users and is not	recom-
	       mended.

       LogVerbose
	       Specify one or more overrides to	LogLevel.   An	override  con-
	       sists of	a pattern lists	that matches the source	file, function
	       and line	number to force	detailed logging for.  For example, an
	       override	pattern	of:

		     kex.c:*:1000,*:kex_exchange_identification():*,packet.c:*

	       would  enable  detailed	logging	for line 1000 of kex.c,	every-
	       thing in	the kex_exchange_identification()  function,  and  all
	       code  in	the packet.c file.  This option	is intended for	debug-
	       ging and	no overrides are enabled by default.

       MACs    Specifies the available MAC (message authentication code) algo-
	       rithms.	The MAC	algorithm is used for data  integrity  protec-
	       tion.   Multiple	 algorithms  must  be comma-separated.	If the
	       specified list begins with a `+'	character, then	the  specified
	       algorithms  will	 be appended to	the default set	instead	of re-
	       placing them.  If the specified list begins with	a `-'  charac-
	       ter,  then  the specified algorithms (including wildcards) will
	       be removed from the default set instead of replacing them.   If
	       the specified list begins with a	`^' character, then the	speci-
	       fied algorithms will be placed at the head of the default set.

	       The  algorithms that contain "-etm" calculate the MAC after en-
	       cryption	(encrypt-then-mac).  These are	considered  safer  and
	       their use recommended.  The supported MACs are:

		     hmac-md5
		     hmac-md5-96
		     hmac-sha1
		     hmac-sha1-96
		     hmac-sha2-256
		     hmac-sha2-512
		     umac-64@openssh.com
		     umac-128@openssh.com
		     hmac-md5-etm@openssh.com
		     hmac-md5-96-etm@openssh.com
		     hmac-sha1-etm@openssh.com
		     hmac-sha1-96-etm@openssh.com
		     hmac-sha2-256-etm@openssh.com
		     hmac-sha2-512-etm@openssh.com
		     umac-64-etm@openssh.com
		     umac-128-etm@openssh.com

	       The default is:

		     umac-64-etm@openssh.com,umac-128-etm@openssh.com,
		     hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com,
		     hmac-sha1-etm@openssh.com,
		     umac-64@openssh.com,umac-128@openssh.com,
		     hmac-sha2-256,hmac-sha2-512,hmac-sha1

	       The list	of available MAC algorithms may	also be	obtained using
	       "ssh -Q mac".

       Match   Introduces  a conditional block.	 If all	of the criteria	on the
	       Match line are satisfied, the keywords on the  following	 lines
	       override	 those	set  in	the global section of the config file,
	       until either another Match line or the end of the file.	 If  a
	       keyword	appears	 in  multiple Match blocks that	are satisfied,
	       only the	first instance of the keyword is applied.

	       The arguments to	Match are one or more  criteria-pattern	 pairs
	       or the single token All which matches all criteria.  The	avail-
	       able  criteria  are User, Group,	Host, LocalAddress, LocalPort,
	       RDomain,	and Address (with RDomain representing the  rdomain(4)
	       on which	the connection was received).

	       The match patterns may consist of single	entries	or comma-sepa-
	       rated lists and may use the wildcard and	negation operators de-
	       scribed in the "PATTERNS" section of ssh_config(5).

	       The  patterns  in  an Address criteria may additionally contain
	       addresses to match in  CIDR  address/masklen  format,  such  as
	       192.0.2.0/24  or	2001:db8::/32.	Note that the mask length pro-
	       vided must be consistent	with the address - it is an  error  to
	       specify	a  mask	length that is too long	for the	address	or one
	       with bits set in	this host portion of the address.   For	 exam-
	       ple, 192.0.2.0/33 and 192.0.2.0/8, respectively.

	       Only  a subset of keywords may be used on the lines following a
	       Match   keyword.	    Available	 keywords    are    AcceptEnv,
	       AllowAgentForwarding,  AllowGroups, AllowStreamLocalForwarding,
	       AllowTcpForwarding,     AllowUsers,	AuthenticationMethods,
	       AuthorizedKeysCommand,		    AuthorizedKeysCommandUser,
	       AuthorizedKeysFile,		  AuthorizedPrincipalsCommand,
	       AuthorizedPrincipalsCommandUser,	     AuthorizedPrincipalsFile,
	       Banner, CASignatureAlgorithms, ChannelTimeout, ChrootDirectory,
	       ClientAliveCountMax,	 ClientAliveInterval,	   DenyGroups,
	       DenyUsers,   DisableForwarding,	ExposeAuthInfo,	 ForceCommand,
	       GatewayPorts,				 GSSAPIAuthentication,
	       HostbasedAcceptedAlgorithms,	      HostbasedAuthentication,
	       HostbasedUsesNameFromPacketOnly,	IgnoreRhosts, Include,	IPQoS,
	       KbdInteractiveAuthentication, KerberosAuthentication, LogLevel,
	       MaxAuthTries,	    MaxSessions,       PasswordAuthentication,
	       PermitEmptyPasswords,	     PermitListen,	   PermitOpen,
	       PermitRootLogin,	   PermitTTY,	 PermitTunnel,	 PermitUserRC,
	       PubkeyAcceptedAlgorithms,		 PubkeyAuthentication,
	       PubkeyAuthOptions,  RekeyLimit,	RevokedKeys,  RDomain, SetEnv,
	       StreamLocalBindMask, StreamLocalBindUnlink,  TrustedUserCAKeys,
	       UnusedConnectionTimeout,	 X11DisplayOffset,  X11Forwarding  and
	       X11UseLocalhost.

       MaxAuthTries
	       Specifies the maximum number of authentication attempts permit-
	       ted per connection.  Once the number of failures	 reaches  half
	       this value, additional failures are logged.  The	default	is 6.

       MaxSessions
	       Specifies  the maximum number of	open shell, login or subsystem
	       (e.g. sftp) sessions permitted per network connection.	Multi-
	       ple sessions may	be established by clients that support connec-
	       tion  multiplexing.   Setting MaxSessions to 1 will effectively
	       disable session multiplexing, whereas setting it	to 0 will pre-
	       vent all	shell, login and subsystem sessions while  still  per-
	       mitting forwarding.  The	default	is 10.

       MaxStartups
	       Specifies the maximum number of concurrent unauthenticated con-
	       nections	 to  the  SSH  daemon.	Additional connections will be
	       dropped until authentication succeeds or	the LoginGraceTime ex-
	       pires for a connection.	The default is 10:30:100.

	       Alternatively, random early drop	can be enabled	by  specifying
	       the   three   colon   separated	values	start:rate:full	 (e.g.
	       "10:30:60").  sshd(8) will refuse connection  attempts  with  a
	       probability of rate/100 (30%) if	there are currently start (10)
	       unauthenticated	connections.   The  probability	increases lin-
	       early and all connection	attempts are refused if	the number  of
	       unauthenticated connections reaches full	(60).

       ModuliFile
	       Specifies  the  moduli(5) file that contains the	Diffie-Hellman
	       groups used for	the  "diffie-hellman-group-exchange-sha1"  and
	       "diffie-hellman-group-exchange-sha256"  key  exchange  methods.
	       The default is /etc/moduli.

       PasswordAuthentication
	       Specifies whether password  authentication  is  allowed.	  Note
	       that	passwords     may     also     be     accepted	   via
	       KbdInteractiveAuthentication.  See also UsePAM.	The default is
	       no.

       PermitEmptyPasswords
	       When password authentication is allowed,	it  specifies  whether
	       the  server  allows  login  to  accounts	 with  empty  password
	       strings.	 The default is	no.

       PermitListen
	       Specifies the addresses/ports on	which a	remote TCP  port  for-
	       warding	may  listen.   The listen specification	must be	one of
	       the following forms:

		     PermitListen port
		     PermitListen host:port

	       Multiple	permissions may	be specified by	separating  them  with
	       whitespace.   An	 argument of any can be	used to	remove all re-
	       strictions and permit any listen	requests.  An argument of none
	       can be used to prohibit all listen requests.  The host name may
	       contain wildcards as  described	in  the	 PATTERNS  section  in
	       ssh_config(5).  The wildcard `*'	can also be used in place of a
	       port number to allow all	ports.	By default all port forwarding
	       listen  requests	are permitted.	Note that the GatewayPorts op-
	       tion may	further	restrict which addresses may be	 listened  on.
	       Note also that ssh(1) will request a listen host	of "localhost"
	       if  no listen host was specifically requested, and this name is
	       treated	differently  to	 explicit   localhost	addresses   of
	       "127.0.0.1" and "::1".

       PermitOpen
	       Specifies the destinations to which TCP port forwarding is per-
	       mitted.	 The  forwarding specification must be one of the fol-
	       lowing forms:

		     PermitOpen	host:port
		     PermitOpen	IPv4_addr:port
		     PermitOpen	[IPv6_addr]:port

	       Multiple	forwards may be	 specified  by	separating  them  with
	       whitespace.   An	 argument of any can be	used to	remove all re-
	       strictions and permit any forwarding requests.  An argument  of
	       none  can  be  used  to	prohibit all forwarding	requests.  The
	       wildcard	`*' can	be used	for host or port to allow all hosts or
	       ports respectively.  Otherwise, no pattern matching or  address
	       lookups	are  performed on supplied names.  By default all port
	       forwarding requests are permitted.

       PermitRootLogin
	       Specifies whether root can log in using ssh(1).	 The  argument
	       must  be	 yes,  prohibit-password, forced-commands-only,	or no.
	       The	default	     is	     no.       Note	 that	    if
	       ChallengeResponseAuthentication	and  UsePAM are	both yes, this
	       setting may be overridden by the	PAM policy.

	       If this option is set to	prohibit-password (or  its  deprecated
	       alias, without-password), password and keyboard-interactive au-
	       thentication are	disabled for root.

	       If  this	option is set to forced-commands-only, root login with
	       public key authentication will be  allowed,  but	 only  if  the
	       command option has been specified (which	may be useful for tak-
	       ing remote backups even if root login is	normally not allowed).
	       All other authentication	methods	are disabled for root.

	       If this option is set to	no, root is not	allowed	to log in.

       PermitTTY
	       Specifies  whether pty(4) allocation is permitted.  The default
	       is yes.

       PermitTunnel
	       Specifies whether tun(4)	device forwarding is allowed.  The ar-
	       gument must be yes, point-to-point (layer 3),  ethernet	(layer
	       2),  or	no.   Specifying  yes  permits both point-to-point and
	       ethernet.  The default is no.

	       Independent of this setting, the	permissions  of	 the  selected
	       tun(4) device must allow	access to the user.

       PermitUserEnvironment
	       Specifies  whether  ~/.ssh/environment and environment= options
	       in ~/.ssh/authorized_keys are processed by sshd(8).  Valid  op-
	       tions  are  yes,	no or a	pattern-list specifying	which environ-
	       ment variable names to accept (for example  "LANG,LC_*").   The
	       default	is  no.	  Enabling  environment	 processing may	enable
	       users to	bypass access restrictions in some configurations  us-
	       ing mechanisms such as LD_PRELOAD.

       PermitUserRC
	       Specifies  whether any ~/.ssh/rc	file is	executed.  The default
	       is yes.

       PerSourceMaxStartups
	       Specifies the number  of	 unauthenticated  connections  allowed
	       from  a	given  source address, or "none" if there is no	limit.
	       This limit is applied in	addition to MaxStartups, whichever  is
	       lower.  The default is none.

       PerSourceNetBlockSize
	       Specifies the number of bits of source address that are grouped
	       together	for the	purposes of applying PerSourceMaxStartups lim-
	       its.   Values  for  IPv4	 and optionally	IPv6 may be specified,
	       separated by a colon.  The default is 32:128, which means  each
	       address is considered individually.

       PidFile
	       Specifies the file that contains	the process ID of the SSH dae-
	       mon,   or   none	  to   not   write   one.    The   default  is
	       /var/run/sshd.pid.

       Port    Specifies the port number that sshd(8) listens on.  The default
	       is 22.  Multiple	options	of this	type are permitted.  See  also
	       ListenAddress.

       PrintLastLog
	       Specifies whether sshd(8) should	print the date and time	of the
	       last user login when a user logs	in interactively.  The default
	       is yes.

       PrintMotd
	       Specifies  whether  sshd(8)  should print /etc/motd when	a user
	       logs in interactively.  (On some	systems	it is also printed  by
	       the shell, /etc/profile,	or equivalent.)	 The default is	yes.

       PubkeyAcceptedAlgorithms
	       Specifies  the  signature  algorithms that will be accepted for
	       public key authentication as a  list  of	 comma-separated  pat-
	       terns.	Alternately  if	 the  specified	list begins with a `+'
	       character, then the specified algorithms	will  be  appended  to
	       the  default  set  instead of replacing them.  If the specified
	       list begins with	a `-' character, then the specified algorithms
	       (including wildcards) will be removed from the default set  in-
	       stead  of  replacing them.  If the specified list begins	with a
	       `^' character, then the specified algorithms will be placed  at
	       the head	of the default set.  The default for this option is:

		  ssh-ed25519-cert-v01@openssh.com,
		  ecdsa-sha2-nistp256-cert-v01@openssh.com,
		  ecdsa-sha2-nistp384-cert-v01@openssh.com,
		  ecdsa-sha2-nistp521-cert-v01@openssh.com,
		  sk-ssh-ed25519-cert-v01@openssh.com,
		  sk-ecdsa-sha2-nistp256-cert-v01@openssh.com,
		  rsa-sha2-512-cert-v01@openssh.com,
		  rsa-sha2-256-cert-v01@openssh.com,
		  ssh-ed25519,
		  ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
		  sk-ssh-ed25519@openssh.com,
		  sk-ecdsa-sha2-nistp256@openssh.com,
		  rsa-sha2-512,rsa-sha2-256

	       The list	of available signature algorithms may also be obtained
	       using "ssh -Q PubkeyAcceptedAlgorithms".

       PubkeyAuthOptions
	       Sets  one  or more public key authentication options.  The sup-
	       ported keywords are: none (the  default;	 indicating  no	 addi-
	       tional	  options    are    enabled),	 touch-required	   and
	       verify-required.

	       The touch-required option causes	public key authentication  us-
	       ing   a	 FIDO	authenticator	algorithm  (i.e.  ecdsa-sk  or
	       ed25519-sk) to always require the signature to  attest  that  a
	       physically present user explicitly confirmed the	authentication
	       (usually	 by  touching the authenticator).  By default, sshd(8)
	       requires	 user  presence	 unless	 overridden  with  an	autho-
	       rized_keys option.  The touch-required flag disables this over-
	       ride.

	       The verify-required option requires a FIDO key signature	attest
	       that the	user was verified, e.g.	via a PIN.

	       Neither	the touch-required or verify-required options have any
	       effect for other, non-FIDO, public key types.

       PubkeyAuthentication
	       Specifies whether public	key authentication  is	allowed.   The
	       default is yes.

       RekeyLimit
	       Specifies the maximum amount of data that may be	transmitted or
	       received	 before	 the  session  key is renegotiated, optionally
	       followed	by a maximum amount of time that may pass  before  the
	       session	key  is	renegotiated.  The first argument is specified
	       in bytes	and may	have a suffix of `K', `M', or `G' to  indicate
	       Kilobytes,  Megabytes, or Gigabytes, respectively.  The default
	       is between `1G' and `4G', depending on  the  cipher.   The  op-
	       tional  second value is specified in seconds and	may use	any of
	       the units documented in the "TIME FORMATS"  section.   The  de-
	       fault  value  for  RekeyLimit is	default	none, which means that
	       rekeying	is performed after the cipher's	default	amount of data
	       has been	sent or	received and no	time based rekeying is done.

       RequiredRSASize
	       Specifies the minimum RSA key size (in bits) that sshd(8)  will
	       accept.	 User  and host-based authentication keys smaller than
	       this limit will be refused.  The	default	is  1024  bits.	  Note
	       that this limit may only	be raised from the default.

       RevokedKeys
	       Specifies  revoked  public  keys	 file, or none to not use one.
	       Keys listed in this file	will be	refused	for public key authen-
	       tication.  Note that if this file is not	readable, then	public
	       key  authentication will	be refused for all users.  Keys	may be
	       specified as a text file, listing one public key	per  line,  or
	       as  an  OpenSSH	Key  Revocation	 List  (KRL)  as  generated by
	       ssh-keygen(1).  For more	information on KRLs, see the KEY REVO-
	       CATION LISTS section in ssh-keygen(1).

       RDomain
	       Specifies an explicit routing domain that is applied after  au-
	       thentication  has  completed.  The user session,	as well	as any
	       forwarded or listening  IP  sockets,  will  be  bound  to  this
	       rdomain(4).   If	 the routing domain is set to %D, then the do-
	       main in which the incoming connection was received will be  ap-
	       plied.

       SecurityKeyProvider
	       Specifies  a  path  to a	library	that will be used when loading
	       FIDO authenticator-hosted keys, overriding the default of using
	       the built-in USB	HID support.

       SetEnv  Specifies one or	more environment variables  to	set  in	 child
	       sessions	 started  by sshd(8) as	"NAME=VALUE".  The environment
	       value may be quoted (e.g. if  it	 contains  whitespace  charac-
	       ters).	Environment  variables	set by SetEnv override the de-
	       fault environment and any variables specified by	the  user  via
	       AcceptEnv or PermitUserEnvironment.

       StreamLocalBindMask
	       Sets the	octal file creation mode mask (umask) used when	creat-
	       ing a Unix-domain socket	file for local or remote port forward-
	       ing.   This  option is only used	for port forwarding to a Unix-
	       domain socket file.

	       The default value is 0177, which	creates	a  Unix-domain	socket
	       file  that  is  readable	 and writable only by the owner.  Note
	       that not	all operating systems honor the	file mode on  Unix-do-
	       main socket files.

       StreamLocalBindUnlink
	       Specifies whether to remove an existing Unix-domain socket file
	       for  local or remote port forwarding before creating a new one.
	       If the socket file already exists and StreamLocalBindUnlink  is
	       not  enabled,  sshd  will  be unable to forward the port	to the
	       Unix-domain socket file.	 This option is	 only  used  for  port
	       forwarding to a Unix-domain socket file.

	       The argument must be yes	or no.	The default is no.

       StrictModes
	       Specifies whether sshd(8) should	check file modes and ownership
	       of  the user's files and	home directory before accepting	login.
	       This is normally	desirable because novices  sometimes  acciden-
	       tally  leave  their directory or	files world-writable.  The de-
	       fault  is  yes.	  Note	 that	this   does   not   apply   to
	       ChrootDirectory,	 whose	permissions  and ownership are checked
	       unconditionally.

       Subsystem
	       Configures an external subsystem	(e.g. file  transfer  daemon).
	       Arguments  should  be  a	subsystem name and a command (with op-
	       tional arguments) to execute upon subsystem request.

	       The command sftp-server implements the SFTP file	transfer  sub-
	       system.

	       Alternately  the	 name  internal-sftp  implements an in-process
	       SFTP  server.	This   may   simplify	configurations	 using
	       ChrootDirectory	 to  force  a  different  filesystem  root  on
	       clients.

	       By default no subsystems	are defined.

       SyslogFacility
	       Gives the facility code that is used when logging messages from
	       sshd(8).	 The possible values are: DAEMON, USER,	AUTH,  LOCAL0,
	       LOCAL1,	LOCAL2,	 LOCAL3,  LOCAL4, LOCAL5, LOCAL6, LOCAL7.  The
	       default is AUTH.

       TCPKeepAlive
	       Specifies whether the system should send	TCP keepalive messages
	       to the other side.  If they are sent, death of  the  connection
	       or crash	of one of the machines will be properly	noticed.  How-
	       ever, this means	that connections will die if the route is down
	       temporarily,  and  some	people find it annoying.  On the other
	       hand, if	TCP keepalives are not sent, sessions may hang indefi-
	       nitely on the  server,  leaving	"ghost"	 users	and  consuming
	       server resources.

	       The  default  is	 yes (to send TCP keepalive messages), and the
	       server will notice if the network goes down or the client  host
	       crashes.	 This avoids infinitely	hanging	sessions.

	       To  disable  TCP	keepalive messages, the	value should be	set to
	       no.

       TrustedUserCAKeys
	       Specifies a file	containing public keys of certificate authori-
	       ties that are trusted to	sign user certificates for authentica-
	       tion, or	none to	not use	one.  Keys are listed  one  per	 line;
	       empty  lines  and comments starting with	`#' are	allowed.  If a
	       certificate is presented	for authentication and has its signing
	       CA key listed in	this file, then	it may be used for authentica-
	       tion for	any user listed	in the certificate's principals	 list.
	       Note  that certificates that lack a list	of principals will not
	       be permitted for	authentication using  TrustedUserCAKeys.   For
	       more  details  on certificates, see the CERTIFICATES section in
	       ssh-keygen(1).

       UnusedConnectionTimeout
	       Specifies whether and how quickly sshd(8) should	 close	client
	       connections  with  no open channels.  Open channels include ac-
	       tive shell, command execution or	subsystem sessions,  connected
	       network,	 socket, agent or X11 forwardings.  Forwarding listen-
	       ers, such as those from the ssh(1) -R flag, are not  considered
	       as  open	 channels and do not prevent the timeout.  The timeout
	       value is	specified in seconds or	may use	any of the units docu-
	       mented in the "TIME FORMATS" section.

	       Note that this timeout starts when the client  connection  com-
	       pletes  user authentication but before the client has an	oppor-
	       tunity to open any channels.  Caution should be used when using
	       short timeout values, as	they may not provide  sufficient  time
	       for  the	 client	to request and open its	channels before	termi-
	       nating the connection.

	       The default none	is to never expire connections for  having  no
	       open  channels.	 This option may be useful in conjunction with
	       ChannelTimeout.

       UseBlacklist
	       Specifies whether sshd(8) attempts to send authentication  suc-
	       cess and	failure	messages to the	blacklistd(8) daemon.  The de-
	       fault  is  no.	For  forward  compatibility  with  an upcoming
	       blacklistd rename, the UseBlocklist alias can be	used instead.

       UseDNS  Specifies whether sshd(8) should	look up	the remote host	 name,
	       and  to check that the resolved host name for the remote	IP ad-
	       dress maps back to the very same	IP address.

	       If this option is set to	no, then only addresses	and  not  host
	       names   may   be	  used	 in  ~/.ssh/authorized_keys  from  and
	       sshd_config Match Host directives.  The default is "yes".

       UsePAM  Enables the Pluggable Authentication Module interface.  If  set
	       to    yes   this	  will	 enable	  PAM	authentication	 using
	       KbdInteractiveAuthentication and	PasswordAuthentication in  ad-
	       dition to PAM account and session module	processing for all au-
	       thentication types.

	       Because	PAM keyboard-interactive authentication	usually	serves
	       an equivalent role to password authentication, you should  dis-
	       able	     either	     PasswordAuthentication	    or
	       KbdInteractiveAuthentication.

	       If UsePAM is enabled, you will not be able to run sshd(8) as  a
	       non-root	user.  The default is yes.

       VersionAddendum
	       Optionally  specifies additional	text to	append to the SSH pro-
	       tocol banner sent by the	server upon connection.	  The  default
	       is  "FreeBSD-20240104".	 The value none	may be used to disable
	       this.

       X11DisplayOffset
	       Specifies the first display number available for	sshd(8)'s  X11
	       forwarding.   This prevents sshd	from interfering with real X11
	       servers.	 The default is	10.

       X11Forwarding
	       Specifies whether X11 forwarding	is  permitted.	 The  argument
	       must be yes or no.  The default is no.

	       When  X11  forwarding is	enabled, there may be additional expo-
	       sure to the server and to client	displays if the	sshd(8)	 proxy
	       display	is  configured	to listen on the wildcard address (see
	       X11UseLocalhost), though	this is	not  the  default.   Addition-
	       ally, the authentication	spoofing and authentication data veri-
	       fication	 and substitution occur	on the client side.  The secu-
	       rity risk of using X11 forwarding is that the client's X11 dis-
	       play server may be exposed to attack when the  SSH  client  re-
	       quests	forwarding   (see   the	 warnings  for	ForwardX11  in
	       ssh_config(5)).	A system administrator may have	 a  stance  in
	       which  they  want to protect clients that may expose themselves
	       to attack by unwittingly	requesting X11 forwarding,  which  can
	       warrant a no setting.

	       Note  that disabling X11	forwarding does	not prevent users from
	       forwarding X11 traffic, as users	can always install  their  own
	       forwarders.

       X11UseLocalhost
	       Specifies whether sshd(8) should	bind the X11 forwarding	server
	       to  the	loopback  address  or to the wildcard address.	By de-
	       fault, sshd binds the forwarding	server to the loopback address
	       and sets	the hostname part of the DISPLAY environment  variable
	       to  localhost.	This  prevents remote hosts from connecting to
	       the proxy display.  However, some older	X11  clients  may  not
	       function	 with  this configuration.  X11UseLocalhost may	be set
	       to no to	specify	that the forwarding server should be bound  to
	       the wildcard address.  The argument must	be yes or no.  The de-
	       fault is	yes.

       XAuthLocation
	       Specifies the full pathname of the xauth(1) program, or none to
	       not use one.  The default is /usr/local/bin/xauth.

TIME FORMATS
       sshd(8)	command-line  arguments	 and  configuration  file options that
       specify	time  may  be  expressed  using	 a  sequence  of   the	 form:
       time[qualifier],	 where	time is	a positive integer value and qualifier
       is one of the following:

	     <none>  seconds
	     s | S   seconds
	     m | M   minutes
	     h | H   hours
	     d | D   days
	     w | W   weeks

       Each member of the sequence is added together to	 calculate  the	 total
       time value.

       Time format examples:

	     600     600 seconds (10 minutes)
	     10m     10	minutes
	     1h30m   1 hour 30 minutes (90 minutes)

TOKENS
       Arguments  to  some keywords can	make use of tokens, which are expanded
       at runtime:

	     %%	   A literal `%'.
	     %C	   Identifies the connection endpoints,	containing four	space-
		   separated  values:  client  address,	 client	 port  number,
		   server address, and server port number.
	     %D	   The routing domain in which the incoming connection was re-
		   ceived.
	     %F	   The fingerprint of the CA key.
	     %f	   The fingerprint of the key or certificate.
	     %h	   The home directory of the user.
	     %i	   The key ID in the certificate.
	     %K	   The base64-encoded CA key.
	     %k	   The base64-encoded key or certificate for authentication.
	     %s	   The serial number of	the certificate.
	     %T	   The type of the CA key.
	     %t	   The key or certificate type.
	     %U	   The numeric user ID of the target user.
	     %u	   The username.

       AuthorizedKeysCommand  accepts  the  tokens %%, %C, %D, %f, %h, %k, %t,
       %U, and %u.

       AuthorizedKeysFile accepts the tokens %%, %h, %U, and %u.

       AuthorizedPrincipalsCommand accepts the tokens %%, %C, %D, %F, %f,  %h,
       %i, %K, %k, %s, %T, %t, %U, and %u.

       AuthorizedPrincipalsFile	accepts	the tokens %%, %h, %U, and %u.

       ChrootDirectory accepts the tokens %%, %h, %U, and %u.

       RoutingDomain accepts the token %D.

FILES
       /etc/ssh/sshd_config
	       Contains	 configuration	data for sshd(8).  This	file should be
	       writable	by root	only, but it is	recommended (though not	neces-
	       sary) that it be	world-readable.

SEE ALSO
       sftp-server(8), sshd(8)

AUTHORS
       OpenSSH is a derivative of the original and free	ssh 1.2.12 release  by
       Tatu  Ylonen.   Aaron  Campbell,	Bob Beck, Markus Friedl, Niels Provos,
       Theo de Raadt and Dug Song removed many bugs, re-added  newer  features
       and  created  OpenSSH.	Markus	Friedl contributed the support for SSH
       protocol	versions 1.5 and 2.0.  Niels Provos  and  Markus  Friedl  con-
       tributed	support	for privilege separation.

FreeBSD	13.2			 July 28, 2023			SSHD_CONFIG(5)

实验介绍

1.1 实验内容

在本节内容中，我们将会给大家简单描述 SSH 的原理，并且尝试 SSH 的几种验证方式。

1.2 实验知识点

  • 基于主机的身份验证
  • 密码验证
  • 公钥验证

1.3 推荐阅读 由于在 SSH 的连接过程中会涉及到有关密钥，签名等一些安全方面的知识，但是在实验内容中并不会涉及太多这方面的有关内容，而要对 SSH 的原理有一定的了解，必然绕不开这些知识，所以推荐大家阅读下列文章：

2. SSH 概述

2.1 SSH 协议

SSH 的全称是 Secure Shell。可以通过其对远程主机进行安全的访问。在最开始设计的时候，SSH 被设计为替代 telnet 以及一些其它不安全远程 shell 协议。

SSH 基于 C/S 架构，即 客户端/服务器。通过客户端与服务器相连，进行安全的远程访问。

2.2 OpenSSH

对于 SSH 协议有很多的实现。而最常用的开源实现为 OpenSSH。在很多操作系统中默认使用，例如在 Ubuntu 和 CentOS 中默认使用 OpenSSH。在下面的安装和配置的操作过程也将基于 OpenSSH 进行操作。

2.3 SSH 的体系结构

根据 RFC4251 关于 SSH 体系结构的定义。SSH 协议主要由三个组件组成：

  • 传输层协议（SSH-TRANS）：该协议提供服务器身份验证，隐私和具有完美转发隐私的完整性。传输层通常是通过 TCP/IP 连接运行。
  • 用户认证协议（SSH-USERAUTH）：验证连接到服务器的客户端用户。运行在传输层的协议之上。
  • 连接协议（SSH-CONNECT）：将加密隧道复用到多个逻辑通道。运行在用户认证协议上。

在 SSH 的三个组件中的 SSH-USERAUTH，用户认证协议。对于用户的身份认证，在 RFC4252 中，有相关的身份认证方式的描述。在本节内容中，我们将带领大家配置以下几种身份认证方式：

  • 基于主机的身份验证（hostbased）
  • 使用密码 （password）
  • 公钥的认证方式（public key）

3. 服务的搭建与配置

3.1 安装与启动

在实验环境中已经有安装相应的组件，若是需要在本机安装相应的内容，可以参考我下面的命令。

在 Centos 上，使用 yum 进行安装：

1
2
3
4
5
# 服务端
$ sudo yum install openssh-server

# 客户端
$ sudo yum install openssh-clients

在 Ubuntu 上，使用 apt 进行安装：

1
2
3
4
5
# 服务端
$ sudo apt-get install openssh-server

# 客户端
$ sudo apt-get install openssh-client

为避免服务未启动，可以手动尝试开启服务，使用如下命令：

1
$ sudo service sshd start

最后，我们可以将要修改的配置文件进行一个备份，以便还原初始值，使用如下命令：

1
2
3
4
5
# 备份服务器端的配置文件
$ sudo cp /etc/ssh/sshd_config /etc/ssh/sshd_config.back

# 备份客户端的配置文件
$ sudo cp /etc/ssh/ssh_config /etc/ssh/ssh_config.back

3.2 配置文件

在安装好相应的软件包之后，会在 /etc/ssh 文件夹下生成一些相应的配置文件，下面我们简单说明：

img

  • ssh_config 为客户端的配置文件，
  • sshd_config 为服务端的配置文件,
  • 其它的文件属于服务端安装时自动生成的密钥文件，以 .pub 结尾的为公钥，对应的不以 .pub 结尾的为私钥文件。

在启动服务的时候，会加载相应的相应的配置文件。对应的还可以在用户目录下的 ~/.ssh/ 目录中编辑配置文件，例如编辑 ~/.ssh/config 文件，则用户在启动客户端时，就会读取 ~/.ssh/config 文件的配置项而不是 /ect/ssh/ssh_config 针对全局的配置。

在这里我们并不会介绍有关于各种加密算法的内容，需要大家自行去了解相关的知识。

3.3 信任模式

如上所示，在安装完服务器端之后，会有相应的密钥对文件（一个公钥一个私钥）生成，由于 SSH 支持多种不同的加密算法，所以可能会根据实际情况会有多个文件被生成。

每个服务器都有一个密钥，由于使用不同的算法可以拥有多个密钥。在密钥交换过程中使用服务器主机密钥来验证客户端是否与正确的服务器交谈，为了做到这一点，客户必须先知道服务器的公共主机密钥（公钥），在 RFC4251 中给出了两种解决的思路或者说信任模式：

  1. 客户端在本地保存主机名与对应的服务器的公钥信息。
  2. 主机关联的公共密钥由受信任的证书机构进行认证（如 CA）

但是对于上述的两种方式来说，第 1 种需要一定的人力维护成本，而第 2 种，对于现在互联网上无数的服务器来说，明显是不可行的。

因此对于 SSH 协议而言，在首次连接到服务器时，允许事先通信而不需要密钥或证书，然而这一过程容易受到中间人攻击。

3.4 hostbased

hostbased 方式允许受信任主机上的指定用户无需输入密码即可直接登录进服务器。服务器会读取配置的可信任主机的信息，一般为 /etc/hosts.equiv。

在默认设置中，SSH 并未启用该验证方式，我们需要修改 /etc/ssh/sshd_config 的配置文件：

1
HostbasedAuthentication yes

上述编辑的 HostbasedAuthentication 可以选择设置是否启用基于主机的身份验证。将会从全局的 /etc/hosts.equiv 文件中匹配指定的主机和用户，以便允许这些主机上的这些用户无密码登录。

除了编辑服务器端的配置文件之外，我们还需编辑客户端的配置文件，这里我们编辑 ~/.ssh/config，则仅针对当前用户的客户端配置文件 ：

1
2
3
4
5
# localhost 属于别名，未指定时只能使用 IP 地址
Host localhost
    HostName 127.0.0.1
    EnableSSHKeysign yes
    HostbasedAuthentication yes

注意，如果 ~/.ssh/config 文件之前不存在，编辑完成之后需要修改其读写权限为 600 ，命令为 chmod 600 ~/.ssh/config，否则会报权限错误。

img

另外全局的客户端配置文件 /etc/ssh/ssh_config 也需要按如下修改：

1
2
3
4
Host *
    GSSAPIAuthentication yes
    EnableSSHKeysign yes
    HostbasedAuthentication yes

img

上述图片中的 Host * 代表的是所有的服务器，就是该配置项下的子项针对所有的服务器。

最后还需要编辑 /etc/hosts.equiv 文件（不存在需要新创建）。该文件的格式为：

1
<IP ADDRESS> <USERNAME>

我们在该文件中添加一行内容，允许本机的 shiyanlou 用户无密码登录本机：

1
127.0.0.1 shiyanlou

重新启动服务器：

1
$ sudo service sshd restart

最后，我们可以使用 ssh 连接到本地的 SSH 服务器上，使用如下命令：

1
$ ssh shiyanlou@localhost

这里需要说明的是，使用 ssh 命令连接远程主机的格式为 ssh [user@]hostname，而 SSH 服务默认运行在 22 端口上，若是配置了使用其它端口，可以通过 -p 参数指定端口号。

我们可以看到显示结果如下图，对于初次连接服务器时，会检测是否已经有相应的记录（检测 known_hosts 文件），若是并未有相应的记录，则会提示是否继续（即在上述信任模式种提到的初次连接时 SSH 允许不需要预先知晓服务器的公共主机密钥）。如果选择继续则代表信任该服务器提供的公钥，并会将其保存到 ~/.ssh/known_hosts 文件中。可以通过在 /etc/ssh/sshd_config 文件中设置 IgnoreUserKnownHosts yes 不进行检测。

img

这时我们可以输入 yes，则会提示保存该公钥并且登陆成功，如下图所示：

img

此时，我们可以查看 ~/.ssh/known_hosts 文件的内容，与刚刚的提示信息进行比对，如下图所示：

img

这里可以看到主机名与对应的公钥的信息，由于提示信息显示的公钥是经过 HASH 算法计算过的值，所以会与文件中的内容不一致，在下面的内容中，我们将会验证两者为同一内容。

这里需要说明的是，由于我们是通过客户端直接连接当前的服务器端，即 localhost，在进行登陆时，登陆成功后不会有太明显的区别。有条件的同学可以在自己电脑上尝试通过客户端连接到服务器端，或者在进行登陆之前切换目录，不要停留在 shiyanlou 用户的家目录，因为登陆成功之后，会默认切换到被登陆用户的家目录。

如下所示，我们首先使用 exit 命令退出登陆，然后切换到 Code 目录，再次尝试登陆，可以看出明显的区别，即登陆成功之后会切换到到 /home/shiyanlou 目录，退出登陆会回到 ~/Code 目录下，如下所示：

img

3.5 password

SSH 默认使用的即是密码验证的方式，即在 /etc/sshd_config 配置文件中的 PasswordAuthentication 配置项默认为 yes ，如下所示，我们查看相关的配置文件：

img

由于刚刚我们配置了基于主机的身份验证，所以需要将之前的设置重新还原，可以手动修改配置，也可以使用我们刚刚备份的配置文件，或者直接删除掉配置的 /etc/hosts.equiv 文件即可。

这里我们删除刚刚配置的 /etc/hosts.equiv 文件，并且停止 sshd 服务，新打开一个终端，使用调试模式启动 sshd，如下所示：

img

此时，我们使用 ssh shiyanlou@localhost 去连接服务器端，由于已经删除掉 /etc/hosts.equiv 配置文件，会提示基于主机的身份验证失败，需要输入密码，如下所示：

img

此时，输入相应的密码即可成功连接，如下所示：

img

注意，实验环境暂不支持密码登录，这里的截图仅做参考。

3.6 public key

最后我们要介绍的验证方式为 public key 即公钥认证，该方式属于上述讨论的信任模式中的第一种实现方式，即通过手动在客户端和服务端配置相应的密钥对，客户端保存私钥，服务器端保存公钥信息。通过公私钥的认证，能够保证客户端是与真正的服务器通信，并且客户端和服务器端互相信任，不需要进行密码认证。

这里所使用的公钥认证的过程与传统意义上公私钥认证方式没有什么不同。具体的原理大家可以参考我在之前给出的链接内容。

对于 SSH 支持的加密算法，我们可以通过 ssh -Q key 来查询，如下所示：

img

下面我们创建一个密钥对，用于配置 public key 认证方式，使用如下命令：

1
2
3
4
5
# 直接使用 ssh-keygen 会默认使用 rsa 算法，并且将生成的密钥文件放到 ~/.ssh/ 目录下
$ ssh-keygen

# 还可以使用 -t 参数指定使用的算法类型， rsa | ecdsa| ed25519 | dsa 等，上述命令等同于下列命令
$ ssh-keygen -t rsa

img

所有提示要输入的地方直接回车即可。生成的公私钥文件，分别保存在 /.ssh/id_rsa，/.ssh/id_rsa.pub 中。

SSH 默认启用了 public key 的认证方式，不需要进行配置，我们直接通过下图查看需要设置的相关配置项，如下所示：

img

我们需要将 ~/.ssh/id_rsa.pub 的公钥信息追加到 ~/.ssh/authorized_keys 文件中，可以在其中添加多个公钥。

使用如下命令：

1
$ cat ~/.ssh/id_rsa.pub | tee -a ~/.ssh/authorized_keys

如果 ~/.ssh/authorized_keys 是首次创建，需要修改其文件权限为 600，命令为 chmod 600 ~/.ssh/authorized_keys。

此时，可以直接使用 ssh shiyanlou@localhost 进行验证，就能够直接进行登陆而不需要输入密码：

img

在客户端，也可以使用 -i 参数，指定要使用的私钥文件，使用如下命令：

1
$ ssh -i ~/.ssh/id_rsa shiyanlou@localhost

img

SSH 使用公私钥进行登录操作视频：

4. 其它配置项

对于 SSH 而言，还有很多其它的可用配置项，针对于 sshd_config 的配置项如下：

1
2
3
4
5
6
7
8
9
10
11
12
13
# 可以自定义端口，默认为 22，在客户端使用 ssh 连接服务端时，可以通过 -p 参数指定端口
Port 22

# 允许 root 用户使用 ssh 登陆，可以设置为 yes ，no ，prohibit-password，without-password  后面两个都代表允许登陆而不使用密码
PermitRootLogin yes

# 指定允许组内的用户登陆，可以指定多个组，以空格分隔。
AllowGroups shiyanlou mysql

# 类似还有 DenyUsers   AllowUsers   DenyGroups  等，它们的优先级顺序从高到低为 DenyUsers,AllowUsers,DenyGroups,AllowGroups。

# 指定应该监听的本地地址，可以指定多个，默认为监听正在使用的路由域上的所有本地地址
ListenAddress hostname:port

5. scp 和 sftp

这里我们将会简单介绍的 scp 和 sftp 两个传输文件的工具。

5.1 scp

scp 可以在多个主机之间复制文件，使用 SSH 传输数据，并且使用它的认证方式进行身份认证。

scp 的使用方式为：

1
scp source target

可以将源(source) 指定为本地路径名，将目标（target）指定为远程主机，代表将本地的文件复制到远程主机。

也可以将源指定为远程主机，将目标指定为本地路径，代表的是将远程主机上的文件复制到本地。

对于远程主机的格式为 [user@]host:[path]，或者 scp://[user@]host[:port][/path]。

本地路径则使用绝对路径或相对路径即可。

如下所示，我们分别在 /home/shiyanlou 目录下创建一个 test1 文件，在 /home/shiyanlou/Code 目录下创建一个 test2 文件，并使用 scp 复制文件：

1
2
3
4
5
6
7
8
9
10
11
# 创建文件
$ touch /home/shiyanlou/test1  /home/shiyanlou/Code/test2

# 切换到 /home/shiyanlou 目录
$ cd ~

# 将当前目录下的 test1 上传到远程服务器上的 /home/shiyanlou/Code 目录下
$ sudo scp test1 shiyanlou@localhost:/home/shiyanlou/Code

# 将远程服务器上的 /home/shiyanlou/Code 目录下的 test2 复制到本地
$ sudo scp shiyanlou@localhost:/home/shiyanlou/Code/test2 /home/shiyanlou

img

如上所示，我们成功进行了文件的复制。scp 使用的是 SSH 进行数据的传输和身份的验证，在之前的内容中，我们配置了 public key 的认证方式，所以能够直接复制成功，而不需要输入密码等验证操作。

除此之外，scp 还有一些常用的参数，例如 -r 递归复制目录，-p 指定端口等。

5.2 sftp

跟 scp 类似的还有 sftp，它 通过SSH 来安全的上传和下载文件，是常用的文件传输工具。关于sftp 详细的使用方法与 ftp 类似，可以参考前面关于 ftp 的章节。

简单示例如下：

img

同样，上述操作也是由于在刚刚的 SSH 身份验证我们启用了公钥验证并设置相关的内容，所以可以直接连接而不需要输入密码。