OpenSSH(OpenBSD Secure Shell)是使用 安全外壳协议(SSH),在计算机网络上提供加密通信会话的软件集。它是 SSH Communications Security 专有 SSH 软件的开源替代方案。OpenSSH 是 OpenBSD 项目的一部分,该项目由 Theo de Raadt 领导。
OpenSSH 有时候会与名字相似的 OpenSSL 相混淆;然而,它们的目的不同,开发团队也不同,名字相似只因为两者目标相似。
安装
客户端用法
连接服务器:
$ ssh -p 端口 用户名@服务器地址
如果服务器只允许公钥认证,参考 SSH 密钥。
配置
客户端可以配置一些通用的参数和主机信息。所有的参数都可以全局配置或者限制为特定主机,例如:
~/.ssh/config
# 全局选项 User 用户名 # 特定于主机的选项 Host myserver Hostname 服务器地址 Port 端口
配置之后,下面的两条命令是等价的:
$ ssh -p 端口 用户名@服务器地址 $ ssh myserver
参考 ssh_config(5) 可以获得更多的信息。
一些参数没有对应的命令行选项,但你可以在命令行中使用 -o
指定配置选项。例如 -oKexAlgorithms=+diffie-hellman-group1-sha1
。
服务端用法
sshd
是 OpenSSH 服务器守护程序,通过 /etc/ssh/sshd_config
配置并由 sshd.service
管理。每次更改配置后,请在重新启动服务前在测试模式下运行 sshd
以确保它能够干净地启动。有效配置将不会产生输出:
# sshd -t
配置
要仅允许某些用户访问,请添加以下一行:
AllowUsers 用户1 用户2
要仅允许某些用户组访问:
AllowGroups 用户组1 用户组2
要添加漂亮的欢迎消息(例如,输出 /etc/issue
文件),请配置 Banner
选项:
Banner /etc/issue
sshdgenkeys 服务会自动在 /etc/ssh
中生成缺失的任何算法的公钥和私钥,即使 sshd_config
中的 HostKeyAlgorithms
选项只允许部分加密算法。OpenSSH 提供了三种基于 ed25519、ecdsa 和 rsa 算法的密钥对。要让 sshd 使用特定的密钥,请指定以下选项:
HostKey /etc/ssh/ssh_host_ed25519_key
如果服务器要暴露在公网下,建议将默认端口从 22 更改为更高的随机端口,例如:
Port 39901
- 要选择尚未分配给常见服务的备用端口,请查看 TCP 和 UDP 端口号列表,还可以在本地的
/etc/services
中查找端口信息。更改 OpenSSH 所使用的端口有助于减少由攻击者使用自动程序尝试通过 SSH 登录你的服务器的情况(但无法彻底解决)。查看 Port knocking 了解相关内容。 - 建议完全禁用密码登录。这将大大提高安全性,更多信息请参见#强制公钥认证。更多推荐的安全方法,请参阅#保护。
- OpenSSH 可以监听多个端口,只需在配置文件中添加多行
Port 端口号
。 - 可以通过从
/etc/ssh
中删除要替换的主机密钥对并以 root 身份运行ssh-keygen -A
来生成新的(或缺失的)主机密钥对。
管理守护程序
启动/启用 sshd.service
。它将保持 SSH 守护程序始终处于活动状态,并为每个传入连接 fork。
sshd.socket
,更多信息请参考 FS#62248。如果在更新到 openssh包 8.0p1-3 时启用了 sshd.socket
,则 sshd.socket
和 sshd@.service
单元将被复制到 /etc/systemd/system/
并重新启用。这样做只是为了不破坏现有设置;仍然建议用户迁移到 sshd.service
。sshd.socket
, 请注意下列问题:
-
sshd.socket
单元可能会失败 (例如:在内存耗尽的情况下) 并且Restart=always
不能被用于 socket 单元. 点击 systemd issue 11553 了解更多内容。 - 使用套接字激活会导致拒绝服务,因为太多的连接会导致拒绝进一步激活服务。见 FS#62248。
sshd.socket
使得 ListenAddress
设置失效,因此它会允许连接任意地址。为了使 ListenAddress
设置起作用,你必须编辑 sshd.socket
来为 ListenStream
指定端口和IP(例如 ListenStream=192.168.1.100:22
)。你还必须在 [Socket]
添加 FreeBind=true
,否则设置 IP 地址会有 ListenAddress
设置一样的缺点:如果网络没有及时启动,套接字会启动失败。sshd@.service
的瞬态实例(实例名称会不同)。因此,不论是 sshd.socket
还是守护进程常规的 sshd.service
都会不被允许在日志中监控连接尝试。可以通过 root 运行 journalctl -u "sshd@*"
或者 journalctl /usr/bin/sshd
查看 SSH 套接字激活实例的日志。保护
允许通过 SSH 远程登录有利于管理,但可能会威胁服务器的安全。由于 SSH 访问通常是暴力攻击的目标,因此需要适当限制 SSH 访问以防止第三方访问服务器。
ssh-audit包 提供对服务端和客户端配置的自动分析。关于这个主题,还有其他几个很好的指南和工具,例如:
强制公钥认证
如果客户端无法通过公钥进行身份验证,SSH 服务端默认会退回到使用密码进行身份验证,从而允许恶意用户尝试通过暴力穷举密码来获取访问权限。防止这种攻击的最有效方法之一是完全禁用密码登录,并强制使用 SSH 密钥。这可以通过在守护程序配置文件中设置以下选项来完成:
/etc/ssh/sshd_config.d/20-force_publickey_auth.conf
PasswordAuthentication no AuthenticationMethods publickey
authorized_keys
文件中设置了公钥身份验证。详细信息请参阅 SSH 密钥#将公钥复制到远程服务器上。双重身份认证和公钥
SSH 可以被设置成需要多重验证。你可以通过 AuthenticationMethods
选项指定使用哪些验证方式,这将同时启用公钥和双重验证。
身份验证提供程序
设置 Google 身份验证器的方法请查看 Google 身份验证器。
对于 Duo,安装提供 pam_duo.so
模块的包 duo_unixAUR 。阅读 Duo Unix 文档以了解如何设置 Duo 凭据(Integration Key,密钥和 API 主机名)。
PAM 设置
要在 OpenSSH 中使用 PAM,请编辑下列文件:
/etc/ssh/sshd_config.d/20-pam.conf
KbdInteractiveAuthentication yes AuthenticationMethods publickey keyboard-interactive:pam
然后你可以通过公钥或在 PAM 中设置的用户身份验证方法登录。
另一方面,如果您想要根据 PAM 设置的要求使用公钥和用户身份验证对用户进行验证,请使用逗号而不是空格来分隔 AuthenticationMethods:
/etc/ssh/sshd_config.d/20-pam.conf
KbdInteractiveAuthentication yes AuthenticationMethods publickey,keyboard-interactive:pam
为了要求同时使用公钥和 pam 身份验证,您可能希望禁用密码登录:
/etc/pam.d/sshd
auth required pam_securetty.so #disable remote root #Require google authenticator auth required pam_google_authenticator.so #But not password #auth include system-remote-login account include system-remote-login password include system-remote-login session include system-remote-login
防止暴力穷举攻击
暴力攻击的概念非常简单:使用大量随机用户名和密码组合尝试登录网页或类似 SSH 等服务。
请参考 ufw#连接速率限制,如果使用的是 iptables,请参考 Simple stateful firewall#Bruteforce attacks。
从 9.8 版本开始,OpenSSH 引入了类似 fail2ban 的基本保护实现:PerSourcePenalties
选项默认配置了一个合理值,会对同一地址上的客户端针对各种条件进行惩罚,导致在一段时间内被拒绝连接。
另外,你也可以使用自动化脚本封禁任何尝试暴力穷举登录的对象。
- 只允许可信源地址进行 SSH 连接
- 使用 fail2ban 或 sshguard 自动封禁多次密码登录失败的 IP
- 使用 pam_shield 封禁一段时间内多次尝试登录的 IP。与 fail2ban 或 sshguard 不同,该程序不会对成功和失败登录请求进行区分。
限制root用户登录
It is generally considered bad practice to allow the root user to log in without restraint over SSH. There are two methods by which SSH root access can be restricted for increased security.
拒绝
Sudo selectively provides root rights for actions requiring these without requiring authenticating against the root account. This allows locking the root account against access via SSH and potentially functions as a security measure against brute force attacks, since now an attacker must guess the account name in addition to the password.
SSH can be configured to deny remote logins with the root user by editing the "Authentication" section in the daemon configuration file. Simply set PermitRootLogin
to no
:
/etc/ssh/sshd_config.d/20-deny_root.conf
PermitRootLogin no
Next, restart the SSH daemon.
You will now be unable to log in through SSH under root, but will still be able to log in with your normal user and use su or sudo to do system administration.
限制
Some automated tasks such as remote, full-system backup require full root access. To allow these in a secure way, instead of disabling root login via SSH, it is possible to only allow root logins for selected commands. This can be achieved by editing ~root/.ssh/authorized_keys
, by prefixing the desired key, e.g. as follows:
command="rrsync -ro /" ssh-ed25519 ...
This will allow any login with this specific key only to execute the command specified between the quotes.
The increased attack surface created by exposing the root user name at login can be compensated by adding the following to sshd_config
:
PermitRootLogin forced-commands-only
This setting will not only restrict the commands which root may execute via SSH, but it will also disable the use of passwords, forcing use of public key authentication for the root account.
A slightly less restrictive alternative will allow any command for root, but makes brute force attacks infeasible by enforcing public key authentication. For this option, set:
PermitRootLogin prohibit-password
对 authorized_keys 文件加锁
If, for whatever reason, you think that the user in question should not be able to add or change existing keys, you can prevent them from manipulating the file.
On the server, make the authorized_keys
file read-only for the user and deny all other permissions:
$ chmod 400 ~/.ssh/authorized_keys
To prevent the user from simply changing the permissions back, set the immutable bit on the authorized_keys
file. To prevent the user from renaming the ~/.ssh
directory and creating a new ~/.ssh
directory and authorized_keys
file, set the immutable bit on the ~/.ssh
directory too. To add or remove keys, you will have to remove the immutable bit from authorized_keys
and make it writable temporarily.
authorized_keys
file via e.g auditd.SSH certificates
While common SSH keys and manual fingerprint verification may be easy to use with a handful of hosts that are managed by a single administrator, this method of authentication does not scale at all. When a number of servers need to be accessed through SSH by several users, manually verifying ssh public key fingerprints of every host becomes nearly impossible to do securely and reliably.
The solution for this is to use SSH certificates that provide automatic verification of public key identities through a chain of trust that scales significantly better than the default trust-on-first-use approach of SSH. SSH certificates are basically nothing else than normal public SSH keys, but with an additional signature from a trusted certificate authority that verifies the key identity.
Create a host certificate authority key for your infrastructure
$ ssh-keygen -t ed25519 -f ~/.ssh/ca_host_key -C 'Host certificate authority for *.example.com'
The private certificate authority key should be stored securely, ideally on a smartcard or hardware token that prevents key extraction like the Nitrokey or YubiKey.
Sign a server's public SSH host key
Copy the public server key to your local system containing the private certificate authority key to sign it:
$ ssh-keygen -h -s ~/.ssh/ca_key -I certLabel -n server01.example.com ./ssh_host_ed25519_key.pub
Move the new certificate and configure sshd to use it
The generated certificate ssh_host_ed25519_key-cert.pub
should be copied to the server at /etc/ssh/
.
/etc/ssh/sshd_config.d/20-ed25519_key.conf
HostCertificate /etc/ssh/ssh_host_ed25519_key-cert.pub
Configure all clients to trust the certificate authority
~/.ssh/known_hosts
@cert-authority *.example.com ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIKL8gB/pjuff005YNazwMCqJpgsXAbQ3r4VStd/CRKwU Host certificate authority for *.example.com
SSH user certificates
Depending on the number of users and method of deployment, SSH User keys can also be used with Certificates. For organizations with many ssh users, this is strongly advised to manage User key deployment securely.
The deployment of user certificates works basically the same as for server identities. More details and instructions can be found at Wikibooks:OpenSSH/Cookbook/Certificate-based Authentication.
Certificate deployment automation
Automated deployment of SSH certificates can be provided by a number of open source tools. Popular examples are:
- Ansible - openssh_cert module
- privacyIDEA - authentication server
- Theo App - authorized keys manager
SSHFP record
The Secure Shell fingerprint record (SSHFP) is an optional resource record in the domain name system that associates SSH keys to a host name. It can be used to verify the SSH fingerprint on public servers by using DNSSEC instead of deploying trusted CA certificates, which allows even unmanaged clients to verify the validity of key fingerprints.
Generate record entry
To generate the required hexadecimal key fingerprint to be stored in the DNS record, create the hash on the target server.
$ ssh-keygen -r host.example.com
This will read all available SSH keys for the specified domain and output valid SSHFP records that can then be stored in the DNS entries of the affected domain.
Client configuration
In order to automatically retrive and trust SSH key fingerprints stored as SSHFP records, add the following to your ssh client configuration file:
~/.ssh/config
# global options Match all VerifyHostKeyDNS yes
If the target host has a valid SSHFP record and this record is verified with a valid DNSSEC signature, the fingerprint is automatically accepted without prompting the user to verify the hosts identity. In case the DNS record is not verified by DNSSEC, the user will be prompted to verify the fingerprint instead.
Generate SSHFP records
To determine the SSH fingerprint of a specific domain, use ssh-keyscan to retrieve the ssh fingerprints in a valid DNS record format. (Note that by default fingerprints for every available key type is provided as both SHA1 and SHA256.)
$ ssh-keyscan -D github.com
; github.com:22 SSH-2.0-babeld-57ca1323 ; github.com:22 SSH-2.0-babeld-57ca1323 github.com IN SSHFP 1 1 6f4c60375018bae0918e37d9162bc15ba40e6365 github.com IN SSHFP 1 2 b8d895ced92c0ac0e171cd2ef5ef01ba3417554a4a6480d331ccc2be3ded0f6b ; github.com:22 SSH-2.0-babeld-57ca1323 github.com IN SSHFP 3 1 3358ab5dd3e306c461c840f7487e93b697e30600 github.com IN SSHFP 3 2 a764003173480b54c96167883adb6b55cf7cfd1d415055aedff2e2c8a8147d03 ; github.com:22 SSH-2.0-babeld-57ca1323 github.com IN SSHFP 4 1 e9619e2ed56c2f2a71729db80bacc2ce9ccce8d4 github.com IN SSHFP 4 2 f83898df0bef57a4ee24985ba598ac17fccb0c0d333cc4af1dd92be14bc23aa5 ; github.com:22 SSH-2.0-babeld-57ca1323
Since the SSHFP record stores the key fingerprints as hexadecimal values while the common output for SSH fingerprints is the base64 encoded SHA256 hash of the public key, it is necessary to convert the record back to the base64 format in order to compare it with values in the known_hosts file or other documentation that commonly stores fingerprints as SHA256.
$ echo "SSHFP-fingerprint" | xxd -r -p | base64
Example for github.com using the hex value for the sha256 fingerprint of the key type ed25519
$ echo "f83898df0bef57a4ee24985ba598ac17fccb0c0d333cc4af1dd92be14bc23aa5" | xxd -r -p | base64
+DiY3wvvV6TuJJhbpZisF/zLDA0zPMSvHdkr4UvCOqU=
Compare with known_hosts entries:
$ ssh-keygen -l -f ~/.ssh/known_hosts
Manually retrieve SSHFP records from DNS
$ dig SSHFP targetdomain.tld +short
小技巧
使用加密的 SOCKS 隧道
这对连接到各种不安全无线连接的笔记本电脑用户非常有用。你唯一需要的是在一个比较安全的位置运行的 SSH 服务器,比如你的家或工作地点。使用动态 DNS 服务可能很有用,如DynDNS,这样你就不必记住你的 IP 地址。
步骤 1:开始连接
你只需要执行这一条命令就可以开始连接:
$ ssh -TND 4711 user@host
其中 user
是你在 host
上运行的 SSH 服务器的用户名。它将询问你的密码,然后建立连接。N
标志禁用了交互式提示,D
标志指定了监听的本地端口(如果你愿意,可以选择任何端口号)。{Ic|T}} 标志禁用伪终端分配。
可以添加 verbose(-v
)标志,这样就可以通过输出验证是否成功连接。
步骤 2 (方法 A):配置你的浏览器(或其它程序)
上述步骤仅与使用新创建的 SOCKS 隧道的网页浏览器或其他程序结合使用时有用。SSH 现在同时支持 SOCKS v4 和 SOCKS v5,你可以选择它们中的任意一种。
- 对于火狐浏览器:进入首选项 > 通用,跳到页面底部并点击网络设置项右边的设置...按钮。接下来,在打开的二级窗口中,选中手动代理配置并在 SOCKS 主机名框内输入
localhost
,并在旁边的端口项输入端口号(上述例子使用4711
)。
- 火狐不会自动通过 SOCKS 隧道发出 DNS 请求,该潜在隐私问题可通过选中页面下方的使用 SOCKS v5 时代理 DNS 查询解决。显然,这只在使用 SOCKS v5 而不是 v4 时有效。
- 重启火狐来应用设置。
- 对于 Chromium,可以将 SOCKS 设置为环境变量或通过命令行选项进行配置。例如,可以在
.bashrc
中添加以下函数:
function secure_chromium { port=4711 export SOCKS_SERVER=localhost:$port export SOCKS_VERSION=5 chromium & exit }
或者是
function secure_chromium { port=4711 chromium --proxy-server="socks://localhost:$port" & exit }
现在打开终端,然后执行:
$ secure_chromium
就可以使用安全隧道了!
步骤 2(方法 B):配置本地 TUN 接口
这种方法在前期稍微复杂,但以后无需手动配置每个应用程序来使用 SOCKS 代理,具体原理是建立一个本地 TUN 接口并通过该接口路由流量。
具体方法请参考 VPN over SSH#Set up badvpn and tunnel interface。
X11 转发
使用 X11 转发可以在本地客户端上显示远程系统运行的 X11 程序图形界面。对于 X11 转发,远程主机不需要安装完整的 X11 环境,但至少要安装 xauth。xauth是维护 Xauthority
配置的工具,该配置被服务端和客户端用于认证 X11 会话(源)。
设置
服务端
- 安装 xorg-xauth包
- 在
/etc/ssh/sshd_config
中:- 设置
X11Forwarding
为 yes - 检查
AllowTcpForwarding
和X11UseLocalhost
选项是否已设为 yes,并且X11DisplayOffset
设为 10(未修改情况下是默认值,参考 sshd_config(5))
- 设置
- 然后重启 sshd daemon.
客户端
- 安装 xorg-xauth包
- 启用
ForwardX11
选项,可以在命令行中使用-X
选项,也可以在客户端配置文件中设置ForwardX11
为 yes。
ForwardX11Trusted
选项(命令行下使用 -Y
),这会防止 X11 转发受到 X11 SECURITY extension 的影响。在操作前,确保你已经看过警告信息。用法
正常登录到远程机器,如果客户端的配置文件中没有启用 ForwardX11,则指定 -X
开关:
$ ssh -X user@host
如果你在试图运行图形应用程序时收到错误,请尝试用 ForwardX11Trusted 代替:
$ ssh -Y user@host
如果出现 X11 forwarding request failed
报错,则需要为你的远程机器重新进行设置。一旦 X11 转发请求成功,你可以在远程服务器上启动任何 X 程序,它将被转发到你的本地会话:
$ xclock
如果出现了 Can't open display
报错,表示 DISPLAY
设置有误。
对一些应用程序要小心,因为它们会检查本地机器上是否有正在运行的实例。Firefox 就是一个例子:要么关闭正在运行的 Firefox 实例,要么使用下面的启动参数在本地机器上启动一个远程实例:
$ firefox --no-remote
如果你在连接时出现“X11 forwarding request failed on channel 0”报错(并且服务器 /var/log/errors.log
显示“Failed to allocate internet-domain X11 display socket”),请确保已安装 xorg-xauth包。如果安装了也还是不行,可以尝试以下方法之一:
- 在服务端的
sshd_config
中启用AddressFamily any
选项,或者 - 在服务端的
sshd_config
中将AddressFamily
选项设置为 inet。
将其设置为 inet 可能会解决 Ubuntu 客户端在 IPv4 上的问题。
对于在 SSH 服务器上以另一个用户身份运行 X 应用程序,you need to xauth add
the authentication line taken from xauth list
of the SSH logged in user.
X11 转发
排障可参考以下链接:
转发其他端口
除了SSH对X11的内置支持外,它还可以通过本地转发或远程转发使用安全隧道传输任何 TCP 连接。
本地转发打开本地计算机上的一个端口,连接将被转发到远程主机,并从那里转发到给定的目的地。通常,转发目的地将与远程主机相同,从而为同一机器提供安全外壳,例如安全 VNC 连接。本地转发会使用到 -L
选项,后接 <tunnel port>:<destination address>:<destination port>
格式的转发规则。
因此:
$ ssh -L 1000:mail.google.com:25 192.168.0.100
将使用 SSH 进行登录到 192.168.0.100
并打开 shell,同时创建本地 TCP 端口 1000 到 mail.google.com 端口 25 的隧道。连接成功后,发送到 localhost:1000
的连接会连到 Gmail 的 SMTP 端口。除非使用了其它手段,否则对于谷歌来说,他们会观察到所有这些连接都是从 192.168.0.100
发送出来的(但对于连接传送的数据又是另一回事),而不是 192.168.0.100
。
类似地:
$ ssh -L 2000:192.168.0.100:6001 192.168.0.100
将允许连接到 localhost:2000
,然后数据会被透明发送到远程主机的 6001 端口。The preceding example is useful for VNC connections using the vncserver utility--part of the tightvnc package--which, though very useful, is explicit about its lack of security.
Remote forwarding allows the remote host to connect to an arbitrary host via the SSH tunnel and the local machine, providing a functional reversal of local forwarding, and is useful for situations where, e.g., the remote host has limited connectivity due to firewalling. It is enabled with the -R
switch and a forwarding specification in the form of <tunnel port>:<destination address>:<destination port>
.
Thus:
$ ssh -R 3000:irc.libera.chat:6667 192.168.0.200
will bring up a shell on 192.168.0.200
, and connections from 192.168.0.200
to itself on port 3000 (the remote host's localhost:3000
) will be sent over the tunnel to the local machine and then on to irc.libera.chat on port 6667, thus, in this example, allowing the use of IRC programs on the remote host to be used, even if port 6667 would normally be blocked to it.
Both local and remote forwarding can be used to provide a secure "gateway", allowing other computers to take advantage of an SSH tunnel, without actually running SSH or the SSH daemon by providing a bind-address for the start of the tunnel as part of the forwarding specification, e.g. <tunnel address>:<tunnel port>:<destination address>:<destination port>
. The <tunnel address>
can be any address on the machine at the start of the tunnel. The address localhost
allows connections via the localhost
or loopback interface, and an empty address or *
allow connections via any interface. By default, forwarding is limited to connections from the machine at the "beginning" of the tunnel, i.e. the <tunnel address>
is set to localhost
. Local forwarding requires no additional configuration; however, remote forwarding is limited by the remote server's SSH daemon configuration. See the GatewayPorts
option in sshd_config(5) and -L address
option in ssh(1) for more information about remote forwarding and local forwarding, respectively.
跳板机
有时,可能无法直接连接到目标的 SSH 守护程序,因而需要使用一个跳转服务器(或堡垒服务器)。为此,我们试图将两个或更多的 SSH 隧道连接在一起,并假设你的本地密钥对链上的每个服务器都有授权。这可以通过 SSH 代理转发(-A
)和使用伪终端分配(-t
)转发你的本地密钥实现:
$ ssh -A -t -l user1 bastion1 \ ssh -A -t -l user2 intermediate2 \ ssh -A -t -l user3 target
这一流程可以使用 ProxyCommand 选项进行自动化:
$ ssh -o ProxyCommand="ssh -W %h:%p bastion.example.org" targetserver.example.org
一个更简单和更安全的方法是将 ProxyJump 选项结合 -J
使用:
$ ssh -J user1@bastion1,user2@intermediate2 user3@target
-J
指令中的多个主机可以用逗号隔开,它们将按照列出的顺序被连接。user...@
部分不是必须的,但可以使用。The host specifications for -J
use the ssh configuration file, so specific per-host options can be set there, if needed.
The main difference between the ProxyCommand and ProxyJump options is that the later does not require a shell on the jumphost. Consequently, this also means that the jumpserver does not require access to the users login credentials or SSH agent forwarding. With the ProxyJump option, the ssh client connects through the jumpserver directly to the target server, establishing an end-to-end encrypted channel between client and target server.
-J
标志相当于配置文件中 ProxyJump
选项,相关细节请参考 ssh_config(5)。
Reverse SSH through a relay
The idea is that the client connects to the server via another relay while the server is connected to the same relay using a reverse SSH tunnel. This is useful when the server is behind a NAT, and the relay is a publicly accessible SSH server used as a proxy to which the user has access. Therefore, the prerequisite is that the client's keys are authorized against both the relay and the server, and the server needs to be authorized against the relay as well for the reverse SSH connection.
The following configuration example assumes that user1 is the user account used on client, user2 on relay and user3 on server. First, assuming we will use port 2222, the server needs to establish the reverse tunnel with:
ssh -R 2222:localhost:22 -N user2@relay
Which can also be automated with a startup script, systemd service, autossh or sidedoorAUR.
At the client side, the connection is established with:
ssh -t user2@relay ssh user3@localhost -p 2222
ssh user3@relay -p 2222
would require you to open this port in the firewall of your relay server, as well as to allow connections to this port from other addresses.The remote command to establish the connection to reverse tunnel can also be defined in relay's ~/.ssh/authorized_keys
by including the command
field as follows:
command="ssh user3@localhost -p 2222" ssh-ed25519 KEY2 user1@client
In this case the connection is established with:
ssh user2@relay
Alternatively, you can add an entry to your ssh configuration that specifies both RemoteCommand
and RequestTTY
:
~/.ssh/config
Host jump-destination Hostname relay User user2 RemoteCommand ssh user3@localhost -p 2222 RequestTTY yes
Which will reduce connecting to:
ssh jump-destination
多路复用
SSH 守护进程通常监听 22 端口。然而,许多公共热点会屏蔽所有不在常规 HTTP/S 端口(80 和 443)的流量,从而阻止了 SSH 连接。最直接解决这个问题的办法是让 sshd
额外监听白名单中的一个端口。
/etc/ssh/sshd_config
Port 22 Port 443
然而,443 端口很可能已经被提供 HTTPS 内容的网页服务器使用,在这种情况下,可以使用类似 sslh包 的多路复用器,它会监听多路复用端口,并智能地将数据包转发给多个服务。
Speeding up SSH
There are several client configuration options which can speed up connections either globally or for specific hosts. See ssh_config(5) for full descriptions of these options.
-
Use a faster cipher: on modern CPUs with AESNI instructions,
aes128-gcm@openssh.com
andaes256-gcm@openssh.com
should offer significantly better performance over openssh's default preferred cipher, usuallychacha20-poly1305@openssh.com
. Cipher can be selected with the-c
flag. For a permanent effect, putCiphers
option in your~/.ssh/config
with ciphers in new preferred order, e.g.:Ciphers aes128-gcm@openssh.com,aes256-gcm@openssh.com,chacha20-poly1305@openssh.com,aes256-ctr,aes192-ctr,aes128-ctr
-
Enable or disable compression: compression can increase speed on slow connections; it is enabled with the
Compression yes
option or the-C
flag. However, the compression algorithm used is the relatively slow gzip(1) which becomes the bottleneck on fast networks. In order to speed up the connection, one should use theCompression no
option on local or fast networks.
-
Connection sharing: you can make all sessions to the same host share a single connection using these options:
ControlMaster auto ControlPersist yes ControlPath ~/.ssh/sockets/socket-%r@%h:%p
- where
~/.ssh/sockets
can be any directory not writable by other users.
-
ControlPersist
specifies how long the master should wait in the background for new clients after the initial client connection has been closed. Possible values are either:-
no
to close the connection immediately after the last client disconnects, - a time in seconds,
-
yes
to wait forever, the connection will never be closed automatically.
-
- Login time can be shortened by bypassing IPv6 lookup using the
AddressFamily inet
option or-4
flag.
- Last, if you intend to use SSH for SFTP or SCP, High Performance SSH/SCP can significantly increase throughput by dynamically raising the SSH buffer sizes. Install the package openssh-hpnAUR to use a patched version of OpenSSH with this enhancement.
使用 SSHFS 挂载远程文件系统
关于将 SSH 可访问的远程系统挂载到本地目录的内容请查看 SSHFS 文章。这样您就可以使用任何工具对挂载的文件执行任何操作(复制、重命名、使用 vim 编辑等)。sshfs 一般优于 shfs,后者自 2004 年以来一直没有更新。
保活
默认情况下,如果 SSH 会话空闲了一段时间,它将自动注销。为了保持会话,如果在一段时间内没有收到数据,客户端可以向服务器发送保持活动的信号,或者如果服务器没有收到客户端的消息,则可以对称地定期发送消息。
- 在服务端,
ClientAliveInterval
以秒为单位设置超时,如果没有从客户端接收到数据,sshd 将发送响应请求。默认值为 0(不发送消息)。例如,要每 60 秒从客户端请求一次响应,请在服务器配置中设置ClientAliveInterval 60
选项。另见ClientAliveCountMax
和TCPKeepAlive
选项。
- 在 '客户端,
ServerAliveInterval
控制从客户端发送到服务器的响应请求之间的间隔。例如,要请求服务器每 120 秒响应一次,请将ServerAliveInterval 120
选项添加到客户端配置中。另请参见ServerAliveCountMax
和TCPKeepAlive
选项。
ServerAliveInterval
,并使其他客户机和服务器保持默认配置。Automatically restart SSH tunnels with systemd
systemd can automatically start SSH connections on boot/login and restart them when they fail. This makes it a useful tool for maintaining SSH tunnels.
The following service can start an SSH tunnel on login using the connection settings in your ssh configuration. If the connection closes for any reason, it waits 10 seconds before restarting it:
~/.config/systemd/user/tunnel.service
[Unit] Description=SSH tunnel to myserver [Service] Type=simple Restart=always RestartSec=10 ExecStart=/usr/bin/ssh -F %h/.ssh/config -N myserver
Then enable and start the Systemd/User service. See #Keep alive for how to prevent the tunnel from timing out. If you wish to start the tunnel on boot, you might want to rewrite the unit as a system service.
Autossh - automatically restarts SSH sessions and tunnels
When a session or tunnel cannot be kept alive, for example due to bad network conditions causing client disconnections, you can use autossh包 to automatically restart them.
Usage examples:
$ autossh -M 0 -o "ServerAliveInterval 45" -o "ServerAliveCountMax 2" username@example.com
Combined with SSHFS:
$ sshfs -o reconnect,compression=yes,transform_symlinks,ServerAliveInterval=45,ServerAliveCountMax=2,ssh_command='autossh -M 0' username@example.com: /mnt/example
Connecting through a SOCKS-proxy set by Proxy settings:
$ autossh -M 0 -o "ServerAliveInterval 45" -o "ServerAliveCountMax 2" -NCD 8080 username@example.com
With the -f
option autossh can be made to run as a background process. Running it this way however means the passphrase cannot be entered interactively.
The session will end once you type exit
in the session, or the autossh process receives a SIGTERM, SIGINT of SIGKILL signal.
Run autossh automatically at boot via systemd
If you want to automatically start autossh, you can create a systemd unit file:
/etc/systemd/system/autossh.service
[Unit] Description=AutoSSH service for port 2222 After=network.target [Service] Environment="AUTOSSH_GATETIME=0" ExecStart=/usr/bin/autossh -M 0 -NL 2222:localhost:2222 -o TCPKeepAlive=yes foo@bar.com [Install] WantedBy=multi-user.target
Here AUTOSSH_GATETIME=0
is an environment variable specifying how long ssh must be up before autossh considers it a successful connection, setting it to 0 autossh also ignores the first run failure of ssh. This may be useful when running autossh at boot. Other environment variables are available at autossh(1). Of course, you can make this unit more complex if necessary (see the systemd documentation for details), and obviously you can use your own options for autossh, but note that the -f
implying AUTOSSH_GATETIME=0
does not work with systemd.
Remember to start and/or enable the service afterwards.
You may also need to disable ControlMaster e.g.
ExecStart=/usr/bin/autossh -M 0 -o ControlMaster=no -NL 2222:localhost:2222 -o TCPKeepAlive=yes foo@bar.com
Alternative service should SSH daemon fail
For remote or headless servers which rely exclusively on SSH, a failure to start the SSH daemon (e.g., after a system upgrade) may prevent administration access. systemd offers a simple solution via OnFailure
option.
Let us suppose the server runs sshd
and telnet is the fail-safe alternative of choice. Create a file as follows. Do not enable telnet.socket
!
/etc/systemd/system/sshd.service.d/override.conf
[Unit] OnFailure=telnet.socket
That's it. Telnet is not available when sshd
is running. Should sshd
fail to start, a telnet session can be opened for recovery.
Terminal background color based on host
为了更好地区分在不同主机上的情况,可以设置基于主机类型的不同背景颜色 。
这种解决方案仅限于 ZSH。
Network specific configuration
You can use host configuration specific to the network you are connected to using a Match exec
.
For example, when using nmcli(1), and the connection is configured (manually or through DHCP) to use a search-domain:
Match exec "nmcli | grep domains: | grep example.com" CanonicalDomains example.com # Should you use a different username on this network #User username # Use a different known_hosts file (for private network or synchronisation) #UserKnownHostsFile <network>_known_hosts
Another example for Match host ... exec "..."
: Consider that connecting to internal.example.com
requires a bastion/proxy (via ProxyJump
) unless you are already connected via VPN. The fragment !exec "host internal.example.com"
applies only when internal.example.com
cannot be looked up via DNS. Various alternatives are discussed at [3].
Match host internal.example.com !exec "host internal.example.com" ProxyJump bastion.example.com Host internal.example.com User foobar
Private networks hostkeys verification
Because different servers on different networks are likely to share a common private IP address, you might want to handle them differently.
The best solution is to use the #Network specific configuration to use a different UserKnownHostsFile
depending on the network you are on. The second solution, best used as default when you are working on new/prototype networks, would be to simply ignore hostkeys for private networks:
Host 10.* 192.168.*.* 172.31.* 172.30.* 172.2?.* 172.1?.* # Disable HostKey verification # Trust HostKey automatically StrictHostKeyChecking no # Do not save the HostKey UserKnownHostsFile=/dev/null # Do not display: "Warning: Permanently Added ..." LogLevel Error
在登录时运行命令
如果你使用的是交互式会话,有多种方法可以在登录时执行一个命令:
- 使用远程主机上的
authorized_keys
文件(参见 sshd(8) § AUTHORIZED_KEYS FILE FORMAT) - 如果服务器启用了
~/.ssh/rc
选项,则使用远程主机上的PermitUserRC
选项 - 使用远程主机上的 shell 配置文件,例如
.bashrc
。
转发代理
SSH 代理转发允许你在连接到一个服务器时使用你的本地密钥。建议只对选定的主机启用代理转发。
~/.ssh/config
Host myserver.com ForwardAgent yes
接下来,配置 SSH 代理并使用 ssh-add 添加你的本地密钥。
如果你现在连接到一个远程服务器,你将能够使用你的本地密钥连接到其他服务。
生成新的密钥对
新的服务器私钥可以通过以下方式生成:
- 删除已有的所有密钥,例如:
# rm /etc/ssh/ssh_host_*_key*
-
重启
sshdgenkeys.service
或者以根用户身份运行ssh-keygen -A
Run sshd as non-privileged user
You may want to run sshd
as non-privileged user in containers, or for testing, etc.
Since non-privileged user cannot read host keys in /etc/ssh
, new host keys must be generated:
$ ssh-keygen -q -N "" -t rsa -b 4096 -f /path/to/host/keys/ssh_host_rsa_key $ ssh-keygen -q -N "" -t ecdsa -f /path/to/host/keys/ssh_host_ecdsa_key $ ssh-keygen -q -N "" -t ed25519 -f /path/to/host/keys/ssh_host_ed25519_key
Create an sshd_config
file. The example below uses a port higher than 1024, provides a new path to the host keys and disables PAM:
/path/to/sshd_config
Port 2022 HostKey /path/to/host/keys/ssh_host_rsa_key HostKey /path/to/host/keys/ssh_host_ecdsa_key HostKey /path/to/host/keys/ssh/ssh_host_ed25519_key UsePAM no
Run sshd with the created config. The -D
flag disables daemon mode and -e
redirects output to stderr to allow easy monitoring.
$ sshd -f /path/to/sshd_config -D -e
排障
Checklist
Check these simple issues before you look any further.
- The configuration directory
~/.ssh
, its contents should be accessible only by the user (check this on both the client and the server), and the user's home directory should only be writable by the user:$ chmod go-w ~ $ chmod 700 ~/.ssh $ chmod 600 ~/.ssh/* $ chown -R $USER ~/.ssh
- Check that the client's public key (e.g.
id_ed25519.pub
) is in~/.ssh/authorized_keys
on the server. - Check that you did not limit SSH access with
AllowUsers
orAllowGroups
in the server config. - Check if the user has set a password. Sometimes new users who have not yet logged in to the server do not have a password.
-
Append
LogLevel DEBUG
to/etc/ssh/sshd_config
. - Run
journalctl -xe
as root for possible (error) messages. -
Restart
sshd
and logout/login on both client and server.
Connection refused or timeout problem
Port forwarding
If you are behind a NAT mode/router (which is likely unless you are on a VPS or publicly addressed host), make sure that your router is forwarding incoming ssh connections to your machine. Find the server's internal IP address with $ ip addr
and set up your router to forward TCP on your SSH port to that IP. portforward.com can help with that.
Is SSH running and listening?
The ss utility shows all the processes listening to a TCP port with the following command line:
$ ss --tcp --listening
If the above command do not show the system is listening to the port ssh
, then SSH is not running: check the journal for errors etc.
Are there firewall rules blocking the connection?
Iptables may be blocking connections on port 22
. Check this with:
# iptables -nvL
and look for rules that might be dropping packets on the INPUT
chain. Then, if necessary, unblock the port with a command like:
# iptables -I INPUT 1 -p tcp --dport 22 -j ACCEPT
For more help configuring firewalls, see firewalls.
Is the traffic even getting to your computer?
Start a traffic dump on the computer you are having problems with:
# tcpdump -lnn -i any port ssh and tcp-syn
This should show some basic information, then wait for any matching traffic to happen before displaying it. Try your connection now. If you do not see any output when you attempt to connect, then something outside of your computer is blocking the traffic (e. g., hardware firewall, NAT router etc.).
Your ISP or a third party blocking default port?
In some cases, your ISP might block the default port (SSH port 22) so whatever you try (opening ports, hardening the stack, defending against flood attacks, et al) ends up useless. To confirm this, create a server on all interfaces (0.0.0.0) and connect remotely.
If you get an error message comparable to this:
ssh: connect to host www.inet.hr port 22: Connection refused
That means the port is not being blocked by the ISP, but the server does not run SSH on that port (See security through obscurity).
However, if you get an error message comparable to this:
ssh: connect to host 111.222.333.444 port 22: Operation timed out
That means that something is rejecting your TCP traffic on port 22. Basically that port is stealth, either by your firewall or 3rd party intervention (like an ISP blocking and/or rejecting incoming traffic on port 22). If you know you are not running any firewall on your computer, and you know that Gremlins are not growing in your routers and switches, then your ISP is blocking the traffic.
To double check, you can run Wireshark on your server and listen to traffic on port 22. Since Wireshark is a Layer 2 Packet Sniffing utility, and TCP/UDP are Layer 3 and above (see IP Network stack), if you do not receive anything while connecting remotely, a third party is most likely to be blocking the traffic on that port to your server.
Diagnosis
Install either tcpdump包 or Wireshark with the wireshark-cli包 package.
For tcpdump:
# tcpdump -ni interface "port 22"
For Wireshark:
$ tshark -f "tcp port 22" -i interface
where interface
is the network interface for a WAN connection (see ip a
to check). If you are not receiving any packets while trying to connect remotely, you can be very sure that your ISP is blocking the incoming traffic on port 22.
Possible solution
The solution is just to use some other port that the ISP is not blocking. Open the /etc/ssh/sshd_config
and configure the file to use different ports. For example, add:
Port 22 Port 1234
Also make sure that other "Port" configuration lines in the file are commented out. Just commenting "Port 22" and putting "Port 1234" will not solve the issue because then sshd will only listen on port 1234. Use both lines to run the SSH server on both ports.
Restart the server sshd.service
and you are almost done. You still have to configure your client(s) to use the other port instead of the default port. There are numerous solutions to that problem, but let us cover two of them here.
Read from socket failed: connection reset by peer
Recent versions of OpenSSH sometimes fail with the above error message when connecting to older ssh servers. This can be worked around by setting various client options for that host. See ssh_config(5) for more information about the following options.
The problem could be the ecdsa-sha2-nistp*-cert-v01@openssh
elliptical host key algorithms. These can be disabled by setting HostKeyAlgorithms
to a list excluding those algorithms. On the client side, the HostKeyAlgorithms
that the client wants to use can also be set by preceding the HostKeyAlgorithms
list with a -
to remove the specified algorithms (including wildcards) from the default set (see ssh_config(5)
man page).
You can check the actually used host key algorithm with ssh -v server_to_connect_to
in the line that contains kex: host key algorithm:
.
If that does not work, it could be that the list of ciphers is too long. Set the Ciphers
option to a shorter list (fewer than 80 characters should be enough). Similarly, you can also try shortening the list of MACs
.
See also the discussion on the OpenSSH bug forum.
"[your shell]: No such file or directory" / ssh_exchange_identification problem
One possible cause for this is the need of certain SSH clients to find an absolute path (one returned by whereis -b [your shell]
, for instance) in $SHELL
, even if the shell's binary is located in one of the $PATH
entries.
"Terminal unknown" or "Error opening terminal" error message
If you receive the above errors upon logging in, this means the server does not recognize your terminal. Ncurses applications like nano may fail with the message "Error opening terminal".
The correct solution is to install the client terminal's terminfo file on the server. This tells console programs on the server how to correctly interact with your terminal. You can get info about current terminfo using $ infocmp
and then find out which package owns it.
If you cannot install it normally, you can copy your terminfo to your home directory on the server:
$ ssh myserver mkdir -p ~/.terminfo/${TERM:0:1} $ scp /usr/share/terminfo/${TERM:0:1}/$TERM myserver:~/.terminfo/${TERM:0:1}/
After logging in and out from the server the problem should be fixed.
TERM hack
Alternatively, you can simply set TERM=xterm
in your environment on the server (e.g. in .bash_profile
). This will silence the error and allow ncurses applications to run again, but you may experience strange behavior and graphical glitches unless your terminal's control sequences exactly match xterm's.
Connection closed by x.x.x.x [preauth]
If you are seeing this error in your sshd logs, make sure you have set a valid HostKey
HostKey /etc/ssh/ssh_host_ed25519_key
subsystem request failed
Since OpenSSH 8.8, scp uses SFTP as the default protocol for data transfers by requesting the subsystem named sftp
. If you run scp in verbose mode, scp -v
, you can determine which subsystem your client is using (e.g. Sending subsystem: <subsystem-name>
). Errors such as subsystem request failed on channel 0
may be fixed by configuring the server's Subsystem settings: sshd_config(5) § Subsystem. The server configuration should resemble the example below.
/etc/ssh/sshd_config
... Subsystem subsystem-name /path/to/subsystem-executable ...
id_dsa refused
OpenSSH 7.0 deprecated DSA public keys for security reasons and OpenSSH 9.8 is built without support for DSA keys by default. The first OpenSSH release of 2025 will remove DSA support entirely. For now, if you absolutely must use them, you will need to rebuild openssh包 while passing --enable-dsa-keys
to configure
.[4]
No matching key exchange method found by OpenSSH 7.0
OpenSSH 7.0 deprecated the diffie-hellman-group1-sha1 key algorithm because it is weak and within theoretical range of the so-called Logjam attack (see https://www.openssh.com/legacy.html). If the key algorithm is needed for a particular host, ssh will produce an error message like this:
Unable to negotiate with 127.0.0.1: no matching key exchange method found. Their offer: diffie-hellman-group1-sha1
The best resolution for these failures is to upgrade/configure the server to not use deprecated algorithms. If that is not possible, you can force the client to reenable the algorithm with the client option KexAlgorithms +diffie-hellman-group1-sha1
.
tmux/screen session killed when disconnecting from SSH
If your processes get killed at the end of the session, it is possible that you are using socket activation and it gets killed by systemd包 when it notices that the SSH session process exited. In that case there are two solutions. One is to avoid using socket activation by using ssh.service
instead of ssh.socket
. The other is to set KillMode=process
in the Service section of ssh@.service
.
The KillMode=process
setting may also be useful with the classic ssh.service
, as it avoids killing the SSH session process or the screen包 or tmux包 processes when the server gets stopped or restarted.
SSH session stops responding
SSH responds to flow control commands XON
and XOFF
. It will freeze/hang/stop responding when you hit Ctrl+s
. Use Ctrl+q
to resume your session.
Broken pipe
If you attempt to create a connection which results in a Broken pipe
response for packet_write_wait
, you should reattempt the connection in debug mode and see if the output ends in error:
debug3: send packet: type 1 packet_write_wait: Connection to A.B.C.D port 22: Broken pipe
The send packet
line above indicates that the reply packet was never received. So, it follows that this is a QoS issue. To decrease the likely-hood of a packet being dropped, set IPQoS
:
/etc/ssh/ssh_config
Match all IPQoS reliability
The reliability
(0x04
) type-of-service should resolve the issue, as well as 0x00
and throughput
(0x08
).
Terminate unresponsive SSH connection
If a client session is no longer responding and cannot be terminated by instructing the running program (e.g. shell), you can still terminate the session by pressing Enter
, ~
and .
one after another in that order.
The ~
is a pseudo-terminal escape character (see ssh(1) § ESCAPE CHARACTERS), which can be added multiple times depending on the client session to terminate. For example, if you connected from A to B and then from B to C and the session from B to C freezes, you can terminate it by pressing Enter
and typing ~~.
, which will leave you in a working session on B.
WARNING: REMOTE HOST IDENTIFICATION HAS CHANGED!
If the client warns that the key of an ssh server has changed, you should verify that the newly offered key really belongs to the server operator via an authenticated (not necessarily encrypted) channel. Then remove the old key from the known_hosts
file with ssh-keygen -R $SSH_HOST
and accept the new key as if it was a new server.
Connecting to a remote without the appropriate terminfo entry
When connecting to hosts that do not have a terminfo entry for your terminal, for example, when using a terminal emulator whose terminfo entry is not shipped with ncurses包 (e.g. kitty and rxvt-unicode), or when connecting to hosts with a limited terminfo database (e.g. systems running OpenWrt), various issues will occur with software that relies on terminfo(5).
A proper solution is to place the appropriate terminfo entry on the host. If that is not feasible, an alternative is to set TERM
to a value that is both supported by the remote host and compatible with the terminal.
Since OpenSSH 8.7, a custom TERM
environment variable can be passed to remote hosts with a simple configuration snippet:
~/.ssh/config
Host example.com SetEnv TERM=xterm-256color
Connection through jump host fails with "bash: No such file or directory"
If you do not have the SHELL
environment variable set to a full valid path (on the jump server), connection will fail with an error message simmilar to this one:
bash: No such file or directory kex_exchange_identification: Connection closed by remote host Connection closed by UNKNOWN port 65535
You can simply solve this by setting your SHELL
to a full path name of a shell that will also be valid on the jump server or by setting a specific SHELL
variable for each server in your ~/.ssh/config
file.
参见
- Wikibooks:OpenSSH
- 防御暴力 ssh 攻击
- OpenSSH 密钥管理:第一部分在 IBM developerWorks 上, 第二部分、第三部分在 funtoo.org 上
- Secure Secure Shell