SSH/zh-cn

SSH (Secure SHell) 是一个加密的终端程序，用于替代类Unix操作系统上传统的 telnet 工具.

SSH已经发展成为一组软件系列，除了提供用于远程终端访问的 ssh 这个主要的程序，还包括其他的工具如 scp(secure copy) 和 sftp (secure file transfer protocol).

最初，SSH并不是免费的. 然而，当今最流行并成为实际标准的SSH实现是 OpenBSD 的OpenSSH，它在Gentoo中已预安装.

Check install
Most deployments of Gentoo Linux will already have OpenSSH installed on the system. This can be checked by running the ssh command. If it is installed a usage statement should be printed:

If no usage statement is printed ssh is either corrupted or not installed. It is also possible that a user is simply rebuilding OpenSSH to include a new USE configuration. Whatever the case, proceed on to view possible USE settings.

Emerge OpenSSH
After changing the necessary USE flags, do not forget to install (or rebuild) OpenSSH:

服务器端
OpenSSH 服务器可以通过 /etc/init.d/ssh start，/etc/init.d/ssh stop 和 /etc/init.d/ssh restart 来控制，就像其它 OpenRC 管理的服务一样.

Add openssh to the default runlevel if this has not been done already:

Start the sshd daemon with:

OpenSSH服务器可以像其他的由OpenRC管理的服务那样控制：

Create keys
In order to provide a secure shell, cryptographic keys are used to manage the encryption, decryption, and hashing functionalities offered by SSH.

On the first start of the SSH service, system keys will be generated. Keys can be (re)generated using the ssh-keygen command.

To generate the key used for SSH protocol version 1 (which usually is not enabled anymore; it has been deprecated in favor of protocol version 2) use:

生成SSH协议2的密钥（DSA和RSA算法）：

服务器端配置
SSH服务器配置文件通常是 ，虽然也有可能通过OpenRC的文件进行进一步的配置，包括修改配置文件的位置. 关于如何配置服务器的详细信息请参考sshd_config man page.

Users should study Sven's OpenSSH guide for a security focused configuration.

客户端配置
The ssh client and related programs (scp, sftp, etc.) can be configured using the following files:

For more information read the manual:

无密码验证
对于管理 git 服务器非常便利.

客户端
在客户端执行下面的命令：

服务器
确保服务器上存在用户的帐号，然后把客户端的文件拷贝到服务器端的文件.

Single machine testing
上面的步骤可以在本地测试：

故障排除
There are 3 different levels of debug modes that can help troubleshooting issues. With the  option SSH prints debugging messages about its progress. This is helpful in debugging connection, authentication, and configuration problems. Multiple  options increase the verbosity. Maximum verbosity is three levels deep.

Death of long-lived connections
Many internet access devices perform Network Address Translation (NAT), a process that enables devices on a private network such as that typically found in a home or business place to access foreign networks, such as the internet, despite only having a single IP address on that network. Unfortunately, not all NAT devices are created equal, and some of them incorrectly close long-lived, occasional-use TCP connections such as those used by SSH. This is generally observable as a sudden inability to interact with the remote server, even though the ssh</tt> client program has not exited.

In order to resolve the issue, OpenSSH clients and servers can be configured to send a 'keep alive', or invisible message aimed at maintaining and confirming the live status of the link.
 * To enable keep alive for all clients connecting to your local server, set  (or some other value, in seconds) within the  file.
 * To enable keep alive for all servers connected to by your local client, set  (or some other value, in seconds) within the  file.

X11 forwarding, not forwarding, or tunneling
Problem: After having made the necessary changes to the configuration files for permitting X11 forwarding, it is discovered X applications are executing on the server and are not being forwarded to the client.

Solution: What is likely occurring during SSH login into the remote server or host, the  variable is either being unset or is being set after the SSH session sets it.

Test for this scenario perform the following after logging in remotely:

The output should be something similar to  or   using server side   setting. If the usual  is not displayed, check to make sure the   variable within  is not being unset or re-initializing. If it is, remove or comment out any custom initialization of  to prevent the code in  from executing during a SSH login:

A trick that works to complete this task would be to define an alias within the users' file.

Intrusion prevention
SSH is a commonly attacked service. Tools such as sshguard and fail2ban monitor logs and black list remote users who have repeatedly attempted, yet failed to login. Utilized them as needed to secure a frequently attacked system.

External resources

 * Securing OpenSSH - Gentoo developer documentation.
 * OpenSSH Key Management Part1 - Gentoo documentation, have a look at all 3 parts.