SSH/zh-cn

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

除了用于远程终端访问的ssh这个主要的二进制文件，SSH已经逐渐扩展成为一套包括像 scp (secure copy) 和 sftp (secure file transfer protocol)等工具的套件.

最初，SSH 并不是免费的. 然而，现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.

安装
Most deployments will already have OpenSSH installed (through the package). This package uses the following USE flags:

After changing USE flags, don't forget to 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 hasn't been done already.

Start sshd daemon with:

The OpenSSH server can be controlled like any other OpenRC-managed service:

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  command.

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

To generate the keys for SSH protocol version 2 (DSA and RSA algorithms):

Server configuration
The SSH server is usually configured in the file, though it is also possible to perform further configuration in OpenRC's, including changing the location of the configuration file. For detailed information on how to configure the server see the sshd_config man page.

您应当也看一看这份偏重于安全的指南.

Client configuration
The ssh client and related programs (scp, sftp, etc.) can be configured in following files:

For more information read the  manual:

Passwordless Authentication
Handy for git server management

Client
On the client run the following command:

Server
Make sure an account for the user exists on the server, and then place the clients' file into the  file.

Single Machine Testing
The above procedure can be tested out locally:

Troubleshooting
There are 3 different levels of debug modes that can help troubleshooting issues. With -v ssh prints debugging messages about its progress. This is helpful in debugging connection, authentication, and configuration problems. Multiple -v options increase the verbosity. The maximum is 3.

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, though the ssh 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 ClientAliveInterval 30 (or some other value, in seconds) within the file.
 * To enable keep alive for all servers connected to by your local client, set ServerAliveInterval 30 (or some other value, in seconds) within the file.

X11 Forwarding Not Forwarding or Tunneling!
Problem: After you have made the necessary changes to the configuration files for permitting X11 Forwarding, you find 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 as follows after logging in remotely:

You should get something similar to "localhost:10.0" or "localhost2.local:10.0" using server side  setting. If you're getting the usual ":0.0", check to make sure you are not unsetting or initializing the  variable within. If so, remove or comment your custom initialization of  or prevent  from executing during SSH login:

A trick would be to alias this within.