Secure Shell (简体中文)

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Secure Shell(SSH)是一个允许两台电脑之间通过安全的连接进行数据交换的网络协议。加密技术保证了数据的保密性和完整性。如果有必要的话,SSH采用公匙加密技术来验证远程主机,以及必要时允许远程主机验证用户。

SSH 通常用于远程访问和执行命令,但是它也支持隧道,转发任意 TCP 端口以及 X11 连接;它还能够用 SFTP 或 SCP 协议来传递文件。

一个 SSH 服务器,默认地,在 TCP 端口 22 进行监听。一个 SSH 客户端程序通常被用来建立一个接受远程连接的 sshd 守护进程。这两者都以专利的,自由软件的以及开源版本的形式和不同的复杂性和完整性,被广泛地应用在现代的操作系统中,包括 Mac OS X,GNU/Linux,Solaris 和 OpenVMS 等。

OpenSSH

OpenSSH (OpenBSD Secure Shell) 是一套用ssh协议,通过电脑网络,提供加密通讯会话的电脑程序。它相对于SSH Communications Security公司拥有专利的Secure Shell软件套装,被创建为开源。OpenSSH是由Theo de Raadt领导的OpenBSD项目的一部分。

人们常把OpenSSH与相似的OpenSSL搞混,但是,这两个项目是由不同的团队开发出来的,有不同的用途,相似的名字只是由于相似的目标。

安装OpenSSH

pacman -S openssh

配置SSH

客户端

SSH客户端的配置文件是Template:Filename

以下是一个范例::

Template:File

推荐将“Protocol”行改为:

Protocol 2

这表示只有协议2将被用到,因为协议1 被认为不太安全。

守护进程

SSH客户端的配置文件是Template:Filename

以下是一个范例::

Template:File

只允许某些用户访问的话,加入这一行:

AllowUsers    user1 user2

要禁止通过SSH进行root用户登录,加入以下行:

PermitRootLogin no

你也可以取消BANNER选项的注释,然后编辑Template:Filename加入友好的欢迎信息内容。

Tip: 你可能想要把默认的端口从22改成其它更高的端口(参考 security through obscurity).

尽管ssh的运行端口可以被像nmap这样的端口扫描器侦测到,但改变它可以减少由于自动验证的尝试造成登录次数。

Tip: 完全取消密码登录方式也可以增强安全性,因为每个访问服务器的用户都需要创建ssh密钥。(参考 Using SSH Keys).

Template:File

允许其它主机进入

Template:Box Note

为了让其他人通过ssh登录到你的机器上,你必须要设置“/etc/hosts.allow”这个文件,加入类似下面的文字:

# 让每个人都能连接到你
sshd: ALL

# 或者限制只能特定的ip能够
sshd: 192.168.0.1

# 或者限制ip段
sshd: 10.0.0.0/255.255.255.0

# 或者限制为IP匹配
sshd: 192.168.1.

现在检查你的“/etc/hosts.deny”文件,找到并确保有类似下面一行的内容:

ALL: ALL
你能SSH出去别人也应该能够SSH进来:)。

开始使用新的配置之前,要先重启守护进程(以root身份):

# rc.d restart sshd

管理SSHD守护进程

只要把sshd加到/etc/rc.conf中的"DAEMONS" section部分:

DAEMONS=(... ... sshd ... ...)

开始/重启/停止这个daemon:

# rc.d {start|stop|restart} sshd

连接到服务器

运行下面的命令:

$ ssh -p port user@server-address

小细节和小戏法

加密Socks通道

对于连接到各种不安全的无线网络上的笔记本电脑用户来说,这个是特别有用的!唯一所需要的就是一个一定程度上处于安全的地点的SSH服务器,比如在家里或办公室。用动态的DNS服务DynDNS也可能是很有用的,这样你就不必记住你的IP了。

第一步:开始连接

你只要在你喜欢的终端中执行这一个命令就能开始你的连接:

$ ssh -ND 4711 user@host

这里的“user”是你在“host”这台SSH服务器上运行的用户名。它会让你输入密码,然后你就能连上了。“N”表示不采用交互提示,而“D”表示指定监听的本地端口(你可以使用任何你喜欢的数字)

一个办法可以让这个过程更简单,那就是在~/.bashrc中加入这样一行:

alias sshtunnel="ssh -ND 4711 -v user@host"

加入冗长的“-v”标志更好,因为这样你可以验证是真的是从那个端口连接的。现在你只要执行命令“sshtunnel”就可以。 :)

第二步:配置你的浏览器(或其它程序)

如果你不配置你的web浏览器以便使用新创建的socks通道的话,上面的一步完全没用!

  • 对于Firefox: Edit -> Preferences -> Advanced -> Network -> Connection -> Setting:
检查"Manual proxy configuration" radio button?, 并且在"SOCKS host" 文本段输入"localhost" , 然后在接下来的一个文本框中输入你的端口数(上面我们用的是4711)。
确定你选择使用SOCKS4。这个程序对不会对SOCKS5起作用.
享受你的安全通道吧!

X11 Forwarding

Tango-preferences-desktop-locale.pngThis article or section needs to be translated.Tango-preferences-desktop-locale.png

Notes: please use the first argument of the template to provide more detailed indications. (Discuss in Talk:Secure Shell (简体中文)#)

为了通过SSH运行图形程序你必须使用X11 Forwarding。一个选项就是需要设置服务器和客户端的配置文件。

在服务器上安装xorg-xauth:

# pacman -S xorg-xauth

Also:

Enable the ForwardX11Trusted can help when gui drawing badly.

To use the forwarding, log on to your server through ssh:

$ ssh -X -p port user@server-address

If you receive errors trying to run graphical applications try trusted forwarding instead:

$ ssh -Y -p port user@server-address

You can now start any X program on the remote server, the output will be forwarded to your local session:

$ xclock

If you get "Cannot open display" errors try the following command as the non root user:

$ xhost +

the above command will allow anybody to forward X11 applications. To restrict forwarding to a particular host type:

$ xhost +hostname

where hostname is the name of the particular host you want to forward to. Type "man xhost" for more details.

Be careful with some applications as they check for a running instance on the local machine. Firefox is an example. Either close running Firefox or use the following start parameter to start a remote instance on the local machine

$ firefox -no-remote

Speed up SSH

You can make all sessions to the same host use a single connection, which will greatly speed up subsequent logins, by adding these lines under the proper host in Template:Filename:

ControlMaster auto
ControlPath ~/.ssh/socket-%r@%h:%p

Changing the ciphers used by SSH to less cpu-demanding ones can improve speed. In this aspect, the best choices are arcfour and blowfish-cbc. Please do not do this unless you know what you are doing; arcfour has a number of known weaknesses. To use them, run SSH with the "c" flag, like this:

$ ssh -c arcfour,blowfish-cbc user@server-address

To use them permanently, add this line under the proper host in Template:Filename:

Ciphers arcfour,blowfish-cbc

Another option to improve speed is to enable compression with the "C" flag. A permanent solution is to add this line under the proper host in Template:Filename:

Compression yes

Login time can be shorten by using the "4" flag, which bypasses IPv6 lookup. This can be made permanent by adding this line under the proper host in Template:Filename:

AddressFamily inet

Another way of making these changes permanent is to create an alias in Template:Filename:

alias ssh='ssh -C4c arcfour,blowfish-cbc'

Troubleshooting

Make sure your DISPLAY string is resolveable on the remote end:

$ ssh -X user@server-address
server $ echo $DISPLAY
localhost:10.0
server $ telnet localhost 6010
localhost/6010: lookup failure: Temporary failure in name resolution   

can be fixed by adding localhost to Template:Filename.

用SSHFS挂载远程文件系统

安装sshfs

# pacman -S sshfs

将你想要允许挂载SSH文件夹的用户添加到fuse组里

# gpasswd -a USER fuse

加载fuse模块(比如说在/etc/rc.conf中)

And then然后,在登录后,你就可以试着挂用sshfs载远程文件夹了:

# mkdir ~/remote_folder
# sshfs USER@remote_server:/tmp ~/remote_folder

上面的命令将把远程服务器上的/tmp文件夹挂载到本地的~/remote_folder目录下。复制任何文件到这个目录将使文件通过SCP通过网络传输。 Same concerns direct file editing, creating or removing.

当我们完成在远程文件夹下的工作,我们可以这样来卸载它:

# fusermount -u ~/remote_folder

如果我们需要经常在这个文件夹下,让它通过/etc/fstab挂载是一个明智的选择。这个办法可以让它在启动的时候挂载或者通过手动挂载(如果是noauto选项的话),而不需要每次都去挂载它。下面是一个简单的样本:

sshfs#USER@remote_server:/tmp /full/path/to/directory fuse    defaults,auto    0 0

Keep Alive

Your ssh session will automatically log out if it is idle. To keep the connection active (alive) add this to Template:Filename or to Template:Filename on the client.

ServerAliveInterval 120

This will send a "keep alive" signal to the server every 120 seconds.

Conversely, to keep incoming connections alive, you can set

ClientAliveInterval 120

(or some other number greater than 0) in Template:Filename on the server.

Save connection data in ssh config

Whenever you want to connect to a ssh server, you usually have to type at least its address and the username. To save that typing work for servers you regularly connect to, you can use the personal Template:Filename or the global Template:Filename files as shown in the following example:

Template:File

Now you can simply connect to the server by using the name you specified:

$ ssh myserver

To see a complete list of the possible options, check out ssh_config's manpage on your system or the ssh_config documentation on the official website.

Change bash prompt when logged over ssh

It can sometimes be useful to be able to make the difference between your local and your remote prompt, in particular when they are both configured in the same way. To do that, just insert this in your bashrc:

Template:File

See Color Bash Prompt for more information about the PS1 variable customization.

Automatically logout all ssh users when the sshd server is shutdown

To automatically log out all remote ssh users when the sshd server system shuts down, for reboot or halt, add this line to /etc/rc.local.shutdown on the sshd server:

who | cut -d " " -f1 | uniq | xargs pkill -KILL -u

This prevents ssh client terminals from hanging during a lengthy timeout, which eventually ends with:

Write failed: Broken pipe

Troubleshooting

Connection Refused Problem

Is SSH running and listening?

# netstat -tnlp | grep ssh

If the above command doesn't display anything, then SSH is NOT running. Check /var/log/messages for errors etc.

Are there firewall rules blocking the connection?

Flush your iptables rules to make sure they are not interfering:

# rc.d stop iptables

or:

# iptables -P INPUT ACCEPT
# iptables -P OUTPUT ACCEPT
# iptables -F INPUT
# iptables -F OUTPUT

Is the traffic even getting to your computer?

Start a traffic dump on the computer you're 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.).

Read from socket failed: Connection reset by peer

Recent versions of openssh sometimes fail with the above error message, due to a bug involving elliptic curve cryptography. In that case, edit the file

~/.ssh/config

or create it, if it doesn't already exist. Add the line

HostKeyAlgorithms ssh-rsa-cert-v01@openssh.com,ssh-dss-cert-v01@openssh.com,ssh-rsa-cert-v00@openssh.com,ssh-dss-cert-v00@openssh.com,ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,ssh-rsa,ssh-dss

With openssh 5.9, the above fix doesn't work. Instead, put the following lines in ~/.ssh/config

Ciphers aes128-ctr,aes192-ctr,aes256-ctr,aes128-cbc,3des-cbc 
MACs hmac-md5,hmac-sha1,hmac-ripemd160

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. Another reason can be that the user is no member of the network group.

See Also

链接和参考