SSH ProxyCommand example: Going through one host to reach another server

Posted on in Categories , , last updated May 17, 2015

How do I use and jump through one server to reach another using ssh on a Linux or Unix-like systems? Is it possible to connect to another host via an intermediary so that the client can act as if the connection were direct using ssh?

You can jump host using ProxyCommand. Some times you can only access a remote server via ssh by first login into an intermediary server (or firewall/jump host). So you first login into to the intermediary server and then ssh to another server. You need to authenticate twice and the chain can be long and is not limited to just two hosts.

Sample setup

     +-------+       +----------+      +-----------+
     | Laptop| <---> | Jumphost | <--> | FooServer |
     +-------+       +----------+      +-----------+


               OR

     +-------+       +----------+      +-----------+
     | Laptop| <---> | Firewall | <--> | FooServer |
     +-------+       +----------+      +-----------+
	192.168.1.5      121.1.2.3         10.10.29.68

I can can only access a remote server named ‘FooServer’ via ssh by first login into an intermediary server called ‘Jumphost’. First, login to Jumphost:
$ ssh [email protected]
Next, I must ssh through the intermediary system as follows:
$ ssh [email protected]

Passing through a gateway or two

Instead of typing two ssh command, I can type the following all-in-one command. This is useful for connecting to FooServer via firewall called ‘Jumphost’ as the jump host:
$ ssh -tt Jumphost ssh -tt FooServer
$ ssh -tt [email protected] ssh -tt [email protected]
$ ssh -tt [email protected] ssh -tt [email protected] command1 arg1 arg2
$ ssh -tt [email protected] ssh -tt [email protected] htop
$ ssh -tt [email protected] ssh -tt [email protected] screen -dR

Where,

  • The -t option passed to the ssh command force pseudo-tty allocation. This can be used to execute arbitrary screen-based programs on a remote machine. Multiple -tt options force tty allocation, even if ssh has no local tty.

Say hello to the ProxyCommand

The syntax is:
$ ssh -o ProxyCommand='ssh firewall nc remote_server1 22' remote_server1
$ ssh -o ProxyCommand='ssh [email protected] nc FooServer 22' [email protected]
##########################################
## -t option is needed to run commands ###
##########################################
$ ssh -t -o ProxyCommand='ssh [email protected] nc FooServer 22' [email protected] htop

The netcat (nc) command is needed to set and establish a TCP pipe between Jumphost (or firewall) and FooServer. Now, my laptop (local system) is connected to Jumphost it now connected FooServer. In this example, the utility netcat (nc) is for reading and writing network connections directly. It can be used to pass connections to a 2nd server such as FooServer.

Update ~/.ssh/config file

Edit the $HOME/.ssh/config file using a text editor such as vi, enter:
$ vi ~/.ssh/config
Append the following configuration:

Host fooserver
HostName FooServer
User vivek
ProxyCommand ssh [email protected] nc %h %p

Save and close the file. Where,

  1. Host fooserver : Set nickname of your choice.
  2. HostName FooServer : Set the real remote server/host name.
  3. User vivek : Set the real user name for remote server/host.
  4. ProxyCommand ssh [email protected] nc %h %p : Specifies the command to use to connect to the server. In this example, I’m using nc command. Any occurrence of %h will be substituted by the host name to connect, %p by the port, and %r by the remote user name.

To test enter:
$ ssh fooserver
To see the details, pass the -v option to the ssh command. Here is another snippet:

Host server1
HostName v.server1
User root
Port 22
ProxyCommand ssh [email protected] nc %h %p %r

Now, run:
$ ssh -v server1
Sample outputs:

OpenSSH_6.2p2, OSSLShim 0.9.8r 8 Dec 2011
debug1: Reading configuration data /Users/veryv/.ssh/config
debug1: /Users/veryv/.ssh/config line 1: Applying options for server1
debug1: Reading configuration data /etc/ssh_config
debug1: /etc/ssh_config line 20: Applying options for *
debug1: /etc/ssh_config line 102: Applying options for *
debug1: Executing proxy command: exec ssh [email protected] nc v.server1 22 root
debug1: permanently_drop_suid: 501
debug1: identity file /Users/veryv/.ssh/id_rsa type 1
debug1: identity file /Users/veryv/.ssh/id_rsa-cert type -1
debug1: identity file /Users/veryv/.ssh/id_dsa type -1
debug1: identity file /Users/veryv/.ssh/id_dsa-cert type -1
debug1: Enabling compatibility mode for protocol 2.0
debug1: Local version string SSH-2.0-OpenSSH_6.2
debug1: Remote protocol version 2.0, remote software version OpenSSH_6.6.1p1 Ubuntu-2ubuntu2
debug1: match: OpenSSH_6.6.1p1 Ubuntu-2ubuntu2 pat OpenSSH*
debug1: SSH2_MSG_KEXINIT sent
debug1: SSH2_MSG_KEXINIT received
debug1: kex: server->client aes128-ctr [email protected] none
debug1: kex: client->server aes128-ctr [email protected] none
debug1: SSH2_MSG_KEX_DH_GEX_REQUEST(1024<1024<8192) sent
debug1: expecting SSH2_MSG_KEX_DH_GEX_GROUP
debug1: SSH2_MSG_KEX_DH_GEX_INIT sent
debug1: expecting SSH2_MSG_KEX_DH_GEX_REPLY
debug1: Server host key: RSA d2:07:84:79:21:a7:84:84:14:ef:f1:7a:84:a5:a1:7s
debug1: Host 'v.server1' is known and matches the RSA host key.
debug1: Found key in /Users/veryv/.ssh/known_hosts:37
debug1: ssh_rsa_verify: signature correct
debug1: SSH2_MSG_NEWKEYS sent
debug1: expecting SSH2_MSG_NEWKEYS
debug1: SSH2_MSG_NEWKEYS received
debug1: Roaming not allowed by server
debug1: SSH2_MSG_SERVICE_REQUEST sent
debug1: SSH2_MSG_SERVICE_ACCEPT received
debug1: Authentications that can continue: publickey,password,keyboard-interactive
debug1: Next authentication method: publickey
debug1: Offering RSA public key: /Users/veryv/.ssh/id_rsa
debug1: Server accepts key: pkalg ssh-rsa blen 279
debug1: Authentication succeeded (publickey).
Authenticated to v.server1 (via proxy).
debug1: channel 0: new [client-session]
debug1: Requesting [email protected]
debug1: Entering interactive session.
debug1: Sending environment.
Welcome to Ubuntu 14.04.2 LTS (GNU/Linux 3.13.0-52-generic x86_64)

 * Documentation:  https://help.ubuntu.com/
Last login: Sun May 17 15:41:26 2015 from 10.70.203.66

The sftp syntax

The syntax is as follows:

sftp -o 'ProxyCommand=ssh %h nc firewall.nixcraft.net.in 22' \
       -o 'HostKeyAlias=firewall.nixcraft.net.in' \
       vivek@server1.nixcraft.net.in

See man pages for more info: ssh(1), ssh_config(5), nc(1)

10 comment

  1. Good article Vivek.

    If you liked this and want to see how to take it to the next level, take a look at my Empowering OpenSSH article

    I use this technique to reach hundreds of servers 2 or 3 or 4 jumps deep in my clients network. It works like a champ.

  2. Why are you using fooserver in your example?? Is that server 1, server 2, a server at a Chinese food restaurant?? Just use common sense terms (server 1, server 2, local computer, etc.) so that people can understand what you are trying to say.

    Please stop the Foo pollution!! Dumbest thing Linux people do.

    1. Actually the fooserver is great. Instantly you can look at it and know it is totally a placeholder and needs to be replaced by a different server. server1 or server2 can be a lot more misleading. Is that the server you were talking about in the previous paragraph? Some global scope, some listed arguments to a function, etc. Foo, bar, baz, etc are standard practice for exactly this reason:
      http://www.quora.com/What-does-foo-mean-in-computer-science
      http://english.stackexchange.com/questions/27843/what-does-foo-mean
      http://en.wikipedia.org/wiki/Metasyntactic_variable

  3. I agree with the use of FOO(server).. At first I wasn’t sure and had to look twice. Caught on finaly (my bad?). SERVER1, SERVER2 etc would allow more folk to understand IMHO (My Humble Opinion:). Thanks for the info however..

  4. To all the people complaining about the use of Foo: Please go read up on unix hacker culture. There is a rich history behind the chosen meta-syntactic variables in this example. The way I see if, if you are trying to learn SSH before FOO you need to take a step back and read a few dusty books. Show some respect for those who came before you (and provided all this great free software).

    1. I don’t know about what hacker culture you’re talking about. And not sure if I really care about it. I am an exclusive UNIX user/sysop/dev since 1994 and always find foo bar bah behehe annoying and more like today’s pseudo-language of lol, rofl, etc, that only shows lack of imagination and shallowness, the same over-confident mindset that gave birth to RTFM partly from inability to verbalize one’s supposed knowledge, partly, and mostly, from the lack of it.
      I bet you’re a perl guy, just because its gibberish syntax makes you feel special.

  5. Great article however you should be using ‘ssh -W’ instead of ‘ssh … nc’ for the “hop” command.

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