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

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

last updated in Categories , ,

 

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. [donotprint][/donotprint] 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 vivek@Jumphost
Next, I must ssh through the intermediary system as follows:
$ ssh vivek@FooServer

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 vivek@Jumphost ssh -tt vivek@FooServer
$ ssh -tt vivek@Jumphost ssh -tt vivek@FooServer command1 arg1 arg2
$ ssh -tt vivek@Jumphost ssh -tt vivek@FooServer htop
$ ssh -tt vivek@Jumphost ssh -tt vivek@FooServer 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 vivek@Jumphost nc FooServer 22' vivek@FooServer
##########################################
## -t option is needed to run commands ###
##########################################
$ ssh -t -o ProxyCommand='ssh vivek@Jumphost nc FooServer 22' vivek@FooServer 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 vivek@Jumphost 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 vivek@Jumphost 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 root@v.backup2 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 root@v.backup2 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 hmac-md5-etm@openssh.com none
debug1: kex: client->server aes128-ctr hmac-md5-etm@openssh.com 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 no-more-sessions@openssh.com
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: nc(1)

0 (0)
Article Rating (No Votes)
Rate this article
Attachments
There are no attachments for this article.
Comments
There are no comments for this article. Be the first to post a comment.
Full Name
Email Address
Security Code Security Code
Related Articles RSS Feed
10 Linux nslookup Command Examples for DNS Lookup
Viewed 9147 times since Sun, Sep 30, 2018
Linux Linux Network Statistics Tools / Commands
Viewed 7583 times since Mon, Sep 21, 2020
How To Use the Linux Auditing System on CentOS 7
Viewed 2255 times since Fri, Apr 5, 2019
OEL 7 – How to disable IPv6 on Oracle Linux 7 – Follow Up
Viewed 8024 times since Wed, Jul 25, 2018
How To Find Largest Top 10 Files and Directories On Linux / UNIX / BSD find
Viewed 1313 times since Mon, Oct 29, 2018
How to Synchronize Directories Using Lsyncd in Linux
Viewed 10351 times since Wed, Oct 31, 2018
Migrate a Linux System from Red Hat Enterprise to CentOS
Viewed 8990 times since Fri, May 15, 2020
Linux Proxy Server Settings – Set Proxy For Command Line
Viewed 1432 times since Mon, Feb 18, 2019
Telnet – Send GET/HEAD HTTP Request
Viewed 1717 times since Mon, Feb 18, 2019
12 Tcpdump Commands – A Network Sniffer Tool
Viewed 7652 times since Fri, Jul 27, 2018