SSH ProxyCommand example: Going through one host to reach another server
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.
+-------+ +----------+ +-----------+ | Laptop| <---> | Jumphost | <--> | FooServer | +-------+ +----------+ +-----------+ OR +-------+ +----------+ +-----------+ | Laptop| <---> | Firewall | <--> | FooServer | +-------+ +----------+ +-----------+ 192.168.1.5 126.96.36.199 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
- 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,
- Host fooserver : Set nickname of your choice.
- HostName FooServer : Set the real remote server/host name.
- User vivek : Set the real user name for remote server/host.
- 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 email@example.com nc %h %p %r
$ ssh -v server1
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 firstname.lastname@example.org 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@example.com none debug1: kex: client->server aes128-ctr firstname.lastname@example.org 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@example.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' \ firstname.lastname@example.org
See man pages for more info: nc(1)