SSH jump host
SSH jump hosts are employed as an alternative to SSH tunneling to access internal machines through a gateway.
The idea is to use ProxyCommand to automatically execute the ssh command on remote host to jump to the next host and forward all traffic through.
Prerequisites
- SSH access to the gateway machine and the internal one.
- Gateway machine has Netcat installed.
Configuration
Single jump
ProxyJump hosts can be defined inside each user's SSH config file.
~/.ssh/configProxyJump Example### First jump host. Directly reachable
Host betajump
HostName jumphost1.example.org
### Host to jump to via jumphost1.example.org
Host behindbeta
HostName behindbeta.example.org
ProxyJump betajump
See the corresponding single jump section under Usage below.
Multiple jump
The same syntax can be used to make jumps over multiple machines:
~/.ssh/configAdd this text### First jump host. Directly reachable
Host alphajump
HostName jumphost1.example.org
### Second jumphost. Only reachable via jumphost1.example.org
Host betajump
HostName jumphost2.example.org
ProxyJump alphajump
### Host only reachable via alphajump and betajump
Host behindalphabeta
HostName behindalphabeta.example.org
ProxyJump betajump
See the corresponding multiple jump section under Usage below.
user $ssh behindalphabetaStatic jump host list
Static jump host list means, that the jump host(s) are known and can be defined before initiating the first ssh connection. Therefore a static jump host 'routing' can be defined in the user's ~/.ssh/config file. The advantage in comparison to the dynamic jump host option is, that you don't have to provide the .ssh config on jump hosts between your machine and all the other jump hosts between you and the final host you want to jump to.
Usage
Single jump
user $ssh behindalphaIf usernames on machines differ, specify them by modifying the correspondent ProxyJump line:
~/.ssh/configModify correspondent ProxyCommandProxyJump otheruser@behindalpha
It works with the scp command, too:
user $scp filename behindalphabeta:~/The colon and path at the end is needed so that scp recognizes it as remote.
Multiple jump
The same syntax can be used to make jumps over multiple machines:
user $ssh -J user1@host1:port1,user2@host2:port2 user3@host3Dynamic jump host list
The -J option can be used to jump through a host:
user $ssh -J host1 host2If usernames or ports on machines differ, specify them:
user $ssh -J user1@host1:port1 user2@host2 -p port2Tips
To ease the connecting even further:
- Set these commands as shell aliases
- To avoid typing passwords use OpenSSH keys
See also
- SSH — the ubiquitous tool for logging into and working on remote machines securely.