SSH cheatsheet

The useful SSH things I forget.

Connection

Start remote shell on host as user, assumes port 22, will prompt for password if required:

ssh user@host

Supply non-standard port:

ssh user@host -p port

Configure connection settings per host in ~/.ssh/config:

Host hostname1
    HostName 192.168.1.1
    User    other
    Port    1234

Host hostname2
    PubkeyAuthentication yes
    IdentityFile ~/.ssh/id_other

Then connect using stored settings:

ssh hostname1
ssh hostname2

Keys

Assuming you have generated a public-private key-pair locally, id_rsa and id_rsa.pub respectively, add your public key to the remote then connect using:

cat id_rsa.pub | ssh user@host 'cat >> .ssh/authorized_keys'
ssh user@host

Find the fingerprint of a public key using:

ssh-keygen -l -f id_rsa.pub

Commands

Run a command on the remote host (and get the stdin and stderr locally):

ssh user@host "ls"

Multiple configurations for a host

Configure fake host names in ~/.ssh/config to allow multiple credentials for same host:

Host fake1.host
    HostName host
    User     user1

Host fake2.host
    HostName host
    User     user2

Then connect using stored settings:

ssh fake1.host
ssh fake2.host

Use case: Github

Normally, if you want to interact with a remote git repo you might register your own key-pair then connect using something like git@github.com:user/repo1.git. However, if you need to automate access to different Github repositories you should use different key-pairs for each repo but since you're not specifying the key-pair on the git url, how can you do that? You can configure different key-pairs against fake hostnames for each repo.

Host repo1.github.com
    HostName github.com
    User git
    IdentityFile ~/.ssh/repo1_rsa

Host repo2.github.com
    HostName github.com
    User git
    IdentityFile ~/.ssh/repo2_rsa

Then we can interact with the remotes via those fake host entries, repo1.github.com:user/repo1.git and repo2.github.com:user/repo1.git, and the SSH config will ensure that we connect using the appropriate key-pair.