SSH config file

The ssh config file is typically in your home directory: ~/.ssh/config

Adding options to this file can simplify your ssh usage, so you don’t have to remember which username and public key go with each computer.

Here’s an example ~/.ssh/config file, with an explanation of each line below.

#
# ssh config file
#
Host *
  ForwardAgent yes
  ForwardX11 yes
  ServerAliveInterval 30
  IdentityFile ~/.ssh/id_ed25519
#
# 
Host sccs
    User csmajor1
    Hostname sccs.swarthmore.edu

Host syrup
    User donaldduck
    Hostname syrup.cs.swarthmore.edu
    Port 2222
    ForwardX11 no

Host rad
    Hostname horseradish.cs.swarthmore.edu

The first “stanza” (the Host * line plus the 4 indented lines) applies to all hosts, unless an option is specified again later for a specific host. These initial “catch all” options include:

ForwardAgent – whether to forward the ssh-agent connection to the remote computer (i.e., if I am using ssh-agent, and I ssh to a remote computer (without having to type my passphrase), I can keep going and ssh from there to another remote computer and still not have to type my passphrase)
ForwardX11 – whether to forward the X11 connection to the remote computer (i.e., if I ssh to a remote computer and run a program that pops up a graphical window, should that window show up on my local computer)
ServerAliveInterval – how many seconds to wait for a response from the server (useful if your idle ssh sessions are being dropped or terminated)
IdentityFile – which ssh public key to use (you can have more than one ssh key)

The next few stanzas allow me to specify:

which username to log in as (User)
a short nickname for the longer hostname (Host and Hostname)
which options to negate for this specific host (ForwardX11 no for syrup)
which port to use (default is port 22, but some computers run ssh on other ports)

Using the above I can type ssh sccs instead of ssh csmajor1@sccs.swarthmore.edu, ssh syrup instead of ssh -p 2222 -Y donaldduck@syrup.cs.swarthmore.edu, and ssh rad instead of ssh horseradish.cs.swarthmore.edu.