SSH Configuration

SSH configuration files control connection defaults and options. The main file is ~/.ssh/config, which lets you define host aliases, authentication options, and forwarding rules without typing them on every command.

Basic syntax:

Host work
    HostName work.example.com
    User alice
    Port 2222
    IdentityFile ~/.ssh/id_ed25519

Host *
    AddKeysToAgent yes
    StrictHostKeyChecking accept-new

Then connect with ssh work instead of ssh -p 2222 [email protected].

Host patterns:

• Host work — matches exactly “work”
• Host *.example.com — matches all subdomains
• Host * — matches all hosts (set defaults here)

Common options:

HostName              actual hostname or IP
User                  username on remote
Port                  SSH port (default 22)
IdentityFile          path to private key
ProxyJump             jump host (see ssh-jump-hosts)
AddKeysToAgent        add keys to SSH agent (yes/no)
StrictHostKeyChecking ask/accept-new/no (host key verification)
ServerAliveInterval   idle timeout in seconds
Compression           enable compression (yes/no)
ForwardAgent          forward SSH agent to remote (yes/no)
LocalForward          local port forwarding
RemoteForward         remote port forwarding

SSH reads config top-to-bottom and uses the first matching value. More specific hosts should appear before Host *.

View effective config for a host: ssh -G work (shows merged settings).

Check syntax: ssh -G hostname (errors show invalid config).