The SSH config file (~/.ssh/config): a practical guide

What the SSH config file is

The SSH config file at ~/.ssh/config lets you save per-host settings — name, user, port, key, and dozens of options — so instead of typing ssh -i ~/.ssh/work_key -p 2222 [email protected] you just type ssh myserver. It's the single biggest quality-of-life upgrade for anyone using SSH from a terminal. This guide shows the format, the options you'll actually use, and how the same idea works on mobile.

The basic format

Create ~/.ssh/config (permissions 600) and add a block per host:

Host myserver
    HostName 203.0.113.7
    User deploy
    Port 2222
    IdentityFile ~/.ssh/work_key

Now ssh myserver expands to the full command. Host is the alias you type; HostName is the real address. That's 90% of the value right there.

The options you'll actually use

Wildcards and defaults

Host accepts patterns, and settings apply top-down, so you can set defaults for everything and override per host:

Host *
    ServerAliveInterval 60
    AddKeysToAgent yes

Host *.internal.example.com
    User admin
    ProxyJump bastion

Host bastion
    HostName 198.51.100.9
    User jump

The Host * block applies sensible defaults everywhere; more specific blocks add to them. Put specific hosts above broad wildcards — the first match for each option wins.

Jump hosts in one line

To reach a private server that's only accessible through a bastion, ProxyJump chains the hops automatically:

Host db
    HostName 10.0.0.5
    User postgres
    ProxyJump bastion

Then ssh db transparently routes through bastion. (A mesh VPN like Tailscale is an alternative that removes the need for a bastion entirely.)

The mobile equivalent

Mobile SSH clients don't use a ~/.ssh/config file — they store the same information per connection in the app: each saved host has its own user, port, key, and keep-alive settings, edited in a form instead of a text file. The benefit is the same (type once, reuse forever); the mechanism is a UI rather than a config file. In TermAI each connection carries its own auth and options, so the equivalent of IdentitiesOnly or a custom port is just a field on that connection.

FAQ

Where is the SSH config file?
At ~/.ssh/config on Linux and macOS (create it if missing, permissions 600). On Windows it's C:\Users\You\.ssh\config.

How do I use a specific key for one host?
Add IdentityFile ~/.ssh/that_key and IdentitiesOnly yes under that host's block — the second line stops the client offering other keys.

What's the difference between Host and HostName?
Host is the alias you type (ssh myserver); HostName is the real address it resolves to.

Do mobile SSH apps use ~/.ssh/config?
No — they store the same settings per connection in the app's UI. The convenience is identical; it's a form instead of a file.

Quick Facts

• Location: ~/.ssh/config (permissions 600); Windows C:\Users\You\.ssh\config
• Core: Host alias → HostName, User, Port, IdentityFile
• Most useful options: IdentitiesOnly yes, ServerAliveInterval 60, ProxyJump
• Patterns: Host * for defaults; first match wins, so specific blocks go above wildcards
• On mobile: the app stores the same settings per connection — a form, not a file