ssh_config/README.md

ssh_config

This is a Go parser for ssh_config files. Importantly, this parser attempts to preserve comments in a given file, so you can manipulate a ssh_config file from a program, if your heart desires.

It's designed to be used with the excellent x/crypto/ssh package, which handles SSH negotiation but isn't very easy to configure.

The ssh_config Get() and GetStrict() functions will attempt to read values from $HOME/.ssh/config and fall back to /etc/ssh/ssh_config. The first argument is the host name to match on, and the second argument is the key you want to retrieve.

port := ssh_config.Get("myhost", "Port")

Certain directives can occur multiple times for a host (such as IdentityFile), so you should use the GetAll or GetAllStrict directive to retrieve those instead.

files := ssh_config.GetAll("myhost", "IdentityFile")

You can also load a config file and read values from it.

var config = `
Host *.test
  Compression yes
`

cfg, err := ssh_config.Decode(strings.NewReader(config))
fmt.Println(cfg.Get("example.test", "Port"))

Some SSH arguments have default values - for example, the default value for KeyboardAuthentication is "yes". If you call Get(), and no value for the given Host/keyword pair exists in the config, we'll return a default for the keyword if one exists.

Reloading SSH config files

Once the first call to Get(), GetStrict(), GetAll(), or GetAllStrict() has been made, the contents of the config files will be cached for all future calls to any of those functions. The ReloadConfigs() function will reset this cache and replace it with the current config file contents.

ssh_config.ReloadConfigs()

Manipulating SSH config files

Here's how you can manipulate an SSH config file, and then write it back to disk.

f, _ := os.Open(filepath.Join(os.Getenv("HOME"), ".ssh", "config"))
cfg, _ := ssh_config.Decode(f)
for _, host := range cfg.Hosts {
    fmt.Println("patterns:", host.Patterns)
    for _, node := range host.Nodes {
        // Manipulate the nodes as you see fit, or use a type switch to
        // distinguish between Empty, KV, and Include nodes.
        fmt.Println(node.String())
    }
}

// Print the config to stdout:
fmt.Println(cfg.String())

Spec compliance

Wherever possible we try to implement the specification as documented in the ssh_config manpage. Unimplemented features should be present in the issues list.

Notably, the Match directive is currently unsupported.

Errata

This is the second comment-preserving configuration parser I've written, after an /etc/hosts parser. Eventually, I will write one for every Linux file format.

Sponsorships

Thank you very much to Tailscale and Indeed for sponsoring development of this library. Sponsors will get their names featured in the README.

You can also reach out about a consulting engagement: https://burke.services