# knownhosts: enhanced Golang SSH known_hosts management

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> This repo is brought to you by [Skeema](https://github.com/skeema/skeema), a
> declarative pure-SQL schema management system for MySQL and MariaDB. Our
> premium products include extensive [SSH tunnel](https://www.skeema.io/docs/options/#ssh)
> functionality, which internally makes use of this package.

Go provides excellent functionality for OpenSSH known_hosts files in its
external package [golang.org/x/crypto/ssh/knownhosts](https://pkg.go.dev/golang.org/x/crypto/ssh/knownhosts). 
However, that package is somewhat low-level, making it difficult to implement full known_hosts management similar to command-line `ssh`'s behavior for `StrictHostKeyChecking=no` configuration.

This repo ([github.com/skeema/knownhosts](https://github.com/skeema/knownhosts)) is a thin wrapper package around [golang.org/x/crypto/ssh/knownhosts](https://pkg.go.dev/golang.org/x/crypto/ssh/knownhosts), adding functions which provide the following functionality:

* Look up known_hosts public keys for any given host
* Auto-populate ssh.ClientConfig.HostKeyAlgorithms easily based on known_hosts
* Write new known_hosts entries to an io.Writer
* Determine if an ssh.HostKeyCallback's error corresponds to a host whose key has changed (indicating potential MitM attack) vs a host that just isn't known yet

## How host key lookup works

Although [golang.org/x/crypto/ssh/knownhosts](https://pkg.go.dev/golang.org/x/crypto/ssh/knownhosts) doesn't directly expose a way to query its known_host map, we use a subtle trick to do so: invoke the HostKeyCallback with a valid host but a bogus key. The resulting KeyError allows us to determine which public keys are actually present for that host.

By using this technique, [github.com/skeema/knownhosts](https://github.com/skeema/knownhosts) doesn't need to duplicate or re-implement any of the actual known_hosts management from [golang.org/x/crypto/ssh/knownhosts](https://pkg.go.dev/golang.org/x/crypto/ssh/knownhosts).

## Populating ssh.ClientConfig.HostKeyAlgorithms based on known_hosts

Hosts often have multiple public keys, each of a different type (algorithm). This can be [problematic](https://github.com/golang/go/issues/29286) in [golang.org/x/crypto/ssh/knownhosts](https://pkg.go.dev/golang.org/x/crypto/ssh/knownhosts): if a host's first public key is *not* in known_hosts, but a key of a different type *is*, the HostKeyCallback returns an error. The solution is to populate `ssh.ClientConfig.HostKeyAlgorithms` based on the algorithms of the known_hosts entries for that host, but 
[golang.org/x/crypto/ssh/knownhosts](https://pkg.go.dev/golang.org/x/crypto/ssh/knownhosts)
does not provide an obvious way to do so.

This package uses its host key lookup trick in order to make ssh.ClientConfig.HostKeyAlgorithms easy to populate:

```golang
import (
	"golang.org/x/crypto/ssh"
	"github.com/skeema/knownhosts"
)

func sshConfigForHost(hostWithPort string) (*ssh.ClientConfig, error) {
	kh, err := knownhosts.New("/home/myuser/.ssh/known_hosts")
	if err != nil {
		return nil, err
	}
	config := &ssh.ClientConfig{
		User:              "myuser",
		Auth:              []ssh.AuthMethod{ /* ... */ },
		HostKeyCallback:   kh.HostKeyCallback(), // or, equivalently, use ssh.HostKeyCallback(kh)
		HostKeyAlgorithms: kh.HostKeyAlgorithms(hostWithPort),
	}
	return config, nil
}
```

## Writing new known_hosts entries

If you wish to mimic the behavior of OpenSSH's `StrictHostKeyChecking=no` or `StrictHostKeyChecking=ask`, this package provides a few functions to simplify this task. For example:

```golang
sshHost := "yourserver.com:22"
khPath := "/home/myuser/.ssh/known_hosts"
kh, err := knownhosts.New(khPath)
if err != nil {
	log.Fatal("Failed to read known_hosts: ", err)
}

// Create a custom permissive hostkey callback which still errors on hosts
// with changed keys, but allows unknown hosts and adds them to known_hosts
cb := ssh.HostKeyCallback(func(hostname string, remote net.Addr, key ssh.PublicKey) error {
	err := kh(hostname, remote, key)
	if knownhosts.IsHostKeyChanged(err) {
		return fmt.Errorf("REMOTE HOST IDENTIFICATION HAS CHANGED for host %s! This may indicate a MitM attack.", hostname)
	} else if knownhosts.IsHostUnknown(err) {
		f, ferr := os.OpenFile(khPath, os.O_APPEND|os.O_WRONLY, 0600)
		if ferr == nil {
			defer f.Close()
			ferr = knownhosts.WriteKnownHost(f, hostname, remote, key)
		}
		if ferr == nil {
			log.Printf("Added host %s to known_hosts\n", hostname)
		} else {
			log.Printf("Failed to add host %s to known_hosts: %v\n", hostname, ferr)
		}
		return nil // permit previously-unknown hosts (warning: may be insecure)
	}
	return err
})

config := &ssh.ClientConfig{
	User:              "myuser",
	Auth:              []ssh.AuthMethod{ /* ... */ },
	HostKeyCallback:   cb,
	HostKeyAlgorithms: kh.HostKeyAlgorithms(sshHost),
}
```

