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118 lines
5.9 KiB
Markdown
118 lines
5.9 KiB
Markdown
# knownhosts: enhanced Golang SSH known_hosts management
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[](https://github.com/skeema/knownhosts/actions)
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[](https://pkg.go.dev/github.com/skeema/knownhosts)
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> This repo is brought to you by [Skeema](https://github.com/skeema/skeema), a
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> declarative pure-SQL schema management system for MySQL and MariaDB. Our
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> premium products include extensive [SSH tunnel](https://www.skeema.io/docs/options/#ssh)
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> functionality, which internally makes use of this package.
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Go provides excellent functionality for OpenSSH known_hosts files in its
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external package [golang.org/x/crypto/ssh/knownhosts](https://pkg.go.dev/golang.org/x/crypto/ssh/knownhosts).
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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.
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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 the following functionality:
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* Look up known_hosts public keys for any given host
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* Auto-populate ssh.ClientConfig.HostKeyAlgorithms easily based on known_hosts, providing a solution for [golang/go#29286](https://github.com/golang/go/issues/29286)
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* Write new known_hosts entries to an io.Writer
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* Properly format/normalize new known_hosts entries containing ipv6 addresses, providing a solution for [golang/go#53463](https://github.com/golang/go/issues/53463)
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* 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
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## How host key lookup works
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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.
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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).
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## Populating ssh.ClientConfig.HostKeyAlgorithms based on known_hosts
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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
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[golang.org/x/crypto/ssh/knownhosts](https://pkg.go.dev/golang.org/x/crypto/ssh/knownhosts)
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does not provide an obvious way to do so.
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This package uses its host key lookup trick in order to make ssh.ClientConfig.HostKeyAlgorithms easy to populate:
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```golang
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import (
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"golang.org/x/crypto/ssh"
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"github.com/skeema/knownhosts"
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)
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func sshConfigForHost(hostWithPort string) (*ssh.ClientConfig, error) {
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kh, err := knownhosts.New("/home/myuser/.ssh/known_hosts")
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if err != nil {
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return nil, err
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}
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config := &ssh.ClientConfig{
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User: "myuser",
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Auth: []ssh.AuthMethod{ /* ... */ },
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HostKeyCallback: kh.HostKeyCallback(), // or, equivalently, use ssh.HostKeyCallback(kh)
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HostKeyAlgorithms: kh.HostKeyAlgorithms(hostWithPort),
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}
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return config, nil
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}
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```
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## Writing new known_hosts entries
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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:
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```golang
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sshHost := "yourserver.com:22"
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khPath := "/home/myuser/.ssh/known_hosts"
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kh, err := knownhosts.New(khPath)
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if err != nil {
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log.Fatal("Failed to read known_hosts: ", err)
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}
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// Create a custom permissive hostkey callback which still errors on hosts
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// with changed keys, but allows unknown hosts and adds them to known_hosts
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cb := ssh.HostKeyCallback(func(hostname string, remote net.Addr, key ssh.PublicKey) error {
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err := kh(hostname, remote, key)
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if knownhosts.IsHostKeyChanged(err) {
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return fmt.Errorf("REMOTE HOST IDENTIFICATION HAS CHANGED for host %s! This may indicate a MitM attack.", hostname)
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} else if knownhosts.IsHostUnknown(err) {
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f, ferr := os.OpenFile(khPath, os.O_APPEND|os.O_WRONLY, 0600)
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if ferr == nil {
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defer f.Close()
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ferr = knownhosts.WriteKnownHost(f, hostname, remote, key)
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}
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if ferr == nil {
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log.Printf("Added host %s to known_hosts\n", hostname)
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} else {
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log.Printf("Failed to add host %s to known_hosts: %v\n", hostname, ferr)
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}
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return nil // permit previously-unknown hosts (warning: may be insecure)
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}
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return err
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})
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config := &ssh.ClientConfig{
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User: "myuser",
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Auth: []ssh.AuthMethod{ /* ... */ },
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HostKeyCallback: cb,
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HostKeyAlgorithms: kh.HostKeyAlgorithms(sshHost),
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}
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```
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## License
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**Source code copyright 2023 Skeema LLC and the Skeema Knownhosts authors**
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```text
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Licensed under the Apache License, Version 2.0 (the "License");
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you may not use this file except in compliance with the License.
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You may obtain a copy of the License at
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http://www.apache.org/licenses/LICENSE-2.0
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Unless required by applicable law or agreed to in writing, software
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distributed under the License is distributed on an "AS IS" BASIS,
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WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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See the License for the specific language governing permissions and
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limitations under the License.
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```
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