SSH Handshake complains about missing host key - go

Question

I'm trying to connect to a remote host and check if a file exist
At this stage I'm trying just to connect but I'm getting an error:
2017/08/01 18:16:39 unable to connect: ssh: handshake failed: ssh: required host key was nil
I've tried to find out if others had issues as mine but I just couldn't find.
I understand that I need to check the knowns_hosts somehow in the process but I just can't figure out how...
var hostKey ssh.PublicKey
// A public key may be used to authenticate against the remote
// server by using an unencrypted PEM-encoded private key file.
//
// If you have an encrypted private key, the crypto/x509 package
// can be used to decrypt it.
key, err := ioutil.ReadFile("/home/user/.ssh/id_rsa")
if err != nil {
log.Fatalf("unable to read private key: %v", err)
}
// Create the Signer for this private key.
signer, err := ssh.ParsePrivateKey(key)
if err != nil {
log.Fatalf("unable to parse private key: %v", err)
}
config := &ssh.ClientConfig{
User: "user",
Auth: []ssh.AuthMethod{
// Use the PublicKeys method for remote authentication.
ssh.PublicKeys(signer),
},
HostKeyCallback: ssh.FixedHostKey(hostKey),
}
// Connect to the remote server and perform the SSH handshake.
client, err := ssh.Dial("tcp", "host.com:22", config)
if err != nil {
log.Fatalf("unable to connect: %v", err)
}
defer client.Close()
}

Answer 1

I would suggest to use knownhosts subpackage
import knownhosts "golang.org/x/crypto/ssh/knownhosts"
...
hostKeyCallback, err := knownhosts.New("/Users/user/.ssh/known_hosts")
if err != nil {
log.Fatal(err)
}
...
config := &ssh.ClientConfig{
User: "user",
Auth: []ssh.AuthMethod{
// Use the PublicKeys method for remote authentication.
ssh.PublicKeys(signer),
},
HostKeyCallback: hostKeyCallback,
}
So that you avoid parsing known_hosts yourself...
hth,

Answer 2

Here what you are looking for:
func getHostKey(host string) (ssh.PublicKey, error) {
file, err := os.Open(filepath.Join(os.Getenv("HOME"), ".ssh", "known_hosts"))
if err != nil {
return nil, err
}
defer file.Close()
scanner := bufio.NewScanner(file)
var hostKey ssh.PublicKey
for scanner.Scan() {
fields := strings.Split(scanner.Text(), " ")
if len(fields) != 3 {
continue
}
if strings.Contains(fields[0], host) {
var err error
hostKey, _, _, _, err = ssh.ParseAuthorizedKey(scanner.Bytes())
if err != nil {
return nil, errors.New(fmt.Sprintf("error parsing %q: %v", fields[2], err))
}
break
}
}
if hostKey == nil {
return nil, errors.New(fmt.Sprintf("no hostkey for %s", host))
}
return hostKey, nil
}
Then replace your hostKey definition line with
hostKey, err := getHostKey("host.com")
if err != nil {
log.Fatal(err)
}
For more information on the subject:
official sample where I took parts of the code from
why a hostKey is necessary now
EDIT:
Also check out Anton's answer below about the golang.org/x/crypto/ssh/knownhosts package.