tailcfg, ipn/ipnlocal: add Hostinfo.SSH_HostKeys, send when SSH enabled

(The name SSH_HostKeys is bad but SSHHostKeys is worse.)

Updates #3802

Change-Id: I2a889019c9e8b065b668dd58140db4fcab868a91
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

ipn/ipnlocal/ssh.go

@@ -11,6 +11,7 @@ import (
 	"errors"
 	"io/ioutil"
 	"os"
+	"strings"
 
 	"golang.org/x/crypto/ssh"
 	"tailscale.com/envknob"
@@ -18,15 +19,16 @@ import (
 
 var useHostKeys = envknob.Bool("TS_USE_SYSTEM_SSH_HOST_KEYS")
 
-func (b *LocalBackend) GetSSHHostKeys() ([]ssh.Signer, error) {
+func (b *LocalBackend) GetSSH_HostKeys() ([]ssh.Signer, error) {
 	// TODO(bradfitz): generate host keys, at least as needed if
 	// an existing SSH server didn't put them on disk. But also
 	// because people may want tailscale-specific ones. For now be
 	// lazy and reuse the host ones.
-	return b.getSystemSSHHostKeys()
+	return b.getSystemSSH_HostKeys()
 }
 
-func (b *LocalBackend) getSystemSSHHostKeys() (ret []ssh.Signer, err error) {
+func (b *LocalBackend) getSystemSSH_HostKeys() (ret []ssh.Signer, err error) {
+	// TODO(bradfitz): cache this?
 	for _, typ := range []string{"rsa", "ecdsa", "ed25519"} {
 		hostKey, err := ioutil.ReadFile("/etc/ssh/ssh_host_" + typ + "_key")
 		if os.IsNotExist(err) {
@@ -46,3 +48,11 @@ func (b *LocalBackend) getSystemSSHHostKeys() (ret []ssh.Signer, err error) {
 	}
 	return ret, nil
 }
+
+func (b *LocalBackend) getSSHHostKeyPublicStrings() (ret []string) {
+	signers, _ := b.GetSSH_HostKeys()
+	for _, signer := range signers {
+		ret = append(ret, strings.TrimSpace(string(ssh.MarshalAuthorizedKey(signer.PublicKey()))))
+	}
+	return ret
+}