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/local.go

@@ -894,7 +894,7 @@ func (b *LocalBackend) Start(opts ipn.Options) error {
 	if b.inServerMode || runtime.GOOS == "windows" {
 		b.logf("Start: serverMode=%v", b.inServerMode)
 	}
-	applyPrefsToHostinfo(hostinfo, b.prefs)
+	b.applyPrefsToHostinfo(hostinfo, b.prefs)
 
 	b.setNetMapLocked(nil)
 	persistv := b.prefs.Persist
@@ -1739,7 +1739,7 @@ func (b *LocalBackend) setPrefsLockedOnEntry(caller string, newp *ipn.Prefs) {
 
 	oldHi := b.hostinfo
 	newHi := oldHi.Clone()
-	applyPrefsToHostinfo(newHi, newp)
+	b.applyPrefsToHostinfo(newHi, newp)
 	b.hostinfo = newHi
 	hostInfoChanged := !oldHi.Equal(newHi)
 	userID := b.userID
@@ -2444,13 +2444,23 @@ func unmapIPPrefixes(ippsList ...[]netaddr.IPPrefix) (ret []netaddr.IPPrefix) {
 	return ret
 }
 
-func applyPrefsToHostinfo(hi *tailcfg.Hostinfo, prefs *ipn.Prefs) {
+// Warning: b.mu might be held. Currently (2022-02-17) both callers hold it.
+func (b *LocalBackend) applyPrefsToHostinfo(hi *tailcfg.Hostinfo, prefs *ipn.Prefs) {
 	if h := prefs.Hostname; h != "" {
 		hi.Hostname = h
 	}
 	hi.RoutableIPs = append(prefs.AdvertiseRoutes[:0:0], prefs.AdvertiseRoutes...)
 	hi.RequestTags = append(prefs.AdvertiseTags[:0:0], prefs.AdvertiseTags...)
 	hi.ShieldsUp = prefs.ShieldsUp
+
+	var sshHostKeys []string
+	if prefs.RunSSH {
+		// TODO(bradfitz): this is called with b.mu held. Not ideal.
+		// If the filesystem gets wedged or something we could block for
+		// a long time. But probably fine.
+		sshHostKeys = b.getSSHHostKeyPublicStrings()
+	}
+	hi.SSH_HostKeys = sshHostKeys
 }
 
 // enterState transitions the backend into newState, updating internal

ipn/ipnlocal/ssh.go

@@ -11,6 +11,7 @@
 	"errors"
 	"io/ioutil"
 	"os"
+	"strings"
 
 	"golang.org/x/crypto/ssh"
 	"tailscale.com/envknob"
@@ -18,15 +19,16 @@
 
 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
+}

ipn/ipnlocal/ssh_stub.go

@@ -0,0 +1,12 @@
+// Copyright (c) 2022 Tailscale Inc & AUTHORS All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build !linux
+// +build !linux
+
+package ipnlocal
+
+func (b *LocalBackend) getSSHHostKeyPublicStrings() []string {
+	return nil
+}

ssh/tailssh/tailssh.go

@@ -49,7 +49,7 @@ func Handle(logf logger.Logf, lb *ipnlocal.LocalBackend, c net.Conn) error {
 	for k, v := range ssh.DefaultSubsystemHandlers {
 		srv.SubsystemHandlers[k] = v
 	}
-	keys, err := lb.GetSSHHostKeys()
+	keys, err := lb.GetSSH_HostKeys()
 	if err != nil {
 		return err
 	}

tailcfg/tailcfg.go

@@ -454,6 +454,7 @@ type Hostinfo struct {
 	RequestTags   []string           `json:",omitempty"` // set of ACL tags this node wants to claim
 	Services      []Service          `json:",omitempty"` // services advertised by this machine
 	NetInfo       *NetInfo           `json:",omitempty"`
+	SSH_HostKeys  []string           `json:"sshHostKeys,omitempty"` // if advertised
 
 	// NOTE: any new fields containing pointers in this type
 	//       require changes to Hostinfo.Equal.
@@ -516,6 +517,10 @@ func (v HostinfoView) RequestTags() views.StringSlice {
 	return views.StringSliceOf(v.ж.RequestTags)
 }
 
+func (v HostinfoView) SSH_HostKeys() views.StringSlice {
+	return views.StringSliceOf(v.ж.SSH_HostKeys)
+}
+
 func (v HostinfoView) Services() ServiceSlice {
 	return ServiceSliceOf(v.ж.Services)
 }

tailcfg/tailcfg_clone.go

@@ -106,6 +106,7 @@ func (src *Hostinfo) Clone() *Hostinfo {
 	dst.RequestTags = append(src.RequestTags[:0:0], src.RequestTags...)
 	dst.Services = append(src.Services[:0:0], src.Services...)
 	dst.NetInfo = src.NetInfo.Clone()
+	dst.SSH_HostKeys = append(src.SSH_HostKeys[:0:0], src.SSH_HostKeys...)
 	return dst
 }
 
@@ -126,6 +127,7 @@ func (src *Hostinfo) Clone() *Hostinfo {
 	RequestTags   []string
 	Services      []Service
 	NetInfo       *NetInfo
+	SSH_HostKeys  []string
 }{})
 
 // Clone makes a deep copy of NetInfo.

tailcfg/tailcfg_test.go

@@ -32,7 +32,7 @@ func TestHostinfoEqual(t *testing.T) {
 		"ShieldsUp", "ShareeNode",
 		"GoArch",
 		"RoutableIPs", "RequestTags",
-		"Services", "NetInfo",
+		"Services", "NetInfo", "SSH_HostKeys",
 	}
 	if have := fieldsOf(reflect.TypeOf(Hostinfo{})); !reflect.DeepEqual(have, hiHandles) {
 		t.Errorf("Hostinfo.Equal check might be out of sync\nfields: %q\nhandled: %q\n",
@@ -181,6 +181,11 @@ func TestHostinfoEqual(t *testing.T) {
 			&Hostinfo{},
 			false,
 		},
+		{
+			&Hostinfo{SSH_HostKeys: []string{"ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIO.... root@bar"}},
+			&Hostinfo{},
+			false,
+		},
 	}
 	for i, tt := range tests {
 		got := tt.a.Equal(tt.b)