cmd/tailscale: write fewer known_hosts, resolve ssh host to FQDN early

Updates #3802

Change-Id: Ic44fa2e6661a9c046e725c04fa6b8213d3d4d2b2
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

cmd/tailscale/cli/ssh.go

@@ -20,6 +20,7 @@ import (
 
 	"github.com/alessio/shellescape"
 	"github.com/peterbourgon/ff/v3/ffcli"
+	"inet.af/netaddr"
 	"tailscale.com/client/tailscale"
 	"tailscale.com/envknob"
 	"tailscale.com/ipn/ipnstate"
@@ -46,6 +47,20 @@ func runSSH(ctx context.Context, args []string) error {
 		}
 		username = lu.Username
 	}
+
+	st, err := tailscale.Status(ctx)
+	if err != nil {
+		return err
+	}
+
+	// hostForSSH is the hostname we'll tell OpenSSH we're
+	// connecting to, so we have to maintain fewer entries in the
+	// known_hosts files.
+	hostForSSH := host
+	if v, ok := nodeDNSNameFromArg(st, host); ok {
+		hostForSSH = v
+	}
+
 	ssh, err := exec.LookPath("ssh")
 	if err != nil {
 		// TODO(bradfitz): use Go's crypto/ssh client instead
@@ -56,10 +71,6 @@ func runSSH(ctx context.Context, args []string) error {
 	if err != nil {
 		return err
 	}
-	st, err := tailscale.Status(ctx)
-	if err != nil {
-		return err
-	}
 	knownHostsFile, err := writeKnownHosts(st)
 	if err != nil {
 		return err
@@ -86,7 +97,7 @@ func runSSH(ctx context.Context, args []string) error {
 		// to use a different one, we'll later be making stock ssh
 		// work well by default too. (doing things like automatically
 		// setting known_hosts, etc)
-		username + "@" + host,
+		username + "@" + hostForSSH,
 	}, argRest...)
 
 	if runtime.GOOS == "windows" {
@@ -135,28 +146,41 @@ func genKnownHosts(st *ipnstate.Status) []byte {
 	var buf bytes.Buffer
 	for _, k := range st.Peers() {
 		ps := st.Peer[k]
-		if len(ps.SSH_HostKeys) == 0 {
-			continue
+		for _, hk := range ps.SSH_HostKeys {
+			hostKey := strings.TrimSpace(hk)
+			if strings.ContainsAny(hostKey, "\n\r") { // invalid
+				continue
+			}
+			fmt.Fprintf(&buf, "%s %s\n", ps.DNSName, hostKey)
 		}
-		// addEntries adds one line per each of p's host keys.
-		addEntries := func(host string) {
-			for _, hk := range ps.SSH_HostKeys {
-				hostKey := strings.TrimSpace(hk)
-				if strings.ContainsAny(hostKey, "\n\r") { // invalid
-					continue
+	}
+	return buf.Bytes()
+}
+
+// nodeDNSNameFromArg returns the PeerStatus.DNSName value from a peer
+// in st that matches the input arg which can be a base name, full
+// DNS name, or an IP.
+func nodeDNSNameFromArg(st *ipnstate.Status, arg string) (dnsName string, ok bool) {
+	if arg == "" {
+		return
+	}
+	argIP, _ := netaddr.ParseIP(arg)
+	for _, ps := range st.Peer {
+		dnsName = ps.DNSName
+		if !argIP.IsZero() {
+			for _, ip := range ps.TailscaleIPs {
+				if ip == argIP {
+					return dnsName, true
 				}
-				fmt.Fprintf(&buf, "%s %s\n", host, hostKey)
 			}
+			continue
 		}
-		if ps.DNSName != "" {
-			addEntries(ps.DNSName)
-		}
-		if base, _, ok := strings.Cut(ps.DNSName, "."); ok {
-			addEntries(base)
+		if strings.EqualFold(strings.TrimSuffix(arg, "."), strings.TrimSuffix(dnsName, ".")) {
+			return dnsName, true
 		}
-		for _, ip := range st.TailscaleIPs {
-			addEntries(ip.String())
+		if base, _, ok := strings.Cut(ps.DNSName, "."); ok && strings.EqualFold(base, arg) {
+			return dnsName, true
 		}
 	}
-	return buf.Bytes()
+	return "", false
 }

net/tsdial/dnsmap.go

@@ -21,9 +21,14 @@ import (
 // It must not be mutated once created.
 //
 // Example keys are "foo.domain.tld.beta.tailscale.net" and "foo",
-// both without trailing dots.
+// both without trailing dots, and both always lowercase.
 type dnsMap map[string]netaddr.IP
 
+// canonMapKey canonicalizes its input s to be a dnsMap map key.
+func canonMapKey(s string) string {
+	return strings.ToLower(strings.TrimSuffix(s, "."))
+}
+
 func dnsMapFromNetworkMap(nm *netmap.NetworkMap) dnsMap {
 	if nm == nil {
 		return nil
@@ -33,9 +38,9 @@ func dnsMapFromNetworkMap(nm *netmap.NetworkMap) dnsMap {
 	have4 := false
 	if nm.Name != "" && len(nm.Addresses) > 0 {
 		ip := nm.Addresses[0].IP()
-		ret[strings.TrimRight(nm.Name, ".")] = ip
+		ret[canonMapKey(nm.Name)] = ip
 		if dnsname.HasSuffix(nm.Name, suffix) {
-			ret[dnsname.TrimSuffix(nm.Name, suffix)] = ip
+			ret[canonMapKey(dnsname.TrimSuffix(nm.Name, suffix))] = ip
 		}
 		for _, a := range nm.Addresses {
 			if a.IP().Is4() {
@@ -52,9 +57,9 @@ func dnsMapFromNetworkMap(nm *netmap.NetworkMap) dnsMap {
 			if ip.Is4() && !have4 {
 				continue
 			}
-			ret[strings.TrimRight(p.Name, ".")] = ip
+			ret[canonMapKey(p.Name)] = ip
 			if dnsname.HasSuffix(p.Name, suffix) {
-				ret[dnsname.TrimSuffix(p.Name, suffix)] = ip
+				ret[canonMapKey(dnsname.TrimSuffix(p.Name, suffix))] = ip
 			}
 			break
 		}
@@ -67,7 +72,7 @@ func dnsMapFromNetworkMap(nm *netmap.NetworkMap) dnsMap {
 		if err != nil {
 			continue
 		}
-		ret[strings.TrimRight(rec.Name, ".")] = ip
+		ret[canonMapKey(rec.Name)] = ip
 	}
 	return ret
 }
@@ -106,7 +111,7 @@ func (m dnsMap) resolveMemory(ctx context.Context, network, addr string) (_ neta
 	// Host is not an IP, so assume it's a DNS name.
 
 	// Try MagicDNS first, otherwise a real DNS lookup.
-	ip := m[host]
+	ip := m[canonMapKey(host)]
 	if !ip.IsZero() {
 		return netaddr.IPPortFrom(ip, port), nil
 	}