wgengine/netstack: add Magic DNS + DNS resolution to SOCKS5 dialing

Updates #707
Updates #504

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

wgengine/netstack/netstack.go

@@ -15,7 +15,9 @@ import (
 	"io"
 	"log"
 	"net"
+	"strconv"
 	"strings"
+	"sync"
 
 	"gvisor.dev/gvisor/pkg/tcpip"
 	"gvisor.dev/gvisor/pkg/tcpip/adapters/gonet"
@@ -34,6 +36,7 @@ import (
 	"tailscale.com/net/socks5"
 	"tailscale.com/types/logger"
 	"tailscale.com/types/netmap"
+	"tailscale.com/util/dnsname"
 	"tailscale.com/wgengine"
 	"tailscale.com/wgengine/filter"
 	"tailscale.com/wgengine/magicsock"
@@ -50,6 +53,9 @@ type Impl struct {
 	e       wgengine.Engine
 	mc      *magicsock.Conn
 	logf    logger.Logf
+
+	mu  sync.Mutex
+	dns map[string]netaddr.IP // Magic DNS names (both base + FQDN) => first IP
 }
 
 const nicID = 1
@@ -120,7 +126,33 @@ func (ns *Impl) Start() error {
 	return nil
 }
 
+func (ns *Impl) updateDNS(nm *netmap.NetworkMap) {
+	ns.mu.Lock()
+	defer ns.mu.Unlock()
+	ns.dns = make(map[string]netaddr.IP)
+	suffix := nm.MagicDNSSuffix()
+
+	if nm.Name != "" && len(nm.Addresses) > 0 {
+		ip := nm.Addresses[0].IP
+		ns.dns[strings.TrimRight(nm.Name, ".")] = ip
+		if dnsname.HasSuffix(nm.Name, suffix) {
+			ns.dns[dnsname.TrimSuffix(nm.Name, suffix)] = ip
+		}
+	}
+	for _, p := range nm.Peers {
+		if p.Name != "" && len(p.Addresses) > 0 {
+			ip := p.Addresses[0].IP
+			ns.dns[strings.TrimRight(p.Name, ".")] = ip
+			if dnsname.HasSuffix(p.Name, suffix) {
+				ns.dns[dnsname.TrimSuffix(p.Name, suffix)] = ip
+			}
+		}
+	}
+}
+
 func (ns *Impl) updateIPs(nm *netmap.NetworkMap) {
+	ns.updateDNS(nm)
+
 	oldIPs := make(map[tcpip.Address]bool)
 	for _, ip := range ns.ipstack.AllAddresses()[nicID] {
 		oldIPs[ip.AddressWithPrefix.Address] = true
@@ -166,10 +198,54 @@ func (ns *Impl) updateIPs(nm *netmap.NetworkMap) {
 	}
 }
 
-func (ns *Impl) dialContextTCP(ctx context.Context, address string) (*gonet.TCPConn, error) {
-	remoteIPPort, err := netaddr.ParseIPPort(address)
+// resolve resolves addr into an IP:port.
+func (ns *Impl) resolve(ctx context.Context, addr string) (netaddr.IPPort, error) {
+	ipp, pippErr := netaddr.ParseIPPort(addr)
+	if pippErr == nil {
+		return ipp, nil
+	}
+	host, port, err := net.SplitHostPort(addr)
 	if err != nil {
-		return nil, fmt.Errorf("could not parse IP:port: %w", err)
+		// addr is malformed.
+		return netaddr.IPPort{}, err
+	}
+	if net.ParseIP(host) != nil {
+		// The host part of addr was an IP, so the netaddr.ParseIPPort above should've
+		// passed. Must've been a bad port number. Return the original error.
+		return netaddr.IPPort{}, pippErr
+	}
+	port16, err := strconv.ParseUint(port, 10, 16)
+	if err != nil {
+		return netaddr.IPPort{}, fmt.Errorf("invalid port in address %q", addr)
+	}
+
+	// Host is not an IP, so assume it's a DNS name.
+
+	// Try MagicDNS first, else otherwise a real DNS lookup.
+	ns.mu.Lock()
+	ip := ns.dns[host]
+	ns.mu.Unlock()
+	if !ip.IsZero() {
+		return netaddr.IPPort{IP: ip, Port: uint16(port16)}, nil
+	}
+
+	// No Magic DNS name so try real DNS.
+	var r net.Resolver
+	ips, err := r.LookupIP(ctx, "ip", host)
+	if err != nil {
+		return netaddr.IPPort{}, err
+	}
+	if len(ips) == 0 {
+		return netaddr.IPPort{}, fmt.Errorf("DNS lookup returned no results for %q", host)
+	}
+	ip, _ = netaddr.FromStdIP(ips[0])
+	return netaddr.IPPort{IP: ip, Port: uint16(port16)}, nil
+}
+
+func (ns *Impl) dialContextTCP(ctx context.Context, addr string) (*gonet.TCPConn, error) {
+	remoteIPPort, err := ns.resolve(ctx, addr)
+	if err != nil {
+		return nil, err
 	}
 	remoteAddress := tcpip.FullAddress{
 		NIC:  nicID,