feature/featuretags: add features for c2n, peerapi, advertise/use routes/exit nodes

Saves 262 KB so far. I'm sure I missed some places, but shotizam says
these were the low hanging fruit.

Updates #12614

Change-Id: Ia31c01b454f627e6d0470229aae4e19d615e45e3
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

net/dns/resolver/forwarder.go

@@ -27,6 +27,7 @@ import (
 	dns "golang.org/x/net/dns/dnsmessage"
 	"tailscale.com/control/controlknobs"
 	"tailscale.com/envknob"
+	"tailscale.com/feature"
 	"tailscale.com/feature/buildfeatures"
 	"tailscale.com/health"
 	"tailscale.com/net/dns/publicdns"
@@ -530,6 +531,9 @@ func (f *forwarder) send(ctx context.Context, fq *forwardQuery, rr resolverAndDe
 		}()
 	}
 	if strings.HasPrefix(rr.name.Addr, "http://") {
+		if !buildfeatures.HasPeerAPIClient {
+			return nil, feature.ErrUnavailable
+		}
 		return f.sendDoH(ctx, rr.name.Addr, f.dialer.PeerAPIHTTPClient(), fq.packet)
 	}
 	if strings.HasPrefix(rr.name.Addr, "https://") {

net/netmon/interfaces_linux.go

@@ -22,6 +22,7 @@ import (
 	"github.com/mdlayher/netlink"
 	"go4.org/mem"
 	"golang.org/x/sys/unix"
+	"tailscale.com/feature/buildfeatures"
 	"tailscale.com/net/netaddr"
 	"tailscale.com/util/lineiter"
 )
@@ -41,6 +42,9 @@ ens18   00000000        0100000A        0003    0       0       0       00000000
 ens18   0000000A        00000000        0001    0       0       0       0000FFFF        0       0       0
 */
 func likelyHomeRouterIPLinux() (ret netip.Addr, myIP netip.Addr, ok bool) {
+	if !buildfeatures.HasPortMapper {
+		return
+	}
 	if procNetRouteErr.Load() {
 		// If we failed to read /proc/net/route previously, don't keep trying.
 		return ret, myIP, false

net/netmon/netmon.go

@@ -14,6 +14,7 @@ import (
 	"sync"
 	"time"
 
+	"tailscale.com/feature/buildfeatures"
 	"tailscale.com/types/logger"
 	"tailscale.com/util/clientmetric"
 	"tailscale.com/util/eventbus"
@@ -181,6 +182,9 @@ func (m *Monitor) SetTailscaleInterfaceName(ifName string) {
 // It's the same as interfaces.LikelyHomeRouterIP, but it caches the
 // result until the monitor detects a network change.
 func (m *Monitor) GatewayAndSelfIP() (gw, myIP netip.Addr, ok bool) {
+	if !buildfeatures.HasPortMapper {
+		return
+	}
 	if m.static {
 		return
 	}

net/netmon/state.go

@@ -573,6 +573,9 @@ var disableLikelyHomeRouterIPSelf = envknob.RegisterBool("TS_DEBUG_DISABLE_LIKEL
 // the LAN using that gateway.
 // This is used as the destination for UPnP, NAT-PMP, PCP, etc queries.
 func LikelyHomeRouterIP() (gateway, myIP netip.Addr, ok bool) {
+	if !buildfeatures.HasPortMapper {
+		return
+	}
 	// If we don't have a way to get the home router IP, then we can't do
 	// anything; just return.
 	if likelyHomeRouterIP == nil {

net/portmapper/portmapper.go

@@ -20,6 +20,7 @@ import (
 
 	"go4.org/mem"
 	"tailscale.com/envknob"
+	"tailscale.com/feature/buildfeatures"
 	"tailscale.com/net/netaddr"
 	"tailscale.com/net/neterror"
 	"tailscale.com/net/netmon"
@@ -262,10 +263,13 @@ func NewClient(c Config) *Client {
 		panic("nil EventBus")
 	}
 	ret := &Client{
-		logf:         c.Logf,
-		netMon:       c.NetMon,
-		ipAndGateway: netmon.LikelyHomeRouterIP, // TODO(bradfitz): move this to method on netMon
-		onChange:     c.OnChange,
+		logf:     c.Logf,
+		netMon:   c.NetMon,
+		onChange: c.OnChange,
+	}
+	if buildfeatures.HasPortMapper {
+		// TODO(bradfitz): move this to method on netMon
+		ret.ipAndGateway = netmon.LikelyHomeRouterIP
 	}
 	ret.pubClient = c.EventBus.Client("portmapper")
 	ret.updates = eventbus.Publish[portmappertype.Mapping](ret.pubClient)

net/tsdial/tsdial.go

@@ -19,6 +19,8 @@ import (
 	"time"
 
 	"github.com/gaissmai/bart"
+	"tailscale.com/feature"
+	"tailscale.com/feature/buildfeatures"
 	"tailscale.com/net/dnscache"
 	"tailscale.com/net/netknob"
 	"tailscale.com/net/netmon"
@@ -135,6 +137,9 @@ func (d *Dialer) TUNName() string {
 //
 // For example, "http://100.68.82.120:47830/dns-query".
 func (d *Dialer) SetExitDNSDoH(doh string) {
+	if !buildfeatures.HasUseExitNode {
+		return
+	}
 	d.mu.Lock()
 	defer d.mu.Unlock()
 	if d.exitDNSDoHBase == doh {
@@ -372,7 +377,7 @@ func (d *Dialer) userDialResolve(ctx context.Context, network, addr string) (net
 	}
 
 	var r net.Resolver
-	if exitDNSDoH != "" {
+	if buildfeatures.HasUseExitNode && buildfeatures.HasPeerAPIClient && exitDNSDoH != "" {
 		r.PreferGo = true
 		r.Dial = func(ctx context.Context, network, address string) (net.Conn, error) {
 			return &dohConn{
@@ -509,6 +514,9 @@ func (d *Dialer) UserDial(ctx context.Context, network, addr string) (net.Conn,
 // network must a "tcp" type, and addr must be an ip:port. Name resolution
 // is not supported.
 func (d *Dialer) dialPeerAPI(ctx context.Context, network, addr string) (net.Conn, error) {
+	if !buildfeatures.HasPeerAPIClient {
+		return nil, feature.ErrUnavailable
+	}
 	switch network {
 	case "tcp", "tcp6", "tcp4":
 	default:
@@ -551,6 +559,9 @@ func (d *Dialer) getPeerDialer() *net.Dialer {
 // The returned Client must not be mutated; it's owned by the Dialer
 // and shared by callers.
 func (d *Dialer) PeerAPIHTTPClient() *http.Client {
+	if !buildfeatures.HasPeerAPIClient {
+		panic("unreachable")
+	}
 	d.peerClientOnce.Do(func() {
 		t := http.DefaultTransport.(*http.Transport).Clone()
 		t.Dial = nil