tailcfg,wgengine/magicsock: set peer relay CapVer (#16531)

Updates tailscale/corp#27502
Updates tailscale/corp#30051

Signed-off-by: Jordan Whited <jordan@tailscale.com>

tailcfg/tailcfg.go

@@ -164,7 +164,8 @@ type CapabilityVersion int
 //   - 117: 2025-05-28: Client understands DisplayMessages (structured health messages), but not necessarily PrimaryAction.
 //   - 118: 2025-07-01: Client sends Hostinfo.StateEncrypted to report whether the state file is encrypted at rest (#15830)
 //   - 119: 2025-07-10: Client uses Hostinfo.Location.Priority to prioritize one route over another.
-const CurrentCapabilityVersion CapabilityVersion = 119
+//   - 120: 2025-07-15: Client understands peer relay disco messages, and implements peer client and relay server functions
+const CurrentCapabilityVersion CapabilityVersion = 120
 
 // ID is an integer ID for a user, node, or login allocated by the
 // control plane.

wgengine/magicsock/debugknobs.go

@@ -62,12 +62,6 @@ var (
 	//
 	//lint:ignore U1000 used on Linux/Darwin only
 	debugPMTUD = envknob.RegisterBool("TS_DEBUG_PMTUD")
-	// debugAssumeUDPRelayCapable forces magicsock to assume that all peers are
-	// UDP relay capable clients and servers. This will eventually be replaced
-	// by a [tailcfg.CapabilityVersion] comparison. It enables early testing of
-	// the UDP relay feature before we have established related
-	// [tailcfg.CapabilityVersion]'s.
-	debugAssumeUDPRelayCapable = envknob.RegisterBool("TS_DEBUG_ASSUME_UDP_RELAY_CAPABLE")
 	// Hey you! Adding a new debugknob? Make sure to stub it out in the
 	// debugknobs_stubs.go file too.
 )

wgengine/magicsock/debugknobs_stubs.go

@@ -31,4 +31,3 @@ func debugRingBufferMaxSizeBytes() int { return 0 }
 func inTest() bool                     { return false }
 func debugPeerMap() bool               { return false }
 func pretendpoints() []netip.AddrPort  { return []netip.AddrPort{} }
-func debugAssumeUDPRelayCapable() bool { return false }

wgengine/magicsock/magicsock.go

@@ -14,7 +14,6 @@ import (
 	"expvar"
 	"fmt"
 	"io"
-	"math"
 	"net"
 	"net/netip"
 	"reflect"
@@ -2616,14 +2615,10 @@ func (c *Conn) SetProbeUDPLifetime(v bool) {
 	})
 }
 
+// capVerIsRelayCapable returns true if version is relay client and server
+// capable, otherwise it returns false.
 func capVerIsRelayCapable(version tailcfg.CapabilityVersion) bool {
-	// TODO(jwhited): implement once capVer is bumped
-	return version == math.MinInt32 || debugAssumeUDPRelayCapable()
-}
-
-func capVerIsRelayServerCapable(version tailcfg.CapabilityVersion) bool {
-	// TODO(jwhited): implement once capVer is bumped & update Test_peerAPIIfCandidateRelayServer
-	return version == math.MinInt32 || debugAssumeUDPRelayCapable()
+	return version >= 120
 }
 
 // onFilterUpdate is called when a [FilterUpdate] is received over the
@@ -2677,10 +2672,16 @@ func peerAPIIfCandidateRelayServer(filt *filter.Filter, self, maybeCandidate tai
 	if filt == nil ||
 		!self.Valid() ||
 		!maybeCandidate.Valid() ||
-		!capVerIsRelayServerCapable(maybeCandidate.Cap()) ||
 		!maybeCandidate.Hostinfo().Valid() {
 		return netip.AddrPort{}
 	}
+	if maybeCandidate.ID() != self.ID() && !capVerIsRelayCapable(maybeCandidate.Cap()) {
+		// If maybeCandidate's [tailcfg.CapabilityVersion] is not relay-capable,
+		// we skip it. If maybeCandidate happens to be self, then this check is
+		// unnecessary as self is always capable from this point (the statically
+		// compiled [tailcfg.CurrentCapabilityVersion]) forward.
+		return netip.AddrPort{}
+	}
 	for _, maybeCandidatePrefix := range maybeCandidate.Addresses().All() {
 		if !maybeCandidatePrefix.IsSingleIP() {
 			continue

wgengine/magicsock/magicsock_test.go

@@ -3399,7 +3399,11 @@ func Test_peerAPIIfCandidateRelayServer(t *testing.T) {
 	}
 
 	selfOnlyIPv4 := &tailcfg.Node{
-		Cap: math.MinInt32,
+		ID: 1,
+		// Intentionally set a value < 120 to verify the statically compiled
+		// [tailcfg.CurrentCapabilityVersion] is used when self is
+		// maybeCandidate.
+		Cap: 119,
 		Addresses: []netip.Prefix{
 			netip.MustParsePrefix("1.1.1.1/32"),
 		},
@@ -3409,13 +3413,17 @@ func Test_peerAPIIfCandidateRelayServer(t *testing.T) {
 	selfOnlyIPv6.Addresses[0] = netip.MustParsePrefix("::1/128")
 
 	peerOnlyIPv4 := &tailcfg.Node{
-		Cap: math.MinInt32,
+		ID:  2,
+		Cap: 120,
 		Addresses: []netip.Prefix{
 			netip.MustParsePrefix("2.2.2.2/32"),
 		},
 		Hostinfo: hostInfo.View(),
 	}
 
+	peerOnlyIPv4NotCapable := peerOnlyIPv4.Clone()
+	peerOnlyIPv4NotCapable.Cap = 119
+
 	peerOnlyIPv6 := peerOnlyIPv4.Clone()
 	peerOnlyIPv6.Addresses[0] = netip.MustParsePrefix("::2/128")
 
@@ -3500,6 +3508,22 @@ func Test_peerAPIIfCandidateRelayServer(t *testing.T) {
 			maybeCandidate: selfOnlyIPv6.View(),
 			want:           netip.AddrPortFrom(selfOnlyIPv6.Addresses[0].Addr(), 6),
 		},
+		{
+			name: "peer incapable",
+			filt: filter.New([]filtertype.Match{
+				{
+					Srcs: []netip.Prefix{netip.MustParsePrefix("2.2.2.2/32")},
+					Caps: []filtertype.CapMatch{
+						{
+							Dst: netip.MustParsePrefix("1.1.1.1/32"),
+							Cap: tailcfg.PeerCapabilityRelayTarget,
+						},
+					},
+				},
+			}, nil, nil, nil, nil, nil),
+			self:           selfOnlyIPv4.View(),
+			maybeCandidate: peerOnlyIPv4NotCapable.View(),
+		},
 		{
 			name: "no match dst",
 			filt: filter.New([]filtertype.Match{