wgengine/filter: support FilterRules matching on srcIP node caps [capver 100]

See #12542 for background.

Updates #12542

Change-Id: Ida312f700affc00d17681dc7551ee9672eeb1789
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

ipn/ipnlocal/local.go

@@ -250,7 +250,7 @@ type LocalBackend struct {
 	// delta node mutations as they come in (with mu held). The map values can
 	// be given out to callers, but the map itself must not escape the LocalBackend.
 	peers            map[tailcfg.NodeID]tailcfg.NodeView
-	nodeByAddr       map[netip.Addr]tailcfg.NodeID
+	nodeByAddr       map[netip.Addr]tailcfg.NodeID // by Node.Addresses only (not subnet routes)
 	nmExpiryTimer    tstime.TimerController        // for updating netMap on node expiry; can be nil
 	activeLogin      string                        // last logged LoginName from netMap
 	engineStatus     ipn.EngineStatus
@@ -2021,7 +2021,7 @@ func (b *LocalBackend) updateFilterLocked(netMap *netmap.NetworkMap, prefs ipn.P
 		b.setFilter(filter.NewShieldsUpFilter(localNets, logNets, oldFilter, b.logf))
 	} else {
 		b.logf("[v1] netmap packet filter: %v filters", len(packetFilter))
-		b.setFilter(filter.New(packetFilter, localNets, logNets, oldFilter, b.logf))
+		b.setFilter(filter.New(packetFilter, b.srcIPHasCapForFilter, localNets, logNets, oldFilter, b.logf))
 	}
 	// The filter for a jailed node is the exact same as a ShieldsUp filter.
 	oldJailedFilter := b.e.GetJailedFilter()
@@ -6839,3 +6839,28 @@ func (b *LocalBackend) startAutoUpdate(logPrefix string) (retErr error) {
 	}()
 	return nil
 }
+
+// srcIPHasCapForFilter is called by the packet filter when evaluating firewall
+// rules that require a source IP to have a certain node capability.
+//
+// TODO(bradfitz): optimize this later if/when it matters.
+func (b *LocalBackend) srcIPHasCapForFilter(srcIP netip.Addr, cap tailcfg.NodeCapability) bool {
+	if cap == "" {
+		// Shouldn't happen, but just in case.
+		// But the empty cap also shouldn't be found in Node.CapMap.
+		return false
+	}
+
+	b.mu.Lock()
+	defer b.mu.Unlock()
+
+	nodeID, ok := b.nodeByAddr[srcIP]
+	if !ok {
+		return false
+	}
+	n, ok := b.peers[nodeID]
+	if !ok {
+		return false
+	}
+	return n.HasCap(cap)
+}

net/tstun/wrap_test.go

@@ -168,7 +168,7 @@ func setfilter(logf logger.Logf, tun *Wrapper) {
 	var sb netipx.IPSetBuilder
 	sb.AddPrefix(netip.MustParsePrefix("1.2.0.0/16"))
 	ipSet, _ := sb.IPSet()
-	tun.SetFilter(filter.New(matches, ipSet, ipSet, nil, logf))
+	tun.SetFilter(filter.New(matches, nil, ipSet, ipSet, nil, logf))
 }
 
 func newChannelTUN(logf logger.Logf, secure bool) (*tuntest.ChannelTUN, *Wrapper) {

tailcfg/tailcfg.go

@@ -140,7 +140,8 @@ type CapabilityVersion int
 //   - 97: 2024-06-06: Client understands NodeAttrDisableSplitDNSWhenNoCustomResolvers
 //   - 98: 2024-06-13: iOS/tvOS clients may provide serial number as part of posture information
 //   - 99: 2024-06-14: Client understands NodeAttrDisableLocalDNSOverrideViaNRPT
-const CurrentCapabilityVersion CapabilityVersion = 99
+//   - 100: 2024-06-18: Client supports filtertype.Match.SrcCaps (issue #12542)
+const CurrentCapabilityVersion CapabilityVersion = 100
 
 type StableID string
 
@@ -1480,6 +1481,7 @@ type FilterRule struct {
 	//     * the string "*" to match everything (both IPv4 & IPv6)
 	//     * a CIDR (e.g. "192.168.0.0/16")
 	//     * a range of two IPs, inclusive, separated by hyphen ("2eff::1-2eff::0800")
+	//     * a string "cap:<capability>" with NodeCapMap cap name
 	SrcIPs []string
 
 	// SrcBits is deprecated; it was the old way to specify a CIDR

wgengine/filter/filter.go

@@ -42,6 +42,10 @@ type Filter struct {
 	logIPs4 func(netip.Addr) bool
 	logIPs6 func(netip.Addr) bool
 
+	// srcIPHasCap optionally specifies a function that reports
+	// whether a given source IP address has a given capability.
+	srcIPHasCap CapTestFunc
+
 	// matches4 and matches6 are lists of match->action rules
 	// applied to all packets arriving over tailscale
 	// tunnels. Matches are checked in order, and processing stops
@@ -157,12 +161,12 @@ func NewAllowAllForTest(logf logger.Logf) *Filter {
 	sb.AddPrefix(any4)
 	sb.AddPrefix(any6)
 	ipSet, _ := sb.IPSet()
-	return New(ms, ipSet, ipSet, nil, logf)
+	return New(ms, nil, ipSet, ipSet, nil, logf)
 }
 
 // NewAllowNone returns a packet filter that rejects everything.
 func NewAllowNone(logf logger.Logf, logIPs *netipx.IPSet) *Filter {
-	return New(nil, &netipx.IPSet{}, logIPs, nil, logf)
+	return New(nil, nil, &netipx.IPSet{}, logIPs, nil, logf)
 }
 
 // NewShieldsUpFilter returns a packet filter that rejects incoming connections.
@@ -174,17 +178,20 @@ func NewShieldsUpFilter(localNets *netipx.IPSet, logIPs *netipx.IPSet, shareStat
 	if shareStateWith != nil && !shareStateWith.shieldsUp {
 		shareStateWith = nil
 	}
-	f := New(nil, localNets, logIPs, shareStateWith, logf)
+	f := New(nil, nil, localNets, logIPs, shareStateWith, logf)
 	f.shieldsUp = true
 	return f
 }
 
-// New creates a new packet filter. The filter enforces that incoming
+// New creates a new packet filter. The filter enforces that incoming packets
-// packets must be destined to an IP in localNets, and must be allowed
+// must be destined to an IP in localNets, and must be allowed by matches.
-// by matches. If shareStateWith is non-nil, the returned filter
+// The optional capTest func is used to evaluate a Match that uses capabilities.
-// shares state with the previous one, to enable changing rules at
+// If nil, such matches will always fail.
-// runtime without breaking existing stateful flows.
+//
-func New(matches []Match, localNets, logIPs *netipx.IPSet, shareStateWith *Filter, logf logger.Logf) *Filter {
+// If shareStateWith is non-nil, the returned filter shares state with the
+// previous one, to enable changing rules at runtime without breaking existing
+// stateful flows.
+func New(matches []Match, capTest CapTestFunc, localNets, logIPs *netipx.IPSet, shareStateWith *Filter, logf logger.Logf) *Filter {
 	var state *filterState
 	if shareStateWith != nil {
 		state = shareStateWith.state
@@ -229,6 +236,7 @@ func matchesFamily(ms matches, keep func(netip.Addr) bool) matches {
 	for _, m := range ms {
 		var retm Match
 		retm.IPProto = m.IPProto
+		retm.SrcCaps = m.SrcCaps
 		for _, src := range m.Srcs {
 			if keep(src.Addr()) {
 				retm.Srcs = append(retm.Srcs, src)
@@ -240,7 +248,7 @@ func matchesFamily(ms matches, keep func(netip.Addr) bool) matches {
 				retm.Dsts = append(retm.Dsts, dst)
 			}
 		}
-		if len(retm.Srcs) > 0 && len(retm.Dsts) > 0 {
+		if (len(retm.Srcs) > 0 || len(retm.SrcCaps) > 0) && len(retm.Dsts) > 0 {
 			retm.SrcsContains = ipset.NewContainsIPFunc(views.SliceOf(retm.Srcs))
 			ret = append(ret, retm)
 		}
@@ -462,7 +470,7 @@ func (f *Filter) runIn4(q *packet.Parsed) (r Response, why string) {
 			//  related to an existing ICMP-Echo, TCP, or UDP
 			//  session.
 			return Accept, "icmp response ok"
-		} else if f.matches4.matchIPsOnly(q) {
+		} else if f.matches4.matchIPsOnly(q, f.srcIPHasCap) {
 			// If any port is open to an IP, allow ICMP to it.
 			return Accept, "icmp ok"
 		}
@@ -478,7 +486,7 @@ func (f *Filter) runIn4(q *packet.Parsed) (r Response, why string) {
 		if !q.IsTCPSyn() {
 			return Accept, "tcp non-syn"
 		}
-		if f.matches4.match(q) {
+		if f.matches4.match(q, f.srcIPHasCap) {
 			return Accept, "tcp ok"
 		}
 	case ipproto.UDP, ipproto.SCTP:
@@ -491,7 +499,7 @@ func (f *Filter) runIn4(q *packet.Parsed) (r Response, why string) {
 		if ok {
 			return Accept, "cached"
 		}
-		if f.matches4.match(q) {
+		if f.matches4.match(q, f.srcIPHasCap) {
 			return Accept, "ok"
 		}
 	case ipproto.TSMP:
@@ -522,7 +530,7 @@ func (f *Filter) runIn6(q *packet.Parsed) (r Response, why string) {
 			//  related to an existing ICMP-Echo, TCP, or UDP
 			//  session.
 			return Accept, "icmp response ok"
-		} else if f.matches6.matchIPsOnly(q) {
+		} else if f.matches6.matchIPsOnly(q, f.srcIPHasCap) {
 			// If any port is open to an IP, allow ICMP to it.
 			return Accept, "icmp ok"
 		}
@@ -538,7 +546,7 @@ func (f *Filter) runIn6(q *packet.Parsed) (r Response, why string) {
 		if q.IPProto == ipproto.TCP && !q.IsTCPSyn() {
 			return Accept, "tcp non-syn"
 		}
-		if f.matches6.match(q) {
+		if f.matches6.match(q, f.srcIPHasCap) {
 			return Accept, "tcp ok"
 		}
 	case ipproto.UDP, ipproto.SCTP:
@@ -551,7 +559,7 @@ func (f *Filter) runIn6(q *packet.Parsed) (r Response, why string) {
 		if ok {
 			return Accept, "cached"
 		}
-		if f.matches6.match(q) {
+		if f.matches6.match(q, f.srcIPHasCap) {
 			return Accept, "ok"
 		}
 	case ipproto.TSMP:

wgengine/filter/filter_test.go

@@ -40,14 +40,31 @@ const (
 	testDeniedProto  ipproto.Proto = 127 // CRUDP, appropriately cruddy
 )
 
-func m(srcs []netip.Prefix, dsts []NetPortRange, protos ...ipproto.Proto) Match {
+// m returnns a Match with the given srcs and dsts.
-	if protos == nil {
+//
+// opts can be ipproto.Proto values (if none, defaultProtos is used)
+// or tailcfg.NodeCapability values. Other values panic.
+func m(srcs []netip.Prefix, dsts []NetPortRange, opts ...any) Match {
+	var protos []ipproto.Proto
+	var caps []tailcfg.NodeCapability
+	for _, o := range opts {
+		switch o := o.(type) {
+		case ipproto.Proto:
+			protos = append(protos, o)
+		case tailcfg.NodeCapability:
+			caps = append(caps, o)
+		default:
+			panic(fmt.Sprintf("unknown option type %T", o))
+		}
+	}
+	if len(protos) == 0 {
 		protos = defaultProtos
 	}
 	return Match{
 		IPProto:      views.SliceOf(protos),
 		Srcs:         srcs,
 		SrcsContains: ipset.NewContainsIPFunc(views.SliceOf(srcs)),
+		SrcCaps:      caps,
 		Dsts:         dsts,
 	}
 }
@@ -65,6 +82,7 @@ func newFilter(logf logger.Logf) *Filter {
 		m(nets("::/0"), netports("::/0:443")),
 		m(nets("0.0.0.0/0"), netports("0.0.0.0/0:*"), testAllowedProto),
 		m(nets("::/0"), netports("::/0:*"), testAllowedProto),
+		m(nil, netports("1.2.3.4:22"), tailcfg.NodeCapability("cap-hit-1234-ssh")),
 	}
 
 	// Expects traffic to 100.122.98.50, 1.2.3.4, 5.6.7.8,
@@ -79,11 +97,17 @@ func newFilter(logf logger.Logf) *Filter {
 	localNetsSet, _ := localNets.IPSet()
 	logBSet, _ := logB.IPSet()
 
-	return New(matches, localNetsSet, logBSet, nil, logf)
+	return New(matches, nil, localNetsSet, logBSet, nil, logf)
 }
 
 func TestFilter(t *testing.T) {
-	acl := newFilter(t.Logf)
+	filt := newFilter(t.Logf)
+
+	ipWithCap := netip.MustParseAddr("10.0.0.1")
+	ipWithoutCap := netip.MustParseAddr("10.0.0.2")
+	filt.srcIPHasCap = func(ip netip.Addr, cap tailcfg.NodeCapability) bool {
+		return cap == "cap-hit-1234-ssh" && ip == ipWithCap
+	}
 
 	type InOut struct {
 		want Response
@@ -139,21 +163,27 @@ func TestFilter(t *testing.T) {
 		{Accept, parsed(testAllowedProto, "2001::1", "2001::2", 0, 0)},
 		{Drop, parsed(testDeniedProto, "1.2.3.4", "5.6.7.8", 0, 0)},
 		{Drop, parsed(testDeniedProto, "2001::1", "2001::2", 0, 0)},
+
+		// Test use of a node capability to grant access.
+		// 10.0.0.1 has the capability; 10.0.0.2 does not (see srcIPHasCap at top of func)
+		{Accept, parsed(ipproto.TCP, ipWithCap.String(), "1.2.3.4", 30000, 22)},
+		{Drop, parsed(ipproto.TCP, ipWithoutCap.String(), "1.2.3.4", 30000, 22)},
 	}
 	for i, test := range tests {
-		aclFunc := acl.runIn4
+		aclFunc := filt.runIn4
 		if test.p.IPVersion == 6 {
-			aclFunc = acl.runIn6
+			aclFunc = filt.runIn6
 		}
 		if got, why := aclFunc(&test.p); test.want != got {
 			t.Errorf("#%d runIn got=%v want=%v why=%q packet:%v", i, got, test.want, why, test.p)
+			continue
 		}
 		if test.p.IPProto == ipproto.TCP {
 			var got Response
 			if test.p.IPVersion == 4 {
-				got = acl.CheckTCP(test.p.Src.Addr(), test.p.Dst.Addr(), test.p.Dst.Port())
+				got = filt.CheckTCP(test.p.Src.Addr(), test.p.Dst.Addr(), test.p.Dst.Port())
 			} else {
-				got = acl.CheckTCP(test.p.Src.Addr(), test.p.Dst.Addr(), test.p.Dst.Port())
+				got = filt.CheckTCP(test.p.Src.Addr(), test.p.Dst.Addr(), test.p.Dst.Port())
 			}
 			if test.want != got {
 				t.Errorf("#%d CheckTCP got=%v want=%v packet:%v", i, got, test.want, test.p)
@@ -165,7 +195,7 @@ func TestFilter(t *testing.T) {
 			}
 		}
 		// Update UDP state
-		_, _ = acl.runOut(&test.p)
+		_, _ = filt.runOut(&test.p)
 	}
 }
 
@@ -264,13 +294,16 @@ func TestParseIPSet(t *testing.T) {
 		{"*", pfx("0.0.0.0/0", "::/0"), ""},
 	}
 	for _, tt := range tests {
-		got, err := parseIPSet(tt.host)
+		got, gotCap, err := parseIPSet(tt.host)
 		if err != nil {
 			if err.Error() == tt.wantErr {
 				continue
 			}
 			t.Errorf("parseIPSet(%q) error: %v; want error %q", tt.host, err, tt.wantErr)
 		}
+		if gotCap != "" {
+			t.Errorf("parseIPSet(%q) cap: %q; want empty", tt.host, gotCap)
+		}
 		compareIP := cmp.Comparer(func(a, b netip.Addr) bool { return a == b })
 		compareIPPrefix := cmp.Comparer(func(a, b netip.Prefix) bool { return a == b })
 		if diff := cmp.Diff(got, tt.want, compareIP, compareIPPrefix); diff != "" {
@@ -278,6 +311,27 @@ func TestParseIPSet(t *testing.T) {
 			continue
 		}
 	}
+
+	capTests := []struct {
+		in   string
+		want tailcfg.NodeCapability
+	}{
+		{"cap:foo", "foo"},
+		{"cap:people-in-8.8.8.0/24", "people-in-8.8.8.0/24"}, // test precedence of "/" search
+	}
+	for _, tt := range capTests {
+		pfxes, gotCap, err := parseIPSet(tt.in)
+		if err != nil {
+			t.Errorf("parseIPSet(%q) error: %v; want no error", tt.in, err)
+			continue
+		}
+		if gotCap != tt.want {
+			t.Errorf("parseIPSet(%q) cap: %q; want %q", tt.in, gotCap, tt.want)
+		}
+		if len(pfxes) != 0 {
+			t.Errorf("parseIPSet(%q) pfxes: %v; want empty", tt.in, pfxes)
+		}
+	}
 }
 
 func BenchmarkFilter(b *testing.B) {
@@ -904,7 +958,7 @@ func TestPeerCaps(t *testing.T) {
 	if err != nil {
 		t.Fatal(err)
 	}
-	filt := New(mm, nil, nil, nil, t.Logf)
+	filt := New(mm, nil, nil, nil, nil, t.Logf)
 	tests := []struct {
 		name     string
 		src, dst string // IP
@@ -1037,7 +1091,7 @@ func benchmarkFile(b *testing.B, file string, opt benchOpt) {
 	logIPs.AddPrefix(tsaddr.CGNATRange())
 	logIPs.AddPrefix(tsaddr.TailscaleULARange())
 
-	f := New(matches, must.Get(localNets.IPSet()), must.Get(logIPs.IPSet()), nil, logger.Discard)
+	f := New(matches, nil, must.Get(localNets.IPSet()), must.Get(logIPs.IPSet()), nil, logger.Discard)
 	var srcIP, dstIP netip.Addr
 	if opt.v4 {
 		srcIP = netip.MustParseAddr("1.2.3.4")

wgengine/filter/filtertype/filtertype.go

@@ -66,11 +66,28 @@ type CapMatch struct {
 // Match matches packets from any IP address in Srcs to any ip:port in
 // Dsts.
 type Match struct {
-	IPProto      views.Slice[ipproto.Proto] // required set (no default value at this layer)
+	// IPProto is the set of IP protocol numbers for which this match applies.
+	// It is required. There is no default value at this layer.
+	// If empty, it doesn't match.
+	IPProto views.Slice[ipproto.Proto]
+
+	// Srcs is the set of source IP prefixes for which this match applies. A
+	// Match can match by either its source IP address being in Srcs (which
+	// SrcsContains tests) or if the source IP is of a known peer self address
+	// that contains a NodeCapability listed in SrcCaps.
 	Srcs []netip.Prefix
+	// SrcsContains is an optimized function that reports whether Addr is in
+	// Srcs, using the best search method for the size and shape of Srcs.
 	SrcsContains func(netip.Addr) bool `json:"-"` // report whether Addr is in Srcs
-	Dsts         []NetPortRange        // optional, if Srcs match
+
-	Caps         []CapMatch            // optional, if Srcs match
+	// SrcCaps is an alternative way to match packets. If the peer's source IP
+	// has one of these capabilities, it's also permitted. The peers are only
+	// looked up by their self address (Node.Addresses) and not by subnet routes
+	// they advertise.
+	SrcCaps []tailcfg.NodeCapability
+
+	Dsts []NetPortRange // optional, if source matches
+	Caps []CapMatch     // optional, if source match
 }
 
 func (m Match) String() string {

wgengine/filter/filtertype/filtertype_clone.go

@@ -23,6 +23,7 @@ func (src *Match) Clone() *Match {
 	*dst = *src
 	dst.IPProto = src.IPProto
 	dst.Srcs = append(src.Srcs[:0:0], src.Srcs...)
+	dst.SrcCaps = append(src.SrcCaps[:0:0], src.SrcCaps...)
 	dst.Dsts = append(src.Dsts[:0:0], src.Dsts...)
 	if src.Caps != nil {
 		dst.Caps = make([]CapMatch, len(src.Caps))
@@ -38,6 +39,7 @@ var _MatchCloneNeedsRegeneration = Match(struct {
 	IPProto      views.Slice[ipproto.Proto]
 	Srcs         []netip.Prefix
 	SrcsContains func(netip.Addr) bool
+	SrcCaps      []tailcfg.NodeCapability
 	Dsts         []NetPortRange
 	Caps         []CapMatch
 }{})

wgengine/filter/match.go

@@ -4,19 +4,23 @@
 package filter
 
 import (
+	"net/netip"
+
 	"tailscale.com/net/packet"
+	"tailscale.com/tailcfg"
 	"tailscale.com/types/views"
 	"tailscale.com/wgengine/filter/filtertype"
 )
 
 type matches []filtertype.Match
 
-func (ms matches) match(q *packet.Parsed) bool {
+func (ms matches) match(q *packet.Parsed, hasCap CapTestFunc) bool {
-	for _, m := range ms {
+	for i := range ms {
+		m := &ms[i]
 		if !views.SliceContains(m.IPProto, q.IPProto) {
 			continue
 		}
-		if !m.SrcsContains(q.Src.Addr()) {
+		if !srcMatches(m, q.Src.Addr(), hasCap) {
 			continue
 		}
 		for _, dst := range m.Dsts {
@@ -32,9 +36,33 @@ func (ms matches) match(q *packet.Parsed) bool {
 	return false
 }
 
-func (ms matches) matchIPsOnly(q *packet.Parsed) bool {
+// srcMatches reports whether srcAddr matche the src requirements in m, either
+// by Srcs (using SrcsContains), or by the node having a capability listed
+// in SrcCaps using the provided hasCap function.
+func srcMatches(m *filtertype.Match, srcAddr netip.Addr, hasCap CapTestFunc) bool {
+	if m.SrcsContains(srcAddr) {
+		return true
+	}
+	if hasCap != nil {
+		for _, c := range m.SrcCaps {
+			if hasCap(srcAddr, c) {
+				return true
+			}
+		}
+	}
+	return false
+}
+
+// CapTestFunc is the function signature of a function that tests whether srcIP
+// has a given capability.
+//
+// It it used in the fast path of evaluating filter rules so should be fast.
+type CapTestFunc = func(srcIP netip.Addr, cap tailcfg.NodeCapability) bool
+
+func (ms matches) matchIPsOnly(q *packet.Parsed, hasCap CapTestFunc) bool {
+	srcAddr := q.Src.Addr()
 	for _, m := range ms {
-		if !m.SrcsContains(q.Src.Addr()) {
+		if !m.SrcsContains(srcAddr) {
 			continue
 		}
 		for _, dst := range m.Dsts {
@@ -43,6 +71,15 @@ func (ms matches) matchIPsOnly(q *packet.Parsed) bool {
 			}
 		}
 	}
+	if hasCap != nil {
+		for _, m := range ms {
+			for _, c := range m.SrcCaps {
+				if hasCap(srcAddr, c) {
+					return true
+				}
+			}
+		}
+	}
 	return false
 }
 

wgengine/filter/tailcfg.go

@@ -58,12 +58,15 @@ func MatchesFromFilterRules(pf []tailcfg.FilterRule) ([]Match, error) {
 		}
 
 		for _, s := range r.SrcIPs {
-			nets, err := parseIPSet(s)
+			nets, cap, err := parseIPSet(s)
 			if err != nil && erracc == nil {
 				erracc = err
 				continue
 			}
 			m.Srcs = append(m.Srcs, nets...)
+			if cap != "" {
+				m.SrcCaps = append(m.SrcCaps, cap)
+			}
 		}
 		m.SrcsContains = ipset.NewContainsIPFunc(views.SliceOf(m.Srcs))
 
@@ -71,11 +74,15 @@ func MatchesFromFilterRules(pf []tailcfg.FilterRule) ([]Match, error) {
 			if d.Bits != nil {
 				return nil, fmt.Errorf("unexpected DstBits; control plane should not send this to this client version")
 			}
-			nets, err := parseIPSet(d.IP)
+			nets, cap, err := parseIPSet(d.IP)
 			if err != nil && erracc == nil {
 				erracc = err
 				continue
 			}
+			if cap != "" {
+				erracc = fmt.Errorf("unexpected capability %q in DstPorts", cap)
+				continue
+			}
 			for _, net := range nets {
 				m.Dsts = append(m.Dsts, NetPortRange{
 					Net: net,
@@ -120,48 +127,52 @@ var (
 //   - the string "*" to match everything (both IPv4 & IPv6)
 //   - a CIDR (e.g. "192.168.0.0/16")
 //   - a range of two IPs, inclusive, separated by hyphen ("2eff::1-2eff::0800")
+//   - "cap:<peer-node-capability>" to match a peer node capability
 //
 // TODO(bradfitz): make this return an IPSet and plumb that all
 // around, and ultimately use a new version of IPSet.ContainsFunc like
 // Contains16Func that works in [16]byte address, so we we can match
 // at runtime without allocating?
-func parseIPSet(arg string) ([]netip.Prefix, error) {
+func parseIPSet(arg string) (prefixes []netip.Prefix, peerCap tailcfg.NodeCapability, err error) {
 	if arg == "*" {
 		// User explicitly requested wildcard.
 		return []netip.Prefix{
 			netip.PrefixFrom(zeroIP4, 0),
 			netip.PrefixFrom(zeroIP6, 0),
-		}, nil
+		}, "", nil
+	}
+	if cap, ok := strings.CutPrefix(arg, "cap:"); ok {
+		return nil, tailcfg.NodeCapability(cap), nil
 	}
 	if strings.Contains(arg, "/") {
 		pfx, err := netip.ParsePrefix(arg)
 		if err != nil {
-			return nil, err
+			return nil, "", err
 		}
 		if pfx != pfx.Masked() {
-			return nil, fmt.Errorf("%v contains non-network bits set", pfx)
+			return nil, "", fmt.Errorf("%v contains non-network bits set", pfx)
 		}
-		return []netip.Prefix{pfx}, nil
+		return []netip.Prefix{pfx}, "", nil
 	}
 	if strings.Count(arg, "-") == 1 {
 		ip1s, ip2s, _ := strings.Cut(arg, "-")
 		ip1, err := netip.ParseAddr(ip1s)
 		if err != nil {
-			return nil, err
+			return nil, "", err
 		}
 		ip2, err := netip.ParseAddr(ip2s)
 		if err != nil {
-			return nil, err
+			return nil, "", err
 		}
 		r := netipx.IPRangeFrom(ip1, ip2)
 		if !r.IsValid() {
-			return nil, fmt.Errorf("invalid IP range %q", arg)
+			return nil, "", fmt.Errorf("invalid IP range %q", arg)
 		}
-		return r.Prefixes(), nil
+		return r.Prefixes(), "", nil
 	}
 	ip, err := netip.ParseAddr(arg)
 	if err != nil {
-		return nil, fmt.Errorf("invalid IP address %q", arg)
+		return nil, "", fmt.Errorf("invalid IP address %q", arg)
 	}
-	return []netip.Prefix{netip.PrefixFrom(ip, ip.BitLen())}, nil
+	return []netip.Prefix{netip.PrefixFrom(ip, ip.BitLen())}, "", nil
 }