diff --git a/client/tailscale/apitype/apitype.go b/client/tailscale/apitype/apitype.go
index cae573a4e..bd10b4d3d 100644
--- a/client/tailscale/apitype/apitype.go
+++ b/client/tailscale/apitype/apitype.go
@@ -11,6 +11,9 @@ import "tailscale.com/tailcfg"
 type WhoIsResponse struct {
 	Node        *tailcfg.Node
 	UserProfile *tailcfg.UserProfile
+
+	// Caps are extra capabilities that the remote Node has to this node.
+	Caps []string `json:",omitempty"`
 }
 
 // FileTarget is a node to which files can be sent, and the PeerAPI
diff --git a/ipn/ipnlocal/local.go b/ipn/ipnlocal/local.go
index 23e1489f5..cc7eea7a9 100644
--- a/ipn/ipnlocal/local.go
+++ b/ipn/ipnlocal/local.go
@@ -526,6 +526,30 @@ func (b *LocalBackend) WhoIs(ipp netaddr.IPPort) (n *tailcfg.Node, u tailcfg.Use
 	return n, u, true
 }
 
+// PeerCaps returns the capabilities that remote src IP has to
+// ths current node.
+func (b *LocalBackend) PeerCaps(src netaddr.IP) []string {
+	b.mu.Lock()
+	defer b.mu.Unlock()
+	if b.netMap == nil {
+		return nil
+	}
+	filt, ok := b.filterAtomic.Load().(*filter.Filter)
+	if !ok {
+		return nil
+	}
+	for _, a := range b.netMap.Addresses {
+		if !a.IsSingleIP() {
+			continue
+		}
+		dstIP := a.IP()
+		if dstIP.BitLen() == src.BitLen() {
+			return filt.AppendCaps(nil, src, a.IP())
+		}
+	}
+	return nil
+}
+
 // SetDecompressor sets a decompression function, which must be a zstd
 // reader.
 //
diff --git a/ipn/localapi/localapi.go b/ipn/localapi/localapi.go
index 2c18a80ac..95fc4dadb 100644
--- a/ipn/localapi/localapi.go
+++ b/ipn/localapi/localapi.go
@@ -223,6 +223,7 @@ func (h *Handler) serveWhoIs(w http.ResponseWriter, r *http.Request) {
 	res := &apitype.WhoIsResponse{
 		Node:        n,
 		UserProfile: &u,
+		Caps:        b.PeerCaps(ipp.IP()),
 	}
 	j, err := json.MarshalIndent(res, "", "\t")
 	if err != nil {
diff --git a/tailcfg/tailcfg.go b/tailcfg/tailcfg.go
index 6349c0c6d..57c991c28 100644
--- a/tailcfg/tailcfg.go
+++ b/tailcfg/tailcfg.go
@@ -68,7 +68,8 @@ type CapabilityVersion int
 //    29: 2022-03-21: MapResponse.PopBrowserURL
 //    30: 2022-03-22: client can request id tokens.
 //    31: 2022-04-15: PingRequest & PingResponse TSMP & disco support
-const CurrentCapabilityVersion CapabilityVersion = 31
+//    32: 2022-04-17: client knows FilterRule.CapMatch
+const CurrentCapabilityVersion CapabilityVersion = 32
 
 type StableID string
 
@@ -1051,6 +1052,18 @@ type NetPortRange struct {
 	Ports PortRange
 }
 
+// CapGrant grants capabilities in a FilterRule.
+type CapGrant struct {
+	// Dsts are the destination IP ranges that this capabilty
+	// grant matches.
+	Dsts []netaddr.IPPrefix
+
+	// Caps are the capabilities the source IP matched by
+	// FilterRule.SrcIPs are granted to the destination IP,
+	// matched by Dsts.
+	Caps []string `json:",omitempty"`
+}
+
 // FilterRule represents one rule in a packet filter.
 //
 // A rule is logically a set of source CIDRs to match (described by
@@ -1081,7 +1094,9 @@ type FilterRule struct {
 
 	// DstPorts are the port ranges to allow once a source IP
 	// matches (is in the CIDR described by SrcIPs & SrcBits).
-	DstPorts []NetPortRange
+	//
+	// CapGrant and DstPorts are mutually exclusive: at most one can be non-nil.
+	DstPorts []NetPortRange `json:",omitempty"`
 
 	// IPProto are the IP protocol numbers to match.
 	//
@@ -1093,6 +1108,18 @@ type FilterRule struct {
 	// Depending on the IPProto values, DstPorts may or may not be
 	// used.
 	IPProto []int `json:",omitempty"`
+
+	// CapGrant, if non-empty, are the capabilities to
+	// conditionally grant to the source IP in SrcIPs.
+	//
+	// Think of DstPorts as "capabilities for networking" and
+	// CapGrant as arbitrary application-defined capabilities
+	// defined between the admin's ACLs and the application
+	// doing WhoIs lookups, looking up the remote IP address's
+	// application-level capabilities.
+	//
+	// CapGrant and DstPorts are mutually exclusive: at most one can be non-nil.
+	CapGrant []CapGrant `json:",omitempty"`
 }
 
 var FilterAllowAll = []FilterRule{
diff --git a/wgengine/filter/filter.go b/wgengine/filter/filter.go
index a54559651..cda28eb30 100644
--- a/wgengine/filter/filter.go
+++ b/wgengine/filter/filter.go
@@ -27,10 +27,12 @@ type Filter struct {
 	// destination within local, regardless of the policy filter
 	// below.
 	local *netaddr.IPSet
+
 	// logIPs is the set of IPs that are allowed to appear in flow
 	// logs. If a packet is to or from an IP not in logIPs, it will
 	// never be logged.
 	logIPs *netaddr.IPSet
+
 	// 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
@@ -38,6 +40,11 @@ type Filter struct {
 	// match is to drop the packet.
 	matches4 matches
 	matches6 matches
+
+	// cap4 and cap6 are the subsets of the matches that are about
+	// capability grants, partitioned by source IP address family.
+	cap4, cap6 matches
+
 	// state is the connection tracking state attached to this
 	// filter. It is used to allow incoming traffic that is a response
 	// to an outbound connection that this node made, even if those
@@ -174,6 +181,8 @@ func New(matches []Match, localNets *netaddr.IPSet, logIPs *netaddr.IPSet, share
 		logf:     logf,
 		matches4: matchesFamily(matches, netaddr.IP.Is4),
 		matches6: matchesFamily(matches, netaddr.IP.Is6),
+		cap4:     capMatchesFunc(matches, netaddr.IP.Is4),
+		cap6:     capMatchesFunc(matches, netaddr.IP.Is6),
 		local:    localNets,
 		logIPs:   logIPs,
 		state:    state,
@@ -205,6 +214,27 @@ func matchesFamily(ms matches, keep func(netaddr.IP) bool) matches {
 	return ret
 }
 
+// capMatchesFunc returns a copy of the subset of ms for which keep(srcNet.IP)
+// and the match is a capability grant.
+func capMatchesFunc(ms matches, keep func(netaddr.IP) bool) matches {
+	var ret matches
+	for _, m := range ms {
+		if len(m.Caps) == 0 {
+			continue
+		}
+		retm := Match{Caps: m.Caps}
+		for _, src := range m.Srcs {
+			if keep(src.IP()) {
+				retm.Srcs = append(retm.Srcs, src)
+			}
+		}
+		if len(retm.Srcs) > 0 {
+			ret = append(ret, retm)
+		}
+	}
+	return ret
+}
+
 func maybeHexdump(flag RunFlags, b []byte) string {
 	if flag == 0 {
 		return ""
@@ -291,6 +321,30 @@ func (f *Filter) CheckTCP(srcIP, dstIP netaddr.IP, dstPort uint16) Response {
 	return f.RunIn(pkt, 0)
 }
 
+// AppendCaps appends to base the capabilities that srcIP has talking
+// to dstIP.
+func (f *Filter) AppendCaps(base []string, srcIP, dstIP netaddr.IP) []string {
+	ret := base
+	var mm matches
+	switch {
+	case srcIP.Is4():
+		mm = f.cap4
+	case srcIP.Is6():
+		mm = f.cap6
+	}
+	for _, m := range mm {
+		if !ipInList(srcIP, m.Srcs) {
+			continue
+		}
+		for _, cm := range m.Caps {
+			if cm.Cap != "" && cm.Dst.Contains(dstIP) {
+				ret = append(ret, cm.Cap)
+			}
+		}
+	}
+	return ret
+}
+
 // ShieldsUp reports whether this is a "shields up" (block everything
 // incoming) filter.
 func (f *Filter) ShieldsUp() bool { return f.shieldsUp }
diff --git a/wgengine/filter/filter_test.go b/wgengine/filter/filter_test.go
index 180b1de2e..00826714b 100644
--- a/wgengine/filter/filter_test.go
+++ b/wgengine/filter/filter_test.go
@@ -872,3 +872,83 @@ func TestMatchesMatchProtoAndIPsOnlyIfAllPorts(t *testing.T) {
 		})
 	}
 }
+
+func TestCaps(t *testing.T) {
+	mm, err := MatchesFromFilterRules([]tailcfg.FilterRule{
+		{
+			SrcIPs: []string{"*"},
+			CapGrant: []tailcfg.CapGrant{{
+				Dsts: []netaddr.IPPrefix{
+					netaddr.MustParseIPPrefix("0.0.0.0/0"),
+				},
+				Caps: []string{"is_ipv4"},
+			}},
+		},
+		{
+			SrcIPs: []string{"*"},
+			CapGrant: []tailcfg.CapGrant{{
+				Dsts: []netaddr.IPPrefix{
+					netaddr.MustParseIPPrefix("::/0"),
+				},
+				Caps: []string{"is_ipv6"},
+			}},
+		},
+		{
+			SrcIPs: []string{"100.199.0.0/16"},
+			CapGrant: []tailcfg.CapGrant{{
+				Dsts: []netaddr.IPPrefix{
+					netaddr.MustParseIPPrefix("100.200.0.0/16"),
+				},
+				Caps: []string{"some_super_admin"},
+			}},
+		},
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+	filt := New(mm, nil, nil, nil, t.Logf)
+	tests := []struct {
+		name     string
+		src, dst string // IP
+		want     []string
+	}{
+		{
+			name: "v4",
+			src:  "1.2.3.4",
+			dst:  "2.4.5.5",
+			want: []string{"is_ipv4"},
+		},
+		{
+			name: "v6",
+			src:  "1::1",
+			dst:  "2::2",
+			want: []string{"is_ipv6"},
+		},
+		{
+			name: "admin",
+			src:  "100.199.1.2",
+			dst:  "100.200.3.4",
+			want: []string{"is_ipv4", "some_super_admin"},
+		},
+		{
+			name: "not_admin_bad_src",
+			src:  "100.198.1.2", // 198, not 199
+			dst:  "100.200.3.4",
+			want: []string{"is_ipv4"},
+		},
+		{
+			name: "not_admin_bad_dst",
+			src:  "100.199.1.2",
+			dst:  "100.201.3.4", // 201, not 200
+			want: []string{"is_ipv4"},
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			got := filt.AppendCaps(nil, netaddr.MustParseIP(tt.src), netaddr.MustParseIP(tt.dst))
+			if !reflect.DeepEqual(got, tt.want) {
+				t.Errorf("got %q; want %q", got, tt.want)
+			}
+		})
+	}
+}
diff --git a/wgengine/filter/match.go b/wgengine/filter/match.go
index dae60870e..6b27f94f2 100644
--- a/wgengine/filter/match.go
+++ b/wgengine/filter/match.go
@@ -47,12 +47,24 @@ func (npr NetPortRange) String() string {
 	return fmt.Sprintf("%v:%v", npr.Net, npr.Ports)
 }
 
+// CapMatch is a capability grant match predicate.
+type CapMatch struct {
+	// Dst is the IP prefix that the destination IP address matches against
+	// to get the capability.
+	Dst netaddr.IPPrefix
+
+	// Cap is the capability that's granted if the destination IP addresses
+	// matches Dst.
+	Cap string
+}
+
 // Match matches packets from any IP address in Srcs to any ip:port in
 // Dsts.
 type Match struct {
 	IPProto []ipproto.Proto // required set (no default value at this layer)
-	Dsts    []NetPortRange
 	Srcs    []netaddr.IPPrefix
+	Dsts    []NetPortRange // optional, if Srcs match
+	Caps    []CapMatch     // optional, if Srcs match
 }
 
 func (m Match) String() string {
diff --git a/wgengine/filter/match_clone.go b/wgengine/filter/match_clone.go
index f47761264..8b77e6fa7 100644
--- a/wgengine/filter/match_clone.go
+++ b/wgengine/filter/match_clone.go
@@ -21,14 +21,16 @@ func (src *Match) Clone() *Match {
 	dst := new(Match)
 	*dst = *src
 	dst.IPProto = append(src.IPProto[:0:0], src.IPProto...)
-	dst.Dsts = append(src.Dsts[:0:0], src.Dsts...)
 	dst.Srcs = append(src.Srcs[:0:0], src.Srcs...)
+	dst.Dsts = append(src.Dsts[:0:0], src.Dsts...)
+	dst.Caps = append(src.Caps[:0:0], src.Caps...)
 	return dst
 }
 
 // A compilation failure here means this code must be regenerated, with the command at the top of this file.
 var _MatchCloneNeedsRegeneration = Match(struct {
 	IPProto []ipproto.Proto
-	Dsts    []NetPortRange
 	Srcs    []netaddr.IPPrefix
+	Dsts    []NetPortRange
+	Caps    []CapMatch
 }{})
diff --git a/wgengine/filter/tailcfg.go b/wgengine/filter/tailcfg.go
index 8f52ff1fa..799fbf21a 100644
--- a/wgengine/filter/tailcfg.go
+++ b/wgengine/filter/tailcfg.go
@@ -70,6 +70,16 @@ func MatchesFromFilterRules(pf []tailcfg.FilterRule) ([]Match, error) {
 				})
 			}
 		}
+		for _, cm := range r.CapGrant {
+			for _, dstNet := range cm.Dsts {
+				for _, cap := range cm.Caps {
+					m.Caps = append(m.Caps, CapMatch{
+						Dst: dstNet,
+						Cap: cap,
+					})
+				}
+			}
+		}
 
 		mm = append(mm, m)
 	}