posture: add network hardware addresses to posture identity

If an optional `hwaddrs` URL parameter is present, add network interface
hardware addresses to the posture identity response.

Just like with serial numbers, this requires client opt-in via MDM or
`tailscale set --posture-checking=true`
(https://tailscale.com/kb/1326/device-identity)

Updates tailscale/corp#21371

Signed-off-by: Anton Tolchanov <anton@tailscale.com>

ipn/ipnlocal/c2n.go

@@ -318,7 +318,7 @@ func handleC2NPostureIdentityGet(b *LocalBackend, w http.ResponseWriter, r *http
 
 	res := tailcfg.C2NPostureIdentityResponse{}
 
-	// Only collect serial numbers if enabled on the client,
+	// Only collect posture identity if enabled on the client,
 	// this will first check syspolicy, MDM settings like Registry
 	// on Windows or defaults on macOS. If they are not set, it falls
 	// back to the cli-flag, `--posture-checking`.
@@ -337,8 +337,17 @@ func handleC2NPostureIdentityGet(b *LocalBackend, w http.ResponseWriter, r *http
 			http.Error(w, err.Error(), http.StatusInternalServerError)
 			return
 		}
-
 		res.SerialNumbers = sns
+
+		// TODO(tailscale/corp#21371, 2024-07-10): once this has landed in a stable release
+		// and looks good in client metrics, remove this parameter and always report MAC
+		// addresses.
+		if r.FormValue("hwaddrs") == "true" {
+			res.IfaceHardwareAddrs, err = posture.GetHardwareAddrs()
+			if err != nil {
+				b.logf("c2n: GetHardwareAddrs returned error: %v", err)
+			}
+		}
 	} else {
 		res.PostureDisabled = true
 	}

posture/hwaddr.go

@@ -0,0 +1,26 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+package posture
+
+import (
+	"net/netip"
+	"slices"
+
+	"tailscale.com/net/netmon"
+)
+
+// GetHardwareAddrs returns the hardware addresses of all non-loopback
+// network interfaces.
+func GetHardwareAddrs() (hwaddrs []string, err error) {
+	err = netmon.ForeachInterface(func(i netmon.Interface, _ []netip.Prefix) {
+		if i.IsLoopback() {
+			return
+		}
+		if a := i.HardwareAddr.String(); a != "" {
+			hwaddrs = append(hwaddrs, a)
+		}
+	})
+	slices.Sort(hwaddrs)
+	return
+}

tailcfg/c2ntypes.go

@@ -55,13 +55,17 @@ type C2NUpdateResponse struct {
 	Started bool
 }
 
-// C2NPostureIdentityResponse contains either a set of identifying serial number
-// from the client or a boolean indicating that the machine has opted out of
-// posture collection.
+// C2NPostureIdentityResponse contains either a set of identifying serial
+// numbers and hardware addresses from the client, or a boolean flag
+// indicating that the machine has opted out of posture collection.
 type C2NPostureIdentityResponse struct {
 	// SerialNumbers is a list of serial numbers of the client machine.
 	SerialNumbers []string `json:",omitempty"`
 
+	// IfaceHardwareAddrs is a list of hardware addresses (MAC addresses)
+	// of the client machine's network interfaces.
+	IfaceHardwareAddrs []string `json:",omitempty"`
+
 	// PostureDisabled indicates if the machine has opted out of
 	// device posture collection.
 	PostureDisabled bool `json:",omitempty"`