net/netmon: add support for `$TS_AVOID_PREFIX`

In addition to the previous commit, add a knob to selectively disallow
Tailscale to use specific addresses as endpoints.

Fixes #1552.

Signed-off-by: Ivan Shapovalov <intelfx@intelfx.name>

net/netmon/state.go

@@ -37,6 +37,10 @@ var avoidInterfaces = envknob.RegisterString("TS_AVOID_INTERFACES")
 // consider when gathering endpoints.
 var onlyInterfaces = envknob.RegisterString("TS_ONLY_INTERFACES")
 
+// In the same vein, avoidPrefix is a debug/power-user knob to exclude any addresses within specific
+// prefixes from being used as endpoints. This is a more granular version of $TS_AVOID_INTERFACES.
+var avoidPrefix = envknob.RegisterString("TS_AVOID_PREFIX")
+
 // LoginEndpointForProxyDetermination is the URL used for testing
 // which HTTP proxy the system should use.
 var LoginEndpointForProxyDetermination = "https://controlplane.tailscale.com/"
@@ -58,6 +62,25 @@ func matchInterfaceName(name string, patterns string) bool {
 	return false
 }
 
+func matchIP(ip netip.Addr, prefixes string) bool {
+	prefixList := strings.Split(prefixes, ",")
+	for _, arg := range prefixList {
+		arg = strings.TrimSpace(arg)
+		if arg == "" {
+			continue
+		}
+		prefix, err := netip.ParsePrefix(arg)
+		if err != nil {
+			continue
+		}
+		if prefix.Contains(ip) {
+			return true
+		}
+	}
+	return false
+
+}
+
 func isProblematicInterface(nif *net.Interface) bool {
 	name := nif.Name
 	// Don't try to send disco/etc packets over zerotier; they effectively
@@ -81,6 +104,13 @@ func isAllowedInterface(nif *net.Interface) bool {
 	return true
 }
 
+func isAllowedAddress(ip netip.Addr) bool {
+	if avoidPrefix() != "" && matchIP(ip, avoidPrefix()) {
+		return false
+	}
+	return true
+}
+
 // LocalAddresses returns the machine's IP addresses, separated by
 // whether they're loopback addresses. If there are no regular addresses
 // it will return any IPv4 linklocal or IPv6 unique local addresses because we
@@ -119,6 +149,10 @@ func LocalAddresses() (regular, loopback []netip.Addr, err error) {
 					continue
 				}
 				ip = ip.Unmap()
+				if !isAllowedAddress(ip) {
+					// Skip addresses that the user does not want to use with Tailscale.
+					continue
+				}
 				// TODO(apenwarr): don't special case cgNAT.
 				// In the general wireguard case, it might
 				// very well be something we can route to