dhcpd: add default options

internal/dhcpd/options.go

@@ -138,6 +138,46 @@ func parseOpt(s string) (opt dhcpv4.Option, err error) {
 func prepareOptions(conf V4ServerConf) (opts dhcpv4.Options) {
 	opts = make(dhcpv4.Options)
 
+	// Set default values for host configuration parameters listed in
+	// Appendix A of RFC-2131.  Those parameters, if requested by client,
+	// should be returned with values defined by Host Requirements Document.
+	//
+	// See https://datatracker.ietf.org/doc/html/rfc2131#appendix-A.
+	//
+	// See also https://datatracker.ietf.org/doc/html/rfc1122,
+	// https://datatracker.ietf.org/doc/html/rfc1123, and
+	// https://datatracker.ietf.org/doc/html/rfc2132.
+	for _, opt := range []dhcpv4.Option{
+		// IP-Layer Per Host
+
+		dhcpv4.OptGeneric(dhcpv4.OptionNonLocalSourceRouting, []byte{0}),
+		// Set the current recommended default time to live for the
+		// Internet Protocol which is 64, see
+		// https://datatracker.ietf.org/doc/html/rfc1700.
+		dhcpv4.OptGeneric(dhcpv4.OptionDefaultIPTTL, []byte{64}),
+
+		// IP-Layer Per Interface
+
+		dhcpv4.OptGeneric(dhcpv4.OptionPerformMaskDiscovery, []byte{0}),
+		dhcpv4.OptGeneric(dhcpv4.OptionMaskSupplier, []byte{0}),
+		dhcpv4.OptGeneric(dhcpv4.OptionPerformRouterDiscovery, []byte{1}),
+		// The all-routers address is preferred wherever possible, see
+		// https://datatracker.ietf.org/doc/html/rfc1256#section-5.1.
+		dhcpv4.OptGeneric(dhcpv4.OptionRouterSolicitationAddress, net.IPv4allrouter),
+		dhcpv4.OptBroadcastAddress(net.IPv4bcast),
+
+		// Link-Layer Per Interface
+
+		dhcpv4.OptGeneric(dhcpv4.OptionTrailerEncapsulation, []byte{0}),
+		dhcpv4.OptGeneric(dhcpv4.OptionEthernetEncapsulation, []byte{0}),
+
+		// TCP Per Host
+		{Code: dhcpv4.OptionTCPKeepaliveInterval, Value: dhcpv4.Duration(0)},
+		dhcpv4.OptGeneric(dhcpv4.OptionTCPKeepaliveGarbage, []byte{0}),
+	} {
+		opts.Update(opt)
+	}
+
 	// Set values for options from server's configuration.
 	for _, opt := range []dhcpv4.Option{
 		dhcpv4.OptRouter(conf.subnet.IP),