Pull request: 3289 freebsd dhcp

Merge in DNS/adguard-home from 3289-freebsd-dhcp to master

Updates #3289.

Squashed commit of the following:

commit 1365d8f17293da611b860525d519a7bbd7851902
Author: Eugene Burkov <e.burkov@adguard.com>
Date:   Fri Jul 30 15:01:13 2021 +0300

    dhcpd: fix doc

commit 26724df27e92d457c39c8bf0fb78179a874e3fb2
Author: Eugene Burkov <e.burkov@adguard.com>
Date:   Fri Jul 30 14:52:58 2021 +0300

    all: imp code & docs

commit 9a9574a885d3d2129ef54fefb9a56857ce060cff
Author: Eugene Burkov <e.burkov@adguard.com>
Date:   Thu Jul 29 15:51:07 2021 +0300

    all: fix broadcasting, sup freebsd dhcp, fix http response

internal/dhcpd/http.go

@@ -412,7 +412,9 @@ func (s *Server) handleDHCPFindActiveServer(w http.ResponseWriter, r *http.Reque
 	result := dhcpSearchResult{
 		V4: dhcpSearchV4Result{
 			OtherServer: dhcpSearchOtherResult{},
-			StaticIP:    dhcpStaticIPStatus{},
+			StaticIP: dhcpStaticIPStatus{
+				Static: "yes",
+			},
 		},
 		V6: dhcpSearchV6Result{
 			OtherServer: dhcpSearchOtherResult{},

internal/dhcpd/server.go

@@ -38,6 +38,9 @@ type V4ServerConf struct {
 
 	GatewayIP  net.IP `yaml:"gateway_ip" json:"gateway_ip"`
 	SubnetMask net.IP `yaml:"subnet_mask" json:"subnet_mask"`
+	// broadcastIP is the broadcasting address pre-calculated from the
+	// configured gateway IP and subnet mask.
+	broadcastIP net.IP
 
 	// The first & the last IP address for dynamic leases
 	// Bytes [0..2] of the last allowed IP address must match the first IP

internal/dhcpd/v4.go

@@ -927,12 +927,30 @@ func (s *v4Server) packetHandler(conn net.PacketConn, peer net.Addr, req *dhcpv4
 		resp.Options.Update(dhcpv4.OptMessageType(dhcpv4.MessageTypeNak))
 	}
 
+	// peer is expected to be of type *net.UDPConn as the server4.NewServer
+	// initializes it.
+	udpPeer, ok := peer.(*net.UDPAddr)
+	if !ok {
+		log.Error("dhcpv4: peer is of unexpected type %T", peer)
+
+		return
+	}
+
+	// Despite the fact that server4.NewIPv4UDPConn explicitly sets socket
+	// options to allow broadcasting, it also binds the connection to a
+	// specific interface.  On FreeBSD conn.WriteTo causes errors while
+	// writing to the addresses that belong to another interface.  So, use
+	// the broadcast address specific for the binded interface in case
+	// server4.Server.Serve sets it to net.IPv4Bcast.
+	if udpPeer.IP.Equal(net.IPv4bcast) {
+		udpPeer.IP = s.conf.broadcastIP
+	}
+
 	log.Debug("dhcpv4: sending: %s", resp.Summary())
 
 	_, err = conn.WriteTo(resp.ToBytes(), peer)
 	if err != nil {
 		log.Error("dhcpv4: conn.Write to %s failed: %s", peer, err)
-		return
 	}
 }
 
@@ -1043,6 +1061,12 @@ func v4Create(conf V4ServerConf) (srv DHCPServer, err error) {
 		Mask: subnetMask,
 	}
 
+	bcastIP := aghnet.CloneIP(routerIP)
+	for i, b := range subnetMask {
+		bcastIP[i] |= ^b
+	}
+	s.conf.broadcastIP = bcastIP
+
 	s.conf.ipRange, err = newIPRange(conf.RangeStart, conf.RangeEnd)
 	if err != nil {
 		return s, fmt.Errorf("dhcpv4: %w", err)