Pull request: 3443 dhcp broadcast

Merge in DNS/adguard-home from 3443-fix-wired-dhcp to master

Updates #3443.

Squashed commit of the following:

commit ec7c3b73e274ba7c05c5856cb2f4ea218ad46870
Author: Eugene Burkov <E.Burkov@AdGuard.COM>
Date:   Tue Sep 7 18:21:07 2021 +0300

    dhcpd: imp docs & naming

commit d87c646af44d837c81aa032e82aad846bde97bc8
Author: Eugene Burkov <E.Burkov@AdGuard.COM>
Date:   Tue Sep 7 16:56:36 2021 +0300

    dhcpd: fix build tags & log changes

commit cbd0b3c1aa0efb32aeaa9130111719ab3861e817
Author: Eugene Burkov <E.Burkov@AdGuard.COM>
Date:   Tue Sep 7 16:26:50 2021 +0300

    dhcpd: revert 3443 & imp broadcasting

internal/dhcpd/broadcast_others.go

@@ -0,0 +1,51 @@
+//go:build aix || darwin || dragonfly || linux || netbsd || solaris
+// +build aix darwin dragonfly linux netbsd solaris
+
+package dhcpd
+
+import (
+	"net"
+
+	"github.com/AdguardTeam/golibs/log"
+	"github.com/insomniacslk/dhcp/dhcpv4"
+)
+
+// broadcast sends resp to the broadcast address specific for network interface.
+func (s *v4Server) broadcast(peer net.Addr, conn net.PacketConn, resp *dhcpv4.DHCPv4) {
+	respData := resp.ToBytes()
+
+	log.Debug("dhcpv4: sending to %s: %s", peer, resp.Summary())
+
+	// This write to 0xffffffff reverts some behavior changes made in
+	// https://github.com/AdguardTeam/AdGuardHome/issues/3289.  The DHCP
+	// server should broadcast the message to 0xffffffff but it's
+	// inconsistent with the actual mental model of DHCP implementation
+	// which requires the network interface selection to bind to.
+	//
+	// See https://github.com/AdguardTeam/AdGuardHome/issues/3480 and
+	// https://github.com/AdguardTeam/AdGuardHome/issues/3366.
+	//
+	// See also https://github.com/AdguardTeam/AdGuardHome/issues/3539.
+	if _, err := conn.WriteTo(respData, peer); err != nil {
+		log.Error("dhcpv4: conn.Write to %s failed: %s", peer, err)
+	}
+
+	// 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
+	}
+
+	// Broadcast the message one more time using the interface-specific
+	// broadcast address.
+	udpPeer.IP = s.conf.broadcastIP
+
+	log.Debug("dhcpv4: sending to %s: %s", peer, resp.Summary())
+
+	if _, err := conn.WriteTo(respData, peer); err != nil {
+		log.Error("dhcpv4: conn.Write to %s failed: %s", peer, err)
+	}
+}