From f606e70250eb2a30842ee7a98bb2d28e923bd530 Mon Sep 17 00:00:00 2001
From: Michael Stapelberg <stapelberg@google.com>
Date: Sun, 7 Oct 2018 17:36:00 +0200
Subject: [PATCH] dhcp4d: respect broadcast bit
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
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Mac’s Internet Recovery DHCP client requires broadcast responses and will ignore
unicast responses outright.
---
 internal/dhcp4d/dhcp4d.go | 12 +++++++++---
 1 file changed, 9 insertions(+), 3 deletions(-)

diff --git a/internal/dhcp4d/dhcp4d.go b/internal/dhcp4d/dhcp4d.go
index 857d1ce..0cba287 100644
--- a/internal/dhcp4d/dhcp4d.go
+++ b/internal/dhcp4d/dhcp4d.go
@@ -163,8 +163,14 @@ func (h *Handler) ServeDHCP(p dhcp4.Packet, msgType dhcp4.MessageType, options d
 		ComputeChecksums: true,
 		FixLengths:       true,
 	}
+	destMAC := p.CHAddr()
+	destIP := reply.YIAddr()
+	if p.Broadcast() {
+		destMAC = []byte{0xff, 0xff, 0xff, 0xff, 0xff, 0xff}
+		destIP = net.IPv4bcast
+	}
 	ethernet := &layers.Ethernet{
-		DstMAC:       p.CHAddr(),
+		DstMAC:       destMAC,
 		SrcMAC:       h.iface.HardwareAddr,
 		EthernetType: layers.EthernetTypeIPv4,
 	}
@@ -173,7 +179,7 @@ func (h *Handler) ServeDHCP(p dhcp4.Packet, msgType dhcp4.MessageType, options d
 		Version:  4,
 		TTL:      255,
 		SrcIP:    h.serverIP,
-		DstIP:    reply.YIAddr(),
+		DstIP:    destIP,
 		Protocol: layers.IPProtocolUDP,
 		Flags:    layers.IPv4DontFragment,
 	}
@@ -188,7 +194,7 @@ func (h *Handler) ServeDHCP(p dhcp4.Packet, msgType dhcp4.MessageType, options d
 		udp,
 		gopacket.Payload(reply))
 
-	if _, err := h.rawConn.WriteTo(buf.Bytes(), &raw.Addr{p.CHAddr()}); err != nil {
+	if _, err := h.rawConn.WriteTo(buf.Bytes(), &raw.Addr{destMAC}); err != nil {
 		log.Printf("WriteTo: %v", err)
 	}