Set src IP on more situations.

If the admin set "ipaddr = *", and no "src_ipaddr = ...", then
we used to send packets from 0.0.0.0.  That's wrong.

We now look for server identifier or server IP address.
If one of those is found, we use that as the source IP

src/modules/proto_dhcp/dhcpd.c

@@ -427,12 +427,25 @@ static int dhcp_process(REQUEST *request)
 	vp->vp_integer = 2; /* BOOTREPLY */
 
 	/*
-	 * Prepare the reply packet for sending through dhcp_socket_send()
+	 *	Prepare the reply packet for sending through dhcp_socket_send()
 	 */
 	request->reply->dst_ipaddr.af = AF_INET;
 	request->reply->src_ipaddr.af = AF_INET;
 	request->reply->src_ipaddr.ipaddr.ip4addr.s_addr = sock->src_ipaddr.ipaddr.ip4addr.s_addr;
 
+	/*
+	 *	They didn't set a proper src_ipaddr, but we want to
+	 *	send the packet with a source IP.  If there's a server
+	 *	identifier, use it.
+	 */
+	if (request->reply->src_ipaddr.ipaddr.ip4addr.s_addr == INADDR_ANY) {
+		vp = pairfind(request->reply->vps, 265, DHCP_MAGIC_VENDOR, TAG_ANY); /* DHCP-Server-IP-Address */
+		if (!vp) vp = pairfind(request->reply->vps, 54, DHCP_MAGIC_VENDOR, TAG_ANY); /* DHCP-DHCP-Server-Identifier */
+		if (vp) {
+			request->reply->src_ipaddr.ipaddr.ip4addr.s_addr = vp->vp_ipaddr;
+		}
+	}
+
 	request->reply->dst_port = request->packet->src_port;
 	request->reply->src_port = request->packet->dst_port;
 
@@ -443,7 +456,7 @@ static int dhcp_process(REQUEST *request)
 	 *	packet to the client.  i.e. the relay may have a
 	 *	public IP, but the gateway a private one.
 	 */
-	vp = pairfind(request->reply->vps, 272, DHCP_MAGIC_VENDOR, TAG_ANY); /* DHCP-Relay-IP-Address */
+
 	if (vp) {
 		RDEBUG("DHCP: Reply will be unicast to giaddr from original packet");
 		request->reply->dst_ipaddr.ipaddr.ip4addr.s_addr = vp->vp_ipaddr;