openwrt-xburst/package/busybox/patches/241-udhcpc-oversized_packets.patch

--- a/networking/udhcp/packet.c
+++ b/networking/udhcp/packet.c
@@ -114,6 +114,10 @@
 	return ~sum;
 }
 
+int udhcp_get_payload_len(struct dhcpMessage *payload)
+{
+	return sizeof(struct dhcpMessage) - DHCP_OPTIONS_BUFSIZE + end_option(payload->options) + sizeof(payload->options[0]);
+}
 
 /* Construct a ip/udp header for a packet, send packet */
 int udhcp_send_raw_packet(struct dhcpMessage *payload,
@@ -125,11 +129,7 @@
 	int fd;
 	int result = -1;
 	const char *msg;
-
-	enum {
-		IP_UPD_DHCP_SIZE = sizeof(struct udp_dhcp_packet) - CONFIG_UDHCPC_SLACK_FOR_BUGGY_SERVERS,
-		UPD_DHCP_SIZE    = IP_UPD_DHCP_SIZE - offsetof(struct udp_dhcp_packet, udp),
-	};
+	int p_len = udhcp_get_payload_len(payload);
 
 	fd = socket(PF_PACKET, SOCK_DGRAM, htons(ETH_P_IP));
 	if (fd < 0) {
@@ -139,7 +139,7 @@
 
 	memset(&dest, 0, sizeof(dest));
 	memset(&packet, 0, sizeof(packet));
-	packet.data = *payload; /* struct copy */
+	memcpy(&(packet.data), payload, p_len);
 
 	dest.sll_family = AF_PACKET;
 	dest.sll_protocol = htons(ETH_P_IP);
@@ -156,23 +156,18 @@
 	packet.ip.daddr = dest_ip;
 	packet.udp.source = htons(source_port);
 	packet.udp.dest = htons(dest_port);
-	/* size, excluding IP header: */
-	packet.udp.len = htons(UPD_DHCP_SIZE);
-	/* for UDP checksumming, ip.len is set to UDP packet len */
+	p_len += sizeof(packet.udp);
+	packet.udp.len = htons(p_len);
 	packet.ip.tot_len = packet.udp.len;
-	packet.udp.check = udhcp_checksum(&packet, IP_UPD_DHCP_SIZE);
-	/* but for sending, it is set to IP packet len */
-	packet.ip.tot_len = htons(IP_UPD_DHCP_SIZE);
+	p_len += sizeof(packet.ip);
+	packet.udp.check = udhcp_checksum(&packet, p_len);
+	packet.ip.tot_len = htons(p_len);
 	packet.ip.ihl = sizeof(packet.ip) >> 2;
 	packet.ip.version = IPVERSION;
 	packet.ip.ttl = IPDEFTTL;
 	packet.ip.check = udhcp_checksum(&packet.ip, sizeof(packet.ip));
 
-	/* Currently we send full-sized DHCP packets (zero padded).
-	 * If you need to change this: last byte of the packet is
-	 * packet.data.options[end_option(packet.data.options)]
-	 */
-	result = sendto(fd, &packet, IP_UPD_DHCP_SIZE, 0,
+	result = sendto(fd, &packet, p_len, 0,
 				(struct sockaddr *) &dest, sizeof(dest));
 	msg = "sendto";
  ret_close:
@@ -224,8 +219,7 @@
 		goto ret_close;
 	}
 
-	/* Currently we send full-sized DHCP packets (see above) */
-	result = safe_write(fd, payload, DHCP_SIZE);
+	result = safe_write(fd, payload, udhcp_get_payload_len(payload));
 	msg = "write";
  ret_close:
 	close(fd);