[PATCH RFC] udhcpc: pad packets to work around broken routers

[Johannes Stezenbach]

Commit b8b72f02 removed all padding from DHCP packets
to fix operation with buggy servers which can't handle
maximum sized packets.  But it introduced a regression
with buggy routers which drop DHCP packets smaller
than 300 bytes (i.e. 342 byte ethernet packets).
Add back some padding to work around.

Signed-off-by: Johannes Stezenbach <js at sig21.net>
---
This is the minimal patch which fixes the issue, but
I'm not sure about hardcoding the 300 byte size.
Maybe add a commandline parameter?


diff --git a/networking/udhcp/packet.c b/networking/udhcp/packet.c
index 33c9585..efee970 100644
--- a/networking/udhcp/packet.c
+++ b/networking/udhcp/packet.c
@@ -146,6 +146,12 @@ int FAST_FUNC udhcp_send_raw_packet(struct dhcp_packet *dhcp_pkt,
 	 */
 	padding = DHCP_OPTIONS_BUFSIZE - 1 - udhcp_end_option(packet.data.options);
 
+	/* work around issue with some routers which drop DHCP packets
+	 * smaller than 300 bytes (342 byte eth packet)
+	 */
+	if (DHCP_SIZE - padding < 300)
+		padding = DHCP_SIZE - 300;
+
 	packet.ip.protocol = IPPROTO_UDP;
 	packet.ip.saddr = source_nip;
 	packet.ip.daddr = dest_nip;
@@ -215,6 +221,13 @@ int FAST_FUNC udhcp_send_kernel_packet(struct dhcp_packet *dhcp_pkt,
 
 	udhcp_dump_packet(dhcp_pkt);
 	padding = DHCP_OPTIONS_BUFSIZE - 1 - udhcp_end_option(dhcp_pkt->options);
+
+	/* work around issue with some routers which drop DHCP packets
+	 * smaller than 300 bytes (342 byte eth packet)
+	 */
+	if (DHCP_SIZE - padding < 300)
+		padding = DHCP_SIZE - 300;
+
 	result = safe_write(fd, dhcp_pkt, DHCP_SIZE - padding);
 	msg = "write";
  ret_close: