[git commit] udhcpc: make DHCP packets to have at least 300 DHCP bytes

[Denys Vlasenko]

commit: http://git.busybox.net/busybox/commit/?id=2d576e2a220e28fe815ea02c38c9bfe9195406c2
branch: http://git.busybox.net/busybox/commit/?id=refs/heads/master

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.

function                                             old     new   delta
udhcp_send_kernel_packet                             268     292     +24
udhcp_send_raw_packet                                462     473     +11
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 2/0 up/down: 35/0)               Total: 35 bytes

Signed-off-by: Johannes Stezenbach <js at sig21.net>
Signed-off-by: Denys Vlasenko <vda.linux at googlemail.com>
---
 networking/udhcp/packet.c |    9 +++++++++
 1 files changed, 9 insertions(+), 0 deletions(-)

diff --git a/networking/udhcp/packet.c b/networking/udhcp/packet.c
index 33c9585..148f525 100644
--- a/networking/udhcp/packet.c
+++ b/networking/udhcp/packet.c
@@ -143,8 +143,15 @@ int FAST_FUNC udhcp_send_raw_packet(struct dhcp_packet *dhcp_pkt,
 	 *
 	 * In order to work with those buggy servers,
 	 * we truncate packets after end option byte.
+	 *
+	 * However, RFC 1542 says "The IP Total Length and UDP Length
+	 * must be large enough to contain the minimal BOOTP header of 300 octets".
+	 * Thus, we retain enough padding to not go below 300 BOOTP bytes.
+	 * Some devices have filters which drop DHCP packets shorter than that.
 	 */
 	padding = DHCP_OPTIONS_BUFSIZE - 1 - udhcp_end_option(packet.data.options);
+	if (padding > DHCP_SIZE - 300)
+		padding = DHCP_SIZE - 300;
 
 	packet.ip.protocol = IPPROTO_UDP;
 	packet.ip.saddr = source_nip;
@@ -215,6 +222,8 @@ 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);
+	if (padding > DHCP_SIZE - 300)
+		padding = DHCP_SIZE - 300;
 	result = safe_write(fd, dhcp_pkt, DHCP_SIZE - padding);
 	msg = "write";
  ret_close: