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

[Vladislav Grishenko]

Hi, have sent here similar patch in August,
http://lists.busybox.net/pipermail/busybox/2013-August/079727.html

> We've discovered a number of dhcp servers and relays check minimal dhcp
> packet size and drops anything less than 300 octets long as per RFC 1542.
> After commit "b8b72f02 dhcp: truncate packets instead of padding them to
574
> bytes. closes bug 1849" dhcpc/dhcpd could generate packets with size less
> than 300 bytes (bootp header + options), and under the circumstances above
> they all will be dropped by that servers/firewalls/relays.
> Refer patch against master attached.

From: Vladislav Grishenko <themiron at mail.ru>
Date: Fri, 30 Aug 2013 13:25:17 +0600
Subject: [PATCH] dhcp: keep padding packets to 300 bytes as per RFC 1542

function                                             old     new   delta
udhcp_send_kernel_packet                             251     272     +21
udhcp_send_raw_packet                                445     448      +3

Signed-off-by: Vladislav Grishenko <themiron at mail.ru>
---
 networking/udhcp/common.h |    3 +++
 networking/udhcp/packet.c |    2 ++
 2 files changed, 5 insertions(+), 0 deletions(-)

diff --git a/networking/udhcp/common.h b/networking/udhcp/common.h
index 0e8e45f..bc44b84 100644
--- a/networking/udhcp/common.h
+++ b/networking/udhcp/common.h
@@ -25,6 +25,9 @@ extern const uint8_t MAC_BCAST_ADDR[6] ALIGN2; /* six
all-ones */
 #define BOOTREQUEST             1
 #define BOOTREPLY               2
 
+/* Minimal BOOTP message size. See RFC 1542 */
+#define DHCP_MIN_SIZE           300
+
 //TODO: rename ciaddr/yiaddr/chaddr
 struct dhcp_packet {
 	uint8_t op;      /* BOOTREQUEST or BOOTREPLY */
diff --git a/networking/udhcp/packet.c b/networking/udhcp/packet.c
index 33c9585..277887c 100644
--- a/networking/udhcp/packet.c
+++ b/networking/udhcp/packet.c
@@ -145,6 +145,7 @@ int FAST_FUNC udhcp_send_raw_packet(struct dhcp_packet
*dhcp_pkt,
 	 * we truncate packets after end option byte.
 	 */
 	padding = DHCP_OPTIONS_BUFSIZE - 1 -
udhcp_end_option(packet.data.options);
+	padding = MIN(DHCP_SIZE - DHCP_MIN_SIZE, padding);
 
 	packet.ip.protocol = IPPROTO_UDP;
 	packet.ip.saddr = source_nip;
@@ -215,6 +216,7 @@ 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);
+	padding = MIN(DHCP_SIZE - DHCP_MIN_SIZE, padding);
 	result = safe_write(fd, dhcp_pkt, DHCP_SIZE - padding);
 	msg = "write";
  ret_close:
-- 
1.7.2.5

Best Regards, Vladislav Grishenko

> -----Original Message-----
> From: busybox-bounces at busybox.net [mailto:busybox-
> bounces at busybox.net] On Behalf Of Johannes Stezenbach
> Sent: Wednesday, October 23, 2013 4:21 PM
> To: busybox at busybox.net
> Subject: [PATCH RFC] udhcpc: pad packets to work around broken routers
> 
> 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:
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