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List:       busybox
Subject:    dhcp: keep padding packets to 300 bytes as per RFC 1542
From:       "Vladislav Grishenko" <themiron () mail ! ru>
Date:       2013-08-30 7:59:25
Message-ID: 001a01cea556$d9b79260$8d26b720$ () mail ! ru
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Hi Denys,

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.

Best Regards, Vladislav Grishenko


["0001-dhcp-keep-padding-packets-to-300-bytes-as-per-RFC-15.patch" (application/octet-stream)]

From c4764a19cb153dc0495c50d76606bc12e54f1185 Mon Sep 17 00:00:00 2001
From: Vladislav Grishenko <themiron@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@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



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