udhcpc: make DHCP packets to have at least 300 DHCP bytes

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@sig21.net>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

diff --git a/networking/udhcp/packet.c b/networking/udhcp/packet.c
index 33c9585cfc6cebb7830927fa48f2824ebb474aca..148f52551a129e179ea8fe031ddb36a81eaa09a2 100644 (file)
--- 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: