udhcp: bind to device even for ucast packets

There are cases where binding to source IP and
destination IP is insufficient to guarantee sane
xmit netdev.

One case where this can fail is when
route-matching netdev carrier is down (cable
unplugged, wifi disconnected), or the netdev is
admin down. Then all the IP based bindings (bind()
+ connect()) will seemingly succeed but the actual
packet can go out through a default gw path.

Depending on the network this happens on
it can create issues or false alarms. It can
also leak some subnet info across networks that
shouldn't be routed.

As such better be safe than sorry and bind to a
netdev to be sure it's used for xmit.

function                                             old     new   delta
udhcp_send_kernel_packet                             293     336     +43
send_packet                                          182     188      +6
bcast_or_ucast                                        37      43      +6
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 3/0 up/down: 55/0)               Total: 55 bytes

Signed-off-by: Michal Kazior <michal@plume.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

1 files changed, 2 insertions, 1 deletions

diff --git a/networking/udhcp/dhcpc.c b/networking/udhcp/dhcpc.c
index 66aa38c20..98720b45b 100644
--- a/networking/udhcp/dhcpc.c
+++ b/networking/udhcp/dhcpc.c
@@ -702,7 +702,8 @@ static int bcast_or_ucast(struct dhcp_packet *packet, uint32_t ciaddr, uint32_t
         if (server)
                 return udhcp_send_kernel_packet(packet,
                         ciaddr, CLIENT_PORT,
-                        server, SERVER_PORT);
+                        server, SERVER_PORT,
+                        client_data.interface);
         return raw_bcast_from_client_data_ifindex(packet, ciaddr);
 }