Fix for DHCP in transmission interface when --bridge-interface in use.

From f3d832b41f44c856003517c583fbd7af4dca722c Mon Sep 17 00:00:00 2001
From: Neil Jerram <Neil.Jerram@metaswitch.com>
Date: Fri, 8 Apr 2016 19:23:47 +0100
Subject: [PATCH] Fix DHCPv4 reply via --bridge-interface alias interface

Sending a DHCPv4 reply through a --bridge-interface alias interface
was inadvertently broken by

  commit 65c72120
  Author: Lung-Pin Chang <changlp@cs.nctu.edu.tw>
  Date:   Thu Mar 19 23:22:21 2015 +0000

      dhcp: set outbound interface via cmsg in unicast reply

        If multiple routes to the same network exist, Linux blindly picks
        the first interface (route) based on destination address, which might not be
        the one we're actually offering leases. Rather than relying on this,
        always set the interface for outgoing unicast DHCP packets.

because in the aliasing case, iface_index is changed from the index of
the interface on which the packet was received, to be the interface
index of the 'bridge' interface (where the DHCP context is expected to
be defined, and so needs to be looked up).

For the cmsg code that the cited commit added, we need the original
iface_index; so this commit saves that off before the aliasing code
can change it, as rcvd_iface_index, and then uses rcvd_iface_index
instead of iface_index for the cmsg code.

src/dhcp.c

@@ -146,6 +146,7 @@ void dhcp_packet(time_t now, int pxe_fd)
   struct iovec iov;
   ssize_t sz; 
   int iface_index = 0, unicast_dest = 0, is_inform = 0;
+  int rcvd_iface_index;
   struct in_addr iface_addr;
   struct iface_param parm;
 #ifdef HAVE_LINUX_NETWORK
@@ -230,6 +231,7 @@ void dhcp_packet(time_t now, int pxe_fd)
      --bridge-interface option), change ifr.ifr_name so that we look
      for DHCP contexts associated with the aliased interface instead
      of with the aliasing one. */
+  rcvd_iface_index = iface_index;
   for (bridge = daemon->bridges; bridge; bridge = bridge->next)
     {
       for (alias = bridge->alias; alias; alias = alias->next)
@@ -387,7 +389,7 @@ void dhcp_packet(time_t now, int pxe_fd)
       msg.msg_controllen = sizeof(control_u);
       cmptr = CMSG_FIRSTHDR(&msg);
       pkt = (struct in_pktinfo *)CMSG_DATA(cmptr);
-      pkt->ipi_ifindex = iface_index;
+      pkt->ipi_ifindex = rcvd_iface_index;
       pkt->ipi_spec_dst.s_addr = 0;
       msg.msg_controllen = cmptr->cmsg_len = CMSG_LEN(sizeof(struct in_pktinfo));
       cmptr->cmsg_level = IPPROTO_IP;