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.

src/dhcp.c

@@ -376,10 +376,9 @@ void dhcp_packet(time_t now, int pxe_fd)
 	}
     } 
 #if defined(HAVE_LINUX_NETWORK)
-  else if ((ntohs(mess->flags) & 0x8000) || mess->hlen == 0 ||
-	   mess->hlen > sizeof(ifr.ifr_addr.sa_data) || mess->htype == 0)
+  else
     {
-      /* broadcast to 255.255.255.255 (or mac address invalid) */
+      /* fill cmsg for outbound interface (both broadcast & unicast) */
       struct in_pktinfo *pkt;
       msg.msg_control = control_u.control;
       msg.msg_controllen = sizeof(control_u);
@@ -389,23 +388,29 @@ void dhcp_packet(time_t now, int pxe_fd)
       pkt->ipi_spec_dst.s_addr = 0;
       msg.msg_controllen = cmptr->cmsg_len = CMSG_LEN(sizeof(struct in_pktinfo));
       cmptr->cmsg_level = IPPROTO_IP;
-      cmptr->cmsg_type = IP_PKTINFO;  
-      dest.sin_addr.s_addr = INADDR_BROADCAST;
-      dest.sin_port = htons(daemon->dhcp_client_port);
-    }
-  else
-    {
-      /* unicast to unconfigured client. Inject mac address direct into ARP cache. 
-	 struct sockaddr limits size to 14 bytes. */
-      dest.sin_addr = mess->yiaddr;
-      dest.sin_port = htons(daemon->dhcp_client_port);
-      memcpy(&arp_req.arp_pa, &dest, sizeof(struct sockaddr_in));
-      arp_req.arp_ha.sa_family = mess->htype;
-      memcpy(arp_req.arp_ha.sa_data, mess->chaddr, mess->hlen);
-      /* interface name already copied in */
-      arp_req.arp_flags = ATF_COM;
-      if (ioctl(daemon->dhcpfd, SIOCSARP, &arp_req) == -1)
-	my_syslog(MS_DHCP | LOG_ERR, _("ARP-cache injection failed: %s"), strerror(errno));
+      cmptr->cmsg_type = IP_PKTINFO;
+
+      if ((ntohs(mess->flags) & 0x8000) || mess->hlen == 0 ||
+         mess->hlen > sizeof(ifr.ifr_addr.sa_data) || mess->htype == 0)
+        {
+          /* broadcast to 255.255.255.255 (or mac address invalid) */
+          dest.sin_addr.s_addr = INADDR_BROADCAST;
+          dest.sin_port = htons(daemon->dhcp_client_port);
+        }
+      else
+        {
+          /* unicast to unconfigured client. Inject mac address direct into ARP cache.
+          struct sockaddr limits size to 14 bytes. */
+          dest.sin_addr = mess->yiaddr;
+          dest.sin_port = htons(daemon->dhcp_client_port);
+          memcpy(&arp_req.arp_pa, &dest, sizeof(struct sockaddr_in));
+          arp_req.arp_ha.sa_family = mess->htype;
+          memcpy(arp_req.arp_ha.sa_data, mess->chaddr, mess->hlen);
+          /* interface name already copied in */
+          arp_req.arp_flags = ATF_COM;
+          if (ioctl(daemon->dhcpfd, SIOCSARP, &arp_req) == -1)
+            my_syslog(MS_DHCP | LOG_ERR, _("ARP-cache injection failed: %s"), strerror(errno));
+        }
     }
 #elif defined(HAVE_SOLARIS_NETWORK)
   else if ((ntohs(mess->flags) & 0x8000) || mess->hlen != ETHER_ADDR_LEN || mess->htype != ARPHRD_ETHER)