commit 65c721200023ef0023114459a8d12f8b0a24cfd8
author Lung-Pin Chang <changlp@cs.nctu.edu.tw> Thu Mar 19 23:22:21 2015 +0000
committer Simon Kelley <simon@thekelleys.org.uk> Thu Mar 19 23:22:21 2015 +0000
tree e4dbdcd3762c014c763c8e70eb84574b300c4acc
parent 979fe86bc8693f660eddea232ae39cbbb50b294c

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

diff --git a/src/dhcp.c b/src/dhcp.c
index 5c3089a..f1f43f8 100644
--- a/src/dhcp.c
+++ b/src/dhcp.c
@@ -376,10 +376,9 @@
 	}
     } 
 #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 @@
       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)