[ewg] [PATCH 2/2] Improve XID generation

[sebastien dugue]

From: Matthieu Hautreux <matthieu.hautreux at cea.fr>

One major drawback of DHCP over IB is the requirement to use broadcasted
replies. The only way for a IB dhclient to find its own reply is by
looking for matching xid (transaction ID). xid are generated using
random() in dhclient. random() is initialized using
srandom(seed+cur_time). Nethertheless, when hw address hlen is 1, the
processed seed is roughly always the same. As a result, IB nodes that
uses dhclient to configure IB interfaces at the same time share the same
xid and as a result use any of the broadcasted replies. The protocol is
then broken.

 The proposed patch build a "backup seed" using all the interfaces available
on the machine and use it if it can not find a good seed for the required
interfaces during real DHCP process. If backup seed construction fails (no
interfaces providing a hw address long enough for seed construction), it try
to build a seed using gethostid().


Signed-off-by: Sebastien Dugue <sebastien.dugue at bull.net>


 dhclient.c |   64 ++++++++++++++++++++++++++++++++++++++++++++++++++-----------
 1 file changed, 53 insertions(+), 11 deletions(-)




Index: dhcp-4.1.0p1/client/dhclient.c
===================================================================
--- dhcp-4.1.0p1.orig/client/dhclient.c	2010-05-17 16:03:33.000000000 +0200
+++ dhcp-4.1.0p1/client/dhclient.c	2010-05-17 16:04:08.000000000 +0200
@@ -906,6 +906,26 @@
 		}
 	}
 
+	/* We create a backup seed before rediscovering interfaces in order to
+	   have a seed built using all of the available interfaces
+	   It's interesting if required interfaces doesn't let us defined
+	   a really unique seed due to a lack of valid HW addr later
+	   (this is the case with DHCP over IB)
+	   We only use the last device as using a sum could broke the
+	   uniqueness of the seed among multiple nodes
+	 */
+	unsigned backup_seed = 0;
+	for (ip = interfaces; ip; ip = ip -> next) {
+		int junk;
+		if ( ip -> hw_address.hlen <= sizeof seed )
+		  continue;
+		memcpy (&junk,
+			&ip -> hw_address.hbuf [ip -> hw_address.hlen -
+						sizeof seed], sizeof seed);
+		backup_seed = junk;
+	}
+
+
 	/* At this point, all the interfaces that the script thinks
 	   are relevant should be running, so now we once again call
 	   discover_interfaces(), and this time ask it to actually set
@@ -920,14 +940,36 @@
 	   Not much entropy, but we're booting, so we're not likely to
 	   find anything better. */
 	seed = 0;
+	int seed_flag = 0;
 	for (ip = interfaces; ip; ip = ip->next) {
 		int junk;
+		if ( ip -> hw_address.hlen <= sizeof seed )
+		  continue;
 		memcpy(&junk,
 		       &ip->hw_address.hbuf[ip->hw_address.hlen -
 					    sizeof seed], sizeof seed);
 		seed += junk;
+		seed_flag = 1;
 	}
-	srandom(seed + cur_time);
+	if ( seed_flag == 0 ) {
+		if ( backup_seed != 0 ) {
+		  seed = backup_seed;
+		  log_info ("xid: rand init seed (%u) built using all"
+			    " available interfaces",seed);
+		}
+		else {
+		  seed = cur_time^((unsigned) gethostid()) ;
+		  log_info ("xid: warning: no netdev with useable HWADDR found"
+			    " for seed's uniqueness enforcement");
+		  log_info ("xid: rand init seed (%u) built using gethostid",
+			    seed);
+		}
+		/* we only use seed and no current time as a broadcast reply */
+		/* will certainly be used by the hwaddrless interface */
+		srandom(seed);
+	}
+	else
+	        srandom(seed + cur_time);
 
 	/* Setup specific Infiniband options */
 	for (ip = interfaces; ip; ip = ip->next) {
@@ -1423,7 +1465,7 @@
 		return;
 	}
 
-	log_info ("DHCPACK from %s", piaddr (packet -> client_addr));
+	log_info ("DHCPACK from %s (xid=%u)", piaddr (packet -> client_addr), client -> xid);
 
 	lease = packet_to_lease (packet, client);
 	if (!lease) {
@@ -2123,7 +2165,7 @@
 		return;
 	}
 
-	log_info ("DHCPNAK from %s", piaddr (packet -> client_addr));
+	log_info ("DHCPNAK from %s (xid=%u)", piaddr (packet -> client_addr), client -> xid);
 
 	if (!client -> active) {
 #if defined (DEBUG)
@@ -2249,10 +2291,10 @@
 		client -> packet.secs = htons (65535);
 	client -> secs = client -> packet.secs;
 
-	log_info ("DHCPDISCOVER on %s to %s port %d interval %ld",
+	log_info ("DHCPDISCOVER on %s to %s port %d interval %ld (xid=%u)",
 	      client -> name ? client -> name : client -> interface -> name,
 	      inet_ntoa (sockaddr_broadcast.sin_addr),
-	      ntohs (sockaddr_broadcast.sin_port), (long)(client -> interval));
+	      ntohs (sockaddr_broadcast.sin_port), (long)(client -> interval), client -> xid);
 
 	/* Send out a packet. */
 	result = send_packet (client -> interface, (struct packet *)0,
@@ -2524,10 +2566,10 @@
 			client -> packet.secs = htons (65535);
 	}
 
-	log_info ("DHCPREQUEST on %s to %s port %d",
+	log_info ("DHCPREQUEST on %s to %s port %d (xid=%u)",
 	      client -> name ? client -> name : client -> interface -> name,
 	      inet_ntoa (destination.sin_addr),
-	      ntohs (destination.sin_port));
+	      ntohs (destination.sin_port), client -> xid);
 
 	if (destination.sin_addr.s_addr != INADDR_BROADCAST &&
 	    fallback_interface)
@@ -2557,10 +2599,10 @@
 
 	int result;
 
-	log_info ("DHCPDECLINE on %s to %s port %d",
+	log_info ("DHCPDECLINE on %s to %s port %d (xid=%u)",
 	      client -> name ? client -> name : client -> interface -> name,
 	      inet_ntoa (sockaddr_broadcast.sin_addr),
-	      ntohs (sockaddr_broadcast.sin_port));
+	      ntohs (sockaddr_broadcast.sin_port), client -> xid);
 
 	/* Send out a packet. */
 	result = send_packet (client -> interface, (struct packet *)0,
@@ -2600,10 +2642,10 @@
 		return;
 	}
 
-	log_info ("DHCPRELEASE on %s to %s port %d",
+	log_info ("DHCPRELEASE on %s to %s port %d (xid=%u)",
 	      client -> name ? client -> name : client -> interface -> name,
 	      inet_ntoa (destination.sin_addr),
-	      ntohs (destination.sin_port));
+	      ntohs (destination.sin_port), client -> xid);
 
 	if (fallback_interface)
 		result = send_packet (fallback_interface,