Don't generate an ip_id for packets with the DF bit set; ip_id is

only meaningful for fragments.  Also don't bother to byte-swap the
ip_id when we do generate it; it is only used at the receiver as a
nonce.  I tried several different permutations of this code with no
measurable difference to each other or to the unmodified version, so
I've settled on the one for which gcc seems to generate the best code.
(If anyone cares to microoptimize this differently for an architecture
where it actually matters, feel free.)

Suggested by:	Steve Bellovin's paper in IMW'02

1 files changed, 17 insertions, 4 deletions

diff --git a/sys/netinet/ip_output.c b/sys/netinet/ip_output.c
index 773768c..69f71d8 100644
--- a/sys/netinet/ip_output.c
+++ b/sys/netinet/ip_output.c
@@ -223,17 +223,30 @@ ip_output(m0, opt, ro, flags, imo, inp)
 	pkt_dst = args.next_hop ? args.next_hop->sin_addr : ip->ip_dst;
 
 	/*
-	 * Fill in IP header.
+	 * Fill in IP header.  If we are not allowing fragmentation,
+	 * then the ip_id field is meaningless, so send it as zero
+	 * to reduce information leakage.  Otherwise, if we are not
+	 * randomizing ip_id, then don't bother to convert it to network
+	 * byte order -- it's just a nonce.  Note that a 16-bit counter
+	 * will wrap around in less than 10 seconds at 100 Mbit/s on a
+	 * medium with MTU 1500.  See Steven M. Bellovin, "A Technique
+	 * for Counting NATted Hosts", Proc. IMW'02, available at
+	 * <http://www.research.att.com/~smb/papers/fnat.pdf>.
 	 */
 	if ((flags & (IP_FORWARDING|IP_RAWOUTPUT)) == 0) {
 		ip->ip_v = IPVERSION;
 		ip->ip_hl = hlen >> 2;
-		ip->ip_off &= IP_DF;
+		if ((ip->ip_off & IP_DF) == 0) {
+			ip->ip_off = 0;
 #ifdef RANDOM_IP_ID
-		ip->ip_id = ip_randomid();
+			ip->ip_id = ip_randomid();
 #else
-		ip->ip_id = htons(ip_id++);
+			ip->ip_id = ip_id++;
 #endif
+		} else {
+			ip->ip_off = IP_DF;
+			ip->ip_id = 0;
+		}
 		ipstat.ips_localout++;
 	} else {
 		hlen = ip->ip_hl << 2;