[PATCH 2/2] tcp: syncookies: reduce mss table to four values

[Florian Westphal <fw@strlen.de>]

Halve mss table size to make blind cookie guessing more difficult.
This is sad since the tables were already small, but there
is little alternative except perhaps adding more precise mss information
in the tcp timestamp.  Timestamps are unfortunately not ubiquitous.

Guessing all possible cookie values still has 8-in 2**32 chance.

Reported-by: Jakob Lell <jakob@jakoblell.com>
Signed-off-by: Florian Westphal <fw@strlen.de>
---
 Changes since v1:
 - add comment explaining mss choices

 net/ipv4/syncookies.c | 23 ++++++++++++-----------
 net/ipv6/syncookies.c |  6 +-----
 2 files changed, 13 insertions(+), 16 deletions(-)

diff --git a/net/ipv4/syncookies.c b/net/ipv4/syncookies.c
index b6ea297..b47f477 100644
--- a/net/ipv4/syncookies.c
+++ b/net/ipv4/syncookies.c
@@ -136,22 +136,23 @@ static __u32 check_tcp_syn_cookie(__u32 cookie, __be32 saddr, __be32 daddr,
 }
 
 /*
- * MSS Values are taken from the 2009 paper
- * 'Measuring TCP Maximum Segment Size' by S. Alcock and R. Nelson:
- *  - values 1440 to 1460 accounted for 80% of observed mss values
- *  - values outside the 536-1460 range are rare (<0.2%).
+ * MSS Values are chosen based on the 2011 paper
+ * 'An Analysis of TCP Maximum Segement Sizes' by S. Alcock and R. Nelson: Values ...
+ *  .. lower than 536 are rare (< 0.2%)
+ *  .. between 537 and 1299 account for less than < 1.5% of observed values
+ *  .. in the 1300-1349 range account for about 15 to 20% of observed mss values
+ *  .. exceeding 1460 are very rare (< 0.04%)
+ *
+ *  1460 is the single most frequently announced mss value (30 to 46% depending
+ *  on monitor location)
  *
  * Table must be sorted.
  */
 static __u16 const msstab[] = {
-	64,
-	512,
-	536,
-	1024,
-	1440,
+	536,
+	1300,
+	1440,	/* 1440, 1452: PPPoE */
 	1460,
-	4312,
-	8960,
 };
 
 /*
diff --git a/net/ipv6/syncookies.c b/net/ipv6/syncookies.c
index 13ca0a0..7198640 100644
--- a/net/ipv6/syncookies.c
+++ b/net/ipv6/syncookies.c
@@ -26,13 +26,9 @@
 
 /* Table must be sorted. */
 static __u16 const msstab[] = {
-	64,
-	512,
-	536,
-	1280 - 60,
+	1280 - 60, /* IPV6_MIN_MTU - 60 */
 	1480 - 60,
 	1500 - 60,
-	4460 - 60,
 	9000 - 60,
 };
 
-- 
1.8.1.5