From: Jiri Bohac <jbohac@suse•cz>
To: Jiri Bohac <jbohac@suse•cz>
Cc: Jakob Lell <jakob@jakoblell•com>,
netdev@vger•kernel.org, davem@davemloft•net
Subject: [PATCH 3/3] [RFC] TCP syncookies: only allow 3 MSS values by default to mitigate spoofing attacks
Date: Fri, 16 Aug 2013 02:05:23 +0200 [thread overview]
Message-ID: <20130816000523.GC11950@midget.suse.cz> (raw)
In-Reply-To: <20130815235743.GA25665@midget.suse.cz>
Jakob Lell discovered that the sequence number that needs to be guessed to
successfully spoof a TCP connection with syncookies only has 27 bits of
entropy. Of the 32 bits, 3 are wasted by the 8 differrent RSS values. [1]
This patch decreases the number of possible MSS values from 8 to 3,
making the spoofing attack 8/3 times more difficult.
Rationale for the new values
- most packets are (1500 - headers); (1450 - headers) is not a huge waste and
prevents fallback to much lower values
- clients will rarely send MSS below 536, so that's a safe fallback
- we need to keep the minimum (64)
[1]: http://www.jakoblell.com/blog/2013/08/13/quick-blind-tcp-connection-spoofing-with-syn-cookies/
Signed-off-by: Jiri Bohac <jbohac@suse•cz>
---
net/ipv4/syncookies.c | 9 ++-------
net/ipv6/syncookies.c | 9 ++-------
2 files changed, 4 insertions(+), 14 deletions(-)
diff --git a/net/ipv4/syncookies.c b/net/ipv4/syncookies.c
index af0692f..0504bbe 100644
--- a/net/ipv4/syncookies.c
+++ b/net/ipv4/syncookies.c
@@ -148,15 +148,10 @@ static __u32 check_tcp_syn_cookie(__u32 cookie, __be32 saddr, __be32 daddr,
int sysctl_tcp4_syncookies_mss[TCP_SYNCOOKIES_MSS_COUNT_MAX] = {
64,
512,
- 536,
- 1024,
- 1440,
- 1460,
- 4312,
- 8960,
+ 1450 - 40,
/* update sysctl_tcp4_syncookies_mss_count accordingly */
};
-int sysctl_tcp4_syncookies_mss_count = 8;
+int sysctl_tcp4_syncookies_mss_count = 3;
/*
* This value is the age (in seconds) of syncookies which will always be
diff --git a/net/ipv6/syncookies.c b/net/ipv6/syncookies.c
index 4268448..ccdb880 100644
--- a/net/ipv6/syncookies.c
+++ b/net/ipv6/syncookies.c
@@ -28,15 +28,10 @@
int sysctl_tcp6_syncookies_mss[TCP_SYNCOOKIES_MSS_COUNT_MAX] = {
64,
512,
- 536,
- 1280 - 60,
- 1480 - 60,
- 1500 - 60,
- 4460 - 60,
- 9000 - 60,
+ 1450 - 60,
/* update sysctl_tcp6_syncookies_mss_count accordingly */
};
-int sysctl_tcp6_syncookies_mss_count = 8;
+int sysctl_tcp6_syncookies_mss_count = 3;
/*
* This value is the age (in seconds) of syncookies which will always be
--
Jiri Bohac <jbohac@suse•cz>
SUSE Labs, SUSE CZ
next prev parent reply other threads:[~2013-08-16 0:05 UTC|newest]
Thread overview: 14+ messages / expand[flat|nested] mbox.gz Atom feed top
2013-08-13 13:57 Quick Blind TCP Connection Spoofing with SYN Cookies Jakob Lell
2013-08-14 21:02 ` some one
2013-08-15 23:57 ` Jiri Bohac
2013-08-16 0:00 ` [PATCH 1/3] [RFC] TCP syncookies: slow down timer to mitigate spoofing attacks Jiri Bohac
2013-08-16 0:34 ` Neal Cardwell
2013-08-16 8:20 ` [PATCH v2 " Jiri Bohac
2013-08-16 21:47 ` [PATCH " Florian Westphal
2013-08-16 0:03 ` [PATCH 2/3] [RFC] TCP syncookies: introduce sysctl to configure the MSS tables Jiri Bohac
2013-08-16 21:40 ` Florian Westphal
2013-08-27 12:55 ` Jiri Bohac
2013-08-16 0:05 ` Jiri Bohac [this message]
2013-08-16 21:31 ` [PATCH 3/3] [RFC] TCP syncookies: only allow 3 MSS values by default to mitigate spoofing attacks Florian Westphal
2013-08-27 13:52 ` Jiri Bohac
2013-08-16 9:21 ` Quick Blind TCP Connection Spoofing with SYN Cookies Florian Westphal
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