[DCCP]: Rate-limit DCCP-Syncs

This implements a SHOULD from RFC 4340, 7.5.4:
 "To protect against denial-of-service attacks, DCCP implementations SHOULD
  impose a rate limit on DCCP-Syncs sent in response to sequence-invalid packets,
  such as not more than eight DCCP-Syncs per second."

The rate-limit is maintained on a per-socket basis. This is a more stringent
policy than enforcing the rate-limit on a per-source-address basis and
protects against attacks with forged source addresses.

Moreover, the mechanism is deliberately kept simple. In contrast to
xrlim_allow(), bursts of Sync packets in reply to sequence-invalid packets
are not supported.  This foils such attacks where the receipt of a Sync
triggers further sequence-invalid packets. (I have tested this mechanism against
xrlim_allow algorithm for Syncs, permitting bursts just increases the problems.)

In order to keep flexibility, the timeout parameter can be set via sysctl; and
the whole mechanism can even be disabled (which is however not recommended).

The algorithm in this patch has been improved with regard to wrapping issues
thanks to a suggestion by Arnaldo.

Commiter note: Rate limited the step 6 DCCP_WARN too, as it says we're
               sending a sync.

Signed-off-by: Gerrit Renker <gerrit@erg.abdn.ac.uk>
Signed-off-by: Ian McDonald <ian.mcdonald@jandi.co.nz>
Signed-off-by: Arnaldo Carvalho de Melo <acme@ghostprotocols.net>

1 files changed, 5 insertions, 0 deletions

diff --git a/Documentation/networking/dccp.txt b/Documentation/networking/dccp.txt
index 4504cc59e405..477026ae0ffb 100644
--- a/Documentation/networking/dccp.txt
+++ b/Documentation/networking/dccp.txt
@@ -112,6 +112,11 @@ tx_qlen = 5
         The size of the transmit buffer in packets. A value of 0 corresponds
         to an unbounded transmit buffer.
 
+sync_ratelimit = 125 ms
+        The timeout between subsequent DCCP-Sync packets sent in response to
+        sequence-invalid packets on the same socket (RFC 4340, 7.5.4). The unit
+        of this parameter is milliseconds; a value of 0 disables rate-limiting.
+
 Notes
 =====