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[DCCP]: Rate-limit DCCP-Syncs

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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>
This commit is contained in:
Gerrit Renker
2007-09-26 11:31:49 -03:00
committed by David S. Miller
parent ee1a15922d
commit a94f0f9705
6 changed files with 39 additions and 9 deletions
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29
net/dccp/input.c
View File

@@ -122,6 +122,23 @@ static int dccp_check_seqno(struct sock *sk, struct sk_buff *skb)
(ackno != DCCP_PKT_WITHOUT_ACK_SEQ))
dp->dccps_gar = ackno;
} else {
unsigned long now = jiffies;
/*
* Step 6: Check sequence numbers
* Otherwise,
* If P.type == Reset,
* Send Sync packet acknowledging S.GSR
* Otherwise,
* Send Sync packet acknowledging P.seqno
* Drop packet and return
*
* These Syncs are rate-limited as per RFC 4340, 7.5.4:
* at most 1 / (dccp_sync_rate_limit * HZ) Syncs per second.
*/
if (time_before(now, (dp->dccps_rate_last +
sysctl_dccp_sync_ratelimit)))
return 0;
DCCP_WARN("DCCP: Step 6 failed for %s packet, "
"(LSWL(%llu) <= P.seqno(%llu) <= S.SWH(%llu)) and "
"(P.ackno %s or LAWL(%llu) <= P.ackno(%llu) <= S.AWH(%llu), "
@@ -132,15 +149,9 @@ static int dccp_check_seqno(struct sock *sk, struct sk_buff *skb)
: "exists",
(unsigned long long) lawl, (unsigned long long) ackno,
(unsigned long long) dp->dccps_awh);
/*
* Step 6: Check sequence numbers
* Otherwise,
* If P.type == Reset,
* Send Sync packet acknowledging S.GSR
* Otherwise,
* Send Sync packet acknowledging P.seqno
* Drop packet and return
*/
dp->dccps_rate_last = now;
if (dh->dccph_type == DCCP_PKT_RESET)
seqno = dp->dccps_gsr;
dccp_send_sync(sk, seqno, DCCP_PKT_SYNC);