Re: xt_recent broken in kernel 3.19.0 + PATCH

[Chris Vine <chris@cvine.freeserve.co.uk>]

On Wed, 11 Feb 2015 09:28:34 +0000
Chris Vine <chris@cvine.freeserve.co.uk> wrote:
> With kernel 3.19.0, the following iptables rule, where SSH_TRIES is
> set to 4:
> 
>   iptables -D SSH_CHAIN -m conntrack --ctstate NEW \
>     -m recent --update --seconds $SSH_LOGIN_PERIOD --hitcount
> $SSH_TRIES -j DROP
> 
> generates this error message in syslog:
> 
>   kernel: xt_recent: hitcount (4) is larger than packets to be
>   remembered (4) for table DEFAULT
> 
> and the rule fails to install in the table.  No error is generated
> with kernel 3.18.6.

The rule provoking this should of course have been the one appending
the rule, which is:

  iptables -A SSH_CHAIN -m conntrack --ctstate NEW \
    -m recent --update --seconds $SSH_LOGIN_PERIOD --hitcount $SSH_TRIES -j DROP

On looking at the code, the changes in the 3.19 kernel seem not to have
been tested and there is an off-by-one error.  The patch below restores
behaviour to be identical to that found in the 3.18 kernel.

Chris

--- linux-3.19.0/net/netfilter/xt_recent.c~	2015-02-10 09:18:44.657376355 +0000
+++ linux-3.19.0/net/netfilter/xt_recent.c	2015-02-11 17:58:33.311608835 +0000
@@ -378,7 +378,7 @@
 	mutex_lock(&recent_mutex);
 	t = recent_table_lookup(recent_net, info->name);
 	if (t != NULL) {
-		if (info->hit_count > t->nstamps_max_mask) {
+		if (info->hit_count > t->nstamps_max_mask + 1) {
 			pr_info("hitcount (%u) is larger than packets to be remembered (%u) for table %s\n",
 				info->hit_count, t->nstamps_max_mask + 1,
 				info->name);