IPTables limits?

[Rick Stevens]

Karl Pearson wrote:
> On Tue, October 21, 2008 11:16 am, Rick Stevens wrote:
>> Karl Pearson wrote:
>>> I'm curious if there's a limit on how many iptables entries it takes
>>> to
>>> hammer a system. Okay, a better question: When am I running the risk
>>> of
>>> messing up my IP traffic if I add DROP entries in the INPUT rule of
>>> iptables?
>> You do ask the damndest questions, Karl!  :-)
>>
>> I've never seen a document that describes any rule limits.  It is a
>> kernel module, so one must assume there is some limit.  It may be that
>> a spelunking sesion through the source might answer that.
>>
>> I've got lots of drop entries on my rules and haven't had any issues,
>> but I'm always guided by the concept that the more rules a packet must
>> traverse, the slower the connection will be at startup.  Therefore I
>> order my rules carefully putting the more generic rules at the top of
>> the list and the more specific ones at the bottom.  That may not work
>> for you...every network is a little different (for example, we blacklist
>> entire /8 networks in some cases because of DOS or hack attacks from
>> those countries).
> 
> I'd love a list of those IPs and how you inserted the rule.

Oh, man, I'd have to go through our firewall configs to dig up the
addresses involved.  As far as the rule insertion, it's pretty much
a standard iptables insert, but we add a "--syn" for TCP and just
simply block UDP.  These are interim...eventually we migrate these
rules out to our Foundry or Cisco routers and remove them from the
individual servers.

>                                                               I also
> wonder about DROP vs REJECT. I know the difference, but what's the
> theory behind using one over the other? I have thought it is about them
> knowing the IP is there vs just a hang. But, I'm wondering if the DROP
> (hang) causes me any headaches in IP traffic limiting? I know, another
> annoying question.

We use DROP.  If we're under attack or someone's probing, why let them
know they found a machine?  If anything, a DROP is easier on the system
than a REJECT since all the system does is drop the connection...it
doesn't send anything back.  And if the prober/attacker's connection
hangs at his end, good!  Screw them in the first place.  Attackers
and probers deserve absolutely no consideration in any way and should
be prosecuted in the same manner as someone trying to break into my home
or office.

> All my DROPs are in the first rule, so they are immediately acted on, or
> in this case, DROPped.
> 
> I used to use sshblack, but the way the logfiles are written now make it
> ineffective for inbound, however I wrote my own blacklist scripts which
> I use with it, so it does do aging checks, which for DDNS is okay, I
> think.

For SSH issues, I have a standard ruleset.  Here it is from the
/etc/sysconfig/iptables file:

# This rejects ssh attempts more than twice in 180 seconds...
# First, mark attempts as part of the "sshattack" group...
-A INPUT -p tcp --syn --dport 22 -m recent --name sshattack --set
# Optional: Include this line if you want to log these attacks...
#-A INPUT -p tcp --syn --dport 22 -m recent --name sshattack --rcheck 
--seconds 180 --hitcount 2 -j LOG --log-prefix "SSH REJECT: "
# Finally, reject the connection if more than one attempt is made in 180 
seconds...
-A INPUT -p tcp --syn --dport 22 -m recent --name sshattack --rcheck 
--seconds 180 --hitcount 2 -j REJECT --reject-with tcp-reset

So, you only get to try to SSH once every 3 minutes.  You can tweak the
"--hitcount" value and "--seconds" value to customize it for your use.
It foils most of the script kiddies out there.

> I have installed fail2ban, which blacklists for a shorter amount of
> time, I suspect under the theory that ssh attacks are almost random, and
> will pass.

Yeah, it works.  Mine has minimal impact on the rulesets, but either
is fine.
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- Rick Stevens, Systems Engineer                      ricks at nerd.com -
- AIM/Skype: therps2        ICQ: 22643734            Yahoo: origrps2 -
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