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
hammer a system. Okay, a better question: When am I running the risk
messing up my IP traffic if I add DROP entries in the INPUT rule of
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
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
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
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
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
For SSH issues, I have a standard ruleset. Here it is from the
# 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
-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.