Question

Using an effective iptables blacklist

Asked by: forrie

I've migrated my mail and web services from a FreeBSD/PF environment to Linux/iptables (CentOS 5.3).  

Being relatively new to iptables, I sought a pre-configured setup to facilitate a quicker rollout - I found a script called "ipkungfu" (which is no longer being developed).   This post is not about that script, so much as it is a question about how to utilize iptables effectively.

Along with my configuration I have a few tables of IPs (mostly /24's) from my PF configuration, tables of:  abuse, spammers and geoip.   The abuse table are IPs that I never, ever want touching any services on my system.  Spammers, blocked from port 25.  Geoip, is generally no access as well.

Iptables has three basic initial chains:

INPUT
FORWARD
OUTPUT

I set up a custom script that would do a for-next loop through the IPs and do something like this:

$iptables -I abuse -s $i -j DROP

I found this isn't working.  I also found that loading large IP lists into iptables is painfully slow (it was lightning fast under FreeBSD/PF).

One particularly annoying host from netsync.net had been slamming my system repeatedly.  The IP was in the DROP directive, but it was getting through anyway.  This, I thought, was due to the fact that some of my services are FORWARDed to an internal host and thus perhaps bypassing something.   I was able to block this host by issuing this command:

# iptables -A FORWARD -s 208.20.34.161 -j DROP
# iptables -A FORWARD -s 208.20.34.150 -j DROP

So my predicament is in correctly understanding how to effectively block these large lists of IPs.  Some of them are BOTNETs some of them are /24's that are dynamic that have shown other patterns of abuse.  Because my server is private, I don't need to be concerned about inadvertently blocking someone.

First, is there a more effective way to block these lists in iptables?  Is there a more efficient way to load these large lists of IPs into the chain(s)?  At this point, it actually causes my system boot time to lag significantly.

Where is the best place to block these IPs in the chains and why doesn't just blocking it in the INPUT chain work.  Or did I configure the DROP incorrectly?

Thanks!

iptables -N abuse
 
 
        for i in `cat $dir/abuse | egrep -hv "^#|$^"`
        do
                $iptables -I abuse -s $i -j DROP
        done

                                  
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Asked On
2009-09-09 at 20:33:28ID24720245
Tags

iptables linux security firewall

Topics

IP Tables/IP Chains

,

Unix Networking

Participating Experts
1
Points
250
Comments
1

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Answers

 

by: BlazPosted on 2009-09-09 at 23:58:43ID: 25297847

Your script looks correct. However you didn't write how you use your abuse chain. I suspect you have a rule like (first rule in INPUT chain):
iptables -I INPUT -j abuse

if you want to block FORWARD table also you should add a rule:
iptables -I FORWARD -j abuse

As far as performance issues are concerned:
Do you see problems only at boot time or at packet processing time? If you see processing time issues (network bandwidth drops or load on machine is high when high network traffic is present) I would suggest:

1. make sure that you have rule order as follows:
iptables -A INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT
iptables -A INPUT -j abuse
iptables -A INPUT -p tcp --dport 25 -j ACCEPT

Most traffic will be affected by rule #1 - established traffic. Only new connections will be tested against abuse IPs and against other rules.

2. If you have REALLY large database of IPs it might be better to create more abuse chains:
iptables -A INPUT -j abuse
iptables -A FORWARD -j abuse
...
iptables -A abuse -s 0.0.0.0/3 -j abuse1
iptables -A abuse -s 32.0.0.0/3 -j abuse32
iptables -A abuse -s 64.0.0.0/3 -j abuse64
...
iptables -A abuse1 -s 1.x.x.x -j DROP
iptables -A abuse1 -s 13.y.y.y -j DROP
...
iptables -A abuse32 -s 33.z.z.z -j DROP
...

You create a sort of search tree for IPs by this method - the packet does not need to traverse the whole IP database but only 1/8 of the database but it has one more chain jump. I have not tested this solution because I have never had so large IP lists that I would notice performance problems


If you have problems only at load time you could try iptables-save/iptables-restore - it is probably faster method to load a large set of rules opposed to issuing iptables command for each rule.

20120131-EE-VQP-002

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