High Performance Firewall

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Imagine this, you have more than two networks separated by Virtual Lans protocols (IEEE 802.1q) or VLANs, carried to you by an intelligent/manageable switch on one troncal line 10/100/1000 MB HD/FD (naturally the best is 1000 MB FD).

You have to share internet to a really BIG numbers of hosts, and maintain a good performance. The first choice is to separate the networks into a equal numbers of ports and maybe a more numbers of firewalls machines. This is not really cost effective, but works.

The second one is what I did. The history of how this begin is related to a some emergency/burn/crash/out of a group of Cisco PIXs. I won't go too deeply into that.

The facts:

I have about 4 networks mask 21 i.e a 8 class C each!!! This is a lots of MACs addresses and even more dangerous a lot of BROADCAST. This is insane but this is the way that my company works.
They gave me 30 publics IPs addresses in 3 groups.
And a machine, a little one, not a big deal.... just for testing. Later (about 8 months) we changed the PC. We put in a real monster.
Later we added a class C network with a routed lot of increasing subnets.... PLUS!!!!

The work

VLAN support

The first thing we have to do is give the kernel the capacity to work with Jumbo Frames. This is done by adding the 8021q module to the kernel

# modprobe 8021q

and/or put in modules in /etc/rc.conf

Next we have to create the virtuals NICs with this command vconfig. Let's suppose we have vlans 20,30,40 and 50 working in our core network.

# vconfig add ethX 20
# vconfig add ethX 30
# vconfig add ethX 50

Where ethX is the troncal NIC Now, if we want to see the interfaces just put ifconfig -a and we will get a list.

So .... next just set the private address as you want. For example (using eth1 like a troncal)

# ifconfig eth1.20 netmask
# ifconfig eth1.30 netmask

I will not explain the number of host neither the mask.... I do a kind of daemon with this, a vlan.conf file in /etc and a vland in /etc/rc.d... I can share this if you want?

The Firewall

It is really easy to make a firewall/nat with iptables there is a lot of information around there...

Just take in mind that you will work with a lot of traffic, this means a lot of CPU usage, so keep your rules tiny, just those necessary. Accept all by default just want to make NAT, no more.... Maybe it is good to accelerate some ports, (80,443,25,110,21,20,53 etc) remember, every packet running through our firewall will past through every rule until it matches to one or fall in the pool (default policy).

The round robin NAT

Let's suppose we have a one ip: 200.aaa.bbb.6 and our gateway is 200.aaa.bbb.1. we can safely put these parameters by default in our configuration. It will not get participation at all in our firewall.

I say I have 3 groups of 10 IPs each to play...... we'll define the NEXT in our firewall script:


And the next important line is:

iptables -t nat -A POSTROUTING -s  -j SNAT --to $Gr1 #ACCESS VLAN 10
iptables -t nat -A POSTROUTING -s  -j SNAT --to $Gr2 #ACCESS VLAN 20
iptables -t nat -A POSTROUTING -s  -j SNAT --to $Gr1 #ACCESS VLAN 30
.... etc

You can repeat the groups for access, subdivide the networks ETC, iptables make a round robin over the Gr1, Gr2 and Gr3 by default, no modification is needed.

It's not necessary to create a virtual card (alias) to every IP in the group.

It's important that every real router knows every group and publishes its via BGP (or similar) to the neighbours.


To accelerate some ports you can put this in the top of FORWARD chain

iptables -A FORWARD -m state --state ESTABLISHED,RELATED -j ACCEPT
iptables -A FORWARD -p icmp -o eth0 -j ACCEPT
iptables -A FORWARD -p tcp -m multiport --dports 80,443,110,53 -j ACCEPT  # FAST FAST FAST 
iptables -A FORWARD -p udp  --dport 53 -j ACCEPT

This mean:

  • the packets incoming will pass only 1 rule if it is an establish connection
  • the packet incoming will pass 2 rules if is a ping or similar
  • the packet will pass 3 rules if is http, mail or similar
  • and the DNS request will pass 3 o 4 rules until go out

The outgoing virus will KILL our machine, and we not need to share "windows" conversations so, kill them!!!!

iptables -A FORWARD -p tcp --dport 135:139 -j DROP
iptables -A FORWARD -p tcp --dport 445 -j DROP
iptables -A FORWARD -p udp --dport 135:139 -j DROP
iptables -A FORWARD -p udp --dport 445 -j DROP

If you can, before they reach our machine.

The High Performance

We get to the real important part of this howto.

In our run to get a really big number of hosts running through our machine we miss some things

  1. We forget that is just one NICs to potentially more than 8000 Mac Addresses. The card shared memory is not prepare for this!!!!!
  2. By default iptables is not prepared to make this number of connections simultaneously !!!!!!


To the first issue... I get some error messages in the logs relative to this, I'm really sorry, I lost these logs and don't remember what they said. But the answer is this, increase the threshold memory to the neighbours. Type this and read:

# cat /proc/sys/net/ipv4/neigh/default/gc_thresh1 
# cat /proc/sys/net/ipv4/neigh/default/gc_thresh2 
# cat /proc/sys/net/ipv4/neigh/default/gc_thresh3 

Next you can put this in the /etc/sysctrl.conf

net.ipv4.neigh.default.gc_thresh1 = 512
net.ipv4.neigh.default.gc_thresh2 = 1024
net.ipv4.neigh.default.gc_thresh3 = 2048

and make sysctl -p to increase to the double!!! (no reboot needed) with this I get no errors!!!!!

The next part will need some comprehension about buckets and conntracks and hashsize (the way how iptables manage the nat connections). There is a very good document about this at here. Read it!!!! Some thing are change since IPtables is know as Netfiler.

In resume!!! Put this in your modules section:

MODULES=(8021q 'nf_conntrack hashsize=1048576' nf_conntrack_ftp 
                               ...and other nf_stuff .......)

The last ones is just to avoid some problems that we have with ftp connections (I thing this is not necessary anymore). The 'nf_conntrack hashsize=1048576' increase the numbers of the hashsize (increase the kernel memory designated to NAT connections) (need reboot or reload module :-) see with dmesg | grep conntrack)

And the next is put some similar to the /etc/sysctrl.conf file

net.netfilter.nf_conntrack_max = 1048576

And do the sysctl -p command

In my case is the same number, that means that I have 1 connection for bucket!!!! I don't need more!!!! by default NetFilter put rate of 1:8. I.E. 8 conections per bucket!! (I think, not remember well)..

In our case we get about 600.000 simultaneous connections in 2 1Giga NICs cards, You can see this with the next command

# cat /proc/sys/net/netfilter/nf_conntrack_count

And put this in a snmpd agent to get and graph it in a MRTG/cacti server ..... uuuuuuu homework

A sample here. (Not actualized, not ever here, but I'll try.)


The iproute2

We have 3 big access to Internet!!! This is because we manage 3 class C groups of IPs (some restrictions of BGP) in this firewall. So, we have 3 incoming traffics that we can manage, but only one outgoing!!! Our default gateway. This can easily fill our outgoing quote, so we have to spare it.

First we have to put some new tables to /etc/iproute2/rt_tables file

# echo 200 PRO_1 >> /etc/iproute2/rt_tables
# echo 205 PRO_2 >> /etc/iproute2/rt_tables
# echo 210 PRO_3 >> /etc/iproute2/rt_tables

Can be more, can be less, depends on traffic

Second we have to give a default gateway to this tables

# ip route add default via 200.aaa.bbb.2 table PRO_1
# ip route add default via 200.aaa.bbb.3 table PRO_2
# ip route add default via 200.aaa.bbb.4 table PRO_3

It's recommended but not necessary put the local interfaces to each table. If you don't put the next few lines you will get not answer of ping in the local network, but you will be able to pass trough.

# ip route add via table PRO_1
# ip route add via table PRO_1
# ip route add via table PRO_1
same PRO_2, same PRO_3

The last thing is to give the order to the incoming packages

# ip rule add from table PRO_1

Again, you can play with the PRO_X and even you can play with the mask and submask For example we want to give only a one class C to outgoing to PRO_3

# ip rule add from table PRO_3

Put this before the <NET>/21

and then TEST IT!!!!

pick a WinPC in one of the private networks and make a tracert to somewhere!!!

Before this, you can browse to a some MYIPsite like www.whatismyip.com and get your "RightNow" address, test it later and get some other address ETC...


Actually we are using a PC with 2G ram , Intel Pentium 4 2.8GHz Dual and 2 NICs 1Gb (one integrated one PCI-E), the top CPU usage don't get the 80% pic...


  • I got a really big surprise when upgrade net-snmp to 5.4.1 (to get statistics)on the Firewall, the CPU usage goes down a lot... from 80% to almost 3%... YES I mean it... I got a confirmation on the other machine (the bridge one, I mention it below) so, we have a much more machine to keep increasing the traffic... this is very good news.

Out traffics is about 180 MB OUT and maybe the half IN.

We get some lost in the cards, about 1 packets per second, it's really low compared with the traffic, so we just don't care.


  • Now we have no errors and no packets lost.

Nevertheless we get a big problem, the MAIL/SPAM outgoing, provoked by VIRUS/MAD_GUYs, can block all the publics IP in a really short time. (in the public Black Lists).

The answer to this may not be ethical, but we have to do it.

We put a similar machine before this one with a bridge configuration blocking every port 25 traffic and sending to another anti SPAM/VIRUS/MAIL machine.

To do this is necessary to give to the bridge the capacity of VLANs and one virtual card (in bridge mode i.e.'br0.30') in every net to answer to the sender...

It works and the customer doesn't notice the difference. Just if you do a telnet anywhere.net 25 you get a Hi I'm a scanner hello answer!!!.. It's really good!!! and works, the mail outgoing is sender from another IP, so we get our 3 groups safe!!!.

Thats all folks, I hope this can help anyone, and it's the only thing I can do to give you something back..

Be strong, be free, 4 ever

CarLost 11:51, 12 October 2007 (EDT) from Chile