Difference between revisions of "High Performance Firewall"

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Revision as of 21:41, 11 October 2007

Hight Performance Firewall/Nat with iptables and VLANs and iproute2


Hi, this is my first intent of making a wiki document, so be warned, this may have some errors or misunderstandings.

Imagine this, you have more than two networks separated by Virtual Lans protocols (IEEE 801.1q) or VLANs, carried to you by a 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 firs choice is 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 will don't deep in 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 really lots 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 big deal.... just for testing. Later (about 8 monts) we change the PC. We put a real monster.
Later we add 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 to the kernel the capacity of 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 put 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 if you want

The Firewall

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

Just make in mind that you will work with a lot of traffic, this mean, a lot of CPU usage, so, keep your rules tiny, just the necessary. Accept all by default just want to make NAT, no more.... Maybe is good accelerate some ports, (80,443,25,110,21,20,53 etc) remember, every packet running trough our firewall will past trough every rule until it match 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 this 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 create a virtual card (alias) to every IP in the group.

It's important that every real router know every group and publish its via BGP (or similar) to the neighbors.


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 eth1 -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 a establish connection
  • the packet incoming will pass 2 rules if is a ping o similar
  • the packet will pass 3 rules if is http, mail or similar
  • and the DNS recuest will pass 3 o 4 rules until go out

The virus outcoming will KILL our machine, and we not need to share "windows" conversatios 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 numbers of host running trough our machine we miss some things

  1. We forget that is just one NICs to potentially more than 1000 Mac Addresses. The card shared memory is not prepare to this!!!!!
  2. By default iptables is not prepare to make this numbers of connections simultaneously !!!!!!


To the first isue... I get some error messages in the logs realtive to this.... I'm really sorry, I lost this logs and don't remember what they say. But the answer is this... increase the treshold memory to the neighbors. 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

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!!! with this I get no errors!!!!!

The next 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 in 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 tha we have with ftp connections (I thing this is no necessary anymore). The 'nf_conntrack hashsize=1048576' increase the numbers of the hashsize (increase the kernel memory designated to NAT connections)

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, thats mean that I have 1 coneccion for bucket!!!! I don't need more!!!! by default NetFilter put rate of 1:8. I.E. 8 conections per bucket!! (I think, not remmember 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 full 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 necesary put the local interfaces to each table

# 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 give the order to the incoming packages

# ip rule add from table PRO_1

Again, you can play 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 someware!!!

Before 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...

PS: English is not my natural language, I ask you forgive me if there is some comprehensiveness stuff, and feel free to improve this if you want!!!!