# Netfilter Overview

### Tom Eastep

Copyright © 2003, 2005 Thomas M. Eastep

Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.2 or any later version published by the Free Software Foundation; with no Invariant Sections, with no Front-Cover, and with no Back-Cover Texts. A copy of the license is included in the section entitled “GNU Free Documentation License”.

2005-01-23

## Netfilter Overview

Netfilter consists of three tables: Filter, Nat and Mangle. Each table has a number of build-in chains: PREROUTING, INPUT, FORWARD, OUTPUT and POSTROUTING.

Rules in the various tables are used as follows:

Filter

  Packet filtering (rejecting, dropping or accepting packets)

Nat

  Network Address Translation including DNAT, SNAT and Masquerading

Mangle

  General packet header modification such as setting the TOS value or marking packets for policy routing and traffic shaping.

The following diagram shows how packets traverse the various builtin chains within Netfilter. Note that not all table/chain combinations are used.

“Local Process” means a process running on the Shorewall system itself.

A more elaborate version of this flow is available here and this one contrasts the Netfilter flow with that of ipchains.

In the above diagram are boxes similar to this:

The above box gives the name of the built-in chain (INPUT) along with the names of the tables (Mangle and Filter) that the chain exists in and in the order that the chains are traversed. The above sample indicates that packets go first through the INPUT chain of the Mangle table then through the INPUT chain of the Filter table. When a chain is enclosed in parentheses, Shorewall does not use the named chain (INPUT) in that table (Mangle).

### Important

Keep in mind that chains in the Nat table are only traversed for new connection requests (including those related to existing connections) while the chains in the other tables are traversed on every packet.

The above diagram should help you understand the output of “shorewall status”. You may also wish to refer to this article that describes the flow of packets through a Shorewall-generated firewall.

Here are some excerpts from “shorewall status” on a server with one interface (eth0):

    [root@lists html]# shorewall status

    Shorewall-1.4.7 Status at lists.shorewall.net - Mon Oct 13 12:51:13 PDT 2003

    Counters reset Sat Oct 11 08:12:57 PDT 2003

The first table shown is the Filter table.

    Chain INPUT (policy DROP 0 packets, 0 bytes)
     pkts bytes target     prot opt in     out     source               destination
     679K  182M ACCEPT     all  --  lo     *       0.0.0.0/0            0.0.0.0/0
     785K   93M accounting  all  --  *      *       0.0.0.0/0            0.0.0.0/0
        0     0 DROP      !icmp --  *      *       0.0.0.0/0            0.0.0.0/0          state INVALID

The following rule indicates that all traffic destined for the firewall that comes into the firewall on eth0 is passed to a chain called “eth0_in”. That chain will be shown further down.

     785K   93M eth0_in    all  --  eth0   *       0.0.0.0/0            0.0.0.0/0
        0     0 common     all  --  *      *       0.0.0.0/0            0.0.0.0/0
        0     0 LOG        all  --  *      *       0.0.0.0/0            0.0.0.0/0          LOG flags 0 level 6 prefix `Shorewall:INPUT:REJECT:'
        0     0 reject     all  --  *      *       0.0.0.0/0            0.0.0.0/0

    Chain FORWARD (policy DROP 0 packets, 0 bytes)
     pkts bytes target     prot opt in     out     source               destination
        0     0 accounting  all  --  *      *       0.0.0.0/0            0.0.0.0/0
        0     0 DROP      !icmp --  *      *       0.0.0.0/0            0.0.0.0/0          state INVALID
        0     0 eth0_fwd   all  --  eth0   *       0.0.0.0/0            0.0.0.0/0
        0     0 common     all  --  *      *       0.0.0.0/0            0.0.0.0/0
        0     0 LOG        all  --  *      *       0.0.0.0/0            0.0.0.0/0          LOG flags 0 level 6 prefix `Shorewall:FORWARD:REJECT:'
        0     0 reject     all  --  *      *       0.0.0.0/0            0.0.0.0/0

    Chain OUTPUT (policy DROP 1 packets, 60 bytes)
     pkts bytes target     prot opt in     out     source               destination
     679K  182M ACCEPT     all  --  *      lo      0.0.0.0/0            0.0.0.0/0
     922K  618M accounting  all  --  *      *       0.0.0.0/0            0.0.0.0/0
        0     0 DROP      !icmp --  *      *       0.0.0.0/0            0.0.0.0/0          state INVALID
     922K  618M fw2net     all  --  *      eth0    0.0.0.0/0            0.0.0.0/0
        0     0 common     all  --  *      *       0.0.0.0/0            0.0.0.0/0
        0     0 LOG        all  --  *      *       0.0.0.0/0            0.0.0.0/0          LOG flags 0 level 6 prefix `Shorewall:OUTPUT:REJECT:'
        0     0 reject     all  --  *      *       0.0.0.0/0            0.0.0.0/0

Here is the eth0_in chain:

    Chain eth0_in (1 references)
     pkts bytes target     prot opt in     out     source               destination
     785K   93M dynamic    all  --  *      *       0.0.0.0/0            0.0.0.0/0
     785K   93M net2fw     all  --  *      *       0.0.0.0/0            0.0.0.0/0

The “dynamic” chain above is where dynamic blacklisting is done.

Next comes the Nat table:

    NAT Table

    Chain PREROUTING (policy ACCEPT 182K packets, 12M bytes)
     pkts bytes target     prot opt in     out     source               destination
    20005 1314K net_dnat   all  --  eth0   *       0.0.0.0/0            0.0.0.0/0

    Chain POSTROUTING (policy ACCEPT 678K packets, 44M bytes)
     pkts bytes target     prot opt in     out     source               destination

    Chain OUTPUT (policy ACCEPT 678K packets, 44M bytes)
     pkts bytes target     prot opt in     out     source               destination

    Chain net_dnat (1 references)
     pkts bytes target     prot opt in     out     source               destination
      638 32968 REDIRECT   tcp  --  *      *       0.0.0.0/0           !206.124.146.177    tcp dpt:80 redir ports 3128

And finally, the Mangle table:

    Mangle Table

    Chain PREROUTING (policy ACCEPT 14M packets, 2403M bytes)
     pkts bytes target     prot opt in     out     source               destination
    1464K  275M pretos     all  --  *      *       0.0.0.0/0            0.0.0.0/0

    Chain INPUT (policy ACCEPT 14M packets, 2403M bytes)
     pkts bytes target     prot opt in     out     source               destination

    Chain FORWARD (policy ACCEPT 0 packets, 0 bytes)
     pkts bytes target     prot opt in     out     source               destination

    Chain OUTPUT (policy ACCEPT 15M packets, 7188M bytes)
     pkts bytes target     prot opt in     out     source               destination
    1601K  800M outtos     all  --  *      *       0.0.0.0/0            0.0.0.0/0

    Chain POSTROUTING (policy ACCEPT 15M packets, 7188M bytes)
     pkts bytes target     prot opt in     out     source               destination

    Chain outtos (1 references)
     pkts bytes target     prot opt in     out     source               destination
        0     0 TOS        tcp  --  *      *       0.0.0.0/0            0.0.0.0/0          tcp dpt:22 TOS set 0x10
     315K  311M TOS        tcp  --  *      *       0.0.0.0/0            0.0.0.0/0          tcp spt:22 TOS set 0x10
        0     0 TOS        tcp  --  *      *       0.0.0.0/0            0.0.0.0/0          tcp dpt:21 TOS set 0x10
      683 59143 TOS        tcp  --  *      *       0.0.0.0/0            0.0.0.0/0          tcp spt:21 TOS set 0x10
     3667 5357K TOS        tcp  --  *      *       0.0.0.0/0            0.0.0.0/0          tcp spt:20 TOS set 0x08
        0     0 TOS        tcp  --  *      *       0.0.0.0/0            0.0.0.0/0          tcp dpt:20 TOS set 0x08

    Chain pretos (1 references)
     pkts bytes target     prot opt in     out     source               destination
     271K   15M TOS        tcp  --  *      *       0.0.0.0/0            0.0.0.0/0          tcp dpt:22 TOS set 0x10
        0     0 TOS        tcp  --  *      *       0.0.0.0/0            0.0.0.0/0          tcp spt:22 TOS set 0x10
      730 41538 TOS        tcp  --  *      *       0.0.0.0/0            0.0.0.0/0          tcp dpt:21 TOS set 0x10
        0     0 TOS        tcp  --  *      *       0.0.0.0/0            0.0.0.0/0          tcp spt:21 TOS set 0x10
        0     0 TOS        tcp  --  *      *       0.0.0.0/0            0.0.0.0/0          tcp spt:20 TOS set 0x08
     2065  111K TOS        tcp  --  *      *       0.0.0.0/0            0.0.0.0/0          tcp dpt:20 TOS set 0x08