## 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”.

2020/09/24

## 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.

Raw

  Used primarily for creating exemptions from connection tracking with the NOTRACK target. Also used for stateless DNAT.

Rawpost

  Used for stateless SNAT.

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 dump”. 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 dump” on a server with one interface (eth0):

    [root@tipper ~]# shorewall dump

    Shorewall 4.4.2.2 Dump at tipper - Fri Oct 16 07:38:16 PDT 2009

    Counters reset Thu Oct  8 00:38:06 PDT 2009

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
     6428 1417K dynamic    all  --  *      *       0.0.0.0/0            0.0.0.0/0           state INVALID,NEW
     967K  629M eth0_in    all  --  eth0   *       0.0.0.0/0            0.0.0.0/0
       49  3896 ACCEPT     all  --  lo     *       0.0.0.0/0            0.0.0.0/0
        0     0 ACCEPT     all  --  *      *       0.0.0.0/0            0.0.0.0/0           state RELATED,ESTABLISHED

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

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 Reject     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 Reject     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
     895K  181M fw2net     all  --  *      eth0    0.0.0.0/0            0.0.0.0/0
       49  3896 ACCEPT     all  --  *      lo      0.0.0.0/0            0.0.0.0/0
        0     0 ACCEPT     all  --  *      *       0.0.0.0/0            0.0.0.0/0           state RELATED,ESTABLISHED
        0     0 Reject     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           [goto]

Here is the eth0_in chain:

    Chain eth0_in (1 references)
     pkts bytes target     prot opt in     out     source               destination
       49  3896 ACCEPT     all  --  lo     *       0.0.0.0/0            0.0.0.0/0
        0     0 ACCEPT     all  --  *      *       0.0.0.0/0            0.0.0.0/0           state RELATED,ESTABLISHED
        0     0 Reject     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           [goto]

Next comes the Nat table:

    NAT Table

    Chain PREROUTING (policy ACCEPT 5593 packets, 1181K bytes)
     pkts bytes target     prot opt in     out     source               destination

    Chain POSTROUTING (policy ACCEPT 11579 packets, 771K bytes)
     pkts bytes target     prot opt in     out     source               destination

    Chain OUTPUT (policy ACCEPT 11579 packets, 771K bytes)
     pkts bytes target     prot opt in     out     source               destination

Next, the Mangle table:

    Mangle Table

    Chain PREROUTING (policy ACCEPT 967K packets, 629M bytes)
     pkts bytes target     prot opt in     out     source               destination
     967K  629M tcpre      all  --  *      *       0.0.0.0/0            0.0.0.0/0

    Chain INPUT (policy ACCEPT 967K packets, 629M 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
        0     0 tcfor      all  --  *      *       0.0.0.0/0            0.0.0.0/0

    Chain OUTPUT (policy ACCEPT 895K packets, 181M bytes)
     pkts bytes target     prot opt in     out     source               destination
     895K  181M tcout      all  --  *      *       0.0.0.0/0            0.0.0.0/0

    Chain POSTROUTING (policy ACCEPT 895K packets, 181M bytes)
     pkts bytes target     prot opt in     out     source               destination
     895K  181M tcpost     all  --  *      *       0.0.0.0/0            0.0.0.0/0

    Chain tcfor (1 references)
     pkts bytes target     prot opt in     out     source               destination

    Chain tcout (1 references)
     pkts bytes target     prot opt in     out     source               destination

    Chain tcpost (1 references)
     pkts bytes target     prot opt in     out     source               destination

    Chain tcpre (1 references)
     pkts bytes target     prot opt in     out     source               destination

And finally, the Raw table:

    Raw Table

    Chain PREROUTING (policy ACCEPT 1004K packets, 658M bytes)
     pkts bytes target     prot opt in     out     source               destination

    Chain OUTPUT (policy ACCEPT 926K packets, 186M bytes)
     pkts bytes target     prot opt in     out     source               destination