This chapter focuses on packet filtering basics, defines the differences
between ipchains and iptables,
explains various options available with iptables
commands, and shows how filtering rules can be preserved between system
reboots.
16.1. Packet Filtering
Traffic moves through a network in packets. A
network packet is a collection of data in a specific size and format.
In order to transmit a file over a network, the sending computer must
first break the file into packets using the rules of the network
protocol. Each of these packets holds a small part of the file
data. Upon receiving the transmission, the target computer reassembles
the packets into the file.
Every packet contains information which helps it navigate the network
and move toward its destination. The packet can tell computers along
the way, as well as the destination machine, where it came from, where
it is going, and what type of packet it is, among other things. Most
packets are designed to carry data, although some protocols use packets
in special ways. For example, the Transmission Control
Protocol (TCP) uses a SYN packet, which contains no data, to
initiate communication between two systems.
The Linux kernel has the built-in ability to filter packets, allowing
some of them into the system while stopping others. The 2.4 kernel's
netfilter has three built-in tables or
rules lists. They are as follows:
filter — The default table
for handling network packets.
nat — Used to alter packets
that create a new connection.
mangle — Used for specific
types of packet alteration.
Each of these tables in turn have a group of built-in
chains which correspond to the actions performed
on the packet by the netfilter.
The built-in chains for the filter table are as
follows:
INPUT — Applies to network packets that are
targeted for the host.
OUTPUT — Applies to
locally-generated network packets.
FORWARD — Applies to network
packets routed through the host.
The built-in chains for the nat table are as follows:
PREROUTING — Alters network packets
when they arrive.
OUTPUT — Alters locally-generated
network packets before they are sent out.
POSTROUTING — Alters network
packets before they are sent out.
The built-in chains for the mangle table are as
follows:
INPUT — Alters network packets
targeted for the host.
OUTPUT — Alters locally-generated
network packets before they are sent out.
FORWARD — Alters network packets
routed through the host.
PREROUTING — Alters incoming
network packets before they are routed.
POSTROUTING — Alters network
packets before they are sent out.
Every network packet received by or sent out of a Linux system is
subject to at least one table.
A packet may be checked against multiple rules within each table before
emerging at the end of the chain. The structure and purpose of these
rules may vary, but they usually seek to identify a packet coming from
or going to a particular IP address or set of addresses when using a
particular protocol and network service.
Regardless of their destination, when packets match a particular rule in
one of the tables, a target or action is applied
to them. If the rule specifies an ACCEPT target for a
matching packet, the packet skips the rest of the rule checks and is
allowed to continue to its destination. If a rule specifies a
DROP target, that packet is refused access to the
system and nothing is sent back to the host that sent the packet. If a
rule specifies a QUEUE target, the packet to be
passed to user-space. If a rule specifies the optional
REJECT target, the packet is dropped, but an error
packet is sent to the packet's originator.
Every chain has a default policy to ACCEPT,
DROP, REJECT, or
QUEUE. If none of the rules in the chain apply to the
packet, then the packet is dealt with in accordance with the default
policy.
The iptables command configures these tables, as well
as sets up new tables if necessary.
