Chapter 5. Network setup

Table of Contents

5.1. The basic network infrastructure
5.1.1. The domain name
5.1.2. The hostname resolution
5.1.3. The network interface name
5.1.4. The network address range for the LAN
5.1.5. The network configuration infrastructure
5.1.6. The network device support
5.2. The network connection method
5.2.1. The DHCP connection with the Ethernet
5.2.2. The static IP connection with the Ethernet
5.2.3. The PPP connection with pppconfig
5.2.4. The alternative PPP connection with wvdialconf
5.2.5. The PPPoE connection with pppoeconf
5.3. The basic network configuration with ifupdown
5.3.1. The command syntax simplified
5.3.2. The basic syntax of "/etc/network/interfaces"
5.3.3. The loopback network interface
5.3.4. The network interface served by the DHCP
5.3.5. The network interface with the static IP
5.3.6. The basics of wireless LAN interface
5.3.7. The wireless LAN interface with WPA/WPA2
5.3.8. The wireless LAN interface with WEP
5.3.9. The PPP connection
5.3.10. The alternative PPP connection
5.3.11. The PPPoE connection
5.3.12. The network configuration state of ifupdown
5.3.13. The basic network reconfiguration
5.3.14. The ifupdown-extra package
5.4. The advanced network configuration with ifupdown
5.4.1. The ifplugd package
5.4.2. The ifmetric package
5.4.3. The virtual interface
5.4.4. The advanced command syntax
5.4.5. The mapping stanza
5.4.6. The manually switchable network configuration
5.4.7. Scripting with the ifupdown system
5.4.8. Mapping with guessnet
5.5. The network configuration for desktop
5.5.1. GUI network configuration tools
5.5.2. Automatic network configuration
5.6. The low level network configuration
5.6.1. Iproute2 commands
5.6.2. Safe low level network operations
5.7. Network optimization
5.7.1. Finding optimal MTU
5.7.2. Setting MTU
5.7.3. WAN TCP optimization
5.8. Netfilter infrastructure
[Tip] Tip

For general guide to the GNU/Linux networking, read the Linux Network Administrators Guide.

The traditional TCP/IP network setup on Debian system uses ifupdown package as a high level tool. There are 2 typical cases:

We will describe these traditional cases in detail here.

We will also touch on some alternative high level tools such as network-manager and wicd which ease configuration of wireless networks (see Section 5.5.2, “Automatic network configuration”).

5.1. The basic network infrastructure

Let's review the basic network infrastructure on the modern Debian system.

5.1.1. The domain name

The naming for the domain name is a tricky one for the normal PC workstation users. The PC workstation may be mobile one hopping around the network or located behind the NAT firewall inaccessible from the Internet. For such case, you may not want the domain name to be a valid domain name to avoid name collision.

According to rfc2606, "invalid" seems to be a choice for the top level domain (TLD) to construct domain names that are sure to be invalid from the Internet.

The mDNS network discovery protocol (Apple Bonjour / Apple Rendezvous, Avahi on Debian) uses "local" as the pseudo-top-level domain. Microsoft also seems to promote "local" for the TLD of local area network.

Other popular choices for the invalid TLD seem to be "localdomain", "lan", "localnet", or "home" according to my incoming mail analysis.

5.1.2. The hostname resolution

The hostname resolution is currently supported by the NSS (Name Service Switch) mechanism too. The flow of this resolution is:

  1. The "/etc/nsswitch.conf" file with stanza like "hosts: files dns" dictates the hostname resolution order. (This replaces the old functionality of the "order" stanza in "/etc/host.conf".)
  2. The files method is invoked first. If the hostname is found in the "/etc/hosts" file, it returns all valid addresses for it and exits. (The "/etc/host.conf" file contains "multi on".)
  3. The dns method is invoked. If the hostname is found by the query to the Internet Domain Name System (DNS) identified by the "/etc/resolv.conf" file, it returns all valid addresses for it and exits.

The "/etc/hosts" file associates IP addresses with hostnames:

127.0.0.1 localhost
127.0.1.1 <host_name>.<domain_name> <host_name>

# The following lines are desirable for IPv6 capable hosts
::1     ip6-localhost ip6-loopback
fe00::0 ip6-localnet
ff00::0 ip6-mcastprefix
ff02::1 ip6-allnodes
ff02::2 ip6-allrouters
ff02::3 ip6-allhosts

Here the <host_name> in this matches the own hostname defined in the "/etc/hostname". The <domain_name> in this is the fully qualified domain name (FQDN) of this host.

[Tip] Tip

For the mobile PC without real FQDN, you may pick a TLD such as bogus "invalid" or mDNS "local" as the TLD part of <domain_name> in this configuration.

The "/etc/resolv.conf" is a static file if the resolvconf package is not installed. If installed, it is a symbolic link. Either way, it contains information that initialize the resolver routines. If the DNS is found at IP="192.168.11.1", it contains:

nameserver 192.168.11.1

The resolvconf package makes this "/etc/resolv.conf" into a symbolic link and manages its contents by the hook scripts automatically.

The hostname resolution via Multicast DNS (using Zeroconf, aka Apple Bonjour / Apple Rendezvous) which effectively allows name resolution by common Unix/Linux programs in the ad-hoc mDNS domain "local", can be provided by installing the libnss-mdns package. The "/etc/nsswitch.conf" file should have stanza like "hosts: files mdns4_minimal [NOTFOUND=return] dns mdns4" to enable this functionality.

5.1.3. The network interface name

The network interface name, e.g. eth0, is assigned to each hardware in the Linux kernel through the user space configuration mechanism, udev (see Section 3.5.10, “The udev system”), as it is found. The network interface name is referred as physical interface in ifup(8) and interfaces(5).

In order to ensure each network interface to be named persistently for each reboot using MAC address etc., there is a record file "/etc/udev/rules.d/70-persistent-net.rules". This file is automatically generated by the "/lib/udev/write_net_rules" program, probably run by the "persistent-net-generator.rules" rules file. You can modify it to change naming rule.

[Caution] Caution

When editing the "/etc/udev/rules.d/70-persistent-net.rules" rules file, you must keep each rule on a single line and the MAC address in lowercase. For example, if you find "Firewire device" and "PCI device" in this file, you probably want to name "PCI device" as eth0 and configure it as the primary network interface.

5.1.4. The network address range for the LAN

Let us be reminded of the IPv4 32 bit address ranges in each class reserved for use on the local area networks (LANs) by rfc1918. These addresses are guaranteed not to conflict with any addresses on the Internet proper.

Table 5.1. List of network address ranges.

Class network addresses net mask net mask /bits # of subnets
A 10.x.x.x 255.0.0.0 /8 1
B 172.16.x.x — 172.31.x.x 255.255.0.0 /16 16
C 192.168.0.x — 192.168.255.x 255.255.255.0 /24 256

[Note] Note

If one of these addresses is assigned to a host, then that host must not access the Internet directly but must access it through a gateway that acts as a proxy for individual services or else does Network Address Translation(NAT). The broadband router usually performs NAT for the consumer LAN environment.

5.1.5. The network configuration infrastructure

There are 2 types of low level networking programs for Linux networking system (see Section 5.6.1, “Iproute2 commands”).

  • Old net-tools programs (ifconfig(8), …) are from the Linux NET-3 networking system. Most of these are obsolete now.
  • New Linux iproute2 programs (ip(8), …) are the current Linux networking system.

Although these low level networking programs are powerful, they are cumbersome to use. So high level network configuration systems have been created.

The ifupdown package is the de facto standard for such high level network configuration system on Debian. It enables you to bring up network simply by doing , e.g., "ifup eth0". Its configuration file is the "/etc/network/interfaces" file and its typical contents are:

auto lo
iface lo inet loopback

auto eth0
iface eth0 inet dhcp

The resolvconf package was created to supplement ifupdown system to support smooth reconfiguration of network address resolution by automating rewrite of resolver configuration file "/etc/resolv.conf". Now, most Debian network configuration packages are modified to use resolvconf package (see "/usr/share/doc/resolvconf/README.Debian").

Helper scripts to the ifupdown package such as ifplugd, guessnet, ifscheme, etc. are created to automate dynamic configuration of network environment such as one for mobile PC on wired LAN. These are relatively difficult to use but play well with existing ifupdown system.

Alternative high level network configuration systems, independent of ifupdown system, such as network-manager, wicd, etc. are created to ease configuration of network environment even for mobile PC on wireless network. Since these are relatively new system and their integration to Debian system is in progress, you may still need to disable the corresponding network interface configuration manually in "/etc/network/interfaces" to avoid conflicts between these and ifupdown (see Section 5.5.2, “Automatic network configuration”).

Table 5.2. List of network configuration tools.

packages popcon size type function
ifupdown V:62, I:99 228 config::ifupdown Standardized tool to bring up and down the network (Debian specific)
ifplugd V:0.4, I:0.9 332 , , Manage the wired network automatically
ifupdown-extra V:0.05, I:0.2 124 , , Network testing script to enhance "ifupdown" package
ifmetric V:0.01, I:0.09 100 , , Set routing metrics for a network interface.
guessnet V:0.09, I:0.4 492 , , Mapping script to enhance "ifupdown" package via "/etc/network/interfaces" file
ifscheme V:0.02, I:0.10 132 , , Mapping scripts to enhance "ifupdown" package
ifupdown-scripts-zg2 V:0.00, I:0.05 220 , , Zugschlus' interface scripts for ifupdown's manual method
network-manager V:27, I:33 2180 config::NetworkManager NetworkManager (daemon): Manage the network automatically
network-manager-gnome V:18, I:29 3372 , , NetworkManager (GNOME frontend)
network-manager-kde V:2, I:4 3088 , , NetworkManager (KDE frontend)
wicd V:1.3, I:1.6 2484 config::wicd Wired and wireless network manager
iptables V:25, I:99 1384 config::Netfilter Administration tools for packet filtering and NAT
iproute V:36, I:76 1000 config::iproute2 IPv6 and other advanced network configuration: ip(8), tc(8), etc.
ifrename V:0.2, I:0.8 108 , , Rename network interfaces based on various static criteria: ifrename(8)
ethtool V:3, I:12 256 , , Display or change Ethernet device settings
iputils-ping V:36, I:99 124 test::iproute2 Tools to test network reachability of a remote host by hostname or IP address
iputils-arping V:1.0, I:15 72 , , Tools to test network reachability of a remote host specified by the ARP address
iputils-tracepath V:0.4, I:2 108 , , Tools to trace the network path to a remote host
net-tools V:72, I:99 1016 config::net-tools The NET-3 networking toolkit (IPv4 network configuration): ifconfig(8) etc.
inetutils-ping V:0.04, I:0.13 268 test::net-tools Tools to test network reachability of a remote host by hostname or IP address (legacy, GNU)
arping V:0.5, I:2 64 , , Tools to test network reachability of a remote host specified by the ARP address (legacy)
traceroute V:14, I:98 184 , , Tools to trace the network path to a remote host (legacy, console)
dhcp3-client V:46, I:93 604 config::low-level DHCP client
wpasupplicant V:12, I:41 960 , , Client support for WPA and WPA2 (IEEE 802.11i)
wireless-tools V:9, I:26 276 , , Tools for manipulating Linux Wireless Extensions
ppp V:7, I:26 1100 , , PPP/PPPoE connection with chat
pppoeconf V:0.5, I:4 200 config::helper Configuration helper for PPPoE connection
pppconfig V:0.3, I:3 900 , , Configuration helper for PPP connection with chat
wvdial V:0.6, I:2 352 , , Configuration helper for PPP connection with wvdial and ppp
mtr-tiny V:4, I:50 120 test::low-level Tools to trace the network path to a remote host (curses)
mtr V:0.5, I:2 176 , , Tools to trace the network path to a remote host (curses and GTK+)
gnome-nettool V:4, I:39 1862 , , Tools for common network information operations (GNOME)
nmap V:6, I:31 3768 , , Network mapper / port scanner (Nmap, console)
zenmap V:0.2, I:1.1 1232 , , Network mapper / port scanner (GTK+)
knmap V:0.12, I:0.7 1980 , , Network mapper / port scanner (KDE)
tcpdump V:3, I:22 796 , , Network traffic analyzer (Tcpdump, console)
wireshark V:1.6, I:10 1720 , , Network traffic analyzer (Wireshark, GTK+)
tshark V:0.6, I:3 308 , , Network traffic analyzer (console)
nagios3 V:0.8, I:1.2 4140 , , Monitoring and management system for hosts, services and networks (Nagios)
tcptrace V:0.06, I:0.4 432 , , Tool to produce a summarization of the connections from tcpdump output
snort V:0.7, I:0.9 1100 , , Flexible network intrusion detection system (Snort)
ntop V:1.2, I:2 15600 , , Display network usage in web browser
dnsutils V:13, I:91 392 , , Network clients provided with BIND: nslookup(8), nsupdate(8), dig(8)
dlint V:0.5, I:7 96 , , Checks DNS zone information using nameserver lookups
dnstracer V:0.10, I:0.6 88 , , Tool to trace a chain of DNS servers to the source

5.1.6. The network device support

Although most hardware devices are supported by the Debian system, there are some network devices which require DSFG non-free external hardware drivers to support them. Please see Section 9.7.7, “Non-free hardware drivers”.

5.2. The network connection method

[Caution] Caution

The connection test method described in this section are meant for testing purposes. It is not meant to be used directly for the daily network connection. You are advised to use them via the ifupdown package (see Section 5.3, “The basic network configuration with ifupdown”).

The typical network connection method and connection path for a PC can be summarized as:

Table 5.3. List of network connection methods and connection paths.

PC connection method connection path
Serial port (ppp0) PPP modem ⇔ POTS ⇔ dial-up access point ⇔ ISP
Ethernet port (eth0) PPPoE/DHCP/Static ⇔ BB-modem ⇔ BB service ⇔ BB access point ⇔ ISP
Ethernet port (eth0) DHCP/Static ⇔ LAN ⇔ BB-router with network address translation (NAT) (⇔ BB-modem …)

Here is the summary of configuration script for each connection method:

Table 5.4. List of network connection configurations.

connection method configuration backend package(s)
PPP pppconfig to create deterministic chat pppconfig, ppp
PPP (alternative) wvdialconf to create heuristic chat ppp, wvdial
PPPoE pppoeconf to create deterministic chat pppoeconf, ppp
DHCP described in "/etc/dhcp3/dhclient.conf" dhcp3-client
static IP (IPv4) described in "/etc/network/interfaces" net-tools
static IP (IPv6) described in "/etc/network/interfaces" iproute

The network connection acronyms mean:

Table 5.5. List of network connection acronyms.

acronym meaning
POTS The plain old telephone service
BB The broadband
BB-service E.g., the digital subscriber line (DSL), the cable TV, or the fiber to the premises (FTTP).
BB-modem E.g., the DSL modem, the cable modem, or the optical network terminal (ONT).
LAN The local area network
WAN The wide area network
DHCP The dynamic host configuration protocol
PPP The point-to-point protocol
PPPoE The point-to-point protocol over Ethernet
ISP The Internet service provider

[Note] Note

The WAN connection services via cable TV are generally served by DHCP or PPPoE. The ones by ADSL and FTTP are generally served by PPPoE. You have to consult your ISP for exact configuration requirements of the WAN connection.

[Note] Note

When BB-router is used to create home LAN environment, PCs on LAN are connected to the WAN via BB-router with network address translation (NAT). For such case, PC's network interfaces on the LAN are served by static IP or DHCP from the BB-router. BB-router must be configured to connect the WAN following the instruction by your ISP.

5.2.1. The DHCP connection with the Ethernet

The typical modern home and small business network, i.e. LAN, are connected to the WAN(Internet) using some consumer grade broadband router. The LAN behind this router is usually served by the dynamic host configuration protocol (DHCP) server running on the router.

Just install the dhcp3-client package for the Ethernet served by the dynamic host configuration protocol (DHCP).

5.2.2. The static IP connection with the Ethernet

No special action is needed for the Ethernet served by the static IP.

5.2.3. The PPP connection with pppconfig

The configuration script pppconfig will configure the PPP connection interactively just by selecting:

  • the telephone number,
  • the ISP user name,
  • the ISP password,
  • the port speed,
  • the modem communication port, and
  • the authentication method.

The configuration files are:

Table 5.6. List of configuration files for the PPP connection with pppconfig.

file function
/etc/ppp/peers/<isp_name> The pppconfig generated configuration file for pppd specific to <isp_name>
/etc/chatscripts/<isp_name> The pppconfig generated configuration file for chat specific to <isp_name>
/etc/ppp/options The general execution parameter for pppd
/etc/ppp/pap-secret Authentication data for the PAP (security risk)
/etc/ppp/chap-secret Authentication data for the CHAP (more secure)

[Caution] Caution

The "<isp_name>" value of "provider" is assumed if pon and poff commands are invoked without arguments.

You can test configuration using low level network configuration tools:

$ sudo pon <isp_name>
...
$ sudo poff <isp_name>

See "/usr/share/doc/ppp/README.Debian.gz" for more information.

5.2.4. The alternative PPP connection with wvdialconf

A different approach to using pppd(8) is to run it from wvdial(1) which comes in the wvdial package. Instead of pppd running chat(8) to dial in and negotiate the connection, wvdial does the dialing and initial negotiating and then starts pppd to do the rest.

The configuration script wvdialconf will configure the PPP connection interactively just by selecting:

  • the telephone number,
  • the ISP user name, and
  • the ISP password.

wvdial succeeds in making the connection in most cases and maintains authentication data list automatically.

The configuration files are:

Table 5.7. List of configuration files for the PPP connection with wvdialconf.

file function
/etc/ppp/peers/wvdial The wvdialconf generated configuration file for pppd specific to wvdial
/etc/wvdial.conf The wvdialconf generated configuration file
/etc/ppp/options The general execution parameter for pppd
/etc/ppp/pap-secret Authentication data for the PAP (security risk)
/etc/ppp/chap-secret Authentication data for the CHAP (more secure)

You can test configuration using low level network configuration tools:

$ sudo wvdial
...
$ sudo killall wvdial

See wvdial(1) and wvdial.conf(5) for more information.

5.2.5. The PPPoE connection with pppoeconf

When your ISP serves you with PPPoE connection and you decide to connect your PC directly to the WAN, the network of your PC must be configured with the PPPoE. The PPPoE stand for PPP over Ethernet. The configuration script pppoeconf will configure the PPPoE connection interactively.

The configuration files are:

Table 5.8. List of configuration files for the PPPoE connection with pppoeconf.

file function
/etc/ppp/peers/dsl-provider The pppoeconf generated configuration file for pppd specific to pppoe
/etc/ppp/options The general execution parameter for pppd
/etc/ppp/pap-secret Authentication data for the PAP (security risk)
/etc/ppp/chap-secret Authentication data for the CHAP (more secure)

You can test configuration using low level network configuration tools:

$ sudo /sbin/ifconfig eth0 up
$ sudo pon dsl-provider
...
$ sudo poff dsl-provider
$ sudo /sbin/ifconfig eth0 down

See "/usr/share/doc/pppoeconf/README.Debian" for more information.

5.3. The basic network configuration with ifupdown

The ifupdown package provides the standardized framework for the high level network configuration in the Debian system. In this section, we learn the basic network configuration with ifupdown with simplified introduction and many typical examples.

5.3.1. The command syntax simplified

The ifupdown package contains 2 commands: ifup(8) and ifdown(8). They offer high level network configuration dictated by the configuration file "/etc/network/interfaces".

Table 5.9. List of basic network configuration commands with ifupdown.

command action
ifup eth0 To bring up a network interface eth0 with the configuration eth0 if "iface eth0" stanza exists.
ifdown eth0 To bring down a network interface eth0 with the configuration eth0 if "iface eth0" stanza exists.

[Warning] Warning

Do not use low level configuration tools such as ifconfig(8) and ip(8) commands to configure an interface in up state.

[Note] Note

There is no command ifupdown.

5.3.2. The basic syntax of "/etc/network/interfaces"

The key syntax of "/etc/network/interfaces" as explained in interfaces(5) can be summarized as:

Table 5.10. List of stanzas in "/etc/network/interfaces"

stanza meaning
"auto <interface_name>" To start interface <interface_name> upon start of the system.
"allow-auto <interface_name>" , ,
"allow-hotplug <interface_name>" To start interface <interface_name> when the kernel detects a hotplug event from the interface.
Lines started with "iface <config_name> …" To define the network configuration <config_name>.
Lines started with "mapping <interface_name_glob> " To define mapping value of <config_name> for the matching <interface_name>.
A line starting with a hash "#" To be ignored as comments. (end-of-line comments are not supported)
A line ending with a backslash "\" To extend the configuration to the next line.

Lines started with iface stanza has the following syntax:

iface <config_name> <address_family> <method_name>
 <option1> <value1>
 <option2> <value2>
 ...

For the basic configuration, the mapping stanza is not used and you use the network interface name as the network configuration name (See Section 5.4.5, “The mapping stanza”).

[Warning] Warning

Do not define duplicates of the "iface" stanza for a network interface in "/etc/network/interfaces".

5.3.3. The loopback network interface

The following configuration entry in the "/etc/network/interfaces" file brings up the loopback network interface lo upon booting the system (via auto stanza).

auto lo
iface lo inet loopback

This one always exists in the "/etc/network/interfaces" file.

5.3.4. The network interface served by the DHCP

After prepairing the system by Section 5.2.1, “The DHCP connection with the Ethernet”, the network interface served by the DHCP is configured by creating the configuration entry in the "/etc/network/interfaces" file as:

allow-hotplug eth0
iface eth0 inet dhcp
 hostname "mymachine"

When the Linux kernel detects the physical interface eth0, the allow-hotplug stanza will cause ifup to bring up the interface and the iface stanza will cause ifup to use DHCP to configure the interface.

5.3.5. The network interface with the static IP

The network interface served by the static IP is configured by creating the configuration entry in the "/etc/network/interfaces" file as, e.g.,:

allow-hotplug eth0
iface eth0 inet static
 address 192.168.11.100
 netmask 255.255.255.0
 broadcast 192.168.11.255
 gateway 192.168.11.1
 dns-domain lan
 dns-nameservers 192.168.11.1

When the Linux kernel detects the physical interface eth0, the allow-hotplug stanza will cause ifup to bring up the interface and the iface stanza will cause ifup to use the static IP to configure the interface.

Here, I assumed:

  • IP address range of the LAN network: 192.168.11.0 - 192.168.11.255
  • IP address of the gateway: 192.168.11.1
  • IP address of the PC: 192.168.11.100
  • The resolvconf package is installed.
  • The domain name as "lan".
  • IP address of the DNS server: 192.168.11.1

When the resolvconf package is not installed, DNS related configuration needs to be done manually by editing the "/etc/resolv.conf" as:

nameserver 192.168.11.1
domain lan
[Caution] Caution

The IP addresses used in the above example are not meant to be copied literally. You have to adjust IP numbers to your actual network configuration.

5.3.6. The basics of wireless LAN interface

The wireless LAN (WLAN for short) provides the fast wireless connectivity through the spread-spectrum communication of unlicensed radio bands based on the set of standards called IEEE 802.11.

The WLAN interfaces are almost like normal Ethernet interfaces but require some network ID and encryption key data to be provided when they are initialized. Their high level network tools are exactly the same as that of Ethernet interfaces except interface names are a bit different like eth1, wlan0, ath0, wifi0, … depending on the kernel drivers used.

[Tip] Tip

The wmaster0 device is the master device which is an internal device used only by SoftMAC with new mac80211 API of Linux.

Here are some keywords to remember for the WLAN:

Table 5.11. List of acronyms for WLAN.

acronym full word meaning
NWID Network ID The 16 bit network ID used by pre-802.11 WaveLAN network. Very much deprecated.
(E)SSID (Extended) Service Set Identifier The network name of the Wireless Access Points (APs) interconnected to form an integrated 802.11 wireless LAN. Domain ID.
WEP, (WEP2) Wired Equivalent Privacy The 1st generation 64-bit (128-bit) wireless encryption standard with 40-bit key. Deprecated.
WPA Wi-Fi Protected Access The 2nd generation wireless encryption standard (most of 802.11i), compatible with WEP.
WPA2 Wi-Fi Protected Access 2 The 3rd generation wireless encryption standard (full 802.11i), non-compatible with WEP.

The actual choice of protocol is usually limited by the wireless router you deploy.

5.3.7. The wireless LAN interface with WPA/WPA2

You need to install the wpasupplicant package to support the WLAN with the new WPA/WPA2.

In case of the DHCP served IP on WLAN connection, the "/etc/network/interfaces" file entry should be:

allow-hotplug ath0
iface ath0 inet dhcp
 wpa-ssid homezone
 # hexadecimal psk is encoded from a plaintext passphrase
 wpa-psk 000102030405060708090a0b0c0d0e0f101112131415161718191a1b1c1d1e1f

See more on "/usr/share/doc/wpasupplicant/README.modes.gz".

5.3.8. The wireless LAN interface with WEP

You need to install the wireless-tools package to support the WLAN with the old WEP. (Your consumer grade router may still be using this insecure infrastructure but this is better than nothing.)

[Caution] Caution

Please note that your network traffic on WLAN with WEP may be sniffed by others.

In case of the DHCP served IP on WLAN connection, the "/etc/network/interfaces" file entry should be:

allow-hotplug eth0
iface eth0 inet dhcp
 wireless-essid Home
 wireless-key1 0123-4567-89ab-cdef
 wireless-key2 12345678
 wireless-key3 s:password
 wireless-defaultkey 2
 wireless-keymode open

See more on "/usr/share/doc/wireless-tools/README.Debian".

5.3.9. The PPP connection

You need to configure the PPP connection first as described before (see Section 5.2.3, “The PPP connection with pppconfig”). Then, add the "/etc/network/interfaces" file entry for the primary PPP device ppp0 as:

iface ppp0 inet ppp
 provider <isp_name>

5.3.10. The alternative PPP connection

You need to configure the alternative PPP connection with wvdial first as described before (see Section 5.2.4, “The alternative PPP connection with wvdialconf”). Then, add the "/etc/network/interfaces" file entry for the primary PPP device ppp0 as:

iface ppp0 inet wvdial

5.3.11. The PPPoE connection

For PC connected directly to the WAN served by the PPPoE, you need to configure system with the PPPoE connection as described before (see Section 5.2.5, “The PPPoE connection with pppoeconf”). Then, add the "/etc/network/interfaces" file entry for the primary PPPoE device eth0 as:

allow-hotplug eth0
iface eth0 inet manual
 pre-up /sbin/ifconfig eth0 up
 up ifup ppp0=dsl
 down ifdown ppp0=dsl
 post-down /sbin/ifconfig eth0 down
# The following is used internally only
iface dsl inet ppp
 provider dsl-provider

5.3.12. The network configuration state of ifupdown

The "/etc/network/run/ifstate" file stores the intended network configuration states for all the currently active network interfaces managed by the ifupdown package are listed. Unfortunately, even if the ifupdown system fails to bring up the interface as intended, the "/etc/network/run/ifstate" file lists it active.

Unless the output of the ifconfig(8) command for an interface does not have a line like following example, it can not be used as a part of IPV4 network:

  inet addr:192.168.11.2  Bcast:192.168.11.255  Mask:255.255.255.0
[Note] Note

For the Ethernet device connected to the PPPoE, the output of the ifconfig(8) command lacks a line which looks like above example.

5.3.13. The basic network reconfiguration

When you try to reconfigure the interface, e.g. eth0, you must disable it first with the "sudo ifdown eth0" command. This will remove the entry of eth0 from the "/etc/network/run/ifstate" file. (This may result in some error message if eth0 is not active or it is configured improperly previously. So far, it seems to be safe to do this for the simple single user work station at any time.)

You are now free to rewrite the "/etc/network/interfaces" contents as needed to reconfigure the network interface, eth0.

Then, you can reactivate eth0 with the "sudo ifup eth0" command.

[Tip] Tip

You can (re)initialize the network interface simply by "sudo ifdown eth0;sudo ifup eth0".

5.3.14. The ifupdown-extra package

The ifupdown-extra package provides the easy network connection test for use with the ifupdown package by:

  • the network-test(1) command from the shell, and
  • the automatic scripts run for each ifup command execution.

The network-test command frees you from the execution of cumbersome low level commands to analyze the network problem.

The automatic scripts are installed in "/etc/network/*/" and:

  • check the network cable connection,
  • check duplicate use of IP address,
  • setup system's static routes based on the "/etc/network/routes" definition,
  • check if network gateway is reachable, and
  • record results in the "/var/log/syslog" file.

This syslog record is quite useful for administration of the network problem on the remote system.

[Tip] Tip

The automatic behavior of the ifupdown-extra package is configurable with the "/etc/default/network-test". Some of these automatic checks slow down the system boot-up a little bit since it takes some time to listen for ARP replies.

5.4. The advanced network configuration with ifupdown

The functionality of the ifupdown package can be improved beyond what was described in Section 5.3, “The basic network configuration with ifupdown” with the advanced knowledge.

The functionalities described here are completely optional. I, being lazy and minimalist, rarely bother to use these.

[Caution] Caution

If you could not set up network connection by information in Section 5.3, “The basic network configuration with ifupdown”, you will make situation worse by using information below.

5.4.1. The ifplugd package

The ifplugd package is older automatic network configuration tool which can manage only Ethernet connections. This solves unplugged/replugged Ethernet cable issues for mobile PC etc.. If you have NetworkManager or Wicd (see Section 5.5.2, “Automatic network configuration”) installled, you do not need this package.

This package runs daemon and replaces auto or allow-hotplug functionalities (see Table 5.10, “List of stanzas in "/etc/network/interfaces"”) and starts interfaces upon their connection to the network.

Here is how to use the ifplugd package for the internal Ethernet port, e.g. eth0:

  • Remove stanza in "/etc/network/interfaces": "auto eth0" or "allow-hotplug eth0",
  • Keep stanza in "/etc/network/interfaces": "iface eth0 inet …" and "mapping …",
  • Install the ifplugd package,
  • Run "sudo dpkg-reconfigure ifplugd", and
  • Put eth0 as the "static interfaces to be watched by ifplugd".

Now, the network reconfiguration works as you desire:

  • Upon power-on or upon hardware discovery, the interface is not brought up by itself.

  • Upon finding the Ethernet cable, the interface is brought up.
  • Upon some time after unplugging the Ethernet cable, the interface is brought down automatically.
  • Upon plugging in another Ethernet cable, the interface will be brought up under the new network environment.
[Tip] Tip

The arguments for the ifplugd(8) command can set its behaviors such as the delay for reconfiguring interfaces.

5.4.2. The ifmetric package

The ifmeric package enables us to manipulate metrics of routes a posteriori even for DHCP.

The following will set the eth0 interface to be preferred over the wlan0 interface:

  • Install the ifmetric package, and
  • Add an option line with "metric 0" just below the "iface eth0 inet dhcp" line.
  • Add an option line with "metric 1" just below the "iface wlan0 inet dhcp" line.

The metric 0 means the highest priority route and is the default one. The larger metric value means lower priority routes. The IP address of the active interface with the lowest metric value becomes the originating one. See ifmetric(8).

5.4.3. The virtual interface

A single physical Ethernet interface can be configured as multiple virtual interfaces with different IP addresses. Usually the purpose is to connect an interface to several IP subnetworks. For example, IP address based virtual web hosting by a single network interface is one such application.

For example, let's suppose that

  • a single Ethernet interface on your host is connected to a Ethernet hub (not to the broadband router),
  • the Ethernet hub is connected to both the Internet and LAN network,
  • the LAN network uses subnet 192.168.0.x/24,
  • your host uses DHCP served IP address with the physical interface eth0 for the Internet, and
  • your host uses 192.168.0.1 with the virtual interface eth0:0 for the LAN,

then following stanzas in "/etc/network/interfaces" will configure your network:

iface eth0 inet dhcp
 metric 0
iface eth0:0 inet static
 address 192.168.0.1
 netmask 255.255.255.0
 network 192.168.0.0
 broadcast 192.168.0.255
 metric 1
[Caution] Caution

Although this configuration example with network address translation (NAT) using netfilter/iptables (see Section 5.8, “Netfilter infrastructure”) can provide cheap router for the LAN with only single interface, there is no real firewall capability with such set up. You should use 2 physical interfaces with NAT to secure the local network from the Internet.

5.4.4. The advanced command syntax

The ifupdown package offers advanced network configuration using the network configuration name and the network interface name. I use slightly different terminology from one used in ifup(8) and interfaces(5).

Table 5.12. List of terminology for network devices.

manpage terminology my terminology examples in the following text explanation
physical interface name network interface name lo, eth0, <interface_name> A name given by the Linux kernel (using udev mechanism).
logical interface name network configuration name config1, config2, <config_name> A name token following iface in the "/etc/network/interfaces".

Basic network configuration commands in Section 5.3.1, “The command syntax simplified” require the network configuration name token of the iface stanza to match the network interface name in the "/etc/network/interfaces".

Advanced network configuration commands enables separation of the network configuration name and the network interface name in the "/etc/network/interfaces":

Table 5.13. List of advanced network configuration commands with ifupdown.

command action
ifup eth0=config1 To bring up a network interface eth0 with the configuration config1.
ifdown eth0=config1 To bring down a network interface eth0 with the configuration config1.
ifup eth0 To bring up a network interface eth0 with the configuration selected by mapping stanza.
ifdown eth0 To bring down a network interface eth0 with the configuration selected by mapping stanza.

5.4.5. The mapping stanza

We skipped explaining the mapping stanza in the "/etc/network/interfaces" in Section 5.3.2, “The basic syntax of "/etc/network/interfaces"” to avoid complication. This stanza has the following syntax:

mapping <interface_name_glob>
 script <script_name>
 map <script_input1>
 map <script_input2>
 map ...

This provides advanced feature to the "/etc/network/interfaces" file by automating the choice of the configuration with the mapping script specified by <script_name>.

When the "<interface_name_glob>" matches "eth0", the execution of

$ sudo ifup eth0

will produce the execution of:

$ sudo ifup eth0=$(echo -e '<script_input1> \n <script_input2> \n ...' | <script_name> eth0)

to configure eth0 automatically. Here, lines with "map" are optional and can be repeated.

[Note] Note

The glob for mapping stanza works like shell filename glob (see Section 1.5.6, “Shell glob”).

5.4.6. The manually switchable network configuration

Here is how to switch manually among several network configurations without rewriting the "/etc/network/interfaces" file as in Section 5.3.13, “The basic network reconfiguration” .

For all the network configuration you need to access, you create a single "/etc/network/interfaces" file, e.g,:

auto lo
iface lo inet loopback

iface config1 inet dhcp
 hostname "mymachine"

iface config2 inet static
 address 192.168.11.100
 netmask 255.255.255.0
 broadcast 192.168.11.255
 gateway 192.168.11.1
 dns-domain lan
 dns-nameservers 192.168.11.1

iface pppoe inet manual
 pre-up /sbin/ifconfig eth0 up
 up ifup ppp0=dsl
 down ifdown ppp0=dsl
 post-down /sbin/ifconfig eth0 down

# The following is used internally only
iface dsl inet ppp
 provider dsl-provider

iface pots inet ppp
 provider provider

Please note the network configuration name which is the token after iface does not use the token for the network interface name. Also, there are no auto stanza nor allow-hotplug stanza to start the network interface eth0 automatically upon events.

Now you are ready to switch the network configuration.

Let's move your PC to a LAN served by the DHCP. You bring up the network interface (the physical interface) eth0 by assigning the network configuration name (the logical interface name) config1 to it:

$ sudo ifup eth0=config1
Password:
...

The interface eth0 is up, configured by DHCP and connected to LAN.

$ sudo ifdown eth0=config1
...

The interface eth0 is down and disconnected from LAN.

Let's move your PC to a LAN served by the static IP. You bring up the network interface eth0 by assigning the network configuration name config2 to it:

$ sudo ifup eth0=config2
...

The interface eth0 is up, configured with static IP and connected to LAN. The additional parameters given as dns-* configures "/etc/resolv.conf" contents. This "/etc/resolv.conf" is better manged if the resolvconf package is installed.

$ sudo ifdown eth0=config2
...

The interface eth0 is down and disconnected from LAN, again.

Let's move your PC to a port on BB-modem connected to the PPPoE served service. You bring up the network interface eth0 by assigning the network configuration name pppoe to it:

$ sudo ifup eth0=pppoe
...

The interface eth0 is up, configured with PPPoE connection directly to the ISP.

$ sudo ifdown eth0=pppoe
...

The interface eth0 is down and disconnected, again.

Let's move your PC to a location without LAN or BB-modem but with POTS and modem. You bring up the network interface ppp0 by assigning the network configuration name pots to it:

$ sudo ifup ppp0=pots
...

The interface ppp0 is up and connected to the Internet with PPP.

$ sudo ifdown ppp0=pots
...

The interface ppp0 is down and disconnected from the Internet.

You should check the "/etc/network/run/ifstate" file for the current network configuration state of the ifupdown system.

[Warning] Warning

You may need to adjust numbers at the end of eth*, ppp*, etc. if you have multiple network interfaces.

5.4.7. Scripting with the ifupdown system

The ifupdown system automatically runs scripts installed in "/etc/network/*/" while exporting environment variables to scripts:

Table 5.14. List of environment variables passed by the ifupdown system

environment variable value passed
"$IFACE" physical name (interface name) of the interface being processed.
"$LOGICAL" logical name (configuration name) of the interface being processed.
"$ADDRFAM" <address_family> of the interface.
"$METHOD" <method_name> of the interface. (e.g., "static")
"$MODE" "start" if run from ifup, "stop" if run from ifdown.
"$PHASE" as per "$MODE", but with finer granularity, distinguishing the pre-up, post-up, pre-down and post-down phases.
"$VERBOSITY" indicates whether "--verbose" was used; set to 1 if so, 0 if not.
"$PATH" the command search path: "/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin"
"$IF_<OPTION>" the value for the corresponding option under the iface stanza.

Here, each environment variable, "$IF_<OPTION>", is created from the name for the corresponding option such as <option1> and <option2> by prepending "$IF_", converting the case to the upper case, replacing hyphens to underscores, and discarding non-alphanumeric characters.

[Tip] Tip

See Section 5.3.2, “The basic syntax of "/etc/network/interfaces"” for <address_family>, <method_name>, <option1> and <option2>.

The ifupdown-extra package (see Section 5.3.14, “The ifupdown-extra package”) uses these environment variables to extend the functionality of the ifupdown package. The ifmetric package (see Section 5.4.2, “The ifmetric package”) installs the "/etc/network/if-up.d/ifmetric" script which sets the metric via the "$IF_METRIC" variable. The guessnet package (see Section 5.4.8, “Mapping with guessnet”), which provides simple and powerful framework for the auto-selection of the network configuration via the mapping mechanism, also uses these.

[Note] Note

For more specific examples of custom network configuration scripts using these environment variables, you should check example scripts in "/usr/share/doc/ifupdown/examples/*" and scripts used in ifscheme and ifupdown-scripts-zg2 packages. These additional scripts have some overlaps of functionalities with basic ifupdown-extra and guessnet packages. If you install these additional scripts, you should customize these scripts to avoid interferences.

5.4.8. Mapping with guessnet

Instead of manually choosing configuration as described in Section 5.4.6, “The manually switchable network configuration”, you can use the mapping mechanism described in Section 5.4.5, “The mapping stanza” to select network configuration automatically with custom scripts.

The guessnet-ifupdown(8) command provided by the guessnet package is designed to be used as a mapping script and provides powerful framework to enhance the ifupdown system.

  • you list test condition as the value for guessnet options for each network configuration under iface stanza.
  • mapping will chose the iface with first non-ERROR result as the network configuration.

This dual usage of the "/etc/network/interfaces" file by the mapping script, guessnet-ifupdown, and the original network configuration infrastructure, ifupdown, does not cause negative impacts since guessnet options only export extra environment variables to scripts run by the ifupdown system. See details in guessnet-ifupdown(8).

[Note] Note

When multiple guessnet option lines are required in "/etc/network/interfaces", use option lines started with guessnet1, guessnet2, and so on, since the ifupdown package does not allow starting strings of option lines to be repeated.

5.5. The network configuration for desktop

5.5.1. GUI network configuration tools

The capability of default GUI network configuration tools for each desktop environments such as GNOME tends to be limited to basic configurations such as static IP or DHCP. They actually overwrite contents of "/etc/network/interfaces" file behind you. Please check how they change "/etc/network/interfaces" file by yourself.

[Caution] Caution

They may not understand complicated advanced configuration done manually in "/etc/network/interfaces" file.

5.5.2. Automatic network configuration

There are independent automatic network configuration tools, such as NetworkManager (NM) (network-manager and associated packages) and Wicd (wicd package) which manage network connection via daemon independen of the ifupdown package. They allow easy management of wireless connections. These come with its own nice GUI user interfaces.

[Warning] Warning

Do not use these automatic network configuration tools for servers. These are aimed primarily for mobile desktop users on laptops.

[Warning] Warning

These automatic network configuration tools are moving targets and documentation here is likely to be incorrect for squeeze. So be warned.

[Caution] Caution

These automatic network configuration tools may not be compatible with esoteric configurations of ifupdown in "/etc/network/interfaces" such as ones in Section 5.3, “The basic network configuration with ifupdown” and Section 5.4, “The advanced network configuration with ifupdown”. Having even "hostname" stanza for DHCP controlled interface as described in Section 5.3.4, “The network interface served by the DHCP” caused NM to ignore such interface in lenny. Check BTS of network-manager and BTS of wicd for current issues and limitations.

The configuration of NM is described in "/usr/share/doc/network-manager/README.Debian". Essentially:

  1. Make desktop user, e.g. foo, belong to group "netdev" by:

    $ sudo adduser foo netdev
  2. Keep configuration of "/etc/network/interfaces" as simple as:

    auto lo
    iface lo inet loopback
    
    auto eth0
    iface eth0 inet dhcp
  3. Restart NM by:

    $ sudo /etc/init.d/network-manager restart
[Note] Note

Only interfaces which are not listed in "/etc/network/interfaces" or which have been configured with "auto …" or "allow-hotplug …" and "iface … inet dhcp" (with no other options) are managed by NM to avoid conflict with ifupdown.

The configuration of Wicd is described in "/usr/share/doc/wicd/README.Debian". Essentially:

  1. Make configuration in "/etc/network/interfaces" only as:

    auto lo
    iface lo inet loopback
  2. Restart Wicd.

    $ sudo /etc/init.d/wicd restart

5.6. The low level network configuration

5.6.1. Iproute2 commands

Iproute2 commands offer complete low-level network configuration capabilities. Here is a translation table from obsolete net-tools commands to new iproute2 etc. commands.

Table 5.15. Translation table from obsolete net-tools commands to new iproute2 commands.

obsolete net-tools new iproute2 etc. manipulation
ifconfig(8) ip addr protocol (IP or IPv6) address on a device.
route(8) ip route routing table entry.
arp(8) ip neigh ARP or NDISC cache entry.
ipmaddr ip maddr multicast address.
iptunnel ip tunnel tunnel over IP.
nameif(8) ifrename(8) name network interfaces based on MAC addresses.
mii-tool(8) ethtool(8) Ethernet device settings.

See ip(8) and IPROUTE2 Utility Suite Howto.

5.6.2. Safe low level network operations

You may use low level network commands as follows safely since they do not change network configuration:

Table 5.16. List of low level network commands.

command description
ifconfig displays the link and address status of active interfaces
ip addr show displays the link and address status of active interfaces
route -n displays all the routing table in numerical addresses
ip route show displays all the routing table in numerical addresses
arp displays the current content of the ARP cache tables
ip neigh displays the current content of the ARP cache tables
plog display ppp daemon log
ping yahoo.com check the Internet connection to "yahoo.com"
whois yahoo.com check who registered "yahoo.com" in the domains database
traceroute yahoo.com trace the Internet connection to "yahoo.com"
tracepath yahoo.com trace the Internet connection to "yahoo.com"
mtr yahoo.com trace the Internet connection to "yahoo.com" (repeatedly)
dig [@dns-server.com] example.com [{a|mx|any}] check DNS records of "example.com" by "dns-server.com" for a "a", "mx", or "any" record
iptables -L -n check packet filter
netstat -a find all open ports
netstat -l --inet find listening ports
netstat -ln --tcp find listening TCP ports (numeric)
dlint example.com check DNS zone information of "examle.org"

[Tip] Tip

Some of these low level network configuration tools reside in "/sbin/". You may need to issue full command path such as "/sbin/ifconfig" or add "/sbin" to the "$PATH" list in your "~/.bashrc".

5.7. Network optimization

Generic network optimization is beyond the scope of this documentation. I will touch only subjects pertinent to the consumer grade connection.

Table 5.17. List of network optimization tools.

packages popcon size description
iftop V:1.1, I:6 72 displays bandwidth usage information on an network interface
iperf V:0.3, I:3 200 Internet Protocol bandwidth measuring tool
apt-spy V:0.2, I:1.5 204 writes a "/etc/apt/sources.list" file based on bandwidth tests
ifstat V:0.2, I:1.0 88 InterFace STATistics Monitoring
bmon V:0.2, I:0.8 188 portable bandwidth monitor and rate estimator
ethstatus V:0.13, I:0.7 84 script that quickly measures network device throughput
bing V:0.10, I:0.7 96 Empirical stochastic bandwidth tester
bwm-ng V:0.19, I:1.0 152 small and simple console-based bandwidth monitor
ethstats V:0.07, I:0.3 52 console-based Ethernet statistics monitor
ipfm V:0.04, I:0.15 156 a bandwidth analysis tool

5.7.1. Finding optimal MTU

The Maximum Transmission Unit (MTU) value can be determined experimentally with ping(8) with "-M do" option which sends ICMP packets with data size starting from 1500 (with offset of 28 bytes for the IP+ICMP header) and finding the largest size without IP fragmentation. For example:

$ ping -c 1 -s $((1500-28)) -M do www.debian.org
PING www.debian.org (194.109.137.218) 1472(1500) bytes of data.
From 192.168.11.2 icmp_seq=1 Frag needed and DF set (mtu = 1454)

--- www.debian.org ping statistics ---
0 packets transmitted, 0 received, +1 errors
  • … try 1454 instead of 1500
  • The ping(8) command succeed

This process is Path MTU (PMTU) discovery (RFC1191) and the tracepath(8) command can automate this.

[Tip] Tip

The above example with PMTU value of 1454 is for my previous FTTP provider which used Asynchronous Transfer Mode (ATM) as its backbone network and served its clients with the PPPoE. The actual PMTU value depends on your environment, e.g., 1500 for the my new FTTP provider.

Table 5.18. Basic guide lines of the optimal MTU value

network environment MTU rationale
Dial-up link (IP: PPP) 576 standard
Ethernet link (IP: DHCP or fixed) 1500 standard and default
Ethernet link (IP: PPPoE) 1492 (=1500-8) 2 bytes for PPP header and 6 bytes for PPPoE header
Ethernet link (ISP's backbone: ATM, IP: DHCP or fixed) 1462 (=48*31-18-8) author's speculation: 18 for Ethernet header, 8 for SAR trailer.
Ethernet link (ISP's backbone: ATM, IP: PPPoE) 1454 (=48*31-8-18-8) see "Optimal MTU configuration for PPPoE ADSL Connections" for rationale.

In addtion to these basic guide lines, you should know:

  • Any use of tunneling methods (VPN etc.) may reduce optimal MTU further by their overheads.
  • The MTU value should not exceed the experimentally determined PMTU value.
  • The bigger MTU value is generally better when other limitations are met.

5.7.2. Setting MTU

Here are examples for setting the MTU value from its default 1500 to 1454.

For the DHCP (see Section 5.3.4, “The network interface served by the DHCP”), you can replace pertinent iface stanza lines in the "/etc/network/interfaces" with, e.g.,:

iface eth0 inet dhcp
 hostname "mymachine"
 pre-up /sbin/ifconfig $IFACE mtu 1454

For the static IP (see Section 5.3.5, “The network interface with the static IP”), you can replace pertinent 'iface' stanza lines in the "/etc/network/interfaces" with, e.g.,:

iface eth0 inet static
 address 192.168.11.100
 netmask 255.255.255.0
 broadcast 192.168.11.255
 gateway 192.168.11.1
 mtu 1454
 dns-domain lan
 dns-nameservers 192.168.11.1

For the direct PPPoE (see Section 5.2.5, “The PPPoE connection with pppoeconf”), you can replace pertinent "mtu" line in the "/etc/ppp/peers/dsl-provider" with:

mtu 1454

The maximum segment size (MSS) is used as an alternative measure of packet size. The relationship between MSS and MTU are:

  • MSS = MTU - 40 for IPv4
  • MSS = MTU - 60 for IPv6
[Note] Note

The iptables(8) (see Section 5.8, “Netfilter infrastructure”) based optimization can clamp packet size by the MSS and is useful for the router.

5.7.3. WAN TCP optimization

The TCP throughput can be maximized by adjusting TCP buffer size parameters as described in "TCP Tuning Guide" and "TCP tuning" for the modern high-bandwidth and high-latency WAN. So far, the current Debian default settings serve well even for my LAN connected by the fast 1G bps FTTP service.

5.8. Netfilter infrastructure

Netfilter provides infrastructure for stateful firewall and network address translation (NAT) with Linux kernel modules (see Section 3.5.11, “The kernel module initialization”).

Table 5.19. List of firewall tools.

packages popcon size description
iptables V:25, I:99 1384 administration tools for netfilter
iptstate V:0.16, I:0.9 156 Tool to continuously monitor netfilter state. (similar to top(1))
shorewall-perl V:0.18, I:0.4 608 Shoreline Firewall, netfilter configuration file generator (Perl-based, recommended for lenny)
shorewall-shell I:1.7 348 Shoreline Firewall, netfilter configuration file generator (shell-based, alternative for lenny)
ipmasq V:0.3, I:0.4 NOT_FOUND Simple set of init script to configure netfilter (old)

Main user space program of netfilter is iptables(8). You can manually configure netfilter interactively from shell, save its state with iptables-save(8), and restore it via init script with iptables-restore(8) upon system reboot.

Configuration helper scripts such as shorewall ease this process.

See documentation at http://www.netfilter.org/documentation/ (or in "/usr/share/doc/iptables/html/"):

[Tip] Tip

Although these were written for Linux 2.4, both iptables(8) command and netfilter kernel function apply for current Linux 2.6.