Chapter 5. Network setup

Table of Contents

5.1. The basic network infrastructure
5.1.1. The hostname resolution
5.1.2. The network interface name
5.1.3. The network address range for the LAN
5.1.4. The network device support
5.2. The modern network configuration for desktop
5.2.1. GUI network configuration tools
5.3. The legacy network connection and configuration
5.4. The network connection method (legacy)
5.4.1. The DHCP connection with the Ethernet
5.4.2. The static IP connection with the Ethernet
5.4.3. The PPP connection with pppconfig
5.4.4. The alternative PPP connection with wvdialconf
5.4.5. The PPPoE connection with pppoeconf
5.5. The basic network configuration with ifupdown (legacy)
5.5.1. The command syntax simplified
5.5.2. The basic syntax of "/etc/network/interfaces"
5.5.3. The loopback network interface
5.5.4. The network interface served by the DHCP
5.5.5. The network interface with the static IP
5.5.6. The basics of wireless LAN interface
5.5.7. The wireless LAN interface with WPA/WPA2
5.5.8. The wireless LAN interface with WEP
5.5.9. The PPP connection
5.5.10. The alternative PPP connection
5.5.11. The PPPoE connection
5.5.12. The network configuration state of ifupdown
5.5.13. The basic network reconfiguration
5.5.14. The ifupdown-extra package
5.6. The advanced network configuration with ifupdown (legacy)
5.6.1. The ifplugd package
5.6.2. The ifmetric package
5.6.3. The virtual interface
5.6.4. The advanced command syntax
5.6.5. The mapping stanza
5.6.6. The manually switchable network configuration
5.6.7. Scripting with the ifupdown system
5.6.8. Mapping with guessnet
5.7. The low level network configuration
5.7.1. Iproute2 commands
5.7.2. Safe low level network operations
5.8. Network optimization
5.8.1. Finding optimal MTU
5.8.2. Setting MTU
5.8.3. WAN TCP optimization
5.9. Netfilter infrastructure
[Tip] Tip

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

[Tip] Tip

Although this document still uses old ifconfig(8) with IPv4 for its network configuration examples, Debian is moving to ip(8) with IPv4+IPv6 in the wheezy release. Patches to update this document are welcomed.

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

Table 5.1. List of network configuration tools

packages popcon size type description
ifupdown V:580, I:996 119 config::ifupdown standardized tool to bring up and down the network (Debian specific)
ifplugd V:3, I:10 352 , , manage the wired network automatically
ifupdown-extra V:0, I:2 120 , , network testing script to enhance "ifupdown" package
ifmetric V:0, I:1 21 , , set routing metrics for a network interface
guessnet V:0, I:1 532 , , mapping script to enhance "ifupdown" package via "/etc/network/interfaces" file
ifscheme V:0, I:0 132 , , mapping scripts to enhance "ifupdown" package
ifupdown-scripts-zg2 V:0, I:0 147 , , Zugschlus' interface scripts for ifupdown's manual method
network-manager V:364, I:483 4650 config::NM NetworkManager (daemon): manage the network automatically
network-manager-gnome V:256, I:428 6267 , , NetworkManager (GNOME frontend)
plasma-widget-networkmanagement V:40, I:66 8067 , , NetworkManager (KDE frontend)
wicd I:24 16 config::wicd wired and wireless network manager (metapackage)
wicd-cli V:0, I:3 28 , , wired and wireless network manager (command-line client)
wicd-curses V:1, I:6 128 , , wired and wireless network manager (Curses client)
wicd-daemon V:26, I:30 1103 , , wired and wireless network manager (daemon)
wicd-gtk V:20, I:27 387 , , wired and wireless network manager (GTK+ client)
iptables V:238, I:993 1400 config::Netfilter administration tools for packet filtering and NAT (Netfilter)
iproute V:592, I:982 39 config::iproute2 iproute2, IPv6 and other advanced network configuration: ip(8), tc(8), etc
ifrename V:2, I:3 192 , , rename network interfaces based on various static criteria: ifrename(8)
ethtool V:116, I:208 314 , , display or change Ethernet device settings
iputils-ping V:322, I:997 153 test::iproute2 test network reachability of a remote host by hostname or IP address (iproute2)
iputils-arping V:3, I:26 76 , , test network reachability of a remote host specified by the ARP address
iputils-tracepath V:26, I:290 110 , , trace the network path to a remote host
net-tools V:667, I:998 910 config::net-tools NET-3 networking toolkit (net-tools, IPv4 network configuration): ifconfig(8) etc.
inetutils-ping V:0, I:1 338 test::net-tools test network reachability of a remote host by hostname or IP address (legacy, GNU)
arping V:3, I:29 46 , , test network reachability of a remote host specified by the ARP address (legacy)
traceroute V:96, I:994 176 , , trace the network path to a remote host (legacy, console)
isc-dhcp-client V:495, I:893 1765 config::low-level DHCP client
wpasupplicant V:388, I:539 1364 , , client support for WPA and WPA2 (IEEE 802.11i)
wpagui V:0, I:5 790 , , Qt GUI client for wpa_supplicant
wireless-tools V:81, I:271 325 , , tools for manipulating Linux Wireless Extensions
ppp V:59, I:537 746 , , PPP/PPPoE connection with chat
pppoeconf V:1, I:20 340 config::helper configuration helper for PPPoE connection
pppconfig V:0, I:9 990 , , configuration helper for PPP connection with chat
wvdial V:2, I:13 276 , , configuration helper for PPP connection with wvdial and ppp
mtr-tiny V:10, I:84 123 test::low-level trace the network path to a remote host (curses)
mtr V:6, I:34 167 , , trace the network path to a remote host (curses and GTK+)
gnome-nettool V:27, I:370 2752 , , tools for common network information operations (GNOME)
nmap V:62, I:465 17329 , , network mapper / port scanner (Nmap, console)
zenmap V:2, I:13 2651 , , network mapper / port scanner (GTK+)
tcpdump V:27, I:200 1037 , , network traffic analyzer (Tcpdump, console)
wireshark V:11, I:76 2569 , , network traffic analyzer (Wireshark, GTK+)
tshark V:3, I:32 338 , , network traffic analyzer (console)
nagios3 V:2, I:14 29 , , monitoring and management system for hosts, services and networks (Nagios)
tcptrace V:0, I:2 389 , , produce a summarization of the connections from tcpdump output
snort V:2, I:3 1707 , , flexible network intrusion detection system (Snort)
ntop V:5, I:11 1549 , , display network usage in web browser
dnsutils V:116, I:930 333 , , network clients provided with BIND: nslookup(8), nsupdate(8), dig(8)
dlint V:2, I:58 96 , , check DNS zone information using nameserver lookups
dnstracer V:0, I:3 81 , , trace a chain of DNS servers to the source

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

  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.

For example, "/etc/hosts" looks like the following.

127.0.0.1 localhost
127.0.1.1 <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

Each line starts with a IP address and it is followed by the associated hostname.

The IP address 127.0.1.1 in the second line of this example may not be found on some other Unix-like systems. The Debian Installer creates this entry for a system without a permanent IP address as a workaround for some software (e.g., GNOME) as documented in the bug #719621.

The <host_name> matches the hostname defined in the "/etc/hostname".

For a system with a permanent IP address, that permanent IP address should be used here instead of 127.0.1.1.

For a system with a permanent IP address and a fully qualified domain name (FQDN) provided by the Domain Name System (DNS), that canonical <host_name>.<domain_name> should be used instead of just <host_name>.

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 the following.

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.

For the PC workstation on the typical adhoc LAN environment, the hostname can be resolved via Multicast DNS (mDNS, Zeroconf) in addition to the basic files and dns methods.

  • Avahi provides a framework for Multicast DNS Service Discovery on Debian.

  • It is equivalent of Apple Bonjour / Apple Rendezvous.

  • The libnss-mdns plugin package provides host name resolution via mDNS for the GNU Name Service Switch (NSS) functionality of the GNU C Library (glibc).

  • The "/etc/nsswitch.conf" file should have stanza like "hosts: files mdns4_minimal [NOTFOUND=return] dns mdns4".

  • Host names ending with the ".local" pseudo-top-level domain (TLD) are resolved.

  • The mDNS IPv4 link-local multicast address "224.0.0.251" or its IPv6 equivalent "FF02::FB" are used to make DNS query for a name ending with ".local".

The hostname resolution via deprecated NETBios over TCP/IP used by the older Windows system can be provided by installing the winbind package. The "/etc/nsswitch.conf" file should have stanza like "hosts: files mdns4_minimal [NOTFOUND=return] dns mdns4 wins" to enable this functionality. (Modern Windows system usually use the dns method for the hostname resolution.)

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 3.3, “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 rules 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.

Debian squeeze and newer can manage the network connection via management daemon software such as NetworkManager (NM) (network-manager and associated packages) or Wicd (wicd and associated packages).

  • They come with their own GUI and command-line programs as their user interfaces.

  • They come with their own daemon as their backend system.

  • They allow easy connection of your system to the Internet.

  • They allow easy management of wired and wireless network configuration.

  • They allow us to configure network independent of the legacy ifupdown package.

[Note] Note

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

These modern network configuration tools need to be configured properly to avoid conflicting with the legacy ifupdown package and its configuration file "/etc/network/interfaces".

[Note] Note

Some features of these automatic network configuration tools may suffer regressions. These are not as robust as the legacy ifupdown package. Check BTS of network-manager and BTS of wicd for current issues and limitations.

Official documentations for NM and Wicd on Debian are provided in "/usr/share/doc/network-manager/README.Debian" and "/usr/share/doc/wicd/README.Debian", respectively.

Essentially, the network configuration for desktop is done as follows.

  1. Make desktop user, e.g. foo, belong to group "netdev" by the following (Alternatively, do it automatically via D-bus under modern desktop environments such as GNOME and KDE).

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

    auto lo
    iface lo inet loopback
  3. Restart NM or Wicd by the following.

    $ sudo /etc/init.d/network-manager restart
    $ sudo /etc/init.d/wicd restart
  4. Configure your network via GUI.

[Note] Note

Only interfaces which are not listed in "/etc/network/interfaces" are managed by NM or Wicd to avoid conflict with ifupdown.

[Tip] Tip

If you wish to extend network configuration capabilities of NM, please seek appropriate plug-in modules and supplemental packages such as network-manager-openconnect, network-manager-openvpn-gnome, network-manager-pptp-gnome, mobile-broadband-provider-info, gnome-bluetooth, etc. The same goes for those of Wicd.

[Caution] Caution

These automatic network configuration tools may not be compatible with esoteric configurations of legacy ifupdown in "/etc/network/interfaces" such as ones in Section 5.5, “The basic network configuration with ifupdown (legacy)” and Section 5.6, “The advanced network configuration with ifupdown (legacy)”. Check BTS of network-manager and BTS of wicd for current issues and limitations.

When the method described in Section 5.2, “The modern network configuration for desktop” does not suffice your needs, you should use the legacy network connection and configuration method which combines many simpler tools.

The legacy network connection is specific for each method (see Section 5.4, “The network connection method (legacy)”).

There are 2 types of programs for the low level network configuration on Linux (see Section 5.7.1, “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 the following.

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.

These are explained in detail with examples (see Section 5.5, “The basic network configuration with ifupdown (legacy)” and Section 5.6, “The advanced network configuration with ifupdown (legacy)”).

[Caution] Caution

The connection test methods 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 NM, Wicd, or the ifupdown package instead (see Section 5.2, “The modern network configuration for desktop” and Section 5.5, “The basic network configuration with ifupdown (legacy)”).

The typical network connection method and connection path for a PC can be summarized as the following.


Here is the summary of configuration scripts for each connection method.


The network connection acronyms mean the following.


[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.

The configuration script pppconfig configures the PPP connection interactively just by selecting the following.

  • The telephone number

  • The ISP user name

  • The ISP password

  • The port speed

  • The modem communication port

  • The authentication method

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 as the following.

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

See "/usr/share/doc/ppp/README.Debian.gz".

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

These traditional setup methods are quite useful if you wish to set up advanced configuration; find details in the following.

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.

After preparing the system by Section 5.4.1, “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 the following.

allow-hotplug eth0
iface eth0 inet dhcp

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


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

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 as the following.

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

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

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 as the following.

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 "/usr/share/doc/wireless-tools/README.Debian".

You need to configure the PPP connection first as described before (see Section 5.4.3, “The PPP connection with pppconfig”). Then, add the "/etc/network/interfaces" file entry for the primary PPP device ppp0 as the following.

iface ppp0 inet ppp
 provider <isp_name>

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 5.4.5, “The PPPoE connection with pppoeconf”). Then, add the "/etc/network/interfaces" file entry for the primary PPPoE device eth0 as the following.

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

The functionality of the ifupdown package can be improved beyond what was described in Section 5.5, “The basic network configuration with ifupdown (legacy)” 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 5.5, “The basic network configuration with ifupdown (legacy)”, you make situation worse by using information below.

The ifplugd package is an 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 5.2, “The modern network configuration for desktop”) installed, you do not need this package.

This package runs a 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.

  1. Remove stanza in "/etc/network/interfaces": "auto eth0" or "allow-hotplug eth0".

  2. Keep stanza in "/etc/network/interfaces": "iface eth0 inet …" and "mapping …".

  3. Install the ifplugd package.

  4. Run "sudo dpkg-reconfigure ifplugd".

  5. 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 is 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.

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


Basic network configuration commands in Section 5.5.1, “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" as the following.


We skipped explaining the mapping stanza in the "/etc/network/interfaces" in Section 5.5.2, “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 features to the "/etc/network/interfaces" file by automating the choice of the configuration with the mapping script specified by <script_name>.

Let's follow the execution of the following.

$ sudo ifup eth0

When the "<interface_name_glob>" matches "eth0", this execution produces the execution of the following command to configure eth0 automatically.

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

Here, script input 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”).

Here is how to switch manually among several network configurations without rewriting the "/etc/network/interfaces" file as in Section 5.5.13, “The basic network reconfiguration” .

For all the network configuration you need to access, you create a separate stanza in "/etc/network/interfaces" file as the following.

auto lo
iface lo inet loopback

iface config1 inet dhcp

iface config2 inet static
 address 192.168.11.100
 netmask 255.255.255.0
 gateway 192.168.11.1
 dns-domain example.com
 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 by the following.

$ 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 by the following.

$ 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 by the following.

$ 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 by the following.

$ 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.

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


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

The ifupdown-extra package (see Section 5.5.14, “The ifupdown-extra package”) uses these environment variables to extend the functionality of the ifupdown package. The ifmetric package (see Section 5.6.2, “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 5.6.8, “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.

Instead of manually choosing configuration as described in Section 5.6.6, “The manually switchable network configuration”, you can use the mapping mechanism described in Section 5.6.5, “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 choses 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.

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


[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".

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


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, try the following

$ 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

You see ping(8) succeed with 1454.

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.


In addtion to these basic guide lines, you should know the following.

  • 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.

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

For the DHCP (see Section 5.5.4, “The network interface served by the DHCP”), you can replace pertinent iface stanza lines in the "/etc/network/interfaces" with the following.

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

For the static IP (see Section 5.5.5, “The network interface with the static IP”), you can replace pertinent iface stanza lines in the "/etc/network/interfaces" with the following.

iface eth0 inet static
 address 192.168.11.100
 netmask 255.255.255.0
 gateway 192.168.11.1
 mtu 1454
 dns-domain example.com
 dns-nameservers 192.168.11.1

For the direct PPPoE (see Section 5.4.5, “The PPPoE connection with pppoeconf”), you can replace pertinent "mtu" line in the "/etc/ppp/peers/dsl-provider" with the following.

mtu 1454

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

  • MSS = MTU - 40 for IPv4

  • MSS = MTU - 60 for IPv6

[Note] Note

The iptables(8) (see Section 5.9, “Netfilter infrastructure”) based optimization can clamp packet size by the MSS and is useful for the router. See "TCPMSS" in iptables(8).

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.

Netfilter provides infrastructure for stateful firewall and network address translation (NAT) with Linux kernel modules (see Section 3.3.1, “The kernel module initialization”).


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 documentations 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 Linux 2.6 and 3.x kernel series.