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.

If you are not sure what to chose for the domain name, choose "localdomain".

Other popular choices for such invalid and safe top-level domain (TLD) seem to be "lan", "localnet", "home", "invalid", or "local" according to my incoming mail analysis. This is consistent with the reported invalid TLD queries. You may also chose some unused second or third level domains.

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

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

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

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 #316099.

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

For <domain_name> of the mobile PC, you may chose invalid and safe domain such as "localdomain". (See Section 5.1.1, “The domain name”)

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> and <domain_name> should be used here.

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.

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.

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 3.5.11, “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.

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.

Although most hardware devices are supported by the Debian system, there are some network devices which require DFSG non-free external hardware drivers to support them. Please see Section 9.7.6, “Non-free hardware drivers”.

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.

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 isc-dhcp-client package for the Ethernet served by the dynamic host configuration protocol (DHCP).

See dhclient.conf(5).

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

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

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".

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 configures the PPP connection interactively just by selecting the following.

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

You can test configuration using low level network configuration tools as the following.

$ sudo wvdial
...
$ sudo killall wvdial

See wvdial(1) and wvdial.conf(5).

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 configures the PPPoE connection interactively.

The configuration files are the following.

You can test configuration using low level network configuration tools as the following.

$ 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".

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".

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

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 5.6.5, “The mapping stanza”).

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.

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 network interface served by the static IP is configured by creating the configuration entry in the "/etc/network/interfaces" file as the following.

allow-hotplug eth0
iface eth0 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

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 the static IP to configure the interface.

Here, I assumed the following.

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

nameserver 192.168.11.1
domain example.com

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.

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.)

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>

You need to configure the alternative PPP connection with wvdial first as described before (see Section 5.4.4, “The alternative PPP connection with wvdialconf”). Then, add the "/etc/network/interfaces" file entry for the primary PPP device ppp0 as the following.

iface ppp0 inet wvdial

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 "/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

When you try to reconfigure the interface, e.g. eth0, you must disable it first with the "sudo ifdown eth0" command. This removes 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.

The ifupdown-extra package provides easy network connection tests for use with the ifupdown package.

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 performs the following.

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

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 5.2, “The modern network configuration for desktop”) installed, 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.

Now, the network reconfiguration works as you desire.

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

The following sets the eth0 interface to be preferred over the wlan0 interface.

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).

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 the following.

The following stanzas in "/etc/network/interfaces" 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
 metric 1

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).

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 feature 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.

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 single "/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.

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.

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.

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.

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).

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

See ip(8) and IPROUTE2 Utility Suite Howto.

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

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.

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

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.

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.