== Network setup
// vim: set sts=2 expandtab:
// Use ":set nowrap" to edit table

TIP: For general guide to the GNU/Linux networking, read the http://www.tldp.org/LDP/nag2/[Linux Network Administrators Guide].

TIP: For modern Debian specific guide to the networking, read https://www.debian.org/doc/manuals/debian-handbook/sect.network-config[The Debian Administrator's Handbook -- Configuring the Network].

WARNING: Instead of using the traditional interface naming scheme ("`eth0`", "`eth1`", "`wlan0`", ...), the newer https://en.wikipedia.org/wiki/Systemd[systemd] uses "https://www.freedesktop.org/wiki/Software/systemd/PredictableNetworkInterfaceNames/[Predictable Network Interface Names]" such as "`enp0s25`".

WARNING: This chapter is getting outdated since this is based on Debian 7.0 (`Wheezy`) released in 2013.

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.

TIP: Under https://en.wikipedia.org/wiki/Systemd[systemd], https://en.wikipedia.org/wiki/Systemd#networkd[networkd] may be used to manage networks.  See `systemd-networkd`(8).

=== The basic network infrastructure

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

.List of network configuration tools
[grid="all"]
`-----------------------------------`-------------`------------`-----------------`--------------------------------------------------------------------
packages                            popcon        size         type              description
------------------------------------------------------------------------------------------------------------------------------------------------------
`ifupdown`                          @-@popcon1@-@ @-@psize1@-@ config::ifupdown  standardized tool to bring up and down the network (Debian specific)
`ifplugd`                           @-@popcon1@-@ @-@psize1@-@ , ,               manage the wired network automatically
`ifupdown-extra`                    @-@popcon1@-@ @-@psize1@-@ , ,               network testing script to enhance "`ifupdown`" package
`ifmetric`                          @-@popcon1@-@ @-@psize1@-@ , ,               set routing metrics for a network interface
`guessnet`                          @-@popcon1@-@ @-@psize1@-@ , ,               mapping script to enhance "`ifupdown`" package via "`/etc/network/interfaces`" file
`ifscheme`                          @-@popcon1@-@ @-@psize1@-@ , ,               mapping scripts to enhance "`ifupdown`" package
`network-manager`                   @-@popcon1@-@ @-@psize1@-@ config::NM        https://en.wikipedia.org/wiki/NetworkManager[NetworkManager] (daemon): manage the network automatically
`network-manager-gnome`             @-@popcon1@-@ @-@psize1@-@ , ,               https://en.wikipedia.org/wiki/NetworkManager[NetworkManager] (GNOME frontend)
`wicd`                              @-@popcon1@-@ @-@psize1@-@ config::wicd      wired and wireless network manager (metapackage)
`wicd-cli`                          @-@popcon1@-@ @-@psize1@-@ , ,               wired and wireless network manager (command-line client)
`wicd-curses`                       @-@popcon1@-@ @-@psize1@-@ , ,               wired and wireless network manager (Curses client)
`wicd-daemon`                       @-@popcon1@-@ @-@psize1@-@ , ,               wired and wireless network manager (daemon)
`wicd-gtk`                          @-@popcon1@-@ @-@psize1@-@ , ,               wired and wireless network manager (GTK+ client)
`iptables`                          @-@popcon1@-@ @-@psize1@-@ config::Netfilter administration tools for packet filtering and NAT (https://en.wikipedia.org/wiki/Netfilter[Netfilter])
`iproute2`                          @-@popcon1@-@ @-@psize1@-@ config::iproute2  http://www.linuxfoundation.org/collaborate/workgroups/networking/iproute2[iproute2], IPv6 and other advanced network configuration: `ip`(8), `tc`(8), etc
`ifrename`                          @-@popcon1@-@ @-@psize1@-@ , ,               rename network interfaces based on various static criteria: `ifrename`(8)
`ethtool`                           @-@popcon1@-@ @-@psize1@-@ , ,               display or change Ethernet device settings
`iputils-ping`                      @-@popcon1@-@ @-@psize1@-@ test::iproute2    test network reachability of a remote host by https://en.wikipedia.org/wiki/Hostname[hostname] or https://en.wikipedia.org/wiki/IP_address[IP address] (http://www.linuxfoundation.org/collaborate/workgroups/networking/iproute2[iproute2])
`iputils-arping`                    @-@popcon1@-@ @-@psize1@-@ , ,               test network reachability of a remote host specified by the https://en.wikipedia.org/wiki/Address_Resolution_Protocol[ARP] address
`iputils-tracepath`                 @-@popcon1@-@ @-@psize1@-@ , ,               trace the network path to a remote host
`net-tools`                         @-@popcon1@-@ @-@psize1@-@ config::net-tools NET-3 networking toolkit (http://www.linuxfoundation.org/collaborate/workgroups/networking/net-tools[net-tools], IPv4 network configuration):  `ifconfig`(8) etc.
`inetutils-ping`                    @-@popcon1@-@ @-@psize1@-@ test::net-tools   test network reachability of a remote host by https://en.wikipedia.org/wiki/Hostname[hostname] or https://en.wikipedia.org/wiki/IP_address[IP address] (legacy, GNU)
`arping`                            @-@popcon1@-@ @-@psize1@-@ , ,               test network reachability of a remote host specified by the https://en.wikipedia.org/wiki/Address_Resolution_Protocol[ARP] address (legacy)
`traceroute`                        @-@popcon1@-@ @-@psize1@-@ , ,               trace the network path to a remote host (legacy, console)
`isc-dhcp-client`                   @-@popcon1@-@ @-@psize1@-@ config::low-level DHCP client
`wpasupplicant`                     @-@popcon1@-@ @-@psize1@-@ , ,               client support for WPA and WPA2 (IEEE 802.11i)
`wpagui`                            @-@popcon1@-@ @-@psize1@-@ , ,               Qt GUI client for wpa_supplicant
`wireless-tools`                    @-@popcon1@-@ @-@psize1@-@ , ,               tools for manipulating Linux Wireless Extensions
`ppp`                               @-@popcon1@-@ @-@psize1@-@ , ,               PPP/PPPoE connection with `chat`
`pppoeconf`                         @-@popcon1@-@ @-@psize1@-@ config::helper    configuration helper for PPPoE connection
`pppconfig`                         @-@popcon1@-@ @-@psize1@-@ , ,               configuration helper for PPP connection with `chat`
`wvdial`                            @-@popcon1@-@ @-@psize1@-@ , ,               configuration helper for PPP connection with `wvdial` and `ppp`
`mtr-tiny`                          @-@popcon1@-@ @-@psize1@-@ test::low-level   trace the network path to a remote host (curses)
`mtr`                               @-@popcon1@-@ @-@psize1@-@ , ,               trace the network path to a remote host (curses and GTK+)
`gnome-nettool`                     @-@popcon1@-@ @-@psize1@-@ , ,               tools for common network information operations (GNOME)
`nmap`                              @-@popcon1@-@ @-@psize1@-@ , ,               network mapper / port scanner (https://en.wikipedia.org/wiki/Nmap[Nmap], console)
`zenmap`                            @-@popcon1@-@ @-@psize1@-@ , ,               network mapper / port scanner (GTK+)
`tcpdump`                           @-@popcon1@-@ @-@psize1@-@ , ,               network traffic analyzer (https://en.wikipedia.org/wiki/Tcpdump[Tcpdump], console)
`wireshark`                         @-@popcon1@-@ @-@psize1@-@ , ,               network traffic analyzer (https://en.wikipedia.org/wiki/Wireshark[Wireshark], GTK+)
`tshark`                            @-@popcon1@-@ @-@psize1@-@ , ,               network traffic analyzer (console)
`tcptrace`                          @-@popcon1@-@ @-@psize1@-@ , ,               produce a summarization of the connections from `tcpdump` output
`snort`                             @-@popcon1@-@ @-@psize1@-@ , ,               flexible network intrusion detection system (https://en.wikipedia.org/wiki/Snort_(software)[Snort])
`ntopng`                            @-@popcon1@-@ @-@psize1@-@ , ,               display network usage in web browser
`dnsutils`                          @-@popcon1@-@ @-@psize1@-@ , ,               network clients provided with https://en.wikipedia.org/wiki/BIND[BIND]: `nslookup`(8), `nsupdate`(8), `dig`(8)
`dlint`                             @-@popcon1@-@ @-@psize1@-@ , ,               check https://en.wikipedia.org/wiki/Domain_Name_System[DNS] zone information using nameserver lookups
`dnstracer`                         @-@popcon1@-@ @-@psize1@-@ , ,               trace a chain of https://en.wikipedia.org/wiki/Domain_Name_System[DNS] servers to the source
------------------------------------------------------------------------------------------------------------------------------------------------------

// removed and current nagios 4 has not been packaged
// `nagios3`                           @-@popcon1@-@ @-@psize1@-@ , ,               monitoring and management system for hosts, services and networks (https://en.wikipedia.org/wiki/Nagios[Nagios])

==== The hostname resolution

The hostname resolution is currently supported by the https://en.wikipedia.org/wiki/Name_Service_Switch[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 https://en.wikipedia.org/wiki/Domain_Name_System[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 https://en.wikipedia.org/wiki/IP_address[IP address] and it is followed by the associated https://en.wikipedia.org/wiki/Hostname[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 https://en.wikipedia.org/wiki/Debian-Installer[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 http://bugs.debian.org/719621[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 https://en.wikipedia.org/wiki/FQDN[fully qualified domain name (FQDN)] provided by the https://en.wikipedia.org/wiki/Domain_Name_System[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, https://en.wikipedia.org/wiki/Zeroconf[Zeroconf]) in addition to the basic `files` and `dns` methods.

- https://en.wikipedia.org/wiki/Avahi_(software)[Avahi] provides a framework for Multicast DNS Service Discovery on Debian.
- It is equivalent of https://en.wikipedia.org/wiki/Bonjour_(software)[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 https://en.wikipedia.org/wiki/.local[".local"] https://en.wikipedia.org/wiki/Pseudo-top-level_domain[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 https://en.wikipedia.org/wiki/NetBIOS_over_TCP/IP[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.)

NOTE: The http://newgtlds.icann.org/en/program-status/delegated-strings[expansion of generic Top-Level Domains (gTLD)] in the https://en.wikipedia.org/wiki/Domain_Name_System[Domain Name System] is underway.  Watch out for the http://icannwiki.com/Name_Collision[name collision] when chosing a domain name used only within LAN.

==== 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 <<_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 https://en.wikipedia.org/wiki/MAC_address[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: When editing the "`/etc/udev/rules.d/70-persistent-net.rules`" rules file, you must keep each rule on a single line and the https://en.wikipedia.org/wiki/MAC_address[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.

==== 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 https://en.wikipedia.org/wiki/Local_area_network[local area networks (LANs)] by http://tools.ietf.org/html/rfc1918[rfc1918].  These addresses are guaranteed not to conflict with any addresses on the Internet proper.

.List of network address ranges
[grid="all"]
`-----`----------------------------`-------------`--------------`------------
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: 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 https://en.wikipedia.org/wiki/Network_address_translation[Network Address Translation (NAT)].  The broadband router usually performs NAT for the consumer LAN environment.

==== The network device support

Although most hardware devices are supported by the Debian system, there are some network devices which require https://www.debian.org/social_contract#guidelines[DFSG] non-free firmware to support them.  Please see <<_hardware_drivers_and_firmware>>.

=== The modern network configuration for desktop

Network interfaces are typically initialized in "`networking.service`" for the `lo` interface and "`NetworkManager.service`" for other interfaces on modern Debian desktop system under `systemd`.

Debian `squeeze` and newer can manage the network connection via management https://en.wikipedia.org/wiki/Daemon_(computer_software)[daemon] software such as https://en.wikipedia.org/wiki/NetworkManager[NetworkManager (NM)] (network-manager and associated packages) or https://en.wikipedia.org/wiki/Wicd_(Linux_Network_Manager)[Wicd] (wicd and associated packages).

- They come with their own https://en.wikipedia.org/wiki/Graphical_user_interface[GUI] and command-line programs as their user interfaces.
- They come with their own https://en.wikipedia.org/wiki/Daemon_(computer_software)[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: 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: Some features of these automatic network configuration tools may suffer regressions.  These are not as robust as the legacy `ifupdown` package.  Check http://bugs.debian.org/cgi-bin/pkgreport.cgi?package=network-manager[BTS of network-manager] and http://bugs.debian.org/cgi-bin/pkgreport.cgi?package=wicd[BTS of wicd] for current issues and limitations.

==== GUI network configuration tools

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 https://en.wikipedia.org/wiki/D-Bus[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: Only interfaces which are **not** listed in "`/etc/network/interfaces`" are managed by NM or Wicd to avoid conflict with `ifupdown`.

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: These automatic network configuration tools may not be compatible with esoteric configurations of legacy `ifupdown` in "`/etc/network/interfaces`" such as ones in <<_the_basic_network_configuration_with_ifupdown_legacy>> and <<_the_advanced_network_configuration_with_ifupdown_legacy>>.  Check http://bugs.debian.org/cgi-bin/pkgreport.cgi?package=network-manager[BTS of network-manager] and http://bugs.debian.org/cgi-bin/pkgreport.cgi?package=wicd[BTS of wicd] for current issues and limitations.

=== The modern network configuration without GUI

Under https://en.wikipedia.org/wiki/Systemd[systemd], the network may be configured in `/etc/systemd/network/` instead.  See `systemd-resolved`(8), `resolved.conf`(5), and `systemd-networkd`(8).

This allows the modern network configuration without GUI.

A DHCP client configuration can be set up by creating "`/etc/systemd/network/dhcp.network`".  E.g.:

---------
[Match]
Name=en*

[Network]
DHCP=yes
---------

A static network configuration can be set up by creating "`/etc/systemd/network/static.network`".  E.g.:

---------
[Match]
Name=en*

[Network]
Address=192.168.0.15/24
Gateway=192.168.0.1
---------

=== The legacy network connection and configuration

When the method described in <<_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 <<_the_network_connection_method_legacy>>).

There are 2 types of programs for the low level network configuration on Linux (see <<_iproute2_commands>>).

- Old http://www.linuxfoundation.org/collaborate/workgroups/networking/net-tools[net-tools] programs (`ifconfig`(8), ...) are from the Linux NET-3 networking system. Most of these are obsolete now.
- New http://www.linuxfoundation.org/collaborate/workgroups/networking/iproute2[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 <<_the_basic_network_configuration_with_ifupdown_legacy>> and <<_the_advanced_network_configuration_with_ifupdown_legacy>>).

=== The network connection method (legacy)

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 <<_the_modern_network_configuration_for_desktop>> and <<_the_basic_network_configuration_with_ifupdown_legacy>>).

// FIXME: Since my MacBook has no build-in modem, and I use the broadband-modem via Ethernet, all POTS-related description needs to be tested with the recent system.

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

.List of network connection methods and connection paths
[grid="all"]
`----------------------`-----------------`------------------------------------------------------------------------------------------------
PC                     connection method connection path
------------------------------------------------------------------------------------------------------------------------------------------
Serial port (`ppp0`)   PPP               <=> https://en.wikipedia.org/wiki/Modem[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 https://en.wikipedia.org/wiki/Network_address_translation[network address translation (NAT)] (<=> BB-modem ...)
------------------------------------------------------------------------------------------------------------------------------------------

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

.List of network connection configurations
[grid="all"]
`-----------------`-----------------------------------------`------------------
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/dhcp/dhclient.conf`"  `isc-dhcp-client`
static IP (IPv4)  described in "`/etc/network/interfaces`"  `iproute` or `net-tools` (obsolete)
static IP (IPv6)  described in "`/etc/network/interfaces`"  `iproute`
-------------------------------------------------------------------------------

The network connection acronyms mean the following.

.List of network connection acronyms
[grid="all"]
`-------------------------------------------------------------------------`---------------------------------------------------------------
acronym                                                                   meaning
------------------------------------------------------------------------------------------------------------------------------------------
https://en.wikipedia.org/wiki/Plain_old_telephone_service[POTS]            plain old telephone service
BB                                                                        https://en.wikipedia.org/wiki/Broadband[broadband]
BB-service                                                                e.g., the digital subscriber line (DSL), the cable TV, or the fiber to the premises (FTTP)
BB-modem                                                                  e.g., https://en.wikipedia.org/wiki/DSL_modem[the DSL modem], https://en.wikipedia.org/wiki/Cable_modem[the cable modem], or https://en.wikipedia.org/wiki/FTTP[the optical network terminal (ONT)]
https://en.wikipedia.org/wiki/Local_area_network[LAN]                      local area network
https://en.wikipedia.org/wiki/Wide_area_network[WAN]                       wide area network
https://en.wikipedia.org/wiki/Dynamic_Host_Configuration_Protocol[DHCP]    dynamic host configuration protocol
https://en.wikipedia.org/wiki/Point-to-Point_Protocol[PPP]                 point-to-point protocol
https://en.wikipedia.org/wiki/Point-to-Point_Protocol_over_Ethernet[PPPoE] point-to-point protocol over Ethernet
https://en.wikipedia.org/wiki/ISP[ISP]                                     Internet service provider
------------------------------------------------------------------------------------------------------------------------------------------

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: When BB-router is used to create home LAN environment, PCs on LAN are connected to the WAN via BB-router with https://en.wikipedia.org/wiki/Network_address_translation[network address translation (NAT)].  For such case, PC@@@sq@@@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 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 https://en.wikipedia.org/wiki/Dynamic_Host_Configuration_Protocol[dynamic host configuration protocol (DHCP)] server running on the router.

Just install the `isc-dhcp-client` package for the Ethernet served by the https://en.wikipedia.org/wiki/Dynamic_Host_Configuration_Protocol[dynamic host configuration protocol (DHCP)].

See `dhclient.conf`(5).

==== The static IP connection with the Ethernet

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

==== The PPP connection with pppconfig

The configuration script `pppconfig` configures the https://en.wikipedia.org/wiki/Point-to-Point_Protocol[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

.List of configuration files for the https://en.wikipedia.org/wiki/Point-to-Point_Protocol[PPP] connection with pppconfig
[grid="all"]
`-----------------------------`-----------------------------------------------------------------------------------------------------------
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 https://en.wikipedia.org/wiki/Password_authentication_protocol[PAP] (security risk)
`/etc/ppp/chap-secret`        Authentication data for the https://en.wikipedia.org/wiki/Challenge-handshake_authentication_protocol[CHAP] (more secure)
------------------------------------------------------------------------------------------------------------------------------------------

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 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` configures the PPP connection interactively just by selecting the following.

- The telephone number
- The ISP user name
- The ISP password

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

.List of configuration files for the PPP connection with wvdialconf
[grid="all"]
`-----------------------`------------------------------------------------------------------------------------------------------------------
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 https://en.wikipedia.org/wiki/Password_authentication_protocol[PAP] (security risk)
`/etc/ppp/chap-secret`  Authentication data for the https://en.wikipedia.org/wiki/Challenge-handshake_authentication_protocol[CHAP] (more secure)
------------------------------------------------------------------------------------------------------------------------------------------

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).

==== 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` configures the PPPoE connection interactively.

The configuration files are the following.

.List of configuration files for the PPPoE connection with pppoeconf
[grid="all"]
`-----------------------------`-----------------------------------------------------------------------------------------------------------
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 https://en.wikipedia.org/wiki/Password_authentication_protocol[PAP] (security risk)
`/etc/ppp/chap-secret`        Authentication data for the https://en.wikipedia.org/wiki/Challenge-handshake_authentication_protocol[CHAP] (more secure)
------------------------------------------------------------------------------------------------------------------------------------------

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 basic network configuration with ifupdown (legacy)

The traditional https://en.wikipedia.org/wiki/Internet_Protocol_Suite[TCP/IP network] setup on the Debian system uses `ifupdown` package as a high level tool.  There are 2 typical cases.

- For **dynamic IP** system such as mobile PCs, you should setup TCP/IP network **with** the `resolvconf` package and enable you to switch your network configuration easily (see <<_the_network_interface_served_by_the_dhcp>>).
- For **static IP** system such as servers, you should setup TCP/IP network **without** the `resolvconf` package and keep your system simple (see <<_the_network_interface_with_the_static_ip>>).

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.

==== 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".

.List of basic network configuration commands with ifupdown
[grid="all"]
`-------------`-------------------------------------------------------------------------------------------------------
command       action
----------------------------------------------------------------------------------------------------------------------
`ifup eth0`   bring up a network interface `eth0` with the configuration `eth0` if "`iface eth0`" stanza exists
`ifdown eth0` bring down a network interface `eth0` with the configuration `eth0` if "`iface eth0`" stanza exists
----------------------------------------------------------------------------------------------------------------------

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

NOTE: There is no command `ifupdown`.

==== The basic syntax of "/etc/network/interfaces"

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

[[list-of-stanzas-in-eni]]
.List of stanzas in "`/etc/network/interfaces`"
[grid="all"]
`-----------------------------------------------------`-----------------------------------------------------------------------------------
stanza                                                meaning
------------------------------------------------------------------------------------------------------------------------------------------
"`auto <interface_name>`"                             start interface  <interface_name> upon start of the system
"`allow-auto <interface_name>`"                       , ,
"`allow-hotplug <interface_name>`"                    start interface <interface_name> when the kernel detects a hotplug event from the interface
Lines started with "`iface <config_name> ...`"        define the network configuration <config_name>
Lines started with "`mapping <interface_name_glob>`"  define mapping value of <config_name> for the matching <interface_name>
A line starting with a hash "`#`"                     ignore as comments (end-of-line comments are **not** supported)
A line ending with a backslash "`\`"                  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 <<_the_mapping_stanza>>).

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

==== 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.

==== The network interface served by the DHCP

After preparing the system by <<_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 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 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.

- 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: installed
- The domain name: "`example.com`"
- 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 the following.

--------------------
nameserver 192.168.11.1
domain example.com
--------------------

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.

==== The basics of wireless LAN interface

The https://en.wikipedia.org/wiki/Wireless_LAN[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 https://en.wikipedia.org/wiki/IEEE_802.11[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: The `wmaster0` device is the master device which is an internal device used only by https://en.wikipedia.org/wiki/SoftMAC[SoftMAC] with new http://linuxwireless.org/[mac80211 API of Linux].

Here are some keywords to remember for the WLAN.

.List of acronyms for WLAN
[grid="all"]
`-----------`--------------------------------------------------------------------------------------`---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
acronym     full word                                                                              meaning
-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
NWID        Network ID                                                                             16 bit network ID used by pre-802.11 https://en.wikipedia.org/wiki/WaveLAN[WaveLAN] network (very deprecated)
(E)SSID     (Extended) https://en.wikipedia.org/wiki/Service_set_identifier[Service Set Identifier] network name of the https://en.wikipedia.org/wiki/Wireless_access_point[Wireless Access Points (APs)] interconnected to form an integrated https://en.wikipedia.org/wiki/IEEE_802.11[802.11 wireless LAN], Domain ID
WEP, (WEP2) https://en.wikipedia.org/wiki/Wired_Equivalent_Privacy[Wired Equivalent Privacy]        1st generation 64-bit (128-bit) wireless encryption standard with 40-bit key (deprecated)
WPA         https://en.wikipedia.org/wiki/Wi-Fi_Protected_Access[Wi-Fi Protected Access]            2nd generation wireless encryption standard (most of 802.11i), compatible with WEP
WPA2        https://en.wikipedia.org/wiki/IEEE_802.11i[Wi-Fi Protected Access 2]                    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.

==== 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 https://en.wikipedia.org/wiki/Dynamic_Host_Configuration_Protocol[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`".

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

In case of the https://en.wikipedia.org/wiki/Dynamic_Host_Configuration_Protocol[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`".

==== The PPP connection

You need to configure the PPP connection first as described before (see <<_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>
--------------------

==== The alternative PPP connection

You need to configure the alternative PPP connection with `wvdial` first as described before (see <<_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
--------------------

==== 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 <<_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 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. 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 https://en.wikipedia.org/wiki/IPv4[IPV4 network].

--------------------
  inet addr:192.168.11.2  Bcast:192.168.11.255  Mask:255.255.255.0
--------------------

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

==== 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 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.

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

==== The ifupdown-extra package

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

- The `network-test`(1) command can be used from the shell.
- The automatic scripts are 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 perform the following.

- Check the network cable connection
- Check duplicate use of IP address
- Setup system@@@sq@@@s static routes based on the "`/etc/network/routes`" definition
- Check if network gateway is reachable
- 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: 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 https://en.wikipedia.org/wiki/Address_Resolution_Protocol[ARP] replies.

=== The advanced network configuration with ifupdown (legacy)

The functionality of the `ifupdown` package can be improved beyond what was described in <<_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: If you could not set up network connection by information in <<_the_basic_network_configuration_with_ifupdown_legacy>>, you make situation worse by using information below.

==== The ifplugd package

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 https://en.wikipedia.org/wiki/NetworkManager[NetworkManager] or https://en.wikipedia.org/wiki/Wicd_(Linux_Network_Manager)[Wicd] (see <<_the_modern_network_configuration_for_desktop>>) installed, you do not need this package.

This package runs a https://en.wikipedia.org/wiki/Daemon_(computer_software)[daemon] and replaces **auto** or **allow-hotplug** functionalities (see <<list-of-stanzas-in-eni>>) 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.
 * Quick boot process without the long DHCP timeout.
 * No funny activated interface without proper IPv4 address (see <<_the_network_configuration_state_of_ifupdown>>).
- 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: The arguments for the `ifplugd`(8) command can set its behaviors such as the delay for reconfiguring interfaces.

==== The ifmetric package

The `ifmetric` 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.

1. Install the `ifmetric` package.
2. Add an option line with "`metric 0`" just below the "`iface eth0 inet dhcp`" line in "`/etc/network/interfaces`".
3. Add an option line with "`metric 1`" just below the "`iface wlan0 inet dhcp`" line in "`/etc/network/interfaces`".

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).

==== 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@@@sq@@@s suppose the following.

- 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.
- Your host uses `192.168.0.1` with the virtual interface `eth0:0` for the LAN.

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
--------------------

CAUTION: Although this configuration example with https://en.wikipedia.org/wiki/Network_address_translation[network address translation (NAT)] using https://en.wikipedia.org/wiki/Netfilter[netfilter/iptables] (see <<_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.

==== The advanced command syntax

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).


.List of terminology for network devices
[grid="all"]
`---------------------------`------------------------------`-------------------------------------`----------------------------------------
manpage terminology         my terminology                 examples in the following text        description
------------------------------------------------------------------------------------------------------------------------------------------
**physical interface** name **network interface** name     `lo`, `eth0`, `<interface_name>`      name given by the Linux kernel (using `udev` mechanism)
**logical interface** name  **network configuration** name `config1`, `config2`, `<config_name>` name token following **`iface`** in the "`/etc/network/interfaces`"
------------------------------------------------------------------------------------------------------------------------------------------

Basic network configuration commands in <<_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.

.List of advanced network configuration commands with ifupdown
[grid="all"]
`---------------------`-------------------------------------------------------------------------------------------------
command               action
------------------------------------------------------------------------------------------------------------------------
`ifup eth0=config1`   bring up a network interface `eth0` with the configuration `config1`
`ifdown eth0=config1` bring down a network interface `eth0` with the configuration `config1`
`ifup eth0`           bring up a network interface `eth0` with the configuration selected by **`mapping`** stanza
`ifdown eth0`         bring down a network interface `eth0` with the configuration selected by **`mapping`** stanza
------------------------------------------------------------------------------------------------------------------------

==== The mapping stanza

We skipped explaining the **`mapping`** stanza in the "`/etc/network/interfaces`" in <<_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@@@sq@@@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: The glob for **`mapping`** stanza works like shell filename glob (see <<_shell_glob>>).

==== The manually switchable network configuration

Here is how to switch manually among several network configurations without rewriting the "`/etc/network/interfaces`" file as in <<_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@@@sq@@@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@@@sq@@@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@@@sq@@@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@@@sq@@@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: You may need to adjust numbers at the end of `eth\*`, `ppp\*`, etc. if you have multiple network interfaces.

==== Scripting with the ifupdown system

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

.List of environment variables passed by the ifupdown system
[grid="all"]
`--------------------`-----------------------------------------------------------------------------------------------------------
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
"`$PATH`"            command search path: "`/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin`"
"`$IF_<OPTION>`"     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: See <<_the_basic_syntax_of_etc_network_interfaces>> for <address_family>, <method_name>, <option1> and <option2>.

The `ifupdown-extra` package (see <<_the_ifupdown_extra_package>>) uses these environment variables to extend the functionality of the `ifupdown` package.  The `ifmetric` package (see <<_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 <<_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: 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.

==== Mapping with guessnet

Instead of manually choosing configuration as described in <<_the_manually_switchable_network_configuration>>, you can use the mapping mechanism described in <<_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: 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.

=== The low level network configuration

==== Iproute2 commands

http://www.linuxfoundation.org/collaborate/workgroups/networking/iproute2[Iproute2] commands offer complete low-level network configuration capabilities. Here is a translation table from obsolete http://www.linuxfoundation.org/collaborate/workgroups/networking/net-tools[net-tools] commands to new http://www.linuxfoundation.org/collaborate/workgroups/networking/iproute2[iproute2] etc. commands.

.Translation table from obsolete `net-tools` commands to new `iproute2` commands
[grid="all"]
`------------------`-----------------`-----------------------------------------------
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 http://www.policyrouting.org/iproute2.doc.html[IPROUTE2 Utility Suite Howto].

==== Safe low level network operations

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

.List of low level network commands
[grid="all"]
`------------------------------------------------`----------------------------------------------------------------------------------------
command                                          description
------------------------------------------------------------------------------------------------------------------------------------------
`ifconfig`                                       display the link and address status of active interfaces
`ip addr show`                                   display the link and address status of active interfaces
`route -n`                                       display all the routing table in numerical addresses
`ip route show`                                  display all the routing table in numerical addresses
`arp`                                            display the current content of the https://en.wikipedia.org/wiki/Address_Resolution_Protocol[ARP] cache tables
`ip neigh`                                       display the current content of the https://en.wikipedia.org/wiki/Address_Resolution_Protocol[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 https://en.wikipedia.org/wiki/Domain_Name_System[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 "`example.com`"
------------------------------------------------------------------------------------------------------------------------------------------

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`".

=== Network optimization

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

.List of network optimization tools
[grid="all"]
`-----------`-------------`------------`---------------------------------------------------------------
packages    popcon        size         description
-------------------------------------------------------------------------------------------------------
`iftop`     @-@popcon1@-@ @-@psize1@-@ display bandwidth usage information on an network interface
`iperf`     @-@popcon1@-@ @-@psize1@-@ Internet Protocol bandwidth measuring tool
`ifstat`    @-@popcon1@-@ @-@psize1@-@ InterFace STATistics Monitoring
`bmon`      @-@popcon1@-@ @-@psize1@-@ portable bandwidth monitor and rate estimator
`ethstatus` @-@popcon1@-@ @-@psize1@-@ script that quickly measures network device throughput
`bing`      @-@popcon1@-@ @-@psize1@-@ empirical stochastic bandwidth tester
`bwm-ng`    @-@popcon1@-@ @-@psize1@-@ small and simple console-based bandwidth monitor
`ethstats`  @-@popcon1@-@ @-@psize1@-@ console-based Ethernet statistics monitor
`ipfm`      @-@popcon1@-@ @-@psize1@-@ bandwidth analysis tool
-------------------------------------------------------------------------------------------------------

==== Finding optimal MTU

The https://en.wikipedia.org/wiki/Maximum_transmission_unit[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 https://en.wikipedia.org/wiki/Path_MTU_discovery[Path MTU (PMTU) discovery] (http://tools.ietf.org/html/rfc1191[RFC1191]) and the `tracepath`(8) command can automate this.

TIP: The above example with PMTU value of 1454 is for my previous FTTP provider which used https://en.wikipedia.org/wiki/Asynchronous_Transfer_Mode[Asynchronous Transfer Mode] (ATM) as its backbone network and served its clients with the https://en.wikipedia.org/wiki/Point-to-Point_Protocol_over_Ethernet[PPPoE].  The actual PMTU value depends on your environment, e.g., 1500 for the my new FTTP provider.

.Basic guide lines of the optimal MTU value
[grid="all"]
`-------------------------------------------------------------`---------------------`-----------------------------------------------------------
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@@@sq@@@s backbone: ATM, IP: DHCP or fixed) 1462 (=48\*31-18-8)   author@@@sq@@@s speculation: 18 bytes for Ethernet header, 8 bytes for SAR trailer
Ethernet link (ISP@@@sq@@@s backbone: ATM, IP: PPPoE)         1454 (=48\*31-8-18-8) see "http://www.mynetwatchman.com/kb/ADSL/pppoemtu.htm[Optimal MTU configuration for PPPoE ADSL Connections]" for rationale
------------------------------------------------------------------------------------------------------------------------------------------

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

- Any use of tunneling methods (https://en.wikipedia.org/wiki/Virtual_private_network[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.

==== Setting MTU

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

For the DHCP (see <<_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 <<_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 <<_the_pppoe_connection_with_pppoeconf>>), you can replace pertinent "`mtu`" line in the "`/etc/ppp/peers/dsl-provider`" with the following.

--------------------
mtu 1454
--------------------

The https://en.wikipedia.org/wiki/Maximum_segment_size[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: The `iptables`(8) (see <<_netfilter_infrastructure>>) based optimization can clamp packet size by the MSS and is useful for the router.  See "TCPMSS" in `iptables`(8).

==== WAN TCP optimization

The TCP throughput can be maximized by adjusting TCP buffer size parameters as described in "http://dsd.lbl.gov/TCP-tuning/[TCP Tuning Guide]" and "https://en.wikipedia.org/wiki/TCP_tuning[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 infrastructure

https://en.wikipedia.org/wiki/Netfilter[Netfilter] provides infrastructure for https://en.wikipedia.org/wiki/Stateful_firewall[stateful firewall] and https://en.wikipedia.org/wiki/Network_address_translation[network address translation (NAT)] with https://en.wikipedia.org/wiki/Linux_kernel[Linux kernel] modules (see <<_the_kernel_module_initialization>>).


.List of firewall tools
[grid="all"]
`-----------------`-------------`------------`--------------------------------------------------------------------------------------------
packages          popcon        size         description
------------------------------------------------------------------------------------------------------------------------------------------
`iptables`        @-@popcon1@-@ @-@psize1@-@ administration tools for https://en.wikipedia.org/wiki/Netfilter[netfilter] (`iptables`(8) for IPv4, `ip6tables`(8) for IPv6)
`arptables`       @-@popcon1@-@ @-@psize1@-@ administration tools for https://en.wikipedia.org/wiki/Netfilter[netfilter] (`arptables`(8) for ARP)
`ebtables`        @-@popcon1@-@ @-@psize1@-@ administration tools for https://en.wikipedia.org/wiki/Netfilter[netfilter] (`ebtables`(8) for Ethernet bridging)
`iptstate`        @-@popcon1@-@ @-@psize1@-@ continuously monitor https://en.wikipedia.org/wiki/Netfilter[netfilter] state (similar to `top`(1))
`shorewall-init`  @-@popcon1@-@ @-@psize1@-@ https://en.wikipedia.org/wiki/Shorewall[Shoreline Firewall] initialization
`shorewall`       @-@popcon1@-@ @-@psize1@-@ https://en.wikipedia.org/wiki/Shorewall[Shoreline Firewall], https://en.wikipedia.org/wiki/Netfilter[netfilter] configuration file generator
`shorewall-lite`  @-@popcon1@-@ @-@psize1@-@ https://en.wikipedia.org/wiki/Shorewall[Shoreline Firewall], https://en.wikipedia.org/wiki/Netfilter[netfilter] configuration file generator (light version)
`shorewall6`      @-@popcon1@-@ @-@psize1@-@ https://en.wikipedia.org/wiki/Shorewall[Shoreline Firewall], https://en.wikipedia.org/wiki/Netfilter[netfilter] configuration file generator (IPv6 version)
`shorewall6-lite` @-@popcon1@-@ @-@psize1@-@ https://en.wikipedia.org/wiki/Shorewall[Shoreline Firewall], https://en.wikipedia.org/wiki/Netfilter[netfilter] configuration file generator (IPv6, light version)
------------------------------------------------------------------------------------------------------------------------------------------

Main user space program of https://en.wikipedia.org/wiki/Netfilter[netfilter] is `iptables`(8).  You can manually configure https://en.wikipedia.org/wiki/Netfilter[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 https://en.wikipedia.org/wiki/Shorewall[shorewall] ease this process.

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

- http://www.netfilter.org/documentation/HOWTO/networking-concepts-HOWTO.html[Linux Networking-concepts HOWTO]
- http://www.netfilter.org/documentation/HOWTO/packet-filtering-HOWTO.html[Linux 2.4 Packet Filtering HOWTO]
- http://www.netfilter.org/documentation/HOWTO/NAT-HOWTO.html[Linux 2.4 NAT HOWTO]

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.

