8.2. Konfiguracja sieci komputerowej

BACK TO BASICS Essential network concepts (Ethernet, IP address, subnet, broadcast)

Most modern local networks use the Ethernet protocol, where data is split into small blocks called frames and transmitted on the wire one frame at a time. Data speeds vary from 10 Mb/s for older Ethernet cards to 100 Gb/s in the newest cards (with the most common rate currently growing from 100 Mb/s to 10 Gb/s). The most widely used cables are called 10BASE-T, 100BASE-T, 1000BASE-T, 10GBASE-T and 40GBASE-T, depending on the throughput they can reliably provide (the T stands for “twisted pair”); those cables end in an RJ45 connector. There are other cable types, used mostly for speeds of 10 Gb/s and above.

An IP address is a number used to identify a network interface on a computer on a local network or the Internet. In the currently most widespread version of IP (IPv4), this number is encoded in 32 bits, and is usually represented as 4 numbers separated by periods (e.g. 192.168.0.1), each number being between 0 and 255 (inclusive, which corresponds to 8 bits of data). The next version of the protocol, IPv6, extends this addressing space to 128 bits, and the addresses are generally represented as a series of hexadecimal numbers separated by colons (e.g., 2001:0db8:13bb:0002:0000:0000:0000:0020, or 2001:db8:13bb:2::20 for short).

A subnet mask (netmask) defines in its binary code which portion of an IP address corresponds to the network, the remainder specifying the machine. In the example of configuring a static IPv4 address given here, the subnet mask, 255.255.255.0 (24 “1”s followed by 8 “0”s in binary representation) indicates that the first 24 bits of the IP address correspond to the network address, and the other 8 are specific to the machine. In IPv6, for readability, only the number of “1”s is expressed; the netmask for an IPv6 network could, thus, be 64.

The network address is an IP address in which the part describing the machine's number is 0. The range of IPv4 addresses in a complete network is often indicated by the syntax, a.b.c.d/e, in which a.b.c.d is the network address and e is the number of bits affected to the network part in an IP address. The example network would thus be written: 192.168.0.0/24. The syntax is similar in IPv6: 2001:db8:13bb:2::/64.

Router jest urządzeniem, które łączy ze sobą wiele różnych sieci, umożliwiając komunikację pomiędzy nimi. Wszystkie dane przechodzące przez router są przez niego kierowane do właściwej sieci. W tym celu router analizuje napływające dane i przekierowuje je w zależności od zawartego w nich docelowego adresu IP. Router jest często nazywany bramą (gateway) - wówczas służy jako urządzenie poprzez które komputer w sieci lokalnej może nawiązać komunikację z siecią zewnętrzną.

Specjalny adres rozgłoszeniowy ("broadcast") łączy wszystkie hosty w sieci, pozwalając na wysłanie danych do wszystkich komputerów w sieci poprzez pojedynczą transmisję. Prawie zawsze jest nieroutowalny (tj. nie podlega routingowi), czyli działa tylko w odniesieniu do sieci z którą jest związany. W praktyce oznacza to, że pakiet danych opatrzony rozgłoszeniowym adresem docelowym nigdy nie przejdzie przez router.

Ten rozdział skupia się na adresach IPv4, jako, że są obecnie najpowszechniej stosowanymi. Szczegóły protokołu IPv6 są omówione w Sekcja 10.6, „IPv6”, aczkolwiek główne koncepcje pozostają takie same.

The network is automatically configured during the initial installation. If Network Manager gets installed (which is generally the case for full desktop installations), then it might be that no configuration is actually required (for example, if you rely on DHCP on a wired connection and have no specific requirements). If a configuration is required (for example, for a WiFi interface), then it will create the appropriate file in /etc/NetworkManager/system-connections/.

NOTE NetworkManager

If Network Manager is particularly recommended in roaming setups (see Sekcja 8.2.5, „Automatic Network Configuration for Roaming Users”), it is also perfectly usable as the default network management tool. You can create “System connections” that are used as soon as the computer boots either manually with a .ini-like file in /etc/NetworkManager/system-connections/ or through a graphical tool (nm-connection-editor). If you were using ifupdown, just remember to deactivate the entries in /etc/network/interfaces that you want Network Manager to handle.

→ https://wiki.gnome.org/Projects/NetworkManager/SystemSettings

→ https://developer.gnome.org/NetworkManager/1.30/ref-settings.html

If Network Manager is not installed, then the installer will configure ifupdown by creating the /etc/network/interfaces file. A line starting with auto gives a list of interfaces to be automatically configured on boot by the networking service. When there are many interfaces, it is good practice to keep the configuration in different files inside /etc/network/interfaces.d/ as described in sidebar WRACAJĄC DO PODSTAW Katalogi zakończone na .d.

In a server context, ifupdown is thus the network configuration tool that you usually get. That is why we will cover it in the next sections. For more information about the syntax of the configuration file please read interfaces(5).

8.2.1. Interfejs Ethernet

Jeśli komputer jest wyposażony w kartę sieciową Ethernet, należy odpowiednio skonfigurować powiązaną z nim sieć IP korzystając z jednej z dwóch metod. Najprostszy sposób to dynamiczna konfiguracja przy użyciu protokołu DHCP; wymaga ona obecności serwera DHCP w lokalnej sieci. W jej ramach można określić pożądaną nazwę hosta, odpowiadającą ustawieniu hostname w poniższym przykładzie. Serwer DHCP odpowiada za przesłanie ustawień sieciowych dla odpowiedniej sieci.

Przykład 8.1. Konfiguracja DHCP

auto enp0s31f6
iface enp0s31f6 inet dhcp
  hostname arrakis

IN PRACTICE Names of network interfaces

By default, the kernel attributes generic names such as eth0 (for wired Ethernet) or wlan0 (for WiFi) to the network interfaces. The number in those names is a simple incremental counter representing the order in which they have been detected. With modern hardware, that order (at least in theory) might change for each reboot and thus the default names are not reliable.

Fortunately, systemd and udev are able to rename the interfaces as soon as they appear. The default name policy is defined by /lib/systemd/network/99-default.link (see systemd.link(5) for an explanation of the NamePolicy entry in that file). In practice, the names are often based on the device's physical location (as guessed by where they are connected) and you will see names starting with en for wired ethernet and wl for WiFi. In the example above, the rest of the name indicates, in abbreviated form, a PCI (p) bus number (0), a slot number (s31), a function number (f6). Thus the name becomes predictable.

Obviously, you are free to override this policy and/or to complement it to customize the names of some specific interfaces. You can find out the names of the network interfaces in the output of ip addr (or as filenames in /sys/class/net/).

In some corner cases it might be necessary to disable the predictable naming of network devices as described above. Besides changing the default udev rule it is also possible to boot the system using the net.ifnames=0 (restores the default kernel behavior) and biosdevname=0 kernel parameters to achieve that.

The predecessor of the predictable name scheme was called “persistent name scheme”. It was done using a udev rule to assign the device name based on the MAC address. This scheme has been abandoned and with the release of Debian 10 Buster users have been encouraged to migrate to predictable network device names.

→ https://www.debian.org/releases/buster/amd64/release-notes/ch-information.en.html#migrate-interface-names

Konfiguracja statyczna polega na ręcznym zdefiniowaniu wszystkich ustawień poprzez nadanie im stałych wartości. Przede wszystkim należy określić adres IP oraz maskę podsieci; czasami również wyznacza się adres sieci oraz adres rozgłoszeniowy. Podaje się również adres bramy domyślnej czyli routera łączącego sieć lokalną z siecią zewnętrzną.

Przykład 8.2. Konfiguracja statyczna

auto enp0s31f6
iface enp0s31f6 inet static
  address 192.168.0.3/24
  broadcast 192.168.0.255
  network 192.168.0.0
  gateway 192.168.0.1

NOTE Multiple addresses

It is possible not only to associate several interfaces to a single, physical network card, but also several IP addresses to a single interface. To assign them, just create several iface entries for the same device. Remember also that an IP address may correspond to any number of names via DNS, and that a name may also correspond to any number of numerical IP addresses.

Jak mozna się domyślić, konfiguracja może byc dość skomplikowana, ale opcje te są jedynie używane w bardzo szczególnych przypadkach. Przykładu tutaj podane są typowe dla najczęstszych konfiguracji.

8.2.2. Wireless Interface

Getting wireless network cards to work can be a bit more challenging. First of all, they often require the installation of proprietary firmwares which are not installed by default in Debian. Then wireless networks rely on cryptography to restrict access to authorized users only, this implies storing some secret key in the network configuration. Let's tackle those topics one by one.

8.2.2.1. Installing the required firmwares

First you have to enable the non-free repository in APT's sources.list file: see Sekcja 6.1, „Wypełnianie pliku sources.list” for details about this file. Many firmware are proprietary and are thus located in this repository. You can try to skip this step if you want, but if the next step doesn't find the required firmware, retry after having enabled the non-free section.

Then you have to install the appropriate firmware-* packages. If you don't know which package you need, you can install the isenkram package and run its isenkram-autoinstall-firmware command. The packages are often named after the hardware manufacturer or the corresponding kernel module: firmware-iwlwifi for Intel wireless cards, firmware-atheros for Qualcomm Atheros, firmware-ralink for Ralink, etc. A reboot is then recommended because the kernel driver usually looks for the firmware files when it is first loaded and no longer afterwards.

8.2.2.2. Wireless specific entries in `/etc/network/interfaces`

ifupdown is able to manage wireless interfaces but it needs the help of the wpasupplicant package which provides the required integration between ifupdown and the wpa_supplicant command used to configure the wireless interfaces (when using WPA/WPA2 encryption). The usual entry in /etc/network/interfaces needs to be extended with two supplementary parameters to specify the name of the wireless network (aka its SSID) and the Pre-Shared Key (PSK).

Przykład 8.3. DHCP configuration for a wireless interface

auto wlp4s0
iface wlp4s0 inet dhcp
  wpa-ssid Falcot
  wpa-psk ccb290fd4fe6b22935cbae31449e050edd02ad44627b16ce0151668f5f53c01b

The wpa-psk parameter can contain either the plain text passphrase or its hashed version generated with wpa_passphrase SSID passphrase. If you use an unencrypted wireless connection, then you should put a wpa-key-mgmt NONE and no wpa-psk entry. For more information about the possible configuration options, have a look at /usr/share/doc/wpasupplicant/README.Debian.gz.

At this point, you should consider restricting the read permissions on /etc/network/interfaces to the root user only since the file contains a private key that not all users should have access to.

8.2.3. Połączenie przy użyciu PPP poprzez modem PSTN

Protokół point-to-point (PPP) tworzy tymczasowe połączenie pomiędzy hostami; jest to najczęściej stosowane rozwiązanie w przypadku połączeń realizowanych poprzez modem telefoniczny (modem "PSTN" - "public switched telephone network" - jako, że połączenie odbywa się przez publiczną komutowaną sieć telefoniczną).

Połączenie poprzez modem telefoniczny wymaga posiadania konta u dostawcy usług, w zakres którego wchodzą numer telefonu, nazwa użytkownika, hasło, oraz czasami protokół uwierzytelniania który ma zostać użyty. Połączenie takie jest konfigurowane przy użyciu narzędzia pppconfig zawartego w pakiecie Debiana o tej samej nazwie. Domyślnie, ustanawiane jest połączenie o nazwie provider (od dostawcy usług internetowych). W razie wątpliwości co do wyboru protokołu uwierzytelniania, należy wybrać PAP, jako oferowany przez większość dostawców internetowych.

Po dokonaniu konfiguracji możliwe jest nawiązanie połączenia przy użyciu polecenia pon, przyjmującego jako parametr nazwę połączenia gdy domyślna wartość ustawienia provider nie jest odpowiednia. Połączenie jest kończone poleceniem poff. Obe te polecenia mogą być wykonane z uprawnieniami roota, bądź przez jakiegokolwiek użytkownika pod warunkiem, że należy on do grupy dip.

8.2.4. Nawiązywanie połączenia poprzez modem ADSL

Ogólna nazwa "modem ADSL" odnosi się do wielu urządzeń o różnych funkcjonalnościach. Modemy najprostsze w użyciu z systemem Linux to te, które poza interfejsem USB posiadają również interfejs Ethernet. Są one dość popularne - większość dostawców usług internetowych stosujących technologię ADSL wynajmuje bądź dzierżawi modem z portem Ethernetowym. Wymagana konfiguracja może różnić się znacznie w zależności od konkretnego typu modemu.

8.2.4.1. Modem obsługujące PPPOE

Niektóre modemy Ethernetowe współpracują z protokołem PPPIE (Point to Point over Ethernet). Narzędzie pppoeconf z pakietu o tej samej nazwie pozwala dokonać konfiguracji takiego połączenia. W tym celu wprowadza ono otrzymane ustawienia do pliku /etc/ppp/peers/dsl-provider, oraz zapisuje informacje logowania w plikach /etc/ppp/pap-secrets i /etc/ppp/chap-secrets. Zaleca się zaakceptowanie i zatwierdzenie wszystkich proponowanych przez nie zmian.

Gdy konfiguracja jest zakończona, można nawiązać połączenie ADSL poleceniem pon dsl-provider oraz zakończyć je poleceniem poff dsl-provider.

TIP Starting ppp at boot

PPP connections over ADSL are, by definition, intermittent. Since they are usually not billed according to time, there are few downsides to the temptation of keeping them always open. The standard means to do so is to use the init system.

With systemd, adding an automatically restarting task for the ADSL connection is a simple matter of creating a “unit file” such as /etc/systemd/system/adsl-connection.service, with contents such as the following:

[Unit]
Description=ADSL connection

[Service]
Type=forking
ExecStart=/usr/sbin/pppd call dsl-provider
Restart=always

[Install]
WantedBy=multi-user.target

Once this unit file has been defined, it needs to be enabled with systemctl enable adsl-connection. Then the loop can be started manually with systemctl start adsl-connection; it will also be started automatically on boot.

On systems not using systemd (including Wheezy and earlier versions of Debian), the standard SystemV init works differently. On such systems, all that is needed is to add a line such as the following at the end of the /etc/inittab file; then, any time the connection is disconnected, init will reconnect it.

adsl:2345:respawn:/usr/sbin/pppd call dsl-provider

For ADSL connections that auto-disconnect on a daily basis, this method reduces the duration of the interruption.

8.2.4.2. Modems Supporting PPTP

The PPTP (Point-to-Point Tunneling Protocol) protocol was created by Microsoft. Deployed at the beginning of ADSL, it was quickly replaced by PPPOE. If this protocol is forced on you, see Sekcja 10.3.4, „PPTP”.

8.2.4.3. Modems Supporting DHCP

When a modem is connected to the computer by an Ethernet cable (crossover cable) you typically configure a network connection by DHCP on the computer; the modem automatically acts as a gateway by default and takes care of routing (meaning that it manages the network traffic between the computer and the Internet).

BACK TO BASICS Crossover cable for a direct Ethernet connection

Computer network cards expect to receive data on specific wires in the cable, and send their data on others. When you connect a computer to a local network, you usually connect a cable (straight or crossover) between the network card and a repeater or switch. However, if you want to connect two computers directly (without an intermediary switch or repeater), you must route the signal sent by one card to the receiving side of the other card, and vice-versa. This is the purpose of a crossover cable, and the reason it is used.

Note that this distinction has become almost irrelevant over time, as modern network cards are able to detect the type of cable present and adapt accordingly, so it won't be unusual that both kinds of cable will work in a given location.

Most “ADSL routers” on the market can be used like this, as do most of the ADSL modems provided by Internet services providers.

8.2.5. Automatic Network Configuration for Roaming Users

Many Falcot engineers have a laptop computer that, for professional purposes, they also use at home. The network configuration to use differs according to location. At home, it may be a wifi network (protected by a WPA key), while the workplace uses a wired network for greater security and more bandwidth.

To avoid having to manually connect or disconnect the corresponding network interfaces, administrators installed the network-manager package on these roaming machines. This software enables a user to easily switch from one network to another using a small icon displayed in the notification area of their graphical desktop. Clicking on this icon displays a list of available networks (both wired and wireless), so they can simply choose the network they wish to use. The program saves the configuration for the networks to which the user has already connected, and automatically switches to the best available network when the current connection drops.

In order to do this, the program is structured in two parts: a daemon running as root handles activation and configuration of network interfaces and a user interface controls this daemon. PolicyKit handles the required authorizations to control this program and Debian configured PolicyKit in such a way so that members of the netdev group can add or change Network Manager connections.

Network Manager knows how to handle various types of connections (DHCP, manual configuration, local network), but only if the configuration is set with the program itself. This is why it will systematically ignore all network interfaces in /etc/network/interfaces and /etc/network/interfaces.d/ for which it is not suited. Since Network Manager doesn't give details when no network connections are shown, the easy way is to delete from /etc/network/interfaces any configuration for all interfaces that must be managed by Network Manager.

Note that this program is installed by default when the “Desktop Environment” task is chosen during initial installation.