Sat, 12 Sep 2009
/Linux/Debian:
Quick Start Networking on Debian
This simple tutorial is meant to show the basics of how to get networking working on Debian Linux
from the command line, and assumes at least that your network card has been recognized, the
appropriate kernel module has been loaded, and a viable network interface initialized for the network
card. This should all happen automatically on boot or card insertion, and if it does not, getting your
card to work is unlikely to be a simple task.
(Note that there are various GUIs that try to make networking "easy" in Linux, but in my experience
they do not work very well, and once you know the command line method you will rarely crave a GUI....)
All commands in this tutorial require a root terminal. The commands we are going to use are:
- ifconfig: query (and modify) status of network interfaces
- ifup: activate a network interface
- ifdown: de-activate a network interface
- iwconfig: query (and modify) the status of a wireless network interface
- iwlist: find out what wireless access points are within range
To find out what live network interfaces you have, enter ifconfig:
<![CDATA[
# ifconfig
eth1 Link encap:Ethernet HWaddr 00:30:bd:d0:16:d0
inet addr:192.168.8.6 Bcast:192.168.8.255 Mask:255.255.255.0
inet6 addr: fe80::230:bdff:fed0:16d0/64 Scope:Link
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
RX packets:41490 errors:57 dropped:0 overruns:0 frame:0
TX packets:43735 errors:10 dropped:15 overruns:0 carrier:0
collisions:0 txqueuelen:1000
RX bytes:29486719 (28.1 MiB) TX bytes:13372215 (12.7 MiB)
Interrupt:3 Base address:0x3100
lo Link encap:Local Loopback
inet addr:127.0.0.1 Mask:255.0.0.0
inet6 addr: ::1/128 Scope:Host
UP LOOPBACK RUNNING MTU:16436 Metric:1
RX packets:12 errors:0 dropped:0 overruns:0 frame:0
TX packets:12 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:0
RX bytes:600 (600.0 B) TX bytes:600 (600.0 B)
]]>
In the above example, "lo" is the loopback interface (important but irrelevant to this tutorial)
and "eth1" is the network interface with the outside world. In eth1, note:
inet addr: 192.168.8.6
which tells you that eth1 has an IP address (and in fact is my wireless card as I write this).
Though not always strictly true, for the purposes of this tutorial, if there is no IP address, the
network is not working. Now enter "ifconfig -a":
<![CDATA[
# ifconfig -a
eth1 Link encap:Ethernet HWaddr 00:30:bd:d0:16:d0
inet addr:192.168.8.6 Bcast:192.168.8.255 Mask:255.255.255.0
inet6 addr: fe80::230:bdff:fed0:16d0/64 Scope:Link
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
RX packets:41860 errors:66 dropped:0 overruns:0 frame:0
TX packets:44166 errors:10 dropped:15 overruns:0 carrier:0
collisions:0 txqueuelen:1000
RX bytes:29644024 (28.2 MiB) TX bytes:13420797 (12.7 MiB)
Interrupt:3 Base address:0x3100
eth4 Link encap:Ethernet HWaddr 00:d0:59:c1:e6:79
BROADCAST MULTICAST MTU:1500 Metric:1
RX packets:0 errors:0 dropped:0 overruns:0 frame:0
TX packets:0 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:1000
RX bytes:0 (0.0 B) TX bytes:0 (0.0 B)
lo Link encap:Local Loopback
inet addr:127.0.0.1 Mask:255.0.0.0
inet6 addr: ::1/128 Scope:Host
UP LOOPBACK RUNNING MTU:16436 Metric:1
RX packets:12 errors:0 dropped:0 overruns:0 frame:0
TX packets:12 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:0
RX bytes:600 (600.0 B) TX bytes:600 (600.0 B)
]]>
Note that "eth4" has now joined the party, and does not have an IP address. eth4 in fact is my
on-board wired network card which I am not using and have turned off. "ifconfig" shows all active
network interfaces, and "ifconfig -a" shows all network interfaces, including those that are not
turned on.
udev is the piece of software that is supposed to assign a unique network name (eth0, eth1, eth3,
wlan0, etc.) to every network card plugged into your system, which should remain stable over reboots.
(Of course, you can change this behavior and even choose what name is assigned to a given card.....)
Now have a look at the file /etc/network/interfaces with your favorite text editor. Mine
looks like this:
<![CDATA[
# The loopback network interface
auto lo
iface lo inet loopback
# wireless card
auto eth1
iface eth1 inet dhcp
wireless-essid any
wireless-mode Managed
# internal wired card
iface eth4 inet dhcp
]]>
Interfaces with an "auto" line should be started automatically on boot, in this case lo and eth1. "man
interfaces" will tell you lots more about the contents of this file, but for the simplest case,
iface eth4 inet dhcp
specifies the configuration of the eth4 wired interface: the network will be queried for a
dynamically assigned IP address via DHCP, and
<![CDATA[
iface eth1 inet dhcp
wireless-essid any
wireless-mode Managed
]]>
will do the same for the eth1 wireless interface. The 2nd and 3rd lines give the wireless card
permission to connect to any wireless access point that it can find. (Of course, one can also not use
DHCP and assign a static IP, and also specify the access point.... not covered here.)
With a /etc/network/interfaces file with the above contents, at the root prompt issuing
ifup eth1
will start the wireless interface. If the card finds a willing and appropriately configured access
point, ifconfig should show eth1 as having an IP address. You can find out more about what is going on
with your wireless card (and reconfigure it) with the iwconfig command:
<![CDATA[
# iwconfig
lo no wireless extensions.
eth4 no wireless extensions.
eth1 IEEE 802.11-DS ESSID:off/any
Mode:Managed Frequency:2.422 GHz Access Point: 00:40:05:D0:7F:9C
Bit Rate:11 Mb/s
Retry short limit:7 RTS thr:off Fragment thr:off
Encryption key:off
Power Management:off
Link Quality=61/100 Signal level=27/100
Rx invalid nwid:0 Rx invalid crypt:0 Rx invalid frag:0
Tx excessive retries:0 Invalid misc:0 Missed beacon:35909
]]>
You can also have a look at what other access points the wireless card is seeing with the iwlist
command:
<![CDATA[
# iwlist eth1 scan
eth1 Scan completed :
Cell 01 - Address: 00:40:05:D0:7F:9C
ESSID:"15hao"
Mode:Managed
Channel:3
Quality:16/100 Signal level:16/100 Noise level:0/0
Encryption key:off
]]>
Just one access point in this neighborhood....
Suppose this is your home configuration, and at work you have a wireless network called 2WIRE162
protected (not very) with a WEP key. Add the following stanza to your /etc/network/interfaces
file:
<![CDATA[
iface work inet dhcp
wireless-essid 2WIRE162
wireless-mode Managed
wireless-key 0642243084
]]>
In our
/etc/network/interfaces file we have three physical network interfaces (lo, eth1, and eth4)
and now we have added a "logical" interface called "work". A physical interface corresponds to a network
card ("lo" is a special case that does not) and a logical interface *can be* associated with any physical
interface. Ie.
ifup eth1
will cause eth1 to connect to any open access point in the vicinity, whereas
ifup eth1=work
will permit eth1 to only connect to an access point with an SSID of "2WIRE162" and a WEP key of
"0642243084".
WPA encryption, unlike WEP, provides real security, but is a little complicated to setup and will not
be covered here. If you want WPA, you need to configure a piece of software called "wpasupplicant".
posted at: 13:13 | path: /Linux/Debian | permanent link to this entry
