Difference between revisions of "Network configuration"

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{{i18n_entry|Česky|Statická IP a DHCP czech}}

Revision as of 22:17, 16 January 2008

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A simple guide to get your network running.

Load the device module

If you use hwdetect it should detect your network card (NIC) module and load it automatically at startup. Otherwise, you will need to know which module is needed for your particular model.

Search the Web for your card model, or try using a Linux LiveCD to find out the name of the needed module - run lsmod to show you all currently loaded modules after booting.

Now when you know which module to use you can load it:

# modprobe <modulename>

If you don't want / can't use some auto-loader like hwdetect you can add it into the modules array in /etc/rc.conf, so you don't need to modprobe it everytime you boot. For example, if tg3 is the network module:

MODULES=(!usbserial tg3 snd-cmipci)

Other common modules are 8139too for cards with the Realtek chipset or sis900 for SiS cards.

Configure IP


For this, you need the dhcpcd package (usually already available on most installation). Edit /etc/rc.conf like this:


For Static IP

If you share your internet connection from a Windows box without a router, be sure to use static IPs on both computers. Otherwise you will have LAN issues.

You need:

  • Your static IP address,
  • The netmask,
  • The broadcast address,
  • Your gateway,
  • Your nameservers' IP addresses,
  • Your domain name.

If you are running a private network, it is safe to use IP addresses in 192.168.*.'* for your IPs, with a netmask of and broadcast address of Unless your network has a router, the gateway address does not matter. Edit /etc/rc.conf like this, substituting your own values for the IP, netmask, broadcast, and gateway:

 eth0="eth0 netmask broadcast"
gateway="default gw"

and your /etc/resolv.conf like this, substituting your nameservers' IPs and your domain name:

nameserver 61.95.849.8
search example.com

You may include as many nameserver lines as you wish.

If you use DHCP and you don't want your DNS servers to change every time you start your network, be sure to add the "-R" option to DHCPCD_ARGS in /etc/conf.d/dhcpcd (used by in /etc/rc.d/network). This prevents DHCP from rewriting your /etc/resolv.conf every time:


Other option

If for some reason dhcpcd eth0 fails, install dhclient (pacman -Sy dhclient) and use 'dhclient eth0' instead.

Set computer name

Edit /etc/rc.conf and set HOSTNAME to your desired computer name:


Set host name/IP

Edit /etc/hosts and add the same HOSTNAME you entered in /etc/rc.conf :      banana.domain.org   localhost.localdomain      localhost    banana

This format, including the localhost entries is required for program compatibility.

Load configuration

To test your settings either reboot the computer, or as root, run /etc/rc.d/network restart. Try pinging your gateway, DNS server, ISP provider and other Internet sites, in that order, to detect any connection problems along the way.

Some more settings

Wireless Setup

The wireless (wlan) configuration is the topic of another wiki page.


You can install and configure a firewall to feel more secure. ;-)


You can install a daemon which will automatically configure your Ethernet device when a cable is plugged in and automatically unconfigure it if the cable is pulled. This is useful on laptops with onboard network adapters, since it will only configure the interface when a cable is really connected. Another use is when you just need to restart the network but don't want to restart the computer or do it from the shell.

Installation is very simple since it's in [extra]:

# pacman -S ifplugd

By default it is configured to work for eth0 device. This and other settings like delays can be configured in /etc/ifplugd/ifplugd.conf.

Start it with

# /etc/rc.d/ifplugd start

or add it into DAEMONS array in /etc/rc.conf.


You can install 'ifenslave' to bind two real Ethernet cables with one IP address. /etc/conf.d/bonding

bond_bond0="eth0 eth1"


MODULES=(... bonding ...)
bond0="bond0 netmask broadcast"

restart network by

/etc/rc.d/network restart


Swapping computers on the cable modem

Most domestic cable ISPs (videotron for example) have the cable modem configured to recognise only one client PC, by the MAC address of its network interface. Once the cable modem has learnt the MAC address of the first PC that talks to it, it will not respond to another MAC address in any way. Thus if you swap one PC for another (or for a router), the new PC (or router) will not work with the cable modem, because the new PC (or router) has a different MAC address to the old one. To reset the cable modem so that it will recognise the new PC, you must power the cable modem off and on again. Once the cable modem has rebooted and gone fully online again (indicator lights settled down), reboot the newly connected PC so that it makes a DHCP request, or manually make it request a new DHCP lease.

The TCP Window Scaling Issue

TCP packets contain a "window" value in their headers indicating how much data the other host may send in return. This value is represented with only 16 bits, hence the window size is at most 64Kb. TCP packets are cached for a while (they have to be reordered), and as memory is (or used to be) limited, one host could easily run out of it.

Back in 1992, as more and more memory became available, RFC 1323 was written to improve the situation: Window Scaling. The "window" value, provided in all packets, will be modified by a Scale Factor defined once, at the very beginning of the connection.

That 8-bit Scale Factor allows the Window to be up to 32 times higher than the initial 64Kb.

It appears that some broken routers and firewalls on the Internet are rewriting the Scale Factor to 0 which causes misunderstandings between hosts.

The Linux kernel 2.6.17 introduced a new calculation scheme generating higher Scale Factors, virtually making the aftermaths of the broken routers and firewalls more visible.

The resulting connection is at best very slow or broken.

How To Diagnose The Problem?

First of all, let's make it clear: this problem is odd. In some cases, you will not be able to use TCP connections (HTTP, FTP, ...) at all and in others, you will be able to communicate with some hosts (very few).

Warning: dmesg's output is OK, logs are clean and ifconfig will report normal status — and actually everything is normal.

If you can't browse any website, but you can ping some rare hosts, chances are great that you're experiencing this issue: ping uses the ICMP protocol and is not affected by TCP issues.

You can try to use WireShark. You might see successful UDP and ICMP communications but unsuccessful TCP communications (only to foreign hosts).

How To Fix It? (The bad way)

To fix it the bad way, you can change the tcp_rmem value, on which Scale Factor calculation is based. Although it should work for most hosts, it's not guaranteed, especially for very distant ones.

echo "4096 87380 174760" > /proc/sys/net/ipv4/tcp_rmem

Or you can try to remove one of your RAM sticks (yes, sir).

How To Fix It? (The good way)

Simply disable Window Scaling. Even if Window Scaling is a nice TCP feature, it may be uncomfortable especially if you can't fix the broken router. There are several ways to disable Window Scaling, and it seems that the most bulletproof (which will work with most kernels) is to add the following lines to your /etc/rc.local:

echo 0 > /proc/sys/net/ipv4/tcp_window_scaling

How To Fix It? (The best way)

This issue is caused by broken routers/firewalls, so let's change them. Some users have reported that the broken router was their very own DSL router.

More about it?

This section is based on the LWN article TCP window scaling and broken routers and a Kernel Trap article: Window Scaling on the Internet.

And more recently, some Archers have been hit by this issue:

There are also several relevant threads on the LKML.

Realtek No Link / WOL issue

Users with Realtek 8168 8169 8101 8111 based NICs (cards / and on board) may notice an issue where the NIC seems to be disabled on boot and has no Link light. This can usually be found on a dual boot system where Windows is also installed. It seems that using the offical realtek drivers (dated anything after May 2007) under windows is the cause. These newer drivers disable the Wake-On-Lan feature by disabling the NIC at Windows shutdown time, where it will remain disabled until the next time Windows boots. You will be able to spot if this is issue is affecting you because the Link light will remain off only until Windows boots, during Windows shutdown it will revert back to the Link light being off. Normal operation should be that the link light is always on as long as the system is on, even during POST. This issue will also affect other OS's without newer drivers (eg. Live CD's). Here's a couple of fixes for this issue.

Method 1 - Rollback/Change Win driver

You can rollback your Windows NIC driver to the Microsoft provided one (if available), or rollback/install to an official Realtek driver pre-dated May 2007 (maybe on the CD that came with your hardware).

Method 2 - Enable WOL in Win driver

Probably the best and fastest fix is to change this setting in the Windows driver. This way it should be fixed system wide and not only under Arch (eg. live CD's, other OSes). In Windows under device manager, find your Realtek Network adapter and double click it. Under the advanced tab change "wake-on-lan after shutdown" to enable.

 In Windows XP (example)
 Right click my computer --> Hardware tab --> Device Manager --> Network Adapters --> "double click" Realtek ... --> Advanced tab --> Wake-On-Lan After Shutdown --> Enable.

Method 3 - Newer Realtek Linux driver

Any newer driver for these Realtek cards can be found for Linux on the realtek site. (untested but believed to also solve the problem).