Difference between revisions of "Network configuration"

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If you use DHCP and you do '''not''' want your DNS servers automatically assigned every time you start your network, be sure to use the "-C" option in <code>DHCPCD_ARGS</code> in <code>/etc/conf.d/dhcpcd</code> (used by <code>/etc/rc.d/network</code>). This prevents dhcpcd from rewriting your <code>/etc/resolv.conf</code> every time:
If you use DHCP and you do '''not''' want your DNS servers automatically assigned every time you start your network, be sure to use the "-C" option in <code>DHCPCD_ARGS</code> in <code>/etc/conf.d/dhcpcd</code> (used by <code>/etc/rc.d/network</code>). This prevents dhcpcd from rewriting your <code>/etc/resolv.conf</code> every time:
  DHCPCD_ARGS="'''-C resolv.conf''' -t 30 -h $HOSTNAME"
  DHCPCD_ARGS="'''-C resolv.conf''' -q"
Note: in previous <code>dhcpcd</code> versions, the now depreciated "-R" option was used instead:
Note: in previous <code>dhcpcd</code> versions, the now depreciated "-R" option was used instead:

Revision as of 21:57, 12 November 2008

{Localized|Configuring_network}Template:I18n links start Template:I18n entry Template:I18n entry Template:I18n entry Template:I18n entry Template:I18n entry Template:I18n entry Template:I18n entry Template:I18n links end


A simple guide to get your network running.

Load the device module

Udev 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:

hwdetect --show-net

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

# modprobe <modulename>

If udev is not detecting and loading the proper module automatically, 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 (already available on most installations). Edit /etc/rc.conf like this:


If you use DHCP and you do not want your DNS servers automatically assigned every time you start your network, be sure to use the "-C" option in DHCPCD_ARGS in /etc/conf.d/dhcpcd (used by /etc/rc.d/network). This prevents dhcpcd from rewriting your /etc/resolv.conf every time:

DHCPCD_ARGS="-C resolv.conf -q"

Note: in previous dhcpcd versions, the now depreciated "-R" option was used instead:


Additonally, make sure to add (a) valid nameserver(s) to /etc/resolv.conf. A sample /etc/resolv.conf:

#DHCP with user-specified DNS 


The new dhcpcd (>= 4.0.2) no longer works with the -R flag since it has been decremented (only works when compiled with compatibility mode). Instead you need place your custom DNS servers in a /etc/resolv.conf.head. dhcpcd will then attach that file to the top of the normal resolv.conf

Finally, if you do not want to reboot, make sure to test your new settings by stopping and starting the /etc/rc.d/network daemon as opposed to bringing down your interface and starting dhcp manually. To restart the network daemon:

/etc/rc.d/network restart

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.

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

See Wireless Setup for more informations.


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"


options bonding miimon=100


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

restart network by

/etc/rc.d/network restart

multiple ip on multiple card

one ip on one card

vi /etc/rc.conf
 eth0="eth0 netmask broadcast"
 INTERFACES=(lo eth0)

two ip on one card (BUG:/etc/rc.d/network stop)

vi /etc/rc.conf
 eth0="eth0 netmask broadcast"
 eth0_0="eth0:0 netmask broadcast"
 INTERFACES=(lo eth0 eth0_0)

one ip on two card

pacman -S ifenslave
vi /etc/rc.conf
 bond0="bond0 netmask broadcast"
 INTERFACES=(lo bond0)
 MODULES=(... bonding ...)

two ip on two card (BUG:/etc/rc.d/network stop)

pacman -S ifenslave
vi /etc/rc.conf
 bond0="bond0 netmask broadcast"
 bond01="bond0:1 netmask broadcast"
 INTERFACES=(lo bond0 bond01)
 MODULES=(... bonding ...)


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).

DLink G604T/DLink G502T DNS issue

Users with a DLink G604T/DLink G502T router, using DHCP and have firmware v2.00+ (typically users with AUS firmware) may have issues with certain programs not resolving the DNS. One of these programs are unfortunatley pacman. The problem is basically the router in certain situations is not sending the DNS properly to DHCP, which causes programs to try and connect to servers with an IP of and fail with a connection timed out error

How To Diagnose The Problem?

The best way to diagnose the problem is to use a firefox/konqueror/links/seamonkey and to enable wget for pacman. If this is a fresh install of Arch Linux, then you may want to consider installing links through the live CD.

Firstly enable wget for pacman (since it gives us info about pacman when its downloading packages) Open /etc/pacman.conf with your favourite editor and uncomment the following line (remove the # if its there)

XferCommand = /usr/bin/wget --passive-ftp -c -O %o %u

While your in pacman.conf, check the default mirror that pacman uses to download packages.

Now open up the default mirror in an internet browser to see if the mirror actually works. If it does work then do pacman -Syy (otherwise pick another working mirror and set it to the pacman default), if you get something similar to the following (notice the

           => `/var/lib/pacman/community.db.tar.gz.part'                            
Resolving mirror.pacific.net.au...

Then you most likely have this problem. The means its unable to resolve the DNS, so we must add it to resolv.conf.

How To Fix It?

Basically what we need to do is to manually add the DNS to our /etc/resolv.conf file, The problem is that DHCP automatically deletes and replaces this file on boot, so we need to edit /etc/conf.d/dhcpcd and change the flags to stop DHCP doing this

When you open up /etc/conf.d/dhcpcd, you should see something close to the following


add the -R flag to the arguments, i.e.


NOTE: If you are using dhcpcd >= 4.0.2 the -R flag has been depreciated, please look here on ([1]) section how to use a custom resolv.conf file

Save and close, now open /etc/resolv.conf. You should see a single namespace (most likely, this is the gateway to your router, which we need to connect to in order to get the DNS of your ISP. Paste the IP into your browser and login to your router. Go to the DNS section and you should see an IP in the Preferred DNS Server, copy it and paste it as a namespace ABOVE the current gateway one.

i.e. a resolv.conf should look something along the lines of


If my Primary DNS Server is then chance resolv.conf to


Now restart the network daemon by doing /etc/rc.d/network restart and do pacman -Syy, if it syncs fine with the server then problem solved

More about it?

This is the whirlpool forum (Australian ISP community) which talks about and gives the same solution to the problem