Difference between revisions of "Network configuration"

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== Check first ==
== Check first ==

Revision as of 19:38, 15 October 2012

zh-CN:Configuring Network Template:Article summary start Template:Article summary text Template:Article summary heading Template:Article summary text Template:Article summary heading Template:Article summary wiki Template:Article summary wiki Template:Article summary wiki Template:Article summary wiki Template:Article summary end

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Reason: This article still references rc.conf, which has been deprecated. (Discuss in Talk:Network configuration#)

Check first

Many times, the basic installation procedure has created a working network configuration. To check if this is so, use the following command:

$ ping -c 3 www.google.com
PING www.l.google.com ( 56(84) bytes of data.
64 bytes from icmp_req=1 ttl=50 time=437 ms
64 bytes from icmp_req=2 ttl=50 time=385 ms
64 bytes from icmp_req=3 ttl=50 time=298 ms

--- www.l.google.com ping statistics ---
3 packets transmitted, 3 received, 0% packet loss, time 1999ms
rtt min/avg/max/mdev = 298.107/373.642/437.202/57.415 ms
Tip: The -c 3 options instruct ping to do so three times. See man ping for more information.

If it works, then you may only wish to personalize your settings from the options below.

If the previous command complains about unknown hosts, it means that your machine was unable to resolve this domain name. It might be related to your service provider or your router/gateway. You can try pinging a static IP address to prove that your machine has access to the Internet.

$ ping -c 3
PING ( 56(84) bytes of data.
64 bytes from icmp_req=1 ttl=53 time=52.9 ms
64 bytes from icmp_req=2 ttl=53 time=72.5 ms
64 bytes from icmp_req=3 ttl=53 time=70.6 ms

--- ping statistics ---
3 packets transmitted, 3 received, 0% packet loss, time 2002ms
rtt min/avg/max/mdev = 52.975/65.375/72.543/8.803 ms
Tip: is a static address that is easy to remember. It is the address of Google's primary DNS server, therefore it can be considered reliable, and is generally not blocked by content filtering systems and proxies.

If you are able to ping this address, you may try adding this nameserver to your resolv.conf file.

Set the host name

A host name is a unique name created to identify a machine on a network. With Arch Linux, a machine's host name is set in /etc/hostname. Host names are restricted to alphanumeric characters. The hyphen (-) can be used, but a host name cannot start or end with it. Length is restricted to 63 characters.

Simply put your host name in /etc/hostname; do not put a domain name in /etc/hostname. Create the file if it does not exist. In this example, myhostname is the host name:


After setting a host name, it is important to include the same host name in /etc/hosts. This will help processes that refer to the computer by its host name to find its IP address, as well as programs that rely on the gethostname() system call to determine the system's host name.

Edit /etc/hosts and add the same name you entered in /etc/hostname:   myhostname localhost
::1         myhostname localhost
Note: The fully qualified domain name (FQDN) should be the first item following the IP address. All of the names on the right side are just aliases for the left-most host/domain name. You can check if this has been properly configured by running hostname --fqdn.

To set the host name temporarily, until the next reboot, use the hostname command from package inetutils as root:

# hostname archlinux

Device Driver

Check Driver Status

Udev should detect your network interface card (NIC) module and load it automatically at start up. Check the "Ethernet controller" entry in the output of lspci -v. It should tell you which kernel module contains the driver of your network device. For example:

$ lspci -v
 02:00.0 Ethernet controller: Attansic Technology Corp. L1 Gigabit Ethernet Adapter (rev b0)
 	Kernel driver in use: atl1
 	Kernel modules: atl1

Next, check that the driver was loaded via dmesg | grep <module name>. For example:

$ dmesg | grep atl1
   atl1 0000:02:00.0: eth0 link is up 100 Mbps full duplex

If driver loads success, skip this section. Otherwise, you will need to know which module is needed for your particular model.

Load the device module

Google for the right module/driver for the chip. Once you know which module to use, you can load it with:

# modprobe <modulename>

If udev is not detecting and loading the proper module automatically during bootup, you can add it into the MODULES array in /etc/rc.conf so you do not need to modprobe it everytime you boot. For example, if tg3 is the network module:

MODULES=(... tg3 snd-cmipci ...)

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

Network Interfaces

Persistent Device Names

For motherboards that have integrated NICs, it is important to know which one is considered the primary NIC (e.g., eth0) and which is considered the secondary NIC (e.g., eth1). Many configuration issues are caused by users incorrectly configuring eth0 in their /etc/rc.conf, when in fact, they have their Ethernet cable plugged into eth1.

Udev is responsible for which device gets which name. With Udev and modular network drivers, the network interface numbering is not persistent across reboots by default, because the drivers are loaded in parallel and, thus, in random order. Configuring your network connection is hard if you do not know if your card will be called eth0 or eth1. You can fix this using ifrename, see Rename network interfaces. It is also possible to manually create udev rules that assign interface names based on the interface's MAC address. See Persistent Device Names.

Get Current Device Names

Current NIC Names can be found with the ip tool.

# ip addr | sed '/^[0-9]/!d;s/: <.*$//'
1: lo
2: eth1
3: eth0
4: firewire0

Enable/disable interface

You can activate or deactivate net interface:

# ip link set <interface> up/down

Check the result with ip addr show dev eth0. For example:

# ip addr show dev eth0
   2: eth0: <BROADCAST,MULTICAST,PROMISC,UP,LOWER_UP> mtu 1500 qdisc vboxnetflt state UP qlen 1000

Configure the IP address

You have two options: a dynamically assigned address using DHCP or an unchanging "static" address. See Wikipedia:DHCP for more information.

Dynamic IP address

Manually run DHCP Client Daemon

Please note that dhcpcd is not dhcpd.

$ dhcpcd eth0
 dhcpcd: version 5.1.1 starting
 dhcpcd: eth0: broadcasting for a lease
 dhcpcd: eth0: leased for 86400 seconds

And now ip addr show dev <interface> should show your inet address.

For some people, the dhclient package (available in [extra]) works where dhcpcd fails.

Run DHCP at booting

For this option, you need the dhcpcd package (already available on most installations). To make use of it, edit /etc/rc.conf like this:


Only the interface has to be defined, as leaving the other options blank will set network to DHCP.

If you use DHCP and you do not want your DNS servers automatically assigned every time you start your network, be sure to add the following to the last section of /etc/dhcpcd.conf:

nohook resolv.conf

To prevent dhcpcd from adding domain name servers to /etc/resolve.conf use the nooption option in /etc/dhcpcd.conf:

nooption domain_name_servers

Then add your own DNS name server to /etc/resolv.conf.

You may use the openresolv package if several different processes want to control /etc/resolv.conf (e.g., dhcpcd and a VPN client). No additional configuration for dhcpcd is needed to use openresolv.

Note: It is possible to have a static IP address using dhcpcd. Simply edit your /etc/conf.d/dhcpcd file to look something like this (where x.x.x.x is your desired IP address):
DHCPCD_ARGS="-q -s x.x.x.x"

Static IP address

There are various reasons why you may wish to assign static IP addresses on your network. For instance, one may gain a certain degree of predictability. Or you may not want the dhcp daemon running all the time.

Note: If you share your Internet connection from a Windows box without a router, be sure to use static IP addresses on both computers to avoid LAN issues.

You need:

  • Static IP address,
  • Subnet mask,
  • Broadcast address,
  • Gateway's IP address,
  • Name servers' IP addresses,
  • Domain name (unless a local LAN, in which case you can make it up).

If you are running a private network, it is safe to use IP addresses in 192.168.*.* for your IP addresses, with a netmask of and a broadcast address of 192.168.*.255. Unless your network has a router, the gateway IP address does not matter. Edit /etc/rc.conf like this, substituting your own values for the IP address, netmask, broadcast, and gateway:


Edit your /etc/resolv.conf like this, substituting your name servers' IP addresses and your local domain name:

nameserver 61.95.849.8
search example.com
Note: Currently, you may include a maximum of 3 nameserver lines.

Manual assignment

You can assign a static IP address in the console:

# ip addr add <ip address>/<netmask> dev <interface>

For example:

# ip addr add dev eth0

For more options, see man ip.

Add your gateway like so:

# ip route add default via <ip address>

(Substitute your own gateway's IP address)

For example:

# ip route add default via

If you the get the error "No such process", it means you have to run # ip link set dev eth0 up.

Calculating Addresses

You can use ipcalc provided by the ipcalc package to calculate IP broadcast, network, netmask, and host ranges for more advanced configurations. For example, I use ethernet over firewire to connect a windows machine to arch. For security and network organization, I placed them on their own network and configured the netmask and broadcast so that they are the only 2 machines on it. To figure out the netmask and broadcast addresses for this, I used ipcalc, providing it with the IP of the arch firewire nic, and specifying ipcalc should create a network of only 2 hosts.

$ ipcalc -nb -s 1

Netmask: = 30
Hosts/Net: 2                     Class A, Private Internet

Load configuration

To test your settings either reboot the computer, or as root:

# rc.d restart network

Try pinging your gateway, DNS server, ISP provider and other Internet sites, in that order, to detect any connection problems along the way, as in this example:

$ ping -c 3 www.google.com

Additional settings

ifplugd for laptops

ifplugd in Official Repositories is 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 do not want to restart the computer or do it from the shell.

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

Start the ifplugd daemon and add ifplugd to your DAEMONS array so it starts automatically on boot.

Alternatively with systemd, enabling net-auto-wired.service should start ifplugd on bootup if you have netcfg installed, otherwise you can use ifplugd@eth0.service.

Bonding or LAG

You will need netcfg from the Official Repositories, as well as the netcfg-bondingAUR package from the AUR.

Edit/create the following files:

 SLAVES="eth0 eth1"
MODULES=(... bonding ...)
 interface=bond0 #comment other lines (address,netmask,gateway,...)
 NETWORKS=(... bonded ...)
 DAEMONS=(... net-profiles ...) #Replace "network".
Note: To change the bonding mode (default is round robin) to, e.g, dynamic link aggregation:

Create /etc/modprobe.d/bonding.conf:

options bonding mode=4
options bonding miimon=100
For more information about the different bonding policies (and other driver settings) see the Linux Ethernet Bonding Driver HOWTO.

To activate the new bonded ports modprobe bonding, stop network and start the net-profiles service:

# modprobe bonding
# rc.d stop network
# rc.d start net-profiles

To check the status and bonding mode:

$ cat /proc/net/bonding/bond0

IP address aliasing

IP aliasing is the process of adding more than one IP address to a network interface. With this, one node on a network can have multiple connections to a network, each serving a different purpose.

To use IP aliasing from netcfg, change POST_UP and PRE_DOWN commands in your network profile to set up the additional IP addresses manually. See here for details.


You will need netcfg from the Official Repositories.

Prepare the configuration:

DESCRIPTION='Five different addresses on the same NIC.'
POST_UP='x=0; for i in 11 12 13 14; do ip addr add 192.168.1.$i/24 brd dev eth0 label eth0:$((x++)); done'
PRE_DOWN='for i in 11 12 13 14; do ip addr del 192.168.1.$i/24 dev eth0; done'


DAEMONS=(... net-profiles ...)

Change MAC/hardware address

Changing your MAC address is not possible anymore via /etc/rc.conf. See MAC Address Spoofing for details.


Swapping computers on the cable modem

Most domestic cable ISPs (videotron for example) have the cable modem configured to recognize only one client PC, by the MAC address of its network interface. Once the cable modem has learned the MAC address of the first PC or equipment 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 MAC address different from 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.

If this method does not work, you will need to clone the MAC address of the original machine. See also Change MAC/hardware address.

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).

When you have this problem, the dmesg's output is OK, logs are clean and ip addr will report normal status... and actually everything appears normal.

If you cannot browse any website, but you can ping some random hosts, chances are great that you're experiencing this issue: ping uses ICMP 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 is not guaranteed, especially for very distant ones.

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

How to fix it (The good way)

Simply disable Window Scaling. Since Window Scaling is a nice TCP feature, it may be uncomfortable to disable it, especially if you cannot fix the broken router. There are several ways to disable Window Scaling, and it seems that the most bulletproof way (which will work with most kernels) is to add the following line to /etc/sysctl.conf (see also sysctl)

net.ipv4.tcp_window_scaling = 0

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.

There are also several relevant threads on the LKML.

Realtek no link / WOL issue

Users with Realtek 8168 8169 8101 8111(C) 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 notice if this issue is affecting you if the Link light remains off until Windows boots up; during Windows shutdown the Link light will switch 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 operative systems without newer drivers (eg. Live CDs). Here are a few fixes for this issue:

Method 1 - Rollback/change Windows driver

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

Method 2 - Enable WOL in Windows driver

Probably the best and the 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 CDs, other operative systems). 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
Note: Newer Realtek Windows drivers (tested with Realtek 8111/8169 LAN Driver v5.708.1030.2008, dated 2009/01/22, available from GIGABYTE) may refer to this option slightly differently, like Shutdown Wake-On-LAN --> Enable. It seems that switching it to Disable has no effect (you will notice the Link light still turns off upon Windows shutdown). One rather dirty workaround is to boot to Windows and just reset the system (perform an ungraceful restart/shutdown) thus not giving the Windows driver a chance to disable LAN. The Link light will remain on and the LAN adapter will remain accessible after POST - that is until you boot back to Windows and shut it down properly again.

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).

Method 4 - Enable LAN Boot ROM in BIOS/CMOS

It appears that setting Integrated Peripherals --> Onboard LAN Boot ROM --> Enabled in BIOS/CMOS reactivates the Realtek LAN chip on system boot-up, despite the Windows driver disabling it on OS shutdown.
This was tested successfully multiple times with GIGABYTE system board GA-G31M-ES2L with BIOS version F8 released on 2009/02/05. YMMV.

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 address of and fail with a connection timed out error

How to diagnose the problem

The best way to diagnose the problem is to use 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 it is downloading packages) Open /etc/pacman.conf with your favourite editor and uncomment the following line (remove the # if it is there)

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

While you are editing /etc/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 it is unable to resolve DNS, so we must add it to /etc/resolv.conf.

How to fix it

Basically what we need to do is to manually add the DNS servers 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 from doing this.

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


Add the -R flag to the arguments, e.g.,

Note: If you are using dhcpcd >= 4.0.2, the -R flag has been deprecated. Please see the #For DHCP assigned IP address section for information on how to use a custom /etc/resolv.conf file.

Save and close the file; now open /etc/resolv.conf. You should see a single nameserver (most likely This is the gateway to your router, which we need to connect to in order to get the DNS servers of your ISP. Paste the IP address into your browser and log in to your router. Go to the DNS section, and you should see an IP address in the Primary DNS Server field; copy it and paste it as a nameserver ABOVE the current gateway one.

For example, /etc/resolv.conf should look something along the lines of:


If my primary DNS server is, then change /etc/resolv.conf to:


Now restart the network daemon by doing rc.d restart network and do pacman -Syy. If it syncs correctly with the server, then the problem is solved.

More about it

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


Check DHCP problem by releasing IP first

Problem may occur when DHCP get wrong IP assignment. For example when two routers are tied together through VPN. The router that is connected to me by VPN may assigning IP address. To fix it. On a console, as root, release IP address:

# dhcpcd -k

Then request a new one:

# dhcpcd

Maybe you had to run those two commands many times.