Network configuration

Check the connection

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

Note: The -c 3 option calls it three times. See man ping for more information.

$ ping -c 3 www.google.com

PING www.l.google.com (74.125.224.146) 56(84) bytes of data.
64 bytes from 74.125.224.146: icmp_req=1 ttl=50 time=437 ms
64 bytes from 74.125.224.146: icmp_req=2 ttl=50 time=385 ms
64 bytes from 74.125.224.146: 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

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 8.8.8.8

PING 8.8.8.8 (8.8.8.8) 56(84) bytes of data.
64 bytes from 8.8.8.8: icmp_req=1 ttl=53 time=52.9 ms
64 bytes from 8.8.8.8: icmp_req=2 ttl=53 time=72.5 ms
64 bytes from 8.8.8.8: icmp_req=3 ttl=53 time=70.6 ms

--- 8.8.8.8 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

Note: 8.8.8.8 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 /etc/resolv.conf file.

Set the hostname

A hostname is a unique name created to identify a machine on a network. To set the hostname:

# hostnamectl set-hostname myhostname

Note: You no longer need to edit /etc/hosts, nss-myhostname will provide host name resolution, and is installed on all systems by default.

To set the hostname temporarily (until a reboot), use the hostname command from inetutils:

# hostname myhostname

Device Driver

Check the driver status

Udev should detect your network interface card (NIC) and automatically load the necessary module at start up. Check the "Ethernet controller" entry (or similar) from the lspci -v output. It should tell you which kernel module contains the driver for 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

Skip the next section if the driver was loaded successfully. 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 chipset. Once you know which module to use, you can load it with:

# modprobe module_name

If udev is not detecting and loading the proper module automatically during bootup, you can add it to a *.conf file from the /etc/modules-load.d/ folder so that you do not need to modprobe it every time you boot. For example, if tg3 is the network module:

# tee /etc/modules-load.d/tg3.conf <<< "tg3"

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

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 the network settings for eth0, 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 Udev#Network device.

Get current device names

Current NIC names can be found using the ip tool:

$ ip addr | sed '/^[0-9]/!d;s/: <.*$//'

1: lo
2: eth1
3: eth0
4: firewire0

Enabling and disabling network interfaces

You can activate or deactivate network interfaces using:

# ip link set eth0 up
# ip link set eth0 down

To check the result:

$ ip addr show dev eth0

2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP qlen 1000
[...]

Configure the IP address

You have two options: a dynamically assigned address using DHCP, or an unchanging "static" address.

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 192.168.1.70 for 86400 seconds

And now, ip addr show dev eth0 should show your inet address.

For some people, dhclient (from the dhclient package) works where dhcpcd fails.

Run DHCP at boot

If you simply want to use DHCP for your Ethernet connection, you can use dhcpcd@.service (provided by the dhcpcd package).

To enable DHCP for eth0, simply use:

# systemctl start dhcpcd@eth0

You can enable the service to automatically start at boot with:

# systemctl enable dhcpcd@eth0

If the dhcpd service starts before your network card module (FS#30235), manually add your network card to /etc/modules-load.d/*.conf. For example, if your Realtek card needs r8169 to be loaded, create:

/etc/modules-load.d/realtek.conf

r8169

Tip: To find out which modules are used by your network card, use lspci -k.

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 dhcpcd.conf:

/etc/dhcpcd.conf

nohook resolv.conf

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

/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 with unchanging addresses, or you may not have a DHCP server available.

Note: If you share your Internet connection from a Windows machine 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

If you are running a private network, it is safe to use IP addresses in 192.168.*.* for your IP addresses, with a subnet mask of 255.255.255.0 and a broadcast address of 192.168.*.255. The gateway is usually 192.168.*.1 or 192.168.*.254.

Manual assignment

You can assign a static IP address in the console:

# ip addr add <IP address>/<subnet mask> dev <interface>

For example:

# ip addr add 192.168.1.2/24 dev eth0

Note: The subnet mask was specified using CIDR notation.

For more options, see man ip.

Add your gateway like so:

# ip route add default via <default gateway IP address>

For example:

# ip route add default via 192.168.1.1

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

Manual connection at boot using systemd

This section details how to manually connect using systemd.

Using dhcpcd

Create the file /etc/systemd/system/network.service using your editor of choice. This example uses wpa_supplicant.

/etc/systemd/system/network.service

[Unit]
Description=Network Connectivity
Wants=network.target
Before=network.target

[Service]
Type=oneshot
RemainAfterExit=yes
ExecStart=/sbin/ip link set dev wlan0 up
ExecStart=/usr/sbin/wpa_supplicant -B -i wlan0 -c /etc/wpa_supplicant.conf
ExecStart=/sbin/dhcpcd wlan0

[Install]
WantedBy=multi-user.target

# systemctl enable network

To test, reboot or stop all other network daemons and run as root:

# systemctl start network

Using a static IP address

Create the file /etc/conf.d/network using your editor of choice. This file will store your interface and static IP address settings.

/etc/conf.d/network

interface=wlan0
address=192.168.0.10
netmask=24
broadcast=192.168.0.255
gateway=192.168.0.1

Create the file /etc/systemd/system/network.service using your editor of choice. This example uses a static IP address and wpa_supplicant.

/etc/systemd/system/network.service

[Unit]
Description=Wireless Static IP Connectivity
Wants=network.target
Before=network.target

[Service]
Type=oneshot
RemainAfterExit=yes
EnvironmentFile=/etc/conf.d/network
ExecStart=/sbin/ip link set dev ${interface} up
ExecStart=/usr/sbin/wpa_supplicant -B -i ${interface} -c /etc/wpa_supplicant.conf # Remove this for wired connections
ExecStart=/sbin/ip addr add ${address}/${netmask} broadcast ${broadcast} dev ${interface}
ExecStart=/sbin/ip route add default via ${gateway}
 
ExecStop=/sbin/ip addr flush dev ${interface}
ExecStop=/sbin/ip link set dev ${interface} down

[Install]
WantedBy=multi-user.target

Do not forget to enable it!

# systemctl enable network

To test, reboot or make sure all other network daemons are stopped and then run as root

# systemctl start network

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 10.66.66.1, and specifying ipcalc should create a network of only 2 hosts.

$ ipcalc -nb 10.66.66.1 -s 1

Address:   10.66.66.1

Netmask:   255.255.255.252 = 30
Network:   10.66.66.0/30
HostMin:   10.66.66.1
HostMax:   10.66.66.2
Broadcast: 10.66.66.3
Hosts/Net: 2                     Class A, Private Internet

Load configuration

To test your settings either reboot the computer or reload the relevant systemd services:

# systemctl restart dhcpcd@eth0

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.

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-bonding^AUR package from the AUR.

Edit/create the following files:

/etc/network.d/bonded

CONNECTION="bonding"
INTERFACE="bond0"
SLAVES="eth0 eth1"
IP="dhcp"
DHCP_TIMEOUT=10

/etc/modules-load.d/bonding.conf

bonding

Set up netcfg to use the bond0 interface.

Start your network:

$ systemctl enable netcfg@bonded

Note: To change the bonding mode (default is round robin) to, e.g, dynamic link aggregation:

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

/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
# systemctl stop network
# systemctl 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.

Example

You will need netcfg from the Official Repositories.

Prepare the configuration:

/etc/network.d/mynetwork

CONNECTION='ethernet'
DESCRIPTION='Five different addresses on the same NIC.'
INTERFACE='eth0'
IP='static'
ADDR='192.168.1.10'
GATEWAY='192.168.1.1'
DNS=('192.168.1.1')
DOMAIN=
POST_UP='x=0; for i in 11 12 13 14; do ip addr add 192.168.1.$i/24 brd 192.168.1.255 dev eth0 label eth0:$((x++)); done'
PRE_DOWN='for i in 11 12 13 14; do ip addr del 192.168.1.$i/24 dev eth0; done'

The simply execute:

$ systemctl enable net-auto-wired.service

Change MAC/hardware address

See MAC Address Spoofing.

Troubleshooting

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 1.0.0.0 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 1.0.0.0),

ftp://mirror.pacific.net.au/linux/archlinux/extra/os/i686/extra.db.tar.gz
           => '/var/lib/pacman/community.db.tar.gz.part'
Resolving mirror.pacific.net.au... 1.0.0.0

then you most likely have this problem. The 1.0.0.0 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:

DHCPCD_ARGS="-t 30 -h $HOSTNAME"

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

DHCPCD_ARGS="-R -t 30 -h $HOSTNAME"

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 10.1.1.1). 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:

nameserver 10.1.1.1

If my primary DNS server is 211.29.132.12, then change /etc/resolv.conf to:

nameserver 211.29.132.12
nameserver 10.1.1.1

Now restart the network daemon by running systemctl restart dhcpcd@<interface> 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:

http://forums.whirlpool.net.au/forum-replies-archive.cfm/461625.html

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.

No eth0 with Atheros AR8161

With the Atheros AR8161 Gigabit Ethernet card, the ethernet connection is not working out-of-the-box (with the installation media of Jan 2013). The module "alx" needs to be loaded but is not present.

The driver from compat-wireless (that should become compat-drives from linux 3.7) need to be installed. You need the firmware version with pc in the filename to have the ethernet drivers.

 $ wget http://www.orbit-lab.org/kernel/compat-wireless-3-stable/v3.6/compat-wireless-3.6.8-1-snpc.tar.bz2
 $ cd compat-wireless-3.6.8-1
 $ ./scripts/driver-select alx
 $ make
 $ sudo make install
 $ sudo modprobe alx