Network configuration

Related articles

• Network Debugging
• Firewalls
• Jumbo frames
• Internet sharing
• Router

This article explains how to set up a network connection.

Check the connection

To troubleshoot a network connection, go through the following conditions and ensure that you meet them:

1. Your network interface is listed, see #Listing network interfaces.
2. Your network interface is enabled, see #Enabling and disabling network interfaces.
3. You are connected to the network. The cable is plugged in or you are connected to the wireless LAN.
4. Your network interface has an IP address, see #IP addresses.
5. Your routing table is correctly set up, see #Routing table.
6. You can ping a local IP address (e.g. your default gateway).
7. You can ping a public IP address (e.g. 8.8.8.8), if you can't it may be related to your default gateway or your internet service provider.
8. You can resolve domain names (e.g. archlinux.org), see #Resolving domain names.

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.

Ping

ping is used to test if you can reach a host.

$ ping www.example.com

PING www.example.com (93.184.216.34): 56(84) data bytes
64 bytes from 93.184.216.34: icmp_seq=0 ttl=56 time=11.632 ms
64 bytes from 93.184.216.34: icmp_seq=1 ttl=56 time=11.726 ms
64 bytes from 93.184.216.34: icmp_seq=2 ttl=56 time=10.683 ms
...

For every reply you receive ping prints a line like above. For more information see the ping(8) manual.

Note that computers can be configured not to respond to ICMP echo requests.[1]

When you receive no reply, you can use a traceroute (traceroute(8) or tracepath(8)) to further diagnose the route to the host.

Note: If you receive an error like ping: icmp open socket: Operation not permitted when executing ping, try to re-install the iputils package.

Device driver

Check the status

udev should detect your network interface controller (NIC) and automatically load the necessary kernel module at startup. 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 module

Search in the Internet for the right module/driver for the chipset. Some common modules are 8139too for cards with a Realtek chipset, or sis900 for cards with a SiS chipset. Once you know which module to use, try to load it manually. If you get an error saying that the module was not found, it is possible that the driver is not included in Arch kernel. You may search the AUR for the module name.

If udev is not detecting and loading the proper module automatically during bootup, see Kernel module#Automatic module handling.

Network management

Setup

To set up a network connection, go through the following steps:

1. Ensure your #network interface is listed and enabled.
2. Connect to the network. Plug in the Ethernet cable or connect to the wireless LAN.
3. Configure your network connection:
   • #static IP address
   • dynamic IP address: use #DHCP

Tip: #Network managers provide automatic network connection and configuration based on network profiles.

The iproute2 package provides the ip(8) command-line utility, used to manage #Network interfaces, #IP addresses and the #Routing table. Be aware that configuration made using ip will be lost after a reboot. You can automate ip commands using scripts and systemd units. Also note that ip commands can generally be abbreviated, for clarity they are however spelled out in this article.

Network interfaces

By default udev assigns names to your network interfaces using Predictable Network Interface Names, which prefixes interfaces names with en (wired/Ethernet), wl (wireless/WLAN), or ww (WWAN).

Tip: To change interface names, see #Change interface name and #Revert to traditional interface names.

Listing network interfaces

Both wired and wireless interface names can be found via ls /sys/class/net or ip link. Note that lo is the loop device and not used in making network connections.

Wireless device names can also be retrieved using iw dev. See also Wireless network configuration#Get the name of the interface.

If your network interface is not listed, make sure your #Device driver was loaded successfully.

Enabling and disabling network interfaces

Network interfaces can be enabled / disabled using ip link set interface up|down, see ip-link(8).

To check the status of the interface eth0:

$ ip link show dev eth0

2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast master br0 state DOWN mode DEFAULT qlen 1000
...

The UP in <BROADCAST,MULTICAST,UP,LOWER_UP> is what indicates the interface is up, not the later state DOWN.

Note: If your default route is through interface eth0, taking it down will also remove the route, and bringing it back up will not automatically reestablish the default route. See #Routing table for reestablishing it.

Static IP address

A static IP address can be configured with most standard #network managers and also dhcpcd.

To manually configure a static IP address, add an IP address as described in #IP addresses, set up your #Routing table and configure your DNS servers.

IP addresses

IP addresses are managed using ip-address(8).

List IP addresses:

$ ip address show

Add an IP address to an interface:

# ip address add address/prefix_len broadcast + dev interface

Note that:

• the address is given in CIDR notation to also supply a subnet mask
• + is a special symbol that makes ip derive the broadcast address from the IP address and the subnet mask

Note: Make sure manually assigned IP addresses do not conflict with DHCP assigned ones. See this forum thread.

Delete an IP address from an interface:

$ ip address del address/prefix_len dev interface

Delete all addresses matching a criteria, e.g. of a specific interface:

$ ip address flush dev interface

Tip: IP addresses can be calculated with ipcalc (ipcalc).

Routing table

The routing table is used to determine if you can reach an IP address directly or what gateway (router) you should use. If no other route matches the IP address, the default gateway is used.

The routing table is managed using ip-route(8).

PREFIX is either a CIDR notation or default for the default gateway.

List routes:

$ ip route show

Add a route:

# ip route add PREFIX via address dev interface

Delete a route:

# ip route del PREFIX via address dev interface

DHCP

A DHCP server provides clients with a dynamic IP address, the subnet mask, the default gateway IP address and optionally also with DNS name servers.

Note: You should not run two DHCP clients simultaneously.

To use DHCP you need a DHCP server in your network and a DHCP client:

Client	Package	Archiso	Note	Systemd units
dhcpcd	dhcpcd	Yes	DHCP, DHCPv6, ZeroConf, static IP	dhcpcd.service, dhcpcd@interface.service
ISC DHCP	dhclient	Yes	DHCP, BOOTP, static IP	dhclient@interface.service
pump	pumpAUR	No	DHCP, BOOTP	?

Note that instead of directly using a DHCP client you can also use a #network manager.

Network managers

A network manager lets you manage network connection settings in so called network profiles to facilitate switching networks.

Note: There are many solutions to choose from, but remember that all of them are mutually exclusive; you should not run two daemons simultaneously.

Network manager	handles wired connections	GUI	Archiso [2]	CLI tools	PPP support (e.g. 3G modem)	#DHCP client	Systemd units
ConnMan	Yes	8 unofficial	No	connmanctl(1)	Yes	internal	connman.service
netctl	Yes	2 unofficial	Yes (base)	netctl(1), wifi-menu	Yes	dhcpcd or dhclient	netctl-ifplugd@interface.service, netctl-auto@interface.service
NetworkManager	Yes	Yes	No	nmcli(1), nmtui(1)	Yes	internal, dhcpcd or dhclient	NetworkManager.service
systemd-networkd	Yes	No	Yes (base)	No	?	internal	systemd-networkd.service, systemd-resolved.service
Wicd	Yes	Yes	No	wicd-cli(8), wicd-curses(8)	No	dhcpcd	wicd.service
Wifi Radar	No	Yes	No	No	No	any (just runs command)

See also List of applications#Network managers.

Resolving domain names

To resolve a domain name using your system configuration, run:

$ getent hosts domain_name

For more fine-grained DNS queries use a dedicated DNS lookup utility, like dig(1) (provided by bind-tools) or drill(1) (provided by ldns).

If you cannot resolve domain names but you are connected to the internet, see resolv.conf and check the hosts line in nsswitch.conf(5).

Set the hostname

A hostname is a unique name created to identify a machine on a network, configured in /etc/hostname—see hostname(5) and hostname(7) for details. The file can contain the system's domain name, if any. To set the hostname, edit /etc/hostname to include a single line with myhostname:

/etc/hostname

myhostname

Tip: For advice on choosing a hostname, see RFC 1178.

Alternatively, using hostnamectl(1):

# hostnamectl set-hostname myhostname

To temporarily set the hostname (until reboot), use hostname(1) from inetutils:

# hostname myhostname

To set the "pretty" hostname and other machine metadata, see machine-info(5) § https://www.freedesktop.org/software/systemd/man/machine-info.html.

Local network hostname resolution

The pre-requisite is to #Set the hostname, after which hostname resolution works on the local system itself:

$ ping myhostname

PING myhostname (192.168.1.2) 56(84) bytes of data.
64 bytes from myhostname (192.168.1.2): icmp_seq=1 ttl=64 time=0.043 ms

To allow other machines to address the host by name, it is necessary to either:

• Configure the hosts(5) file, or
• Enable a service which resolves the hostname.

Note: systemd provides hostname resolution via the myhostname nss module, enabled by default in /etc/nsswitch.conf. However, clients may still rely on /etc/hosts, see [3] [4] for examples.

To configure the hosts file, add the following line to /etc/hosts:

127.0.1.1	myhostname.localdomain	myhostname

Note: The order of hostnames/aliases that follow the IP address in /etc/hosts is significant. The first string is considered the canonical hostname and may be appended with parent domains, where domain components are separated by a dot (ie. .localdomain above). All following strings on the same line are considered aliases. See hosts(5) for more info.

As a result the system resolves to both entries:

$ getent hosts

127.0.0.1       localhost
127.0.1.1       myhostname.localdomain	myhostname

For a system with a permanent IP address, that permanent IP address should be used instead of 127.0.1.1.

Note: Another option is to set up a full DNS server such as BIND or Unbound, but that is overkill and too complex for most systems. For small networks and dynamic flexibility with hosts joining and leaving the network zero-configuration networking services may be more applicable:

• Samba provides hostname resolution via Microsoft's NetBIOS. It only requires installation of samba and enabling of the nmbd.service service. Computers running Windows, macOS, or Linux with nmbd running, will be able to find your machine.
• Avahi provides hostname resolution via zeroconf, also known as Avahi or Bonjour. It requires slightly more complex configuration than Samba: see Avahi#Hostname resolution for details. Computers running macOS, or Linux with an Avahi daemon running, will be able to find your machine. Windows does not have an built-in Avahi client or daemon.

Tips and tricks

Change interface name

Note: When changing the naming scheme, do not forget to update all network-related configuration files and custom systemd unit files to reflect the change.

You can change the device name by defining the name manually with an udev-rule. For example:

/etc/udev/rules.d/10-network.rules

SUBSYSTEM=="net", ACTION=="add", ATTR{address}=="aa:bb:cc:dd:ee:ff", NAME="net1"
SUBSYSTEM=="net", ACTION=="add", ATTR{address}=="ff:ee:dd:cc:bb:aa", NAME="net0"

These rules will be applied automatically at boot.

A couple of things to note:

• To get the MAC address of each card, use this command: cat /sys/class/net/device_name/address
• Make sure to use the lower-case hex values in your udev rules. It does not like upper-case.

If the network card has a dynamic MAC, you can use DEVPATH, for example:

/etc/udev/rules.d/10-network.rules

SUBSYSTEM=="net", DEVPATH=="/devices/platform/wemac.*", NAME="int"
SUBSYSTEM=="net", DEVPATH=="/devices/pci*/*1c.0/*/net/*", NAME="en"

The device path should match both the new and old device name, since the rule may be executed more than once on bootup. For example, in the second rule, "/devices/pci*/*1c.0/*/net/enp*" would be wrong since it will stop matching once the name is changed to en. Only the system-default rule will fire the second time around, causing the name to be changed back to e.g. enp1s0.

To test your rules, they can be triggered directly from userspace, e.g. with udevadm --debug test /sys/DEVPATH. Remember to first take down the interface you are trying to rename (e.g. ip link set enp1s0 down).

Note: When choosing the static names it should be avoided to use names in the format of "ethX" and "wlanX", because this may lead to race conditions between the kernel and udev during boot. Instead, it is better to use interface names that are not used by the kernel as default, e.g.: net0, net1, wifi0, wifi1. For further details please see the systemd documentation.

Revert to traditional interface names

If you would prefer to retain traditional interface names such as eth0, Predictable Network Interface Names can be disabled by masking the udev rule:

# ln -s /dev/null /etc/udev/rules.d/80-net-setup-link.rules

Alternatively, add net.ifnames=0 to the kernel parameters.

Set device MTU and queue length

You can change the device MTU and queue length by defining manually with an udev-rule. For example:

/etc/udev/rules.d/10-network.rules

ACTION=="add", SUBSYSTEM=="net", KERNEL=="wl*", ATTR{mtu}="1500", ATTR{tx_queue_len}="2000"

Note:

• mtu: For PPPoE, the MTU should be no larger than 1492. You can also set MTU via systemd.netdev(5).
• tx_queue_len: Small value for slower devices with a high latency like modem links and ISDN. High value is recommend for server connected over the high-speed Internet connections that perform large data transfers.

ifplugd for laptops

Tip: dhcpcd provides the same feature out of the box.

ifplugd 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.

Note: netctl package includes netctl-ifplugd@.service, otherwise you can use ifplugd@.service from ifplugd package. For example, enable ifplugd@eth0.service.

Bonding or LAG

See netctl#Bonding or Wireless bonding.

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. Typical uses are virtual hosting of Web and FTP servers, or reorganizing servers without having to update any other machines (this is especially useful for nameservers).

Example

To manually set an alias, for some NIC, use iproute2 to execute

# ip addr add 192.168.2.101/24 dev eth0 label eth0:1

To remove a given alias execute

# ip addr del 192.168.2.101/24 dev eth0:1

Packets destined for a subnet will use the primary alias by default. If the destination IP is within a subnet of a secondary alias, then the source IP is set respectively. Consider the case where there is more than one NIC, the default routes can be listed with ip route.

Promiscuous mode

Toggling promiscuous mode will make a (wireless) NIC forward all traffic it receives to the OS for further processing. This is opposite to "normal mode" where a NIC will drop frames it is not intended to receive. It is most often used for advanced network troubleshooting and packet sniffing.

/etc/systemd/system/promiscuous@.service

[Unit]
Description=Set %i interface in promiscuous mode
After=network.target

[Service]
Type=oneshot
ExecStart=/usr/bin/ip link set dev %i promisc on
RemainAfterExit=yes

[Install]
WantedBy=multi-user.target

If you want to enable promiscuous mode on interface eth0 run enable promiscuous@eth0.service.

Troubleshooting

Swapping computers on the cable modem

Some cable ISPs (Vidéotron 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 MAC address spoofing.

The TCP window scaling problem

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 us 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 are experiencing this problem: ping uses ICMP and is not affected by TCP problems.

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

Ways of fixing it

Bad

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

Good

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.d/99-disable_window_scaling.conf (see also sysctl):

net.ipv4.tcp_window_scaling = 0

Best

This problem is caused by broken routers/firewalls, so let us 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 problem

Users with Realtek 8168 8169 8101 8111(C) based NICs (cards / and on-board) may notice a problem 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 official 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 problem 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 problem will also affect other operating systems without newer drivers (eg. Live CDs). Here are a few fixes for this problem.

Enable the NIC directly in Linux

Follow #Enabling and disabling network interfaces to enable the interface.

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).

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 operating 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 and choose "Properties"
--> "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.

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).

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.

Note: This was tested several times on a GIGABYTE GA-G31M-ES2L motherboard, BIOS version F8 released on 2009/02/05.

No interface with Atheros chipsets

Users of some Atheros ethernet chips are reporting it does not work out-of-the-box (with installation media of February 2014). The working solution for this is to install backports-patched^AUR.

Broadcom BCM57780

This Broadcom chipset sometimes does not behave well unless you specify the order of the modules to be loaded. The modules are broadcom and tg3, the former needing to be loaded first.

These steps should help if your computer has this chipset:

• Find your NIC in lspci output:

$ lspci | grep Ethernet

02:00.0 Ethernet controller: Broadcom Corporation NetLink BCM57780 Gigabit Ethernet PCIe (rev 01)

• If your wired networking is not functioning in some way or another, unplug your cable then do the following:

# modprobe -r tg3
# modprobe broadcom
# modprobe tg3

• Plug your network cable back in and check whether the module succeeded with:

$ dmesg | greg tg3

• If this procedure solved the issue you can make it permanent by adding broadcom and tg3 (in this order) to the MODULES array:

/etc/mkinitcpio.conf

MODULES=(.. broadcom tg3 ..)

• Regenerate the initramfs
• Alternatively, you can create an /etc/modprobe.d/broadcom.conf:

softdep tg3 pre: broadcom

Note: These methods may work for other chipsets, such as BCM57760.

Realtek RTL8111/8168B

# lspci | grep Ethernet

03:00.0 Ethernet controller: Realtek Semiconductor Co., Ltd. RTL8111/8168B PCI Express Gigabit Ethernet controller (rev 02)

The adapter should be recognized by the r8169 module. However, with some chip revisions the connection may go off and on all the time. The alternative r8168 should be used for a reliable connection in this case. Blacklist r8169, if r8168 is not automatically loaded by udev, see Kernel modules#Automatic module handling.

Another fault in the drivers for some revisions of this adapter is poor IPv6 support. IPv6#Disable functionality can be helpful if you encounter issues such as hanging webpages and slow speeds.

Gigabyte Motherboard with Realtek 8111/8168/8411

With motherboards such as the Gigabyte GA-990FXA-UD3, booting with IOMMU off (which can be the default) will cause the network interface to be unreliable, often failing to connect or connecting but allowing no throughput. This will apply to the onboard NIC and to any other pci-NIC in the box because the IOMMU setting affects the entire network interface on the board. Enabling IOMMU and booting with the install media will throw AMD I-10/xhci page faults for a second, but then boots normally, resulting in a fully functional onboard NIC (even with the r8169 module).

When configuring the boot process for your installation, add iommu=soft as a kernel parameter to eliminate the error messages on boot and restore USB3.0 functionality.

See also

• Debian Reference: Network setup
• RHEL7: Networking Guide
• Linux Home Networking
• Monitoring and tuning the Linux Networking Stack: Receiving data
• Monitoring and tuning the Linux Networking Stack: Sending data
• Tracing a packet journey using tracepoints, perf and eBPF