Network configuration – ArchWiki

This article describes how to configure network connections on OSI layer 3 and above. Medium-specifics are handled in the /Ethernet and /Wireless subpages.

Check the connection

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

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 received, the ping utility will print a line like the above until you interrupt (Ctrl+c) it interactively. For more information see the manual. Note that computers can be configured not to respond to ICMP echo requests. [1]

If you receive an error message (see ping error indications) or no reply, this may be related to incomplete configuration, but also your default gateway or your Internet Service Provider (ISP). You can run a traceroute to further diagnose the route to the host.

Network management

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

Note: The installation image uses

The installation image uses systemd-resolved and systemd-networkd , which is configured as a DHCP client for Ethernet WLAN and WWAN network interfaces.

iproute2

iproute2 is a dependency of the meta package and provides the command-line interface, used to manage network interfaces, IP addresses and the routing table. Be aware that configuration made using ip will be lost after a reboot. For persistent configuration, you can use a network manager or 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 interface controllers using Predictable Network Interface Names, which prefixes interfaces names with en (wired/Ethernet), wl (wireless/WLAN), or ww (WWAN). See .

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 virtual loopback interface and not used in making network connections.

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

If your network interface is not listed, make sure your device driver was loaded successfully. See /Ethernet#Device driver or /Wireless#Device driver.

Enabling and disabling network interfaces

Network interfaces can be enabled or disabled using ip link set interface up|down, see .

To check the status of the interface enp2s0:

$ ip link show dev enp2s0

2: enp2s0: <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 enp2s0, taking it down will also remove the route, and bringing it back up will not automatically re-establish the default route. See

If your default route is through interface, taking it down will also remove the route, and bringing it back up will not automatically re-establish the default route. See #Routing table for re-establishing it.

Static or dynamic IP address?

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If you are using a Wi-FI or a router, for example, at home, you will most likely be using a dynamic IP address. The IP address is assigned by the Wi-Fi or router and it is what your computer should be configured to use. Or, if you are at home and your computer is connected to your ISP’s modem, for example, a cable modem, that will also be using a dynamic IP address. Dynamic IP addresses can change each time you turn your computer on. In a work environment you may have a static IP address or a dynamic IP address. At home you can configure your router to always assign your computer the same IP address in which case you are using a static IP address. When you are using a dynamic IP address you will need to use DHCP so that it can set up your network interface with the correct IP address. In addition to configuring your IP address, DHCP can also configure your routing (how to get from where you are to wherever on the network you are going) as well as your name servers, which convert the host name, for example, google.com, into its IP address, that number with dots in 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 .

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.

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

IP addresses can be calculated with 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 .

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

List IPv4 routes:

$ ip route show

List IPv6 routes:

$ ip -6 route

Add a route:

# ip route add PREFIX via address dev interface

Delete a route:

# ip route del PREFIX via address dev interface

DHCP

A Dynamic Host Configuration Protocol (DHCP) server provides clients with a dynamic IP address, the subnet mask, the default gateway IP address and optionally also with DNS name servers.

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

Client
Package
Archiso
Note
Systemd units

dhcpcd

Yes
DHCP, DHCPv6, ZeroConf, static IP
dhcpcd.service, dhcpcd@interface.service
ISC dhclient

Yes
DHCP, DHCPv6, BOOTP, static IP
dhclient@interface.service
Note:

• ISC has ended development on the ISC DHCP client as of early 2022. This client implementation is no longer maintained and should not be used in production any longer.
• You should not run two DHCP clients simultaneously.
• Instead of directly using a standalone DHCP client you can also use a network manager, some of which have a built-in DHCP client.
• Alternatively, iwd has a built-in DHCP client that can be used with some configuration: iwd#Enable built-in network configuration.

Tip:

• You can check if a DHCP server is running with .
• While waiting for an IP to be assigned you can run something like watch -n 1 ping -c 1 archlinux.org to confirm that the network is connected.

Servers

Server
Package
IPv4
IPv6
GUI
Interfaces
Storage backend(s)
Note

dhcpd

Yes
Yes
Glass-ISC-DHCP
?
File
Superseded by Kea.

dnsmasq

Yes
Yes
No
?
File
Also DNS, PXE and TFTP.

Kea

Yes
Yes
Stork
REST, RADIUS, NETCONF
File, MySQL, PostgreSQL, Cassandra
Also DNS. Supersedes dhcpd.

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
GUI
Archiso [3]
CLI tools
PPP support
(e.g. 3G modem)
DHCP client
systemd units

ConnMan

8 unofficial
No

Yes (with AUR)
internal
connman.service
netctl

2 unofficial
No
, wifi-menu
Yes
dhcpcd or
netctl-ifplugd@interface.service, netctl-auto@interface.service
NetworkManager

Yes
No
,
Yes
internal or
NetworkManager.service
systemd-networkd

No
Yes ( )

No
internal
systemd-networkd.service, systemd-resolved.service

Set the hostname

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A hostname is a unique name created to identify a machine on a network, configured in /etc/hostname—see and 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

For advice on choosing a hostname, see RFC 1178

Alternatively, using :

# hostnamectl set-hostname myhostname

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

# hostname myhostname

To set the “pretty” hostname and other machine metadata, see .

Local network hostname resolution

To make your machine accessible in your LAN via its hostname you can:

• edit the /etc/hosts file for every device in your LAN, see
• set up a DNS server to resolve your hostname and make the LAN devices use it (e.g. via #DHCP)
• or the easy way: use a Zero-configuration networking service:
  • Hostname resolution via Microsoft’s NetBIOS. Provided by Samba on Linux. It only requires the nmb.service. Computers running Windows, macOS, or Linux with nmb running, will be able to find your machine.
  • Hostname resolution via mDNS. Provided by either nss_mdns with Avahi (see Avahi#Hostname resolution for setup details) or systemd-resolved. Computers running macOS, or Linux with Avahi or systemd-resolved running, will be able to find your machine. The older Win32 API does not support mDNS, which may prevent some older Windows applications from accessing your device.

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"

To get the DEVPATH of all currently-connected devices, see where the symlinks in /sys/class/net/ lead. For example:

file /sys/class/net/*

/sys/class/net/enp0s20f0u4u1: symbolic link to ../../devices/pci0000:00/0000:00:14.0/usb2/2-4/2-4.1/2-4.1:1.0/net/enp0s20f0u4u1
/sys/class/net/enp0s31f6:     symbolic link to ../../devices/pci0000:00/0000:00:1f.6/net/enp0s31f6
/sys/class/net/lo:            symbolic link to ../../devices/virtual/net/lo
/sys/class/net/wlp4s0:        symbolic link to ../../devices/pci0000:00/0000:00:1c.6/0000:04:00.0/net/wlp4s0

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.

If you are using a USB network device (e.g. Android phone tethering) that has a dynamic MAC address and you want to be able to use different USB ports, you could use a rule that matched depending on vendor and product ID instead:

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

SUBSYSTEM=="net", ACTION=="add", ATTRS{idVendor}=="12ab", ATTRS{idProduct}=="3cd4", NAME="net2"

To test your rules, they can be triggered directly from userspace, e.g. with udevadm --debug test /sys/class/net/*. 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

When choosing the static names, 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.:. 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"

mtu: Using a value larger than 1500 (so called jumbo frames) can significantly speed up your network transfers. Note that all network interfaces, including switches in the local network, must support the same MTU in order to use jumbo frames. For PPPoE, the MTU should not be larger than 1492. You can also set MTU via .

tx_queue_len: Small value for slower devices with a high latency like modem links and ISDN. High value is recommended for server connected over the high-speed internet connections that perform large data transfers.

Note: The example rule above is for wireless interfaces. In case of wired ones, using KERNEL=="eth*" is required instead of KERNEL=="wl*".

Bonding or LAG

See netctl or systemd-networkd, 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 to execute

# ip addr add 192.168.2.101/24 dev enp2s0 label enp2s0:1

To remove a given alias execute

# ip addr del 192.168.2.101/24 dev enp2s0:1

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/[email protected]

[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 enp2s0, enable [email protected].

Investigate sockets

ss is a utility to investigate network ports and is part of the package. It has a similar functionality to the deprecated netstat utility.

Common usage includes:

Display all TCP Sockets with service names:

$ ss -at

Display all TCP Sockets with port numbers:

$ ss -atn

Display all UDP Sockets:

$ ss -au

For more information see .

Troubleshooting

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 64KiB. 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 64KiB.

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 output from dmesg is okay, 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 an archived Kernel Trap article: Window Scaling on the Internet.

There are also several relevant threads on the LKML.

Connected second PC unable to use bridged LAN

First PC have two LAN. Second PC have one LAN and connected to first PC. Lets go second PC to give all access to LAN after bridged interface:

This article or section needs expansion.

Reason: Explain what the settings actually do. (Discuss in

Explain what the settings actually do. (Discuss in Talk:Network configuration

# sysctl net.bridge.bridge-nf-filter-pppoe-tagged=0
# sysctl net.bridge.bridge-nf-filter-vlan-tagged=0
# sysctl net.bridge.bridge-nf-call-ip6tables=0
# sysctl net.bridge.bridge-nf-call-iptables=0
# sysctl net.bridge.bridge-nf-call-arptables=0

localhost is resolved over the network

(an NSS module provided by systemd and enabled by default in /etc/nsswitch.conf) provides localhost and the local hostname resolution to an IP address. Some software may, however, still instead read /etc/hosts directly; see [4] [5] for examples.

To prevent such software from unsafely resolving localhost over the network, add entries for localhost to the file:

/etc/hosts

127.0.0.1        localhost
::1              localhost

Note: Report any software affected by this issue in localhost entries added to the default /etc/hosts.

Report any software affected by this issue in FS#56684 . This may help in gettingentries added to the default

To allow resolving the local hostname, additionally add it to the file:

/etc/hosts

127.0.0.1        localhost
::1              localhost
127.0.1.1        myhostname

For a system with a permanent IP address, replace 127.0.1.1 with that permanent IP address. For a system with a fully qualified domain name, insert the fully qualified domain name before the hostname (see the following link for the reasoning). For example:

/etc/hosts

127.0.0.1        localhost
::1              localhost
203.0.113.45     host1.fqdomain.example host1

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. All following strings on the same line are considered aliases. See for more info.

As a result the system resolves to both entries:

$ getent hosts

127.0.0.1       localhost
127.0.0.1       localhost
127.0.1.1       myhostname

See also