Network configuration - ArchWiki

# Network configuration

Related articles

- Jumbo frames
- Firewalls
- Wireless network configuration
- Network bridge
- List of applications/Internet#Network managers

This page explains how to set up a wired connection to a network. If you need to set up wireless networking see the Wireless network configuration page.

## Check the connection

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

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 `8.8.8.8` but not `www.google.com`, check your DNS configuration. See resolv.conf for details.

## Set the hostname

A hostname is a unique name created to identify a machine on a network: it is configured in `/etc/hostname`. The file can contain the system's domain name, if any. To set the hostname, do:

    # hostnamectl set-hostname myhostname

This will put `myhostname` into `/etc/hostname`. See `man 5 hostname` and `man 1 hostnamectl` for details.

Note:

systemd provides hostname resolution via the `myhostname` nss module (enabled by default in `/etc/nsswitch.conf`). This means changing hostnames in `/etc/hosts` is usually not necessary. See #Local network hostname resolution if there are problems with hostname resolution, such as delays with network-based applications.

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

    # hostname myhostname

## Device Driver

### Check the status

udev should detect your network interface card (see Wikipedia:Network_interface_controller) 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 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's 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 modules#Loading.

## Network Interfaces

### Device names

For computers with multiple NICs, it is important to have fixed device names. Many configuration problems are caused by interface name changing.

udev is responsible for which device gets which name. Systemd v197 introduced Predictable Network Interface Names, which automatically assigns static names to network devices. Interfaces are now prefixed with `en` (ethernet), `wl` (WLAN), or `ww` (WWAN) followed by an automatically generated identifier, creating an entry such as `enp0s25`.

This behavior may be disabled by adding `net.ifnames=0` in your kernel command line.

Tip: You can run `ip link` or `ls /sys/class/net` to list all available interfaces.
Note: When changing the interface naming scheme, do not forget to update all network-related configuration files and custom systemd unit files to reflect the change. Specifically, if you have netctl static profiles enabled, run `netctl reenable profile` to update the generated service file.

#### Get current device names

Current NIC names can be found via `sysfs`:

    $ ls /sys/class/net

    lo eth0 eth1 firewire0

#### Change device name

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"

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 doesn't 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"

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.

### 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}="1480", ATTR{tx_queue_len}="2000"

### 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 link show dev eth0

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

## Configure the IP address

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

### Dynamic IP address

#### systemd-networkd

An easy way to setup DHCP for simple requirements is to use systemd-networkd service provided by systemd. See systemd-networkd#Basic DHCP network.

#### dhcpcd

dhcpcd is used as default client in Arch Linux to setup DHCP on the installation ISO. It is a more powerful tool and allows to configure more DHCP client options. See dhcpcd#Running on how to activate it for an interface.

### 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.

A static address can be configured with most networking tools standard in Arch Linux, for example see netctl, systemd-networkd, dhcpcd.

The following describes how to configure a static IP address manually. You need:

- Static IP address
- Subnet mask in CIDR notation, for example `/24` is the CIDR notation of `255.255.255.0` netmask.
- Broadcast address
- Gateway's IP address
- Name server (DNS) IP addresses. See also resolv.conf.

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.

Warning:

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

#### Manual assignment

Enable the network interface:

    # ip link set interface up

Assign a static IP address in the console:

    # ip addr add IP_address/subnet_mask broadcast broadcast_address dev interface

For example:

    # ip addr add 192.168.1.2/24 broadcast 192.168.1.255 dev interface

For more options, see `man ip`.

Add your gateway IP address like so:

    # ip route add default via default_gateway

For example:

    # ip route add default via 192.168.1.1

#### Persistent configuration on boot using systemd

First create a configuration file for the systemd service, replace `interface` with the proper network interface name:

    /etc/conf.d/net-conf-interface

    address=192.168.1.2
    netmask=24
    broadcast=192.168.1.255
    gateway=192.168.1.1

Create a network start script:

    /usr/local/bin/net-up.sh

    #!/bin/bash
    ip link set dev "$1" up
    ip addr add ${address}/${netmask} broadcast ${broadcast} dev "$1"

    [[ -z ${gateway} ]] || {
      ip route add default via ${gateway}
    }

Network stop script:

    /usr/local/bin/net-down.sh

    #!/bin/bash
    ip addr flush dev "$1"
    ip route flush dev "$1"
    ip link set dev "$1" down

Make both scripts executable:

    # chmod +x /usr/local/bin/net-{up,down}.sh

systemd service file:

    /etc/systemd/system/network@.service

    [Unit]
    Description=Network connectivity (%i)
    Wants=network.target
    Before=network.target
    BindsTo=sys-subsystem-net-devices-%i.device
    After=sys-subsystem-net-devices-%i.device

    [Service]
    Type=oneshot
    RemainAfterExit=yes
    EnvironmentFile=/etc/conf.d/net-conf-%i
    ExecStart=/usr/local/bin/net-up.sh %i
    ExecStop=/usr/local/bin/net-down.sh %i

    [Install]
    WantedBy=multi-user.target

Enable and start the unit `network@interface`, replacing `interface` with the name of your interface.

Tip: If you prefer so, you can skip the scripts and add the commands as additional `ExecStart=` and `ExecStop=` lines; see the wireless network example.

#### Calculating addresses

You can use `ipcalc` provided by the ipcalc package to calculate IP broadcast, network, netmask, and host ranges for more advanced configurations. An example is using Ethernet over Firewire to connect a Windows machine to Linux. To improve security and organization, both machines have their own network with the netmask and broadcast configured accordingly.

Finding out the respective netmask and broadcast addresses is done with `ipcalc`, by specifying the IP of the Linux NIC `10.66.66.1` and the number of hosts (here two):

    $ 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. Then 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

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

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`.

Note: netctl package includes `netctl-ifplugd@.service`, otherwise you can use `ifplugd@.service` from ifplugd package. Use for example `systemctl enable ifplugd@eth0.service`.

### Bonding or LAG

See netctl#Bonding.

### IP address aliasing

This article or section needs expansion.

Reason: Manual method using ip should be added; then move current example using netctl into netctl. (Discuss)

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

You will need netctl from the official repositories.

Prepare the configuration:

    /etc/netctl/mynetwork

    Connection='ethernet'
    Description='Six different addresses on the same NIC.'
    Interface='eth0'
    IP='static'
    Address=('192.168.1.10' '192.168.178.11' '192.168.1.12' '192.168.1.13' '192.168.1.14' '192.168.1.15')
    Gateway='192.168.1.1'
    DNS=('192.168.1.1')

Then simply execute:

    $ netctl start mynetwork

### Change MAC/hardware address

See MAC address spoofing.

### Internet sharing

See Internet sharing.

### Router configuration

See Router.

### 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 enable other machines to address the host by name, either a manual configuration of the respective `/etc/hosts` files or a service to propagate/resolve the name is required. With systemd the latter is done via the `myhostname` nss module. However, not all network services (on the same system; examples: [1], [2]) or other clients with different operating systems use the same methods to try resolve the hostname.

A first work-around that can be tried is to add the following line to `/etc/hosts`:

    127.0.1.1	myhostname.localdomain	myhostname

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`.

Another possibility 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. There are two options available:

- Samba provides hostname resolution via Microsoft's NetBIOS. It only requires installation of samba and enabling of the `nmbd.service` service. Computers running Windows, OS X, 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 OS X, 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.

### 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:

    # systemctl enable promiscuous@eth0.service

## Troubleshooting

### Swapping computers on the cable modem

Some 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 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'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 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'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 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 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 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.

#### Method 1: enable the NIC directly in Linux

Get the ethernet NIC name from the output of:

    $ ip a

Bring up the device as root using the NIC name:

    # ip link set dev NIC_name up

For ex, if NIC_name is enp7s0:

    # ip link set dev enp7s0 up

If it worked and the card is powered on, you should see `state UP` for the given interface in the output of `ip link`.

#### Method 2: 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 3: 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.

#### Method 4: 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 5: 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 the package backports-patched from 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, try unplugging your cable then doing the following:

    # modprobe -r tg3
    # modprobe broadcom
    # modprobe tg3

- Plug you network cable in. If this solves your problems you can make this permanent by adding `broadcom` and `tg3` (in this order) to the `MODULES` array in `/etc/mkinitcpio.conf`:

    MODULES=".. broadcom tg3 .."

- Rebuild the initramfs:

    # mkinitcpio -p linux

- 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 can be found in the official repositories and should be used for a reliable connection in this case. Blacklist `r8169`, if r8168 is not automatically loaded by udev add it to your list of user specified modules.

Category:

- Networking