Difference between revisions of "Network configuration"

Revision as of 22:11, 9 March 2014

zh-CN:Network Configuration zh-TW:Network Configuration

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

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

$ ping -c 3 www.google.com

PING www.l.google.com (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

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.

To set the hostname temporarily (until a reboot), use hostname 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. 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 name. 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 a symlink:

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

Users upgrading from an earlier systemd version will have a blank rules file created automatically. So if you want to use persistent device names, just delete the file.

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 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"

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"

Get current device names

Current NIC names can be found via sysfs

$ ls /sys/class/net

lo eth0 eth1 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 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

dhcpcd

The easiest is to use dhcpcd, which is included in the base group. Either use the provided service file dhcpcd@.service, passing the interface name as an argument, or start it manually by running dhcpcd interface.

systemd-networkd

With systemd version >= 209, it's possible to manage network connections with systemd-networkd. With this approach, only a single configuration file per interface is needed, for example:

/etc/systemd/network/20-dhcp.network

[Match]
Name=*

[Network]
DHCP=yes

Name = * matches all available network interfaces, and e.g. Name = en* would only match ethernet interfaces. See Network configuration#Device names for the naming scheme and ip link for a list of available interfaces.

systemd doesn't update /etc/resolv.conf - instead it updates /run/systemd/network/resolv.conf. A quick fix is to symlink the two, ie:

# ln -sf /run/systemd/network/resolv.conf /etc/resolv.conf

Then start/enable systemd-networkd.service.

Further examples: https://coreos.com/blog/intro-to-systemd-networkd/

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.

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 broadcast broadcast_address dev interface

For example:

# ip addr add 192.168.1.2/24 broadcast 192.168.1.255 dev eth0

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

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

Persistent configuration on boot using systemd and udev rules

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

/etc/conf.d/network@interface

address=192.168.0.15
netmask=24
broadcast=192.168.0.255
gateway=192.168.0.1

Create a systemd unit 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/network@%i

ExecStart=/usr/bin/ip link set dev %i up
ExecStart=/usr/bin/ip addr add ${address}/${netmask} broadcast ${broadcast} dev %i
ExecStart=/usr/bin/sh -c 'test -n ${gateway} && /usr/bin/ip route add default via ${gateway}'

ExecStop=/usr/bin/ip addr flush dev %i
ExecStop=/usr/bin/ip link set dev %i down

[Install]
WantedBy=multi-user.target

Enable the unit and start it, passing the name of the interface:

# systemctl enable network@interface.service
# systemctl start network@interface.service

Persistent configuration on boot using systemd-networkd

With systemd version >= 209, it's possible to manage network connections with the integrated systemd-networkd service (which is disabled by default with version 210). With this approach, only a single configuration file per interface is needed, for example:

/etc/systemd/network/10-static-ethernet.network

[Match]
Name=enp0s25

[Network]
Address=192.168.0.15/24
Gateway=192.168.0.1

Name has to be the name of the interface you want to configure, see ip link for the names of available interfaces.

Then start/enable systemd-networkd.service.

Further examples: https://coreos.com/blog/intro-to-systemd-networkd/

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. 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

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.

Bonding or LAG

See netctl#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

You will need netctl from the official repositories.

Prepare the configuration:

/etc/netctl/mynetwork

Connection='ethernet'
Description='Five 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

$ ping hostname

PING hostname (192.168.1.2) 56(84) bytes of data.
64 bytes from hostname (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.

When setting up a DNS server such as BIND or Unbound is overkill, manually editing your /etc/hosts is too cumbersome, or when you want more flexibility with dynamic leaving and joining of hosts to the network, it is possible to handle hostname resolution on your local network using zero-configuration networking. 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.

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).

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

net.ipv4.tcp_window_scaling = 0

How to fix it (The best way)

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 operative 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, a new interface should appear in the output of ifconfig

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 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

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.

No eth0 with Atheros AR9485

The ethernet (eth0) for Atheros AR9485 are not working out-of-the-box (with installation media of February 2014). The working solution for this is to install the package backports-patched^AUR 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:

$ 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 (as root):

# modprobe -r tg3
# modprobe broadcom
# modprobe tg3

Now 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 .."

Then rebuild the initramfs:

# mkinitcpio -p linux