Related articles * [Network Debugging](/index.php/Network_Debugging "Network Debugging") * [Firewalls](/index.php/Firewalls "Firewalls") * [Jumbo frames](/index.php/Jumbo_frames "Jumbo frames") * [Internet sharing](/index.php/Internet_sharing "Internet sharing") * [Router](/index.php/Router "Router") This article explains how to configure a network connection. ## Contents * [1 Check the connection](#Check_the_connection) * [1.1 Ping](#Ping) * [2 Device driver](#Device_driver) * [2.1 Check the status](#Check_the_status) * [2.2 Load the module](#Load_the_module) * [3 Network management](#Network_management) * [3.1 Network interfaces](#Network_interfaces) * [3.1.1 Listing network interfaces](#Listing_network_interfaces) * [3.1.2 Enabling and disabling network interfaces](#Enabling_and_disabling_network_interfaces) * [3.2 Static IP address](#Static_IP_address) * [3.3 IP addresses](#IP_addresses) * [3.4 Routing table](#Routing_table) * [3.5 DHCP](#DHCP) * [3.6 Network managers](#Network_managers) * [4 Set the hostname](#Set_the_hostname) * [4.1 Local hostname resolution](#Local_hostname_resolution) * [4.2 Local network hostname resolution](#Local_network_hostname_resolution) * [5 Tips and tricks](#Tips_and_tricks) * [5.1 Change interface name](#Change_interface_name) * [5.2 Revert to traditional interface names](#Revert_to_traditional_interface_names) * [5.3 Set device MTU and queue length](#Set_device_MTU_and_queue_length) * [5.4 ifplugd for laptops](#ifplugd_for_laptops) * [5.5 Bonding or LAG](#Bonding_or_LAG) * [5.6 IP address aliasing](#IP_address_aliasing) * [5.6.1 Example](#Example) * [5.7 Promiscuous mode](#Promiscuous_mode) * [6 Troubleshooting](#Troubleshooting) * [6.1 Swapping computers on the cable modem](#Swapping_computers_on_the_cable_modem) * [6.2 The TCP window scaling problem](#The_TCP_window_scaling_problem) * [6.2.1 How to diagnose the problem](#How_to_diagnose_the_problem) * [6.2.2 Ways of fixing it](#Ways_of_fixing_it) * [6.2.2.1 Bad](#Bad) * [6.2.2.2 Good](#Good) * [6.2.2.3 Best](#Best) * [6.2.3 More about it](#More_about_it) * [6.3 Realtek no link / WOL problem](#Realtek_no_link_.2F_WOL_problem) * [6.3.1 Enable the NIC directly in Linux](#Enable_the_NIC_directly_in_Linux) * [6.3.2 Rollback/change Windows driver](#Rollback.2Fchange_Windows_driver) * [6.3.3 Enable WOL in Windows driver](#Enable_WOL_in_Windows_driver) * [6.3.4 Newer Realtek Linux driver](#Newer_Realtek_Linux_driver) * [6.3.5 Enable LAN Boot ROM in BIOS/CMOS](#Enable_LAN_Boot_ROM_in_BIOS.2FCMOS) * [6.4 No interface with Atheros chipsets](#No_interface_with_Atheros_chipsets) * [6.5 Broadcom BCM57780](#Broadcom_BCM57780) * [6.6 Realtek RTL8111/8168B](#Realtek_RTL8111.2F8168B) * [6.7 Gigabyte Motherboard with Realtek 8111/8168/8411](#Gigabyte_Motherboard_with_Realtek_8111.2F8168.2F8411) * [7 See also](#See_also) ## Check the connection To troubleshoot a network connection, go through the following conditions and ensure that you meet them: 1. Your [network interface](#Network_interfaces) is listed and enabled. 2. You are connected to the network. The cable is plugged in or you are [connected to the wireless LAN](/index.php/Wireless_network_configuration "Wireless network configuration"). 3. Your network interface has an [IP address](#IP_addresses) 4. Your [routing table](#Routing_table) is correctly set up. 5. You can [ping](#Ping) a local IP address (e.g. your default gateway). 6. You can [ping](#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. 7. [Check if you can resolve domain names](/index.php/Check_if_you_can_resolve_domain_names "Check if you can resolve domain names") (e.g. `archlinux.org`). **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](https://en.wikipedia.org/wiki/Ping_(networking_utility) 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)](https://jlk.fjfi.cvut.cz/arch/manpages/man/ping.8) manual. Note that computers can be configured not to respond to ICMP echo requests.[[1]](https://unix.stackexchange.com/questions/412446/how-to-disable-ping-response-icmp-echo-in-linux-all-the-time) When you receive no reply, you can use a [traceroute](https://en.wikipedia.org/wiki/Traceroute "wikipedia:Traceroute") ([traceroute(8)](https://jlk.fjfi.cvut.cz/arch/manpages/man/traceroute.8) or [tracepath(8)](https://jlk.fjfi.cvut.cz/arch/manpages/man/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](https://www.archlinux.org/packages/?name=iputils) package. ## Device driver ### Check the status [udev](/index.php/Udev "Udev") should detect your [network interface controller](https://en.wikipedia.org/wiki/Network_interface_controller "wikipedia:Network interface controller") (NIC) and automatically load the necessary [kernel module](/index.php/Kernel_module "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](/index.php/Kernel_modules#Manual_module_handling "Kernel modules"). 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](/index.php/AUR "AUR") for the module name. If udev is not detecting and loading the proper module automatically during bootup, see [Kernel module#Automatic module handling](/index.php/Kernel_module#Automatic_module_handling "Kernel module"). ## Network management To set up a network connection, go through the following steps: 1. Ensure your [network interface](#Network_interfaces) is listed and enabled. 2. Connect to the network. Plug in the Ethernet cable or [connect to the wireless LAN](/index.php/Wireless_network_configuration "Wireless network configuration"). 3. Configure your network connection: * [static IP address](#Static_IP_address) * dynamic IP address: use [DHCP](#DHCP) **Tip:** [#Network managers](#Network_managers) provide automatic network connection and configuration based on network profiles. The [iproute2](https://www.archlinux.org/packages/?name=iproute2) package provides the [ip(8)](https://jlk.fjfi.cvut.cz/arch/manpages/man/ip.8) command-line utility, used to manage [network interfaces](#Network_interfaces), [IP addresses](#IP_addresses) and the [routing table](#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](/index.php/Systemd#Writing_unit_files "Systemd"). Also note that `ip` commands can generally be abbreviated, for clarity they are however spelled out in this article. ### Network interfaces By default [udev](/index.php/Udev "Udev") assigns names to your network interfaces using [Predictable Network Interface Names](http://www.freedesktop.org/wiki/Software/systemd/PredictableNetworkInterfaceNames), which prefixes interfaces names with `en` (wired/[Ethernet](https://en.wikipedia.org/wiki/Ethernet "wikipedia:Ethernet")), `wl` (wireless/WLAN), or `ww` ([WWAN](https://en.wikipedia.org/wiki/Wireless_WAN "wikipedia:Wireless WAN")). **Tip:** To change interface names, see [#Change interface name](#Change_interface_name) and [#Revert to traditional interface names](#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](https://en.wikipedia.org/wiki/loop_device "wikipedia: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](/index.php/Wireless_network_configuration#Get_the_name_of_the_interface "Wireless network configuration"). If your network interface is not listed, make sure your [device driver](#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)](https://jlk.fjfi.cvut.cz/arch/manpages/man/ip-link.8). To check the status of the interface `eth0`: `$ ip link show dev eth0` ``` 2: eth0: mtu 1500 qdisc pfifo_fast master br0 state DOWN mode DEFAULT qlen 1000 ... ``` The `UP` in `` 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](#Routing_table) for reestablishing it. ### Static IP address A static IP address can be configured with most standard [network managers](#Network_managers) and also [dhcpcd](/index.php/Dhcpcd "Dhcpcd"). To manually configure a static IP address, add an IP address as described in [#IP addresses](#IP_addresses), set up your [routing table](#Routing_table) and [configure your DNS servers](/index.php/Domain_name_resolution "Domain name resolution"). ### IP addresses [IP addresses](https://en.wikipedia.org/wiki/IP_address "wikipedia:IP address") are managed using [ip-address(8)](https://jlk.fjfi.cvut.cz/arch/manpages/man/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](https://en.wikipedia.org/wiki/Classless_Inter-Domain_Routing#CIDR_notation "wikipedia:Classless Inter-Domain Routing") to also supply a [subnet mask](https://en.wikipedia.org/wiki/Subnetwork "wikipedia:Subnetwork") * `+` is a special symbol that makes `ip` derive the [broadcast address](https://en.wikipedia.org/wiki/Broadcast_address "wikipedia: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](http://www.raspberrypi.org/forums/viewtopic.php?f=28&t=16797). 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](http://jodies.de/ipcalc) ([ipcalc](https://www.archlinux.org/packages/?name=ipcalc)). ### Routing table The [routing table](https://en.wikipedia.org/wiki/Routing_table "wikipedia: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](https://en.wikipedia.org/wiki/Default_gateway "wikipedia:Default gateway") is used. The routing table is managed using [ip-route(8)](https://jlk.fjfi.cvut.cz/arch/manpages/man/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 [Dynamic Host Configuration Protocol](https://en.wikipedia.org/wiki/Dynamic_Host_Configuration_Protocol "wikipedia: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. **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](/index.php/Archiso "Archiso") | Note | Systemd units | | [dhcpcd](/index.php/Dhcpcd "Dhcpcd") | [dhcpcd](https://www.archlinux.org/packages/?name=dhcpcd) | Yes | DHCP, DHCPv6, ZeroConf, static IP | `dhcpcd.service`, `dhcpcd@*interface*.service` | | [ISC dhclient](https://www.isc.org/downloads/dhcp/) | [dhclient](https://www.archlinux.org/packages/?name=dhclient) | Yes | DHCP, BOOTP, static IP | `dhclient@*interface*.service` | Note that instead of directly using a DHCP client you can also use a [network manager](#Network_managers). **Tip:** You can check if a DHCP server is running with [dhcping](https://www.archlinux.org/packages/?name=dhcping). ### 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](/index.php/Archiso "Archiso") [[2]](https://git.archlinux.org/archiso.git/tree/configs/releng/packages.x86_64) | CLI tools | [PPP](https://en.wikipedia.org/wiki/Point-to-Point_Protocol "wikipedia:Point-to-Point Protocol") support (e.g. 3G modem) | [#DHCP](#DHCP) client | Systemd units | | [ConnMan](/index.php/ConnMan "ConnMan") | 8 unofficial | No | [connmanctl(1)](https://jlk.fjfi.cvut.cz/arch/manpages/man/connmanctl.1) | Yes | internal | `connman.service` | | [netctl](/index.php/Netctl "Netctl") | 2 unofficial | Yes ([base](https://www.archlinux.org/groups/x86_64/base/)) | [netctl(1)](https://jlk.fjfi.cvut.cz/arch/manpages/man/netctl.1), wifi-menu | Yes | [dhcpcd](/index.php/Dhcpcd "Dhcpcd") or [dhclient](https://www.archlinux.org/packages/?name=dhclient) | `netctl-ifplugd@*interface*.service`, `netctl-auto@*interface*.service` | | [NetworkManager](/index.php/NetworkManager "NetworkManager") | Yes | No | [nmcli(1)](https://jlk.fjfi.cvut.cz/arch/manpages/man/nmcli.1), [nmtui(1)](https://jlk.fjfi.cvut.cz/arch/manpages/man/nmtui.1) | Yes | internal, [dhcpcd](/index.php/Dhcpcd "Dhcpcd") or [dhclient](https://www.archlinux.org/packages/?name=dhclient) | `NetworkManager.service` | | [systemd-networkd](/index.php/Systemd-networkd "Systemd-networkd") | No | Yes ([base](https://www.archlinux.org/groups/x86_64/base/)) | No |  ? | internal | `systemd-networkd.service`, `systemd-resolved.service` | | [Wicd](/index.php/Wicd "Wicd") | Yes | No | [wicd-cli(8)](https://jlk.fjfi.cvut.cz/arch/manpages/man/wicd-cli.8), [wicd-curses(8)](https://jlk.fjfi.cvut.cz/arch/manpages/man/wicd-curses.8) | No | [dhcpcd](/index.php/Dhcpcd "Dhcpcd") | `wicd.service` | There also is [Wifi Radar](/index.php/Wifi_Radar "Wifi Radar"), a GUI application to manage WiFi networks with [wireless_tools](https://www.archlinux.org/packages/?name=wireless_tools), it however does not handle wired connections. See also [List of applications#Network managers](/index.php/List_of_applications#Network_managers "List of applications"). ## Set the hostname A [hostname](https://en.wikipedia.org/wiki/Hostname "wikipedia:Hostname") is a unique name created to identify a machine on a network, configured in `/etc/hostname`—see [hostname(5)](https://jlk.fjfi.cvut.cz/arch/manpages/man/hostname.5) and [hostname(7)](https://jlk.fjfi.cvut.cz/arch/manpages/man/hostname.7) for details. The file can contain the system's domain name, if any. To set the hostname, [edit](/index.php/Textedit "Textedit") `/etc/hostname` to include a single line with `*myhostname*`: `/etc/hostname` ``` *myhostname* ``` **Tip:** For advice on choosing a hostname, see [RFC 1178](https://tools.ietf.org/html/rfc1178). Alternatively, using [hostnamectl(1)](https://jlk.fjfi.cvut.cz/arch/manpages/man/hostnamectl.1): ``` # hostnamectl set-hostname *myhostname* ``` To temporarily set the hostname (until reboot), use [hostname(1)](https://jlk.fjfi.cvut.cz/arch/manpages/man/hostname.1) from [inetutils](https://www.archlinux.org/packages/?name=inetutils): ``` # hostname *myhostname* ``` To set the "pretty" hostname and other machine metadata, see [machine-info(5)](https://jlk.fjfi.cvut.cz/arch/manpages/man/machine-info.5#https%3A.2F.2Fwww.freedesktop.org.2Fsoftware.2Fsystemd.2Fman.2Fmachine-info.html). ### Local hostname resolution The `myhostname` [Name Service Switch](/index.php/Name_Service_Switch "Name Service Switch") (NSS) module of [systemd](/index.php/Systemd "Systemd") provides local hostname resolution without having to edit `/etc/hosts` ([hosts(5)](https://jlk.fjfi.cvut.cz/arch/manpages/man/hosts.5)). It is enabled by default. Some clients may however still rely on `/etc/hosts`, see [[3]](https://lists.debian.org/debian-devel/2013/07/msg00809.html) [[4]](https://bugzilla.mozilla.org/show_bug.cgi?id=87717#c55) 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)](https://jlk.fjfi.cvut.cz/arch/manpages/man/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`. ### 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 [hosts(5)](https://jlk.fjfi.cvut.cz/arch/manpages/man/hosts.5) * set up a [DNS server](/index.php/DNS_server "DNS server") to resolve your hostname and make the LAN devices use it (e.g. via [#DHCP](#DHCP)) * or the easy way: use a [Zero-configuration networking](https://en.wikipedia.org/wiki/Zero-configuration_networking "wikipedia:Zero-configuration networking") service: * [Samba](/index.php/Samba "Samba") provides hostname resolution via Microsoft's **NetBIOS**. It only requires installation of [samba](https://www.archlinux.org/packages/?name=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](/index.php/Avahi "Avahi") provides hostname resolution via **zeroconf**, also known as Avahi or Bonjour. It requires slightly more complex configuration than Samba: see [Avahi#Hostname resolution](/index.php/Avahi#Hostname_resolution "Avahi") for details. Computers running macOS, or Linux with an Avahi daemon running, will be able to find your machine. Windows does not have a 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](/index.php/Udev#Testing_rules_before_loading "Udev") 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 "eth*X*" and "wlan*X*"**, 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](http://www.freedesktop.org/wiki/Software/systemd/PredictableNetworkInterfaceNames) documentation. ### Revert to traditional interface names If you would prefer to retain traditional interface names such as eth0, [Predictable Network Interface Names](http://www.freedesktop.org/wiki/Software/systemd/PredictableNetworkInterfaceNames) 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](/index.php/Kernel_parameters "Kernel parameters"). ### Set device MTU and queue length You can change the device [MTU](https://en.wikipedia.org/wiki/Maximum_transmission_unit "wikipedia:Maximum transmission unit") 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)](https://jlk.fjfi.cvut.cz/arch/manpages/man/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](/index.php/Dhcpcd "Dhcpcd") provides the same feature out of the box. [ifplugd](https://www.archlinux.org/packages/?name=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](/index.php/Netctl "Netctl") package includes `netctl-ifplugd@.service`, otherwise you can use `ifplugd@.service` from [ifplugd](https://www.archlinux.org/packages/?name=ifplugd) package. For example, [enable](/index.php/Enable "Enable") `ifplugd@eth0.service`. ### Bonding or LAG See [netctl#Bonding](/index.php/Netctl#Bonding "Netctl") or [Wireless bonding](/index.php/Wireless_bonding "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](https://www.archlinux.org/packages/?name=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](https://en.wikipedia.org/wiki/Promiscuous_mode "wikipedia: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](https://en.wikipedia.org/wiki/Packet_sniffing "wikipedia: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](/index.php/Enable "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](/index.php/MAC_address_spoofing "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](http://www.faqs.org/rfcs/rfc1323.html) 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](/index.php/Wireshark "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](/index.php/Sysctl "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](http://lwn.net/Articles/92727/) and a Kernel Trap article: [Window Scaling on the Internet](http://kerneltrap.org/node/6723). 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](#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](https://aur.archlinux.org/packages/backports-patched/). ### 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](/index.php/Regenerate_the_initramfs "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](https://www.archlinux.org/packages/?name=r8168) should be used for a reliable connection in this case. [Blacklist](/index.php/Blacklist "Blacklist") `r8169`, if [r8168](https://www.archlinux.org/packages/?name=r8168) is not automatically loaded by [udev](/index.php/Udev "Udev"), see [Kernel modules#Automatic module handling](/index.php/Kernel_modules#Automatic_module_handling "Kernel modules"). Another fault in the drivers for some revisions of this adapter is poor IPv6 support. [IPv6#Disable functionality](/index.php/IPv6#Disable_functionality "IPv6") 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](/index.php/PCI_passthrough_via_OVMF#Setting_up_IOMMU "PCI passthrough via OVMF") 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](/index.php/Kernel_parameter "Kernel parameter") to eliminate the error messages on boot and restore USB3.0 functionality. ## See also * [Linux Network Administrators Guide](https://www.tldp.org/LDP/nag2/index.html) * [Debian Reference: Network setup](https://www.debian.org/doc/manuals/debian-reference/ch05.en.html) * [RHEL7: Networking Guide](https://access.redhat.com/documentation/en-US/Red_Hat_Enterprise_Linux/7/html/Networking_Guide/) * [Linux Home Networking](http://www.linuxhomenetworking.com/wiki/) * [Monitoring and tuning the Linux Networking Stack: Receiving data](https://blog.packagecloud.io/eng/2016/06/22/monitoring-tuning-linux-networking-stack-receiving-data/) * [Monitoring and tuning the Linux Networking Stack: Sending data](https://blog.packagecloud.io/eng/2017/02/06/monitoring-tuning-linux-networking-stack-sending-data/) * [Tracing a packet journey using tracepoints, perf and eBPF](http://blog.yadutaf.fr/2017/07/28/tracing-a-packet-journey-using-linux-tracepoints-perf-ebpf/)