DHCP Client¶

Debian¶

Installation¶

There are different DHCP client packages around. The default for Debian seems to be dhcp-client. It is probably installed already, as you can check within aptitude. But to be sure you can run as root

aptitude install dhcp-client

Note that the traditional DHCP client, isc-dhcp-client, is now EOL and the dhcp-client virtual package doesn’t provide many alternatives. Here is a set of DHCP clients that might work as an alternative:

• coredhcp-client

• dhcpcanon

• dhcpcd

• NetworkManager

• odhcp6c

• systemd-networkd

• udhcpc

Note that ifupdown can use udhcpc if available.

Users of systemd-networkd or NetworkManager do not need to worry about that deprecation since they include their own DHCP clients.

Configuration¶

Basic configuration¶

For a basic configuration you have to edit the file /etc/network/interfaces, which defines the interfaces of your computer. If you want to use eth0 as the interface to be configured through DHCP during boot up, add/edit the eth0-entry

auto eth0
iface eth0 inet dhcp

The different keywords have the following meaning:

auto: the interface should be configured during boot time.

inet: interface uses TCP/IP networking.

dhcp: the interface can be configured through DHCP.

That is all.

To change the system to use a static IP address (not dhcp), change the /etc/network/interfaces file to something like:

iface eth0 inet static
    address 192.168.12.2
    netmask 255.255.255.0
    gateway 192.168.12.254

Arch Linux¶

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 ip-link(8).

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``.

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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 #Routing table for re-establishing it.

Static or dynamic IP address?

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Too convoluted. (Discuss in Talk:Network configuration)

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 ip-address(8).

List IP addresses:

$ ip address show

Add an IP address to an interface:

# ip address add address/prefix_len broadcast + dev interface

Note that:

• the address is given in CIDR notation to also supply a subnet mask

• + is a special symbol that makes ip derive the broadcast address from the IP address and the subnet mask

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  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

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

Routing table

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

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

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

List IPv4 routes:

$ ip route show

List IPv6 routes:

$ ip -6 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 (DHCP) server provides clients with a dynamic or static 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:

Set the hostname

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Reason: The myhostname pseudo-variable may cause confusion since that name is also used by nss-myhostname(8). (Discuss in Talk:Network configuration#myhostname → yourhostname)

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

/etc/hostname

myhostname

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.. tip:: For advice on choosing a hostname, see
   `RFC 1178 <https://tools.ietf.org/html/rfc1178>`__.

   Alternatively, using `hostnamectl(1) <https://man.archlinux.org/man/hostnamectl.1>`__:

# hostnamectl set-hostname myhostname

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

# hostname myhostname

To set the "pretty" hostname and other machine
metadata, see
`machine-info(5) <https://man.archlinux.org/man/machine-info.5>`__.

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)

• 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

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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 systemd.link(5) file. For example:

/etc/systemd/network/10-net0.link

[Match]
PermanentMACAddress=aa:bb:cc:dd:ee:ff

[Link]
Name=net0

Alternatively, a udev rule can be used:

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

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

These rules will be applied automatically at boot. To apply the change immediately, do a manual trigger of the udev rule on the net subsystem:

# udevadm trigger --verbose --subsystem-match=net --action=add

If you want to run a test on the changes made, udevadm --debug test /sys/class/net/* can be of help.

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Note:

• The priority of Name is lower than NamePolicy, so make sure the latter is unset/empty or the name will not be changed. 99-default.link sets NamePolicy, requiring the custom config to be ordered before it, i.e. to have a numerical prefix.

• The network interface must be down before changing its name. [4]

• To get the MAC address of each card, run ip link.

• 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 Path (which can be checked using networkctl status interface_name):

/etc/systemd/network/10-net1.link

[Match]
Path=pci-0000:01:00.0

[Link]
Name=net1

Or, use a udev rule with DEVPATH:

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

SUBSYSTEM=="net", DEVPATH=="/devices/pci*/*1c.0/*/net/*", NAME="net1"

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 given rule, "/devices/pci*/*1c.0/*/net/en*" would be wrong since it will stop matching once the name is changed to net1. Only the system-default rule will fire the second time around, causing the name to be changed back.

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 model ID instead:

/etc/systemd/network/20-net2.link

[Match]
Property=ID_VENDOR_ID=12ab ID_MODEL_ID=3cd4

[Link]
Name=net2

or

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

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

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

Revert to traditional interface names

If you would prefer to retain traditional interface names such as eth0, Predictable Network Interface Names can be disabled by changing the default NamePolicy for udev’s net_setup_link built-in:

/etc/systemd/network/99-default.link.d/traditional-naming.conf

[Link]
NamePolicy=keep kernel

Alternatively, net_setup_link can be completely disabled by masking the corresponding udev rule:

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

or by adding net.ifnames=0 to the kernel parameters.

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Warning: systemd.link(5) relies on net_setup_link to work. Prefer to use the first approach unless you fully understand what you’re doing.

Set device MTU and queue length

You can change the device MTU and queue length by defining manually with a systemd.link(5) config. For example:

/etc/systemd/network/30-mtu.link

[Match]
Type=wlan

[Link]
MTUBytes=1500
TransmitQueueLength=2000

Or through a udev rule:

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

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

MTUBytes: 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 systemd.netdev(5).

TransmitQueueLength: 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.

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 iproute2 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/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 enp2s0, enable promiscuous@enp2s0.service.

Investigate sockets

ss is a utility to investigate network ports and is part of the iproute2 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 ss(8).

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:

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Reason: 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

nss-myhostname(8) (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 [5] [6] for examples.

To prevent such software from unsafely resolving localhost over the network, add entries for localhost to the hosts(5) file:

/etc/hosts

127.0.0.1        localhost
::1              localhost

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Note: Report any software affected by this issue in FS#56684. This may help in getting localhost entries added to the default /etc/hosts.

To allow resolving the local hostname, additionally add it to the hosts(5) 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

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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 hosts(5) 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