Network configuration

This short document is a quick reference to some commands you would like to know when you’re connecting to a network and asking “What’s the conf, here?” If there is a kind of “automatic configuration” taking place, you may want to know what information the network is giving you. In the other case, you need to know how to set up a manual configuration by hand.

IPv4 configuration

Automatic IPv4 configuration is quite straightforward: run a DHCP client! For example, to discover a DHCP server on eth0 interface:

dhclient eth0
# or
dhcpcd eth0

Static IPv4 configuration can be achieved thanks to ip command:

ip addr add dev eth0
ip route add default via dev eth0

This presupposes that you know what IP address your gateway uses. Without this knowledge, you need to sniff and/or scan the network to find it.

IPv6 configuration

Automatic IPv6 configuration is more straightforward than IPv4: it is done by the kernel and you have nothing to do! Sometimes you may want to ignore the automatic configuration. This command disables it on eth0:

sysctl -w

Automatic configuration works because when a host sends a Router Solicitation message (ICMPv6 type 133), the gateways answer with a Router Advertisement (ICMPv6 type 134) containing information similar to DHCP. To send a RS by hand, install ndisc6 package and run:

rdisc6 eth0

Like with IPv4, static IPv6 can be configured with ip command:

ip addr add 2001:db8::42/64 dev eth0
ip -6 route add default via 2001:db8::1 dev eth0

However, unlike IPv4, there is a smart way to find the gateway: multicast. More precisely, IPv6 defines ff02::2 as being a multicast address for all routers on a link (a Linux machine answers as a router when is 1). If your gateway answers to ping request, you can run this command to get its IPv6 address:

ping6 -c1 ff02::2%eth0

The result would look like:

PING ff02::2%eth0(ff02::2) 56 data bytes
64 bytes from fe80::4242:42ff:fe42:4242: icmp_seq=1 ttl=64 time=0.420 ms

In such case, you may configure your default route with this link-local address (in fe80::/64):

ip -6 route add default via fe80::4242:42ff:fe42:4242 dev eth0

DNS configuration

In automatic configurations, the IP addresses of the local DNS servers and the search domains are found in DHCP headers and RA options. If you use a software like NetworkManager or resolvconf or wicd, this information is directly written in /etc/resolv.conf.

In manual configuration, edit this file with lines like these:

# and

Systemd network configuration

On a server which provides an ens42 wired connection with a static IP configuration, it is possible to use systemd-networkd with the following configuration in /etc/systemd/network/



Broadcast ping

To discover pingable hosts on your network, send an Echo Request (ping) to every host. In IPv4 this broadcasts an ICMP type 8 message:

ping -b -I eth0

In IPv6 this multicasts an ICMPv6 type 128 message to all nodes:

ping6 ff02::1%eth0

After such command, you may list every link-layer addresses of your neighbors by issuing:

ip neigh show