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.
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 192.168.0.42/24 dev eth0 ip route add default via 192.168.0.1 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.
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 sys.net.ipv6.conf.accept_ra=0
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
sys.net.ipv6.conf.forwarding 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
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:
# resolver1.opendns.com and resolver2.opendns.com nameserver 208.67.222.222 nameserver 208.67.220.220 domain example.com search example.com
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/wired.network:
[Match] Name=ens42 [Network] Address=192.168.0.42/24 Gateway=192.168.0.1
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 255.255.255.255 -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
When the network don't give your host an IP address, you need to find an unused
one. In IPv4 you can test whether an IP address is free or used by sending ARP
requests (what doesn't require an IPv4 address to be configured, unlike
ping):
arping 192.168.0.42 -I eth0
In IPv6, ARP requests are replaced with Neighbor Solicitation (ICMPv6 type 135 messages):
ndisc6 2001:db8::42 eth0