Linux Networking

Some important commands and settings that are needed frequently.

Add IPv4/IPv6 address to interface

In the past, ifconfig was used to add ip addresses. Nowadays, the ip command is used instead.

ip [-6] a a address dev iface

Examples:

ip a a 10.0.0.1 dev enp2s0
ip -6 a a fd40:cafe:affe:1234::1 dev enp0s3

Set default IPv6 gateway

ip -6 r a gateway dev iface

gateway must not be a link local address (that is one that starts with "fe80")!

IPv4 forwarding and NAT

Check if IPv4 forwarding is activated:

sysctl net.ipv4.conf.all.forwarding

On demand run

sysctl -w net.ipv4.conf.all.forwarding=1

Perform NAT on the interface that has Internet connection using iptables:

iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE

IPv6 forwarding

Query:

sysctl net.ipv6.conf.all.forwarding

Or using procfs:

cat /proc/sys/net/ipv6/conf/all/forwarding

Activating:

sysctl -w net.ipv6.conf.all.forwarding=1

Remark: "all" can be substituted by any network interface on the system, for example, cat /proc/sys/net/ipv6/conf/eth0/forwarding prints the value for interface "eth0".