ip link: Usage, Options & Common CI Errors

ip link shows and configures the network interfaces themselves (state, MTU, MAC).

ip link operates on the link layer - bringing interfaces up or down and setting MTU. The classic container gotcha is an MTU mismatch (overlay networks use < 1500) that silently stalls large transfers.

What it does

ip link displays and configures network devices at the link layer: administrative state (up/down), MTU, MAC address, and flags. It does not touch IP addresses (that is ip addr) - only the interface itself.

Common usage

Terminal

ip link show                   # all interfaces and their state
ip link show eth0
ip link set eth0 up            # bring an interface up
ip link set eth0 mtu 1450      # set MTU (overlay networks)
ip -br link                    # brief one-line-per-iface view

Options

Subcommand	What it does
show [dev]	List interfaces / one interface
set <if> up\|down	Bring the interface up or down
set <if> mtu <N>	Set the maximum transmission unit
set <if> address <mac>	Change the MAC address
-br	Brief, columnar output

Common errors in CI

"RTNETLINK answers: Operation not permitted" on set - the step needs NET_ADMIN (--cap-add=NET_ADMIN or privileged). The subtle one is MTU: Docker/VPN/overlay networks often use an MTU below 1500, so packets larger than the path MTU are dropped and a clone or pull hangs at the same byte every time. Check ip link show <if> for the mtu value and match it. An interface in state DOWN means it was never brought up - ip link set <if> up.