sysctl

Linux only forwards packets between its cards when net.ipv4.ip_forward is 1: that setting is the difference between a host with two cards and a router. Other settings worth knowing: rp_filter (drop packets arriving on a card the reply would not leave by) and net.ipv6.conf.all.forwarding. The same values are files under /proc/sys/.

sysctl command usage

sysctl NAME
sysctl -w NAME=VALUE
sysctl -a

Options

-w

write a new value

-a

print every setting (pipe it through grep)

-n

print the value only

sysctl examples

sysctl net.ipv4.ip_forward

Shows whether this device forwards IPv4 packets between its cards.

sysctl -w net.ipv4.ip_forward=1

Turns packet forwarding on: the device now routes.

sysctl -a | grep rp_filter

Shows the reverse-path filter setting of every card.

Reading the output

$ sysctl net.ipv4.ip_forward
net.ipv4.ip_forward = 1

• 1: packets arriving for another address are routed on. 0: they are dropped, silently.

Pitfalls

• A router with ip_forward = 0 still answers pings to its own addresses, so it looks alive while it passes nothing on.
• In a container some settings are read-only (/proc/sys is mounted read-only): they are set when it is created.