3.2.2 Ensure ICMP redirects are not accepted - 'net.ipv6.conf.all.accept_redirects = 0'

Information

ICMP redirect messages are packets that convey routing information and tell your host
(acting as a router) to send packets via an alternate path. It is a way of allowing an outside
routing device to update your system routing tables. By setting
net.ipv4.conf.all.accept_redirects and net.ipv6.conf.all.accept_redirects to 0,
the system will not accept any ICMP redirect messages, and therefore, won't allow
outsiders to update the system's routing tables.

Rationale:

Attackers could use bogus ICMP redirect messages to maliciously alter the system routing
tables and get them to send packets to incorrect networks and allow your system packets
to be captured.

Solution

Set the following parameters in /etc/sysctl.conf or a /etc/sysctl.d/* file:

net.ipv4.conf.all.accept_redirects = 0
net.ipv4.conf.default.accept_redirects = 0
net.ipv6.conf.all.accept_redirects = 0
net.ipv6.conf.default.accept_redirects = 0

Run the following commands to set the active kernel parameters:

# sysctl -w net.ipv4.conf.all.accept_redirects=0
# sysctl -w net.ipv4.conf.default.accept_redirects=0
# sysctl -w net.ipv6.conf.all.accept_redirects=0
# sysctl -w net.ipv6.conf.default.accept_redirects=0
# sysctl -w net.ipv4.route.flush=1
# sysctl -w net.ipv6.route.flush=1

See Also

https://workbench.cisecurity.org/files/2420