Bind unable to restart due to TCP_FASTOPEN

Symptoms

When restarting the Named DNS service, you may encounter the following error preventing the DNS Service from restarting.

named[8400]: ../../../../lib/isc/unix/socket.c:5682: unexpected error:
named[8400]: setsockopt(23, TCP_FASTOPEN) failed with Protocol not available

Description 

When the kernel feature TCP_FASTOPEN is not enabled, this can cause the DNS service fail to restart. Most kernels do enable this, and you can easily check and verify if this is enabled with a quick check from the command line interface. You can also do this via the terminal interface in the WHM. Home » Server Configuration » Terminal.

Workaround

To verify if this is enabled on the server run the following:

cat /proc/sys/net/ipv4/tcp_fastopen

If this returns a value of 0, it is disabled. If this returns a 1, this means it is only enabled for outgoing client connections, and a 2 indicates it is available on the server. Setting this to 3 will enable both of those options.

echo "3" > /proc/sys/net/ipv4/tcp_fastopen

If you would like to keep this through a reboot, echo the following into a new .conf file.

echo "net.ipv4.tcp_fastopen=3" > /etc/sysctl.d/30-tcp_fastopen.conf