Linux bind IP that doesn’t exist with net.ipv4.ip_nonlocal_bind

net.ipv4.ip_nonlocal_bind

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Why use net.ipv4.ip_nonlocal_bind under Linux operating systems?

How do I allow Linux processes to bind to IP address that doesn’t exist yet on my Linux systems or server?You need to set up, which allows processes to bind() to non-local IP addresses, which can be quite useful for application such as load balancer such as Nginx, HAProxy, keepalived and others. This page explains how to bind IP address that doesn’t exist with net.ipv4.ip_nonlocal_bind Linux kernel option.

HAProxy acts as a load balancer (LB) and a proxy server for TCP and HTTP-based applications. Similarly, Keepalived software provides High-Availability (HA) and Load Balancing features for Linux using VRRP protocol. It acts as an IP failover (Virtual IP) software to route traffic to the correct backend. We can combine HAProxy (or Nginx) along with Keepalived to build a two-node high availability cluster for our applications.

However, LB in HAPorxy, Nginx, and Keepalived need the ability to bind to a non-local IP address. The problem is we can have an IP (Virtual IP) address assigned to one node at a time. So other nodes running Nginx/HAProxy will refuse to start. You will often see an error that as follows:

Nginx: cannot bind socket.

We want to allows a running LB instance to bind to an IP that is not local for failover.

Linux bind IP that doesn’t exist with net.ipv4.ip_nonlocal_bind

Use the sysctl command to find the current value of net.ipv4.ip_nonlocal_bind:

# sysctl net.ipv4.ip_nonlocal_bind

# sysctl net.ipv6.ip_nonlocal_bind

We can use the cat command as follows too:

cat /proc/sys/net/ipv4/ip_nonlocal_bind

To bind IP that doesn’t exist yet under Linux, run:

sudo sysctl -w net.ipv4.ip_nonlocal_bind=1

Linux bind IP net.ipv4.ip_nonlocal_bind

We can use the following syntax too:

echo 1 > /proc/sys/net/ipv4/ip_nonlocal_bind

For IPv6:

sudo sysctl -w net.ipv6.ip_nonlocal_bind=1

Binding to Non-local IP addresses in Linux permanently

Edit the /etc/sysctl.conf or /etc/sysctl.d/99-custom.conf

sudo vi /etc/sysctl.d/99-custom.conf

Append the following line:

## allow Nginx to start and bind to non local IP ##

net.ipv4.ip_nonlocal_bind=1



Save and close the file. To load changes, run:

sudo sysctl -f /etc/sysctl.d/99-custom.conf



Understanding sysctl command options

The -w option enable writing a value to Linux kernel variable. The -a option show all variables. For more info type the following man command:

man sysctl

OR

sysctl --help

Sample outputs:

Options: -a, --all display all variables -A alias of -a -X alias of -a --deprecated include deprecated parameters to listing -b, --binary print value without new line -e, --ignore ignore unknown variables errors -N, --names print variable names without values -n, --values print only values of a variables -p, --load [ = <file> ] read values from file -f alias of -p --system read values from all system directories -r, --pattern <expression> select setting that match expression -q, --quiet do not echo variable set -w, --write enable writing a value to variable -o does nothing -x does nothing -d alias of -h -h, --help display this help and exit -V, --version output version information and exit Options: -a, --all display all variables -A alias of -a -X alias of -a --deprecated include deprecated parameters to listing -b, --binary print value without new line -e, --ignore ignore unknown variables errors -N, --names print variable names without values -n, --values print only values of a variables -p, --load[=<file>] read values from file -f alias of -p --system read values from all system directories -r, --pattern <expression> select setting that match expression -q, --quiet do not echo variable set -w, --write enable writing a value to variable -o does nothing -x does nothing -d alias of -h -h, --help display this help and exit -V, --version output version information and exit

Conclusion

You learned how to set net.ipv4.ip_nonlocal_bind to 1 to configure a high available load-balancer (LB) under Linux. See “Handling nginx Failover With KeepAlived” and Linux kernel docs here for more info.