The new virtual host serving you this website is now capable of IPv6, which is great. In the days where IPv4 addresses are rare and also not very cheap (>15€ per month for more than 4 addresses), it's good to have a /64 IPv6 subnet. 2^64 addresses is way to much but well.. it is for free.

Nginx is my webserver of choice right now and I want to enable IPv6 in dual stack mode. My first naive approach was to just uncomment the listen [::]:80 directive for all servers and that's it. I was wrong, this is the result:

Starting nginx: the configuration file /etc/nginx/nginx.conf syntax is ok
configuration file /etc/nginx/nginx.conf test is successful
[emerg]: bind() to [::]:80 failed (98: Address already in use)
[emerg]: bind() to [::]:80 failed (98: Address already in use)
[emerg]: bind() to [::]:80 failed (98: Address already in use)
[emerg]: bind() to [::]:80 failed (98: Address already in use)
[emerg]: bind() to [::]:80 failed (98: Address already in use)
[emerg]: still could not bind()

The point here is, that if net.ipv6.bindv6only (in /etc/sysctl.conf) is disabled - which it is by default - nginx listens in dual stack mode. And as we have the directive listen 80 in our configuration as well, it crashes.

There are two solutions to this: Disable listen 80 for every server or change ipv6only to 1.

Replace all listen 80 with listen [::]:80

server {
# listen   80; ## listen for ipv4
  listen   [::]:80; ## listen for ipv6
}

$> netstat -nlp | grep nginx
tcp6  0   0 :::80     :::*    LISTEN   8960/nginx
tcp6  0   0 :::443    :::*    LISTEN   8960/nginx

No tcp (IPv4) listen directives for nginx. Despite that, the sockets are configured to accept both IPv4 and IPv6 connections via in6addr_any.

Enable ipv6only

To explicitly create to sockets for each protocol, you'd have to to configure nginx the following way:

server {
  listen 80;
  listen [::]:80 ipv6only=on;
}

The default on Linux is disabled ipv6only. So you have to enable it manually with sysctl -w net.ipv6.bindv6only="1" or put the following into /etc/sysctl.conf and reboot.

net.ipv6.bindv6only = 1