nginx: [emerg] bind() to [::]:80 failed (98: Address already in use)

NGINX Config IPv6 Issue

Error Message:

```
Redirecting to /bin/systemctl status  -l nginx.service
● nginx.service - The nginx HTTP and reverse proxy server
   Loaded: loaded (/usr/lib/systemd/system/nginx.service; enabled; vendor preset: disabled)
   Active: failed (Result: exit-code) since Sat 2018-03-31 15:54:13 CEST; 16h ago
  Process: 18761 ExecReload=/bin/kill -s HUP $MAINPID (code=exited, status=0/SUCCESS)
  Process: 13076 ExecStart=/usr/sbin/nginx (code=exited, status=1/FAILURE)
  Process: 13072 ExecStartPre=/usr/sbin/nginx -t (code=exited, status=0/SUCCESS)
  Process: 13070 ExecStartPre=/usr/bin/rm -f /run/nginx.pid (code=exited, status=0/SUCCESS)
 Main PID: 6121 (code=exited, status=0/SUCCESS)

Mar 31 15:54:12 CentOS-72-64-minimal nginx[13076]: nginx: [emerg] bind() to 0.0.0.0:80 failed (98: Address already in use)
Mar 31 15:54:12 CentOS-72-64-minimal nginx[13076]: nginx: [emerg] bind() to [::]:80 failed (98: Address already in use)
Mar 31 15:54:12 CentOS-72-64-minimal nginx[13076]: nginx: [emerg] bind() to 0.0.0.0:443 failed (98: Address already in use)
Mar 31 15:54:12 CentOS-72-64-minimal nginx[13076]: nginx: [emerg] bind() to [::]:443 failed (98: Address already in use)
Mar 31 15:54:12 CentOS-72-64-minimal nginx[13076]: nginx: [emerg] bind() to [::]:444 failed (98: Address already in use)
Mar 31 15:54:13 CentOS-72-64-minimal nginx[13076]: nginx: [emerg] still could not bind()
Mar 31 15:54:13 CentOS-72-64-minimal systemd[1]: nginx.service: control process exited, code=exited status=1
Mar 31 15:54:13 CentOS-72-64-minimal systemd[1]: Failed to start The nginx HTTP and reverse proxy server.
Mar 31 15:54:13 CentOS-72-64-minimal systemd[1]: Unit nginx.service entered failed state.
Mar 31 15:54:13 CentOS-72-64-minimal systemd[1]: nginx.service failed.
```

Solution:

[::]:80 is a ipv6 address. This error can be caused if you have a nginx configuration that is listening on port
80 and also on port [::]:80. I had the following in my default sites-available file:

```
listen 80;
listen [::]:80 default_server;
```

You can fix this by adding ipv6only=on to the [::]:80 like this:

```
listen 80;
listen [::]:80 ipv6only=on default_server;
```

__Make sure that only the default server listens over IPv6!__