Nginx not spawning both ipv4 and ipv6 workers

[Charles Lawrence]

I'm in the process of setting up a new server built on ubuntu 16.04
using nginx 1.10.0.

The specific issue is that while my new configuration essentially
matches my old nginx configuration from an ubuntu 13.10 server using
nginx 1.4.4, nginx 1.10.0 is only creating either ipv4 or ipv6
workers, but not both. This behavior is not present on the old server.
Not sure what else to try at this point.

I've verified that my nginx installation was built with ipv6.

> nginx version: nginx/1.10.0 (Ubuntu)
> built with OpenSSL 1.0.2g-fips  1 Mar 2016
> TLS SNI support enabled
> configure arguments: --with-cc-opt='-g -O2 -fPIE -fstack-protector-strong -Wformat -Werror=format-security -Wdate-time -D_FORTIFY_SOURCE=2' --with-ld-opt='-Wl,-Bsymbolic-functions -fPIE -pie -Wl,-z,relro -Wl,-z,now' --prefix=/usr/share/nginx --conf-path=/etc/nginx/nginx.conf --http-log-path=/var/log/nginx/access.log --error-log-path=/var/log/nginx/error.log --lock-path=/var/lock/nginx.lock --pid-path=/run/nginx.pid --http-client-body-temp-path=/var/lib/nginx/body --http-fastcgi-temp-path=/var/lib/nginx/fastcgi --http-proxy-temp-path=/var/lib/nginx/proxy --http-scgi-temp-path=/var/lib/nginx/scgi --http-uwsgi-temp-path=/var/lib/nginx/uwsgi --with-debug --with-pcre-jit --with-ipv6 --with-http_ssl_module --with-http_stub_status_module --with-http_realip_module --with-http_auth_request_module --with-http_addition_module --with-http_dav_module --with-http_geoip_module --with-http_gunzip_module --with-http_gzip_static_module --with-http_image_filter_module --with-http_v2_module --with-http_sub_module --with-http_xslt_module --with-stream --with-stream_ssl_module --with-mail --with-mail_ssl_module --with-threads

Below are my current configurations for the new server:

># /etc/nginx/nginx.conf> user www-data;
> worker_rlimit_nofile 30000;
> worker_processes 8;
> pid /run/nginx.pid;
>
> events {
>   worker_connections 500000;
> }
>
> http {
>   sendfile on;
>   tcp_nopush on;
>   tcp_nodelay on;
>   keepalive_timeout 65;
>   types_hash_max_size 2048;
>
>   include /etc/nginx/mime.types;
>   default_type application/octet-stream;
>
>   ssl_protocols TLSv1 TLSv1.1 TLSv1.2; # Dropping SSLv3, ref: POODLE
>   ssl_prefer_server_ciphers on;
>
>   access_log /var/log/nginx/access.log;
>   error_log /var/log/nginx/error.log;
>
>   gzip on;
>   gzip_disable "msie6";
>   gzip_vary on;
>   gzip_proxied any;
>   gzip_comp_level 6;
>   gzip_buffers 16 8k;
>   gzip_http_version 1.1;
>   gzip_types text/plain text/css application/json application/javascript text/xml application/xml application/xml+rss text/javascript;
>
>   include /etc/nginx/conf.d/*.conf;
>   include /etc/nginx/sites-enabled/*;
> }

Lastly, the weird thing is whether the workers get bound to ipv4 or
ipv6 entirely depends on the order in which the listen directives are
placed. In the following data, I've switched the order and tried
different configurations multiple times. After each change to
/etc/nginx/sites-enabled/blog I did sudo service nginx stop; sudo
service nginx start; sudo lsof -i;to get the data.

Also note that I changed the workers count to 8 after performing these
steps. However while the number of workers increased, the same
behavior was seen where all workers were either ipv4 or ipv6.

> listen [::]:80;
> listen 80;
> nginx    27675     root    6u  IPv4 204423      0t0  TCP *:http (LISTEN)
> nginx    27676 www-data    6u  IPv4 204423      0t0  TCP *:http (LISTEN)
>
> listen 80;
> listen [::]:80;
> nginx    27747     root    6u  IPv6 205134      0t0  TCP *:http (LISTEN)
> nginx    27748 www-data    6u  IPv6 205134      0t0  TCP *:http (LISTEN)
>
> listen 80;
> listen [::]:80 default ipv6only=on;
> nginx    27819     root    6u  IPv6 205849      0t0  TCP *:http (LISTEN)
> nginx    27820 www-data    6u  IPv6 205849      0t0  TCP *:http (LISTEN)
>
> listen 80;
> listen [::]:80 default ipv6only=off;
> nginx    27885     root    6u  IPv6 206495      0t0  TCP *:http (LISTEN)
> nginx    27886 www-data    6u  IPv6 206495      0t0  TCP *:http (LISTEN)
>
> listen 80;
> listen [::]:80 default;
> nginx    27953     root    6u  IPv6 207184      0t0  TCP *:http (LISTEN)
> nginx    27954 www-data    6u  IPv6 207184      0t0  TCP *:http (LISTEN)