Much slower than expected but IPv6 seems to be gaining ground. I don't have any general statistics, but all of the VPS hosting companies I work with support IPv6 and even at homes networks I have encountered IPv6.

So on the network layer many of my users have IPv6 enabled, and my servers also have it, but how can I make sure the web server also serves both IPv4 and IPv6?

I'll start with the verification as you will also need to do that both before you make changes - to see what are the problems - and after you make changes - to see if you have managed to fix them.

IPv6 on the network

$ ping6 code-maven.com

PING code-maven.com(s7.hostlocal.com (2600:3c00::f03c:91ff:fedf:456f)) 56 data bytes
64 bytes from s7.hostlocal.com (2600:3c00::f03c:91ff:fedf:456f): icmp_seq=1 ttl=54 time=265 ms
64 bytes from s7.hostlocal.com (2600:3c00::f03c:91ff:fedf:456f): icmp_seq=2 ttl=54 time=286 ms
...


$ curl -I -4 http://code-maven.com/

HTTP/1.1 301 Moved Permanently
Server: nginx/1.4.6 (Ubuntu)
Date: Sun, 05 May 2019 15:44:47 GMT
Content-Type: text/html
Content-Length: 193
Connection: keep-alive
Location: https://code-maven.com/

Then verify the same over Ipv6: 

$ curl -I -6 http://code-maven.com/

HTTP/1.1 301 Moved Permanently
Server: nginx/1.4.6 (Ubuntu)
Date: Sun, 05 May 2019 15:48:09 GMT
Content-Type: text/html
Content-Length: 193
Connection: keep-alive
Location: https://code-maven.com/

I can check the redirection for the www.code-maven.com hostname as well.

Then I can verify if the https version serves the page. First over IPv4 and then over IPv6. In each case I can use -I to see the header or leave out the -I and see the actual html content returned. 

$ curl -I -4 https://code-maven.com/

HTTP/1.1 200 OK
Server: nginx/1.4.6 (Ubuntu)
Date: Sun, 05 May 2019 15:49:24 GMT
Content-Type: text/html; charset=UTF-8
Content-Length: 11559
Connection: keep-alive


$ curl -I -6 https://code-maven.com/

HTTP/1.1 200 OK
Server: nginx/1.4.6 (Ubuntu)
Date: Sun, 05 May 2019 15:51:39 GMT
Content-Type: text/html; charset=UTF-8
Content-Length: 11510
Connection: keep-alive

Wrong configuration

Before I fixed this domain I had results like this: 

$ curl -I -4 http://code-maven.com/

HTTP/1.1 301 Moved Permanently
Server: nginx/1.4.6 (Ubuntu)
Date: Sun, 05 May 2019 15:44:47 GMT
Content-Type: text/html
Content-Length: 193
Connection: keep-alive
Location: https://code-maven.com/


$ curl -I -6 http://code-maven.com/

HTTP/1.1 301 Moved Permanently
Server: nginx/1.4.6 (Ubuntu)
Date: Sun, 05 May 2019 15:48:09 GMT
Content-Type: text/html
Content-Length: 193
Connection: keep-alive
Location: http://hostlocal.com/

That is, the http version over IPv4 redirected to the right place while http over IPv6 redirected to a different site. (That was the default site on the specific server.)

How to configure Nginx to serve over both Ipv4 and IPv6

I am using an old version of Ubuntu and thus an old version of Nginx. I think the defaults have changed since then and you might not need all this configuration, but as far as I understand explicitly configuring this won't cause problems even in newer versions of nginx.

Tell each https server section in your Nginx configuration to work on both IPv6 (the first row) and on IPv4 (second row): 

  listen    [::]:443 ssl;
  listen    443 ssl;

Tell each http server section in your Nginx configuration to work on both IPv6 (the first row) and on IPv4 (second row): 

  listen     [::]:80;
  listen     80;

Once you made the changes, reload nginx, and verify with curl.