I've seen various config examples for handling dual-stack IPv4 and IPv6 virtual hosts on nginx. Many suggest this pattern:

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

As far as I can see, this achieves exactly the same thing as:

listen [::]:80 ipv6only=off;

Why would you use the former? The only reason I can think of is if you need additional params that are specific to each protocol, for example if you only wanted to set deferred on IPv4.

  • Deffered as nothing to do with IP stack version it's a TCP option. – Xavier Lucas Oct 20 '14 at 16:33
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    Sure, but you set it in listen directives, and the options are applied per host:port pair. – Synchro Oct 20 '14 at 16:35
  • Hum I really can't imagine a case in which you would want to do that. I think the only reason is historical and Michael Hampton nailed it. – Xavier Lucas Oct 20 '14 at 16:44

That probably is about the only reason you would use the former construct, these days.

The reason you're seeing this is probably that the default of ipv6only changed in nginx 1.3.4. Prior to that, it defaulted to off; in newer versions it defaults to on.

This happens to interact with the IPV6_V6ONLY socket option on Linux, and similar options on other operating systems, whose defaults aren't necessarily predictable. Thus the former construct was required pre-1.3.4 to ensure that you were actually listening for connections on both IPv4 and IPv6.

The change to the nginx default for ipv6only ensures that the operating system default for dual stack sockets is irrelevant. Now, nginx either explicitly binds to IPv4, IPv6, or both, never depending on the OS to create a dual stack socket by default.

Indeed, my standard nginx configs for pre-1.3.4 have the first configuration, and post-1.3.4 all have the second configuration.

Though, since binding a dual stack socket is a Linux-only thing, my current configurations now look more like the first example, but without ipv6only set, to wit:

listen [::]:80;
listen 80;
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    Some operating systems do not do dual ipv4 and ipv6 sockets at all, like OpenBSD, so for that you are going to have to listen twice. – Justin Cormack Mar 9 '15 at 20:47
  • @JustinCormack Yes, you're right, and I've taken that into account for some time. Just hadn't updated this post until now. – Michael Hampton Feb 2 '17 at 1:59
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    listen localhost:8080; seems to listen to both (1.12.2) and using proxy_pass http://localhost:8080 would load balance between ::1 and - I had to add a line for ipv6 to get real ip in logs set_real_ip_from; set_real_ip_from ::1; real_ip_header X-Forwarded-For; – Antony Gibbs Feb 6 '18 at 17:02

If you host multiple vhost domains with a single Nginx instance, you can't use the single combined listen directive

listen [::]:80 ipv6only=off;

for each of them. Nginx has a weird quirk where you can only specify the ipv6only parameter once for each port, or it will fail to start. That means you can't specify it for each vhost domain server block.

As Michael mentioned, starting with Nginx 1.3.4, the ipv6only parameter defaults to on.

Therefore, if you want to host multiple domains on both IPv4 and IPv6 with a single Nginx server, you are forced to use two listen directives for each domain server block:

listen 80;
listen [::]:80; 

Additionally, as Sander mentioned, using ipv6only=off has the drawback that IPv4 addresses are translated to IPv6. This can cause problems if your app does IP checking against blacklists like Akismet or StopForumSpam because unless you build in a reverse translation layer, your app will check the IPv6 translation of the spammer's IPv4 address, which won't match any of the IPv4 addresses in the blacklist.

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    Yes, that's the same as I mentioned about deferred, and other per-protocol directives. It would be useful if they could be specified separately from the listen directive for the reason you say. – Synchro Apr 24 '15 at 10:22
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    And the core of the matter is, you need to specify listen directive for each domain separately. Otherwise what would happen ? site would work fine via ipv4 and via ipv6 it would show the nginx welcome page. ROFL – Silver Moon Dec 9 '15 at 6:57
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    Thank you for the thorough explanation! I was getting a confusing error when I specified ipv6only=off for the same port twice. Your answer solved the problem! – user243345 Apr 24 '16 at 14:38
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    Also if you want to use 2 vhosts both listening to 443: listen 443; listen [::]:443; . Using listen [::]:80 ipv6only=off; will throw an nginx error that port is already in use – lukeaus Jul 20 '16 at 6:37

With the ipv6only=off configuration style the IPv4 addresses might be shown as IPv6 addresses using the (software-only) IPv4-mapped IPv6 addresses in for example log files, environment variables (REMOTE_ADDR) etc.


To my understanding (and according to the docs at http://nginx.org/en/docs/http/ngx_http_core_module.html#listen), using just

listen 80;

...is sufficient if you wish to channel both IPv4 & IPv6 traffic at the same port.

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    That has already been established, and mentioned in the question. Please see the other answers for the difference. – Synchro Mar 9 '17 at 7:46
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    It did not for me, I needed both. wget and curl where failing when using ipv6 until I added the line "listen [::]:80 ipv6only=on;" – Basil A Mar 30 '17 at 14:55
  • This doesn't work for me, and I can't find where in the docs it says it would work. – gsingh2011 Jul 23 '20 at 7:15

One pesky problem I have encountered while adding IPv6 support to a site with the listen [::]:80 ipv6only=off; snippet, was when I added it to a vhost and the default_server was already configured to listen for both 80 and [::]:80.

nginx refused to start, complaining that the address was already in use!

Replacing the magic listen [::]:80 ipv6only=off; with the two traditional listen lines allows nginx to start just fine.

As much as listen [::]:80 ipv6only=off; may be convenient in manual configuration, it may cause nasty troubles when used in automated configuration systems.

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