Why listening "::" (all IPv6) also get "0.0.0.0"(all IPv4) listened?

[jjqq2013]

• Version:v7.0.0
• Platform:Windows 7 Pro
• Subsystem:

I found a strange problem, i created a simple TCP server to listen at port 5555 of "::"(all IPv6 interfaces)

net.createServer(c=>console.log).listen(5555,'::')

Then i found all IPv4 interfaces are also listening port 5555.

> netstat -an | findstr /I :5555
  TCP    0.0.0.0:5555           0.0.0.0:0              LISTENING
  TCP    [::]:5555              [::]:0                 LISTENING

Another relevant problem is, if i do not specify host, or use undefined, or empty string,

net.createServer(c=>console.log).listen(5555)
//or net.createServer(c=>console.log).listen(5555,undefined)
//or net.createServer(c=>console.log).listen(5555,'')

It should listen :: OR 0.0.0.0, not both, according the document:

server.listen([port][, hostname][, backlog][, callback])
If the hostname is omitted, the server will accept connections on any IPv6 address (::) when IPv6 is available, or any IPv4 address (0.0.0.0) otherwise

But the fact is both :: and 0.0.0.0 get listened.

I don't know which is correct? document or implementation? Anyway, this is a bit confusing.
Or maybe this is just a trick implementation of the Windows OS itself ?

[bnoordhuis]

Or maybe this is just a trick implementation of the Windows OS itself ?

It's called dual-stack mode and it's platform-specific. Linux lets you enable/disable it through the net.ipv6.bindv6only sysctl, I assume Windows has something similar.

I'll add doc tags because it doesn't look like the documentation currently mentions this. Libuv has UV_TCP_IPV6ONLY and UV_UDP_IPV6ONLY flags that we could leverage along the lines of net.createServer({ ipv6only: true }).

[jjqq2013]

Thank you. You saved my time.

[silverwind]

This dual stack mode is certainly confusing, as most people would expect :: (The alias for all v6 addresses) to bind to just v6, not magically also binding v4 on certain platforms.

The user has no way of knowing if the platform supports dual-stack, so if one wants to listen on v4 and v6, you'd have to try listening on both :: and 0.0.0.0 and catch the EADDRINUSE from the v4 listen on platforms that don't support it (I think macOS is an example).

I'd argue that ipv6only behavior should be made the default behaviour when hostname is ::. The dual-stack mode could still be in effect when hostname is undefined.

[mika-fischer]

Two things to note about this bug:

1. It makes it more difficult than necessary to create portable code.
2. It makes it impossible to create a IPv6-only socket with nodejs on Linux, and that just seems wrong.