adapter-node: IPv4-only bind (0.0.0.0) default diverges from Node.js http.Server, why?

[therealPaulPlay]

I noticed occasional NGINX errors on my server after switching a container (which uses --network host in docker) from Express.js to SvelteKit and realized that while the Node.js http.Server.listen() defaults to binding :: and thereby allows for dual-stack connections, SvelteKit defaults to binding 0.0.0.0 and only supports IPv4.

This is a bit of a footgun, because the expectation is for the node-adapter to behave like the Node.js HTTP server or at least similar to it.

On Linux with net.ipv6.bindv6only=0 (which is the default), :: supports both IPv4 and IPv6 via IPv4-mapped IPv6 addresses.

So while NGINX happily uses both, and the Node.js http module's server by default does too (which Express uses under the hood), SvelteKit with the node adapter only supports IPv4, resulting in errors whenever NGINX uses IPv6 for a request.

Changing the default to :: would allow for parity with Node.js and less issues for consumers since both IPv4 and IPv6 just work on most OSes.

[rafshanDev90]

Problem Analysis

• Root Cause: @sveltejs/adapter-node defaults to 0.0.0.0 (IPv4-only), diverging from Node.js default :: (dual-stack).
• Impact: Under Docker --network host, NGINX upstream requests via IPv6 (::1) fail with 502/Connection Refused.

---

Solutions## Option 1: Restore Dual-Stack Binding (SvelteKit)

Override the default host binding in your container environment to enable both IPv4 and IPv6 support:

docker-compose.yml or container envenvironment:

• HOST=::
• PORT=3000

Option 2: Force IPv4 Upstream (NGINX)

If the host environment does not cleanly support IPv6 bindings, explicitly restrict NGINX to the IPv4 loopback:

nginx.conf

location / {
proxy_pass http://127.0.0.1:3000; # Avoids resolving to [::1]
proxy_set_header Host $host;
}

[therealPaulPlay]

Yes, thanks? I'm not struggling with this anymore as mentioned in my post. Setting HOST in the .env to :: did the trick. I'm just challenging the default.