fetch() fails with "Network request failed: closed before established" when first DNS server is unavailable

[pashangshangpo]

Hi,

I encountered an issue where fetch() always fails with Network request failed: closed before established even though the network and DNS are working fine.

Environment:

• macOS
• No proxy environment variables set

Root Cause:

After some investigation, I found that my system had two DNS servers configured:

• nameserver[0]: 45.14.70.54 (a locally running DNS by Clash proxy, which was offline)
• nameserver[1]: 223.6.6.6 (a valid public DNS)

The first DNS server 45.14.70.54 was unreachable (connection timed out), but the second one 223.6.6.6 worked perfectly fine.

The problem:

It seems that lws (libwebsockets) does not fall back to the next DNS server when the first one fails or times out. Instead, it immediately throws an error.

Comparison:

Tool	Behavior
curl	Tries first DNS → fails → automatically falls back to second DNS → succeeds ✅
lws	Tries first DNS → fails → throws error immediately ❌

Verified with:

# First DNS - times out
nslookup www.baidu.com 45.14.70.54
# → connection timed out; no servers could be reached

# Second DNS - works fine
nslookup www.baidu.com 223.6.6.6
# → returns correct IP address

Expected behavior:

When the first DNS server is unavailable, lws should fall back to the next configured DNS server, similar to how curl handles this situation.

Thanks!

[lws-team]

Are we using lws async DNS, or the libc DNS? lws async DNS until now just picks the first DNS server and that's it. libc (blocking) DNS should try both. You don't mention what the build options for Async DNS are.

When we say, "connection timed out" with the first DNS server, this is talking about the effect on the lws tcp connection to somewhere else? What did the first server actually reply (no reply? SERVFAIL?)

[saghul]

Hey there, I'm the author of txiki.js. In the build I'm configuring the async DNS resolver, not the libc one.

[lws-team]

Right now main branch has the stuff needed to have async DNS properly use multiple servers. That includes initially broadsiding queries to all servers, tracking performance and round-robin for the servers with reasonable performance.

main and v4.5 are very close api-wise, it should just build against your project with your existing settings. If any problems, I will be interested to hear about it.

[saghul]

Nice! I'll give it a try!