In libcurl 8.17.0, setting both CURLOPT_TIMEOUT and CURLOPT_CONNECT_ONLY simultaneously will consistently cause a timeout.

[Wyuer]

I did this

In my project, I need to connect to an FTP server to test connectivity. I used (void)curl_easy_setopt(self->curl, CURLOPT_CONNECT_ONLY, 1L); and also set (void)curl_easy_setopt(self->curl, CURLOPT_TIMEOUT, 10L);.
During local testing, it worked fine. However, timeout issues (*server response timeout, easy_perform result: 28) were gradually reported in the project.
After trying different libcurl versions, I found that version 7.84.0 had no such problem, while the issue occurred after upgrading to libcurl 8.17.0 in the project.

While adding debug logs in libcurl to reproduce this issue, I found that when setting TIMEOUT, Curl_pp_state_timeout calls timediff_t timeout2_ms = Curl_timeleft(data, &now, FALSE);
and then takes the smaller value between timeout_ms and timeout2_ms:
timeout_ms = CURLMIN(timeout_ms, timeout2_ms).

However, in Curl_timeleft, if CONNECT_ONLY is set, it returns 0:
if((!data->set.timeout || data->set.connect_only) && !duringconnect) {
return 0; /* no timeout in place or checked, return "no limit" */
}

After taking the minimum value, Curl_pp_state_timeout returns 0, which causes Curl_pp_statemach to return a timeout error:
if(timeout_ms <= 0) {
failf(data, "server response timeout");
return CURLE_OPERATION_TIMEDOUT; /* already too little time */
}

I expected the following

This issue can be resolved, or it should be noted that these two options cannot be used simultaneously.

curl/libcurl version

curl 8.17.0

operating system

Linux (none) 4.9.37 #1 SMP PREEMPT Tue Dec 30 16:37:52 CST 2025 armv7l GNU/Linux

[icing]

Thanks for the report. I agree with your finding that CURLOPT_TIMEOUT is handled differently in the two places you found.

You do not mention the use of CURLOPT_CONNECTTIMEOUT which means that the default of 5 minutes would be in place. The intent for CURLOPT_CONNECT_ONLY transfers is that CURLOPT_TIMEOUT does not really apply, indeed. That is why Curl_timeleft() behaves that way.

The bug seems to be that the FTP "protocol connect" (user auth, etc.) does check CURLOPT_TIMEOUT which should only apply when the actual FTP transfer happens.

One question for understanding your intent: why do you set CURLOPT_TIMEOUT and what do you expect to happen with it?

[Wyuer]

Actually, I originally used CURLOPT_NOBODY + CURLOPT_TIMEOUT, and only recently changed CURLOPT_NOBODY to CURLOPT_CONNECT_ONLY. I assumed CURLOPT_TIMEOUT would also apply to the timeout of CURLOPT_CONNECT_ONLY, so I kept it.

After discovering the issue, I have already replaced CURLOPT_TIMEOUT with CURLOPT_CONNECTTIMEOUT. However, I believe the timeout behavior of CURLOPT_TIMEOUT + CURLOPT_CONNECT_ONLY is also worth attention—especially since the behavior differs between the old and new versions.

[icing]

If you can build from #20354, I would be interested to know if that behaves correctly for you. Thanks!