While investigating HELP-25377, I noticed that _mongoc_get_rr_search uses strerror to print an error message from h_errno. The latter is set when an error occurs in the res_nsearch or res_search calls earlier. However, h_errno is not designed to be run through strerror, and the actual error is different from what we see in the error message. In HELP-25377 in particular, the error message seen was "Interrupted system call". We can see its mapping:

#define	EINTR		 4	/* Interrupted system call */

Looking at the error section for h_errno in the manual, this is not at all what's happening:

Errors

The variable h_errno can have the following values:

HOST_NOT_FOUND
The specified host is unknown.
NO_ADDRESS or NO_DATA
The requested name is valid but does not have an IP address.
NO_RECOVERY
A nonrecoverable name server error occurred.
TRY_AGAIN
A temporary error occurred on an authoritative name server. Try again later.

h_errno.h also defines hstrerror to retrieve the error string for a given error code, but this has been marked obsolete. With that in mind, I'd suggest adding _mongoc_hstrerror to get an error string for an error, taken from the list above.

I'll note that whether on purpose or by oversight, the function also ignores the TRY_AGAIN error. One could argue that "Try again later" does not suggest retrying the lookup right away, and it also wouldn't have fixed the problem in HELP-25377 as h_errno is set to NO_DATA. However, it might be beneficial to try again to protect against transient failures.