While the daemons run res_init() on startup to initialize glibc's name resolution mechanisms, libcurl uses its own backend to resolve names. I'm not an expert with libcurl, but as I understand it, the resolving configuration is basically initialized on every call to curl_easy_init().

This results in the artefact that web checks pick up changes in /etc/resolv.conf and presumably nsswitch.conf, gai.conf and host.conf (I don't mean "hosts"), while the daemons otherwise don't.

I first thought that Zabbix could maybe resolve the name and just hand it over to libcurl, which is possible. However, I realized that this is no solution, because a redirect may require another name resolution, so this effect won't go away. I don't know if there's another solution.

I would therefore ask you to document this behaviour, so we can point people somewhere, when the question arises.