mod_isolate_host: handle server-generated stanzas The hook for setting the no_host_isolation is only called for c2s sessions. This does not work for stanzas generated by the server, such as PEP notifications or presence probe answers. To handle that, we do per-stanza checks for the case that the origin is local.

This module lets you block connections to any remote servers not on a
whitelist.

``` {.lua}
modules_enabled = {
    -- other modules --
    "s2s_whitelist",

}
s2s_whitelist = {
    "example.org",
}
```