mod_s2s_keepalive: Isolate source host of pings The incoming_s2s table is not restricted to the current virtualhost so this prevents opening more connections than what's needed. Also prevents useless double sending of one whitespace per local host.

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

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