Selector: add a function nslookup.

[dalf]

The current selectors about the source IP and Host are

• IP
• Host

Behind a reverse proxy they are useless.

The HTTP header X-Forwarded-For allows to filter the real user IP:
"filters": ["Header:X-Forwarded-For=<ip>

In the searx-docker project, there is this rule:

    {
        "name": "searx.space",
        "filters": ["Header:X-Forwarded-For=(2001:41d0:8:de3::1|176.31.252.227)"],
        "stop": true,
        "actions": [{ "name": "log"}]
    },

It would useful to have something like this:

    {
        "name": "searx.space",
        "filters": ["Header:X-Forwarded-For=nslookup(check.searx.space)"],
        "stop": true,
        "actions": [{ "name": "log"}]
    },

So even if at one point in time, check.searx.space resolves to different IPs, the rule would still work as expected.
The nslookup function can resolve the IPs addresses when filtron starts. The result of that function would be a regex similar to the first snippet.

Extend version: filtron resolves every day the IPs addresses without restart.