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## `IP_FILTER_RDNS_SERVER`

Option | `IP_FILTER_RDNS_SERVER`
Description | IP filter for filtering which DNS servers may be specified in the `rdns()` function or `reverseDns()` function.
Introduced Version | 1.37
Default | ``

IP filter for filtering which DNS servers may be specified in the `rdns()` function or `reverseDns()` function.

The syntax of these list is a sequence, separated by newlines or `;`, of `allow` or `deny` actions, along with a CIDR block or `all`. An IP address is allowed or denied by finding the first match and applying the corresponding action. The `all` value matches any IP-address (i.e., IPv4 or IPv6).

### Note

This only restricts what value may be specified - if no server is specified then the system default or the one specified by `RDNS_DEFAULT_SERVER` will be used irrespective of whether it is blocked by this filter.

For example, here we restrict the server to `8.8.8.8` and `1.1.1.1` and deny all others.

ini

    IP_FILTER_RDNS_SERVER=allow 8.8.8.8; allow 1.1.1.1; deny all

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