# Implement max-len filter

To implement a max-length filter, we need a rule/policy/filter file that contains the corresponding prefixes with their maximum length. The entitydb contains a bash script `build_maxlen_filter.sh` for this purpose. If you run this script, you will get a bird-compatible list.

```
define CRXN_IPs = [
    include "<path-to-file>";
];

function is_crxn_net() {
  return net ~ CRXN_IPs;
}
```
This bird configuration can be used to load the file. You have to replace `<path-to-file>` with the path of the filter file. The function `is_crxn_net` then checks whether a route complies with the rules or not.

```
if (! is_crxn_net() ) then {
    print "[CRXN] Invalid crxn route: ", net;
    reject;
}
```
This configuration can then be added to your import filter before 'accept'. The line with `print` is optional. If you leave this line, every hjack attempt will be logged in the bird log output.