mirror of https://codeberg.org/CRXN/docs.git
34 lines
1.6 KiB
Markdown
34 lines
1.6 KiB
Markdown
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# Implement max-len filter
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To implement a max-length filter, we need a rule/policy/filter file that contains the corresponding prefixes with their maximum length and a list with the prefixes without maxlen to distinguish if the prefix belongs to CRXN or to dn42. The entitydb contains a bash script `build_maxlen_filter.sh` for this purpose. If you run this script with the argument `bird`, you will get a bird-compatible list with the maximum lengths. If you run the script with the `bird prefix-list` parameter, you will get a prefix list without the maximum lengths.
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```
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define CRXN_IPs = [
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include "<path-to-file-1>";
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];
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define CRXN_MAXLEN = [
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include "<path-to-file-2>";
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];
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function is_crxn_net() {
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return net ~ CRXN_IPs;
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}
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function is_maxlen_valid() {
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return net ~ CRXN_MAXLEN;
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}
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```
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This bird configuration can be used to load the file. You have to replace `<path-to-file-1>` with the path of the filter file without maximum lengths and `<path-to-file-2>` with the path of the filter file with maximum lengths. Function `is_crxn_net` then checks if the prefix belongs to the CRXN network and function `is_maxlen_valid` then checks if the prefix is maxlen-valid.
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```
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if (is_crxn_net() && (! is_maxlen_valid())) then {
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print "[CRXN] Invalid crxn route: ", net;
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reject;
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}
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```
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This instruction checks whether a prefix belongs to the CRXN network and then whether the prefix is maxlen-valid. If it is not maxlen-valid, it is filtered and a message is issued.
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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.
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