Page MenuHomeVyOS Platform
Create Task
Maniphest T7337

Documentation for as-path-list regular expressions
Open, NormalPublicFEATURE REQUEST
Actions

Description

Summary

Add documentation for regular expressions for as-path-lists.

Use case

Writing regular expression is very hard, if the syntax is unspecified. https://docs.vyos.io/en/latest/configuration/policy/as-path-list.html gives a trivial example. I assume, that this gets sent as is to FRR, however as far as I can tell FRR also has not documented what they are doing here. The only thing that I found is this example here: https://docs.frrouting.org/en/latest/bgp.html#bogon-asn-filter-policy-configuration-example

Additional information

The NLNOG route filtering guide gives examples for matching ranges of ASNs, however these examples show a different syntax that what the "documentation" at FRR shows. https://bgpfilterguide.nlnog.net/guides/bogon_asns/#frr-vtysh
There are also examples for VyOS there, which are basically the FRR config wrapped in VyOS commands with the same expressions.

Details

Version
-
Is it a breaking change?
Perfectly compatible
Issue type
Documentation update

Event Timeline

991jo created this task.Apr 9 2025, 3:22 PM
pasik subscribed.Apr 9 2025, 3:37 PM
Viacheslav triaged this task as Normal priority.Apr 9 2025, 3:40 PM
talmakion subscribed.EditedJun 26 2025, 10:47 PM

@991jo As far as I can tell, the documentation for both VyOS and FRR are simply correct - both bgp as-path access-list and show bgp ipvX regex render AS paths as text and run a POSIX regular expression against them. There's no range expansion or other fanciness beyond special handling of the '_' match - just a space-separated text string of ASNs vs a regexec()'d pattern.

The relevant code: https://github.com/FRRouting/frr/blob/master/bgpd/bgp_filter.c, bgp_filter.h, bgp_aspath.h and bgp_regex.c.

However, if you are keen, it is possible to generate a generic regex match from a number range. If you wished to introduce some extended syntax to the VyOS config generator, that could be bolted on to give FRR a nice regex to use, what you're asking about would be possible. I'd suggest something that doesn't clash with existing regex or AS-path syntax - so not 123-456, maybe a 123:456?

However, the result of this range expansion would look pretty similar to the NLNOG mikrotik v6 examples, excessively large ranges may cause performance problems.