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Lua Userscripting

Implement C <-> Lua interface to support userscripting on routers!

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Details

FRR, as a network routing suite, has a large variety of user-configurable policy settings. These include ACLs, “route-maps” (small data transform programs defined in FRR’s configuration DSL that change attributes of IP routes), and various protocol-specific knobs that influence anything from the path cost metric on routes to node preference values in high availability failover protocols such as VRRP. Presently these are all implemented as bespoke options within FRR’s configuration language, and have to be statically defined in its configuration file. We would like to vastly increase the flexibility and extensibility of FRR by adding support for hooking into user-defined Lua programs to change runtime behavior on the fly. If you are familiar with eBPF, you can think of this project as implementing eBPF, but for a userspace network routing stack. Or, if you are more familiar with Web technologies - Lua is to C as Javascript is to HTML.

FRR currently has some sparse glue code to connect to Lua via its C interface, but this is all exploratory / testing code and is not shipped in release builds. The ideal result of this project would be an interface that allows FRR developers to define hook points in the code that transfer control to a user’s Lua program, run the program, and pass any results back to FRR. This would allow extremely advanced functionality and effectively make FRR a completely programmable routing stack. New features such as monitoring, policy definitions, flow control, deployment management etc. could be shipped as plain text Lua scripts, shared among users, and allow FRR to operate in any number of environments and configurations with minimal or no patching to the core C codebase.

FRR has high performance constraints and operates in production networks, so the implementation of this Lua interface should be highly performant. Students proficient in writing safe, efficient C - or who are willing to learn - are a must. Since we have no serious implementation of this feature currently, this is effectively a green field project, meaning overhead and implementation constraints should be relatively minimal.

Bonus Goal

If the Lua interface is completed, FRR has a slew of features that are excellent candidates for integration with it. These include route-maps, various patchwork monitoring features, and flow control protocols. We would love to see any of these, or anything else, converted to use the new interface so we can pass the fruits of your labor on to users!

Quick Start

• If you’re unfamiliar with Lua, a good summary of what Lua can provide to C programs is located here.
• Have a look at some of FRR’s exploratory Lua code; this example offloads basic route-map functionality to a Lua script: https://github.com/FRRouting/frr/blob/master/bgpd/bgp_routemap.c#L344
• Read our user docs on routemaps; this is a good working example of the sort of functionality FRR would like to offload to Lua.

Mentors

Stephen Worley (@sworleys)
Quentin Young (@qlyoung)

Helpful Links

FRR dev guide
FRR user guide