Run external script after accepting BGP update or withdraw

[Alex Bligh]

Stanislav,

On 20 Jul 2014, at 08:05, Stanislav Datskevich <sdatskevich at gmail.com> wrote:

> I'm not a C programmer, so, could you please explain why fork() is the problem? I see this feature work this way:
> 1. Place "run external command" statement into "if" statement inside the filter or function, so it will be ran only on the specific updates/withdraws.
> 2. When update/withdraw message is arrived, BIRD runs external command, passes message details to it as environment variables and forks it. After fork BIRD is not blocked by child execution, so it will not be a performance impact.

If you are carrying a large routing table of many hundreds of thousands of routes, losing an interface (or regaining it) will result in hundreds of thousands (if not millions) of withdrawals or advertisements, which will in turn result in hundreds of thousands of calls to fork() in order to run the external script. That will be very slow. Of course one answer is 'don't do that then'. But another possibility is to allow whatever hook is put in not only to run a script, but also (e.g.) to call a little bit of lua. Provided you already have your luaState (meaning the lua VM is already initialised), the overhead in calling one function in lua is tiny.

This would allow this idea to scale to hundreds of thousands or millions of routes without difficulty. The lua could be used to (e.g.) log certain route changes to a file. For instance, anyone interested in logging any advertisement of a route which has been both withdrawn and advertised more than twice in the last 10 minutes (a.k.a. a flap)? Or in logging receipt from other than the customer of a more specific route than one of one's customers routes that are not in a pre-agreed list? Or do some matching on the above and only in respect of /some/ of the routes, run a script? These would be simple little programs in lua (tens of lines maximum), but doubtless are not things that the members of the Ondrej Aggregate will want to take code for on a case by case basis into bird.

-- 
Alex Bligh