[PATCH] Mail: add the "reuseport" option of the "listen" directive

[Robert Mueller]

> Could you please test if compiling with 
> --with-cc-opt="-DNGX_HAVE_EPOLLEXCLUSIVE=0" 
> improves things, notably on production systems?  In my limited 
> testing it seems to be improve things, and if this is indeed the 
> case, we can consider removing use of EPOLLEXCLUSIVE.

I can try this tomorrow, but did you see the link Jan posted to the cloudflare blog?

https://blog.cloudflare.com/the-sad-state-of-linux-socket-balancing/

This explains the problem we're seeing exactly and why reuseport fixes it.

> > As you can see, without the reuseport option, this causes severe 
> > scalability problems for us.
> 
> I tend to think that reuseport is a bad option for load balancing 
> between worker processes, as it can be easily tricked by an outside 
> actor to select a particular worker process, and this opens an 
> obvious DoS attack vector.

Really? Can you explain how this is possible?

Also given that cloudflare use this option, and I expect cloudflare are literally the largest users of nginx in the world and also have to deal with extreme adversarial environments given they run a service to protect against DDoS, I would expect they would be aware of any potential DoS vector in this regard, or if not aware, extremely interested in hearing about it!

-- 
Rob Mueller
robm at fastmail.fm