Re: [exim] Rate limiting connections

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Author: Jeremy Harris
Date:  
To: exim-users
Subject: Re: [exim] Rate limiting connections
On 26/09/17 11:55, Robert Bannocks wrote:
> In respect of smtp_accept_max is there some upper limit and, for RH Linux, how do I find out what this may be. There is presumably some Kernel limit, but there are other things to consider too. Is this just something people adjust hubristically or is there a more scientific approach?


I like the suggestion of hubris...

The kernel limitation is essentially 64k minus any reserved via
ip_local_port_range and ip_local_reserved_ports sysctls.

Exim has a hardcoded limit of 4k (obviously individual distros could
vary the code, but there's no support in build documentation for doing
so. Memory is pre-allocated for connection-tracking purposes in
proportion to the configured value, so you don't want it too high.

The practical limit will be your system load, dealing with the number
of concurrent connections.

>
> Same question for smtp_connect_backlog.


This is just the value given with the "listen" syscall to the kernel.
See the comments in the docs.

>
> Do not assume what I have inherited makes any sense.


If your problem is with certain individual source systems, consider
the smtp_max_per_host config option.

--
Cheers,
Jeremy