In message <41848ab9-339a-41a8-9a20-b1533eb77...@dukhovni.org>, Viktor Dukhovni <postfix-us...@dukhovni.org> wrote:
>> On Mar 3, 2019, at 2:56 AM, Ronald F. Guilmette ><r...@tristatelogic.com> wrote: >> >> But this other fellow I've been taking to offered an unexpectedobservation: >> If a given Postfix installation was attempting to support, say, 1 million >> unique domain names (correponding to 1 million unique customers) and if >> just 11,000 of those were to all simultaneously attempt to send -outbound- >> emails to six (6) different destinations apiece, then... this other fellow >> asserted... all of the 65536 maximum available IPv4 port numbers would be >> exhausted, and then havoc would ensue. > >This mental model is deeply flawed. Thank you for the response Vicktor, but could you please be more specific, just so that I have it on the record? Whose mental model is it that you are saying is "deeply flawed"? Mine or the other guy's? >Postfix has a queue manager, that >limits the concurrency per destination, and the active queue size. And >a master(8) process that limits the process count per transport. Postfix >also accepts messages at a finite rate, so 66,000 messages will not arrive >instantaneously. Once the active queue is full further accepted messages >will accumulate in the incoming queue on disk, but will not consume network >resources or RAM. Paraphrasing, it sounds to me like you just said that Postfix is designed to behave well, and in fact does behave well, even under very high loads. But I, for one, already knew that. (And I suspect that most folks who use Postfix at "big" places knew that already also.) I still would like to know if the total number of outbound SMTP connections which Postfix may have open, at any one given point in time, may or may not exceed 65536. (I admit that this is really rather entirely a matter of academic curiosity on my part and that it may have little or no practical implications. I just have this running disagreement going about how many angels can dance on the head of... I'm sorry... about how many domain names can, in practice be hosted on a single IPv4 address. I say "millions". Others are telling me that I'm delusional and need to seek immediate treatment. I am not yet favorably inclined to acecpt their judgement on the matter. The key point of disagreement seens to be our differing evaluations about how many simultaneous outbound SMTP a good quality... or best quality... SMTP server could in practice support.) >But the port number exhaustion scenario is not even close. I'm not at all sure how to interpret that. May I assume that your intent was to say that a hosting company could tell all of its 1 million customers to use a single shared mail server for all of their outbound needs, and that even though this might possibly create a unsustainable load, the unsustainability would *not* become evident, in the first instance, as an exhaustion of outbound IPv4 port numbers?
