Re: question for those who run mail servers

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On 31/05/12 14:09, Bob Hoffman wrote:
> Not technically a centos question, but a lot of you guys seem to manage
> some large systems
> and I could use some clarification on a postfix setting.*
> *reject_unknown_client_hostname
> (in postfix<  2.3 reject_unknown_client)
> When I first used this there were issues with users trying to send mail
> through the server
> from hotels, wireless spots, etc. This was solved by pushing up permit
> sasl_authenticated.
> I took it out after those issues. I read many online posts from 2008
> saying too many
> false positives. (though none were clear if those were incoming mail or
> from mail users)
> Do you use reject_unknown_client_hostname?

I don't use it because as you already say the false positive rate is too 
high. This is caused largely by incorrectly configured entries in dns.

For example, suppose a client connects from a given IP address.

Postfix will do a rDNS lookup on that IP address to get the client 
hostname. If that lookup fails then the mail will get temp rejected.

Then Postfix will do a DNS lookup on the client hostname it just 
retrieved. If that lookup fails then the mail will get temp rejected.

The above two conditions result in temp rejections in case of temporary 
dns lookup failures which provides a bit of a safety net allowing 5 days 
(by default) for folks to notice (and fix) issues in their logs. From my 
experience I'd say most people do not bother reading their logs on a 
daily basis, at best only when they are made aware of a problem.

Finally, Postfix will check that the DNS lookup on the client hostname 
matches the client IP that is connecting to the server. If it doesn't 
match then the message will be permanently rejected. This is where FPs 
will result as far too many people do not understand how to correctly 
configure their server in DNS.

To summarise, you are looking for IP -> hostname -> IP to match.

Mail admins typically take two lines of approach on this:

1. I can't afford the potential FPs from idiots who don't know how to 
configure their mail servers.

2. I have no sympathy for idiots who don't know how to configure their 
mail servers and to hell with the FPs, - I'm going to teach them a 
lesson and reject their mail.

It's your mail server and you are free to configure it as you see fit. 
Decide which of the two camps above best describes your view and act 

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