On Fri, February 13, 2015 11:04 am, Les Mikesell wrote:

On Fri, Feb 13, 2015 at 9:57 AM, Ken Smith kens@kensnet.org wrote:

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Hi All,

I'm just wanting to check that my understanding of the settings is correct as my web searches are finding a lot of dated information.

If I want a Centos 6 sendmail system act as the secondary MX for domain bbbbb.co.uk do I just add a

Connect:bbbbb.co.uk RELAY

statement into /etc/mail/access and restart sendmail

Obviously I have the DNS MX records for the domain are already established.

I've been getting "/config error/: /mail loops back to me/ " errors.

I think I may be stumbling into a variant of cname problem where the hostname as far as the sendmail machine is concerned is aaaaa.com but the DNS setting for the secondary MX is smtp1.bbbbb.co.uk.

They both resolve to the same IP but when sendmail looks up the MX records for bbbbb.co.uk it will find smtp.bbbbb.co.uk and smtp1.bbbbb.co.uk listed and it may relay the mail off to smtp1.bbbbb.co.uk without recognising that aaaaa.com = smtp1.bbbbb.co.uk. Am I on the right track here, as I then just need to change the secondary MX setting in DNS to aaaaa.com?

I'd recommend not having a secondary MX at all unless it is equipped to reject invalid users and spam in all the same ways as your primary.

Agree, but...

Otherwise it accept junk that your primary rejects

Not exactly. If greylisting on primary is set, but on backup MX is not, still what is killed by greylisting by primary MX, almost never will come through backup MX. This is due to the same reason why greylisting is efficient: it trows off all that doesn't behave as mail server (thus never comes for re-delivery, and definitely doesn't try backup MX which real servers always do even before attempt of re-delivery). Still, it is good to have the same greylisting on backup MX. And all other blows and whistles.

and then you are obligated to send a bounce message which is always a bad thing - you want the authoritative receiver to reject at the smtp level instead of accepting at all.

I agree, it is wrongful behavior to accept something which later you discover you can not deliver. I would call it bad MX setup, as you are making yourself potential source of backscatter (which though is not as much exploited yet as open relays, but still is bad setup).

If you set backup MX based on postfix, there is relay_recipients you have to specify, which lists all e-mail addresses that are legitimate on primary MX. Nothing else is being accepted by default, thus secondary MX does not become a source of backscatter.

<rant> I've seen at least at some point that google mail accepts everything. Then, (after they parsed and filed information in that message I would speculate) they send non-delivery notification. That was a real incident after which I have a policy on my servers: I do not forward e-mail of users who left department to their google mail. As I don't want _my_ server to become a source of backscatter as a result of the crap they do. </rant>

Valeri

++++++++++++++++++++++++++++++++++++++++ Valeri Galtsev Sr System Administrator Department of Astronomy and Astrophysics Kavli Institute for Cosmological Physics University of Chicago Phone: 773-702-4247 ++++++++++++++++++++++++++++++++++++++++

Les Mikesell wrote:

On Fri, Feb 13, 2015 at 11:39 AM, Valeri Galtsev galtsev@kicp.uchicago.edu wrote:

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Otherwise it accept junk that your primary rejects

Not exactly. If greylisting on primary is set, but on backup MX is not, still what is killed by greylisting by primary MX, almost never will come through backup MX. This is due to the same reason why greylisting is efficient: it trows off all that doesn't behave as mail server (thus never comes for re-delivery, and definitely doesn't try backup MX which real servers always do even before attempt of re-delivery).

I'm not convinced. Spam is big business and trying a 2nd MX is cheap.

...

Still, it is good to have the same greylisting on backup MX. And all other blows and whistles.

Greylisting would be kind of hard to do right. You'd have to keep the known-good senders in sync across the receivers. But my bigger worry would be a dictionary-type attack on user names as recipients if you don't have access to the real user list on the secondary. Aside from the blowback of the bounces, if you've ever accepted an address it is likely to get on lists of known-good spam and cause extra traffic forever after.

In this case the secondary MX has the same RBL's etc etc as the primary. I do see the spammers sending their junk to the secondary more than the primary MX. Agree the secondary does not know the difference between valid and invalid addresses.

Thoughts on my configuration?? I might just change the DNS name in the secondary MX anyway.

Ken

On Fri, Feb 13, 2015 at 12:45 PM, Valeri Galtsev galtsev@kicp.uchicago.edu wrote:

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In this case the secondary MX has the same RBL's etc etc as the primary. I do see the spammers sending their junk to the secondary more than the primary MX. Agree the secondary does not know the difference between valid and invalid addresses.

What software the secondary MX is based on in whose case you say secondary MX doesn't know legitimate addresses of primary MX?

I know about postfix. And all my servers are based on postfix. And even in the most trivial configuration of secondary MX based on postfix secondary MX _does_ have to have all legitimate addressed of primary MX. These are in relay_recipients table. Any address that is not in that table, will not be accepted by secondary MX. Postfix even in the most trivial configuration is sane and does not "accept everything".

So, what is the secondary MX server that you are describing that "accepts everything" is based on?

I think he means that the secondary does not know the user names on the primary. Which it won't, unless someone maintains it, regardless of the server software.

On Fri, Feb 13, 2015 at 1:11 PM, Valeri Galtsev galtsev@kicp.uchicago.edu wrote:

So even though sendmail I heard is not a security disaster for long time already I'm quite happy with postfix.

Sendmail was pretty much all fixed by the time postfix was released, and made even better with the addition of the milter interface that lets you run scanning, etc. processes under different uids but able to participate in the smtp conversation. Postfix eventually got around to copying that too.

At some point even RedHat switched to postfix as default MX software on their system (not long ago though...).

Just another change for change's sake as far as I'm concerned. Sendmail continues to work just fine and the configs as shipped rarely take more than a few lines of change in the m4 file to do normal operations.

I guess, backup MX example makes me even happier: postfix really prevents you from doing wrong thing (making your backup MX a source of backscatter).

It's not postfix doing that, it is you, doing whatever has to be done to keep your lists in sync. Still, I don't see the point of even having a secondary MX. The days are long gone when chunks of the internet can't reach each other for long periods of time and anything sending should do its own queuing and retries. In fact if you do greylisting, you have forced all of your senders to prove it.

Karanbir Singh wrote:

On 13/02/15 18:45, Valeri Galtsev wrote:

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So, what is the secondary MX server that you are describing that "accepts everything" is based on?

if you actually read the thread you are replying to blindly, you might find out ?

:-)

On Fri, February 13, 2015 11:52 am, Les Mikesell wrote:

On Fri, Feb 13, 2015 at 11:39 AM, Valeri Galtsev galtsev@kicp.uchicago.edu wrote:

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Otherwise it accept junk that your primary rejects

Not exactly. If greylisting on primary is set, but on backup MX is not, still what is killed by greylisting by primary MX, almost never will come through backup MX. This is due to the same reason why greylisting is efficient: it trows off all that doesn't behave as mail server (thus never comes for re-delivery, and definitely doesn't try backup MX which real servers always do even before attempt of re-delivery).

I'm not convinced. Spam is big business and trying a 2nd MX is cheap.

I stated pure observation on at least two pairs of primary - backup MX I maintain. Still I made backup MXes with greylisting as well (they are separately hit by same bad spammers scripts, at a rate about 10 times smaller than primary MXes are and absolutely independently).

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Still, it is good to have the same greylisting on backup MX. And all other blows and whistles.

Greylisting would be kind of hard to do right. You'd have to keep the known-good senders in sync across the receivers. But my bigger worry would be a dictionary-type attack on user names as recipients if you don't have access to the real user list on the secondary.

With standard backup MX based on postix (with rather trivial configuration) you always do have list of legitimate recipients of primary MX on the secondary MX. Sorry if my previous e-mail is not explicit enough about it. It's a work, however, to maintain that table on backup MX (so your backup MX does accept mail for newly added users to primary MX). But having backup MX receiving everything is wrong configuration prone to backscatter - at least I see we agree on that. So, just don't roll out badly configured backup MX, I would say.

Valeri

Aside from the blowback of the bounces, if you've ever accepted an address it is likely to get on lists of known-good spam and cause extra traffic forever after.

-- Les Mikesell lesmikesell@gmail.com _______________________________________________ CentOS mailing list CentOS@centos.org http://lists.centos.org/mailman/listinfo/centos

++++++++++++++++++++++++++++++++++++++++ Valeri Galtsev Sr System Administrator Department of Astronomy and Astrophysics Kavli Institute for Cosmological Physics University of Chicago Phone: 773-702-4247 ++++++++++++++++++++++++++++++++++++++++

On Fri, February 13, 2015 12:41 pm, Les Mikesell wrote:

On Fri, Feb 13, 2015 at 12:32 PM, Valeri Galtsev galtsev@kicp.uchicago.edu wrote:

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I stated pure observation on at least two pairs of primary - backup MX I maintain. Still I made backup MXes with greylisting as well (they are separately hit by same bad spammers scripts, at a rate about 10 times smaller than primary MXes are and absolutely independently).

I think that's unusual - spammers often target the secondaries as a preference on the premise that they are likely to not be as well-configured as the primary. But it has been a while since I ran one so maybe things have changed.

Consider me lucky...

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Still, it is good to have the same greylisting on backup MX. And all other blows and whistles.

Greylisting would be kind of hard to do right. You'd have to keep the known-good senders in sync across the receivers. But my bigger worry would be a dictionary-type attack on user names as recipients if you don't have access to the real user list on the secondary.

With standard backup MX based on postix (with rather trivial configuration) you always do have list of legitimate recipients of primary MX on the secondary MX.

Doing greylisting right means you also have to keep the table of already-known senders up to date and that may be very dynamic.

If you are kind person, yes. Sqlgrey is designed to work simultaneously for primary, secondary (and tretary maybe - didn't check) MXes. Yet, even if they are independent, all will work, you are just not being nice to other servers and make them make 3 delivery attempts (the last is successful) instead of two (that is: primary MX - "temporary failure", secondary - "temporary failure", primary after some time - accepted; instead of primary MX - "temporary failure", secondary - accepted which will be in nice configuration common for both MXes greylisting engine and database).

Valeri

++++++++++++++++++++++++++++++++++++++++ Valeri Galtsev Sr System Administrator Department of Astronomy and Astrophysics Kavli Institute for Cosmological Physics University of Chicago Phone: 773-702-4247 ++++++++++++++++++++++++++++++++++++++++

On Fri, 2015-02-13 at 11:04 -0600, Les Mikesell wrote:

I'd recommend not having a secondary MX at all unless it is equipped to reject invalid users and spam in all the same ways as your primary. Otherwise it accept junk that your primary rejects and then you are obligated to send a bounce message which is always a bad thing - you want the authoritative receiver to reject at the smtp level instead of accepting at all. There's a whole category of spam where the real target is the apparent sender where a bounce will go. Also anything sending valid mail should be prepared to queue and retry on temporary failures just as well as your own secondary would.

On some domains I have 3 MXs - primary, secondary and tertiary - all share exactly the same coding, configuration and reporting. Absolutely no sense is weakening security for any MX although some spammers think the highest numbered MX is the weakest !