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(March 1996)
 

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Subject: Cascading help messages loop
From: bollow @ math . ethz . ch
Date: Thu, 7 Mar 1996 11:04:45 +0100
To: list-managers @ greatcircle . com
Cc: drobayo @ posh . internext . com

Greetings!

I've recently moved the mailing list which I run from MXSERVER@BIBLE.ACU.EDU
to Majordomo@genesis.acu.edu. Soon after than, a mail loop between the two
computers started. If have now understood how this came about, and I've
decided to share this experience, for the warning of the unwary :)

a) A user sees information about the CHURCHPLANTERS list on a web
   page and is interested. That web page allows to create a subscription
   request and send it to the old server address. (I really hate those
   WWW-email gateways, they've caused me trouble before.)

b) The user gets a message from the old, disabled list which states that
   the list has moved and that he can subscribe to CHURCHPLANTERS by
   sending mail to Majordomo@genesis.acu.edu 

c) Instead of composing a new message, the user uses the "forward" feature 
   of his mailreader to send the following command to Majordomo:

   subscribe churchplanters

   The mailreader sends this message with a "From:" header field like this:

   From: MX mailing list processor <MXserver@BIBLE.ACU.EDU> (by way of drobayo@posh.internext.com (Daniel D. Robayo))

   Not being experienced in the ways of e-mail, the user suspects nothing.

d) Majordomo sees the From: header and interprets the subscription request
   as follows: 

subscribe churchplanters MX mailing list processor <MXserver@BIBLE.ACU.EDU> (by way of drobayo@posh.internext.com (Daniel D. Robayo))

   As the maillog indicates, the envelope-From of this request was 
   <drobayo@posh.internext.com>. Unfortunately, the address
   drobayo@posh.internext.com was included in a comment to the subscribed
   address, and for this reason Majordomo accepted the request. I consider 
   this behavior to be a bug of Majordomo, because RFC 822 specifies:

   RFC822> Comments should be retained while the message is subject to 
   RFC822> interpretation according to this standard. However, comments
   RFC822> must NOT be included in other cases, such as during protocol
   RFC822> exchanges with mail servers.

   This means (by my interpretation) that Majordomo is REQUIRED by the
   standard to strip comments from the address before checking whether
   it matches with the envelope-From. For my list, the subscription mode
   is set to "open", hence subscription requests are automatically
   honoured if the subscribed address "matches" the envelope-From.

e) A welcome message is mailed out to the new subscriber
   MXserver@BIBLE.ACU.EDU. This welcome message contains examples of
   commands which people could send to Majordomo.

f) This starts a nasty loop between Majordomo@genesis.acu.edu and
   MXSERVER@bible.acu.edu in which the two computers frantically
   scream their help messages at each other. With every iteration of
   the loop, the number of messages is doubled. (I call such a thing
   a loop cascade.) Hundreds of error messages were generated and sent
   to the majordomo admins, but fortunately not to the list subscribers.

NEEDLESS TO SAY, I've now taken some measures which should suffice to
prevent this kind of thing from happening again.

Shalom, Norbert.

-- 
Norbert Bollow, Zuerich, Switzerland.    Admin of the CHURCHPLANTERS list
Email-To: bollow@math.ethz.ch                  PGP encrypted mail welcome
PGP public key & WWW homepage of CHURCHPLANTERS  at  http://pobox.com/~nb

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