forgeries(7) [debian man page]

forgeries(7)						 Miscellaneous Information Manual					      forgeries(7)

NAME

       forgeries - how easy it is to forge mail

SUMMARY

       An electronic mail message can easily be forged.  Almost everything in it, including the return address, is completely under the control of
       the sender.

       An electronic mail message can be manually traced to its origin if (1) all system administrators of intermediate machines are both coopera-
       tive and competent, (2) the sender did not break low-level TCP/IP security, and (3) all intermediate machines are secure.

       Users  of  cryptography	can  automatically  ensure  the integrity and secrecy of their mail messages, as long as the sending and receiving
       machines are secure.

FORGERIES

       Like postal mail, electronic mail can be created entirely at the whim of the sender.  From, Sender, Return-Path,  and  Message-ID  can  all
       contain whatever information the sender wants.

       For  example, if you inject a message through sendmail or qmail-inject or SMTP, you can simply type in a From field.  In fact, qmail-inject
       lets you set up MAILUSER, MAILHOST, and MAILNAME environment variables to produce your desired From field on every message.

TRACING FORGERIES

       Like postal mail, electronic mail is postmarked when it is sent.  Each machine that receives an electronic mail	message  adds  a  Received
       line to the top.

       A modern Received line contains quite a bit of information.  In conjunction with the machine's logs, it lets a competent system administra-
       tor determine where the machine received the message from, as long as the sender did not break low-level TCP/IP	security  or  security	on
       that machine.

       Large  multi-user  machines often come with inadequate logging software.  Fortunately, a system administrator can easily obtain a copy of a
       931/1413/Ident/TAP server, such as pidentd.  Unfortunately, some system administrators fail to do this, and are thus unable to  figure  out
       which local user was responsible for generating a message.

       If  all intermediate system administrators are competent, and the sender did not break machine security or low-level TCP/IP security, it is
       possible to trace a message backwards.  Unfortunately, some traces are stymied by intermediate system administrators who are  uncooperative
       or untrustworthy.

CRYPTOGRAPHY

       The  sender  of	a  mail message may place his message into a cryptographic envelope stamped with his seal.  Strong cryptography guarantees
       that any two messages with the same seal were sent by the same cryptographic entity: perhaps a single person, perhaps a group of  cooperat-
       ing people, but in any case somebody who knows a secret originally held only by the creator of the seal.  The seal is called a public key.

       Unfortunately,  the  creator  of  the seal is often an insecure machine, or an untrustworthy central agency, but most of the time seals are
       kept secure.

       One popular cryptographic program is pgp.

SEE ALSO

       pgp(1), identd(8), qmail-header(8)

																      forgeries(7)

envelopes(5)							File Formats Manual						      envelopes(5)

NAME

       envelopes - sender/recipient lists attached to messages

INTRODUCTION

       Electronic mail messages are delivered in envelopes.

       An  envelope  lists a sender and one or more recipients.  Usually these envelope addresses are the same as the addresses listed in the mes-
       sage header:

	  (envelope) from djb to root
	  From: djb
	  To: root

       In more complicated situations, though, the envelope addresses may differ from the header addresses.

ENVELOPE EXAMPLES

       When a message is delivered to several people at different locations, it is first photocopied and placed into several envelopes:

	  (envelope) from djb to root
	  From: djb			     Copy #1 of message
	  To: root, god@brl.mil

	  (envelope) from djb to god@brl.mil
	  From: djb			     Copy #2 of message
	  To: root, god@brl.mil

       When a message is delivered to several people at the same location, the sender doesn't have to photocopy it.  He can instead stuff it  into
       one envelope with several addresses; the recipients will make the photocopy:

	  (envelope) from djb to god@brl.mil, angel@brl.mil
	  From: djb
	  To: god@brl.mil, angel@brl.mil, joe, frde

       Bounced	mail  is sent back to the envelope sender address.  The bounced mail doesn't list an envelope sender, so bounce loops are impossi-
       ble:

	  (envelope) from <> to djb
	  From: MAILER-DAEMON
	  To: djb
	  Subject: unknown user frde

       The recipient of a message may make another copy and forward it in a new envelope:

	  (envelope) from djb to joe
	  From: djb			     Original message
	  To: joe

	  (envelope) from joe to fred
	  From: djb			     Forwarded message
	  To: joe

       A mailing list works almost the same way:

	  (envelope) from djb to sos-list
	  From: djb			     Original message
	  To: sos-list

	  (envelope) from sos-owner to god@brl.mil
	  From: djb			     Forwarded message
	  To: sos-list			     to recipient #1

	  (envelope) from sos-owner to frde
	  From: djb			     Forwarded message
	  To: sos-list			     to recipient #2

       Notice that the mailing list is set up to replace the envelope sender with something new, sos-owner.  So bounces will  come  back  to  sos-
       owner:

	  (envelope) from <> to sos-owner
	  From: MAILER-DAEMON
	  To: sos-owner
	  Subject: unknown user frde

       It's  a	good  idea  to	set  up  an extra address, sos-owner, like this: the original envelope sender (djb) has no way to fix bad sos-list
       addresses, and of course bounces must not be sent to sos-list itself.

HOW ENVELOPE ADDRESSES ARE STORED

       Envelope sender and envelope recipient addresses are transmitted and recorded in several ways.

       When a user injects mail through qmail-inject, he can supply a Return-Path line or a -f option for the  envelope  sender;  by  default  the
       envelope  sender  is  his  login  name.	The envelope recipient addresses can be taken from the command line or from various header fields,
       depending on the options to qmail-inject.  Similar comments apply to sendmail.

       When a message is transferred from one machine to another through SMTP, the envelope sender is given in a MAIL FROM command,  the  envelope
       recipients are given in RCPT TO commands, and the message is supplied separately by a DATA command.

       When a message is delivered by qmail to a single local recipient, qmail-local records the recipient in Delivered-To and the envelope sender
       in Return-Path.	It uses Delivered-To to detect mail forwarding loops.

       sendmail normally records the envelope sender in Return-Path.  It does not record envelope recipient addresses, on the theory that they are
       redundant: you received the mail, so you must have been one of the envelope recipients.

       Note  that, if the header doesn't have any recipient addresses, sendmail will move envelope recipient addresses back into the header.  This
       situation occurs if all addresses were originally listed as Bcc, since Bcc is automatically removed.  When sendmail sees this, it creates a
       new  Apparently-To  header  field with the envelope recipient addresses.  This has the strange effect that each blind-carbon-copy recipient
       will see a list of all recipients on the same machine.

       When a message is stored in mbox format, the envelope sender is recorded at the top of the message as a UUCP-style From	(no  colon)  line.
       Note that this line is less reliable than the Return-Path line added by qmail-local or sendmail.

SEE ALSO

       qmail-header(5), qmail-local(8), qmail-inject(8)

																      envelopes(5)