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.TH s/qmail: forgeries 7
.SH "NAME"
forgeries \- how easy it is to forge mail
.SH "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 cooperative and competent,
(2) the sender did not break low-level TCP/IP security,
and
(3) all intermediate machines are secure.
Users of
.I cryptography
can automatically ensure the integrity and secrecy
of their mail messages, as long as
the sending and receiving machines are secure.
.SH "FORGERIES"
Like postal mail,
electronic mail can be created entirely at the whim of the sender.
.BR From ,
.BR Sender ,
.BR Return-Path ,
and
.BR Message-ID
can all contain whatever information the sender wants.
For example, if you inject a message through
.B sendmail
or
.B qmail-inject
or
.BR SMTP ,
you can simply type in a
.B From
field.
In fact,
.B qmail-inject
lets you set up
.BR MAILUSER ,
.BR MAILHOST ,
and
.B MAILNAME
environment variables
to produce your desired
.B From
field on every message.
.SH "TRACING FORGERIES"
Like postal mail,
electronic mail is postmarked when it is sent.
Each machine that receives an electronic mail message
adds a
.B Received
line to the top.
A modern
.B Received
line contains quite a bit of information.
In conjunction with the machine's logs,
it lets a competent system administrator
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
.BR 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.
.SH "CRYPTOGRAPHY"
The sender of a mail message may place his message into a
.I 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 cooperating people,
but in any case somebody who knows a secret originally held
only by the creator of the seal.
The seal is called a
.I public key\fR.
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
.BR pgp .
.SH "SEE ALSO"
pgp(1),
identd(8),
qmail-header(8)
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