08-29-2001
Actually I thought about this myself lately... the reason you'd maybe want to change your ip is if your machine is on a multi-homed natted box and recieving mail from an internal LAN. Then it will report the internal address on the header, unless you do some more natting (madness in my eyes, since multiple NATS can be dodgy).
I don't want MR.Hacker to know the ip address of my internal machines ... do I .
Just change the macro in the sendmail.cf... oh and buy the Bat book just for extreme measure
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LEARN ABOUT ULTRIX
forgeries
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)