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forgeries.7 (2958B)

      1 .TH forgeries 7
      2 .SH "NAME"
      3 forgeries \- how easy it is to forge mail
      4 .SH "SUMMARY"
      5 An electronic mail message can easily be forged.
      6 Almost everything in it,
      7 including the return address,
      8 is completely under the control of the sender.
      9 
     10 An electronic mail message can be manually traced to its origin
     11 if (1) all system administrators of intermediate machines
     12 are both cooperative and competent,
     13 (2) the sender did not break low-level TCP/IP security,
     14 and
     15 (3) all intermediate machines are secure.
     16 
     17 Users of
     18 .I cryptography
     19 can automatically ensure the integrity and secrecy
     20 of their mail messages, as long as
     21 the sending and receiving machines are secure.
     22 .SH "FORGERIES"
     23 Like postal mail,
     24 electronic mail can be created entirely at the whim of the sender.
     25 .BR From ,
     26 .BR Sender ,
     27 .BR Return-Path ,
     28 and
     29 .BR Message-ID
     30 can all contain whatever information the sender wants.
     31 
     32 For example, if you inject a message through
     33 .B sendmail
     34 or
     35 .B qmail-inject
     36 or
     37 .BR SMTP ,
     38 you can simply type in a
     39 .B From
     40 field.
     41 In fact,
     42 .B qmail-inject
     43 lets you set up
     44 .BR MAILUSER ,
     45 .BR MAILHOST ,
     46 and
     47 .B MAILNAME
     48 environment variables
     49 to produce your desired
     50 .B From
     51 field on every message.
     52 .SH "TRACING FORGERIES"
     53 Like postal mail,
     54 electronic mail is postmarked when it is sent.
     55 Each machine that receives an electronic mail message
     56 adds a
     57 .B Received
     58 line to the top.
     59 
     60 A modern
     61 .B Received
     62 line contains quite a bit of information.
     63 In conjunction with the machine's logs,
     64 it lets a competent system administrator
     65 determine where the machine received the message from,
     66 as long as the sender did not break low-level TCP/IP security
     67 or security on that machine.
     68 
     69 Large multi-user machines often come with inadequate logging software.
     70 Fortunately, a system administrator can easily obtain a copy of a
     71 931/1413/Ident/TAP server, such as
     72 .BR pidentd .
     73 Unfortunately,
     74 some system administrators fail to do this,
     75 and are thus unable to figure out which local user
     76 was responsible for generating a message.
     77 
     78 If all intermediate system administrators are competent,
     79 and the sender did not break machine security or low-level TCP/IP security,
     80 it is possible to trace a message backwards.
     81 Unfortunately, some traces are stymied by intermediate system
     82 administrators who are uncooperative or untrustworthy.
     83 .SH "CRYPTOGRAPHY"
     84 The sender of a mail message may place his message into a
     85 .I cryptographic
     86 envelope stamped with his seal.
     87 Strong cryptography guarantees that any two messages with the same seal
     88 were sent by the same cryptographic entity:
     89 perhaps a single person, perhaps a group of cooperating people,
     90 but in any case somebody who knows a secret originally held
     91 only by the creator of the seal.
     92 The seal is called a
     93 .I public key\fR.
     94 
     95 Unfortunately, the creator of the seal is often an insecure machine,
     96 or an untrustworthy central agency,
     97 but most of the time seals are kept secure.
     98 
     99 One popular cryptographic program is
    100 .BR pgp .
    101 .SH "SEE ALSO"
    102 pgp(1),
    103 identd(8),
    104 qmail-header(8)