January 2005
Intermediate to advanced
544 pages
23h 44m
English
Content preview from Linux Server Security, Second Edition
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Chapter 9: Securing Internet Email
The main difference between abuse and probing is intent: those who relay UCE
through your server probably don’t care about the server itself or the networks to
which it’s connected; they care only about whether they can use them for their own
purposes. But somebody who probes an SMTP server for usernames, group member-
ships, or debugging information is almost certainly interested in compromising that
SMTP server and the network on which it resides.
Historically, two SMTP commands specified by RFC 2821 (Simple Mail Transfer Pro-
tocol, available at ftp://ftp.isi.edu/in-notes/rfc2821.txt) have been prolific leakers of
such information: VRFY, which verifies whether a given username is valid on the sys-
tem and, if so, what the user’s full name is; and EXPN, which expands the specified
mailing-list name into a list of individual account names.
A third SMTP command, VERB, can be used to put some MTAs into “verbose”
mode. VERB is an Extended SMTP command and was introduced in RFC 1700
(Assigned Numbers). Since one of the guiding principles in IS security is “never reveal
anything to strangers unnecessarily,” you should not allow any publicly accessible
MTA server to run in verbose mode.
EXPN, VRFY,andVERB are throwbacks to a simpler time when legitimate ...
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