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Network Warrior, 2nd Edition by Gary A. Donahue

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Inspects

Fixups were the features on a PIX firewall that inspected application protocols. They have been replaced with application inspections, or inspects for short. While fixups were much easier to configure, inspects follow a more IOS-like method of implementation in that they are embedded into a global service policy. It might look difficult, but it doesn’t need to be.

Inspects are used to enable complex protocols, such as FTP, that have multiple streams. They are also used to make protocols more secure. For example, the SMTP inspect limits the commands that can be run through the ASA within the SMTP protocol.

To illustrate one of the common application inspections, I’ve connected through an ASA to a mail server using Telnet. This ASA is not running the SMTP fixup, which you should rarely see in the wild, as it is enabled by default. When I issue the SMTP command EHLO someserver, I get a list of information regarding the capabilities of the mail server:

[GAD@someserver GAD]$telnet mail.myserver.net 25
Trying 10.10.10.10...
Connected to mail.myserver.net.
Escape character is '^]'.
220 mail.myserver.net ESMTP Postfix
EHLO someserver
250-mail.myserver.net
250-PIPELINING
250-SIZE 10240000
250-ETRN
250 8BITMIME

This information is not necessary for the successful transfer of email messages, and could be useful to a hacker. For example, a hacker could try to pull email messages off the server using the ETRN deque command. The SMTP inspect intercepts and disables the ETRN command.

Note ...

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