Cisco ASA 5505 - Manual (Page 910)

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Cisco ASA 5500 Series Configuration Guide using the CLI
Chapter 43 Configuring Inspection of Basic Internet Protocols
TFTP Inspection
TFTP Inspection
TFTP inspection is enabled by default.
TFTP, described in RFC 1350, is a simple protocol to read and write files between a TFTP server and
client.
The ASA inspects TFTP traffic and dynamically creates connections and translations, if necessary, to
permit file transfer between a TFTP client and server. Specifically, the inspection engine inspects TFTP
read request (RRQ), write request (WRQ), and error notification (ERROR).
A dynamic secondary channel and a PAT translation, if necessary, are allocated on a reception of a valid
read (RRQ) or write (WRQ) request. This secondary channel is subsequently used by TFTP for file
transfer or error notification.
Only the TFTP server can initiate traffic over the secondary channel, and at most one incomplete
secondary channel can exist between the TFTP client and server. An error notification from the server
closes the secondary channel.
TFTP inspection must be enabled if static PAT is used to redirect TFTP traffic.