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Cisco PIX Firewall and VPN Configuration Guide
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Chapter 5 Configuring Application Inspection (Fixup)
Basic Internet Protocols
PIX Firewall Version 6.2 introduces full support for NAT and PAT of DNS messages originating from
either inside (more secure) or outside (less secure) interfaces. This means that if a client on an inside
network requests DNS resolution of an inside address from a DNS server on an outside interface, the
DNS A-record is translated correctly.
For example, in Figure 5-2, a client on the inside network issues an HTTP request to server
192.168.100.1, using its host name server.example.com. The address of this server is mapped through
PAT to a single ISP-assigned address 209.165.200.5. The DNS server resides on the ISP network.
Figure 5-2 NAT/PAT of DNS Messages
67605
Webserver
192.168.100.1
Webclient
PIX Firewall
ISP Internet
DNS server
When the request is made to the DNS server, the PIX Firewall translates the non-routable source address
in the IP header and forwards the request to the ISP network on its outside interface. When the DNS
A-record is returned, the PIX
Firewall applies address translation not only to the destination address, but
also to the embedded IP address of the web server. This address is contained in the user data portion of
the DNS reply packet. As a result, the web client on the inside network gets the address it needs to
connect to the web server on the inside network.
The transparent support for DNS in PIX Firewall Version 6.2 and higher means that the same process
works if the client making the DNS request is on a DMZ (or other less secure) network and the DNS
server is on an inside (or other more secure) interface.
FTP
You can use the fixup command to change the default port assignment for the File Transfer Protocol
(FTP). The command syntax is as follows:
[no] fixup protocol ftp [strict] [port]
The port parameter lets you configure the port at which the PIX Firewall listens for FTP traffic.
The strict option prevents web browsers from sending embedded commands in FTP requests. Each ftp
command must be acknowledged before a new command is allowed. Connections sending embedded
commands are dropped. The strict option only lets an FTP server generate the 227 command and only
lets an FTP client generate the PORT command. The 227 and PORT commands are checked to ensure
they do not appear in an error string.
If you disable FTP fixups with the no fixup protocol ftp command, outbound users can start connections
only in passive mode, and all inbound FTP is disabled.
Note The use of the strict option may break FTP clients that do not comply with the RFC standards.
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