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Suppose now that we decide to allow FTP services on our DMZ as well as HTTP It sounds simple enough Based on our HTTP example, all we need to know is the port number used by FTP, right We know that FTP uses TCP port 21, so we create an access list entry as shown: Access-list 101 permit TCP any 1601011 0000 eq 21 Outside clients can reach the FTP server, however, but their session hangs when they issue any commands Why Well, we must understand the way the FTP protocol operates Although it is true that FTP uses TCP port 21, it also uses TCP port 20 Port 21 is used for the transfer of FTP commands, while port 20 is used to transfer data To complicate matters, instead of the client simply opening an additional connection to port 20, the server initiates the connection from port 20 to the clients' randomly chosen port above 1023 The client then sends the port information to the server over the established data channel Therefore, we need to modify our access list to account for the use of this additional port number: Access-list 101 permit TCP any 1601011 0000 eq 21 Access-list 101 permit TCP any 1601011 0000 eq 20 A point worth noting is that since we are allowing access from outside FTP clients to our FTP server, the access list entries are fairly simple What if our internal clients attempted to contact FTP servers on the Internet Recall that earlier we used the following access list entry to permit return traffic to our internal hosts:
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Access-list 101 permit tcp any 1601100 00255255 gt 1023 established Will this entry permit our clients to use the FTP protocol Unfortunately, the answer is no To see why, remember that we stated previously that the FTP server initiates the return connection to the clients This would mean that FTP servers on the Internet would initiate a new connection back to our FTP clients Although the destination port number would be greater than 1023, the ACK or RST bit would not be set Consequently, the packet would fail to pass the established keyword check and would be denied We must add a new access list entry: Access-list 101 permit tcp any eq 20 1601100 00255255 gt 1023 Access-list 101 permit tcp any 1601100 00255255 gt 1023 established Note the use of 20 as the source port and the lack of the established keyword in this example Although we show the new access list entry prior to the previous entry, we could have placed the new entry after the previous entry Recall our earlier discussion regarding the placement of access list entries We see that the new entry appears to be more specific than our previous entry, yet the previous entry used the established keyword and the new entry does not Therefore, the new entry does not match a subset of packets matched by the previous entry Many protocols use multiple ports in a manner similar to FTP As stated earlier, these applications are called "multi-channel" Creating filters for these applications is very challenging with traditional access lists When we examine next-generation access lists in 8, "Advanced Cisco Router Security Features," we will see that context-based access control provides features to deal securely with many multi-channel protocols Let's now examine the access list in its entirety up to this point Figure 7 8 can be referenced as a reminder Interface serial 0 Ip access-group 101 in Access-list 101 permit tcp any 1601011 eq 80 Access-list 101 permit tcp any 1601011 eq 21 Access-list 101 permit tcp any 1601011 eq 20 Access-list 101 permit tcp any eq 20 1601100 00255255 gt 1023 Access-list 101 permit tcp any 1601100 00255255 gt 1023 established So far, our access list has dealt only with applications using TCP as their transport Let's now turn to an application that uses UDP as its transport protocol
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