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Best way to allow FTP in IOS access-list


what is the best way to allow FTP xfer's on high ports numbers through an IOS based ACL? I want to tell the router only to allow FTP transfer on the high port numbers, not anything else. Is this possible?

For example I use the following extended ACL commands for a particualr host :

access-list 120 permit tcp any host range ftp-data ftp

access-list 120 permit tcp any host gt 1400

Without the second line I am not able to get the data through. Ie I can log onto a FTP site, but can't get any data, since FTP "overflows" onto higher port numbers. I thought there was a particualr range that FTP uses for this "overflow" but I don't think that this is true.

The problem is that the router/hosts are now obviously volunerable to attacks on port numbers higher than 1400.

Anyway, hope this is possible in IOS ...

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Cisco Employee

Re: Best way to allow FTP in IOS access-list

Your ACL is right, but as you said, you are going to open for all ports even when no FTP traffic and anyone can exploit the ports.

The best way to do this on IOS, is use IOS Firewall (CBAC) which inspects on protocol-basis such as TCP, UDP, FTP, HTTP etc and dynamically opens holes as traffic returns back to your router, you do not have to permit all the ports. Here is few example and reading URLs about CBAC



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