December 2006
Intermediate to advanced
1188 pages
72h 8m
English
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Filtering Based on TCP Header Flags
Problem
You want to filter on the flag bits in the TCP header.
Solution
The following ACL blocks several illegal combinations of TCP header flags:
Router1#configure terminalEnter configuration commands, one per line. End with CNTL/Z. Router1(config)#access-list161deny tcp any any ack fin psh rst syn urgRouter1(config)#access-list161deny tcp any any rst synRouter1(config)#access-list161deny tcp any any rst syn finRouter1(config)#access-list161deny tcp any any rst syn fin ackRouter1(config)#access-list161deny tcp any any syn finRouter1(config)#access-list161deny tcp any any syn fin ackRouter1(config)#endRouter1#
Beginning with IOS Version 12.3(4)T, Cisco changed the format of TCP flag filtering, while still maintaining support for the legacy method. The following is the same example as above using the new TCP flag filtering method:
Router2#configure terminalEnter configuration commands, one per line. End with CNTL/Z. Router2(config)#ip access-list extended TCPFLAGFILTERRouter2(config-ext-nacl)#deny tcp any any match-all +ack +fin +psh +rst +syn +urgRouter2(config-ext-nacl)#deny tcp any any match-all +rst +synRouter2(config-ext-nacl)#deny tcp any any match-all +rst +syn +finRouter2(config-ext-nacl)#deny tcp any any match-all +rst +syn +fin +ackRouter2(config-ext-nacl)#deny tcp any any match-all +syn +finRouter2(config-ext-nacl)#deny tcp any any match-all +syn +fin +ackRouter2(config-ext-nacl)#endRouter2#
Discussion
There ...
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O’Reilly covers everything we've got, with content to help us build a world-class technology community, upgrade the capabilities and competencies of our teams, and improve overall team performance as well as their engagement.Julian F.
I wanted to learn C and C++, but it didn't click for me until I picked up an O'Reilly book. When I went on the O’Reilly platform, I was astonished to find all the books there, plus live events and sandboxes so you could play around with the technology.Addison B.
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I'm always learning. So when I got on to O'Reilly, I was like a kid in a candy store. There are playlists. There are answers. There's on-demand training. It's worth its weight in gold, in terms of what it allows me to do.