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Cisco IOS firewall - ip inspect name OUTSIDE TCP

Posted on 2008-10-20
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Last Modified: 2008-11-18
I am trying to make sense of the IP INSPECT statement of the Cisco IOS firewall. What does it mean when I have IP INSPECT NAME OUTSIDE TCP? or IP INSPECT NAME OUTSIDE ICMP? Does it mean that after the packet pass through the access list, it will be inspected by the IOS firewall and it the packet is not a TCP connection or an ICMP packet, the packet will be dropped? Thx
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Question by:netdoc01
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by:kyleb84
ID: 22764264
It means that it will keep track of TCP connections, inspecting packets for abnormalities and/or possible attacks.

Same applies to ICMP, the firewall will inspect the packet, make sure everything looks ok in it, then pass it on.

If the packet is deemed bad, corrupt or invalid - even part of a possible attack, it will then be dropped.
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by:kyleb84
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For example, if you do not include a

 "IP INSPECT NAME OUTSIDE UDP"

The firewall will not take any notice of UDP packets, and just pass them straight on to the destination.
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by:netdoc01
ID: 22764328
"inspecting packets for abnormalities and/or possible attacks"

How can it determine if a packet is bad like you have mentioned above?
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kyleb84 earned 500 total points
ID: 22764370
For TCP connections, each packet has a set of "flags" in the header that control a connection, you wouldn't - for example see all SYN + RST + ACK flags set in the one packet (SYN = do a connect, RST = reset, ACK = acknowledge).

ICMP packets can contain invalid requests (Can't think of an example).

Some mixture of invalid flags / requests are well known attacks, other instances might just be data corruption that happened on the way.

The bottom line is that when it inspect TCP - it only looks at the TCP info header, not the data therein.

The firewall CAN do layer 4+ inspection as well, which actually takes a look at the data inside the packet and not just the header.

Layer 4+ protocols like SIP, HTTP, and FTP can all be monitored as well. This involves keeping track of such connections deep inside your TCP / UDP packet.

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