IOS DoS defense causes DoS to itself:)

May 12, 2006 2 Replies

Can I somehow skip IOS fw maximum tcp half-open "sessions" control (DoS countermeasure) for certain amounts of traffic (matched by ACL)? I saw several times (including today) that internal hosts (mostly infected by virus) reaches upper threshold defined for half-opened connections and then router run into trouble with forwarding other legal traffic. If you then just remove ip inspect rule from interface then, for example web browsing performance comes to normal. So, it would be nice if I could only log excessive number of half-opened connections instead of terminating it.



Of course, Cisco TAC suggests that you block unnecessary outbound connections to keep half-opened conn. rate below upper threshold, but sometimes it's not acceptable - you don't want to block any traffic if you are not sure that this is a virus and this is my situation in which my routers are used in small ISP, so it's "unethically" to block customer traffic:)



B.R. Igor


You can adjust the max value for half-open sessions, and most other ip inspect values.

On a side note: If your policy is not to block traffic, then why use ip inspect on your customer traffic at all?

If you go with tuning (as I do) then you have to make these ip inspect values very high, but it would be nice if you could set up different values for a different types of traffic selected by acl or route-map.

I need ip inspect since my customers are using the same interfaces as I do and this IOS firewall protects my internal network.

B.R. Igor

Join the Discussion

Have something to add? Share your thoughts — no account required.

Didn't find your answer?

Ask the community — no account required