Hi everyone,
Recently our network experience a Internal DoS attack. One internal server ( the network/security team doesnt have any access to the adninistration of these server) starts to send a lot of DNS bogus request to some DNS servers on the Internet. With sh conn detail we saw the IP of these server and blocked it with an ACL in the Internal ASA 5520 interface. After that, the server team disconnect the server, and made their job cleaning these infected device. Everything goes normal again....
Today, the same server starts again with the same problem. But a lot worst thant the first time. The ASA starts to drops packets in the internal interface, the overruns was increasing dramatically ( like 10000 per second), the asp-drop table shows the same amount of traffic than interface overruns in the ACL-Drop line , and the CNT blocks for 16xxx with sh blocks was in zero. The sh acess-list INSIDE shows near 9 million hints in the line that deny the DNS request from the server to the Internet. Again, we disconnect the server and the problem was solved by the server team.
It seems that our ASA cant handle in their internal interface the amount of traffic that these server send outbound. IS there anyway to raise the blocks in the firewall? What is the best way to deny the servers connections ( ACL, or MPF or threat detection maybe), and avoid the ASA interface overruns even when the server sends these large amount of request.
Regards.
