Managing Deny Flows - Cisco PIX 500 Series Configuration Manual

Chapter 16 Identifying Traffic with Access Lists
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For example, you configure the following access list:
hostname(config)# access-list outside-acl permit ip host 1.1.1.1 any log 7 interval 600
hostname(config)# access-list outside-acl permit ip host 2.2.2.2 any
hostname(config)# access-list outside-acl deny ip any any log 2
hostname(config)# access-group outside-acl in interface outside
When a packet is permitted by the first ACE of outside-acl, the security appliance generates the
following system message:
%ASA|PIX-7-106100: access-list outside-acl permitted tcp outside/1.1.1.1(12345) ->
inside/192.168.1.1(1357) hit-cnt 1 (first hit)
Although 20 additional packets for this connection arrive on the outside interface, the traffic does not
have to be checked against the access list, and the hit count does not increase.
If one more connection by the same host is initiated within the specified 10 minute interval (and the
source and destination ports remain the same), then the hit count is incremented by 1 and the following
message is displayed at the end of the 10 minute interval:
%ASA|PIX-7-106100: access-list outside-acl permitted tcp outside/1.1.1.1(12345)->
inside/192.168.1.1(1357) hit-cnt 2 (600-second interval)
When a packet is denied by the third ACE, the security appliance generates the following system
message:
%ASA|PIX-2-106100: access-list outside-acl denied ip outside/3.3.3.3(12345) ->
inside/192.168.1.1(1357) hit-cnt 1 (first hit)
20 additional attempts within a 5 minute interval (the default) result in the following message at the end
of 5 minutes:
%ASA|PIX-2-106100: access-list outside-acl denied ip outside/3.3.3.3(12345) ->
inside/192.168.1.1(1357) hit-cnt 21 (300-second interval)

Managing Deny Flows

When you enable logging for message 106100, if a packet matches an ACE, the security appliance
creates a flow entry to track the number of packets received within a specific interval. The security
appliance has a maximum of 32 K logging flows for ACEs. A large number of flows can exist
concurrently at any point of time. To prevent unlimited consumption of memory and CPU resources, the
security appliance places a limit on the number of concurrent deny flows; the limit is placed only on deny
flows (and not permit flows) because they can indicate an attack. When the limit is reached, the security
appliance does not create a new deny flow for logging until the existing flows expire.
For example, if someone initiates a DoS attack, the security appliance can create a large number of deny
flows in a short period of time. Restricting the number of deny flows prevents unlimited consumption of
memory and CPU resources.
When you reach the maximum number of deny flows, the security appliance issues system message
106100:
%ASA|PIX-1-106101: The number of ACL log deny-flows has reached limit (numbe r).
OL-12172-03
interval secs—The time interval in seconds between system messages, from 1 to 600. The default
is 300. This value is also used as the timeout value for deleting an inactive flow.
disable—Disables all access list logging.
default—Enables logging to message 106023. This setting is the same as having no log option.
Cisco Security Appliance Command Line Configuration Guide
Logging Access List Activity
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