Cisco ASA Packet Process Algorithm

The interface that receives the packet is called the ingress interface and the interface through which the packet exits is called the egress interface. When referring to the packet flow through any device, it can be easily simplified by looking at the task in terms of these two interfaces.

Here is a diagram of how the Cisco ASA processes the packet that it receives:

Here are the individual steps in detail:

1. Packet is reached at the ingress interface.

2. Once the packet reaches the internal buffer of the interface, the input counter of the interface is incremented by one.

3. Cisco ASA will first verify if this is an existing connection by looking at its internal connection table details. If the packet flow matches an existing connection, then the access-control list (ACL) check is bypassed, and the packet is moved forward.

If packet flow does not match an existing connection, then TCP state is verified. If it is a SYN packet or UDP packet, then the connection counter is incremented by one and the packet is sent for an ACL check. If it is not a SYN packet, the packet is dropped and the event is logged.

4. The packet is processed as per the interface ACLs. It is verified in sequential order of the ACL entries and if it matches any of the ACL entries, it moves forward. Otherwise, the packet is dropped and the information is logged. The ACL hit count will be incremented by one when the packet matches the ACL entry.

5. The packet is verified for the translation rules. If a packet passes through this check, then a connection entry is created for this flow, and the packet moves forward. Otherwise, the packet is dropped and the information is logged.

6. The packet is subjected to an Inspection Check. This inspection verifies whether or not this specific packet flow is in compliance with the protocol. Cisco ASA has a built-in inspection engine that inspects each connection as per its pre-defined set of application-level functionalities. If it passed the inspection, it is moved forward. Otherwise, the packet is dropped and the information is logged.

Additional Security-Checks will be implemented if a CSC module is involved.

7. The IP header information is translated as per the NAT/PAT rule and checksums are updated accordingly. The packet is forwarded to AIP-SSM for IPS related security checks, when the AIP module is involved.

8. The packet is forwarded to the egress interface based on the translation rules. If no egress interface is specified in the translation rule, then the destination interface is decided based on global route lookup.

9. On the egress interface, the interface route lookup is performed. Remember, the egress interface is determined by the translation rule that will take the priority.

10. Once a Layer 3 route has been found and the next hop identified, Layer 2 resolution is performed. Layer 2 rewrite of MAC header happens at this stage.

11. The packet is transmitted on wire, and Interface counters increment on the egress interface.

Show Commands

Here are some useful commands that help in tracking the packet flow details at different stages of processing:

Show interface
Show conn
Show access-list
Show xlate
Show service-policy inspect
Show run static
Show run nat
Show run global
Show nat
Show route
Show arp

Syslog Messages

Syslog messages provide useful information about packet processing. Here are some example syslog messages for your reference:

Syslog message when there is no connection entry:

%ASA-6-106015: Deny TCP (no connection) from
IP_address/port to IP_address/port flags tcp_flags on interface
interface_name

Syslog message when the packet is denied by an access-list:

%ASA-4-106023: Deny protocol src
[interface_name:source_address/source_port] dst
interface_name:dest_address/dest_port by access_group
acl_ID

Syslog message when there is no translation rule is found:

%ASA-3-305005: No translation group found for protocol
src interface_name: source_address/source_port dst interface_name:
dest_address/dest_port

Syslog message when a packet is denied by Security Inspection:

%ASA-4-405104: H225 message received from
outside_address/outside_port to inside_address/inside_port before
SETUP

Syslog message when there is no route information:

%ASA-6-110003: Routing failed to locate next-hop for
protocol from src interface:src IP/src port to dest interface:dest IP/dest
port