cisco-asa-syslog

Parse, interpret, and analyze Cisco ASA (Adaptive Security Appliance) firewall syslog messages. Use this skill whenever working with Cisco ASA log files, syslog streams from ASA devices, firewall event analysis, or security investigations involving ASA-generated events. Covers the syslog protocol foundation and the ASA-specific message format with message ID categorization.

Cisco ASA Syslog Analysis

This skill teaches you how to read, parse, and interpret log messages generated by Cisco ASA firewalls. All ASA logs are delivered via the syslog protocol, so understanding syslog is foundational before working with ASA-specific content.

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Part 1: Syslog Protocol Foundation

All Cisco ASA log messages are transported and formatted using the syslog standard (RFC 3164 / RFC 5424). You must understand syslog before interpreting ASA messages.

Severity Levels

Every syslog message carries a numeric severity (0 = most critical, 7 = least critical):

Level	Keyword	Meaning
0	Emergency	System is unusable
1	Alert	Immediate action required
2	Critical	Critical conditions
3	Error	Error conditions
4	Warning	Warning conditions
5	Notice	Normal but significant events
6	Informational	Routine informational messages
7	Debug	Verbose diagnostic output

Facility Codes

Facility codes indicate the source subsystem (0-23). Cisco ASA uses local4 (code 20) by default, but this is configurable on the device.

PRI Value

The PRI header encodes both facility and severity:

PRI = (Facility x 8) + Severity

Example: local4 (20) + Informational (6) -> (20 x 8) + 6 = 166, shown as <166> in the log header. This is the most common PRI value seen in ASA logs.

RFC 3164 Format (Traditional)

TIMESTAMP HOSTNAME TAG: MESSAGE

Example: <166>Jan 15 10:22:01 fw01 : %ASA-6-302013: Built outbound TCP connection...

RFC 5424 Format (ASA 9.10+)

Enabled via "logging timestamp rfc5424". Uses ISO 8601 UTC timestamps:

VERSION TIMESTAMP HOSTNAME APP-NAME PROCID MSGID STRUCTURED-DATA MSG

Example: <166>1 2018-06-27T12:17:46Z asa - - - %ASA-6-110002: Failed to locate egress interface...

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Part 2: Cisco ASA Log Format

Core Message Structure

Every ASA event message follows this pattern, regardless of syslog envelope:

%ASA-severity-message_id: message_text

Field	Description
%ASA	Fixed literal prefix on all ASA messages
severity	Single digit 0-7 matching syslog severity levels above
message_id	6-digit numeric ID uniquely identifying the event type
message_text	Human-readable description with variable substitutions

Full Log Line with Syslog Envelope

TIMESTAMP HOSTNAME : %ASA-severity-message_id: message_text

Real example (RFC 3164): <166>Jan 15 10:22:01 fw01 : %ASA-6-302013: Built outbound TCP connection 12345 for outside:203.0.113.1/443 (203.0.113.1/443) to inside:192.168.1.100/54321 (192.168.1.100/54321)

Real example (RFC 5424): <166>1 2024-01-15T10:22:01Z fw01 - - - %ASA-6-302013: Built outbound TCP connection...

Message ID Numbering and Categories

The 6-digit message ID encodes the functional category in its first 3 digits:

Range	Category
1xxxxx	System, interfaces, failover, hardware
2xxxxx	VPN failover, high availability
3xxxxx	Firewall, connection tracking, NAT
4xxxxx	Security, IPS, ACL, threat detection
5xxxxx	User authentication, identity
6xxxxx	VPN crypto, IPSec, SSL, remote access
7xxxxx	Application inspection, protocol handling
8xxxxx	Management, platform, clustering

Common Variable Tokens

ASA message text uses descriptive named placeholders:

Token	Meaning
interface_name	ASA interface (e.g., inside, outside)
src_ip/port	Source IP and port
dst_ip/port	Destination IP and port
protocol	IP protocol (TCP, UDP, ICMP, etc.)
conn_id	Unique connection tracking number
direction	inbound or outbound
reason	Cause of event (e.g., SYN Timeout, FIN)
username	Authenticated user identity
group	VPN tunnel group or policy name
access_list	Name of the ACL that matched
action	permit or deny

Parsing Regex

Extract the core ASA fields from any log line:

%ASA-(?P\d)-(?P<message_id>\d{6}):\s+(?P<message_text>.+)

Full line including syslog envelope:

^(?P\S+\s+\S+\s+\S+|\S+T\S+Z)\s+(?P\S+)\s+:\s+%ASA-(?P\d)-(?P<message_id>\d{6}):\s+(?P<message_text>.+)$

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Guidelines

• Always extract the message_id first -- it uniquely identifies the event type
• Check severity immediately: levels 1-3 indicate critical/error conditions requiring attention; 6-7 are routine
• Use the first 3 digits of the message_id to identify the functional area before looking up details
• ASA logs are interface-aware -- always note which interface (inside, outside, dmz) events occur on, as it provides essential network direction context
• Watch for paired messages: ASA often emits build/teardown pairs (e.g., 302013 "Built" + 302014 "Teardown") that together describe the full lifecycle of a connection
• For detailed per-message-ID definitions (message text templates, variable meanings, recommended actions), consult the reference files indexed below

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Reference Files

Detailed documentation for each message ID is maintained in separate files organized by range, matching Cisco's official documentation structure:

File	Message ID Range
references/Syslog Messages 101001 to 199027.md	101001 - 199027
references/Syslog Messages 201002 to 219002.md	201002 - 219002
references/Syslog Messages 302003 to 342008.md	302003 - 342008
references/Syslog Messages 400000 to 450002.md	400000 - 450002
references/Syslog Messages 500001 to 520025.md	500001 - 520025
references/Syslog Messages 602101 to 622102.md	602101 - 622102
references/Syslog Messages 701001 to 714011.md	701001 - 714011
references/Syslog Messages 715001 to 721019.md	715001 - 721019
references/Syslog Messages 722001 to 776020.md	722001 - 776020
references/Syslog Messages 776201 to 833333.md	776201 - 833333

For serverity levels, refer to references/Messages Listed by Severity Level.md

For overall information of Cisco ASA syslog messages, refer to references/About Cisco Secure Firewall ASA.md

Official Cisco source: https://www.cisco.com/c/en/us/td/docs/security/asa/syslog/asa-syslog.html