Parsing Syslog Messages

March 10, 2024 by Prabhat Sharma


In the world of information technology and cybersecurity, the ability to collect, analyze, and understand log data is crucial for maintaining the health and security of IT environments. Syslog is a standard for message logging that allows a computer system to send event notification messages across IP networks to event message collectors – also known as syslog servers or syslog daemons. Parsing these syslog messages is vital for several reasons, including security monitoring, troubleshooting, and compliance with regulatory standards. This blog will explore the importance of syslog parsing, how to use the parse_syslog function in OpenObserve, address challenges with non-RFC compliant syslog messages from devices like MikroTik routers, and demonstrate parsing them using the parse_regex function. syslog messages can be ingested into OpenObserve using syslog-ng, otel-collector, fluentbit or vector.

Why is Syslog Parsing Important?

Syslog parsing is important for several reasons:

Security Monitoring: By analyzing syslog messages, organizations can detect suspicious activities and potential security threats in real-time. This allows for immediate response to mitigate risks.

Troubleshooting: Syslog messages provide valuable insights into system behaviors and errors. Parsing these logs can help in quickly identifying and resolving issues affecting system performance and availability.

Compliance: Many regulatory standards require the collection, analysis, and archiving of logs to ensure data integrity and security. Proper syslog parsing aids in meeting these compliance requirements.

Operational Efficiency: Through the systematic analysis of syslog data, organizations can optimize their operations, improve system performance, and reduce downtime.

Let's take an example syslog message:

<34>1 2024-03-09T14:55:22.123Z evntslog - ID47 [exampleSDID@32473 iut="3" eventSource="Application" eventID="1011"] An application event occurred.

This can be broken down into the following components:

  1. Priority (<34>): This value is encapsulated within angle brackets at the very beginning of the message. The priority value is calculated by (facility value * 8) + severity value. Facilities represent the type of program that is logging the message, while severity indicates the importance of the event. In this case, 34 indicates a specific facility and severity level.
  2. Version (1): This number immediately follows the priority and indicates the syslog protocol version used. 1 represents the use of the RFC 5424 syntax.
  3. Timestamp (2024-03-09T14:55:22.123Z): This is the date and time when the event was generated, following the ISO 8601 format. The Z indicates that this time is in UTC.
  4. Hostname ( The name of the device or server generating the event. This helps in identifying the source of the message in a network.
  5. Application Name (evntslog): The name of the application or process that generated the message.
  6. Process ID (-): A hyphen in this position indicates that the process ID is not provided. If available, this field helps to identify the specific process within the source that generated the message.
  7. Message ID (ID47): An identifier for the type of message being reported. This helps in classifying and filtering messages based on their nature or importance.
  8. Structured Data (exampleSDID@32473 iut="3" eventSource="Application" eventID="1011"): This optional field provides a mechanism to transmit event information in a structured format. It can include multiple pieces of data, such as event source, event ID, and any other context-relevant information, encapsulated within square brackets. The exampleSDID@32473 is a unique identifier for the structured data element, followed by a series of key-value pairs.
  9. Message Text (An application event occurred.): The free-form message text provides a human-readable description of the event or error. This part of the syslog message conveys the specific details about what occurred.

Using parse_syslog in OpenObserve

OpenObserve offers the parse_syslog function through Vector Remap Language (VRL), enabling users to extract structured data from syslog messages easily. This function automatically parses the priority, facility, severity, timestamp, hostname, and message from a syslog string, according to the RFC 6587, RFC 5424 and RFC 3164 standards.

Consider a syslog example message discussed earlier:

"event": "<34>1 2024-03-09T14:55:22.123Z evntslog - ID47 [exampleSDID@32473 iut=\"3\" eventSource=\"Application\" eventID=\"1011\"] An application event occurred."

To parse this message using the parse_syslog function in OpenObserve, you would simply apply the function to the message:

.event = parse_syslog!(.event)

will result in:

    "event": {
        "appname": "evntslog",
        "exampleSDID@32473": {
            "eventID": "1011",
            "eventSource": "Application",
            "iut": "3"
        "facility": "auth",
        "hostname": "",
        "message": "An application event occurred.",
        "msgid": "ID47",
        "severity": "crit",
        "timestamp": "2024-03-09T14:55:22.123Z",
        "version": 1

There are cases where the built-in parse_syslog function may not be effective in parsing syslog messages, especially when the messages are not RFC compliant. In such cases, the parse_regex function can be used to extract meaningful data from syslog messages.

Parsing Non-RFC Compliant Syslog Messages

Some deices like MikroTik routers are known to generate non-RFC compliant syslog messages. These messages do not adhere to the standard syslog format and require custom parsing to extract meaningful data. Let's consider the following syslog messages generated by a MikroTik router:

2024-03-04T21:58:06.625318+00:00 testing-router firewall,info input: in:ether5-S2S out:(unknown 0), connection-mark:to_ether5 connection-state:established src-mac 00:01:5c:9f:f0:46, proto 50, xxx.yyy.zzz.www->aaa.bbb.ccc.ddd, len 140
2024-03-04T22:58:13+00:00 testing-router input: in:ether5-S2S out:(unknown 0), connection-mark:to_ether5 connection-state:established src-mac 00:01:5c:9f:f0:46, proto 50, xxx.yyy.zzz.www->aaa.bbb.ccc.ddd, len 156
2024-03-04T22:58:13+00:00 testing-router input: in:ether5-S2S out:(unknown 0), connection-mark:to_ether5 connection-state:established proto 47, xxx.yyy.zzz.www->aaa.bbb.ccc.ddd, len 100

Once ingested into OpenObserve, they may appear as JSON objects like this:

    "log": "2024-03-04T22:58:13+00:00 testing-router input: in:ether5-S2S out:(unknown 0), connection-mark:to_ether5 connection-state:established proto 47, xxx.yyy.zzz.www->aaa.bbb.ccc.ddd, len 100"

The Power of parse_regex in VRL

You can create a regular expression pattern and use it for parsing the sample logs. The parse_regex function then applies this pattern to the .log field of the incoming JSON object:

pattern = r'(?P<timestamp>\d{4}-\d{2}-\d{2}T\d{2}:\d{2}:\d{2}(?:\.\d+)?(?:\+\d{2}:\d{2})?)\s+(?P<router>\S+)\s+(?:(?P<message_type>[^,]+),\s+)?input:\s+in:(?P<in_iface>[^ ]+)\s+out:\((?P<out_iface>[^)]+)\),\s+connection-mark:(?P<connection_mark>\S+)\s+connection-state:(?P<connection_state>\S+)(?:\s+src-mac\s+(?P<src_mac>[^,]+),)?\s+proto\s+(?P<proto>\d+),\s+(?P<src_ip>[^->]+)->(?P<dst_ip>[^,]+),\s+len\s+(?P<length>\d+)'

# Parse the log using the regex pattern
.log = parse_regex!(.log, pattern)

Breaking Down the Regex Pattern

The regex pattern is a sequence of named groups that correspond to the different parts of the syslog message we want to extract:

  • (?P\d{4}-\d{2}-\d{2}T\d{2}:\d{2}:\d{2}(?:.\d+)?(?:+\d{2}:\d{2})?): Captures the timestamp. It matches a date and time in ISO 8601 format, optionally including milliseconds and a time zone offset. The (?P...) syntax is used to create a named capturing group.
  • \s+: Matches one or more whitespace characters. It separates different parts of the log.
  • (?P\S+): Captures the router name. \S+ matches one or more non-whitespace characters.
  • (?:(?P<message_type>^\s+)\s+)?: Optionally matches and captures the message type, which is any sequence of characters except whitespace, followed by at least one whitespace character. The ? after the group makes this entire section optional.
  • input:\s+in:(?P<in_iface>^+): Matches the literal input: followed by in: and captures the input interface name, which is any sequence of characters except a space.
  • \s+out:((?P<out_iface>^)+)): Matches out: followed by a pair of parentheses containing the output interface name, capturing any characters until the closing parenthesis.
  • ,: Literal commas and spaces are used as separators between different parts of the log.
  • connection-mark:(?P<connection_mark>\S+): Captures the connection mark, which is a sequence of one or more non-whitespace characters.
  • connection-state:(?P<connection_state>\S+): Captures the connection state, also as a sequence of non-whitespace characters.
  • (?:\s+src-mac\s+(?P<src_mac>^,+),)?: Optionally matches and captures the source MAC address. This part of the pattern is optional, as indicated by the trailing ?.
  • proto\s+(?P\d+): Matches the protocol number, capturing one or more digits.
  • (?P<src_ip>^->+)->(?P<dst_ip>^,+): Captures the source and destination IP addresses. It looks for a sequence of characters not including -> for the source IP and characters not including , for the destination IP, separated by ->.
  • len\s+(?P\d+): Captures the length of the packet or payload as a sequence of digits.

Example of Parsed Data

When the parse_regex function is applied to the syslog message, the output is a structured JSON object that clearly delineates the extracted data:

    "log": {
        "timestamp": "2024-03-04T22:58:13+00:00",
        "router": "testing-router",
        "message_type": null,
        "in_iface": "ether5-S2S",
        "out_iface": "unknown 0",
        "connection_mark": "to_ether5",
        "connection_state": "established",
        "src_mac": null,
        "proto": "47",
        "src_ip": "xxx.yyy.zzz.www",
        "dst_ip": "aaa.bbb.ccc.ddd",
        "length": "100"

This structured data is now ready for further analysis or alerting within OpenObserve. By leveraging the parse_regex function, network administrators can transform MikroTik's complex syslog messages into actionable insights, contributing to more effective network monitoring and management.


You should save the VRL function and associate it to the stream. This will ensure that the logs are parsed and structured as they are ingested into OpenObserve.


Syslog parsing is a critical aspect of network monitoring and security. By effectively parsing syslog messages, organizations can gain valuable insights into system behaviors, security events, and operational issues. OpenObserve provides powerful tools like parse_syslog and parse_regex to facilitate the extraction of structured data from syslog messages, enabling network administrators to make informed decisions and take proactive measures to ensure the health and security of their IT environments. By leveraging these parsing capabilities, organizations can enhance their security posture, improve operational efficiency, and meet compliance requirements effectively.



Prabhat Sharma is the founder of OpenObserve. He has deep background in cloud, kubernetes, observability and more. He has interests in machine learning, liberal arts, economics and systems architecture. In his spare time he spends time playing with his kids.

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