Segment7 Blog

Legacy syslog in Grafana Alloy

3 min read tags:

Grafana Alloy is an OpenTelemetry Collector wrapper that seems to have slightly better documentation, especially for processing pipelines. It's useful for collecting logs from my homelab and storing them in Loki for easier monitoring.

Alloy 1.13+ supports "raw" format syslog source allowing collection and processing of devices that don't send modern RFC5424 or RFC3164 syslog data. Unifi Network and Protect support an "SIEM Server" integration which uses legacy syslog format, and outputs Common Event Format logs. Unfortunately Alloy doesn't support CEF directly, but a processing pipeline can extract known fields for structured logs.

Pipeline

Alloy allows one syslog format per source so it will listen on one port, process the logs, then write them into Loki.

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---
flowchart LR
  A([unifi device])
  B([loki.source.syslog "unifi"])
  C([loki.process "unifi"])
  D([loki.write "unifi"])
  A -- port 9516 --> B
  B --> C
  C --> D

Syslog source

I have loki.source.syslog listening on UDP port 9516 using the "raw" format. It forwards to the unifi processor.

loki.source.syslog "unifi" {
  listener {
    address = "0.0.0.0:9516"
    labels = {
      component = "loki.source.syslog",
      protocol = "udp",
    }
    syslog_format = "raw"
  }

  forward_to = [
    loki.process.unifi.receiver,
  ]
}

Debugging

Use loki.echo as a forward_to entry for any of these to output the current log line from alloy to check your work. For the syslog receiver:

loki.echo "debug" { }

loki.source.syslog "unifi" {
  // …

  forward_to = [
    loki.echo.debug.receiver,
    loki.process.unifi.receiver,
  ]
}

Log processor

The loki.process converts the log line into structured output. I use a set of stages to extract fields from the raw log line, or a extract further fields.

To start, the log processor will output to the unifi write receiver. I'll fill in the rest as its discussed.

loki.process "unifi" {
  // stages

  forward_to = [
    loki.write.unifi.receiver,
  ]
}

CEF header

First, parse the CEF header using a stage.regex. We'll use these capture groups in further stages.

  stage.regex {
    expression = "CEF:(?P<v>.*?)\\|(?P<vendor>.*?)\\|(?P<product>.*?)\\|(?P<version>.*?)\\|(?P<event_id>.*?)\\|(?P<event_name>.*?)\\|(?P<severity>.*?)\\|(?P<extension>.*)$"
  }

Extension pairs

Next I extract some of the extension pairs. I found these for UniFi Network 11 by enabling debug for the loki.source.syslog forward_to. Alternatively you can tcpdump -s0 -A port 9516 if you don't want lines logged twice.

  stage.regex {
    expression = "UNIFIcategory=(?P<category>\\S+)"
  }

  stage.regex {
    expression = "UNIFIdeviceIp=(?P<device_ip>\\S+)"
  }

  stage.regex {
    expression = "UNIFIdeviceModel=(?P<device_model>\\S+)"
  }

  stage.regex {
    expression = "UNIFIdeviceName=(?P<device>\\S+)"
  }

Timestamp

Alloy will default to the time the message arrived at the source. Since the logs include their own timestamp we can extract it and parse it using stage.timestamp. UNIFIutcTime from UniFi Network is RFC3339, timestamp from UniFi Protect is milliseconds since the epoch.

  stage.regex {
    expression = "(UNIFIutcTime|timestamp)=(?P<timestamp>\\S+)"
  }

  stage.timestamp {
    format = "RFC3339"
    fallback_formats = [
      "UnixMs",
    ]
    source = "timestamp"
    action_on_failure = "skip"
  }

Severity

CEF uses numbers for severity, but the rest of my logs use syslog severities. I mapped them using stage.replace to something that seemed reasonable:

  // CEF low
  stage.replace {
    expression = "(0|1|2|3)"
    source = "severity"
    replace = "informational"
  }

  // CEF medium
  stage.replace {
    expression = "(4|5|6)"
    source = "severity"
    replace = "notice"
  }

  // CEF high
  stage.replace {
    expression = "(7|8)"
    source = "severity"
    replace = "warning"
  }

  // CEF very high
  stage.replace {
    expression = "(9|10)"
    source = "severity"
    replace = "critical"
  }

Unquote message

For UniFi Protect the message is wrapped in quotes. I haven't seen escaped quotes so I only remove the wrapping quotes.

  // clean wrapping quotes
  stage.replace {
    expression = "(^\")"
    source = "message"
    replace = ""
  }

  stage.replace {
    expression = "(\"$)"
    source = "message"
    replace = ""
  }

Output

Now that all the processing is complete I set the labels with stage.labels and the output message with stage.output. An empty labels value will use the capture group set above.

  stage.labels {
    values = {
      application = "product",
      category = "",
      device = "",
      hostname = "",
      device_ip = "",
      device_model = "",
      event_id = "",
      event_name = "",
      product = "",
      severity = "",
      version = "",
    }
  }

  stage.output {
    source = "message"
  }

  forward_to = [
    loki.write.unifi.receiver,
  ]

Writer

In order to make the messages easier to filter I set the alloy_source label on the loki.write that forwrds the messages to loki.

loki.write "unifi" {
  endpoint {
    url = "https://otel.example:3100/loki/api/v1/push"
  }

  external_labels = {
    alloy_source = "unifi",
  }
}

All together

syslog source, processor, writer 
loki.source.syslog "unifi" {
  listener {
    address = "0.0.0.0:9516"
    protocol = "udp"
    labels = {
      component = "loki.source.syslog",
      protocol = "udp",
    }
    syslog_format = "raw"
  }

  forward_to = [
    loki.process.unifi.receiver,
  ]
}

loki.process "unifi" {
  forward_to = [
    loki.write.unifi.receiver,
  ]

  stage.regex {
    expression = "CEF:(?P<v>.*?)\\|(?P<vendor>.*?)\\|(?P<product>.*?)\\|(?P<version>.*?)\\|(?P<event_id>.*?)\\|(?P<event_name>.*?)\\|(?P<severity>.*?)\\|(?P<extension>.*)$"
  }

  stage.regex {
    expression = "UNIFIcategory=(?P<category>\\S+)"
  }

  stage.regex {
    expression = "UNIFIdeviceIp=(?P<device_ip>\\S+)"
  }

  stage.regex {
    expression = "UNIFIdeviceModel=(?P<device_model>\\S+)"
  }

  stage.regex {
    expression = "UNIFIdeviceName=(?P<device>\\S+)"
  }

  stage.regex {
    expression = "(UNIFIutcTime|timestamp)=(?P<timestamp>\\S+)"
  }

  stage.timestamp {
    format = "RFC3339"
    fallback_formats = [
      "UnixMs",
    ]
    source = "timestamp"
    action_on_failure = "skip"
  }

  // CEF low
  stage.replace {
    expression = "(0|1|2|3)"
    source = "severity"
    replace = "informational"
  }

  // CEF medium
  stage.replace {
    expression = "(4|5|6)"
    source = "severity"
    replace = "notice"
  }

  // CEF high
  stage.replace {
    expression = "(7|8)"
    source = "severity"
    replace = "warning"
  }

  // CEF very high
  stage.replace {
    expression = "(9|10)"
    source = "severity"
    replace = "critical"
  }

  // use message from extension
  stage.regex {
    expression = "msg=(?P<message>.*)$"
    source = "extension"
  }

  // clean wrapping quotes
  stage.replace {
    expression = "(^\")"
    source = "message"
    replace = ""
  }

  stage.replace {
    expression = "(\"$)"
    source = "message"
    replace = ""
  }

  stage.labels {
    values = {
      application = "product",
      category = "",
      device = "",
      hostname = "",
      device_ip = "",
      device_model = "",
      event_id = "",
      event_name = "",
      product = "",
      severity = "",
      version = "",
    }
  }

  stage.output {
    source = "message"
  }
}

loki.write "unifi" {
  endpoint {
    url = "https://loki.example:3100/loki/api/v1/push"
  }

  external_labels = {
    alloy_source = "unifi",
  }
}

Processed event 

{
  "timestamp": "2026-11-11 17:11:50.996",
  "logLevel": "",
  "displayLevel": "",
  "line": "Side yard has recorded motion.",
  "labels": {
    "alloy_source": "unifi",
    "application": "UniFi Protect",
    "category": "detection",
    "component": "loki.source.syslog",
    "event_id": "2159",
    "event_name": "motion",
    "product": "UniFi Protect",
    "protocol": "udp",
    "severity": "informational",
    "version": "7.3.74"
  }
}

(Duplicate timstamps removed)

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