Configure Splunk Universal Forwarder to ingest JSON files

Parse single-line JSON into separate events

  1. UF inputs.conf
  2. Forwarder props.conf
  3. Search head props.conf
  4. Ingesting API response

The recommended logging format according to Splunk best practice looks like this:

{ "datetime": 1672531212123456, "event_id": 1, "key1": "value1", "key2": "value2", "key3": "value3" } { "datetime": 1672531213789012, "event_id": 2, "key1": "value1", "key2": "value2", "key3": "value3" } { "datetime": 1672531214345678, "event_id": 3, "key1": "value1", "key2": "value2", "key3": "value3" }
  • Each event is in JSON, not the file.
    • This also means the log file is not a valid JSON file.
  • Each event is separated by newline.

The format can be achieved by exporting live event in JSON and append to a log file. However, I encountered a situation where the log file can only be generated by batch. Exporting the equivalent of the previous “example.log” in JSON without string manipulation looks like this:

[{"datetime": 1672531212123456, "event_id": 1, "key1": "value1", "key2": "value2", "key3": "value3"}, {"datetime": 1672531213789012, "event_id": 2, "key1": "value1", "key2": "value2", "key3": "value3"}, {"datetime": 1672531214345678, "event_id": 3, "key1": "value1", "key2": "value2", "key3": "value3"}]

I will detail the required configurations in this post, so that Splunk is able to parse it correctly even though “example.json” is not a valid JSON file.

UF inputs.conf §

[monitor:///var/log/app_a] disabled = 0 index = index_name sourcetype = app_a_event

monitor directive is made up of two parts: monitor:// and the path, e.g. /var/log/app_a. Unlike most Splunk configs, this directive does’t require the backslash (used in Windows path) to be escaped, e.g. monitor://C:\foo\bar.

A path can be a file or a folder. When (*) wildcard matching is used to match multiple folders, another wildcard needs to be specified again to match files in those matched folders. The wildcard works for a single path segment only. For example, to match all the following files, use monitor:///var/log/app_*/*. Splunk also supports “…” for recursive matching.

├── app_a
│   ├── 1.log
│   ├── 2.log
│   └── 3.log
├── app_b
│   ├── 1.log
│   ├── 2.log
│   └── 3.log
└── app_c
    ├── 1.log
    ├── 2.log
    └── 3.log

Specify an appropriate value in sourcetype config, the value will be the value of sourcetype field in the ingested events under the “monitor” directive. Take note of the value you have configured, it will be used in the rest of configurations.

Forwarder props.conf §

[app_a_event] description = App A logs INDEXED_EXTRACTIONS = JSON # separate each object into a line LINE_BREAKER = }(,){\"datetime\" # a line represents an event SHOULD_LINEMERGE = 0 TIMESTAMP_FIELDS = datetime TIME_FORMAT = %s ## default is 2000 # MAX_DAYS_AGO = 3560

The directive name should be the sourcetype value specified in the inputs.conf. The following configs apply to the universal forwarder is because INDEXED_EXTRACTIONS is used.

  • LINE_BREAKER: Search for string that matches the regex and replace only the capturing group with newline (\n). This is to separate each event into separate line.
    • }(,){\"datetime\" searches for },{"datetime" and replaces “,” with “\n”.
  • SHOULD_LINEMERGE: only used for event that spans multiple lines. In this case, it’s the reverse, the log file has all events in one line.
  • TIMESTAMP_FIELDS: Refers to datetime key in the example.json.
  • MAX_DAYS_AGO (optional): Specify the value if there are events older than 2,000 days.
  • TIME_FORMAT: Optional if Unix time is used, but recommended to specify whenever possible. When Unix time is used, it is not necessary to specify %s%3N when there is subsecond.

The location of “props.conf” depends on whether the universal forwarder is centrally managed by a deployment server.

Path A: $SPLUNK_HOME/etc/deployment-apps/foo/local/props.conf
Path B: $SPLUNK_HOME/etc/apps/foo/local/props.conf

If there is a deployment server, then the config file should be in path A, in which the server will automatically deploy it to path B in the UF. If the UF is not centrally managed, it should head straight to path B.

Search head props.conf §

[app_a_event] description = App A logs KV_MODE = none AUTO_KV_JSON = 0 SHOULD_LINEMERGE = 0

Since index-time field extraction is already enabled using INDEXED_EXTRACTIONS, search-time field extraction is no longer necessary. If KV_MODE and AUTO_KV_JSON are not disabled, there will be duplicate fields in the search result.

In Splunk Enterprise, the above file can be saved in a custom app, e.g. “$SPLUNK_HOME/etc/app/custom-app/default/props.conf”

For Splunk Cloud deployment, the above configuration can be added through a custom app or Splunk Web: Settings > Source types.

Ingesting API response §

It is important to note SEDCMD runs after INDEXED_EXTRACTIONS. I noticed this behaviour when I tried to ingest API response of LibreNMS.

{"status": "ok", "devices": [{"device_id": 1, "key1": "value1", "key2": "value2"}, {"device_id": 2, "key1": "value1", "key2": "value2"}, {"device_id": 3, "key1": "value1", "key2": "value2"}], "count": 3}

In this scenario, I only wanted to ingest “devices” array where each item is an event. The previous approach not only did not split the array, but “status” and “count” fields still existed in each event despite the use of SEDCMD to remove them.

The solution is not to use INDEXED_EXTRACTIONS (index-time field extraction), but use KV_MODE (search-time field extraction) instead. INDEXED_EXTRACTIONS is not enabled so that SEDCMD works more reliably. If it’s enabled, the JSON parser can unpredictably split part of the prefix (in this case {"status": "ok", "devices": [) or suffix into separate events and SEDCMD does not work across events. SEDCMD does work with INDEXED_EXTRACTIONS, but you have to make sure the replacement is within an event

# heavy forwarder or indexer [api_a_response] description = API A response # remove bracket at the start and end of each line SEDCMD-remove_prefix = s/^\{"status": "ok", "devices": \[//g SEDCMD-remove_suffix = s/\], "count": [0-9]+\}$//g # separate each object into a line LINE_BREAKER = }(, ){\"device_id\" # if each line/event is very long # TRUNCATE = 0 # a line represents an event SHOULD_LINEMERGE = 0
# search head [api_a_response] description = API A response KV_MODE = json AUTO_KV_JSON = 1