Splunk HEC

The Splunk HEC Destination can stream data to a Splunk HEC (HTTP Event Collector) receiver through the event endpoint. The data arrives to Splunk cooked and parsed, so it enters at the Splunk data pipeline’s indexing segment.

Type: Streaming | TLS Support: Yes | PQ Support: Yes

Events sent to the Splunk HEC Destination will show higher outbound data volume than the same events sent to the Splunk Single Instance or Splunk Load Balanced Destinations, which use the S2S binary protocol.

Configuring Cribl Stream to Output to Splunk HEC Destinations​

From the top nav, click Manage, then select a Worker Group to configure. Next, you have two options:

To configure via the graphical QuickConnect UI, click Routing > QuickConnect (Stream) or Collect (Edge). Next, click Add Destination at right. From the resulting drawer’s tiles, select Splunk > HEC. Next, click either Add Destination or (if displayed) Select Existing. The resulting drawer will provide the options below.

Or, to configure via the Routing UI, click Data > Destinations (Stream) or More > Destinations (Edge). From the resulting page’s tiles or the Destinations left nav, select Splunk > HEC. Next, click Add Destination to open a New Destination modal that provides the options below.

General Settings​

Output ID: Enter a unique name to identify this Splunk HEC definition. If you clone this Destination, Cribl Stream will add -CLONE to the original Output ID.

Load balancing: When enabled (default), lets you specify multiple Splunk HEC endpoints and load weights. With the default No setting, if you notice that Cribl Stream is not sending data to all possible IP addresses, enable Advanced Settings > Round-robin DNS.

Splunk HEC endpoint: URL of a Splunk HEC endpoint to send events to (e.g., http://myhost.example.com:8088/services/collector/event). This setting appears only when Load balancing is toggled to No.

Splunk HEC Endpoints​

Use the Splunk HEC Endpoints table to specify a known set of receivers on which to load-balance data. It appears only when Load balancing is toggled to Yes.

To specify more receivers on new rows, click Add Endpoint. Each row provides the following fields:

HEC Endpoint: Specify the URL to a Splunk HEC endpoint to send events to – e.g., http://localhost:8088/services/collector/event.

Load weight: Set the relative traffic-handling capability for each connection by assigning a weight (> 0). This column accepts arbitrary values, but for best results, assign weights in the same order of magnitude to all connections. Cribl Stream will attempt to distribute traffic to the connections according to their relative weights.

The final column provides an X button to delete any row from the table.

When you first enable load balancing, or if you edit the load weight once your data is load–balanced, give the logic time to settle. The changes might take a few seconds to register.

For details on configuring all these options, see About Load Balancing.

For Splunk Cloud endpoints, change the Splunk HEC endpoint’s default http: prefix to: https:.

Authentication Settings ​

Use the Authentication method drop-down to select one of these options:

  • Manual: Displays an HEC Auth token field for you to enter your Splunk HEC API access token.

  • Secret: This option exposes an HEC Auth token (text secret) drop-down, in which you can select a stored secret that references the API access token described above. A Create link is available to store a new, reusable secret.

This Destination does not support Mutual TLS (mTLS).

Indexer Acknowledgement​

Do not set Enable Indexer Acknowledgement on the Splunk token. With this setting enabled, Splunk receivers expect the Channel GUID to be passed in, and requests will fail with errors of this form:

channel: output:splunk_cloud_hec
cid: w2
level: error
message: Request failed
reason: Received status code=400, method=POST, url=https://http-inputs-bazookatron.splunkcloud.com/services/collector/event
response: {"text":"Data channel is missing","code":10}
time: 2023-02-14T15:26:03.413Z

Optional Settings​

Exclude current host IPs: This toggle appears when Load balancing is set to Yes. It determines whether to exclude all IPs of the current host from the list of any resolved hostnames. Defaults to No, which keeps the current host available for load balancing.

Backpressure behavior: Select whether to block, drop, or queue events when all receivers are exerting backpressure. (Causes might include a broken or denied connection, or a rate limiter.) Defaults to Block.

Tags: Optionally, add tags that you can use to filter and group Destinations in Cribl Stream’s Manage Destinations page. These tags aren’t added to processed events. Use a tab or hard return between (arbitrary) tag names.

Persistent Queue Settings​

This section is displayed when the Backpressure behavior is set to Persistent Queue.

Max file size: The maximum data volume to store in each queue file before closing it. Enter a numeral with units of KB, MB, etc. Defaults to 1 MB.

Max queue size: The maximum amount of disk space that the queue is allowed to consume on each Worker Process. Once this limit is reached, this Destination will stop queueing data and apply the Queue‑full behavior. Required, and defaults to 5 GB. Accepts positive numbers with units of KB, MB, GB, etc. Can be set as high as 1 TB, unless you’ve configured a different Max PQ size per Worker Process in Group Settings.

Queue file path: The location for the persistent queue files. Defaults to $CRIBL_HOME/state/queues. To this value, Cribl Stream will append /<worker‑id>/<output‑id>.

Compression: Codec to use to compress the persisted data, once a file is closed. Defaults to None; Gzip is also available.

Queue-full behavior: Whether to block or drop events when the queue is exerting backpressure (because disk is low or at full capacity). Block is the same behavior as non-PQ blocking, corresponding to the Block option on the Backpressure behavior drop-down. Drop new data throws away incoming data, while leaving the contents of the PQ unchanged.

Clear Persistent Queue: Click this “panic” button if you want to delete the files that are currently queued for delivery to this Destination. A confirmation modal will appear - because this will free up disk space by permanently deleting the queued data, without delivering it to downstream receivers. (Appears only after Output ID has been defined.)

Strict ordering: The default Yes position enables FIFO (first in, first out) event forwarding. When receivers recover, Cribl Stream will send earlier queued events before forwarding newly arrived events. To instead prioritize new events before draining the queue, toggle this off. Doing so will expose this additional control:

  • Drain rate limit (EPS): Optionally, set a throttling rate (in events per second) on writing from the queue to receivers. (The default 0 value disables throttling.) Throttling the queue’s drain rate can boost the throughput of new/active connections, by reserving more resources for them. You can further optimize Workers’ startup connections and CPU load at Group Settings > Worker Processes.

Processing Settings​

Post‑Processing​

Pipeline: Pipeline to process data before sending the data out using this output.

System fields: A list of fields to automatically add to events that use this output. By default, includes cribl_pipe (identifying the Cribl Stream Pipeline that processed the event). Supports wildcards. Other options include:

  • cribl_host – Cribl Stream Node that processed the event.
  • cribl_input – Cribl Stream Source that processed the event.
  • cribl_output – Cribl Stream Destination that processed the event.
  • cribl_route – Cribl Stream Route (or QuickConnect) that processed the event.
  • cribl_wp – Cribl Stream Worker Process that processed the event.

Retries​

Honor Retry-After header: Whether to honor a Retry-After header, provided that the header specifies a delay no longer than 180 seconds. Cribl Stream limits the delay to 180 seconds even if the Retry-After header specifies a longer delay. When enabled, any Retry-After header received takes precedence over all other options configured in the Retries section. When disabled, all Retry-After headers are ignored.

Settings for failed HTTP requests: When you want to automatically retry requests that receive particular HTTP response status codes, use these settings to list those response codes.

For any HTTP response status codes that are not explicitly configured for retries, Cribl Stream applies the following rules:

Status CodeAction
Greater than or equal to 400 and less than or equal to 500.Drop the request.
Greater than 500.Retry the request.

Upon receiving a response code that’s on the list, Cribl Stream first waits for a set time interval called the Pre-backoff interval and then begins retrying the request. Time between retries increases based on an exponential backoff algorithm whose base is the Backoff multiplier, until the backoff multiplier reaches the Backoff limit (ms). At that point, Cribl Stream continues retrying the request without increasing the time between retries any further.

By default, this Destination has no response codes configured for automatic retries. For each response code you want to add to the list, click Add Setting and configure the following settings:

  • HTTP status code: A response code that indicates a failed request, for example 429 (Too Many Requests) or 503 (Service Unavailable).
  • Pre-backoff interval (ms): The amount of time to wait before beginning retries, in milliseconds. Defaults to 1000 (one second).
  • Backoff multiplier: The base for the exponential backoff algorithm. A value of 2 (the default) means that Cribl Stream will retry after 2 seconds, then 4 seconds, then 8 seconds, and so on.
  • Backoff limit (ms): The maximum backoff interval Cribl Stream should apply for its final retry, in milliseconds. Default (and minimum) is 10,000 (10 seconds); maximum is 180,000 (180 seconds, or 3 minutes).

Retry timed-out HTTP requests: When you want to automatically retry requests that have timed out, toggle this control on to display the following settings for configuring retry behavior:

  • Pre-backoff interval (ms): The amount of time to wait before beginning retries, in milliseconds. Defaults to 1000 (one second).
  • Backoff multiplier: The base for the exponential backoff algorithm. A value of 2 (the default) means that Cribl Stream will retry after 2 seconds, then 4 seconds, then 8 seconds, and so on.
  • Backoff limit (ms): The maximum backoff interval Cribl Stream should apply for its final retry, in milliseconds. Default (and minimum) is 10,000 (10 seconds); maximum is 180,000 (180 seconds, or 3 minutes).

Advanced Settings​

Output multi–metrics: Toggle to Yes to output multiple-measurement metric data points. (Supported in Splunk 8.0 and above, this format enables sending multiple metrics in a single event, improving the efficiency of your Splunk capacity.)

Validate server certs: Reject certificates that are not authorized by a trusted CA (e.g., the system’s CA). Defaults to Yes.

Round–robin DNS: Toggle to Yes to use round-robin DNS lookup across multiple IPv6 addresses. When a DNS server returns multiple addresses, this will cause Cribl Stream to cycle through them in the order returned. (This setting is available only when General Settings > Load balancing is set to No.)

Compress: Compresses the payload body before sending. Defaults to Yes (recommended).

Request timeout: Amount of time (in seconds) to wait for a request to complete before aborting it. Defaults to 30.

Request concurrency: Maximum number of concurrent requests per Worker Process. When Cribl Stream hits this limit, it begins throttling traffic to the downstream service. Defaults to 5. Minimum: 1. Maximum: 32. Each request can potentially hit a different HEC receiver.

Max body size (KB): Maximum size, in KB, of the request body. Defaults to 4096. Lowering the size can potentially result in more parallel requests and also cause outbound requests to be made sooner.

If you’re sending data to Splunk Cloud, Cribl recommends a maximum value of 1 MB for Max body size (KB). This upper limit is defined by the maximum content length for the default HTTP Event Collector (HEC) provided by Splunk Cloud.

Max events per request: Maximum number of events to include in the request body. The 0 default allows unlimited events.

Flush period (sec): Maximum time between requests. Low values can cause the payload size to be smaller than the configured Max body size. Defaults to 1.

  • Retries happen on this flush interval.
  • Any HTTP response code in the 2xx range is considered success.
  • Any response code in the 5xx range is considered a retryable error, which will not trigger Persistent Queue (PQ) usage.
  • Any other response code will trigger PQ (if PQ is configured as the Backpressure behavior).

Extra HTTP headers: Click Add Header to add Name/Value pairs to pass as additional HTTP headers. Values will be sent encrypted.

The next two fields take effect only in the Cribl App for Splunk. (Cribl Stream ignores their values.)

Next processing queue: Specify the next Splunk processing queue to send the events to, after HEC processing. Defaults to indexQueue.

Default _TCP_ROUTING: Specify the value of the _TCP_ROUTING field for events that do not have _ctrl._TCP_ROUTING set. Defaults to nowhere. This is useful only when you expect the HEC receiver to route this data on to another destination.

The next two fields appear only when the General Settings > Load balancing option is set to Yes.

DNS resolution period (seconds): Re-resolve any hostnames after each interval of this many seconds, and pick up destinations from A records. Defaults to 600 seconds.

Load balance stats period (seconds): Lookback traffic history period. Defaults to 300 seconds. (Note that if multiple receivers are behind a hostname – i.e., multiple A records – all resolved IPs will inherit the weight of the host, unless each IP is specified separately. In Cribl Stream load balancing, IP settings take priority over those from hostnames.)

Failed request logging mode: Use this drop-down to determine which data should be logged when a request fails. Select among None (the default), Payload, or Payload + Headers. With this last option, Cribl Stream will redact all headers, except non-sensitive headers that you declare below in Safe headers.

Safe headers: Add headers to declare them as safe to log in plaintext. (Sensitive headers such as authorization will always be redacted, even if listed here.) Use a tab or hard return to separate header names.

Environment: If you’re using GitOps, optionally use this field to specify a single Git branch on which to enable this configuration. If empty, the config will be enabled everywhere.

Notes on HTTP-Based Outputs​

  • To proxy outbound HTTP/S requests, see System Proxy Configuration.

  • Cribl Stream will attempt to use keepalives to reuse a connection for multiple requests. After two minutes of the first use, the connection will be thrown away, and a new connection will be reattempted. This is to prevent sticking to a particular Destination when there is a constant flow of events.

  • If the server does not support keepalives – or if the server closes a pooled connection while idle – a new connection will be established for the next request.

  • When resolving the Destination’s hostname with load balancing disabled, Cribl Stream will pick the first IP in the list for use in the next connection. Enable Round-robin DNS to better balance distribution of events between Splunk HEC servers.

  • See Splunk’s documentation on editing fields.conf to ensure the visibility of index-time fields sent to Splunk by Cribl Stream.

Troubleshooting Resources​

Cribl University offers an Advanced Troubleshooting > Destination Integrations: Splunk Cloud short course. To follow the direct course link, first log into your Cribl University account. (To create an account, click the Sign up link. You’ll need to click through a short Terms & Conditions presentation, with chill music, before proceeding to courses – but Cribl’s training is always free of charge.) Once logged in, check out other useful Advanced Troubleshooting short courses and Troubleshooting Criblets.