Configuring Cribl Stream to Receive Splunk TCP Data

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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 Source at left. From the resulting drawer’s tiles, select [Push > ] Splunk > Splunk TCP. 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 > Sources (Stream) or More > Sources (Edge). From the resulting page’s tiles or left nav, select [Push > ] Splunk > Splunk TCP. Next, click New Source to open a New Source modal that provides the options below.

Cribl Stream ships with a Splunk TCP Source preconfigured to listen on Port 9997. You can clone or directly modify this Source to further configure it, and then enable it.

General Settings

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

Address: Enter hostname/IP to listen for Splunk data. For example, localhost or 0.0.0.0.

Port: Enter port number.

Optional Settings

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

TLS Settings (Server Side)

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Enabled defaults to No. When toggled to Yes:

Certificate name: Name of the predefined certificate.

Private key path: Server path containing the private key (in PEM format) to use. Path can reference $ENV_VARS.

Passphrase: Passphrase to use to decrypt private key.

Certificate path: Server path containing certificates (in PEM format) to use. Path can reference $ENV_VARS.

CA certificate path: Server path containing CA certificates (in PEM format) to use. Path can reference $ENV_VARS.

Authenticate client (mutual auth): Require clients to present their certificates. Used to perform mutual authentication using SSL certs. Defaults to No. When toggled to Yes:

• Validate client certs: Reject certificates that are not authorized by a CA in the CA certificate path, or by another trusted CA (for example, the system’s CA). Defaults to Yes.

• Common Name: Regex that a peer certificate’s subject attribute must match in order to connect. Defaults to .*. Matches on the substring after CN=. As needed, escape regex tokens to match literal characters. (For example, to match the subject CN=worker.cribl.local, you would enter: worker\.cribl\.local.) If the subject attribute contains Subject Alternative Name (SAN) entries, the Source will check the regex against all of those but ignore the Common Name (CN) entry (if any). If the certificate has no SAN extension, the Source will check the regex against the single name in the CN.

Minimum TLS version: Optionally, select the minimum TLS version to accept from connections.

Maximum TLS version: Optionally, select the maximum TLS version to accept from connections.

Persistent Queue Settings

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In this section, you can optionally specify persistent queue storage, using the following controls. This will buffer and preserve incoming events when a downstream Destination is down, or exhibiting backpressure.

On Cribl-managed Cribl.Cloud Workers (with an Enterprise plan), this tab exposes only the Enable Persistent Queue toggle. If enabled, PQ is automatically configured in Always On mode, with a maximum queue size of 1 GB disk space allocated per PQ‑enabled Source, per Worker Process.

The 1 GB limit is on uncompressed inbound data, and no compression is applied to the queue. This limit is not configurable. For configurable queue size, compression, mode, and other options below, use a hybrid Group.

Enable Persistent Queue: Defaults to No. When toggled to Yes:

Mode: Select a condition for engaging persistent queues.

• Always On: This default option will always write events to the persistent queue, before forwarding them to Cribl Stream’s data processing engine.
• Smart: This option will engage PQ only when the Source detects backpressure from Cribl Stream’s data processing engine.

Max buffer size: The maximum number of events to hold in memory before reporting backpressure to the sender and writing the queue to disk. Defaults to 1000. (This buffer is per connection, not just per Worker Process – and this can dramatically expand memory usage.)

Commit frequency: The number of events to send downstream before committing that Stream has read them. Defaults to 42.

Max file size: The maximum data volume to store in each queue file before closing it and (optionally) applying the configured Compression. Enter a numeral with units of KB, MB, and so forth. If not specified, Cribl Stream applies the default 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 Source will stop queueing data and block incoming data. Required, and defaults to 5 GB. Accepts positive numbers with units of KB, MB, GB, and so forth. 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 field’s specified path, Cribl Stream will append /<worker-id>/inputs/<input-id>.

Compression: Optional codec to compress the persisted data after a file is closed. Defaults to None; Gzip is also available.

As of Cribl Stream 4.1, Source-side PQ’s default Mode changed from Smart to Always on. This option more reliably ensures events’ delivery, and the change does not affect existing Sources’ configurations. However:

• If you create Stream Sources programmatically, and you want to enforce the previous Smart mode, you’ll need to update your existing code.
• If you enable Always on, this can reduce data throughput. As a trade-off for data durability, you might need to either accept slower throughput, or provision more machines/faster disks.
• You can optimize Workers’ startup connections and CPU load at Group Settings > Worker Processes.

Processing Settings

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Event Breakers

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Event Breaker rulesets: A list of event breaking rulesets that will be applied to the input data stream before the data is sent through the Routes. Defaults to System Default Rule.

Event Breaker buffer timeout: How long (in milliseconds) the Event Breaker will wait for new data to be sent to a specific channel, before flushing out the data stream, as-is, to the Routes. Minimum 10 ms, default 10000 (10 sec), maxiumum 43200000 (12 hours).

Fields

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In this section, you can add Fields to each event, using Eval-like functionality.

Name: Field name.

Value: JavaScript expression to compute field’s value, enclosed in quotes or backticks. (Can evaluate to a constant.)

Pre-Processing

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In this section’s Pipeline drop-down list, you can select a single existing Pipeline to process data from this input before the data is sent through the Routes.

Auth Tokens

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Add Token : Click to add authorization tokens. Each token’s section provides the fields listed below. If no tokens are specified, unauthenticated access will be permitted.

Token: Shared secrets to be provided by any Splunk forwarder (Authorization: <token>). Click Generate to create a new secret.

Description: Optional description of this token.

Advanced Settings

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Enable Proxy Protocol: Toggle to Yes if the connection is proxied by a device that supports Proxy Protocol v1 or v2.

IP allowlist regex: Regex matching IP addresses that are allowed to establish a connection. Defaults to .* (such as, all IPs).

Max active connections: Maximum number of active connections allowed per Worker Process. Defaults to 1000. Set a lower value if connection storms are causing the Source to hang. Set 0 for unlimited connections.

Max S2S version: The highest version of the Splunk-to-Splunk protocol to expose during handshake. Defaults to v4; v3 is also available.

Use Universal Forwarder time zone: Displayed (and enabled by default) only when Max S2S version is set to v4. Provides Event Breakers with a __TZ field, which derives events’ time zone from UF-provided metadata. See Using the UF Time Zone and Configuring a Splunk Forwarder, below.

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.

Connected Destinations

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Select Send to Routes to enable conditional routing, filtering, and cloning of this Source’s data via the Routing table.

Select QuickConnect to send this Source’s data to one or more Destinations via independent, direct connections.

Using the UF Time Zone

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Under Advanced Settings, the Use Universal Forwarder time zone toggle mitigates cases where incoming events have timestamp strings but no time zone information. For example:

12-15-2022 14:57:22.080 WARN TcpOutputFd [1607 TcpOutEloop] - Connect to 172.17.0.1:9997 failed. Connection refused

This gap can be problematic, especially if the originating Universal Forwarder is in a different time zone from the processing Worker Node.

The __TZ field is the solution. Event Breakers use the __TZ field to derive time zone information, enabling them to set the _time field correctly. Derived time zone information will appear in Cribl Stream’s own logs as shown below:

Setting the Log Level for Connection Messages

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When a Splunk forwarder connects to Cribl Stream, Cribl Stream logs the following message at the debug level: Connection with forwarder has been established successfully.

To see this message, set the :forwarders level to debug.

Each message contains details specific to the forwarder, such as the protocol, Splunk version, or remote host, to name a few.

In some situations, logging each incoming connection can produce many messages, which can make it hard to find other messages.

You can adjust the level of these connection messages. To do so, follow these steps:

1. Select Settings > Global Settings > Logging > Levels.

2. Search for the channel that logs the connection messages. It will have a name in the form input:<source-id>:forwarders. For example: input:in_splunk_tcp:forwarders.

3. Set the channel to the log level you prefer, such as debug or silly.

4. Click Save to save your setting.

5. Commit and deploy the change.

Internal Fields

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Cribl Stream uses a set of internal fields to assist in handling of data. These “meta” fields are not part of an event, but they are accessible, and Functions can use them to make processing decisions.

Fields for this Source:

• __inputId
• __s2sVersion – value can be either v3 or v4
• __source
• __srcIpPort
• __TZ – see above

Configuring a Splunk Forwarder

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To configure a Splunk forwarder (UF, HF) use the following sample outputs.conf stanzas:

[tcpout]
disabled = false 
defaultGroup = cribl, <optional_clone_target_group>, ...
enableOldS2SProtocol = true

[tcpout:cribl]
server = [<cribl_ip>|<cribl_host>]:<port>, [<cribl_ip>|<cribl_host>]:<port>, ...
compressed = false
sendCookedData = true
# As of Splunk 6.5, using forceTimebasedAutoLB is no longer recommended. Ensure this is left at default for UFs
# forceTimebasedAutoLB = false 

[tcpout]
disabled = false
defaultGroup = cribl
enableOldS2SProtocol = true

[tcpout:cribl]
server = default.main.<Your-Org-ID>.cribl.cloud:9997
# sslVerifyServerCert = true
sslRootCAPath = $SPLUNK_HOME/etc/auth/cacert.pem
compressed = false
useSSL = true
sendCookedData = true

With a Cribl.Cloud Enterprise plan, generalize the above URL’s default.main substring to <group-name>.main when sending to other Worker Groups.

If your use case requires compression, use SSL forwarding to compress the data stream.

Preventing Data Loss with v3

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If you set Max S2S version to v3 and are using Splunk 9.1.0 or later, Cribl recommends that you use the enableOldS2SProtocol = true setting shown above to avoid data loss. If you are working with v3 and a Splunk version earlier than 9.1.0, you should use negotiateProtocolLevel = 0. Depending on your environment, enabling negotiateProtocolLevel with a non-0 value could cause Cribl Stream to not accept data from the forwarder.

If you set Max S2S version to v4, these settings are not necessary. The Splunk receiver will detect which version is in use and automatically use the correct handler.

See Internal Fields for information on the __s2sVersion field.

Troubleshooting

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The Source’s configuration modal has helpful tabs for troubleshooting:

Live Data: Try capturing live data to see real-time events as they are ingested. On the Live Data tab, click Start Capture to begin viewing real-time data.

Logs: Review and search the logs that provide detailed information about the ingestion process, including any errors or warnings that may have occurred.

You can also view the Monitoring page that provides a comprehensive overview of data volume and rate, helping you identify ingestion issues. Analyze the graphs showing events and bytes in/out over time.

Splunk Forwarder Performance Issues

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If you encounter performance issues with a Splunk Forwarder, Cribl recommends increasing the number of parallel ingestion Pipelines or increasing forwarder throughput. You can experiment with either or both of these settings.

To increase the number of parallel ingestion Pipelines, adjust the setting for parallelIngestionPipelines in server.conf. Experiment with values ranging from 2–4.

To adjust forwarder throughput, increase the maxKBps value in limits.conf. The default value is 256. A value of 0 removes all throttling from the forwarder.

“Connection rejected” with a reason of “Too many connections”

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The maximum number of active Splunk TCP connections has been exceeded per Worker Process. The default is 1000.

Recommendation

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In the Splunk TCP input’s Advanced Settings configuration, increase the Max Active Connections value, set it to 0 for unlimited, and/or increase the # of Worker Processes the Worker Node(s) are using.