Configuring Cribl Stream to Receive Raw UDP 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 > ] Raw UDP. 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 > ] Raw UDP. Next, click Add Source to open a New Source modal that provides the options below.

Sending large numbers of UDP events per second can cause Cribl.Cloud to drop some of the data. This results from restrictions of the UDP protocol.

To minimize the risk of data loss, deploy a hybrid Stream Worker Group with customer-managed Worker Nodes as close as possible to the UDP sender. Cribl also recommends tuning the OS UDP buffer size.

General Settings

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Input ID: Enter a unique name to identify this raw UDP Source definition.

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

Port: Enter the 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.

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, etc. 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, 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 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.

In Cribl Stream 4.1 and later, Source-side PQ’s default Mode is Always on, to best ensure events’ delivery. For details on optimizing this selection, see Always On versus Smart Mode.

You can optimize Workers’ startup connections and CPU load at Group Settings > Worker Processes.

Processing Settings

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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.

Advanced Settings

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Single msg per UDP: Toggle to Yes if each UDP message should be treated as an independent event. Leave set to the default No if the message should be broken on newlines to create multiple events.

Ingest raw bytes: Toggle to Yes to add a __rawBytes field to each event containing an array of the bytes received as the UDP message.

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

Max buffer size (events): Maximum number of events to buffer when downstream is blocking. The buffer is only in memory.

UDP socket buffer size (bytes): Optionally, set the SO_RCVBUF socket option for the UDP socket. This value tells the operating system how many bytes can be buffered in the kernel before events are dropped. Leave blank to use the OS default. Min: 256. Max: 4294967295.

It may also be necessary to increase the size of the buffer available to the SO_RCVBUF socket option. Consult the documentation for your operating system for a specific procedure.

Setting this value will affect OS memory utilization.

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.

What Fields to Expect

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The Raw UDP Source does minimal processing of the incoming UDP messages to remain consistent with the internal Cribl event model. For each UDP message received on the socket, you can expect an event with the following fields:

• _raw: Contains the UTF-8 representation of the entire message received (if Single msg per UDP is set to Yes), or of the given line that was split out of the message.
• _time: The UNIX timestamp (in seconds) at which the message was received by Cribl Stream.
• source: A string in the form udp|<remote IP address>|<remote port>, indicating the remote sender.
• host: The hostname of the machine running Cribl Stream that ingested this event.

Also, the internal fields listed below will be present.

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 accessible for this Source:

• __inputId
• __srcIpPort
• __rawBytes: When Ingest raw bytes is set to Yes, this field will be an array containing the bytes of the UDP message.

UDP Tuning

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Incoming UDP traffic is put into a buffer by the Linux kernel. Cribl will drain from that buffer as resources are available. At lower throughput levels, and with plenty of available processes, this isn’t an issue. As you scale up, however, the default size of that buffer may be too small.

You can check the current buffer size with:

$ sysctl net.core.rmem_max

A typical value of about 200 KB is far too small for an even moderately busy syslog server. You can check the health of UDP with the following command. Check the packet receive errors line.

$ netstat -su

If packet receive errors is more than zero, you have lost events, which is a particularly serious problem if the number of errors is increasing rapidly. This means you need to increase your net.core.rmem_max setting (see earlier).

You can update the live settings, but you’ll also need to change the boot-time setting so next time you reboot everything is ready to roll.

Live change, setting to 25 MB:

$ sysctl -w net.core.rmem_max=26214400
net.core.rmem_max = 26214400
$ sysctl -w net.core.rmem_default=26214400
net.core.rmem_default = 26214400

For the permanent settings change, add the following lines to /etc/sysctl.conf:

net.core.rmem_max=26214400
net.core.rmem_default=26214400

We recommend you track a few things related to UDP receiving:

• The netstat -su command, watching for errors.
• The Status tab in the UDP (Raw) Source. In particular, watch for dropped messages. They could indicate you need a bigger buffer under Advanced Settings (default: 1000 events). They could also indicate your Worker is encountering pressure further down the Pipeline.
• Especially if you increase your kernel receive buffer as above, watch your Worker processes’ memory usage.

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.