Configure Cribl Stream to Output Data in Syslog Format

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1. On the top bar, select Products, and then select Cribl Stream. Under Worker Groups, select a Worker Group. Next, you have two options:

   • To configure via QuickConnect, navigate to Routing > QuickConnect (Stream) or Collect (Edge). Select Add Destination and select the Destination you want from the list, choosing either Select Existing or Add New.
   • To configure via the Routes, select Data > Destinations or More > Destinations (Edge). Select the Destination you want. Next, select Add Destination.

2. In the New Destination modal, configure the following under General Settings:

   • Output ID: Enter a unique name to identify this Syslog definition. If you clone this Destination, Cribl Stream will add -CLONE to the original Output ID.
   • Description: Optionally, enter a description.
   • Protocol: The network protocol to use for sending out syslog messages. Defaults to TCP; UDP is also available.
   • Load balancing: Cribl Stream displays this option only when the Protocol is set to TCP. Toggle on to specify multiple destinations. See Load Balancing Settings below.

   
   Turning Load balancing off displays the following two additional fields:
   

   • Address: Address/hostname of the receiver.

   • Port: Port number to connect to on the host.

   
   If Load balancing is disabled and you notice that Cribl Stream is not sending data to all possible IP addresses, enable Advanced Settings > Round-robin DNS.
   

   • Record size limit: Displayed when Protocol is set to UDP. Sets the maximum size of syslog messages. Defaults to 1500, and must be ≤ 2048. To avoid packet fragmentation, keep this value less than or equal to the MTU (maximum transmission unit) for the network path to the downstream receiver. When the message length exceeds the size limit, it is truncated and followed with an ellipsis ("…").

3. Next, you can configure the following Optional Settings:

   • Facility: Default value for message facility. If set, will be overwritten by the value of __facility. Defaults to user.

   • Severity: Default value for message severity. If set, will be overwritten by the value of __severity. Defaults to notice.

   • App name: Default value for application name. If set, will be overwritten by the value of __appname. Defaults to Cribl.

   • Message format: The syslog message format supported by the receiver. Defaults to RFC3164.

   • Timestamp format: The timestamp format to use when serializing an event’s time field. Options include Syslog (default) and ISO8601. For details, see Timestamp format below.

   • Throttling: Throttle rate, in bytes per second. Defaults to 0, meaning no throttling. Multiple-byte units such as KB, MB, or GB are also allowed – for example, 42 MB. When throttling is engaged, your Backpressure behavior selection determines whether Cribl Stream will handle excess data by blocking it, dropping it, or queueing it to disk. For details, see Throttling below.

   • 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 on the Destinations page. These tags aren’t added to processed events. Use a tab or hard return between (arbitrary) tag names.

4. Optionally, you can adjust the Persistent Queue, TLS, Processing, Timeout, and Advanced settings outlined in the sections below.

5. Select Save, then Commit & Deploy.

Load Balancing Settings

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Load balancing is available only when the Protocol is set to TCP.

Toggling Load balancing on replaces the static General Settings > Address and Port fields with the following controls:

Exclude current host IPs: Appears when Load balancing is toggled on. It determines whether to exclude all IPs of the current host from the list of any resolved hostnames. Default is toggled off, which keeps the current host available for load balancing.

Destinations

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Use the Destinations table to specify a known set of receivers on which to load-balance data. To specify more receivers on new rows, click Add Destination. Each row provides the following fields:

Address: Hostname of the receiver. Optionally, you can paste in a comma-separated list, in <host>:<port> format.

Port: Port number to send data to on this host.

TLS: Whether to inherit TLS configs from group setting, or disable TLS. Defaults to inherit.

TLS servername: Servername to use if establishing a TLS connection. If not specified, defaults to connection host (if not an IP). Otherwise, uses the global TLS settings.

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.

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

Timestamp Format

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For Syslog, the timestamp format is Mmm dd hh:mm:ss, using the timezone of the syslog device. For example: Jan 18 16:56:36.

For ISO8601, the basic timestamp format is YYYY‑MM‑DD. For example: 2023‑01‑18. For extended formats, add hours, minutes, seconds, decimal fractions (optional), and timezone. For example: 2023‑01‑18T16:56:36.996‑08:00.

Throttling

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When Throttling is used in conjunction with Max connections, the throttling rate is applied per connection. For instance, if Max connections is set to 2 and throttling is set to 3 MB, each of the two connections can send data at a rate of up to 3 MB per second, resulting in a total potential throughput of 6 MB per second across both connections.

Persistent Queue Settings

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The Persistent Queue Settings tab displays when the Backpressure behavior option in General settings is set to Persistent Queue. Persistent queue buffers and preserves incoming events when a downstream Destination has an outage or experiences backpressure.

Before enabling persistent queue, learn more about persistent queue behavior and how to optimize it with your system:

• About Persistent Queues
• Optimize Destination Persistent Queues (dPQ)
• Destination Backpressure Triggers

On Cribl-managed Cloud Workers (with an Enterprise plan), this tab exposes only the destructive Clear Persistent Queue button (described at the end of this section). A maximum queue size of 1 GB disk space is automatically allocated per PQ‑enabled Destination, per Worker Process. The 1 GB limit is on outbound uncompressed data, and no compression is applied to the queue.

This limit is not configurable. If the queue fills up, Cribl Stream/Edge will block outbound data. To configure the queue size, compression, queue-full fallback behavior, and other options below, use a hybrid Group.

Mode: Use this menu to select when Cribl Stream/Edge engages the persistent queue in response to backpressure events from this Destination. The options are:

If a Worker/Edge Node starts with an invalid Mode setting, it automatically switches to Error mode. This might happen if the Worker/Edge Node is running a version that does not support other modes (older than 4.9.0), or if it encounters a nonexistent value in YAML configuration files.

File size limit: 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 can consume on each Worker Process. When the queue reaches this limit, the Destination stops queueing data and applies the Queue‑full behavior. Defaults to 5 GB. This field accepts positive numbers with units of KB, MB, GB, and so on. You can set it as high as 1 TB, unless you’ve configured a different Worker Process PQ size limit on the Group Settings/Fleet Settings page.

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

Compression: Set the codec to use when compressing the persisted data after closing a file. Defaults to None. Gzip is also available.

Queue-full behavior: Whether to block or drop events when the queue begins to exert backpressure. A queue begins to exert backpressure when the disk is low or at full capacity. This setting has two options:

• Block: The output will refuse to accept new data until the receiver is ready. The system will return block signals back to the sender.
• Drop new data: Discard all new events until the backpressure event has resolved and the receiver is ready.

Backpressure duration Limit: When Mode is set to Backpressure, this setting controls how long to wait during network slowdowns before activating queues. A shorter duration enhances critical data loss prevention, while a longer duration helps avoid unnecessary queue transitions in environments with frequent, brief network fluctuations. The default value is 30 seconds.

Strict ordering: Toggle on (default) to enable FIFO (first in, first out) event forwarding, ensuring Cribl Stream/Edge sends earlier queued events first when receivers recover. The persistent queue flushes every 10 seconds in this mode. Toggle off to prioritize new events over queued events, configure a custom drain rate for the queue, and display this option:

• 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 drain rate can boost the throughput of new and active connections by reserving more resources for them. You can further optimize Worker startup connections and CPU load in the Worker Processes settings.

Clear Persistent Queue: For Cloud Enterprise only, click this button if you want to delete the files that are currently queued for delivery to this Destination. If you click this button, a confirmation modal appears. Clearing the queue frees up disk space by permanently deleting the queued data, without delivering it to downstream receivers. This button only appears after you define the Output ID.

Use the Clear Persistent Queue button with caution to avoid data loss. See Steps to Safely Disable and Clear Persistent Queues for more information.

TLS Settings (Client Side)

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Enabled: Defaults to toggled off. When toggled on:

Validate server 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 toggled on.

Server name (SNI): Server name for the SNI (Server Name Indication) TLS extension. This must be a host name, not an IP address.

Minimum TLS version: Optionally, select the minimum TLS version to use when connecting.

Maximum TLS version: Optionally, select the maximum TLS version to use when connecting.

Certificate name: The name of the predefined certificate.

CA certificate path: Path on client containing CA certificates (in PEM format) to use to verify the server’s cert. Path can reference $ENV_VARS.

Private key path (mutual auth): Path on client containing the private key (in PEM format) to use. Path can reference $ENV_VARS. Use only if mutual auth is required.

Certificate path (mutual auth): Path on client containing certificates in (PEM format) to use. Path can reference $ENV_VARS. Use only if mutual auth is required.

Passphrase: Passphrase to use to decrypt private key.

Timeout Settings

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These timeout settings apply only to the TCP protocol.

Connection timeout: Amount of time (in milliseconds) to wait for the connection to establish, before retrying. Defaults to 10000.

Write timeout: Amount of time (milliseconds) to wait for a write to complete, before assuming connection is dead. Defaults to 60000.

Processing Settings

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Post‑Processing

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Pipeline: Pipeline or Pack 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.

Advanced Settings

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The first three options are always displayed:

Octet count framing: When enabled, Cribl Stream prefixes messages with their byte count, regardless of whether the messages are constructed from message or __syslogout. When disabled, Cribl Stream omits prefixes, and instead appends a \n to messages. RFCs 5425 and 6587 describe how octet counting works in syslog.

Log failed requests to disk: Toggle on to make the payload of the most recently failed request available for inspection. See Inspect Payload to Troubleshoot Closed Connections 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.

The following options are added if you enable the General Settings tab’s Load balancing option:

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

• For TCP: This helps maintain up-to-date IP addresses for your TCP connections. Defaults to 600 seconds.
• For UDP: This reduces the frequency of DNS lookups for UDP destinations, improving performance. Defaults to 0 seconds, meaning DNS lookups occur for every outgoing message.

Load balance stats period (seconds): Lookback traffic history period. Defaults to 300 seconds. If multiple receivers – that is, multiple A records – are behind a hostname, 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.

Max connections: Constrains the number of concurrent receiver connections per Worker Process, to limit memory utilization. If you set this to a number greater than 0, Cribl Stream will randomly select that many discovered IPs to connect to on every DNS resolution period. Cribl Stream will rotate IPs in future resolution periods – monitoring weight and historical data, to ensure fair load balancing of events among IPs.

Inspect Payload to Troubleshoot Closed Connections

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When a downstream receiver closes connections from this Destination (or just stops responding), inspecting the payload of the most recently failed request can help you find the cause. For example:

• Suppose you send an event whose size is larger than the downstream receiver can handle.
• Suppose you send an event that has a number field, but the value exceeds the highest number that the downstream receiver can handle.

When Log failed requests to disk is enabled, you can inspect the last failed request payload. Here is how:

1. In the Destination UI, navigate to the Logs tab.
2. Find a log entry with a connection error message.
3. Expand the log entry.
4. If the message includes the phrase See payload file for more info, note the path in the file field on the next line.

Now you have the path to the directory where Cribl Stream is storing the payload from the last failed request.

Internal Fields

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Cribl Stream uses a set of internal fields to assist in forwarding data to downstream services. Fields for this Destination:

• __priority
• __facility
• __severity
• __procid
• __appname
• __msgid
• __syslogout

Syslog Format Options

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Unlike other Cribl Stream Destinations, the Syslog Destination applies an additional post-processing step after the Pipeline(s) transform events. This additional step structures the data for compliance with the syslog transport protocol (RFC 3164 and/or RFC 5424) before it is transmitted to downstream services.

• With RFC 3164-compliant messages, you’ll get priority, timestamp, host, and message, such that message includes appname and procid. You will not get msgid or STRUCTURED-DATA.
• With RFC 5424-compliant messages – that is, structured data – Cribl Stream will send the contents of event.structuredData, whether or not that content is valid according to the RFC. Cribl Stream will make no attempt to frame, enclose, or validate the data. If present, structured data should be in the form [ SD-ID *PARAM-NAME=PARAM-VALUE] but Cribl Stream will not enforce this. Cribl Stream will send whatever is in that field, or, if the field is missing, send a dash (-).

In General Settings, you can format the timestamps, format the message delivering the event, and set defaults for Facility, Severity, and App name (in RFC 5424, these are Facility, Severity, and APP-NAME, respectively).

Here is an example of an RFC 3164-compliant syslog event:

• <13>Jul 11 10:34:35 testbox testing[42]

Here is an example of an RFC 5424-compliant syslog event:

• <214>1 2022-07-11T18:58:45.000Z testbox testapp 42 - [exampleSDID@32473 iut="3" foo="bar" this="that"] Here is a message

Structure Syslog Output

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Two approaches are available for specifying how the Syslog Destination will structure its output. One relies on the message field, and the other relies on the __syslogout field. The Syslog Destination determines what to do based on whether or not those fields are set, as the flowchart below makes clear.

• Use message (Simplest Approach): Cribl recommends using this option, which requires you to configure message and to leave __syslogout unset. When message is defined, Cribl Stream discards _raw and composes a new payload using the other syslog-related fields. Cribl Stream automatically processes the values of the message, _time, host, and other fields, and creates an ISO-compliant timestamp and a properly formatted event. In the flowchart, the rounded rectangle that reads “Use message + other fields …” represents this approach.

• Export __syslogout (Alternative Approach): When you define __syslogout, Cribl Stream sends this field and its value as the entire syslog message, discards _raw, and ignores all the other fields. In the flowchart, the rounded rectangle that reads “Send __syslogout …” represents this approach.

As the flowchart shows, if neither message nor __syslogout are set, Cribl Stream uses the existing _raw field as the message field (with or without a header), and prepends all the other syslog-related fields to the event. While this could be considered a third approach, Cribl does not recommend it, and we will not describe it further in this topic.

Before you configure this Destination, review the subsection below that explains the approach that you prefer.

Use message (Simplest Approach)

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Cribl recommends this approach for environments where events enter Cribl Stream from the Syslog Source, because that Source supplies all required fields.

This approach is the simplest and least error-prone because Cribl Stream creates the payload for you. The following fields are required:

• facility
• severity
• _time
• host
• message

Cribl Stream will create a new syslog-adherent payload using these required fields. See Field Logic and Behavior below. To specify which RFC your events should adhere to, use General Settings > Optional Settings > Message format.

If these optional fields are included in the event, Cribl Stream will use them appropriately when constructing the payload:

• appname
• host
• procid
• msgid (RFC 5424 only)
• structuredData (RFC 5424 only)

Field Logic and Behavior

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Let’s examine the fields in a syslog-formatted event. Keep in mind that if Severity, Facility, or APP-NAME are not set on the event, Cribl Stream defaults to the corresponding value(s) configured in the Syslog Destination (Severity, Facility, and App name, respectively).

• __priority: If this field is present Cribl Stream will use it and ignore facility and severity.

• The version (used for RFC 5424 only, and not a field that you can set) follows priority (with no spaces in between).

• __facility or facility: Cribl Stream uses this field to calculate priority. The RFCs define Facility codes.

• __severity or severity: Cribl Stream also uses this field to calculate priority. The RFCs define Severity levels.

Use either (a) __facility and __severity or (b) facility and severity. For these fields, avoid mixing double-underscore naming with no-underscore naming, because if you mix them, Cribl Stream will ignore one of them and you will get unintended results.

If __priority is not set, Cribl Stream calculates the Priority from the values of Severity and Facility. Cribl Stream uses the formula: Priority = ((8 * Facility) + Severity).

• For example, if the facility is 13 (Security) and the severity is 2 (Critical), the priority will be (13 * 8) + 2 = 106.

Now let’s look at the rest of the fields. The list below shows field names in priority order: if both __appname and appname are present, Cribl Stream uses __appname.

• _time: The value of _time in Cribl events is in epoch format, but the syslog RFCs dictate that each event’s timestamp must be in human-readable format (see TIMESTAMP in RFC 3164 or RFC 5424). When defining a Syslog Destination, you configure this with General Settings > Timestamp format. The ISO8601 option (ISO 8601 format) defines the method for specifying time zone and year, while the older Syslog format lacks this information. For this reason, Cribl recommends ISO8601.

• host: the value for the required host field in a syslog event (see HOSTNAME in RFC 3164 or RFC 5424), following the timestamp.

• __appname or appname: The required application name (APP-NAME in RFC 5424). This is typically the name of the daemon or process that is logging any given event.

• __procid or procid: This optional field (PROCID in RFC 5424) is the Process ID. Use a numeric value, optionally surrounded with brackets – for example, [4321]. Cribl Stream will automatically adjust the spaces and syntax to ensure RFC-compliant formatting.

• __msgid or msgid: This optional field (MSGID in RFC 5424 only) is the Message ID.

• structuredData: This optional field (STRUCTURED-DATA in RFC 5424 only) contains a namespace and zero or more key-value pairs. Cribl Stream does not validate whether it adheres to the RFC.

Export __syslogout (Alternative Approach)

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The most common use of this approach is to send to a non-syslog downstream receiver such as a raw TCP listener. While there is no native “raw TCP” Destination in Cribl Stream, this approach – where you configure a Syslog Destination with the TCP protocol and __syslogout – can be an effective method for delivering raw data over TCP to a downstream receiver that does not enforce syslog RFCs.

When you set __syslogout, that field becomes the entire syslog message sent. Neither _raw nor any other metadata are sent downstream.

If you enable Octet count framing, Cribl Stream will prepend the number of bytes of the constructed __syslogout field to the message before sending it.

Cribl Stream does not check whether the value of __syslogout is RFC-compliant. This means that to send proper, RFC-compliant syslog messages, you must manually construct the __syslogout payload, starting with _time, for all the fields that Cribl Stream would automatically handle with the first approach above.

If the value of __syslogout fails to adhere to the syslog RFCs, a downstream syslog server may misinterpret the event; drop non-compliant events entirely; or, try to “fix” the format by supplying missing fields.

Troubleshooting

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

Live Data: Try capturing live data to see real-time events as they flow through the Destination. 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 delivery process, including any errors or warnings that may have occurred.

Test: Ensures that the Destination is correctly set up and reachable. Verify that sample events are sent correctly by clicking Run Test.

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