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SC4S Configuration Variables

Other than device filter creation, SC4S is almost entirely controlled by environment variables. Here are the categories and variables needed to properly configure SC4S for your environment.

Global Configuration

Variable Values Description
SPLUNK_HEC_URL url URL(s) of the Splunk endpoint, can be a single URL space seperated list
SPLUNK_HEC_TOKEN string Splunk HTTP Event Collector Token
SC4S_USE_REVERSE_DNS yes or no(default) use reverse DNS to identify hosts when HOST is not valid in the syslog header
SC4S_CONTAINER_HOST string variable passed to the container to identify the actual log host for container implementations
  • NOTE: Do not configure HEC Acknowledgement when deploying the HEC token on the Splunk side; the underlying syslog-ng http destination does not support this feature. Moreover, HEC Ack would significantly degrade performance for streaming data such as syslog.

  • NOTE: Use of the SC4S_USE_REVERSE_DNS variable can have a significant impact on performance if the reverse DNS facility (typically a caching nameserver) is not performant. If you notice events being indexed far later than their actual timestamp in the event (latency between _indextime and _time), this is the first place to check.

Splunk HEC Destination Configuration

Variable Values Description
SC4S_DEST_SPLUNK_HEC_GLOBAL yes Send events to Splunk using HEC. This applies only to the primary HEC destination.
SC4S_DEST_SPLUNK_HEC_CIPHER_SUITE comma separated list Open SSL cipher suite list
SC4S_DEST_SPLUNK_HEC_SSL_VERSION comma separated list Open SSL version list
SC4S_DEST_SPLUNK_HEC_TLS_CA_FILE container path /opt/syslog-ng/tls/server.pem Custom trusted cert file, specified as a full path in the container filesystem: /opt/syslog-ng/tls/<ca-file>
Ensure that the container TLS directory /opt/syslog-ng/tls is available locally via container mount in the docker-compose.yml or systemd unit file, and that you place the CA file in the locally-mounted directory.
SC4S_DEST_SPLUNK_HEC_TLS_VERIFY yes(default) or no verify HTTP(s) certificate
SC4S_DEST_SPLUNK_HEC_WORKERS numeric Number of destination workers (default: 10 threads). This should rarely need to be changed; consult sc4s community for advice on appropriate setting in extreme high- or low-volume environments.

List of sc4s indexed fields that will be included with each event in Splunk (default is the entire list except “none”). Two other indexed fields, sc4s_vendor_product and sc4s_syslog_format, will also appear along with the fields selected via the list and cannot be turned on or off individually. If no indexed fields are desired (including the two internal ones), set the value to the single value of “none”. When setting this variable, separate multiple entries with commas and do not include extra spaces.

This list maps to the following indexed fields that will appear in all Splunk events:
facility: sc4s_syslog_facility
severity: sc4s_syslog_severity
container: sc4s_container
loghost: sc4s_loghost
dport: sc4s_destport
fromhostip: sc4s_fromhostip
proto: sc4s_proto
  • NOTE: When using alternate HEC destinations, the destination operating paramaters outlined above (CIPHER_SUITE, SSL_VERSION, etc.) can be individually controlled per DESTID (see “Configuration of Additional Splunk HEC Destinations” immediately below). For example, to set the number of workers for the alternate HEC destination d_hec_FOO to 24, set SC4S_DEST_SPLUNK_HEC_FOO_WORKERS=24.

Creation of Additional Splunk HEC Destinations

Additional Splunk HEC destinations can be dynamically created through environment variables. When set, the destinations will be created with the DESTID appended, for example: d_hec_FOO. These destinations can then be specified for use (along with any other destinations created locally) either globally or per source. See the “Alternate Destination Use” in the next section for details.

Variable Values Description
SPLUNK_HEC_ALT_DESTS Comma or space-separated UPPER case list of destination IDs Destination IDs are UPPER case, single-word friendly strings used to identify the new destinations which will be named with the DESTID appended, for example d_hec_FOO
SPLUNK_HEC_<DESTID>_URL url Example: SPLUNK_HEC_FOO_URL=https://splunk:8088 DESTID must be a member of the list specified in SPLUNK_HEC_ALT_DESTS configured above
SPLUNK_HEC_<DESTID>_TOKEN string Example: SPLUNK_HEC_BAR_TOKEN=<token> DESTID must be a member of the list specified in SPLUNK_HEC_ALT_DESTS configured above
  • NOTE: The DESTID specified in the URL and TOKEN variables above must match the DESTID entries enumerated in the SPLUNK_HEC_ALT_DESTS list. For each DESTID value specified in SPLUNK_HEC_ALT_DESTS there must be a corresponding URL and TOKEN variable set as well. Failure to do so will cause destinations to be created without proper HEC parameters which will result in connection failure.

  • NOTE: Additional Splunk HEC destinations will not be tested at startup. It is the responsiblity of the admin to ensure that additional destinations are provisioned with the correct URL(s) and tokens to ensure proper connectivity.

  • NOTE: The disk and CPU requirements will increase proportionally depending on the number of additional HEC destinations in use (e.g. each HEC destination will have its own disk buffer by default).

Alternate Destination Use

All alternate destinations (including alternate HEC destinations) are configured for use in SC4S through the variables below. Global and/or source-specific forms of the variables below can be used to send data to additional and/or alternate destinations.

  • NOTE: The administrator is responsible for ensuring that any non-HEC alternate destinations are configured in the local mount tree, and that the underlying syslog-ng process in sc4s properly parses them.

  • NOTE: Do not include the primary HEC destination (d_hec) in any list of alternate destinations. The configuration of the primary HEC destination is configured separately from that of the alternates below. However, alternate HEC destinations (e.g. d_hec_FOO) should be configured below, just like any other user-supplied destination.

Variable Values Description
SC4S_DEST_GLOBAL_ALTERNATES Comma or space-separated list of destinations Send all sources to alternate destinations
SC4S_DEST_<VENDOR_PRODUCT>_ALTERNATES Comma or space-separated list of syslog-ng destinations Send specific sources to alternate syslog-ng destinations using the VENDOR_PRODUCT syntax, e.g. SC4S_DEST_CISCO_ASA_ALTERNATES

SC4S Disk Buffer Configuration

Disk buffers in SC4S are allocated per destination. Keep this in mind when using additional destinations that have disk buffering configured. By default, when alternate HEC destinations are configured as outlined above disk buffering will be configured identically to that of the main HEC destination (unless overriden individually).

Important Notes Regarding Disk Buffering:

  • “Reliable” disk buffering offers little advantage over “normal” disk buffering, at a significant performance penalty. For this reason, normal disk buffering is recommended.

  • If you add destinations locally in your configuration, pay attention to the cumulative buffer requirements when allocating local disk.

  • Disk buffer storage is configured via container volumes and is persistent between restarts of the conatiner. Be sure to account for disk space requirements on the local sc4s host when creating the container volumes in your respective runtime environment (outlined in the “getting started” runtime docs). These volumes can grow significantly if there is an extended outage to the SC4S destinations (HEC endpoints). See the “SC4S Disk Buffer Configuration” section on the Configruation page for more info.

  • The values for the variables below represent the total sizes of the buffers for the destination. These sizes are divded by the number of workers (threads) when setting the actual syslog-ng buffer options, because the buffer options apply to each worker rather than the entire destination. Pay careful attention to this when using the “BYOE” version of SC4S, where direct access to the syslog-ng config files may hide this nuance. Lastly, be sure to factor in the syslog-ng data structure overhead (approx. 2x raw message size) when calculating the total buffer size needed. To determine the proper size of the disk buffer, consult the “Data Resilience” section below.

  • When changing the disk buffering directory, the new directory must exist. If it doesn’t, then syslog-ng will fail to start.

  • When changing the disk buffering directory, if buffering has previously occurd on that instance, a persist file may exist which will prevent syslog-ng from changing the directory.

Disk Buffer Variables

Variable Values/Default Description
SC4S_DEST_SPLUNK_HEC_DISKBUFF_ENABLE yes(default) or no Enable local disk buffering
SC4S_DEST_SPLUNK_HEC_DISKBUFF_RELIABLE yes or no(default) Enable reliable/normal disk buffering (normal recommended)
SC4S_DEST_SPLUNK_HEC_DISKBUFF_MEMBUFSIZE bytes (10241024) Memory buffer size in bytes (used with reliable disk buffering)
SC4S_DEST_SPLUNK_HEC_DISKBUFF_MEMBUFLENGTH messages (15000) Memory buffer size in message count (used with normal disk buffering)
SC4S_DEST_SPLUNK_HEC_DISKBUFF_DISKBUFSIZE bytes (53687091200) Size of local disk buffer in bytes (default 50 GB)
SC4S_DEST_SPLUNK_HEC_DEFAULT_DISKBUFF_DIR path Location to store the disk buffer files. This variable should only be set when using BYOE; this location is fixed when using the Container.

Archive File Configuration

This feature is designed to support compliance or “diode mode” archival of all messages. Instructions for mounting the appropriate local directory to use this feature are included in each “getting started” runtime document. The files will be stored in a folder structure at the mount point using the pattern shown in the table below depending on the value of the SC4S_GLOBAL_ARCHIVE_MODE variable. All events for both modes are formatted using syslog-ng’s EWMM template.

Variable Value/Default Location/Pattern
SC4S_GLOBAL_ARCHIVE_MODE compliance(default) <archive mount>/${YEAR}/${MONTH}/${DAY}/${fields.sc4s_vendor_product}_${YEAR}${MONTH}${DAY}${HOUR}${MIN}.log"
SC4S_GLOBAL_ARCHIVE_MODE diode <archive mount>/${.splunk.sourcetype}/${HOST}/$YEAR-$MONTH-$DAY-archive.log


Use the following variables to select global archiving or per-source archiving. C4S does not prune the files that are created; therefore the administrator must provide a means of log rotation to prune files and/or move them to an archival system to avoid exhaustion of disk space.

Variable Values Description
SC4S_ARCHIVE_GLOBAL yes or undefined Enable archive of all vendor_products
SC4S_ARCHIVE_<VENDOR_PRODUCT> yes(default) or undefined See sources section of documentation enables selective archival

Syslog Source Configuration

Variable Values/Default Description
SC4S_LISTEN_DEFAULT_TLS_PORT undefined or 6514 Enable a TLS listener on port 6514
SC4S_SOURCE_TLS_OPTIONS See openssl List of SSl/TLS protocol versions to support
SC4S_SOURCE_TLS_CIPHER_SUITE See openssl List of Ciphers to support
SC4S_SOURCE_TCP_MAX_CONNECTIONS 2000 Max number of TCP Connections
SC4S_SOURCE_TCP_IW_SIZE 20000000 Initial Window size
SC4S_SOURCE_TCP_FETCH_LIMIT 2000 Number of events to fetch from server buffer at once
SC4S_SOURCE_UDP_SO_RCVBUFF 1703936 UDP server buffer size in bytes. Make sure that the host OS kernel is configured similarly.
SC4S_SOURCE_LISTEN_UDP_SOCKETS 1 Number of kernel sockets per active UDP port, which configures multi-threading of the UDP input buffer in the kernel to prevent packet loss. Total UDP input buffer is the multiple of SC4S_SOURCE_LISTEN_UDP_SOCKETS * SC4S_SOURCE_UDP_SO_RCVBUFF
SC4S_SOURCE_STORE_RAWMSG undefined or “no” Store unprocessed “on the wire” raw message in the RAWMSG macro for use with the “fallback” sourcetype. Do not set this in production; substantial memory and disk overhead will result. Use for log path/filter development only.

Syslog Source TLS Certificate Configuration

  • Create a folder /opt/sc4s/tls if not already done as part of the “getting started” process.
  • Uncomment the appropriate mount line in the unit or yaml file (again, documented in the “getting started” runtime documents).
  • Save the server private key in PEM format with NO PASSWORD to /opt/sc4s/tls/server.key
  • Save the server certificate in PEM format to /opt/sc4s/tls/server.pem
  • Add the following line to /opt/sc4s/env_file

SC4S metadata configuration

Log Path overrides of index or metadata

A key aspect of SC4S is to properly set Splunk metadata prior to the data arriving in Splunk (and before any TA processing takes place). The filters will apply the proper index, source, sourcetype, host, and timestamp metadata automatically by individual data source. Proper values for this metadata, including a recommended index and output format (template), are included with all “out-of-the-box” log paths included with SC4S and are chosen to properly interface with the corresponding TA in Splunk. The administrator will need to ensure all recommneded indexes be created to accept this data if the defaults are not changed.

It is understood that default values will need to be changed in many installations. To accomodate this, each filter consults a lookup file that is mounted to the container (by default /opt/sc4s/local/context/splunk_metadata.csv) and is populated with defaults on the first run of SC4S after being set up according to the “getting started” runtime documents. This is a CSV file containing a “key” that is referenced in the log path for each data source. These keys are documented in the individual source files in this section, and allow one to override Splunk metadata either in whole or part. The use of this file is best shown by example. Here is the “Sourcetype and Index Configuration” table from the Juniper Netscreen source documentation page in this section:

key sourcetype index notes
juniper_netscreen netscreen:firewall netfw none

Here is a snippet from the splunk_metadata.csv file:


The columns in this file are key, metadata, and value. Defaults are populated into this file at initial startup, and any changes made will be preserved on subsequent startups. Changes can be made by modifying and/or adding rows in the table and specifying one or more of the following metadata/value pairs for a given key:

  • index to specify an alternate value for index
  • source to specify an alternate value for source
  • host to specify an alternate value for host
  • sourcetype to specify an alternate value for sourcetype (be very careful when changing this; only do so if an upstream TA is not being used, or a custom TA (built by you) is being used.)
  • sc4s_template to specify an alternate value for the syslog-ng template that will be used to format the event that will be indexed by Splunk. Changing this carries the same warning as the sourcetype above; this will affect the upstream TA. The template choices are documented elsewhere in this “Configuration” section.

In this case, the juniper_netscreen key references a new index used for that data source called ns_index.

In general, for most deployments the index should be the only change needed; other default metadata should almost never be overridden (particularly for the “Out of the Box” data sources). Even then, care should be taken when considering any alternates, as the defaults for SC4S were chosen with best practices in mind.

The splunk_metadata.csv file should also be appended to with an appropriate default for the index when building a custom SC4S log path (filter). Care should be taken during filter design to choose appropriate index, sourctype and template defaults, so that admins are not compelled to override them.

Override index or metadata based on host, ip, or subnet (compliance overrides)

In other cases it is appropriate to provide the same overrides but based on PCI scope, geography, or other criterion rather than globally. This is accomplished by the use of a file that uniquely identifies these source exceptions via syslog-ng filters, which maps to an associated lookup of alternate indexes, sources, or other metadata. In addition, (indexed) fields can also be added to futher classify the data.

  • The conf and csv files referenced below will be populated into the /opt/sc4s/local/context directory when SC4S is run for the first time after being set up according to the “getting started” runtime documents, in a similar fashion to splunk_metadata.csv. After this first-time population of the files takes place, they can be edited (and SC4S restarted) for the changes to take effect. To get started:

  • Edit the file compliance_meta_by_source.conf to supply uniquely named filters to identify events subject to override.

  • Edit the file compliance_meta_by_source.csv to supply appropriate field(s) and values.

The three columns in the csv file are filter name, field name, and value. Filter names in the conf file must match one or more corresonding filter name rows in the csv file. The field name column obeys the following convention:

  • .splunk.index to specify an alternate value for index
  • .splunk.source to specify an alternate value for source
  • .splunk.sourcetype to specify an alternate value for sourcetype (be very careful when changing this; only do so if a downstream TA is not being used, or a custom TA (built by you) is being used.)
  • fields.fieldname where fieldname will become the name of an indexed field sent to Splunk with the supplied value

This file construct is best shown by an example. Here is a sample compliance_meta_by_source.conf file:

filter f_test_test {
   host("something-*" type(glob)) or

and the corresponding compliance_meta_by_source.csv file:


First off, ensure that the filter name(s) in the conf file match one or more rows in the csv file. In this case, any incoming message with a hostname starting with something- or arriving from a netmask of will match the f_test_test filter, and the corresponding entries in the csv file will be checked for overrides. In this case, the new index is pciindex, and an indexed field named compliance will be sent to Splunk, with it’s value set to pci. To add additional overrides, simply add another filter foo_bar {}; stanza to the conf file, and add appropriate entries to the csv file that match the filter name(s) to the overrides you deisre.

  • IMPORTANT: The files above are actual syslog-ng config file snippets that get parsed directly by the underlying syslog-ng process. Take care that your syntax is correct; for more information on proper syslog-ng syntax, see the syslog-ng documentation. A syntax error will cause the runtime process to abort in the “preflight” phase at startup.

Finally, to update your changes for the systemd-based runtimes, restart SC4S using the commands:

sudo systemctl daemon-reload
sudo systemctl restart sc4s

For the Docker Swarm runtime, redeploy the updated service using the command:

docker stack deploy --compose-file docker-compose.yml sc4s

Dropping all data by ip or subnet

In some cases rogue or port-probing data can be sent to SC4S from misconfigured devices or vulnerability scanners. Update the vendor_product_by_source.conf filter f_null_queue with one or more ip/subnet masks to drop events without logging. Note that drop metrics will be recorded.

Fixing (overriding) the host field

In some cases the host value is not present in an event (or an IP address is in its place). For administrators who require a true hostname be attached to each event, SC4S provides an optional facilty to perform a reverse IP to name lookup. If the variable SC4S_USE_REVERSE_DNS is set to “yes”, SC4S will first check host.csv and replace the value of host with the value specified that matches the incoming IP address. If a value is not found in host.csv then a reverse DNS lookup will be attempted against the configured nameserver. The IP address will only be used as the host value as a last resort.

  • NOTE: Use of this variable can have a significant impact on performance if the reverse DNS facility (typically a caching nameserver) is not performant. If you notice events being indexed far later than their actual timestamp in the event (latency between _indextime and _time), this is the first place to check.

Splunk Connect for Syslog output templates (syslog-ng templates)

Splunk Connect for Syslog utilizes the syslog-ng template mechanism to format the output payload (event) that will be sent to Splunk. These templates can format the messages in a number of ways (straight text, JSON, etc.) as well as utilize the many syslog-ng “macros” (fields) to specify what gets placed in the payload that is delivered to the destination. Here is a list of the templates used in SC4S, which can be used in the metadata override section immediately above. New templates can also be added by the administrator in the “local” section for local destinations; pay careful attention to the syntax as the templates are “live” syslog-ng config code.

Template name Template contents Notes
t_standard ${DATE} ${HOST} ${MSGHDR}${MESSAGE} Standard template for most RFC3164 (standard syslog) traffic
t_msg_only ${MSGONLY} syslog-ng $MSG is sent, no headers (host, timestamp, etc.)
t_msg_trim $(strip $MSGONLY) As above with whitespace stripped
t_everything ${ISODATE} ${HOST} ${MSGHDR}${MESSAGE} Standard template with ISO date format
t_hdr_msg ${MSGHDR}${MESSAGE} Useful for non-compliant syslog messages
t_legacy_hdr_msg ${LEGACY_MSGHDR}${MESSAGE} Useful for non-compliant syslog messages
t_hdr_sdata_msg ${MSGHDR}${MSGID} ${SDATA} ${MESSAGE} Useful for non-compliant syslog messages
t_program_msg ${PROGRAM}[${PID}]: ${MESSAGE} Useful for non-compliant syslog messages
t_program_nopid_msg ${PROGRAM}: ${MESSAGE} Useful for non-compliant syslog messages
t_JSON_3164 $(format-json –scope rfc3164
–pair PRI=”<$PRI>”
–exclude FACILITY
–exclude PRIORITY)
JSON output of all RFC3164-based syslog-ng macros. Useful with the “fallback” sourcetype to aid in new filter development.
t_JSON_5424 $(format-json –scope rfc5424
–pair PRI=”<$PRI>”
–exclude DATE
–exclude FACILITY
–exclude PRIORITY)
JSON output of all RFC5424-based syslog-ng macros; for use with RFC5424-compliant traffic.
t_JSON_5424_SDATA $(format-json –scope rfc5424
–pair PRI=”<$PRI>”
–exclude DATE
–exclude FACILITY
–exclude PRIORITY)
–exclude MESSAGE
JSON output of all RFC5424-based syslog-ng macros except for MESSAGE; for use with RFC5424-compliant traffic.

Data Resilience - Local Disk Buffer Configuration

SC4S provides capability to minimize the number of lost events if the connection to all the Splunk Indexers goes down. This capability utilizes the disk buffering feature of Syslog-ng. SC4S receives a response from the Splunk HTTP Event Collector (HEC) when a message is received successfully. If a confirmation message from the HEC endpoint is not received (or a “server busy” reply, such as a “503” is sent), the load balancer will try the next HEC endpoint in the pool. If all pool members are exhausted (such as would occur if there were a full network outage to the HEC endpoints), events will queue to the local disk buffer on the SC4S Linux host. SC4S will continue attempting to send the failed events while it buffers all new incoming events to disk. If the disk space allocated to disk buffering fills up then SC4S will stop accepting new events and subsequent events will be lost. Once SC4S gets confirmation that events are again being received by one or more indexers, events will then stream from the buffer using FIFO queueing. The number of events in the disk buffer will reduce as long as the incoming event volume is less than the maximum SC4S (with the disk buffer in the path) can handle. When all events have been emptied from the disk buffer, SC4S will resume streaming events directly to Splunk.

For more detail on the Syslog-ng behavior the documentation can be found here:

SC4S has disk buffering enabled by default and it is strongly recommended that you keep it on, however this feature does have a performance cost. Without disk buffering enabled SC4S can handle up to 345K EPS (800 bytes/event avg) With “Normal” disk buffering enabled SC4S can handle up to 60K EPS (800 bytes/event avg) – This is still a lot of data!

To guard against data loss it is important to configure the appropriate type and amount of storage for SC4S disk buffering. To estimate the storage allocation, follow these steps:

  • Start with your estimated maximum events per second that each SC4S server will experience. Based on the maximum throughput of SC4S with disk buffering enabled, the conservative estimate for maximum events per second would be 60K (however, you should use the maximum rate in your environment for this calculation, not the max rate SC4S can handle).
  • Next is your average estimated event size based on your data sources. It is common industry practice to estimate log events as 800 bytes on average.
  • Then, factor in the maximum length of connectivity downtime you want disk buffering to be able to handle. This measure is very much dependent on your risk tolerance.
  • Lastly, syslog-ng imposes significant overhead to maintain its internal data structures (primarily macros) so that the data can be properly “played back” upon network restoration. This overhead currently runs at about 1.7x above the total storage size for the raw messages themselves, and can be higher for “fallback” data sources due to the overlap of syslog-ng macros (data fields) containing some or all of the original message.

For example, to protect against a full day of lost connectivity from SC4S to all your indexers at maximum throughput the calculation would look like the following:

60,000 EPS * 86400 seconds * 800 bytes * 1.7 = 6.4 TB of storage

To configure storage allocation for the SC4S disk buffering, do the following:

  • Edit the file /opt/sc4s/default/env_file
  • Add the SC4S_DEST_SPLUNK_HEC_DISKBUFF_DISKBUFSIZE variable to the file and set the value to the number of bytes based on your estimation (e.g. 7050240000000 in the example above)
  • Splunk does not recommend reducing the disk allocation below 500 GB
  • Restart SC4S

Given that in a connectivity outage to the Indexers events will be saved and read from disk until the buffer is emptied, it is ideal to use the fastest type of storage available. For this reason, NVMe storage is recommended for SC4S disk buffering.

It is best to design your deployment so that the disk buffer will drain after connectivity is restored to the Splunk Indexers (while incoming data continues at the same general rate). Since “your mileage may vary” with different combinations of data load, instance type, and disk subsystem performance, it is good practice to provision a box that performs twice as well as is required for your max EPS. This headroom will allow for rapid recovery after a connectivity outage.