Snort FAQ


Stream5 High Availability (HA)

Michael Altizer

Documentation last updated 2013-11-21


# THIS CODE IS STILL EXPERIMENTAL! # DO NOT USE IN PRODUCTION ENVIRONMENTS. # Please send any issues to the Snort team ##########################################

== Overview == Starting in Snort 2.9.5, there is limited support for sharing of session state for the purposes of high availability. Currently, it supports sharing the following state from Stream5 lightweight sessions: * The Session Key * Session Flags * IP Protocol (requires TARGET_BASED) * Application Protocol (requires TARGET_BASED) * Direction * Ignore Direction

There are two types of Stream5 HA messages: updates and deletions. Both types use a serialized session key to describe the target session. In the case of a deletion message being received, there is no additional data and the target session will deleted from the session cache if it is present. Update messages, on the other hand, may contain a number of data records, including the non-key HA state enumerated above. If the session to be updated was not present in the cache, it will be created, otherwise it will be updated. In either case, the session will be placed into “HA standby” mode, indicating that the current Snort instance is not the owner of the session. Sessions remain in “HA standby” mode until Snort receives a packet on them and will not time out normally, although they will be pruned as normal when a cache is being overrun.

An HA update message will be generated when a session is marked as requiring synchronization (the criteria for which differ per session type) and the time is appropriate based on the configuration. The two configurable time constraints are Minimum Session Lifetime and Minimum Synchronization Interval. In general, no HA update messages will be sent for a session prior to the Minimum Session Lifetime having elapsed, and update messages on a given session will not be sent more often than once per Minimum Synchronization Interval. Certain critical changes to a session will result in these time constraints being ignored. A configured value of 0 for either time constraint will also effectively result in it being ignored.

== Extensibility == There is a framework for preprocessors to register to generate and consume HA data records built into the current implementation. In order to register, a preprocessor must call RegisterStreamHAFuncs() with a preprocessor ID, a subcode unique to that preprocessor, the maximum size of its data record, and pointers to the functions for producing and consuming its own data records. Multiple different data records may be generated and consumed by a preprocessor simply by registering additional functions with a different subcode. The return value from RegisterStreamHAFuncs() is a bit index that must be passed in to Stream5SetHAPendingBit() to indicate that a session has changed and that an update message should be generated.

NOTE: The current implementation is limited to a maximum of 8 such data records being registered across all preprocessors. (Determined by the width of ha_pending_mask in the Stream5LWSession structure.)

== Configuration == In order to utilize the functionality, Snort must be configured and compiled with –enable-ha.

In order to enable Stream5 HA, the ‘enable_ha’ option must be given to the ‘stream5_global’ directive.

There is a new preprocessor ‘stream5_ha’ configuration directive which accepts the following options: * min_session_lifetime - The number of milliseconds a session must have lived before it will generate HA update messages. It must be an integer between 0 and 65535. Default: 0. * min_sync_interval - The number of milliseconds since the last update message was generated before a session will generate another. It must be an integer between 0 and 65535. Default: 0. * startup_input_file - The name of a file for Snort to read HA messages from at startup to prime the session caches with saved state. * runtime_output_file - The name of a file to which Snort will write all HA messages that are generated while it is running. * use_side_channel - Indicates that all HA messages should also be sent to the side channel for processing. No value. NOTE: Requires that Snort has been compiled with the Side Channel feature and it has been enabled.

== Example/Default Configuration == preprocessor stream5_global: track_udp yes, track_tcp yes, enable_ha preprocessor stream5_ha: min_session_lifetime 1000, min_sync_interval 100, startup_input_file, runtime_output_file ha.out, use_side_channel