[[ml-info-functions]]
= Information Content Functions

The information content functions detect anomalies in the amount of information
that is contained in strings within a bucket. These functions can be used as
a more sophisticated method to identify incidences of data exfiltration or
C2 (Command and Control) activity, when analyzing the size in bytes of the data might not be sufficient.

The {ml-features} include the following information content functions:

* `info_content`, `high_info_content`, `low_info_content`

[discrete]
[[ml-info-content]]
== Info_content, High_info_content, Low_info_content

The `info_content` function detects anomalies in the amount of information that
is contained in strings in a bucket.

If you want to monitor for unusually high amounts of information,
use `high_info_content`.
If want to look at drops in information content, use `low_info_content`.

These functions support the following properties:

* `field_name` (required)
* `by_field_name` (optional)
* `over_field_name` (optional)
* `partition_field_name` (optional)

For more information about those properties, see the
{ref}/ml-put-job.html#ml-put-job-request-body[create {anomaly-jobs} API].

.Example 1: Analyzing subdomain strings with the info_content function
[source,js]
--------------------------------------------------
{
  "function" : "info_content",
  "field_name" : "subdomain",
  "over_field_name" : "highest_registered_domain"
}
--------------------------------------------------
// NOTCONSOLE

If you use this `info_content` function in a detector in your {anomaly-job}, it
models information that is present in the `subdomain` string. It detects
anomalies where the information content is unusual compared to the other
`highest_registered_domain` values. An anomaly could indicate an abuse of the
DNS protocol, such as malicious command and control activity.

NOTE: In this example, both high and low values are considered anomalous.
In many use cases, the `high_info_content` function is often a more appropriate
choice.

.Example 2: Analyzing query strings with the high_info_content function
[source,js]
--------------------------------------------------
{
  "function" : "high_info_content",
  "field_name" : "query",
  "over_field_name" : "src_ip"
}
--------------------------------------------------
// NOTCONSOLE

If you use this `high_info_content` function in a detector in your {anomaly-job},
it models information content that is held in the DNS query string. It detects
`src_ip` values where the information content is unusually high compared to
other `src_ip` values. This example is similar to the example for the
`info_content` function, but it reports anomalies only where the amount of
information content is higher than expected.

.Example 3: Analyzing message strings with the low_info_content function
[source,js]
--------------------------------------------------
{
  "function" : "low_info_content",
  "field_name" : "message",
  "by_field_name" : "logfilename"
}
--------------------------------------------------
// NOTCONSOLE

If you use this `low_info_content` function in a detector in your {anomaly-job},
it models information content that is present in the message string for each
`logfilename`. It detects anomalies where the information content is low
compared to its past behavior. For example, this function detects unusually low
amounts of information in a collection of rolling log files. Low information
might indicate that a process has entered an infinite loop or that logging
features have been disabled.