Grokking grok

The Elastic Stack ships with numerous predefined grok patterns that simplify working with grok. The syntax for reusing grok patterns takes one of the following forms:

For example, let’s say you have message data that looks like this:

3.44 55.3.244.1

The first value is a number, followed by what appears to be an IP address. You can match this text by using the following grok expression:

%{NUMBER:duration} %{IP:client}

To ease migration to the Elastic Common Schema (ECS), a new set of ECS-compliant patterns is available in addition to the existing patterns. The new ECS pattern definitions capture event field names that are compliant with the schema.

The ECS pattern set has all of the pattern definitions from the legacy set, and is a drop-in replacement. Use the ecs-compatability setting to switch modes.

New features and enhancements will be added to the ECS-compliant files. The legacy patterns may still receive bug fixes which are backwards compatible.

You can incorporate predefined grok patterns into Painless scripts to extract data. To test your script, use either the field contexts of the Painless execute API or create a runtime field that includes the script. Runtime fields offer greater flexibility and accept multiple documents, but the Painless execute API is a great option if you don’t have write access on a cluster where you’re testing a script.

For example, if you’re working with Apache log data, you can use the %{COMMONAPACHELOG} syntax, which understands the structure of Apache logs. A sample document might look like this:

"timestamp":"2020-04-30T14:30:17-05:00","message":"40.135.0.0 - -
[30/Apr/2020:14:30:17 -0500] \"GET /images/hm_bg.jpg HTTP/1.0\" 200 24736"

To extract the IP address from the message field, you can write a Painless script that incorporates the %{COMMONAPACHELOG} syntax. You can test this script using the ip field context of the Painless execute API, but let’s use a runtime field instead.

Based on the sample document, index the @timestamp and message fields. To remain flexible, use wildcard as the field type for message:

PUT /my-index/
{
  "mappings": {
    "properties": {
      "@timestamp": {
        "format": "strict_date_optional_time||epoch_second",
        "type": "date"
      },
      "message": {
        "type": "wildcard"
      }
    }
  }
}

Next, use the bulk API to index some log data into my-index.

POST /my-index/_bulk?refresh
{"index":{}}
{"timestamp":"2020-04-30T14:30:17-05:00","message":"40.135.0.0 - - [30/Apr/2020:14:30:17 -0500] \"GET /images/hm_bg.jpg HTTP/1.0\" 200 24736"}
{"index":{}}
{"timestamp":"2020-04-30T14:30:53-05:00","message":"232.0.0.0 - - [30/Apr/2020:14:30:53 -0500] \"GET /images/hm_bg.jpg HTTP/1.0\" 200 24736"}
{"index":{}}
{"timestamp":"2020-04-30T14:31:12-05:00","message":"26.1.0.0 - - [30/Apr/2020:14:31:12 -0500] \"GET /images/hm_bg.jpg HTTP/1.0\" 200 24736"}
{"index":{}}
{"timestamp":"2020-04-30T14:31:19-05:00","message":"247.37.0.0 - - [30/Apr/2020:14:31:19 -0500] \"GET /french/splash_inet.html HTTP/1.0\" 200 3781"}
{"index":{}}
{"timestamp":"2020-04-30T14:31:22-05:00","message":"247.37.0.0 - - [30/Apr/2020:14:31:22 -0500] \"GET /images/hm_nbg.jpg HTTP/1.0\" 304 0"}
{"index":{}}
{"timestamp":"2020-04-30T14:31:27-05:00","message":"252.0.0.0 - - [30/Apr/2020:14:31:27 -0500] \"GET /images/hm_bg.jpg HTTP/1.0\" 200 24736"}
{"index":{}}
{"timestamp":"2020-04-30T14:31:28-05:00","message":"not a valid apache log"}

Now you can define a runtime field in the mappings that includes your Painless script and grok pattern. If the pattern matches, the script emits the value of the matching IP address. If the pattern doesn’t match (clientip != null), the script just returns the field value without crashing.

PUT my-index/_mappings
{
  "runtime": {
    "http.clientip": {
      "type": "ip",
      "script": """
        String clientip=grok('%{COMMONAPACHELOG}').extract(doc["message"].value)?.clientip;
        if (clientip != null) emit(clientip);
      """
    }
  }
}

Alternatively, you can define the same runtime field but in the context of a search request. The runtime definition and the script are exactly the same as the one defined previously in the index mapping. Just copy that definition into the search request under the runtime_mappings section and include a query that matches on the runtime field. This query returns the same results as if you defined a search query for the http.clientip runtime field in your index mappings, but only in the context of this specific search:

GET my-index/_search
{
  "runtime_mappings": {
    "http.clientip": {
      "type": "ip",
      "script": """
        String clientip=grok('%{COMMONAPACHELOG}').extract(doc["message"].value)?.clientip;
        if (clientip != null) emit(clientip);
      """
    }
  },
  "query": {
    "match": {
      "http.clientip": "40.135.0.0"
    }
  },
  "fields" : ["http.clientip"]
}

Using the http.clientip runtime field, you can define a simple query to run a search for a specific IP address and return all related fields. The fields parameter on the _search API works for all fields, even those that weren’t sent as part of the original _source:

GET my-index/_search
{
  "query": {
    "match": {
      "http.clientip": "40.135.0.0"
    }
  },
  "fields" : ["http.clientip"]
}

The response includes the specific IP address indicated in your search query. The grok pattern within the Painless script extracted this value from the message field at runtime.

{
  "hits" : {
    "total" : {
      "value" : 1,
      "relation" : "eq"
    },
    "max_score" : 1.0,
    "hits" : [
      {
        "_index" : "my-index",
        "_id" : "1iN2a3kBw4xTzEDqyYE0",
        "_score" : 1.0,
        "_source" : {
          "timestamp" : "2020-04-30T14:30:17-05:00",
          "message" : "40.135.0.0 - - [30/Apr/2020:14:30:17 -0500] \"GET /images/hm_bg.jpg HTTP/1.0\" 200 24736"
        },
        "fields" : {
          "http.clientip" : [
            "40.135.0.0"
          ]
        }
      }
    ]
  }
}