Detection: Detection of DNS Tunnels

Description

This search is used to detect DNS tunneling, by calculating the sum of the length of DNS queries and DNS answers. The search also filters out potential false positives by filtering out queries made to internal systems and the queries originating from internal DNS, Web, and Email servers. Endpoints using DNS as a method of transmission for data exfiltration, Command And Control, or evasion of security controls can often be detected by noting an unusually large volume of DNS traffic. NOTE:Deprecated because existing detection is doing the same. This detection is replaced with two other variations, if you are using MLTK then you can use this search ESCU - DNS Query Length Outliers - MLTK - Rule or use the standard deviation version ESCU - DNS Query Length With High Standard Deviation - Rule, as an alternantive.

Search

 1
 2| tstats `security_content_summariesonly` dc("DNS.query") as count  from datamodel=Network_Resolution  where nodename=DNS "DNS.message_type"="QUERY" NOT (`cim_corporate_web_domain_search("DNS.query")`) NOT "DNS.query"="*.in-addr.arpa" NOT ("DNS.src_category"="svc_infra_dns" OR "DNS.src_category"="svc_infra_webproxy" OR "DNS.src_category"="svc_infra_email*"   ) by "DNS.src","DNS.query" 
 3| rename "DNS.src" as src  "DNS.query" as message 
 4| eval length=len(message) 
 5| stats sum(length) as length by src 
 6| append [ tstats `security_content_summariesonly` dc("DNS.answer") as count  from datamodel=Network_Resolution  where nodename=DNS "DNS.message_type"="QUERY" NOT (`cim_corporate_web_domain_search("DNS.query")`) NOT "DNS.query"="*.in-addr.arpa" NOT ("DNS.src_category"="svc_infra_dns" OR "DNS.src_category"="svc_infra_webproxy" OR "DNS.src_category"="svc_infra_email*"   ) by "DNS.src","DNS.answer" 
 7| rename "DNS.src" as src  "DNS.answer" as message 
 8| eval message=if(message=="unknown","", message) 
 9| eval length=len(message) 
10| stats sum(length) as length by src ] 
11| stats sum(length) as length by src 
12| where length > 10000 
13| `detection_of_dns_tunnels_filter`

Data Source

Macros Used

Annotations

Default Configuration

This detection is configured by default in Splunk Enterprise Security to run with the following settings:

Implementation

To successfully implement this search, we must ensure that DNS data is being ingested and mapped to the appropriate fields in the Network_Resolution data model. Fields like src_category are automatically provided by the Assets and Identity Framework shipped with Splunk Enterprise Security. You will need to ensure you are using the Assets and Identity Framework and populating the src_category field. You will also need to enable the cim_corporate_web_domain_search() macro which will essentially filter out the DNS queries made to the corporate web domains to reduce alert fatigue.

Known False Positives

It's possible that normal DNS traffic will exhibit this behavior. If an alert is generated, please investigate and validate as appropriate. The threshold can also be modified to better suit your environment.

Associated Analytic Story

• Command And Control

• Data Protection

• Suspicious DNS Traffic

Risk Based Analytics (RBA)

Detection Testing

Replay any dataset to Splunk Enterprise by using our replay.py tool or the UI. Alternatively you can replay a dataset into a Splunk Attack Range

Source: GitHub | Version: 3