Zeekurity Zen – Part VIII: How to Send Zeek Logs to Elastic

This is part of the Zeekurity Zen Zeries on building a Zeek (formerly Bro) network sensor.

Overview

In our Zeek journey thus far, we’ve:

• Set up Zeek to monitor some network traffic.
• Used Zeek Package Manager to install packages.
• Configured Zeek to send logs to Splunk for analysis.
• Uncovered notable events through basic threat hunting.
• Leveraged threat intelligence to elevate our threat hunting game.
• Detected malicious files by hash and extracted commonly exploited file types.
• Analyzed encrypted traffic through handshake details and fingerprinting.

In Part III, we sent our Zeek logs to Splunk and unlocked a new level of analysis and visualization capabilities.  But what if we don’t have access to Splunk? What if instead of Splunk, we use Elastic?

For those new to Elastic, the “Elastic Stack” or “ELK Stack” is a popular open source data analytics platform.  Elastic has become increasingly popular not just because it offers a free tier but also through the addition of significant security-focused features and an active user community.

Fortunately for us, Elastic natively supports Zeek logs and is a great way to analyze and visualize Zeek data.

To do this, we’ll walkthrough these steps:

1. Configure Zeek to output logs in JSON format.
2. Install and configure Filebeat to consume Zeek logs.
3. Write simple Kibana queries.

Output Zeek logs to JSON

1. Stop Zeek if it is currently running.

   zeekctl stop

2. Edit /opt/zeek/share/zeek/site/local.zeek and add the following.

   # Output to JSON
   @load policy/tuning/json-logs.zeek

3. Restart Zeek and view the logs in /opt/zeek/logs/current to confirm they are now in JSON format.

   zeekctl deploy
   cd /opt/zeek/logs/current
   less conn.log

Configure Filebeat

This guide assumes you have already installed Filebeat. If not, refer to Elastic’s documentation and then come back here when you’re done.

1. Switch back to your normal user.

   su eric

2. Stop Filebeat if it is currently running.

   sudo systemctl stop filebeat

3. Enable Filebeat’s Zeek module.

   sudo filebeat modules enable zeek

4. Add Zeek’s log paths (e.g., /opt/zeek/logs/current/http.log) to the Zeek Filebeat configuration file. Assuming you installed Filebeat from the standard Elastic repositories, the configuration file will be in /etc/filebeat/modules.d/zeek.yml. For each log type you want to capture:
   • Set the enabled field to true.
   • Set the var.paths field to the log’s specific path.

   The sample configuration file below enables all log types currently available in the Filebeat Zeek module. Note that while the “signature” log type is listed, it is currently not available and will result in an error if enabled. This Github issue discusses this in further detail.

   # Module: zeek
   # Docs: https://www.elastic.co/guide/en/beats/filebeat/7.x/filebeat-module-zeek.html
   - module: zeek
     capture_loss:
       enabled: true
       var.paths: ["/opt/zeek/logs/current/capture_loss.log"]
     connection:
       enabled: true
       var.paths: ["/opt/zeek/logs/current/conn.log"]
     dce_rpc:
       enabled: true
       var.paths: ["/opt/zeek/logs/current/dce_rpc.log"]
     dhcp:
       enabled: true
       var.paths: ["/opt/zeek/logs/current/dhcp.log"]
     dnp3:
       enabled: true
       var.paths: ["/opt/zeek/logs/current/dnp3.log"]
     dns:
       enabled: true
       var.paths: ["/opt/zeek/logs/current/dns.log"]
     dpd:
       enabled: true
       var.paths: ["/opt/zeek/logs/current/dpd.log"]
     files:
       enabled: true
       var.paths: ["/opt/zeek/logs/current/files.log"]
     ftp:
       enabled: true
       var.paths: ["/opt/zeek/logs/current/ftp.log"]
     http:
       enabled: true
       var.paths: ["/opt/zeek/logs/current/http.log"]
     intel:
       enabled: true
       var.paths: ["/opt/zeek/logs/current/intel.log"]
     irc:
       enabled: true
       var.paths: ["/opt/zeek/logs/current/irc.log"]
     kerberos:
       enabled: true
       var.paths: ["/opt/zeek/logs/current/kerberos.log"]
     modbus:
       enabled: true
       var.paths: ["/opt/zeek/logs/current/modbus.log"]
     mysql:
       enabled: true
       var.paths: ["/opt/zeek/logs/current/mysql.log"]
     notice:
       enabled: true
       var.paths: ["/opt/zeek/logs/current/notice.log"]
     ntlm:
       enabled: true
       var.paths: ["/opt/zeek/logs/current/ntlm.log"]
     ntp:
       enabled: true
       var.paths: ["/opt/zeek/logs/current/ntp.log"]
     ocsp:
       enabled: true
       var.paths: ["/opt/zeek/logs/current/oscp.log"]
     pe:
       enabled: true
       var.paths: ["/opt/zeek/logs/current/pe.log"]
     radius:
       enabled: true
       var.paths: ["/opt/zeek/logs/current/radius.log"]
     rdp:
       enabled: true
       var.paths: ["/opt/zeek/logs/current/rdp.log"]
     rfb:
       enabled: true
       var.paths: ["/opt/zeek/logs/current/rfb.log"]
     signature:
       enabled: false
       var.paths: ["/opt/zeek/logs/current/signature.log"]
     sip:
       enabled: true
       var.paths: ["/opt/zeek/logs/current/sip.log"]
     smb_cmd:
       enabled: true
       var.paths: ["/opt/zeek/logs/current/smb_cmd.log"]
     smb_files:
       enabled: true
       var.paths: ["/opt/zeek/logs/current/smb_files.log"]
     smb_mapping:
       enabled: true
       var.paths: ["/opt/zeek/logs/current/smb_mapping.log"]
     smtp:
       enabled: true
       var.paths: ["/opt/zeek/logs/current/smtp.log"]
     snmp:
       enabled: true
       var.paths: ["/opt/zeek/logs/current/snmp.log"]
     socks:
       enabled: true
       var.paths: ["/opt/zeek/logs/current/socks.log"]
     ssh:
       enabled: true
       var.paths: ["/opt/zeek/logs/current/ssh.log"]
     ssl:
       enabled: true
       var.paths: ["/opt/zeek/logs/current/ssl.log"]
     stats:
       enabled: true
       var.paths: ["/opt/zeek/logs/current/stats.log"]
     syslog:
       enabled: true
       var.paths: ["/opt/zeek/logs/current/syslog.log"]
     traceroute:
       enabled: true
       var.paths: ["/opt/zeek/logs/current/traceroute.log"]
     tunnel:
       enabled: true
       var.paths: ["/opt/zeek/logs/current/tunnel.log"]
     weird:
       enabled: true
       var.paths: ["/opt/zeek/logs/current/weird.log"]
     x509:
       enabled: true
       var.paths: ["/opt/zeek/logs/current/x509.log"]
       # Set custom paths for the log files. If left empty,
       # Filebeat will choose the paths depending on your OS.
       #var.paths:
   

5. Restart Filebeat to apply the configuration and confirm your Zeek logs are now properly ingested into Elasticsearch and available for analysis in Kibana.

   sudo systemctl restart filebeat

   

Simple Kibana Queries

Once Zeek logs are flowing into Elasticsearch, we can write some simple Kibana queries to analyze our data. Let’s convert some of our previous sample threat hunting queries from Splunk SPL into Elastic KQL. Try taking each of these queries further by creating relevant visualizations using Kibana Lens.

Connections To Destination Ports Above 1024

event.module:zeek AND event.dataset:zeek.connection AND destination.port>1024

Query Responses With NXDOMAIN

event.module:zeek AND event.dataset:zeek.dns AND dns.response_code:NXDOMAIN

Expired Certificates

event.module:zeek AND event.dataset:zeek.x509 AND file.x509.not_after < now

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