Zeek

Zeek is formerly known as Bro. From https://www.zeek.org/:

Zeek is a powerful network analysis framework that is much different from the typical IDS you may know. (Zeek is the new name for the long-established Bro system. Note that parts of the system retain the “Bro” name, and it also often appears in the documentation and distributions.)

Zeek logs are sent to Elasticsearch for parsing and storage and can then be found in Hunt and Kibana. Here’s an example of Zeek conn (connection) logs in Hunt:

Community ID

Security Onion enables Zeek’s native support for Community ID.

Performance

Zeek uses AF-PACKET so that you can spin up multiple Zeek workers to handle more traffic.

To change the number of AF-PACKET workers for Zeek:

• Stop Zeek:

  sudo so-zeek-stop
  

• Edit /opt/so/saltstack/local/pillar/minions/$SENSORNAME_$ROLE.sls and change the zeek_lbprocs variable to the desired number of cores.

• Start Zeek:

  sudo so-zeek-start
  

For best performance, Zeek should be pinned to specific CPUs. In most cases, you’ll want to pin sniffing processes to the same CPU that your sniffing NIC is bound to. You can do this using the pin_cpus setting as shown at https://docs.zeek.org/en/stable/configuration/#using-pf-ring.

Email

To configure email notifications, please see the Email Configuration section.

Syslog

To forward Zeek logs to an external syslog collector, please see the Syslog Output section.

Intel

You can add your own intel to /opt/so/saltstack/local/salt/zeek/policy/intel/intel.dat on the manager and then run sudo salt $SENSORNAME_$ROLE state.highstate. When writing this file, ensure there are no leading/trailing spaces or lines, and that only a single tab is used to separate fields. If you experience an error, or do not notice /nsm/zeek/logs/current/intel.log being generated, try having a look in /nsm/zeek/logs/current/reporter.log for clues. You may also want to restart Zeek after making changes by running sudo so-zeek-restart.

For more information, please see:

https://docs.zeek.org/en/latest/frameworks/intel.html

http://blog.bro.org/2014/01/intelligence-data-and-bro_4980.html

https://github.com/weslambert/securityonion-misp

Custom Scripts

Custom scripts can be added to /opt/so/saltstack/local/salt/zeek/policy/custom/<$custom-module> on the manager. The custom folder is mapped to Zeek through Docker on the minions. Once the script module is created, the configuration for local.zeek will need to be updated. In Security Onion 2, this configuration is abstracted into a SaltStack pillar. For example, we would copy /opt/so/saltstack/default/pillar/zeek/init.sls to /opt/so/saltstack/local/pillar/zeek/init.sls, and add our custom module to be loaded by Zeek (alternatively, the pillar could be modified in the global.sls file. More details can be found here here: https://docs.securityonion.net/en/latest/zeek.html#configuration):

zeek:
  local:
    '@load':
      - misc/loaded-scripts
      - tuning/defaults
      - misc/capture-loss
      - misc/stats
      - frameworks/software/vulnerable
      - frameworks/software/version-changes
      - protocols/ftp/software
      - protocols/smtp/software
      - protocols/ssh/software
      - protocols/http/software
      - protocols/dns/detect-external-names
      - protocols/ftp/detect
      - protocols/conn/known-hosts
      - protocols/conn/known-services
      - protocols/ssl/known-certs
      - protocols/ssl/validate-certs
      - protocols/ssl/log-hostcerts-only
      - protocols/ssh/geo-data
      - protocols/ssh/detect-bruteforcing
      - protocols/ssh/interesting-hostnames
      - protocols/http/detect-sqli
      - frameworks/files/hash-all-files
      - frameworks/files/detect-MHR
      - policy/frameworks/notice/extend-email/hostnames
      - ja3
      - hassh
      - intel
      - cve-2020-0601
      - securityonion/bpfconf
      - securityonion/communityid
      - securityonion/file-extraction
      - custom/$module-name

One the configuration has been updated, Zeek can be restarted with sudo so-zeek-restart on applicable nodes to pick up the changes. Finally, /nsm/zeek/logs/current/loaded_scripts.log can be checked to ensure the new module has been loaded. For example:

grep mynewmodule /nsm/zeek/logs/current/loaded_scripts.log

Logs

Zeek logs are stored in /nsm/zeek/logs. They are collected by Filebeat, parsed by and stored in Elasticsearch, and viewable in Hunt and Kibana.

We configure Zeek to output logs in JSON format. If you need to parse those JSON logs from the command line, you can use jq.

If you want to specify what Zeek logs are ingested, you can use so-zeek-logs.

Zeek monitors your network traffic and creates logs, such as:

conn.log

• TCP/UDP/ICMP connections
• For more information, see:

https://docs.zeek.org/en/latest/scripts/base/protocols/conn/main.zeek.html#type-Conn::Info

dns.log

• DNS activity
• For more information, see:

https://docs.zeek.org/en/latest/scripts/base/protocols/dns/main.zeek.html#type-DNS::Info

ftp.log

• FTP activity
• For more information, see:

https://docs.zeek.org/en/latest/scripts/base/protocols/ftp/info.zeek.html#type-FTP::Info

http.log

• HTTP requests and replies
• For more information, see:

https://docs.zeek.org/en/latest/scripts/base/protocols/http/main.zeek.html#type-HTTP::Info

ssl.log

• SSL/TLS handshake info
• For more information, see:

https://docs.zeek.org/en/latest/scripts/base/protocols/ssl/main.zeek.html#type-SSL::Info

notice.log

• Zeek notices
• For more information, see:

https://docs.zeek.org/en/latest/scripts/base/frameworks/notice/main.zeek.html#type-Notice::Info

…and others, which can be researched here:

https://docs.zeek.org/en/latest/script-reference/log-files.html

As you can see, Zeek log data can provide a wealth of information to the analyst, all easily accessible through Hunt or Kibana.

Configuration

You can use Salt to manage Zeek’s local.zeek, node.cfg and zeekctl.cfg:

local.zeek: The allowed options for this file are @load, @load-sigs and redef. An example of configuring this pillar can be seen below.

node.cfg: The pillar items to modify this file are located under the sensor pillar in the minion pillar file. The options that can be customized in the file include: interface, lb_procs, pin_cpus, and af_packet_buffer_size.

zeekctl.cfg: An example of customizing this can be seen below. The allowed options can be seen in https://github.com/Security-Onion-Solutions/securityonion/blob/master/salt/zeek/files/zeekctl.cfg.jinja.

Here is an example of how we would modify local.zeek. We can see the default pillar assignments used for local.zeek in /opt/so/saltstack/default/pillar/zeek/init.sls. This file should never be modified as it could be updated in the future and any modification made would be overwritten. The global or minion pillar files should be used for making changes as they are stored in /opt/so/saltstack/local/, and that directory isn’t overwritten during a Security Onion code update.

zeek:
  zeekctl:
    MailTo: root@localhost
    MailConnectionSummary: 1
    MinDiskSpace: 5
    MailHostUpDown: 1
    LogRotationInterval: 3600
    LogExpireInterval: 0
    StatsLogEnable: 1
    StatsLogExpireInterval: 0
    StatusCmdShowAll: 0
    CrashExpireInterval: 0
    SitePolicyScripts: local.zeek
    LogDir: /nsm/zeek/logs
    SpoolDir: /nsm/zeek/spool
    CfgDir: /opt/zeek/etc
    CompressLogs: 1
  local:
    '@load':
      - misc/loaded-scripts
      - tuning/defaults
      - misc/capture-loss
      - misc/stats
      - frameworks/software/vulnerable
      - frameworks/software/version-changes
      - protocols/ftp/software
      - protocols/smtp/software
      - protocols/ssh/software
      - protocols/http/software
      - protocols/dns/detect-external-names
      - protocols/ftp/detect
      - protocols/conn/known-hosts
      - protocols/conn/known-services
      - protocols/ssl/known-certs
      - protocols/ssl/validate-certs
      - protocols/ssl/log-hostcerts-only
      - protocols/ssh/geo-data
      - protocols/ssh/detect-bruteforcing
      - protocols/ssh/interesting-hostnames
      - protocols/http/detect-sqli
      - frameworks/files/hash-all-files
      - frameworks/files/detect-MHR
      - policy/frameworks/notice/extend-email/hostnames
      - ja3
      - hassh
      - intel
      - cve-2020-0601
      - securityonion/bpfconf
      - securityonion/communityid
      - securityonion/file-extraction
    '@load-sigs':
      - frameworks/signatures/detect-windows-shells
    redef:
      - LogAscii::use_json = T;
      - LogAscii::json_timestamps = JSON::TS_ISO8601;

In this file, there are two keys under zeek, zeekctl and local. We will be using zeek:local for this example since we are modifying the zeek.local file. We will address zeek:zeekctl in another example where we modify the zeekctl.cfg file.

Under zeek:local, there are three keys: @load, @load-sigs, and redef. In the pillar definition, @load and @load-sigs are wrapped in quotes due to the @ character. Under each of the keys, there is a list of items that will be added to the local.zeek file with the appropriate directive of either @load, @load-sigs or redef. In order to modify either of the lists, the entire list must redefined in either the global or minion pillar file.

If we have a node where protocols/ssh/detect-bruteforcing is generating a lot of noise and we want to tell Zeek to stop loading that script, we would do the following. Since we just want to turn it off for that specific node, we would open /opt/so/saltstack/local/pillar/minions/$SENSORNAME_$ROLE.sls. At the bottom, we would append the following:

zeek:
  local:
    '@load':
      - misc/loaded-scripts
      - tuning/defaults
      - misc/capture-loss
      - misc/stats
      - frameworks/software/vulnerable
      - frameworks/software/version-changes
      - protocols/ftp/software
      - protocols/smtp/software
      - protocols/ssh/software
      - protocols/http/software
      - protocols/dns/detect-external-names
      - protocols/ftp/detect
      - protocols/conn/known-hosts
      - protocols/conn/known-services
      - protocols/ssl/known-certs
      - protocols/ssl/validate-certs
      - protocols/ssl/log-hostcerts-only
      - protocols/ssh/geo-data
      - protocols/ssh/interesting-hostnames
      - protocols/http/detect-sqli
      - frameworks/files/hash-all-files
      - frameworks/files/detect-MHR
      - policy/frameworks/notice/extend-email/hostnames
      - ja3
      - hassh
      - intel
      - cve-2020-0601
      - securityonion/bpfconf
      - securityonion/communityid
      - securityonion/file-extraction

We redefined the @load list in the minion pillar file, but we left out the `protocols/ssh/detect-bruteforcing. This will override the value defined in the /opt/so/saltstack/default/pillar/zeek/init.sls and the global pillar file if it is defined there, and prevent the script from being added to the local.zeek file. If we wanted to add a script to be loaded, then we would add out script to the list. Since we aren’t changing @load-sigs or redef, then we do not need to add them here. Once the file is saved, and the node checks in the with manager, the local.zeek file will be updated and the so-zeek docker container will be restarted.

Let’s see an example of how we would modify the zeekctl.cfg file. From the example above, we know that the default pillar values are set for zeek in /opt/so/saltstack/default/pillar/zeek/init.sls. The default pillar values for zeekctl.cfg are as follows:

zeek:
  zeekctl:
    MailTo: root@localhost
    MailConnectionSummary: 1
    MinDiskSpace: 5
    MailHostUpDown: 1
    LogRotationInterval: 3600
    LogExpireInterval: 0
    StatsLogEnable: 1
    StatsLogExpireInterval: 0
    StatusCmdShowAll: 0
    CrashExpireInterval: 0
    SitePolicyScripts: local.zeek
    LogDir: /nsm/zeek/logs
    SpoolDir: /nsm/zeek/spool
    CfgDir: /opt/zeek/etc
    CompressLogs: 1

For anything not defined here, Zeek will use its own defaults. The options that are allowed to be managed with the pillar can be found at https://github.com/Security-Onion-Solutions/securityonion/blob/master/salt/zeek/files/zeekctl.cfg.jinja.

In order to add or modify an option in zeekctl, we will need to modify either the global or minion pillar file. For example, if we wanted to turn log compression off and change the timeout for Broker communication events to 20 seconds globally, we would add the following to the global pillar file.

zeek:
  zeekctl:
    compresslogs: 0
    commtimeout: 20

Since zeek:zeekctl is a dictionary with dictionary values, we do not need to redefine the entire pillar here like we did for zeek:local above. Once the pillar file is saved and the node checks in with the manager, the zeekctl.cfg file will be updated and the so-zeek container will be restarted.

More Information

See also

For more information about Zeek, please see https://www.zeek.org/.