Network analysis with the Bro Network Security Monitor

Got Your Back


With the standard install on CentOS, Bro will be installed in /opt/bro/, which contains the following subdirectories: bin, etc, include, lib, and share.

The bin subdirectory contains the executables that make up the Bro distribution as follows:

  • adtrace is responsible for tracking network addresses. It basically gives Bro the ability to trace source and destination Layer 2 (MAC) and Layer 3 (IP) addresses. It functions by reading the .pcap file and outputs the source Layer 2 address, the destination Layer 2 address, the source IP, and the destination IP.
  • bro-cut is responsible for formatting Bro's output into columns. It functions by reading Bro's logins and printing the specified columns. This allows simple reports to be run on the basis of field names, instead of having to count columns with Awk, for example.
  • broccoli-config is the configuration program for Broccoli (Bro Client COmmunications LIbrary). Broccoli lets developers create client sensors for Bro.
  • broctl, as seen earlier in the article, is an interactive shell used to start, stop, and manage Bro's engines, as well as support a help system (Listing 1). See also the "Interactive Shell" box.

Listing 1

Bro Help System

[BroControl] > help
BroControl Version 1.3
  capstats [<nodes>] [<secs>]      - Report interface statistics with capstats
  check [<nodes>]                  - Check configuration before installing it
  cleanup [--all] [<nodes>]        - Delete working dirs (flush state) on nodes
  config                           - Print broctl configuration
  cron [--no-watch]                - Perform jobs intended to run from cron
  cron enable|disable|?            - Enable/disable "cron" jobs
  df [<nodes>]                     - Print nodes' current disk usage
  diag [<nodes>]                   - Output diagnostics for nodes
  exec <shell cmd>                 - Execute shell command on all hosts
  exit                             - Exit shell
  install                          - Update broctl installation/configuration
  netstats [<nodes>]               - Print nodes' current packet counters
  nodes                            - Print node configuration
  peerstatus [<nodes>]             - Print status of nodes' remote connections
  print <id> [<nodes>]             - Print values of script variable at nodes
  process <trace> [<op>] [-- <sc>] - Run Bro (with options and scripts) on trace
  quit                             - Exit shell
  restart [--clean] [<nodes>]      - Stop and then restart processing
  scripts [-c] [<nodes>]           - List the Bro scripts the nodes will load
  start [<nodes>]                  - Start processing
  status [<nodes>]                 - Summarize node status
  stop [<nodes>]                   - Stop processing
  top [<nodes>]                    - Show Bro processes ala top
  update [<nodes>]                 - Update configuration of nodes on the fly
Commands provided by plugins:
  ps.bro [<nodes>]                 - Show Bro processes on nodes' systems

Interactive Shell

Besides the install and start commands used earlier, the ps.bro and restart commands within the BroControl interactive shell are also quite useful. To see which process Bro is running, you can use ps.bro (Listing 2). For a quick restart of Bro, restart is the command to use. Bro doesn't require a lot of restarts for maintenance issues, but this command does restart counters, which can be handy when working on event correlations or with the netstats command (Listing 3).

Listing 2


[BroControl] > ps.bro
       USER       PID  PPID %CPU %MEM    VSZ   RSS TT       S  STARTED       TIME COMMAND
>>> localhost
   (+) root     11766 11757 34.8  6.4 862984 96872 ?        S   Oct 19 7-00:03:58 bro
   (+) root     11792 11766 11.4  3.3 150888 51400 ?        S   Oct 19 2-07:05:05 bro

Listing 3


[BroControl] > netstats
        bro: 1415509669.641986 recvd=431 dropped=0 link=431
[BroControl] > restart
stopping ...
stopping bro ...
starting ...
starting bro ...
[BroControl] > netstats
        bro: 1415509677.525993 recvd=35 dropped=0 link=35
[BroControl] >
  • captsats collects statistics on the network interface and is useful for monitoring traffic loads. You can define the logging interval with the -I parameter and the number of intervals with the -n parameter. For example, to run a report every three seconds for five iterations on eth0, you would use the command in Listing 4.

Listing 4


/opt/bro/bin/capstats -I 3 -n 5 -i eth0
1415510235.518025 pkts=2 kpps=0.0 kbytes=0 mbps=0.0 nic_pkts=2  nic_drops=0 u=0 t=0 i=0 o=0 nonip=2
1415510238.518111 pkts=8 kpps=0.0 kbytes=1 mbps=0.0 nic_pkts=10 nic_drops=0 u=5 t=2 i=0 o=0 nonip=1
1415510241.519354 pkts=6 kpps=0.0 kbytes=0 mbps=0.0 nic_pkts=16 nic_drops=0 u=2 t=2 i=0 o=0 nonip=2
1415510244.519597 pkts=6 kpps=0.0 kbytes=0 mbps=0.0 nic_pkts=22 nic_drops=0 u=2 t=2 i=0 o=0 nonip=2
1415510247.520546 pkts=7 kpps=0.0 kbytes=1 mbps=0.0 nic_pkts=29 nic_drops=0 u=3 t=2 i=0 o=0 nonip=2

The capstats report shows (left to right) the relative time stamp, the number of packets (pkts ) seen by capstats during a particular interval, the number of packets per second (kpps x1000), the kilobytes seen during the interval (kbytes ), megabits per second (mbps ), libpcap's reported packets (nic_pkts ), the number of packets reported by libpcap as dropped (nic_drops ), the number of packets via UDP, TCP, ICMP, and other protocols (u , t , i, o ), and the number of non-IP packets (nonip ).

  • ftwire2bro is a command-line utility that takes Netflow files and converts them to Bro format. Another utility in the bin directory, nfcollector, captures Netflow data from a socket and converts it to Bro format.
  • rst, short for TCP reset, is used by Bro to terminate established TCP connections. One valuable use would be for configuring Bro to deal with a zero day attack in which the traffic simply needs to be shutdown. An example application would be to halt the replication of a worm for which virus software vendors have not yet provided a signature or the means for removal.
  • trace-summary is another reporting tool that can be used for analysis against Bro's logs.

Configuration and Output Files

Another subdirectory worth understanding in /opt/bro/ is etc. This directory contains four configuration files: broccoli.conf, broctl.cfg, networls.cfg, and nodes.cfg.

Unless you are developing applications that interact with Bro, the broccoli.conf file can be ignored. The broctl.cfg file isn't typically needed on an RPM-installed version of Bro; however, you might need it if you have to restructure disk volumes at some point in time.

The networks.cfg file defines networks and typically contains the private address space as default. You can add additional networks for Bro to search when using descriptions of networks in reports.

The nodes.cfg file is where you can add additional nodes or change interfaces.

Besides the /opt/bro/ directory, Bro also manages /var/opt/bro/. This directory contains two subdirectories: logs and spool.

The logs directory comprises archived logs containing historical data (all logs previous to today); however, it also contains the current log linked to the current directory. The subdirectories in logs are in the format YYYY-MM-DD and, as mentioned, the current directory (Listing 5).

Listing 5

var/opt/bro/logs Subdirectories

ls -l /var/opt/bro/logs/
total 244
drwxr-xr-x. 2 root root  4096 Oct 20 00:00 2014-10-19
drwxr-xr-x. 2 root root 12288 Oct 21 00:00 2014-10-20
drwxr-xr-x. 2 root root 12288 Oct 22 00:00 2014-10-21
drwxr-xr-x. 2 root root 12288 Oct 23 00:00 2014-10-22
drwxr-xr-x. 2 root root 12288 Oct 24 00:00 2014-10-23
drwxr-xr-x. 2 root root 12288 Oct 25 00:00 2014-10-24
drwxr-xr-x. 2 root root 12288 Nov  5 00:00 2014-11-04
drwxr-xr-x. 2 root root 12288 Nov  6 00:00 2014-11-05
drwxr-xr-x. 2 root root 12288 Nov  7 00:00 2014-11-06
drwxr-xr-x. 2 root root 12288 Nov  8 00:00 2014-11-07
drwxr-xr-x. 2 root root 12288 Nov  8 23:07 2014-11-08
lrwxrwxrwx. 1 root root    22 Oct 19 21:22 current -> /var/opt/bro/spool/bro

The spool subdirectory not only contains the working directory but also is used for scratch space by Bro (tmp subdirectory), as well as the broctl database (broctl.dat).

Now that Bro is up and running, I can take a look at one of the output types that Bro supports. Luckily, Bro uses intuitive names. For example, the log for October 19 is shown in Listing 6.

Listing 6

Bro Log for One Day

[root@centos6-5 2014-10-19]# ls -l /var/opt/bro/logs/2014-10-19/
total 268
-rw-r--r--. 1 root root   2360 Oct 19 22:00 communication.21:34:11-22:00:00.log.gz
-rw-r--r--. 1 root root   4787 Oct 19 23:00 communication.22:00:00-23:00:00.log.gz
-rw-r--r--. 1 root root   4787 Oct 20 00:00 communication.23:00:00-00:00:00.log.gz
-rw-r--r--. 1 root root  15157 Oct 19 22:00 conn.21:34:39-22:00:00.log.gz
-rw-r--r--. 1 root root  33060 Oct 19 23:00 conn.22:00:00-23:00:00.log.gz
-rw-r--r--. 1 root root    875 Oct 19 23:00 files.22:05:05-23:00:00.log.gz
-rw-r--r--. 1 root root    626 Oct 19 23:00 http.22:05:05-23:00:00.log.gz
-rw-r--r--. 1 root root    174 Oct 19 22:00 known_hosts.21:54:41-22:00:00.log.gz
-rw-r--r--. 1 root root    233 Oct 19 22:00 known_services.21:54:41-22:00:00.log.gz
-rw-r--r--. 1 root root   2006 Oct 19 22:00 loaded_scripts.21:34:11-22:00:00.log.gz
-rw-r--r--. 1 root root    434 Oct 20 00:00 notice.23:02:17-00:00:00.log.gz
-rw-r--r--. 1 root root    197 Oct 19 22:00 packet_filter.21:34:11-22:00:00.log.gz
-rw-r--r--. 1 root root    393 Oct 19 22:00 reporter.21:34:21-22:00:00.log.gz
-rw-r--r--. 1 root root    270 Oct 19 22:00 software.21:54:41-22:00:00.log.gz
-rw-r--r--. 1 root root    336 Oct 19 22:00 ssh.21:55:11-22:00:00.log.gz
-rw-r--r--. 1 root root    400 Oct 20 00:00 ssh.23:00:00-00:00:00.log.gz
-rw-r--r--. 1 root root   1268 Oct 19 22:00 weird.21:34:12-22:00:00.log.gz
-rw-r--r--. 1 root root   2477 Oct 19 23:00 weird.22:00:00-23:00:00.log.gz
-rw-r--r--. 1 root root   2992 Oct 20 00:00 weird.23:00:00-00:00:00.log.gz

Bro has identified all the protocols it analyzed on October 19 and named them according to the protocol. For example, HTTP and SSH protocols clearly stand out. Bro also provided additional logs, such as the known_hosts , connections logs (conn ), and others.

DHCP Compromise

As an example of Bro's abilities, I'll look at a specific incident. On October 30, the DHCP service on the network at was "knocked over" via a distributed denial of service, and a rogue service was put in place at; however, the server team was unaware of when addresses were assigned by the rogue DHCP server. Bro can show when the first assignment was made:

ls dh*

Two logs concerning DHCP exist: one from 4:00am and one from 3:00pm. With the bro-cut command, I can determine that the earliest assignment by the rogue server was at 4:04am and assigned the address (Listing 7). The field names used in this command, id_resp_h and assigned_ip, are found in the first few lines of all Bro logs. By using Bro's known services logs,

Listing 7


zcat dhcp.* | /opt/bro/bin/bro-cut -d ts id.resp_h assigned_ip lease_time
2014-10-30T04:04:04-0500   86400.000000
2014-10-30T15:54:52-0500   86400.000000
ls known_services*

I can confirm that Bro picked up on a new service in the time frame of interest, so I can check the known_services log before the address assignment identified in the previous command (Listing 8).

Listing 8

known_services Log of Interest

zcat known_services.03\:00\:00-04\:00\:00.log.gz | /opt/bro/bin/bro-cut -d
2014-10-30T03:00:29-0500    67 udp   DHCP
2014-10-30T03:12:12-0500    80 tcp   HTTP
2014-10-30T03:14:29-0500    53 tcp   HTTP

According to Listing 8, the DHCP was first recognized on the network at 3:00am, much earlier than existing logs could have provided. However, notice that it picked up on two additional services: two web servers – one running on the well-known port 80 and another on port 53. These steps, then, could be used to track down traffic to the server, checking and validating the HTTP logs to which Bro connects.

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