Baby Bro, Part 2: Conditionals, Address Types

Bro has native types for addresses and networks, making it much easier to work with network data. Today's Baby Bro script shows global variable definition, the use of the address and subnet types, and a simple conditional:

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# declaring global variables # no need to put quotes around addr or subnet variable definitions global ipv4_host:addr = 1.1.1.1; global ipv4_net:subnet = 1.1.0.0/16; event bro_init() { if (ipv4_host in ipv4_net) { # addr and subnet types are autoconverted to strings with fmt print fmt("%s is in network %s",ipv4_host,ipv4_net); } else { print fmt("host %s is not in network %s",ipv4_host,ipv4_net); } }

Running this from the CLI, we get the expected output:

jswan@so12a:~/bro$ bro addr_net_types.bro
1.1.1.1 is in network 1.1.0.0/16

Bro also has several interesting built-in functions for working with network data that we'll explore in upcoming posts. For now, we'll take a look at the mask_addr function, which allows you to use Bro as an improvised subnet calculator. You can run a Bro micro-script from the CLI with with  the -e option, just like the -e flag in Perl or the -c flag in Python:

jswan@so12a:~/bro$ bro -e "print mask_addr(10.18.32.199,14);"
10.16.0.0/14
jswan@so12a:~/bro$ bro -e "print mask_addr(10.18.32.199,31);"
10.18.32.198/31

Great for those late-night subnetting sessions after too many microbrews!

Just in case you were wondering: all of this works natively for IPv6, with some changes to the syntax:

jswan@so12a:~/bro$ bro -e "print [fe80::1db9] in [fe80::]/64;"
T # T is the way Bro outputs "True" in a Boolean test

We'll look at some more IPv6 stuff in an upcoming post.