Types

The Bro scripting language supports the following built-in types:

Name Description
bool Boolean
count, int, double Numeric types
time, interval Time types
string String
pattern Regular expression
port, addr, subnet Network types
enum Enumeration (user-defined type)
table, set, vector, record Container types
function, event, hook Executable types
file File type (only for writing)
opaque Opaque type (for some built-in functions)
any Any type (for functions or containers)

Here is a more detailed description of each type:

bool

Reflects a value with one of two meanings: true or false. The two “bool” constants are T and F.

The “bool” type supports the following operators: equality/inequality (==, !=), logical and/or (&&, ||), logical negation (!), and absolute value (where |T| is 1, and |F| is 0, and in both cases the result type is count).

int

A numeric type representing a 64-bit signed integer. An “int” constant is a string of digits preceded by a “+” or “-” sign, e.g. -42 or +5 (the “+” sign is optional but see note about type inferencing below). An “int” constant can also be written in hexadecimal notation (in which case “0x” must be between the sign and the hex digits), e.g. -0xFF or +0xabc123.

The “int” type supports the following operators: arithmetic operators (+, -, *, /, %), comparison operators (==, !=, <, <=, >, >=), assignment operators (=, +=, -=), pre-increment (++), pre-decrement (--), unary plus and minus (+, -), and absolute value (e.g., |-3| is 3, but the result type is count).

When using type inferencing use care so that the intended type is inferred, e.g. “local size_difference = 0” will infer “count”, while “local size_difference = +0” will infer “int”.

count

A numeric type representing a 64-bit unsigned integer. A “count” constant is a string of digits, e.g. 1234 or 0. A “count” can also be written in hexadecimal notation (in which case “0x” must precede the hex digits), e.g. 0xff or 0xABC123.

The “count” type supports the same operators as the “int” type, but a unary plus or minus applied to a “count” results in an “int”.

double

A numeric type representing a double-precision floating-point number. Floating-point constants are written as a string of digits with an optional decimal point, optional scale-factor in scientific notation, and optional “+” or “-” sign. Examples are -1234, -1234e0, 3.14159, and .003E-23.

The “double” type supports the following operators: arithmetic operators (+, -, *, /), comparison operators (==, !=, <, <=, >, >=), assignment operators (=, +=, -=), unary plus and minus (+, -), and absolute value (e.g., |-3.14| is 3.14).

When using type inferencing use care so that the intended type is inferred, e.g. “local size_difference = 5” will infer “count”, while “local size_difference = 5.0” will infer “double”.

time

A temporal type representing an absolute time. There is currently no way to specify a time constant, but one can use the double_to_time, current_time, or network_time built-in functions to assign a value to a time-typed variable.

Time values support the comparison operators (==, !=, <, <=, >, >=). A time value can be subtracted from another time value to produce an interval value. An interval value can be added to, or subtracted from, a time value to produce a time value. The absolute value of a time value is a double with the same numeric value.

interval

A temporal type representing a relative time. An interval constant can be written as a numeric constant followed by a time unit where the time unit is one of usec, msec, sec, min, hr, or day which respectively represent microseconds, milliseconds, seconds, minutes, hours, and days. Whitespace between the numeric constant and time unit is optional. Appending the letter “s” to the time unit in order to pluralize it is also optional (to no semantic effect). Examples of interval constants are 3.5 min and 3.5mins. An interval can also be negated, for example -12 hr represents “twelve hours in the past”.

Intervals support addition and subtraction, the comparison operators (==, !=, <, <=, >, >=), the assignment operators (=, +=, -=), and unary plus and minus (+, -).

Intervals also support division (in which case the result is a double value). An interval can be multiplied or divided by an arithmetic type (count, int, or double) to produce an interval value. The absolute value of an interval is a double value equal to the number of seconds in the interval (e.g., |-1 min| is 60.0).

string

A type used to hold bytes which represent text and also can hold arbitrary binary data.

String constants are created by enclosing text within a pair of double quotes (”). A string constant cannot span multiple lines in a Bro script. The backslash character (\) introduces escape sequences. Bro recognizes the following escape sequences: \\, \n, \t, \v, \b, \r, \f, \a, \ooo (where each ‘o’ is an octal digit), \xhh (where each ‘h’ is a hexadecimal digit). If Bro does not recognize an escape sequence, Bro will ignore the backslash (“\g” becomes “g”).

Strings support concatenation (+), and assignment (=, +=). Strings also support the comparison operators (==, !=, <, <=, >, >=). The number of characters in a string can be found by enclosing the string within pipe characters (e.g., |"abc"| is 3). Substring searching can be performed using the “in” or ”!in” operators (e.g., “bar” in “foobar” yields true).

The subscript operator can extract a substring of a string. To do this, specify the starting index to extract (if the starting index is omitted, then zero is assumed), followed by a colon and index one past the last character to extract (if the last index is omitted, then the extracted substring will go to the end of the original string). However, if both the colon and last index are omitted, then a string of length one is extracted. String indexing is zero-based, but an index of -1 refers to the last character in the string, and -2 refers to the second-to-last character, etc. Here are a few examples:

local orig = "0123456789";
local second_char = orig[1];         # "1"
local last_char = orig[-1];          # "9"
local first_two_chars = orig[:2];    # "01"
local last_two_chars = orig[8:];     # "89"
local no_first_and_last = orig[1:9]; # "12345678"
local no_first = orig[1:];           # "123456789"
local no_last = orig[:-1];           # "012345678"
local copy_orig = orig[:];           # "0123456789"

Note that the subscript operator cannot be used to modify a string (i.e., it cannot be on the left side of an assignment operator).

pattern

A type representing regular-expression patterns which can be used for fast text-searching operations. Pattern constants are created by enclosing text within forward slashes (/) and is the same syntax as the patterns supported by the flex lexical analyzer. The speed of regular expression matching does not depend on the complexity or size of the patterns. Patterns support two types of matching, exact and embedded.

In exact matching the == equality relational operator is used with one “pattern” operand and one “string” operand (order of operands does not matter) to check whether the full string exactly matches the pattern. In exact matching, the ^ beginning-of-line and $ end-of-line anchors are redundant since the pattern is implicitly anchored to the beginning and end of the line to facilitate an exact match. For example:

/foo|bar/ == "foo"

yields true, while:

/foo|bar/ == "foobar"

yields false. The != operator would yield the negation of ==.

In embedded matching the in operator is used with one “pattern” operand (which must be on the left-hand side) and one “string” operand, but tests whether the pattern appears anywhere within the given string. For example:

/foo|bar/ in "foobar"

yields true, while:

/^oob/ in "foobar"

is false since “oob” does not appear at the start of “foobar”. The !in operator would yield the negation of in.

port

A type representing transport-level port numbers (besides TCP and UDP ports, there is a concept of an ICMP “port” where the source port is the ICMP message type and the destination port the ICMP message code). A port constant is written as an unsigned integer followed by one of /tcp, /udp, /icmp, or /unknown.

Ports support the comparison operators (==, !=, <, <=, >, >=). When comparing order across transport-level protocols, unknown < tcp < udp < icmp, for example 65535/tcp is smaller than 0/udp.

Note that you can obtain the transport-level protocol type of a port with the get_port_transport_proto built-in function, and the numeric value of a port with the port_to_count built-in function.

addr

A type representing an IP address.

IPv4 address constants are written in “dotted quad” format, A1.A2.A3.A4, where Ai all lie between 0 and 255.

IPv6 address constants are written as colon-separated hexadecimal form as described by RFC 2373 (including the mixed notation with embedded IPv4 addresses as dotted-quads in the lower 32 bits), but additionally encased in square brackets. Some examples: [2001:db8::1], [::ffff:192.168.1.100], or [aaaa:bbbb:cccc:dddd:eeee:ffff:1111:2222].

Note that IPv4-mapped IPv6 addresses (i.e., addresses with the first 80 bits zero, the next 16 bits one, and the remaining 32 bits are the IPv4 address) are treated internally as IPv4 addresses (for example, [::ffff:192.168.1.100] is equal to 192.168.1.100).

Addresses can be compared for equality (==, !=), and also for ordering (<, <=, >, >=). The absolute value of an address gives the size in bits (32 for IPv4, and 128 for IPv6). Addresses can also be masked with / to produce a subnet:

local a: addr = 192.168.1.100;
local s: subnet = 192.168.0.0/16;
if ( a/16 == s )
    print "true";

And checked for inclusion within a subnet using in or !in:

local a: addr = 192.168.1.100;
local s: subnet = 192.168.0.0/16;
if ( a in s )
    print "true";

You can check if a given addr is IPv4 or IPv6 using the is_v4_addr and is_v6_addr built-in functions.

Note that hostname constants can also be used, but since a hostname can correspond to multiple IP addresses, the type of such a variable is “set[addr]”. For example:

local a = www.google.com;
subnet

A type representing a block of IP addresses in CIDR notation. A subnet constant is written as an addr followed by a slash (/) and then the network prefix size specified as a decimal number. For example, 192.168.0.0/16 or [fe80::]/64.

Subnets can be compared for equality (==, !=). An “addr” can be checked for inclusion in a subnet using the in or !in operators.

enum

A type allowing the specification of a set of related values that have no further structure. An example declaration:

type color: enum { Red, White, Blue, };

The last comma after Blue is optional. Both the type name color and the individual values (Red, etc.) have global scope.

Enumerations do not have associated values or ordering. The only operations allowed on enumerations are equality comparisons (==, !=) and assignment (=).

table

An associate array that maps from one set of values to another. The values being mapped are termed the index or indices and the result of the mapping is called the yield. Indexing into tables is very efficient, and internally it is just a single hash table lookup.

The table declaration syntax is:

table [ type^+ ] of type

where type^+ is one or more types, separated by commas. The index type cannot be any of the following types: pattern, table, set, vector, file, opaque, any.

Here is an example of declaring a table indexed by “count” values and yielding “string” values:

global a: table[count] of string;

The yield type can also be more complex:

global a: table[count] of table[addr, port] of string;

which declares a table indexed by “count” and yielding another “table” which is indexed by an “addr” and “port” to yield a “string”.

One way to initialize a table is by enclosing a set of initializers within braces, for example:

global t: table[count] of string = {
    [11] = "eleven",
    [5] = "five",
};

A table constructor can also be used to create a table:

global t2 = table(
    [192.168.0.2, 22/tcp] = "ssh",
    [192.168.0.3, 80/tcp] = "http"
);

Table constructors can also be explicitly named by a type, which is useful when a more complex index type could otherwise be ambiguous:

type MyRec: record {
    a: count &optional;
    b: count;
};

type MyTable: table[MyRec] of string;

global t3 = MyTable([[$b=5]] = "b5", [[$b=7]] = "b7");

Accessing table elements is provided by enclosing index values within square brackets ([]), for example:

print t[11];

And membership can be tested with in or !in:

if ( 13 in t )
    ...
if ( [192.168.0.2, 22/tcp] in t2 )
    ...

Add or overwrite individual table elements by assignment:

t[13] = "thirteen";

Remove individual table elements with delete:

delete t[13];

Nothing happens if the element with index value 13 isn’t present in the table.

The number of elements in a table can be obtained by placing the table identifier between vertical pipe characters:

|t|

See the for statement for info on how to iterate over the elements in a table.

set

A set is like a table, but it is a collection of indices that do not map to any yield value. They are declared with the syntax:

set [ type^+ ]

where type^+ is one or more types separated by commas. The index type cannot be any of the following types: pattern, table, set, vector, file, opaque, any.

Sets can be initialized by listing elements enclosed by curly braces:

global s: set[port] = { 21/tcp, 23/tcp, 80/tcp, 443/tcp };
global s2: set[port, string] = { [21/tcp, "ftp"], [23/tcp, "telnet"] };

A set constructor (equivalent to above example) can also be used to create a set:

global s3 = set(21/tcp, 23/tcp, 80/tcp, 443/tcp);

Set constructors can also be explicitly named by a type, which is useful when a more complex index type could otherwise be ambiguous:

type MyRec: record {
    a: count &optional;
    b: count;
};

type MySet: set[MyRec];

global s4 = MySet([$b=1], [$b=2]);

Set membership is tested with in or !in:

if ( 21/tcp in s )
    ...

if ( [21/tcp, "ftp"] !in s2 )
    ...

Elements are added with add:

add s[22/tcp];

Nothing happens if the element with value 22/tcp was already present in the set.

And removed with delete:

delete s[21/tcp];

Nothing happens if the element with value 21/tcp isn’t present in the set.

The number of elements in a set can be obtained by placing the set identifier between vertical pipe characters:

|s|

See the for statement for info on how to iterate over the elements in a set.

vector

A vector is like a table, except it’s always indexed by a count (and vector indexing is always zero-based). A vector is declared like:

global v: vector of string;

And can be initialized with the vector constructor:

local v = vector("one", "two", "three");

Vector constructors can also be explicitly named by a type, which is useful for when a more complex yield type could otherwise be ambiguous.

type MyRec: record {
    a: count &optional;
    b: count;
};

type MyVec: vector of MyRec;

global v2 = MyVec([$b=1], [$b=2], [$b=3]);

Accessing vector elements is provided by enclosing index values within square brackets ([]), for example:

print v[2];

An element can be added to a vector by assigning the value (a value that already exists at that index will be overwritten):

v[3] = "four";

The number of elements in a vector can be obtained by placing the vector identifier between vertical pipe characters:

|v|

Vectors of integral types (int or count) support the pre-increment (++) and pre-decrement operators (--), which will increment or decrement each element in the vector.

Vectors of arithmetic types (int, count, or double) can be operands of the arithmetic operators (+, -, *, /, %), but both operands must have the same number of elements (and the modulus operator % cannot be used if either operand is a vector of double). The resulting vector contains the result of the operation applied to each of the elements in the operand vectors.

Vectors of bool can be operands of the logical “and” (&&) and logical “or” (||) operators (both operands must have same number of elements). The resulting vector of bool is the logical “and” (or logical “or”) of each element of the operand vectors.

See the for statement for info on how to iterate over the elements in a vector.

record

A “record” is a collection of values. Each value has a field name and a type. Values do not need to have the same type and the types have no restrictions. Field names must follow the same syntax as regular variable names (except that field names are allowed to be the same as local or global variables). An example record type definition:

type MyRecordType: record {
    c: count;
    s: string &optional;
};

Records can be initialized or assigned as a whole in three different ways. When assigning a whole record value, all fields that are not &optional or have a &default attribute must be specified. First, there’s a constructor syntax:

local r: MyRecordType = record($c = 7);

And the constructor can be explicitly named by type, too, which is arguably more readable:

local r = MyRecordType($c = 42);

And the third way is like this:

local r: MyRecordType = [$c = 13, $s = "thirteen"];

Access to a record field uses the dollar sign ($) operator, and record fields can be assigned with this:

local r: MyRecordType;
r$c = 13;

To test if a field that is &optional has been assigned a value, use the ?$ operator (it returns a bool value of T if the field has been assigned a value, or F if not):

if ( r ?$ s )
    ...
function

Function types in Bro are declared using:

function( argument*  ): type

where argument is a (possibly empty) comma-separated list of arguments, and type is an optional return type. For example:

global greeting: function(name: string): string;

Here greeting is an identifier with a certain function type. The function body is not defined yet and greeting could even have different function body values at different times. To define a function including a body value, the syntax is like:

function greeting(name: string): string
    {
    return "Hello, " + name;
    }

Note that in the definition above, it’s not necessary for us to have done the first (forward) declaration of greeting as a function type, but when it is, the return type and argument list (including the name of each argument) must match exactly.

Here is an example function that takes no parameters and does not return a value:

function my_func()
    {
    print "my_func";
    }

Function types don’t need to have a name and can be assigned anonymously:

greeting = function(name: string): string { return "Hi, " + name; };

And finally, the function can be called like:

print greeting("Dave");

Function parameters may specify default values as long as they appear last in the parameter list:

global foo: function(s: string, t: string &default="abc", u: count &default=0);

If a function was previously declared with default parameters, the default expressions can be omitted when implementing the function body and they will still be used for function calls that lack those arguments.

function foo(s: string, t: string, u: count)
    {
    print s, t, u;
    }

And calls to the function may omit the defaults from the argument list:

foo("test");
event

Event handlers are nearly identical in both syntax and semantics to a function, with the two differences being that event handlers have no return type since they never return a value, and you cannot call an event handler.

Example:

event my_event(r: bool, s: string)
{
    print "my_event", r, s;
}

Instead of directly calling an event handler from a script, event handler bodies are executed when they are invoked by one of three different methods:

  • From the event engine

    When the event engine detects an event for which you have defined a corresponding event handler, it queues an event for that handler. The handler is invoked as soon as the event engine finishes processing the current packet and flushing the invocation of other event handlers that were queued first.

  • With the event statement from a script

    Immediately queuing invocation of an event handler occurs like:

    event password_exposed(user, password);
    

    This assumes that password_exposed was previously declared as an event handler type with compatible arguments.

  • Via the schedule expression in a script

    This delays the invocation of event handlers until some time in the future. For example:

    schedule 5 secs { password_exposed(user, password) };
    

Multiple event handler bodies can be defined for the same event handler identifier and the body of each will be executed in turn. Ordering of execution can be influenced with &priority.

hook

A hook is another flavor of function that shares characteristics of both a function and an event. They are like events in that many handler bodies can be defined for the same hook identifier and the order of execution can be enforced with &priority. They are more like functions in the way they are invoked/called, because, unlike events, their execution is immediate and they do not get scheduled through an event queue. Also, a unique feature of a hook is that a given hook handler body can short-circuit the execution of remaining hook handlers simply by exiting from the body as a result of a break statement (as opposed to a return or just reaching the end of the body).

A hook type is declared like:

hook( argument* )

where argument is a (possibly empty) comma-separated list of arguments. For example:

global myhook: hook(s: string)

Here myhook is the hook type identifier and no hook handler bodies have been defined for it yet. To define some hook handler bodies the syntax looks like:

hook myhook(s: string) &priority=10
    {
    print "priority 10 myhook handler", s;
    s = "bye";
    }

hook myhook(s: string)
    {
    print "break out of myhook handling", s;
    break;
    }

hook myhook(s: string) &priority=-5
    {
    print "not going to happen", s;
    }

Note that the first (forward) declaration of myhook as a hook type isn’t strictly required. Argument types must match for all hook handlers and any forward declaration of a given hook.

To invoke immediate execution of all hook handler bodies, they are called similarly to a function, except preceded by the hook keyword:

hook myhook("hi");

or

if ( hook myhook("hi") )
    print "all handlers ran";

And the output would look like:

priority 10 myhook handler, hi
break out of myhook handling, bye

Note how the modification to arguments can be seen by remaining hook handlers.

The return value of a hook call is an implicit bool value with T meaning that all handlers for the hook were executed and F meaning that only some of the handlers may have executed due to one handler body exiting as a result of a break statement.

file

Bro supports writing to files, but not reading from them (to read from files see the Input Framework). Files can be opened using either the open or open_for_append built-in functions, and closed using the close built-in function. For example, declare, open, and write to a file and finally close it like:

local f = open("myfile");
print f, "hello, world";
close(f);

Writing to files like this for logging usually isn’t recommended, for better logging support see Logging Framework.

opaque

A data type whose actual representation/implementation is intentionally hidden, but whose values may be passed to certain built-in functions that can actually access the internal/hidden resources. Opaque types are differentiated from each other by qualifying them like “opaque of md5” or “opaque of sha1”.

An example use of this type is the set of built-in functions which perform hashing:

local handle = md5_hash_init();
md5_hash_update(handle, "test");
md5_hash_update(handle, "testing");
print md5_hash_finish(handle);

Here the opaque type is used to provide a handle to a particular resource which is calculating an MD5 hash incrementally over time, but the details of that resource aren’t relevant, it’s only necessary to have a handle as a way of identifying it and distinguishing it from other such resources.

any

Used to bypass strong typing. For example, a function can take an argument of type any when it may be of different types. The only operation allowed on a variable of type any is assignment.

Note that users aren’t expected to use this type. It’s provided mainly for use by some built-in functions and scripts included with Bro.

void

An internal Bro type (i.e., “void” is not a reserved keyword in the Bro scripting language) representing the absence of a return type for a function.


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Copyright 2016, The Bro Project. Last updated on October 17, 2017. Created using Sphinx 1.5.2.