2 releases

0.1.1 May 20, 2021
0.1.0 May 18, 2021

#87 in #bitfields

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178 downloads per month
Used in 7 crates (via libpacket)

MIT/Apache

87KB
2K SLoC

libpacket

Packet dissector for various protocols.

Ripped out from the unmaintained libpnet.


lib.rs:

The pnet_macros crate provides the #[packet] macro and compiler plugin, which is used to specify the format of on-the-wire packets, and automatically generate zero-copy accessors and mutators for the fields. It is used as follows:

/// Import the `Packet` custom derive attribute
use libpacket_derive::Packet;
/// This module contains a list of type aliases which may be used
use libpacket_core::types::{u4, u12be};

/// Packets are specified in the same way as normal Rust structs, but with a `#[packet]`
/// attribute.
#[derive(Packet)]
pub struct Example {
    // This is a simple field which contains a 4-bit, unsigned integer.
    // Note that `u4` is simply an alias for `u8` - the name is a hint
    // to the compiler plugin, it is NOT a usable 4 bit type!
    simple_field1: u4,

    // This specifies that `simple_field2` should be a 12-bit field,
    // with bits stored in big endian
    simple_field2: u12be,

    // All packets must specify a `#[payload]`, which should be a
    // `Vec<u8>`. This represents the packet's payload, for example in
    // an IPv4 packet, the payload could be a UDP packet, or in a UDP
    // packet the payload could be the application data. All the
    // remaining space in the packet is considered to be the payload
    // (this doesn't have to be the case, see the documentation for
    // `#[payload]` below.
    #[payload]
    payload: Vec<u8>
}

A number of things will then be generated. You can see this in action in the documentation and source of each of the packet types in the pnet::packet module. Things generated include (assuming the Example struct from above):

  • An ExamplePacket<'p> structure, which is used for receiving packets on the network. This structure contains:
    • A method, pub fn new<'p>(packet: &'p [u8]) -> ExamplePacket<'p>, used for the construction of an ExamplePacket, given a buffer to store it. The buffer should be long enough to contain all the fields in the packet.
    • A method, pub fn to_immutable<'p>(&'p self) -> ExamplePacket<'p>, which is simply an identity function. It exists for consistency with MutableExamplePacket.
    • A number of accessor methods, of the form pub get_{field_name}(&self) -> {field_type}, which will retrieve the host representation of the on-the-wire value.
  • A MutableExamplePacket<'p> structure, which is used when sending packets on the network. This structure contains:
    • A method, pub fn new<'p>(packet: &'p mut [u8]) -> MutableExamplePacket<'p>, used for the construction of a MutableExamplePacket, given a buffer to store it. The buffer should be long enough to contain all the fields in the packet.
    • A method, pub fn to_immutable<'p>(&'p self) -> ExamplePacket<'p>, which converts from a MutableExamplePacket to an ExamplePacket
    • A method, pub fn populate(&mut self, packet: Example), which, given an Example struct, will populate the MutableExamplePacket with the values from the Example struct.
    • A number of accessor methods, of the form pub get_{field_name}(&self) -> {field_type}, which will retrieve the host representation of the on-the-wire value.
    • A number of mutator methods, of the form pub set_{field_name}(&mut self, val: {field_type}), which will take a host value, convert it to the required on-the-wire format, and store it in the buffer which backs the MutableExamplePacket.
  • A number of trait implementations for each of the MutableExamplePacket and ExamplePacket structures. These include:
    • pnet::packet::Packet (ExamplePacket and MutableExamplePacket)
    • pnet::packet::MutablePacket (MutableExamplePacket only)
    • std::fmt::Debug (ExamplePacket and MutableExamplePacket)
    • pnet::packet::FromPacket (ExamplePacket and MutableExamplePacket)
    • pnet::packet::PacketSize (ExamplePacket and MutableExamplePacket)
  • An ExampleIterator structure, which implements std::iter::Iterator, to allow iterating over vectors of ExamplePacket contained within another packet. Used internally.

Attributes

There are a number of attributes which fields may have, these include:

  • #[length_fn = "function_name"]

    This attribute is used to enable variable length fields. To specify a variable length field, it should have the type Vec<T>. It must have the #[length_fn] (or #[length]) attribute, which specifies a function name to calculate the length of the field. The signature for the length function should be fn {function_name}<'a>(example_packet: &ExamplePacket<'a>) -> usize, substituting &ExamplePacket<'a> for the appropriately named packet type for your structure. You may access whichever fields are required to calculate the length of the field. The returned value should be a number of bytes that the field uses.

    The type contained in the vector may either be one of the primitive types specified in pnet_macros::types, or another structure marked with #[derive(Packet)], for example Vec<Example>.

  • #[length = "arithmetic expression"]

    This attribute is used to enable variable length fields. To specify a variable length field, it should have the type Vec<T>. It must have the #[length] (or #[length_fn]) attribute, which specifies an arithmetic expression to calculate the length of the field. Only field names, constants, integers, basic arithmetic expressions (+ - * / %) and parentheses are in the expression. An example would be #[length = "field_name + CONSTANT - 4].

    The type contained in the vector may either be one of the primitive types specified in pnet_macros::types, or another structure marked with #[derive(Packet)], for example Vec<Example>.

  • #[payload]

    This attribute specifies the payload associated with the packet. This should specify the data associated with the packet. It may be used in two places:

    • The last field in the packet, in which case it is assumed to use the remaining length of the buffer containing the packet
    • Another location in the packet, in which case the #[length_fn] attribute must also be specified to give the length of the payload. If the packet has no payload, you must still specify this attribute, but you can provide a #[length_fn] attribute returning zero.
  • #[construct_with(, ...)]

    Unfortunately, compiler plugins do not currently have access to type information during the decoration stage (where all of the above is generated), so this attribute is required. This must be used for all fields which are neither primitive types, nor vectors of primitive types. Three things are required when using #[construct_with]:

    • The field type must have a method new, which takes one or more parameters of primitive types.
    • The field must be annotated with the #[construct_with(...)] attribute, specifying a list of types identical to those taken by the new method.
    • The pnet::packet::ToPrimitiveValues trait must be implemented for the field type, which must return a tuple of the primitive types specified in the parameters to the #[construct_with(...)] attribute, and in the new method.

Dependencies

~3–4.5MB
~87K SLoC