Friday, February 6, 2015

Building a Snort3.0 Codec

The Snort3.0 Codecs decipher raw packets. These Codecs are now completely pluggable; almost every Snort3.0 Codec can be built dynamically and replaced with an alternative, customized Codec. The pluggable nature has also made it easier to build new Codecs for protocols without having to touch the Snort3.0 code base.

The first step in creating a Codec is defining its class and protocol. Every Codec must inherit from the Snort3.0 Codec class defined in "framework/codec.h". The following is an example Codec named "example" and has an associated struct that is 14 bytes long.

#include <cstdint>
#include <arpa/inet.h>
#include “framework/codec.h”
#include "main/snort_types.h"

#define EX_NAME “example”
#define EX_HELP “example codec help string”

struct Example
    uint8_t dst[6];
    uint8_t src[6];
    uint16_t ethertype;

    static inline uint8_t size()
    { return 14; }

class ExCodec : public Codec
    ExCodec() : Codec(EX_NAME) { }
    ~ExCodec() { }

    bool decode(const RawData&, CodecData&, DecodeData&) override;
    void get_protocol_ids(std::vector<uint16_t>&) override;

}; /* class ExCodec : public Codec */

After defining ExCodec, the next step is adding the Codec's decode functionality. The function below does this by implementing a valid decode function. The first parameter, which is the RawData struct, provides both a pointer to the raw data that has come from a wire and the length of that raw data. The function takes this information and validates that there are enough bytes for this protocol. If the raw data's length is less than 14 bytes, the function returns false and Snort3.0 discards the packet; the packet is neither inspected nor processed. If the length is greater than 14 bytes, the function populates two fields in the CodecData struct, next_prot_id and lyr_len. The lyr_len field tells Snort3.0 the number of bytes that this layer contains. The next_prot_id field provides Snort3.0 the value of the next EtherType or IP protocol number.

bool ExCodec::decode(const RawData& raw, CodecData& codec, DecodeData&)
   if ( raw.len < Example::size() )
         return false;

    const Example* const ex = reinterpret_cast<const Example*>(;
    codec.next_prot_id = ntohs(ex->ethertype);
    codec.lyr_len = ex->size();
    return true;

For instance, assume this decode function receives the following raw data with a validated length of 32 bytes:

    00 11 22 33 44 55 66 77    88 99 aa bb 08 00 45 00
    00 38 00 01 00 00 40 06    5c ac 0a 01 02 03 0a 09

The Example struct's EtherType field is the 13 and 14 bytes. Therefore, this function tells Snort that the next protocol has an EtherType of 0x0800. Additionally, since the lyr_len is set to 14, Snort knows that the next protocol begins 14 bytes after the beginning of this protocol. The Codec with EtherType 0x0800, which happens to be the IPv4 Codec, will receive the following data with a validated length of 18 ( == 32 – 14):

    45 00 00 38 00 01 00 00    40 06 5c ac 0a 01 02 03
    0a 09

How does Snort3.0 know that the IPv4 Codec has an EtherType of 0x0800? The Codec class has a second virtual function named get_protocol_ids(). When implementing the function, a Codec can register for any number of values between 0x0000 - 0xFFFF. Then, if the next_proto_id is set to a value for which this Codec has registered, this Codec's decode function will be called. As a general note, the protocol ids between [0, 0x00FF] are IP protocol numbers, [0x0100, 0x05FF] are custom types, and [0x0600, 0xFFFF] are EtherTypes.

For example, in the get_protocol_ids function below, the ExCodec registers for the protocols numbers 17, 787, and 2054. 17 happens to be the protocol number for UDP while 2054 is ARP's EtherType. Therefore, this Codec will now attempt to decode UDP and ARP data. Additionally, if any Codec sets the next_protocol_id to 787, ExCodec's decode function will be called. Some custom protocols are already defined in the file "protocols/protocol_ids.h"

void ExCodec::get_protocol_ids(std::vector<uint16_t>&v)
    v.push_back(0x0011); // == 17  == UDP
    v.push_back(0x1313); // == 787  == custom
    v.push_back(0x0806); // == 2054  == ARP

To register a Codec for Data Link Type's rather than protocols, the function get_data_link_type() can be similarly implemented.

The final step to creating a pluggable Codec is the snort_plugins array. This array is important because when Snort3.0 loads a dynamic library, the program only find plugins that are inside the snort_plugins array. In other words, if a plugin has not been added to the snort_plugins array, that plugin will not be loaded into Snort3.0.

Although the details will not be covered in this post, the following code snippet is a basic CodecApi that Snort3.0 can load. This snippet can be copied and used with only three minor changes. First, in the function ctor, ExCodec should be replaced with the name of the Codec that is being built. Second, EX_NAME must match the Codec's name or Snort will be unable to load this Codec. Third, EX_HELP should be replaced with the general description of this Codec. Once this code snippet has been added, ExCodec is ready to be compiled and plugged into Snort3.0.

static Codec* ctor(Module*)
{ return new ExCodec; }

static void dtor(Codec *cd)

{ delete cd; }

static const CodecApi ex_api =

    nullptr, // pointer to a function called during Snort's startup.
    nullptr, // pointer to a function called during Snort's exit.
    nullptr, // pointer to a function called during thread's startup.
    nullptr, // pointer to a function called during thread's destruction.
    ctor, // pointer to the codec constructor.
    dtor, // pointer to the codec destructor.

SO_PUBLIC const BaseApi* snort_plugins[] =


Two example Codecs are available in the extra directory on git and the extra tarball on the Snort3.0 page. One of those examples is the Token Ring Codec while the other example is the PIM Codec.

As a final note, there are four more virtual functions that a Codec should implement: encodeformatupdate, and log. If the functions are not implemented Snort will not throw any errors. However, Snort may also be unable to accomplish some of its basic functionality.
  • encode is called whenever Snort actively responds and needs to builds a packet, i.e. whenever a rule using an IPS ACTION like react, reject, or rewrite is triggered. This function is used to build the response packet protocol by protocol.
  • format is called when Snort is rebuilding a packet. For instance, every time Snort reassembles a TCP stream or IP fragment, format is called. Generally, this function either swaps any source and destination fields in the protocol or does nothing.
  • update is similar to format in that it is called when Snort is reassembling a packet. Unlike format, this function only sets length fields.
  • log is called when either the log_codecs logger or a custom logger that calls PacketManager::log_protocols is used when running Snort3.0.

Happy (de)coding!