\subsection howto_newpacket_parser_skeleton The PacketParser skeleton
\code
+ #include <senf/Packets.hh>
+
struct GREPacketParser : public senf::PacketParser
{
# include SENF_PARSER()
which does just that: It calculates the checksum of the GRE packet.
\code
+ #include <senf/Utils/IpChecksum.hh>
+
checksum_t::value_type calculateChecksum() const
{
if (!checksumEnabled())
return 0;
senf::IpChecksum cs;
- cs.feed( i(), i()+4 );
+ cs.feed( i(), i(4) );
// Skip even number of 0 bytes (the 2 bytes checksum field)
// cs.feed(0); cs.feed(0);
- cs.feed( i()+6, data().end() );
+ cs.feed( i(6), data().end() );
return cs.sum()
}
packet including it's header with the checksum field temporarily set to 0. Instead of really
changing the checksum field we manually pass the correct data to \a cs.
+ We use the special <tt>i(</tt><i>offset</i><tt>)</tt> helper to get iterators \a offset number
+ of bytes into the data. This helper has the additional benefit of range-checking the returned
+ iterator and thereby safe os from errors due to truncated packets: If the offset is out of
+ range, a TruncatedPacketException will be thrown.
+
In this code we utilize some additional information provided by senf::PacketParserBase. The \a
i() member returns an iterator to the first byte the parser is interpreting whereas \a data()
returns a reference to the packet data container for the packet being parsed. Access to \a
\code
#include <senf/Packets.hh>
- #include <senf/Utils/IpChecksum.hh>
struct GREPacketParser_OptFields : public senf::PacketParser
{
checksum_t::value_type calculateChecksum() const;
};
+
+ // In the implementation (.cc) file:
+
+ #include <senf/Utils/IpChecksum.hh>
GREPacketParser::checksum_t::value_type GREPacketParser::calculateChecksum() const
{
if (!checksumEnabled())
return 0;
+ validate(6);
senf::IpChecksum cs;
cs.feed( i(), i()+4 );
// Skip even number of 0 bytes (the 2 bytes checksum field)
utilizes a registry and is not hopelessly complex, the packet type will almost always look like
this:
\code
+ #include <senf/Packets.hh>
+
struct GREPacketType
: public senf::PacketTypeBase,
public senf::PacketTypeMixin<GREPacketType, EtherTypes>
\c .cc file):
\code
+ #include <senf/Packets/DefaultBundle/EthernetPacket.hh>
+
SENF_PACKET_REGISTRY_REGISTER( senf::EtherTypes, 0x6558, senf::EthernetPacket );
\endcode
GRE has been assigned the value 47:
\code
+ #include <senf/Packets/DefaultBundle/IPv4Packet.hh>
+
SENF_PACKET_REGISTRY_REGISTER( senf::IpTypes, 47, GREPacket );
\endcode
{
if (p->checksumPresent())
p->checksum() << p->calculateChecksum();
- p->protocolType() << key(p->next(senf::nothrow));
+ p->protocolType() << key(p.next(senf::nothrow));
}
\endcode
assign parsers to each other efficiently and it supports 'optional values' (as provided by <a
href="http://www.boost.org/libs/optional/doc/optional.html">Boost.Optional</a> and as returned
by \c key()).
+
+
+ \subsection howto_newpacket_type_dump Writing out a complete packet: The 'dump()' member
+
+ For diagnostic purposes, every packet should provide a meaningful \a dump() member which writes
+ out the complete packet. This member is simple to implement and is often very helpful when
+ tracking down problems.
+
+ \code
+ #include <boost/io/ios_state.hpp>
+
+ static void dump(packet p, std::ostream & os)
+ {
+ boost::io::ios_all_saver ias(os);
+ os << "General Routing Encapsulation:\n"
+ << " checksum present : " << p->checksumPresent() ? "true" : "false" << "\n"
+ << " version : " << p->version() << "\n"
+ << " protocol type : 0x" << std::hex << std::setw(4) << std::setfill('0')
+ << p->protocolType() << "\n";
+ if (p->checksumPresent())
+ os << " checksum : 0x" << std::hex << std::setw(4)
+ << std::setfill('0') << p->checksum() << "\n";
+ }
+ \endcode
+
+ This member is quite straight forward. We should try to adhere to the formating standard shown
+ above: The first line should be the type of packet/header being dumped followed by one line for
+ each protocol field. The colon's should be aligned at column 33 with the field name indented by
+ 2 spaces.
+
+ The \c boost::ios_all_saver is just used to ensure, that the stream formatting state is restored
+ correctly at the end of the method. An instance of this type will save the stream state when
+ constructed and will restore that state when destructed.
+
+ \subsection howto_newpacket_type_final Final touches
+
+ The \c GREPacket implementation is now almost complete. The only thing missing is the \c
+ GREPacket itself. \c GREPacket is just a typedef for a specific senf::ConcretePacket template
+ instantiation. Here the complete GREPacket definition:
+
+ \code
+ #include <senf/Packets.hh>
+
+ struct GREPacketType
+ : public senf::PacketTypeBase,
+ public senf::PacketTypeMixin<GREPacketType, EtherTypes>
+ {
+ typedef senf::PacketTypeMixin<GREPacketType, EtherTypes> mixin;
+ typedef senf::ConcretePacket<GREPacketType> packet;
+ typedef senf::GREPacketParser parser;
+
+ using mixin::nextPacketRange;
+ using mixin::nextPacketType;
+ using mixin::init;
+ using mixin::initSize;
+
+ static key_t nextPacketKey(packet p) { return p->protocolType(); }
+
+ static void finalize(packet p) {
+ if (p->checksumPresent()) p->checksum() << p->calculateChecksum();
+ p->protocolType() << key(p.next(senf::nothrow));
+ }
+
+ static void dump(packet p, std::ostream & os);
+ };
+
+ typedef GREPacketType::packet GREPacket;
+
+ // In the implementation (.cc) file:
+
+ #include <senf/Packets/DefaultBundle/EthernetPacket.hh>
+ #include <senf/Packets/DefaultBundle/IPv4Packet.hh>
+
+ SENF_PACKET_REGISTRY_REGISTER( senf::EtherTypes, 0x6558, senf::EthernetPacket );
+ SENF_PACKET_REGISTRY_REGISTER( senf::IpTypes, 47, GREPacket );
+
+ void GREPacketType::dump(packet p, std::ostream & os)
+ {
+ boost::io::ios_all_saver ias(os);
+ os << "General Routing Encapsulation:\n"
+ << " checksum present : " << p->checksumPresent() ? "true" : "false" << "\n"
+ << " version : " << p->version() << "\n"
+ << " protocol type : 0x" << std::hex << std::setw(4) << std::setfill('0')
+ << p->protocolType() << "\n";
+ if (p->checksumPresent())
+ os << " checksum : 0x" << std::hex << std::setw(4)
+ << std::setfill('0') << p->checksum() << "\n";
+ }
+ \endcode
+
+
+ \section howto_newpacket_advanced Going further
+
+ \subsection howto_newpacket_advanced_valid Checking the GRE packet for validity
+
+ We have implemented the \c GREPacket completely by now. There is however still room for
+ improvement. Reading the RFC, there are some restrictions which a packet needs to obey to be
+ considered valid. If the packet is not valid it cannot be parsed and should be dropped. There
+ are two conditions which lead to this: If one of the \a reserved0 fields first 5 bits is set or
+ if the version is not 0. We will add a \a valid() check to the parser and utilize this check in
+ the packet type.
+
+ So first lets update the parser. We will need to change the fields a little bit so we have
+ access to the first 5 bits of \a reserved0. We therefore replace the first three field
+ statements with
+
+ \code
+ SENF_PARSER_BITFIELD_RO ( checksumPresent, 1, bool );
+ SENF_PARSER_PRIVATE_BITFIELD ( reserved0_5bits_, 5, unsigned );
+ SENF_PARSER_SKIP_BITS ( 7 );
+ SENF_PARSER_PRIVATE_BITFIELD ( version_, 3, unsigned );
+ \endcode
+
+ We have added an additional private bitfield \a reserved0_5bits_() and we made the \a version_()
+ field private since we do not want the user to change the value. We could have used a read-only
+ field in this special case (since the valid value is 0) but generally in a case like this since
+ the field will be initialized by the parser (see next section on how). the field should be
+ private with an additional public read-only accessor.
+
+ We will now add two additional simple members to the parser
+
+ \code
+ typedef version__t version_t;
+ version_t::value_type version() const { return version_(); }
+
+ bool valid() const { return version() == 0 && reserved0_5bits_() == 0; }
+ \endcode
+
+ I think, both are quite straight forward: \a version() will allow the user to read out the value
+ of the version field. However, since it does \e not return a parser but a numeric value, the
+ access is read-only. The \a valid() member will just check the restrictions as defined in the RFC.
+
+ Now to the packet type. We want to refrain from parsing the payload if the packet is
+ invalid. This is important: If the packet is not valid, we have no idea, whether the payload is
+ what we surmise it to be (if any of the \a reserved0_5bits_() are set, the packet is from an
+ older GRE RFC and the header is quite a bit longer so the payload will be incorrect).
+
+ So we need to change the logic which is used by the packet library to find the type of the next
+ packet. We have two ways to do this: We keep using the default \c nextPacketType()
+ implementation as provided by the senf::PacketTypeMixin and have our \a nextPacketKey()
+ implementation return a key value which is guaranteed never to be registered in the registry.
+
+ The more flexible possibility is implementing \c nextPacketType() ourselves. In this case, the
+ first method would suffice, but we will choose to go the second route to show how to write the
+ \c nextPacketType() member. We therefore remove the \c using declaration of \c nextPacketType()
+ and also remove the \a nextPacketKey() implementation. Instead we add the following to the
+ packet type
+
+ \code
+ // disabled: using Min::nextPacketType;
+
+ factory_t nextPacketType(packet p) { return p->valid() ? lookup(p->protocolType()) : no_factory(); }
+ \endcode
+
+ As we see, this is still quite simple. \c factory_t is provided by senf::PacketTypeBase. For our
+ purpose it is an opaque type which somehow enables the packet library to create a new packet of
+ a specified packet type. The \c factory_t has a special value, \c no_factory() which stands for
+ the absence of any concrete factory. In a boolean context this (and only this) \c factory_t
+ value tests \c false.
+
+ The \c lookup() member is provided by the senf::PacketTypeMixin. It looks up the key passed as
+ argument in the registry and returns the factory or \c no_factory(), if the key was not found in
+ the registry.
+
+ In this case this is all. We can however return the factory for some specific type easily as in
+ the following example:
+
+ \code
+ factory_t nextPacketType(packet p) {
+ if (p->valid()) {
+ if (p->protocolType() == 0x6558) return senf::EthernetPacket::factory();
+ else return lookup(p->protocolType());
+ }
+ else return no_factory();
+ }
+ \endcode
+
+ Of course, this example is academic since senf::EthernetPacket is correctly registered in the
+ senf::EtherTypes registry but you get the picture.
+
+
+ \subsection howto_newpacket_advanced_init Non-trivial packet initialization
+
+ When we create a new GRE packet using the packet library, the library will initialize the packet
+ with all 0 bytes. It just happens, that this is correct for our GRE packet. Lets just for the
+ sake of experiment assume, the GRE packet would have to set \a version() to 1 not 0. In this
+ case, the default initialization would not suffice. It is however very simple to explicitly
+ initialize the packet. The initialization happens within the parser. We just add
+
+ \code
+ SENF_PARSER_INIT() { version_() << 1u; }
+ \endcode
+
+ to \c GREPacketParser. Here we see, why we have defined \a version_() as a private and not a
+ read-only field.
+
+
+ \section howto_newpacket_final The ultimate GRE packet implementation completed
+
+ So here we now have \c GREPacket finally complete in all it's glory. First the header file \c
+ GREPacket.hh:
+
+ \code
+ #ifndef GRE_PACKET_HH
+ #define GRE_PACKET_HH
+
+ #include <senf/Packets.hh>
+
+ struct GREPacketParser_OptFields : public senf::PacketParser
+ {
+ # include SENF_FIXED_PARSER()
+
+ SENF_PARSER_FIELD ( checksum, senf::UInt16Parser );
+ SENF_PARSER_SKIP ( 2 );
+
+ SENF_PARSER_FINALIZE(GREPacketParser_OptFields);
+ };
+
+ struct GREPacketParser : public senf::PacketParser
+ {
+ # include SENF_PARSER()
+
+ SENF_PARSER_BITFIELD_RO ( checksumPresent, 1, bool );
+ SENF_PARSER_PRIVATE_BITFIELD ( reserved0_5bits_, 5, unsigned );
+ SENF_PARSER_SKIP_BITS ( 7 );
+ SENF_PARSER_PRIVATE_BITFIELD ( version_, 3, unsigned );
+
+ SENF_PARSER_FIELD ( protocolType, senf::UInt16Parser );
+
+ SENF_PARSER_PRIVATE_VARIANT ( optionalFields_, checksumPresent,
+ (senf::VoidPacketParser)
+ (GREPacketParser_OptFields) );
+
+ typedef version__t version_t;
+ version_t::value_type version() const { return version_(); }
+
+ bool valid() const { return version() == 0 && reserved0_5bits_() == 0; }
+
+ typedef GREPacketParser_OptFields::checksum_t checksum_t;
+ checksum_t checksum() const
+ { return optionalFields_().get<1>().checksum(); }
+
+ void enableChecksum() const { optionalFields_().init<1>(); }
+ void disableChecksum() const { optionalFields_().init<0>(); }
+
+ SENF_PARSER_FINALIZE(GREPacketParser);
+
+ checksum_t::value_type calculateChecksum() const;
+ };
+
+ struct GREPacketType
+ : public senf::PacketTypeBase,
+ public senf::PacketTypeMixin<GREPacketType, EtherTypes>
+ {
+ typedef senf::PacketTypeMixin<GREPacketType, EtherTypes> mixin;
+ typedef senf::ConcretePacket<GREPacketType> packet;
+ typedef senf::GREPacketParser parser;
+
+ using mixin::nextPacketRange;
+ using mixin::init;
+ using mixin::initSize;
+
+ factory_t nextPacketType(packet p)
+ { return p->valid() ? lookup(p->protocolType()) : no_factory(); }
+
+ static void finalize(packet p) {
+ if (p->checksumPresent()) p->checksum() << p->calculateChecksum();
+ p->protocolType() << key(p.next(senf::nothrow));
+ }
+
+ static void dump(packet p, std::ostream & os);
+ };
+
+ typedef GREPacketType::packet GREPacket;
- \fixme Document the needed \c \#include files
- \fixme Provide an advanced section with additional info: How to ensure, that the first 5 bits in
- reserver0 are not set. How to enforce version == 0 (that is, make version() read-only and
- return no_factory for the next packet type if any of the conditions is violated)
+ #endif
+ \endcode
+
+ And the implementation file \c GREPacket.cc:
+
+ \code
+ #include "GREPacket.hh"
+ #include <senf/Utils/IpChecksum.hh>
+ #include <senf/Packets/DefaultBundle/EthernetPacket.hh>
+ #include <senf/Packets/DefaultBundle/IPv4Packet.hh>
+
+ SENF_PACKET_REGISTRY_REGISTER( senf::EtherTypes, 0x6558, senf::EthernetPacket );
+ SENF_PACKET_REGISTRY_REGISTER( senf::IpTypes, 47, GREPacket );
+
+ GREPacketParser::checksum_t::value_type GREPacketParser::calculateChecksum() const
+ {
+ if (!checksumEnabled())
+ return 0;
+
+ validate(6);
+ senf::IpChecksum cs;
+ cs.feed( i(), i()+4 );
+ // Skip even number of 0 bytes (the 2 bytes checksum field)
+ // cs.feed(0); cs.feed(0);
+ cs.feed( i()+6, data().end() );
+
+ return cs.sum()
+ }
+
+ void GREPacketType::dump(packet p, std::ostream & os)
+ {
+ boost::io::ios_all_saver ias(os);
+ os << "General Routing Encapsulation:\n"
+ << " checksum present : " << p->checksumPresent() ? "true" : "false" << "\n"
+ << " version : " << p->version() << "\n"
+ << " protocol type : 0x" << std::hex << std::setw(4) << std::setfill('0')
+ << p->protocolType() << "\n";
+ if (p->checksumPresent())
+ os << " checksum : 0x" << std::hex << std::setw(4)
+ << std::setfill('0') << p->checksum() << "\n";
+ }
+ \endcode
+
+
+ \section howto_newpacket_using Using the newly created GRE packet type
+
+ The GRE packet is now fully integrated into the packet library framework. For example, we can
+ read GRE packets from a raw INet socket and forward decapsulated Ethernet frames to a packet
+ socket.
+
+ \code
+ #include <iostream>
+ #include <senf/Packets.hh>
+ #include <senf/Packets/DefaultBundle/EthernetPacket.hh>
+ #include <senf/Socket/Protocols/INet/RawINetProtocol.hh>
+ #include <senf/Socket/Protocols/Raw/PacketSocketHandle.hh>
+ #include "GREPacket.hh"
+
+ int main(int, char const **)
+ {
+ senf::RawV6ClientSocketHandle isock (47u); // 47 = Read GRE packets
+ senf::PacketSocketHandle osock;
+
+ while (1) {
+ try {
+ GREPacket gre (GREPacket::create(senf::noinit));
+ isock.read(gre.data(),0u);
+ if (gre->checksumPresent() && gre->checksum() != gre->calculateChecksum())
+ throw InvalidPacketChainException();
+ osock.write(gre.next<EthernetPacket>().data())
+ }
+ catch (senf::TruncatedPacketException & ex) {
+ std::cerr << "Malformed packet dropped\n";
+ }
+ catch (senf::InvalidPacketChainException & ex) {
+ std::cerr << "Invalid GRE packet dropped\n";
+ }
+ }
+ }
+ \endcode
+
+ Or we can do the opposite: Read ethernet packets from a \c tap device and send them out GRE
+ encapsulated.
+
+ \code
+ #include <iostream>
+ #include <senf/Packets.hh>
+ #include <senf/Packets/DefaultBundle/EthernetPacket.hh>
+ #include <senf/Socket/Protocols/INet/RawINetProtocol.hh>
+ #include <senf/Socket/Protocols/Raw/TunTapSocketHandle.hh>
+ #include "GREPacket.hh"
+
+ int main(int argc, char const ** argv)
+ {
+ if (argc != 2) {
+ std::cerr << "Usage: " << argv[0] << " <tunnel endpoint>\n";
+ return 1;
+ }
+
+ senf::TapSocketHandle tap ("tap0");
+ senf::ConnectedRawV6ClientSocketHandle osock (47u, senf::INet6SocketAddress(argv[1]));
+
+ while (1) {
+ senf::EthernetPacket eth (senf::EthernetPacket::create(senf::noinit));
+ isock.read(eth.data(),0u);
+ GREPacket gre (senf::GREPacket::createBefore(eth));
+ gre.finalize();
+ osock.write(gre.data());
+ }
+ }
+ \endcode
+
+
+ \section howto_newpacket_further Further reading
+
+ Lets start with references to the important API's (Use the 'Show all members' link to get the
+ complete API of one of the classes and templates):
+
+ \li senf::ConcretePacket : this is the API provided by the packet handles.
+ \li senf::PacketData : this API provides raw data access accessible via the handles 'data'
+ member.
+ \li senf::PacketParserBase : this is the generic parser API. This API is accessible via the
+ packets \c -> operator or via the sub-parsers returned by the field accessors.
+
+ When implementing new packet's, the following information will be helpful:
+
+ \li senf::PacketTypeBase : here you find a description of the members which need to be
+ implemented to provide a 'packet type'. Most of these members will normally be provided by
+ the mixin helper.
+ \li senf::PacketTypeMixin : here you find all about the packet type mixin and how to use it.
+ \li \ref packetparser : This section describes the packet parser facility.
+ \li \ref packetparsermacros : A complete list and documentation of all the packet parser macros.
+ \li There are several lists of available reusable packet parsers: \ref parseint, \ref
+ parsecollection. However, this list is not complete as there are other protocol specific
+ reusable parsers (without claiming to be exhaustive: senf::INet4AddressParser,
+ senf::INet6AddressParser, senf::MACAddressParser)
+
*/
\f
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