Current File : //backups/router/usr/local/include/kea/dns/message.h
// Copyright (C) 2009-2024 Internet Systems Consortium, Inc. ("ISC")
//
// This Source Code Form is subject to the terms of the Mozilla Public
// License, v. 2.0. If a copy of the MPL was not distributed with this
// file, You can obtain one at http://mozilla.org/MPL/2.0/.
#ifndef MESSAGE_H
#define MESSAGE_H
#include <stdint.h>
#include <iterator>
#include <string>
#include <ostream>
#include <dns/exceptions.h>
#include <util/buffer.h>
#include <dns/edns.h>
#include <dns/question.h>
#include <dns/rrset.h>
namespace isc {
namespace dns {
class TSIGContext;
class TSIGRecord;
///
/// \brief A standard DNS module exception that is thrown if a wire format
/// message parser encounters a short length of data that don't even contain
/// the full header section.
///
class MessageTooShort : public isc::dns::Exception {
public:
MessageTooShort(const char* file, size_t line, const char* what) :
isc::dns::Exception(file, line, what) {}
};
///
/// \brief A standard DNS module exception that is thrown if a section iterator
/// is being constructed for an incompatible section. Specifically, this
/// happens RRset iterator is being constructed for a Question section.
///
class InvalidMessageSection : public isc::dns::Exception {
public:
InvalidMessageSection(const char* file, size_t line, const char* what) :
isc::dns::Exception(file, line, what) {}
};
///
/// \brief A standard DNS module exception that is thrown if a \c Message
/// class method is called that is prohibited for the current mode of
/// the message.
///
class InvalidMessageOperation : public isc::dns::Exception {
public:
InvalidMessageOperation(const char* file, size_t line, const char* what) :
isc::dns::Exception(file, line, what) {}
};
///
/// \brief A standard DNS module exception that is thrown if a UDP buffer size
/// smaller than the standard default maximum (DEFAULT_MAX_UDPSIZE) is
/// being specified for the message.
///
class InvalidMessageUDPSize : public isc::dns::Exception {
public:
InvalidMessageUDPSize(const char* file, size_t line, const char* what) :
isc::dns::Exception(file, line, what) {}
};
typedef uint16_t qid_t;
class AbstractMessageRenderer;
class Message;
class MessageImpl;
class Opcode;
class Rcode;
template <typename T>
struct SectionIteratorImpl;
/// \c SectionIterator is a templated class to provide standard-compatible
/// iterators for Questions and RRsets for a given DNS message section.
/// The template parameter is either \c QuestionPtr (for the question section)
/// or \c RRsetPtr (for the answer, authority, or additional section).
template <typename T>
class SectionIterator {
public:
// Aliases used to enable iterator behavior on this class
using iterator_category = std::input_iterator_tag;
using value_type = T;
using difference_type = std::ptrdiff_t;
using pointer = T*;
using reference = T&;
SectionIterator() : impl_(0) {
}
SectionIterator(const SectionIteratorImpl<T>& impl);
~SectionIterator();
SectionIterator(const SectionIterator<T>& source);
void operator=(const SectionIterator<T>& source);
SectionIterator<T>& operator++();
SectionIterator<T> operator++(int);
const T& operator*() const;
const T* operator->() const;
bool operator==(const SectionIterator<T>& other) const;
bool operator!=(const SectionIterator<T>& other) const;
private:
SectionIteratorImpl<T>* impl_;
};
typedef SectionIterator<QuestionPtr> QuestionIterator;
typedef SectionIterator<RRsetPtr> RRsetIterator;
class MessageImpl;
/// @brief Pointer to the @ref MessageImpl object.
typedef boost::shared_ptr<MessageImpl> MessageImplPtr;
/// \brief The \c Message class encapsulates a standard DNS message.
///
/// Details of the design and interfaces of this class are still in flux.
/// Here are some notes about the current design.
///
/// Since many realistic DNS applications deal with messages, message objects
/// will be frequently used, and can be performance sensitive. To minimize
/// the performance overhead of constructing and destructing the objects,
/// this class is designed to be reusable. The \c clear() method is provided
/// for this purpose.
///
/// A \c Message class object is in either the \c PARSE or the \c RENDER mode.
/// A \c PARSE mode object is intended to be used to convert wire-format
/// message data into a complete \c Message object.
/// A \c RENDER mode object is intended to be used to convert a \c Message
/// object into wire-format data.
/// Some of the method functions of this class are limited to a specific mode.
/// In general, "set" type operations are only allowed for \c RENDER mode
/// objects.
/// The initial mode must be specified on construction, and can be changed
/// through some method functions.
///
/// This class uses the "pimpl" idiom, and hides detailed implementation
/// through the \c impl_ pointer. Since a \c Message object is expected to
/// be reused, the construction overhead of this approach should be acceptable.
///
/// Open issues (among other things):
/// - We may want to provide an "iterator" for all RRsets/RRs for convenience.
/// This will be for applications that do not care about performance much,
/// so the implementation can only be moderately efficient.
/// - We may want to provide a "find" method for a specified type
/// of RR in the message.
class Message {
public:
/// Constants to specify the operation mode of the \c Message.
enum Mode {
PARSE = 0, // Parse mode (handling an incoming message)
RENDER = 1 // Render mode (building an outgoing message)
};
/// \brief Constants for flag bit fields of a DNS message header.
///
/// Only the defined constants are valid where a header flag is required
/// in this library (e.g., in \c Message::setHeaderFlag()).
/// Since these are enum constants, however, an invalid value could be
/// passed via casting without an error at compilation time.
/// It is generally the callee's responsibility to check and reject invalid
/// values.
/// Of course, applications shouldn't pass invalid values even if the
/// callee does not perform proper validation; the result in such usage
/// is undefined.
///
/// In the current implementation, the defined values happen to be
/// a 16-bit integer with one bit being set corresponding to the
/// specified flag in the second 16 bits of the DNS Header section
/// in order to make the internal implementation simpler.
/// For example, \c HEADERFLAG_QR is defined to be 0x8000 as the QR
/// bit is the most significant bit of the second 16 bits of the header.
/// However, applications should not assume this coincidence and
/// must solely use the enum representations.
/// Any usage based on the assumption of the underlying values is invalid
/// and the result is undefined.
///
/// Likewise, bit wise operations such as AND or OR on the flag values
/// are invalid and are not guaranteed to work, even if it could compile
/// with casting.
/// For example, the following code will compile:
/// \code const uint16_t combined_flags =
/// static_cast<uint16_t>(Message::HEADERFLAG_AA) |
/// static_cast<uint16_t>(Message::HEADERFLAG_CD);
/// message->setHeaderFlag(static_cast<Message::HeaderFlag>(combined_flags));
/// \endcode
/// and (with the current definition) happens to work as if it were
/// validly written as follows:
/// \code message->setHeaderFlag(Message::HEADERFLAG_AA);
/// message->setHeaderFlag(Message::HEADERFLAG_CD);
/// \endcode
/// But the former notation is invalid and may not work in future versions.
/// We did not try to prohibit such usage at compilation time, e.g., by
/// introducing a separately defined class considering the balance
/// between the complexity and advantage, but hopefully the cast notation
/// is sufficiently ugly to prevent proliferation of the usage.
enum HeaderFlag {
HEADERFLAG_QR = 0x8000, // Query (if cleared) or response (if set)
HEADERFLAG_AA = 0x0400, // Authoritative answer
HEADERFLAG_TC = 0x0200, // Truncation
HEADERFLAG_RD = 0x0100, // Recursion desired
HEADERFLAG_RA = 0x0080, // Recursion available
HEADERFLAG_AD = 0x0020, // Authentic %data (RFC4035)
HEADERFLAG_CD = 0x0010 // DNSSEC checking disabled (RFC4035)
};
/// \brief Constants to specify sections of a DNS message.
///
/// The sections are those defined in RFC 1035 excluding the Header
/// section; the fields of the Header section are accessed via specific
/// methods of the \c Message class (e.g., \c getQid()).
///
/// <b>Open Design Issue:</b>
/// In the current implementation the values for the constants are
/// sorted in the order of appearance in DNS messages, i.e.,
/// from %Question to Additional.
/// So, for example,
/// code <code>section >= Message::SECTION_AUTHORITY</code> can be
/// used to do something in or after the Authority section.
/// This would be convenient, but it is not clear if it's really a good
/// idea to rely on relationship between the underlying values of enum
/// constants. At the moment, applications are discouraged to rely on
/// this implementation detail. We will see if such usage is sufficiently
/// common to officially support it.
///
/// Note also that since we don't define \c operator++ for this enum,
/// the following code intending to iterate over all sections will
/// \b not compile:
/// \code for (Section s; s <= SECTION_ADDITIONAL; ++s) { // ++s undefined
/// // do something
/// } \endcode
/// This is intentional at this moment, and we'll see if we need to allow
/// that as we have more experiences with this library.
///
/// <b>Future Extension:</b> We'll probably also define constants for
/// the section names used in dynamic updates in future versions.
enum Section {
SECTION_QUESTION = 0, // Question section
SECTION_ANSWER = 1, // Answer section
SECTION_AUTHORITY = 2, // Authority section
SECTION_ADDITIONAL = 3 // Additional section
};
///
/// \name Constructors and Destructor
///
/// Note: The copy constructor and the assignment operator are
/// intentionally defined as private.
/// The intended use case wouldn't require copies of a \c Message object;
/// once created, it would normally be expected to be reused, changing the
/// mode from \c PARSE to \c RENDER, and vice versa.
//@{
public:
/// \brief The constructor.
/// The mode of the message is specified by the \c mode parameter.
Message(Mode mode);
/// \brief The destructor.
~Message() = default;
private:
Message(const Message& source);
Message& operator=(const Message& source);
//@}
public:
/// \brief Return whether the specified header flag bit is set in the
/// header section.
///
/// This method is basically exception free, but if
/// \c flag is not a valid constant of the \c HeaderFlag type,
/// an exception of class \c InvalidParameter will be thrown.
///
/// \param flag The header flag constant to test.
/// \return \c true if the specified flag is set; otherwise \c false.
bool getHeaderFlag(const HeaderFlag flag) const;
/// \brief Set or clear the specified header flag bit in the header
/// section.
///
/// The optional parameter \c on indicates the operation mode,
/// set or clear; if it's \c true the corresponding flag will be set;
/// otherwise the flag will be cleared.
/// In either case the original state of the flag does not affect the
/// operation; for example, if a flag is already set and the "set"
/// operation is attempted, it effectively results in no operation.
///
/// The parameter \c on can be omitted, in which case a value of \c true
/// (i.e., set operation) will be assumed.
/// This is based on the observation that the flag would have to be set
/// in the vast majority of the cases where an application needs to
/// use this method.
///
/// This method is only allowed in the \c RENDER mode;
/// if the \c Message is in other mode, an exception of class
/// InvalidMessageOperation will be thrown.
///
/// If \c flag is not a valid constant of the \c HeaderFlag type,
/// an exception of class \c InvalidParameter will be thrown.
///
/// \param flag The header flag constant to set or clear.
/// \param on If \c true the flag will be set; otherwise the flag will be
/// cleared.
void setHeaderFlag(const HeaderFlag flag, const bool on = true);
/// \brief Return the query ID given in the header section of the message.
qid_t getQid() const;
/// \brief Set the query ID of the header section of the message.
///
/// This method is only allowed in the \c RENDER mode;
/// if the \c Message is in other mode, an exception of class
/// InvalidMessageOperation will be thrown.
void setQid(qid_t qid);
/// \brief Return the Response Code of the message.
///
/// This includes extended codes specified by an EDNS OPT RR (when
/// included). In the \c PARSE mode, if the received message contains
/// an EDNS OPT RR, the corresponding extended code is identified and
/// returned.
///
/// The message must have been properly parsed (in the case of the
/// \c PARSE mode) or an \c Rcode has been set (in the case of the
/// \c RENDER mode) beforehand. Otherwise, an exception of class
/// \c InvalidMessageOperation will be thrown.
const Rcode& getRcode() const;
/// \brief Set the Response Code of the message.
///
/// This method is only allowed in the \c RENDER mode;
/// if the \c Message is in other mode, an exception of class
/// InvalidMessageOperation will be thrown.
///
/// If the specified code is an EDNS extended RCODE, an EDNS OPT RR will be
/// included in the message.
void setRcode(const Rcode& rcode);
/// \brief Return the OPCODE given in the header section of the message.
///
/// The message must have been properly parsed (in the case of the
/// \c PARSE mode) or an \c Opcode has been set (in the case of the
/// \c RENDER mode) beforehand. Otherwise, an exception of class
/// \c InvalidMessageOperation will be thrown.
const Opcode& getOpcode() const;
/// \brief Set the OPCODE of the header section of the message.
///
/// This method is only allowed in the \c RENDER mode;
/// if the \c Message is in other mode, an exception of class
/// InvalidMessageOperation will be thrown.
void setOpcode(const Opcode& opcode);
/// \brief Return, if any, the EDNS associated with the message.
///
/// This method never throws an exception.
///
/// \return A shared pointer to the EDNS. This will be a null shared
/// pointer if the message is not associated with EDNS.
ConstEDNSPtr getEDNS() const;
/// \brief Set EDNS for the message.
///
/// This method is only allowed in the \c RENDER mode;
/// if the \c Message is in other mode, an exception of class
/// InvalidMessageOperation will be thrown.
///
/// \param edns A shared pointer to an \c EDNS object to be set in
/// \c Message.
void setEDNS(ConstEDNSPtr edns);
/// \brief Return, if any, the TSIG record contained in the received
/// message.
///
/// Currently, this method is only intended to return a TSIG record
/// for an incoming message built via the \c fromWire() method in the
/// PARSE mode. A call to this method in the RENDER mode is invalid and
/// result in an exception. Also, calling this method is meaningless
/// unless \c fromWire() is performed.
///
/// The returned pointer is valid only during the lifetime of the
/// \c Message object and until \c clear() is called. The \c Message
/// object retains the ownership of \c TSIGRecord; the caller must not
/// try to delete it.
///
/// \exception InvalidMessageOperation Message is not in the PARSE mode.
///
/// \return A pointer to the stored \c TSIGRecord or null.
const TSIGRecord* getTSIGRecord() const;
/// \brief Returns the number of RRs contained in the given section.
///
/// In the \c PARSE mode, the returned value may not be identical to
/// the actual number of RRs of the incoming message that is parsed.
/// The \c Message class handles some "meta" RRs such as EDNS OPT RR
/// separately. This method doesn't include such RRs.
/// Also, a future version of the parser will detect and unify duplicate
/// RRs (which should be rare in practice though), in which case
/// the stored RRs in the \c Message object will be fewer than the RRs
/// originally contained in the incoming message.
///
/// Likewise, in the \c RENDER mode, even if \c EDNS is set in the
/// \c Message, this method doesn't count the corresponding OPT RR
/// in the Additional section.
///
/// This method is basically exception free, but if
/// \c section is not a valid constant of the \c Section type,
/// an exception of class \c OutOfRange will be thrown.
///
/// \param section The section in the message where RRs should be
/// counted.
/// \return The number of RRs stored in the specified section of the
/// message.
unsigned int getRRCount(const Section section) const;
/// \brief Return an iterator corresponding to the beginning of the
/// Question section of the message.
const QuestionIterator beginQuestion() const;
/// \brief Return an iterator corresponding to the end of the
/// Question section of the message.
const QuestionIterator endQuestion() const;
/// \brief Return an iterator corresponding to the beginning of the
/// given section (other than Question) of the message.
///
/// \c section must be a valid constant of the \c Section type;
/// otherwise, an exception of class \c OutOfRange will be thrown.
const RRsetIterator beginSection(const Section section) const;
/// \brief Return an iterator corresponding to the end of the
/// given section (other than Question) of the message.
///
/// \c section must be a valid constant of the \c Section type;
/// otherwise, an exception of class \c OutOfRange will be thrown.
const RRsetIterator endSection(const Section section) const;
/// \brief Add a (pointer like object of) Question to the message.
///
/// This method is only allowed in the \c RENDER mode;
/// if the \c Message is in other mode, an exception of class
/// InvalidMessageOperation will be thrown.
void addQuestion(QuestionPtr question);
/// \brief Add a (pointer like object of) Question to the message.
///
/// This version internally creates a \c QuestionPtr object from the
/// given \c question and calls the other version of this method.
/// So this is inherently less efficient, but is provided because this
/// form may be more intuitive and may make more sense for performance
/// insensitive applications.
///
/// This method is only allowed in the \c RENDER mode;
/// if the \c Message is in other mode, an exception of class
/// InvalidMessageOperation will be thrown.
void addQuestion(const Question& question);
/// \brief Add a (pointer like object of) RRset to the given section
/// of the message.
///
/// Note that \c addRRset() does not currently check for duplicate
/// data before inserting RRsets. The caller is responsible for
/// checking for these (see \c hasRRset() below).
///
/// \throw InvalidParameter rrset is null
/// \throw InvalidMessageOperation The message is not in the \c RENDER
/// mode.
/// \throw OutOfRange \c section doesn't specify a valid \c Section value.
///
/// \param section The message section to which the rrset is to be added
/// \param rrset The rrset to be added. Must not be null.
void addRRset(const Section section, RRsetPtr rrset);
/// \brief Determine whether the given section already has an RRset
/// matching the given name, RR class and RR type.
///
/// \c section must be a valid constant of the \c Section type;
/// otherwise, an exception of class \c OutOfRange will be thrown.
///
/// This should probably be extended to be a "find" method that returns
/// a matching RRset if found.
bool hasRRset(const Section section, const Name& name,
const RRClass& rrclass, const RRType& rrtype) const;
/// \brief Determine whether the given section already has an RRset
/// matching the one pointed to by the argument
///
/// \c section must be a valid constant of the \c Section type;
/// otherwise, an exception of class \c OutOfRange will be thrown.
bool hasRRset(const Section section, const RRsetPtr& rrset) const;
/// \brief Remove RRSet from Message
///
/// Removes the RRset identified by the section iterator from the message.
/// Note: if,.for some reason, the RRset is duplicated in the section, only
/// one occurrence is removed.
///
/// If the operation is successful, all iterators into the section are
/// invalidated.
///
/// \param section Section to which the iterator belongs
/// \param iterator Iterator pointing to the element to be removed
///
/// \return true if the element was removed, false if the iterator was not
/// found in the specified section.
bool removeRRset(const Section section, RRsetIterator& iterator);
/// \brief Remove all RRSets from the given Section
///
/// This method is only allowed in the \c RENDER mode, and the given
/// section must be valid.
///
/// \throw InvalidMessageOperation Message is not in the \c RENDER mode
/// \throw OutOfRange The specified section is not valid
///
/// \param section Section to remove all rrsets from
void clearSection(const Section section);
// The following methods are not currently implemented.
//void removeQuestion(QuestionPtr question);
// notyet:
//void addRR(const Section section, const RR& rr);
//void removeRR(const Section section, const RR& rr);
/// \brief Clear the message content (if any) and reinitialize it in the
/// specified mode.
void clear(Mode mode);
/// \brief Adds all rrsets from the source the given section in the
/// source message to the same section of this message
///
/// \param section the section to append
/// \param source The source Message
void appendSection(const Section section, const Message& source);
/// \brief Prepare for making a response from a request.
///
/// This will clear the DNS header except those fields that should be kept
/// for the response, and clear answer and the following sections.
/// See also dns_message_reply() of BIND9.
void makeResponse();
/// \brief Convert the Message to a string.
///
/// At least \c Opcode and \c Rcode must be validly set in the \c Message
/// (as a result of parse in the \c PARSE mode or by explicitly setting
/// in the \c RENDER mode); otherwise, an exception of
/// class \c InvalidMessageOperation will be thrown.
std::string toText() const;
/// \brief Render the message in wire formant into a message renderer
/// object with (or without) TSIG.
///
/// This \c Message must be in the \c RENDER mode and both \c Opcode and
/// \c Rcode must have been set beforehand; otherwise, an exception of
/// class \c InvalidMessageOperation will be thrown.
///
/// If a non-null \c tsig_ctx is passed, it will also add a TSIG RR
/// with (in many cases) the TSIG MAC for the message along with the
/// given TSIG context (\c tsig_ctx). The TSIG RR will be placed at
/// the end of \c renderer. The \c TSIGContext at \c tsig_ctx will
/// be updated based on the fact it was used for signing and with
/// the latest MAC.
///
/// \exception InvalidMessageOperation The message is not in the Render
/// mode, or either Rcode or Opcode is not set.
/// \exception InvalidParameter The allowable limit of \c renderer is too
/// small for a TSIG or the Header section. Note that this shouldn't
/// happen with parameters as defined in the standard protocols,
/// so it's more likely a program bug.
/// \exception Unexpected Rendering the TSIG RR fails. The implementation
/// internally makes sure this doesn't happen, so if that ever occurs
/// it should mean a bug either in the TSIG context or in the renderer
/// implementation.
///
/// \note The renderer's internal buffers and data are automatically
/// cleared, keeping the length limit and the compression mode intact.
/// In case truncation is triggered, the renderer is cleared completely.
///
/// \param renderer DNS message rendering context that encapsulates the
/// output buffer and name compression information.
/// \param tsig_ctx A TSIG context that is to be used for signing the
/// message
void toWire(AbstractMessageRenderer& renderer,
TSIGContext* tsig_ctx = 0);
/// Parse options.
///
/// describe PRESERVE_ORDER: note doesn't affect EDNS or TSIG.
///
/// The option values are used as a parameter for \c fromWire().
/// These are values of a bitmask type. Bitwise operations can be
/// performed on these values to express compound options.
enum ParseOptions {
PARSE_DEFAULT = 0, // The default options
PRESERVE_ORDER = 1 // Preserve RR order and don't combine them
};
/// \brief Parse the header section of the \c Message.
///
/// NOTE: If the header has already been parsed by a previous call
/// to this method, this method simply returns (i.e., it does not
/// read from the \c buffer).
void parseHeader(isc::util::InputBuffer& buffer);
/// \brief (Re)build a \c Message object from wire-format data.
///
/// This method parses the given wire format data to build a
/// complete Message object. On success, the values of the header section
/// fields can be accessible via corresponding get methods, and the
/// question and following sections can be accessible via the
/// corresponding iterators. If the message contains an EDNS or TSIG,
/// they can be accessible via \c getEDNS() and \c getTSIGRecord(),
/// respectively.
///
/// This \c Message must be in the \c PARSE mode.
///
/// This method performs strict validation on the given message based
/// on the DNS protocol specifications. If the given message data is
/// invalid, this method throws an exception (see the exception list).
///
/// By default, this method combines RRs of the same name, RR type and
/// RR class in a section into a single RRset, even if they are interleaved
/// with a different type of RR (though it would be a rare case in
/// practice). If the \c PRESERVE_ORDER option is specified, it handles
/// each RR separately, in the appearing order, and converts it to a
/// separate RRset (so this RRset should contain exactly one Rdata).
/// This mode will be necessary when the higher level protocol is
/// ordering conscious. For example, in AXFR and IXFR, the position of
/// the SOA RRs are crucial.
///
/// \exception InvalidMessageOperation \c Message is in the RENDER mode
/// \exception DNSMessageFORMERR The given message data is syntactically
/// \exception MessageTooShort The given data is shorter than a valid
/// header section
/// \exception std::bad_alloc Memory allocation failure
/// \exception Others \c Name, \c Rdata, and \c EDNS classes can also throw
///
/// \param buffer A input buffer object that stores the wire
/// data. This method reads from position 0 in the passed buffer.
/// \param options Parse options
void fromWire(isc::util::InputBuffer& buffer, ParseOptions options = PARSE_DEFAULT);
///
/// \name Protocol constants
///
//@{
/// \brief The default maximum size of UDP DNS messages that don't cause
/// truncation.
///
/// With EDNS the maximum size can be increased per message.
static const uint16_t DEFAULT_MAX_UDPSIZE = 512;
/// \brief The default maximum size of UDP DNS messages we can handle
static const uint16_t DEFAULT_MAX_EDNS0_UDPSIZE = 4096;
//@}
private:
MessageImplPtr impl_;
};
/// \brief Pointer-like type pointing to a \c Message
///
/// This type is expected to be used as an argument in asynchronous
/// callback functions. The internal reference-counting will ensure that
/// that ongoing state information will not be lost if the object
/// that originated the asynchronous call falls out of scope.
typedef boost::shared_ptr<Message> MessagePtr;
typedef boost::shared_ptr<const Message> ConstMessagePtr;
/// Insert the \c Message as a string into stream.
///
/// This method convert \c message into a string and inserts it into the
/// output stream \c os.
///
/// \param os A \c std::ostream object on which the insertion operation is
/// performed.
/// \param message A \c Message object output by the operation.
/// \return A reference to the same \c std::ostream object referenced by
/// parameter \c os after the insertion operation.
std::ostream& operator<<(std::ostream& os, const Message& message);
} // namespace dns
} // namespace isc
#endif // MESSAGE_H
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