[master] Merge branch 'trac3082'

src/lib/dhcp/Makefile.am

@@ -23,6 +23,7 @@ libb10_dhcp___la_SOURCES += iface_mgr_linux.cc
 libb10_dhcp___la_SOURCES += iface_mgr_sun.cc
 libb10_dhcp___la_SOURCES += libdhcp++.cc libdhcp++.h
 libb10_dhcp___la_SOURCES += option4_addrlst.cc option4_addrlst.h
+libb10_dhcp___la_SOURCES += option4_client_fqdn.cc option4_client_fqdn.h
 libb10_dhcp___la_SOURCES += option6_ia.cc option6_ia.h
 libb10_dhcp___la_SOURCES += option6_iaaddr.cc option6_iaaddr.h
 libb10_dhcp___la_SOURCES += option6_addrlst.cc option6_addrlst.h

src/lib/dhcp/option4_client_fqdn.cc

+// Copyright (C) 2013 Internet Systems Consortium, Inc. ("ISC")
+//
+// Permission to use, copy, modify, and/or distribute this software for any
+// purpose with or without fee is hereby granted, provided that the above
+// copyright notice and this permission notice appear in all copies.
+//
+// THE SOFTWARE IS PROVIDED "AS IS" AND ISC DISCLAIMS ALL WARRANTIES WITH
+// REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF MERCHANTABILITY
+// AND FITNESS.  IN NO EVENT SHALL ISC BE LIABLE FOR ANY SPECIAL, DIRECT,
+// INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM
+// LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE
+// OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR
+// PERFORMANCE OF THIS SOFTWARE.
+
+#include <dhcp/dhcp4.h>
+#include <dhcp/option4_client_fqdn.h>
+#include <dns/labelsequence.h>
+#include <util/buffer.h>
+#include <util/io_utilities.h>
+#include <util/strutil.h>
+#include <sstream>
+
+namespace isc {
+namespace dhcp {
+
+/// @brief Implements the logic for the Option6ClientFqdn class.
+///
+/// The purpose of the class is to separate the implementation details
+/// of the Option4ClientFqdn class from the interface. This implementation
+/// uses libdns classes to process FQDNs. At some point it may be
+/// desired to split libdhcp++ from libdns. In such case the
+/// implementation of this class may be changed. The declaration of the
+/// Option6ClientFqdn class holds the pointer to implementation, so
+/// the transition to a different implementation would not affect the
+/// header file.
+class Option4ClientFqdnImpl {
+public:
+    /// Holds flags carried by the option.
+    uint8_t flags_;
+    /// Holds RCODE1 and RCODE2 values.
+    Option4ClientFqdn::Rcode rcode1_;
+    Option4ClientFqdn::Rcode rcode2_;
+    /// Holds the pointer to a domain name carried in the option.
+    boost::shared_ptr<isc::dns::Name> domain_name_;
+    /// Indicates whether domain name is partial or fully qualified.
+    Option4ClientFqdn::DomainNameType domain_name_type_;
+
+    /// @brief Constructor, from domain name.
+    ///
+    /// @param flags A value of the flags option field.
+    /// @param rcode An object representing the RCODE1 and RCODE2 values.
+    /// @param domain_name A domain name carried by the option given in the
+    /// textual format.
+    /// @param name_type A value which indicates whether domain-name is partial
+    /// or fully qualified.
+    Option4ClientFqdnImpl(const uint8_t flags,
+                          const Option4ClientFqdn::Rcode& rcode,
+                          const std::string& domain_name,
+                          const Option4ClientFqdn::DomainNameType name_type);
+
+    /// @brief Constructor, from wire data.
+    ///
+    /// @param first An iterator pointing to the begining of the option data
+    /// in the wire format.
+    /// @param last An iterator poiting to the end of the option data in the
+    /// wire format.
+    Option4ClientFqdnImpl(OptionBufferConstIter first,
+                          OptionBufferConstIter last);
+
+    /// @brief Copy constructor.
+    ///
+    /// @param source An object being copied.
+    Option4ClientFqdnImpl(const Option4ClientFqdnImpl& source);
+
+    /// @brief Assignment operator.
+    ///
+    /// @param source An object which is being assigned.
+    Option4ClientFqdnImpl& operator=(const Option4ClientFqdnImpl& source);
+
+    /// @brief Set a new domain name for the option.
+    ///
+    /// @param domain_name A new domain name to be assigned.
+    /// @param name_type A value which indicates whether the domain-name is
+    /// partial or fully qualified.
+    void setDomainName(const std::string& domain_name,
+                       const Option4ClientFqdn::DomainNameType name_type);
+
+    /// @brief Check if flags are valid.
+    ///
+    /// In particular, this function checks if the N and S bits are not
+    /// set to 1 in the same time.
+    ///
+    /// @param flags A value carried by the flags field of the option.
+    /// @param check_mbz A boolean value which indicates if this function should
+    /// check if the MBZ bits are set (if true). This parameter should be set
+    /// to false when validating flags in the received message. This is because
+    /// server should ignore MBZ bits in received messages.
+    /// @throw InvalidOption6FqdnFlags if flags are invalid.
+    static void checkFlags(const uint8_t flags, const bool check_mbz);
+
+    /// @brief Parse the Option provided in the wire format.
+    ///
+    /// @param first An iterator pointing to the begining of the option data
+    /// in the wire format.
+    /// @param last An iterator poiting to the end of the option data in the
+    /// wire format.
+    void parseWireData(OptionBufferConstIter first,
+                       OptionBufferConstIter last);
+
+    /// @brief Parse domain-name encoded in the canonical format.
+    ///
+    void parseCanonicalDomainName(OptionBufferConstIter first,
+                                  OptionBufferConstIter last);
+
+    /// @brief Parse domain-name emcoded in the deprecated ASCII format.
+    ///
+    /// @param first An iterator pointing to the begining of the option data
+    /// where domain-name is stored.
+    /// @param last An iterator poiting to the end of the option data where
+    /// domain-name is stored.
+    void parseASCIIDomainName(OptionBufferConstIter first,
+                              OptionBufferConstIter last);
+
+};
+
+Option4ClientFqdnImpl::
+Option4ClientFqdnImpl(const uint8_t flags,
+                      const Option4ClientFqdn::Rcode& rcode,
+                      const std::string& domain_name,
+                      // cppcheck 1.57 complains that const enum value is not passed
+                      // by reference. Note that, it accepts the non-const enum value
+                      // to be passed by value. In both cases it is unneccessary to
+                      // pass the enum by reference.
+                      // cppcheck-suppress passedByValue
+                      const Option4ClientFqdn::DomainNameType name_type)
+    : flags_(flags),
+      rcode1_(rcode),
+      rcode2_(rcode),
+      domain_name_(),
+      domain_name_type_(name_type) {
+
+    //  Check if flags are correct. Also, check that MBZ bits are not set. If
+    // they are, throw exception.
+    checkFlags(flags_, true);
+    // Set domain name. It may throw an exception if domain name has wrong
+    // format.
+    setDomainName(domain_name, name_type);
+}
+
+Option4ClientFqdnImpl::Option4ClientFqdnImpl(OptionBufferConstIter first,
+                                             OptionBufferConstIter last)
+    : rcode1_(Option4ClientFqdn::RCODE_CLIENT()),
+      rcode2_(Option4ClientFqdn::RCODE_CLIENT()) {
+    parseWireData(first, last);
+    // Verify that flags value was correct. This constructor is used to parse
+    // incoming packet, so don't check MBZ bits. They are ignored because we
+    // don't want to discard the whole option because MBZ bits are set.
+    checkFlags(flags_, false);
+}
+
+Option4ClientFqdnImpl::
+Option4ClientFqdnImpl(const Option4ClientFqdnImpl& source)
+    : flags_(source.flags_),
+      rcode1_(source.rcode1_),
+      rcode2_(source.rcode2_),
+      domain_name_(),
+      domain_name_type_(source.domain_name_type_) {
+    if (source.domain_name_) {
+        domain_name_.reset(new isc::dns::Name(*source.domain_name_));
+    }
+}
+
+Option4ClientFqdnImpl&
+// This assignment operator handles assignment to self, it copies all
+// required values.
+// cppcheck-suppress operatorEqToSelf
+Option4ClientFqdnImpl::operator=(const Option4ClientFqdnImpl& source) {
+    if (source.domain_name_) {
+        domain_name_.reset(new isc::dns::Name(*source.domain_name_));
+
+    } else {
+        domain_name_.reset();
+    }
+
+    // Assignment is exception safe.
+    flags_ = source.flags_;
+    rcode1_ = source.rcode1_;
+    rcode2_ = source.rcode2_;
+    domain_name_type_ = source.domain_name_type_;
+
+    return (*this);
+}
+
+void
+Option4ClientFqdnImpl::
+setDomainName(const std::string& domain_name,
+              // cppcheck 1.57 complains that const enum value is not passed
+              // by reference. Note that, it accepts the non-const enum
+              // to be passed by value. In both cases it is unneccessary to
+              // pass the enum by reference.
+              // cppcheck-suppress passedByValue
+              const Option4ClientFqdn::DomainNameType name_type) {
+    // domain-name must be trimmed. Otherwise, string comprising spaces only
+    // would be treated as a fully qualified name.
+    std::string name = isc::util::str::trim(domain_name);
+    if (name.empty()) {
+        if (name_type == Option4ClientFqdn::FULL) {
+            isc_throw(InvalidOption4FqdnDomainName,
+                      "fully qualified domain-name must not be empty"
+                      << " when setting new domain-name for DHCPv4 Client"
+                      << " FQDN Option");
+        }
+        // The special case when domain-name is empty is marked by setting the
+        // pointer to the domain-name object to NULL.
+        domain_name_.reset();
+
+    } else {
+        try {
+            domain_name_.reset(new isc::dns::Name(name));
+            domain_name_type_ = name_type;
+
+        } catch (const Exception& ex) {
+            isc_throw(InvalidOption4FqdnDomainName,
+                      "invalid domain-name value '"
+                      << domain_name << "' when setting new domain-name for"
+                      << " DHCPv4 Client FQDN Option");
+
+        }
+    }
+}
+
+void
+Option4ClientFqdnImpl::checkFlags(const uint8_t flags, const bool check_mbz) {
+    // The Must Be Zero (MBZ) bits must not be set.
+    if (check_mbz && ((flags & ~Option4ClientFqdn::FLAG_MASK) != 0)) {
+        isc_throw(InvalidOption4FqdnFlags,
+                  "invalid DHCPv4 Client FQDN Option flags: 0x"
+                  << std::hex << static_cast<int>(flags) << std::dec);
+    }
+
+    // According to RFC 4702, section 2.1. if the N bit is 1, the S bit
+    // MUST be 0. Checking it here.
+    if ((flags & (Option4ClientFqdn::FLAG_N | Option4ClientFqdn::FLAG_S))
+        == (Option4ClientFqdn::FLAG_N | Option4ClientFqdn::FLAG_S)) {
+        isc_throw(InvalidOption4FqdnFlags,
+                  "both N and S flag of the DHCPv4 Client FQDN Option are set."
+                  << " According to RFC 4702, if the N bit is 1 the S bit"
+                  << " MUST be 0");
+    }
+}
+
+void
+Option4ClientFqdnImpl::parseWireData(OptionBufferConstIter first,
+                                     OptionBufferConstIter last) {
+
+    // Buffer must comprise at least one byte with the flags.
+    // The domain-name may be empty.
+    if (std::distance(first, last) < Option4ClientFqdn::FIXED_FIELDS_LEN) {
+        isc_throw(OutOfRange, "DHCPv4 Client FQDN Option ("
+                  << DHO_FQDN << ") is truncated");
+    }
+
+    // Parse flags
+    flags_ = *(first++);
+
+    // Parse RCODE1 and RCODE2.
+    rcode1_ = Option4ClientFqdn::Rcode(*(first++));
+    rcode2_ = Option4ClientFqdn::Rcode(*(first++));
+
+    try {
+        if ((flags_ & Option4ClientFqdn::FLAG_E) != 0) {
+            parseCanonicalDomainName(first, last);
+
+        } else {
+            parseASCIIDomainName(first, last);
+
+        }
+    } catch (const Exception& ex) {
+        isc_throw(InvalidOption4FqdnDomainName,
+                  "failed to parse the domain-name in DHCPv4 Client FQDN"
+                  << " Option: " << ex.what());
+    }
+}
+
+void
+Option4ClientFqdnImpl::parseCanonicalDomainName(OptionBufferConstIter first,
+                                                OptionBufferConstIter last) {
+    // Parse domain-name if any.
+    if (std::distance(first, last) > 0) {
+        // The FQDN may comprise a partial domain-name. In this case it lacks
+        // terminating 0. If this is the case, we will need to add zero at
+        // the end because Name object constructor requires it.
+        if (*(last - 1) != 0) {
+            // Create temporary buffer and add terminating zero.
+            OptionBuffer buf(first, last);
+            buf.push_back(0);
+            // Reset domain name.
+            isc::util::InputBuffer name_buf(&buf[0], buf.size());
+            domain_name_.reset(new isc::dns::Name(name_buf));
+            // Terminating zero was missing, so set the domain-name type
+            // to partial.
+            domain_name_type_ = Option4ClientFqdn::PARTIAL;
+        } else {
+            // We are dealing with fully qualified domain name so there is
+            // no need to add terminating zero. Simply pass the buffer to
+            // Name object constructor.
+            isc::util::InputBuffer name_buf(&(*first),
+                                            std::distance(first, last));
+            domain_name_.reset(new isc::dns::Name(name_buf));
+            // Set the domain-type to fully qualified domain name.
+            domain_name_type_ = Option4ClientFqdn::FULL;
+        }
+    }
+}
+
+void
+Option4ClientFqdnImpl::parseASCIIDomainName(OptionBufferConstIter first,
+                                            OptionBufferConstIter last) {
+    if (std::distance(first, last) > 0) {
+        std::string domain_name(first, last);
+        domain_name_.reset(new isc::dns::Name(domain_name));
+        domain_name_type_ = domain_name[domain_name.length() - 1] == '.' ?
+            Option4ClientFqdn::FULL : Option4ClientFqdn::PARTIAL;
+    }
+}
+
+Option4ClientFqdn::Option4ClientFqdn(const uint8_t flag, const Rcode& rcode)
+    : Option(Option::V4, DHO_FQDN),
+      impl_(new Option4ClientFqdnImpl(flag, rcode, "", PARTIAL)) {
+}
+
+Option4ClientFqdn::Option4ClientFqdn(const uint8_t flag,
+                                     const Rcode& rcode,
+                                     const std::string& domain_name,
+                                     const DomainNameType domain_name_type)
+    : Option(Option::V4, DHO_FQDN),
+      impl_(new Option4ClientFqdnImpl(flag, rcode, domain_name,
+                                      domain_name_type)) {
+}
+
+Option4ClientFqdn::Option4ClientFqdn(OptionBufferConstIter first,
+                                     OptionBufferConstIter last)
+    : Option(Option::V4, DHO_FQDN, first, last),
+      impl_(new Option4ClientFqdnImpl(first, last)) {
+}
+
+Option4ClientFqdn::~Option4ClientFqdn() {
+    delete(impl_);
+}
+
+Option4ClientFqdn::Option4ClientFqdn(const Option4ClientFqdn& source)
+    : Option(source),
+      impl_(new Option4ClientFqdnImpl(*source.impl_)) {
+}
+
+Option4ClientFqdn&
+// This assignment operator handles assignment to self, it uses copy
+// constructor of Option4ClientFqdnImpl to copy all required values.
+// cppcheck-suppress operatorEqToSelf
+Option4ClientFqdn::operator=(const Option4ClientFqdn& source) {
+    Option4ClientFqdnImpl* old_impl = impl_;
+    impl_ = new Option4ClientFqdnImpl(*source.impl_);
+    delete(old_impl);
+    return (*this);
+}
+
+bool
+Option4ClientFqdn::getFlag(const uint8_t flag) const {
+    // Caller should query for one of the: E, N, S or O flags. Any other value
+    /// is invalid and results in the exception.
+    if (flag != FLAG_S && flag != FLAG_O && flag != FLAG_N && flag != FLAG_E) {
+        isc_throw(InvalidOption4FqdnFlags, "invalid DHCPv4 Client FQDN"
+                  << " Option flag specified, expected E, N, S or O");
+    }
+
+    return ((impl_->flags_ & flag) != 0);
+}
+
+void
+Option4ClientFqdn::setFlag(const uint8_t flag, const bool set_flag) {
+    // Check that flag is in range between 0x1 and 0x7. Although it is
+    // discouraged this check doesn't preclude the caller from setting
+    // multiple flags concurrently.
+    if (((flag & ~FLAG_MASK) != 0) || (flag == 0)) {
+        isc_throw(InvalidOption4FqdnFlags, "invalid DHCPv4 Client FQDN"
+                  << " Option flag " << std::hex
+                  << static_cast<int>(flag) << std::dec
+                  << "is being set. Expected combination of E, N, S and O");
+    }
+
+    // Copy the current flags into local variable. That way we will be able
+    // to test new flags settings before applying them.
+    uint8_t new_flag = impl_->flags_;
+    if (set_flag) {
+        new_flag |= flag;
+    } else {
+        new_flag &= ~flag;
+    }
+
+    // Check new flags. If they are valid, apply them. Also, check that MBZ
+    // bits are not set.
+    Option4ClientFqdnImpl::checkFlags(new_flag, true);
+    impl_->flags_ = new_flag;
+}
+
+void
+Option4ClientFqdn::setRcode(const Rcode& rcode) {
+    impl_->rcode1_ = rcode;
+    impl_->rcode2_ = rcode;
+}
+
+void
+Option4ClientFqdn::resetFlags() {
+    impl_->flags_ = 0;
+}
+
+std::string
+Option4ClientFqdn::getDomainName() const {
+    if (impl_->domain_name_) {
+        return (impl_->domain_name_->toText(impl_->domain_name_type_ ==
+                                            PARTIAL));
+    }
+    // If an object holding domain-name is NULL it means that the domain-name
+    // is empty.
+    return ("");
+}
+
+void
+Option4ClientFqdn::packDomainName(isc::util::OutputBuffer& buf) const {
+    // If domain-name is empty, do nothing.
+    if (!impl_->domain_name_) {
+        return;
+    }
+
+    if (getFlag(FLAG_E)) {
+        // Domain name, encoded as a set of labels.
+        isc::dns::LabelSequence labels(*impl_->domain_name_);
+        if (labels.getDataLength() > 0) {
+            size_t read_len = 0;
+            const uint8_t* data = labels.getData(&read_len);
+            if (impl_->domain_name_type_ == PARTIAL) {
+                --read_len;
+            }
+            buf.writeData(data, read_len);
+        }
+
+    } else {
+        std::string domain_name = impl_->domain_name_->toText();
+        buf.writeData(&domain_name[0], domain_name.size());
+
+    }
+}
+
+void
+Option4ClientFqdn::setDomainName(const std::string& domain_name,
+                                 const DomainNameType domain_name_type) {
+    impl_->setDomainName(domain_name, domain_name_type);
+}
+
+void
+Option4ClientFqdn::resetDomainName() {
+    setDomainName("", PARTIAL);
+}
+
+Option4ClientFqdn::DomainNameType
+Option4ClientFqdn::getDomainNameType() const {
+    return (impl_->domain_name_type_);
+}
+
+void
+Option4ClientFqdn::pack(isc::util::OutputBuffer& buf) {
+    // Header = option code and length.
+    packHeader(buf);
+    // Flags field.
+    buf.writeUint8(impl_->flags_);
+    // RCODE1 and RCODE2
+    buf.writeUint8(impl_->rcode1_.getCode());
+    buf.writeUint8(impl_->rcode2_.getCode());
+    // Domain name.
+    packDomainName(buf);
+}
+
+void
+Option4ClientFqdn::unpack(OptionBufferConstIter first,
+                          OptionBufferConstIter last) {
+    setData(first, last);
+    impl_->parseWireData(first, last);
+    // Check that the flags in the received option are valid. Ignore MBZ bits,
+    // because we don't want to discard the whole option because of MBZ bits
+    // being set.
+    impl_->checkFlags(impl_->flags_, false);
+}
+
+std::string
+Option4ClientFqdn::toText(int indent) {
+    std::ostringstream stream;
+    std::string in(indent, ' '); // base indentation
+    stream << in  << "type=" << type_ << " (CLIENT_FQDN), "
+           <<  "flags: ("
+           << "N=" << (getFlag(FLAG_N) ? "1" : "0") << ", "
+           << "E=" << (getFlag(FLAG_E) ? "1" : "0") << ", "
+           << "O=" << (getFlag(FLAG_O) ? "1" : "0") << ", "
+           << "S=" << (getFlag(FLAG_S) ? "1" : "0") << "), "
+           << "domain-name='" << getDomainName() << "' ("
+           << (getDomainNameType() == PARTIAL ? "partial" : "full")
+           << ")";
+
+    return (stream.str());
+}
+
+uint16_t
+Option4ClientFqdn::len() {
+    uint16_t domain_name_length = 0;
+    // If domain name is partial, the NULL terminating character
+    // is not included and the option length have to be adjusted.
+    if (impl_->domain_name_) {
+        domain_name_length = impl_->domain_name_type_ == FULL ?
+            impl_->domain_name_->getLength() : impl_->domain_name_->getLength() - 1;
+    }
+
+    return (getHeaderLen() + FIXED_FIELDS_LEN + domain_name_length);
+}
+
+} // end of isc::dhcp namespace
+} // end of isc namespace

src/lib/dhcp/option4_client_fqdn.h

+// Copyright (C) 2013 Internet Systems Consortium, Inc. ("ISC")
+//
+// Permission to use, copy, modify, and/or distribute this software for any
+// purpose with or without fee is hereby granted, provided that the above
+// copyright notice and this permission notice appear in all copies.
+//
+// THE SOFTWARE IS PROVIDED "AS IS" AND ISC DISCLAIMS ALL WARRANTIES WITH
+// REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF MERCHANTABILITY
+// AND FITNESS.  IN NO EVENT SHALL ISC BE LIABLE FOR ANY SPECIAL, DIRECT,
+// INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM
+// LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE
+// OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR
+// PERFORMANCE OF THIS SOFTWARE.
+
+#ifndef OPTION4_CLIENT_FQDN_H
+#define OPTION4_CLIENT_FQDN_H
+
+#include <dhcp/option.h>
+#include <dns/name.h>
+
+#include <string>
+
+namespace isc {
+namespace dhcp {
+
+/// @brief Exception thrown when invalid flags have been specified for
+/// DHCPv4 Client FQDN %Option.
+class InvalidOption4FqdnFlags : public Exception {
+public:
+    InvalidOption4FqdnFlags(const char* file, size_t line, const char* what) :
+        isc::Exception(file, line, what) {}
+};
+
+/// @brief Exception thrown when invalid domain name is specified.
+class InvalidOption4FqdnDomainName : public Exception {
+public:
+    InvalidOption4FqdnDomainName(const char* file, size_t line,
+                                 const char* what) :
+        isc::Exception(file, line, what) {}
+};
+
+/// Forward declaration to implementation of @c Option4ClientFqdn class.
+class Option4ClientFqdnImpl;
+
+/// @brief Represents DHCPv4 Client FQDN %Option (code 81).
+///
+/// This option has been defined in the RFC 4702 and it has a following
+/// structure:
+/// - Code (1 octet) - option code (always equal to 81).
+/// - Len (1 octet) - a length of the option.
+/// - Flags (1 octet) - a field carrying "NEOS" flags described below.
+/// - RCODE1 (1 octet) - deprecated field which should be set to 0 by the client
+/// and set to 255 by the server.
+/// - RCODE2 (1 octet) - deprecated, should be used in the same way as RCODE1.
+/// - Domain Name - variable length field comprising partial or fully qualified
+/// domain name.
+///
+/// The flags field has the following structure:
+/// @code
+///        0 1 2 3 4 5 6 7
+///       +-+-+-+-+-+-+-+-+
+///       |  MBZ  |N|E|O|S|
+///       +-+-+-+-+-+-+-+-+
+/// @endcode
+/// where:
+/// - N flag specifies whether server should (0) or should not (1) perform DNS
+///  Update,
+/// - E flag specifies encoding of the Domain Name field. If this flag is set
+/// to 1 it indicates canonical wire format without compression. 0 indicates
+/// the deprecated ASCII format.
+/// - O flag is set by the server to indicate that it has overridden client's
+/// preference set with the S bit.
+/// - S flag specifies whether server should (1) or should not (0) perform
+/// forward (FQDN-to-address) updates.
+///
+/// This class exposes a set of functions to modify flags and check their
+/// correctness.
+///