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package com.sun.jndi.dns;


import javax.naming.*;


The Header class represents the header of a DNS message.
Author:Scott Seligman
/** * The Header class represents the header of a DNS message. * * @author Scott Seligman */
class Header { static final int HEADER_SIZE = 12; // octets in a DNS header // Masks and shift amounts for DNS header flag fields. static final short QR_BIT = (short) 0x8000; static final short OPCODE_MASK = (short) 0x7800; static final int OPCODE_SHIFT = 11; static final short AA_BIT = (short) 0x0400; static final short TC_BIT = (short) 0x0200; static final short RD_BIT = (short) 0x0100; static final short RA_BIT = (short) 0x0080; static final short RCODE_MASK = (short) 0x000F; int xid; // ID: 16-bit query identifier boolean query; // QR: true if query, false if response int opcode; // OPCODE: 4-bit opcode boolean authoritative; // AA boolean truncated; // TC boolean recursionDesired; // RD boolean recursionAvail; // RA int rcode; // RCODE: 4-bit response code int numQuestions; int numAnswers; int numAuthorities; int numAdditionals; /* * Returns a representation of a decoded DNS message header. * Does not modify or store a reference to the msg array. */ Header(byte[] msg, int msgLen) throws NamingException { decode(msg, msgLen); } /* * Decodes a DNS message header. Does not modify or store a * reference to the msg array. */ private void decode(byte[] msg, int msgLen) throws NamingException { try { int pos = 0; // current offset into msg if (msgLen < HEADER_SIZE) { throw new CommunicationException( "DNS error: corrupted message header"); } xid = getShort(msg, pos); pos += 2; // Flags short flags = (short) getShort(msg, pos); pos += 2; query = (flags & QR_BIT) == 0; opcode = (flags & OPCODE_MASK) >>> OPCODE_SHIFT; authoritative = (flags & AA_BIT) != 0; truncated = (flags & TC_BIT) != 0; recursionDesired = (flags & RD_BIT) != 0; recursionAvail = (flags & RA_BIT) != 0; rcode = (flags & RCODE_MASK); // RR counts numQuestions = getShort(msg, pos); pos += 2; numAnswers = getShort(msg, pos); pos += 2; numAuthorities = getShort(msg, pos); pos += 2; numAdditionals = getShort(msg, pos); pos += 2; } catch (IndexOutOfBoundsException e) { throw new CommunicationException( "DNS error: corrupted message header"); } } /* * Returns the 2-byte unsigned value at msg[pos]. The high * order byte comes first. */ private static int getShort(byte[] msg, int pos) { return (((msg[pos] & 0xFF) << 8) | (msg[pos + 1] & 0xFF)); } }