1 /*

     2  * Copyright (c) 2019, Oracle and/or its affiliates. All rights reserved.

     3  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.

     4  *

     5  * This code is free software; you can redistribute it and/or modify it

     6  * under the terms of the GNU General Public License version 2 only, as

     7  * published by the Free Software Foundation.  Oracle designates this

     8  * particular file as subject to the "Classpath" exception as provided

     9  * by Oracle in the LICENSE file that accompanied this code.

    10  *

    11  * This code is distributed in the hope that it will be useful, but WITHOUT

    12  * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or

    13  * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License

    14  * version 2 for more details (a copy is included in the LICENSE file that

    15  * accompanied this code).

    16  *

    17  * You should have received a copy of the GNU General Public License version

    18  * 2 along with this work; if not, write to the Free Software Foundation,

    19  * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.

    20  *

    21  * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA

    22  * or visit www.oracle.com if you need additional information or have any

    23  * questions.

    24  */

    25 

    26 package jdk.dns.client.internal;

    27 

    28 import jdk.dns.client.ex.DnsResolverException;

    29 

    30 class Header {

    31 

    32     static final int HEADER_SIZE = 12;  // octets in a DNS header

    33 

    34     // Masks and shift amounts for DNS header flag fields.

    35     static final short QR_BIT = (short) 0x8000;

    36     static final short OPCODE_MASK = (short) 0x7800;

    37     static final int OPCODE_SHIFT = 11;

    38     static final short AA_BIT = (short) 0x0400;

    39     static final short TC_BIT = (short) 0x0200;

    40     static final short RD_BIT = (short) 0x0100;

    41     static final short RA_BIT = (short) 0x0080;

    42     static final short RCODE_MASK = (short) 0x000F;

    43 

    44     int xid;                    // ID:  16-bit query identifier

    45     boolean query;              // QR:  true if query, false if response

    46     int opcode;                 // OPCODE:  4-bit opcode

    47     boolean authoritative;      // AA

    48     boolean truncated;          // TC

    49     boolean recursionDesired;   // RD

    50     boolean recursionAvail;     // RA

    51     int rcode;                  // RCODE:  4-bit response code

    52     int numQuestions;

    53     int numAnswers;

    54     int numAuthorities;

    55     int numAdditionals;

    56 

    57     /*

    58      * Returns a representation of a decoded DNS message header.

    59      * Does not modify or store a reference to the msg array.

    60      */

    61     Header(byte[] msg, int msgLen) throws DnsResolverException {

    62         decode(msg, msgLen);

    63     }

    64 

    65     /*

    66      * Decodes a DNS message header.  Does not modify or store a

    67      * reference to the msg array.

    68      */

    69     private void decode(byte[] msg, int msgLen) throws DnsResolverException {

    70 

    71         try {

    72             int pos = 0;        // current offset into msg

    73 

    74             if (msgLen < HEADER_SIZE) {

    75                 throw new DnsResolverException(

    76                         "DNS error: corrupted message header");

    77             }

    78 

    79             xid = getShort(msg, pos);

    80             pos += 2;

    81 

    82             // Flags

    83             short flags = (short) getShort(msg, pos);

    84             pos += 2;

    85             query = (flags & QR_BIT) == 0;

    86             opcode = (flags & OPCODE_MASK) >>> OPCODE_SHIFT;

    87             authoritative = (flags & AA_BIT) != 0;

    88             truncated = (flags & TC_BIT) != 0;

    89             recursionDesired = (flags & RD_BIT) != 0;

    90             recursionAvail = (flags & RA_BIT) != 0;

    91             rcode = (flags & RCODE_MASK);

    92 

    93             // RR counts

    94             numQuestions = getShort(msg, pos);

    95             pos += 2;

    96             numAnswers = getShort(msg, pos);

    97             pos += 2;

    98             numAuthorities = getShort(msg, pos);

    99             pos += 2;

   100             numAdditionals = getShort(msg, pos);

   101             pos += 2;

   102 

   103         } catch (IndexOutOfBoundsException e) {

   104             throw new DnsResolverException("DNS error: corrupted message header");

   105         }

   106     }

   107 

   108     /*

   109      * Returns the 2-byte unsigned value at msg[pos].  The high

   110      * order byte comes first.

   111      */

   112     private static int getShort(byte[] msg, int pos) {

   113         return (((msg[pos] & 0xFF) << 8) |

   114                 (msg[pos + 1] & 0xFF));

   115     }

   116 }