網絡協議之mDNS20170217

DNS（Domain Name System，域名系統）因特網上作為域名和IP地址相互映射的一個分布式數據庫，能夠使用戶更方便的訪問互聯網，而不用去記住能夠被機器直接讀取的IP數串。通過主機名，最終得到該主機名對應的IP地址的過程叫做域名解析（或主機名解析）。DNS協議運行在UDP協議之上，使用端口號53。在RFC文檔中RFC 2181對DNS有規范說明，RFC 2136對DNS的動態更新進行說明，RFC 2308對DNS查詢的反向緩存進行說明。

一、mDNS    具體協議規范地址如下 ： http://www.ietf.org/rfc/rfc6762.txt

mdns 即多播dns（Multicast DNS），mDNS主要實現了在沒有傳統DNS服務器的情況下使局域網內的主機實現相互發現和通信，使用的端口為5353，遵從dns協議，使用現有的DNS信息結構、名語法和資源記錄類型。並且沒有指定新的操作代碼或響應代碼。

 

在局域網中，設備和設備之前相互通信需要知道對方的ip地址的，大多數情況，設備的ip不是靜態ip地址，而是通過dhcp 協議動態分配的ip 地址，如何設備發現呢，就是要mdns大顯身手，例如：現在物聯網設備和app之間的通信，要么app通過廣播，要么通過組播，發一些特定信息，感興趣設備應答，實現局域網設備的發現，當然mdns 比這強大的多

1.mDNS 基於 UDP 協議。

組播地址: 組播地址使用的是D類地址，地址范圍為：224.0.0.0—239.255.255.255

 

2.mdns 工作原理簡單描述：

 

mdns 使用組播地址為： 224.0.0.251 （ipv6： FF02::FB） 端口為5353，mdns 是用於局域網內部的，並且主機的域名為.local 結尾,每個進入局域網的主機，如果開啟了mDNS服務的話，都會向局域網內的所有主機組播一個消息，我是誰（域名），和我的IP地址是多少。然后其他有mdns服務的主機就會響應，也會告訴你，它是誰（域名），它的IP地址是多少。 當然設備需要服務時，就是使用mdns 查詢域名對對應的ip地址，對應的設備收到該報文后同樣通過組播方式應答，此時其他主機設備也是可以收到該應答報文，其他主機也會記錄域名和ip 以及ttl 等，更新緩存

 

比如，A主機進入局域網，開啟了 mDNS 服務，並向 mDNS 服務注冊以下信息：我提供 FTP 服務，我的IP是 192.168.1.101，端口是 21。當B主機進入局域網，並向 B 主機的 mDNS 服務請求，我要找局域網內 FTP 服務器，B主機的 mDNS 就會去局域網內向其他的 mDNS 詢問，並且最終告訴你，有一個IP地址為 192.168.1.101，端口號是 21 的主機，也就是 A 主機提供 FTP 服務，所以 B 主機就知道了 A 主機的 IP 地址和端口號了。

 

大概的原理就是這樣子，mDNS提供的服務要遠遠多於這個，當然服務多但並不復雜。

3.mDNSResponder與Bonjour的關系:

The mDNSResponder project is a component of Bonjour,

Apple's ease-of-use IP networking initiative:

<http://developer.apple.com/bonjour/>

Bonjour是法語中的Hello之意。它是Apple公司為基於組播域名服務(multicast DNS)的開放性零配置網絡標准所起的名字。使用Bonjour的設備在網絡中自動組播它們自己的服務信息並監聽其它設備的服務信息。設備之間就像在打招呼，這也是該技術命名為Bonjour的原因。Bonjour使得局域網中的系統和服務即使在沒有網絡管理員的情況下也很容易被找到。

舉一個簡單的例子：在局域網中，如果要進行打印服務，必須先知道打印服務器的IP地址。此IP地址一般由IT部門的人負責分配，然后他還得全員發郵件以公示此地址。有了Bonjour以后，打印服務器自己會依據零配置網絡標准在局域網內部找到一個可用的IP並注冊一個打印服務，名為“print service”之類的。當客戶端需要打印服務時，會先搜索網絡內部的打印服務器。由於不知道打印服務器的IP地址，客戶端只能根據諸如"print service"的名字去查找打印機。在Bonjour的幫助下，客戶端最終能找到這台注冊了“print service”名字的打印機，並獲得它的IP地址以及端口號。

從Bonjour角度來看，該技術主要解決了三個問題：

• Addressing：即為主機分配IP。Bonjour的Addressing處理比較簡單，即每個主機在網絡內部的地址可選范圍內找一個IP，然后查看網絡內部是否有其他主機再用。如果該IP沒有被分配的話，它將使用此IP。
• Naming：Naming解決的是host名和IP地址的對應關系。Bonjour采用的是Multiple DNS技術，即DNS查詢消息將通過UDP組播方式發送。一旦網絡內部某個機器發現查詢的機器名和自己設置的一樣，就回復這條請求。此外，Bonjour還拓展了MDNS的用途，即除了能查找host外，還支持對service的查找。不過，Bonjour的Naming有一個限制，即網絡內部不能有重名的host或service。
• Service Discovery：SD基於上面的Naming工作，它使得應用程序能查找到網絡內部的服務，並解析該服務對應的IP地址和端口號。應用程序一旦得到服務的IP地址和端口號，就可以直接和該服務建立交互關系。

Bonjour技術在Mac OS以及Itunes、Iphone上都得到了廣泛應用。為了進一步推廣，Apple通過開源工程mdnsresponder將其開源出來。在Windows平台上，它將生成一個后台程序mdnsresponder。在Android平台上（或者說支持POSIX的Linux平台）它是一個名為mdnsd的程序。不過，不論是mdnsresponder還是mdnsd，應用開發者要做的僅僅是利用Bonjour的API向它們發起服務注冊、服務查詢和服務解析等請求並接收來自它們的處理結果。

下面我們將介紹Bonjour API中使用最多的三個函數，它們分別是服務注冊、服務查詢和服務解析。理解這三個函數的功能也是理解MDnsSdListener的基礎。

使用Bonjour API必須包含如下的頭文件和動態庫，並連接到：

#include <dns_sd.h>  //必須包含此頭文件

libmdnssd.so  //鏈接到此so

Bonjour中，服務注冊的API為DNSServiceRegister，原型如圖1所示：

　　　　　　　　　　　　　　　　　　　　

圖1  DNSServiceRegister原型

該函數的解釋如下：

• sdRef：代表一個未初始化的DNSService實體。其類型DNSServiceRef是指針。該參數最終由DNSServiceRegister函數分配內存並初始化。
• flags：表示當網絡內部有重名服務時的沖突處理。默認是按順序修改服務名。例如要注冊的服務名為“printer”，當檢測到重名沖突時，就可改名為“printer(1)”。
• interfaceIndex：表示該服務輸出到主機的哪些網絡接口上。值-1表示僅對本機支持，也就是該服務的用在loop接口上。
• name：表示服務名，為空的話就取機器名。
• regtype：服務類型，用字符串表達。Bonjour要求格式為"_服務名._傳輸協議"，例如"_ftp._tcp"。目前傳輸協議僅支持TCP和UDP。
• domian和host一般都為空。
• port表示該服務的端口。如果為0的話，Bonjour會自動分配一個。
• txtLen以及txtRecord字符串用來描述該服務。一般都設置為空。
• callBack：設置回調函數。該服注冊的請求結果都會通過它回調給客戶端。
• context：上下文指針，由應用程序設置。

當客戶端需要搜索網絡內部特定服務時，需要使用DNSServiceBrowser API，其原型如圖2所示：

　　　　　　　　　　　　　　　　　　　　　

圖2  DNSServiceBrowser原型

其中：

• sdref、interfaceIndex、regtype、domain以及context含義與DNSServiceRegister一樣。
• flags：在本函數中沒有作用。
• callBack：為DNSServiceBrowser處理結果的回調通知接口。

當客戶端想獲得指定服務的IP和端口號時，需要使用DNSServiceResolve API，其原型如圖3所示：

 　　　　　　　　　　　　　　　　　　　　　

圖3  DNSServiceResolve原型

其中：

• name、regtype和domain都從DNSServiceBrowse函數的處理結果中獲得。
• callBack用於通知DNSServiceResolve的處理結果。該回調函數將返回服務的IP地址和端口號。

如果需要了解Bonjour安卓中的使用方法及原理，請閱讀該部分的原文: http://blog.csdn.net/innost/article/details/8629139

 

4.Linux中的使用方法：

附件提供了mDNS的源碼，分析源碼我們就可以知道如何編譯安裝以及如何使用:

在mDNSResponder-107.5\mDNSPosix目錄中的Responder.c文件中的main我們可以看到

調用了PrintUsage函數中提供了用法說明：

根據上面的描述，使用shell命令調用的例子:

//mDNSResponderPosix來源於bonjour,服務注冊

sprintf(buf, "mDNSResponderPosix -n %s -t _ipc_http._tcp. " "-d local. -p 5959 &", gpCC->ccArg.pDevId);//-n 服務名,-t 服務類型,-d域名,-p端口號，

    system(buf);//后台運行

再看main函數中執行過程:

初始化后調用了RegisterOurServices函數把要提供的服務注冊進去（追蹤下去就可以看到最終調用了mdnscore.c中的mDNS_RegisterService函數）：

最后如果想詳細了解可以閱讀如下文件，或者網上搜索下載mDNS的源碼閱讀

/* -*- Mode: C; tab-width: 4 -*-
 *
 * Copyright (c) 2002-2004 Apple Computer, Inc. All rights reserved.
 *
 * @APPLE_LICENSE_HEADER_START@
 * 
 * This file contains Original Code and/or Modifications of Original Code
 * as defined in and that are subject to the Apple Public Source License
 * Version 2.0 (the 'License'). You may not use this file except in
 * compliance with the License. Please obtain a copy of the License at
 * http://www.opensource.apple.com/apsl/ and read it before using this
 * file.
 * 
 * The Original Code and all software distributed under the License are
 * distributed on an 'AS IS' basis, WITHOUT WARRANTY OF ANY KIND, EITHER
 * EXPRESS OR IMPLIED, AND APPLE HEREBY DISCLAIMS ALL SUCH WARRANTIES,
 * INCLUDING WITHOUT LIMITATION, ANY WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE, QUIET ENJOYMENT OR NON-INFRINGEMENT.
 * Please see the License for the specific language governing rights and
 * limitations under the License.
 * 
 * @APPLE_LICENSE_HEADER_END@

    Change History (most recent first):

$Log: Responder.c,v $
Revision 1.30  2005/10/26 22:21:16  cheshire
<rdar://problem/4149841> Potential buffer overflow in mDNSResponderPosix

Revision 1.29  2005/03/04 21:35:33  cheshire
<rdar://problem/4037201> Services.txt file not parsed properly when it contains more than one service

Revision 1.28  2005/01/11 01:55:26  ksekar
Fix compile errors in Posix debug build

Revision 1.27  2004/12/01 04:28:43  cheshire
<rdar://problem/3872803> Darwin patches for Solaris and Suse
Use version of daemon() provided in mDNSUNP.c instead of local copy

Revision 1.26  2004/11/30 22:37:01  cheshire
Update copyright dates and add "Mode: C; tab-width: 4" headers

Revision 1.25  2004/11/11 02:00:51  cheshire
Minor fixes to getopt, error message

Revision 1.24  2004/11/09 19:32:10  rpantos
Suggestion from Ademar de Souza Reis Jr. to allow comments in services file

Revision 1.23  2004/09/17 01:08:54  cheshire
Renamed mDNSClientAPI.h to mDNSEmbeddedAPI.h
  The name "mDNSClientAPI.h" is misleading to new developers looking at this code. The interfaces
  declared in that file are ONLY appropriate to single-address-space embedded applications.
  For clients on general-purpose computers, the interfaces defined in dns_sd.h should be used.

Revision 1.22  2004/09/16 01:58:22  cheshire
Fix compiler warnings

Revision 1.21  2004/06/15 03:48:07  cheshire
Update mDNSResponderPosix to take multiple name=val arguments in a sane way

Revision 1.20  2004/05/18 23:51:26  cheshire
Tidy up all checkin comments to use consistent "<rdar://problem/xxxxxxx>" format for bug numbers

Revision 1.19  2004/03/12 08:03:14  cheshire
Update comments

Revision 1.18  2004/01/25 00:00:55  cheshire
Change to use mDNSOpaque16fromIntVal() instead of shifting and masking

Revision 1.17  2003/12/11 19:11:55  cheshire
Fix compiler warning

Revision 1.16  2003/08/14 02:19:55  cheshire
<rdar://problem/3375491> Split generic ResourceRecord type into two separate types: AuthRecord and CacheRecord

Revision 1.15  2003/08/12 19:56:26  cheshire
Update to APSL 2.0

Revision 1.14  2003/08/06 18:20:51  cheshire
Makefile cleanup

Revision 1.13  2003/07/23 00:00:04  cheshire
Add comments

Revision 1.12  2003/07/15 01:55:16  cheshire
<rdar://problem/3315777> Need to implement service registration with subtypes

Revision 1.11  2003/07/14 18:11:54  cheshire
Fix stricter compiler warnings

Revision 1.10  2003/07/10 20:27:31  cheshire
<rdar://problem/3318717> mDNSResponder Posix version is missing a 'b' in the getopt option string

Revision 1.9  2003/07/02 21:19:59  cheshire
<rdar://problem/3313413> Update copyright notices, etc., in source code comments

Revision 1.8  2003/06/18 05:48:41  cheshire
Fix warnings

Revision 1.7  2003/05/06 00:00:50  cheshire
<rdar://problem/3248914> Rationalize naming of domainname manipulation functions

Revision 1.6  2003/03/08 00:35:56  cheshire
Switched to using new "mDNS_Execute" model (see "mDNSCore/Implementer Notes.txt")

Revision 1.5  2003/02/20 06:48:36  cheshire
<rdar://problem/3169535> Xserve RAID needs to do interface-specific registrations
Reviewed by: Josh Graessley, Bob Bradley

Revision 1.4  2003/01/28 03:07:46  cheshire
Add extra parameter to mDNS_RenameAndReregisterService(),
and add support for specifying a domain other than dot-local.

Revision 1.3  2002/09/21 20:44:53  zarzycki
Added APSL info

Revision 1.2  2002/09/19 04:20:44  cheshire
Remove high-ascii characters that confuse some systems

Revision 1.1  2002/09/17 06:24:35  cheshire
First checkin

*/

#include "mDNSEmbeddedAPI.h"// Defines the interface to the client layer above
#include "mDNSPosix.h"    // Defines the specific types needed to run mDNS on this platform

#include <assert.h>
#include <stdio.h>            // For printf()
#include <stdlib.h>            // For exit() etc.
#include <string.h>            // For strlen() etc.
#include <unistd.h>            // For select()
#include <errno.h>            // For errno, EINTR
#include <signal.h>
#include <fcntl.h>

#if COMPILER_LIKES_PRAGMA_MARK
#pragma mark ***** Globals
#endif

static mDNS mDNSStorage;       // mDNS core uses this to store its globals
static mDNS_PlatformSupport PlatformStorage;  // Stores this platform's globals

static const char *gProgramName = "mDNSResponderPosix";

#if COMPILER_LIKES_PRAGMA_MARK
#pragma mark ***** Signals
#endif

static volatile mDNSBool gReceivedSigUsr1;
static volatile mDNSBool gReceivedSigHup;
static volatile mDNSBool gStopNow;

// We support 4 signals.
//
// o SIGUSR1 toggles verbose mode on and off in debug builds
// o SIGHUP  triggers the program to re-read its preferences.
// o SIGINT  causes an orderly shutdown of the program.
// o SIGQUIT causes a somewhat orderly shutdown (direct but dangerous)
// o SIGKILL kills us dead (easy to implement :-)
//
// There are fatal race conditions in our signal handling, but there's not much 
// we can do about them while remaining within the Posix space.  Specifically, 
// if a signal arrives after we test the globals its sets but before we call 
// select, the signal will be dropped.  The user will have to send the signal 
// again.  Unfortunately, Posix does not have a "sigselect" to atomically 
// modify the signal mask and start a select.

static void HandleSigUsr1(int sigraised)
    // If we get a SIGUSR1 we toggle the state of the 
    // verbose mode.
{
    assert(sigraised == SIGUSR1);
    gReceivedSigUsr1 = mDNStrue;
}

static void HandleSigHup(int sigraised)
    // A handler for SIGHUP that causes us to break out of the 
    // main event loop when the user kill 1's us.  This has the 
    // effect of triggered the main loop to deregister the 
    // current services and re-read the preferences.
{
    assert(sigraised == SIGHUP);
    gReceivedSigHup = mDNStrue;
}

static void HandleSigInt(int sigraised)
    // A handler for SIGINT that causes us to break out of the 
    // main event loop when the user types ^C.  This has the 
    // effect of quitting the program.
{
    assert(sigraised == SIGINT);
    
    if (gMDNSPlatformPosixVerboseLevel > 0) {
        fprintf(stderr, "\nSIGINT\n");
    }
    gStopNow = mDNStrue;
}

static void HandleSigQuit(int sigraised)
    // If we get a SIGQUIT the user is desperate and we 
    // just call mDNS_Close directly.  This is definitely 
    // not safe (because it could reenter mDNS), but 
    // we presume that the user has already tried the safe 
    // alternatives.
{
    assert(sigraised == SIGQUIT);

    if (gMDNSPlatformPosixVerboseLevel > 0) {
        fprintf(stderr, "\nSIGQUIT\n");
    }
    mDNS_Close(&mDNSStorage);
    exit(0);
}

#if COMPILER_LIKES_PRAGMA_MARK
#pragma mark ***** Parameter Checking
#endif

static mDNSBool CheckThatRichTextNameIsUsable(const char *richTextName, mDNSBool printExplanation)
    // Checks that richTextName is reasonable 
    // label and, if it isn't and printExplanation is true, prints 
    // an explanation of why not.
{
    mDNSBool result = mDNStrue;
    if (result && strlen(richTextName) > 63) {
        if (printExplanation) {
            fprintf(stderr, 
                    "%s: Service name is too long (must be 63 characters or less)\n", 
                    gProgramName);
        }
        result = mDNSfalse;
    }
    if (result && richTextName[0] == 0) {
        if (printExplanation) {
            fprintf(stderr, "%s: Service name can't be empty\n", gProgramName);
        }
        result = mDNSfalse;
    }
    return result;
}

static mDNSBool CheckThatServiceTypeIsUsable(const char *serviceType, mDNSBool printExplanation)
    // Checks that serviceType is a reasonable service type 
    // label and, if it isn't and printExplanation is true, prints 
    // an explanation of why not.
{
    mDNSBool result;
    
    result = mDNStrue;
    if (result && strlen(serviceType) > 63) {
        if (printExplanation) {
            fprintf(stderr, 
                    "%s: Service type is too long (must be 63 characters or less)\n", 
                    gProgramName);
        }
        result = mDNSfalse;
    }
    if (result && serviceType[0] == 0) {
        if (printExplanation) {
            fprintf(stderr, 
                    "%s: Service type can't be empty\n", 
                    gProgramName);
        }
        result = mDNSfalse;
    }
    return result;
}

static mDNSBool CheckThatPortNumberIsUsable(long portNumber, mDNSBool printExplanation)
    // Checks that portNumber is a reasonable port number
    // and, if it isn't and printExplanation is true, prints 
    // an explanation of why not.
{
    mDNSBool result;
    
    result = mDNStrue;
    if (result && (portNumber <= 0 || portNumber > 65535)) {
        if (printExplanation) {
            fprintf(stderr, 
                    "%s: Port number specified by -p must be in range 1..65535\n", 
                    gProgramName);
        }
        result = mDNSfalse;
    }
    return result;
}

#if COMPILER_LIKES_PRAGMA_MARK
#pragma mark ***** Command Line Arguments
#endif

static const char kDefaultPIDFile[]     = "/var/run/mDNSResponder.pid";
static const char kDefaultServiceType[] = "_afpovertcp._tcp.";
static const char kDefaultServiceDomain[] = "local.";
enum {
    kDefaultPortNumber = 548
};

static void PrintUsage()
{
    fprintf(stderr, 
            "Usage: %s [-v level ] [-r] [-n name] [-t type] [-d domain] [-p port] [-f file] [-b] [-P pidfile] [-x name=val ...]\n", 
            gProgramName);
    fprintf(stderr, "          -v verbose mode, level is a number from 0 to 2\n");
    fprintf(stderr, "             0 = no debugging info (default)\n");
    fprintf(stderr, "             1 = standard debugging info\n");
    fprintf(stderr, "             2 = intense debugging info\n");
    fprintf(stderr, "             can be cycled kill -USR1\n");
    fprintf(stderr, "          -r also bind to port 53 (port 5353 is always bound)\n");
    fprintf(stderr, "          -n uses 'name' as the service name (required)\n");
    fprintf(stderr, "          -t uses 'type' as the service type (default is '%s')\n", kDefaultServiceType);
    fprintf(stderr, "          -d uses 'domain' as the service domain (default is '%s')\n", kDefaultServiceDomain);
    fprintf(stderr, "          -p uses 'port' as the port number (default is '%d')\n",  kDefaultPortNumber);
    fprintf(stderr, "          -f reads a service list from 'file'\n");
    fprintf(stderr, "          -b forces daemon (background) mode\n");
    fprintf(stderr, "          -P uses 'pidfile' as the PID file\n");
    fprintf(stderr, "             (default is '%s')\n",  kDefaultPIDFile);
    fprintf(stderr, "             only meaningful if -b also specified\n");
    fprintf(stderr, "          -x stores name=val in TXT record (default is empty).\n");
    fprintf(stderr, "             MUST be the last command-line argument;\n");
    fprintf(stderr, "             all subsequent arguments after -x are treated as name=val pairs.\n");
}

static   mDNSBool  gAvoidPort53      = mDNStrue;
static const char *gServiceName      = "";
static const char *gServiceType      = kDefaultServiceType;
static const char *gServiceDomain    = kDefaultServiceDomain;
static mDNSu8      gServiceText[sizeof(RDataBody)];
static mDNSu16     gServiceTextLen   = 0;
static        int  gPortNumber       = kDefaultPortNumber;
static const char *gServiceFile      = "";
static   mDNSBool  gDaemon           = mDNSfalse;
static const char *gPIDFile          = kDefaultPIDFile;

static void ParseArguments(int argc, char **argv)
    // Parses our command line arguments into the global variables 
    // listed above.
{
    int ch;
    
    // Set gProgramName to the last path component of argv[0]
    
    gProgramName = strrchr(argv[0], '/');
    if (gProgramName == NULL) {
        gProgramName = argv[0];
    } else {
        gProgramName += 1;
    }
    
    // Parse command line options using getopt.
    
    do {
        ch = getopt(argc, argv, "v:rn:t:d:p:f:dP:bx");
        if (ch != -1) {
            switch (ch) {
                case 'v':
                    gMDNSPlatformPosixVerboseLevel = atoi(optarg);
                    if (gMDNSPlatformPosixVerboseLevel < 0 || gMDNSPlatformPosixVerboseLevel > 2) {
                        fprintf(stderr, 
                                "%s: Verbose mode must be in the range 0..2\n", 
                                gProgramName);
                        exit(1);
                    }
                    break;
                case 'r':
                    gAvoidPort53 = mDNSfalse;
                    break;
                case 'n':
                    gServiceName = optarg;
                    if ( ! CheckThatRichTextNameIsUsable(gServiceName, mDNStrue) ) {
                        exit(1);
                    }
                    break;
                case 't':
                    gServiceType = optarg;
                    if ( ! CheckThatServiceTypeIsUsable(gServiceType, mDNStrue) ) {
                        exit(1);
                    }
                    break;
                case 'd':
                    gServiceDomain = optarg;
                    break;
                case 'p':
                    gPortNumber = atol(optarg);
                    if ( ! CheckThatPortNumberIsUsable(gPortNumber, mDNStrue) ) {
                        exit(1);
                    }
                    break;
                case 'f':
                    gServiceFile = optarg;
                    break;
                case 'b':
                    gDaemon = mDNStrue;
                    break;
                case 'P':
                    gPIDFile = optarg;
                    break;
                case 'x':
                    while (optind < argc)
                        {
                        gServiceText[gServiceTextLen] = strlen(argv[optind]);
                        memcpy(gServiceText+gServiceTextLen+1, argv[optind], gServiceText[gServiceTextLen]);
                        gServiceTextLen += 1 + gServiceText[gServiceTextLen];
                        optind++;
                        }
                    ch = -1;
                    break;
                case '?':
                default:
                    PrintUsage();
                    exit(1);
                    break;
            }
        }
    } while (ch != -1);

    // Check for any left over command line arguments.
    
    if (optind != argc) {
        PrintUsage();
        fprintf(stderr, "%s: Unexpected argument '%s'\n", gProgramName, argv[optind]);
        exit(1);
    }
    
    // Check for inconsistency between the arguments.
    
    if ( (gServiceName[0] == 0) && (gServiceFile[0] == 0) ) {
        PrintUsage();
        fprintf(stderr, "%s: You must specify a service name to register (-n) or a service file (-f).\n", gProgramName);
        exit(1);
    }
}

#if COMPILER_LIKES_PRAGMA_MARK
#pragma mark ***** Registration
#endif

typedef struct PosixService PosixService;

struct PosixService {
    ServiceRecordSet coreServ;
    PosixService *next;
    int serviceID;
};

static PosixService *gServiceList = NULL;

static void RegistrationCallback(mDNS *const m, ServiceRecordSet *const thisRegistration, mStatus status)
    // mDNS core calls this routine to tell us about the status of 
    // our registration.  The appropriate action to take depends 
    // entirely on the value of status.
{
    switch (status) {

        case mStatus_NoError:      
            debugf("Callback: %##s Name Registered",   thisRegistration->RR_SRV.resrec.name->c); 
            // Do nothing; our name was successfully registered.  We may 
            // get more call backs in the future.
            break;

        case mStatus_NameConflict: 
            debugf("Callback: %##s Name Conflict",     thisRegistration->RR_SRV.resrec.name->c); 

            // In the event of a conflict, this sample RegistrationCallback 
            // just calls mDNS_RenameAndReregisterService to automatically 
            // pick a new unique name for the service. For a device such as a 
            // printer, this may be appropriate.  For a device with a user 
            // interface, and a screen, and a keyboard, the appropriate response 
            // may be to prompt the user and ask them to choose a new name for 
            // the service.
            //
            // Also, what do we do if mDNS_RenameAndReregisterService returns an 
            // error.  Right now I have no place to send that error to.
            
            status = mDNS_RenameAndReregisterService(m, thisRegistration, mDNSNULL);
            assert(status == mStatus_NoError);
            break;

        case mStatus_MemFree:      
            debugf("Callback: %##s Memory Free",       thisRegistration->RR_SRV.resrec.name->c); 
            
            // When debugging is enabled, make sure that thisRegistration 
            // is not on our gServiceList.
            
            #if !defined(NDEBUG)
                {
                    PosixService *cursor;
                    
                    cursor = gServiceList;
                    while (cursor != NULL) {
                        assert(&cursor->coreServ != thisRegistration);
                        cursor = cursor->next;
                    }
                }
            #endif
            free(thisRegistration);
            break;

        default:                   
            debugf("Callback: %##s Unknown Status %ld", thisRegistration->RR_SRV.resrec.name->c, status); 
            break;
    }
}

static int gServiceID = 0;

static mStatus RegisterOneService(const char *  richTextName, 
                                  const char *  serviceType, 
                                  const char *  serviceDomain, 
                                  const mDNSu8  text[],
                                  mDNSu16       textLen,
                                  long          portNumber)
{
    mStatus             status;
    PosixService *      thisServ;
    domainlabel         name;
    domainname          type;
    domainname          domain;
    
    status = mStatus_NoError;
    thisServ = (PosixService *) malloc(sizeof(*thisServ));
    if (thisServ == NULL) {
        status = mStatus_NoMemoryErr;
    }
    if (status == mStatus_NoError) {
        MakeDomainLabelFromLiteralString(&name,  richTextName);
        MakeDomainNameFromDNSNameString(&type, serviceType);
        MakeDomainNameFromDNSNameString(&domain, serviceDomain);
        status = mDNS_RegisterService(&mDNSStorage, &thisServ->coreServ,
                &name, &type, &domain,                // Name, type, domain
                NULL, mDNSOpaque16fromIntVal(portNumber),
                text, textLen,                        // TXT data, length
                NULL, 0,                            // Subtypes
                mDNSInterface_Any,                    // Interface ID
                RegistrationCallback, thisServ);    // Callback and context
    }
    if (status == mStatus_NoError) {
        thisServ->serviceID = gServiceID;
        gServiceID += 1;

        thisServ->next = gServiceList;
        gServiceList = thisServ;

        if (gMDNSPlatformPosixVerboseLevel > 0) {
            fprintf(stderr, 
                    "%s: Registered service %d, name '%s', type '%s', port %ld\n", 
                    gProgramName, 
                    thisServ->serviceID, 
                    richTextName,
                    serviceType,
                    portNumber);
        }
    } else {
        if (thisServ != NULL) {
            free(thisServ);
        }
    }
    return status;
}

static mDNSBool ReadALine(char *buf, size_t bufSize, FILE *fp)
// Read a line, skipping over any blank lines or lines starting with '#'
{
    mDNSBool good, skip;
    do {
        good = (fgets(buf, bufSize, fp) != NULL);
        skip = (good && (buf[0] == '#'));
    } while (good && skip);
    if (good)
    {
        int        len = strlen( buf);
        if ( buf[len - 1] == '\r' || buf[len - 1] == '\n')
            buf[len - 1] = '\0';
    }
    return good;
}

static mStatus RegisterServicesInFile(const char *filePath)
{
    mStatus     status = mStatus_NoError;
    FILE *      fp = fopen(filePath, "r");
    int         junk;
    
    if (fp == NULL) {
        status = mStatus_UnknownErr;
    }
    if (status == mStatus_NoError) {
        mDNSBool good = mDNStrue;
        do {
            int         ch;
            char name[256];
            char type[256];
            const char *dom = kDefaultServiceDomain;
            char rawText[1024];
            mDNSu8  text[sizeof(RDataBody)];
            unsigned int textLen = 0;
            char port[256];

            // Skip over any blank lines.
            do ch = fgetc(fp); while ( ch == '\n' || ch == '\r' );
            if (ch != EOF) good = (ungetc(ch, fp) == ch);
            
            // Read three lines, check them for validity, and register the service.
            good = ReadALine(name, sizeof(name), fp);               
            if (good) {
                good = ReadALine(type, sizeof(type), fp);
            }
            if (good) {
                char *p = type;
                while (*p && *p != ' ') p++;
                if (*p) {
                    *p = 0;
                    dom = p+1;
                }
            }
            if (good) {
                good = ReadALine(port, sizeof(port), fp);
            }
            if (good) {
                good =     CheckThatRichTextNameIsUsable(name, mDNSfalse)
                        && CheckThatServiceTypeIsUsable(type, mDNSfalse)
                        && CheckThatPortNumberIsUsable(atol(port), mDNSfalse);
            }
            if (good) {
                while (1) {
                    int len;
                    if (!ReadALine(rawText, sizeof(rawText), fp)) break;
                    len = strlen(rawText);
                    if (len <= 255)
                        {
                        unsigned int newlen = textLen + 1 + len;
                        if (len == 0 || newlen >= sizeof(text)) break;
                        text[textLen] = len;
                        memcpy(text + textLen + 1, rawText, len);
                        textLen = newlen;
                        }
                    else
                        fprintf(stderr, "%s: TXT attribute too long for name = %s, type = %s, port = %s\n", 
                            gProgramName, name, type, port);
                }
            }
            if (good) {
                status = RegisterOneService(name, type, dom, text, textLen, atol(port));
                if (status != mStatus_NoError) {
                    fprintf(stderr, "%s: Failed to register service, name = %s, type = %s, port = %s\n", 
                            gProgramName, name, type, port);
                    status = mStatus_NoError;       // keep reading
                }
            }
        } while (good && !feof(fp));

        if ( ! good ) {
            fprintf(stderr, "%s: Error reading service file %s\n", gProgramName, filePath);
        }
    }
    
    if (fp != NULL) {
        junk = fclose(fp);
        assert(junk == 0);
    }
    
    return status;
}

static mStatus RegisterOurServices(void)
{
    mStatus status;
    
    status = mStatus_NoError;
    if (gServiceName[0] != 0) {
        status = RegisterOneService(gServiceName, 
                                    gServiceType, 
                                    gServiceDomain, 
                                    gServiceText, gServiceTextLen, 
                                    gPortNumber);
    }
    if (status == mStatus_NoError && gServiceFile[0] != 0) {
        status = RegisterServicesInFile(gServiceFile);
    }
    return status;
}

static void DeregisterOurServices(void)
{
    PosixService *thisServ;
    int thisServID;
    
    while (gServiceList != NULL) {
        thisServ = gServiceList;
        gServiceList = thisServ->next;

        thisServID = thisServ->serviceID;
        
        mDNS_DeregisterService(&mDNSStorage, &thisServ->coreServ);

        if (gMDNSPlatformPosixVerboseLevel > 0) {
            fprintf(stderr, 
                    "%s: Deregistered service %d\n",
                    gProgramName, 
                    thisServ->serviceID);
        }
    }
}

#if COMPILER_LIKES_PRAGMA_MARK
#pragma mark **** Main
#endif

int main(int argc, char **argv)
{
    mStatus status;
    int     result;

    // Parse our command line arguments.  This won't come back if there's an error.
    
    ParseArguments(argc, argv);

    // If we're told to run as a daemon, then do that straight away.
    // Note that we don't treat the inability to create our PID 
    // file as an error.  Also note that we assign getpid to a long 
    // because printf has no format specified for pid_t.
    
    if (gDaemon) {
        if (gMDNSPlatformPosixVerboseLevel > 0) {
            fprintf(stderr, "%s: Starting in daemon mode\n", gProgramName);
        }
        daemon(0,0);
        {
            FILE *fp;
            int  junk;
            
            fp = fopen(gPIDFile, "w");
            if (fp != NULL) {
                fprintf(fp, "%ld\n", (long) getpid());
                junk = fclose(fp);
                assert(junk == 0);
            }
        }
    } else {
        if (gMDNSPlatformPosixVerboseLevel > 0) {
            fprintf(stderr, "%s: Starting in foreground mode, PID %ld\n", gProgramName, (long) getpid());
        }
    }

    status = mDNS_Init(&mDNSStorage, &PlatformStorage,
        mDNS_Init_NoCache, mDNS_Init_ZeroCacheSize,
        mDNS_Init_AdvertiseLocalAddresses,
        mDNS_Init_NoInitCallback, mDNS_Init_NoInitCallbackContext);
    if (status != mStatus_NoError) return(2);

    status = RegisterOurServices();
    if (status != mStatus_NoError) return(2);
    
    signal(SIGHUP,  HandleSigHup);      // SIGHUP has to be sent by kill -HUP <pid>
    signal(SIGINT,  HandleSigInt);      // SIGINT is what you get for a Ctrl-C
    signal(SIGQUIT, HandleSigQuit);     // SIGQUIT is what you get for a Ctrl-\ (indeed)
    signal(SIGUSR1, HandleSigUsr1);     // SIGUSR1 has to be sent by kill -USR1 <pid>

    while (!gStopNow)
        {
        int nfds = 0;
        fd_set readfds;
        struct timeval timeout;
        int result;
        
        // 1. Set up the fd_set as usual here.
        // This example client has no file descriptors of its own,
        // but a real application would call FD_SET to add them to the set here
        FD_ZERO(&readfds);
        
        // 2. Set up the timeout.
        // This example client has no other work it needs to be doing,
        // so we set an effectively infinite timeout
        timeout.tv_sec = 0x3FFFFFFF;
        timeout.tv_usec = 0;
        
        // 3. Give the mDNSPosix layer a chance to add its information to the fd_set and timeout
        mDNSPosixGetFDSet(&mDNSStorage, &nfds, &readfds, &timeout);
        
        // 4. Call select as normal
        verbosedebugf("select(%d, %d.%06d)", nfds, timeout.tv_sec, timeout.tv_usec);
        result = select(nfds, &readfds, NULL, NULL, &timeout);
        
        if (result < 0)
            {
            verbosedebugf("select() returned %d errno %d", result, errno);
            if (errno != EINTR) gStopNow = mDNStrue;
            else
                {
                if (gReceivedSigUsr1)
                    {
                    gReceivedSigUsr1 = mDNSfalse;
                    gMDNSPlatformPosixVerboseLevel += 1;
                    if (gMDNSPlatformPosixVerboseLevel > 2)
                        gMDNSPlatformPosixVerboseLevel = 0;
                    if ( gMDNSPlatformPosixVerboseLevel > 0 )
                        fprintf(stderr, "\nVerbose level %d\n", gMDNSPlatformPosixVerboseLevel);
                    }
                if (gReceivedSigHup)
                    {
                    if (gMDNSPlatformPosixVerboseLevel > 0)
                        fprintf(stderr, "\nSIGHUP\n");
                    gReceivedSigHup = mDNSfalse;
                    DeregisterOurServices();
                    status = mDNSPlatformPosixRefreshInterfaceList(&mDNSStorage);
                    if (status != mStatus_NoError) break;
                    status = RegisterOurServices();
                    if (status != mStatus_NoError) break;
                    }
                }
            }
        else
            {
            // 5. Call mDNSPosixProcessFDSet to let the mDNSPosix layer do its work
            mDNSPosixProcessFDSet(&mDNSStorage, &readfds);
            
            // 6. This example client has no other work it needs to be doing,
            // but a real client would do its work here
            // ... (do work) ...
            }
        }

    debugf("Exiting");
    
    DeregisterOurServices();
    mDNS_Close(&mDNSStorage);

    if (status == mStatus_NoError) {
        result = 0;
    } else {
        result = 2;
    }
    if ( (result != 0) || (gMDNSPlatformPosixVerboseLevel > 0) ) {
        fprintf(stderr, "%s: Finished with status %ld, result %d\n", gProgramName, status, result);
    }
    
    return result;
}

ReadMe About mDNSPosix
----------------------

mDNSPosix is a port of Apple's Multicast DNS and DNS Service Discovery
code to Posix platforms.

Multicast DNS and DNS Service Discovery are technologies that allow you
to register IP-based services and browse the network for those services.
For more information about mDNS, see the mDNS web site.

  <http://www.multicastdns.org/>

Multicast DNS is part of a family of technologies resulting from the
efforts of the IETF Zeroconf working group.  For information about
other Zeroconf technologies, see the Zeroconf web site.

  <http://www.zeroconf.org/>

Apple uses the trade mark "Bonjour" to describe our implementation of
Zeroconf technologies.  This sample is designed to show how easy it is
to make a device "Bonjour compatible".

The "Bonjour" trade mark can also be licensed at no charge for
inclusion on your own products, packaging, manuals, promotional
materials, or web site. For details and licensing terms, see

  <http://developer.apple.com/bonjour/>

The code in this sample was compiled and tested on Mac OS X (10.1.x,
10.2, 10.3), Solaris (SunOS 5.8), Linux (Redhat 2.4.9-21, Fedora Core 1), 
and OpenBSD (2.9). YMMV.


Packing List
------------

The sample uses the following directories:

o mDNSCore -- A directory containing the core mDNS code.  This code
  is written in pure ANSI C and has proved to be very portable.
  Every platform needs this core protocol engine code.

o mDNSShared -- A directory containing useful code that's not core to
  the main protocol engine itself, but nonetheless useful, and used by
  more than one (but not necessarily all) platforms.

o mDNSPosix -- The files that are specific to Posix platforms: Linux,
  Solaris, FreeBSD, NetBSD, OpenBSD, etc. This code will also work on
  OS X, though that's not its primary purpose.

o Clients -- Example client code showing how to use the API to the
  services provided by the daemon.


Building the Code
-----------------

The sample does not use autoconf technology, primarily because I didn't
want to delay shipping while I learnt how to use it.  Thus the code
builds using a very simple make file.  To build the sample you should
cd to the mDNSPosix directory and type "make os=myos", e.g.

    make os=panther

For Linux you would change that to:

    make os=linux

There are definitions for each of the platforms I ported to.  If you're
porting to any other platform please add appropriate definitions for it
and send us the diffs so they can be incorporated into the main
distribution.


Using the Sample
----------------
When you compile, you will get:

o Main products for general-purpose use (e.g. on a desktop computer):
  - mdnsd
  - libmdns
  - nss_mdns (See nss_ReadMe.txt for important information about nss_mdns)

o Standalone products for dedicated devices (printer, network camera, etc.)
  - mDNSClientPosix
  - mDNSResponderPosix
  - mDNSProxyResponderPosix

o Testing and Debugging tools
  - dns-sd command-line tool (from the "Clients" folder)
  - mDNSNetMonitor
  - mDNSIdentify

As root type "make install" to install eight things:
o mdnsd                   (usually in /usr/sbin)
o libmdns                 (usually in /usr/lib)
o dns_sd.h                (usually in /usr/include)
o startup scripts         (e.g. in /etc/rc.d)
o manual pages            (usually in /usr/share/man)
o dns-sd tool             (usually in /usr/bin)
o nss_mdns                (usually in /lib)
o nss configuration files (usually in /etc)

The "make install" concludes by executing the startup script
(usually "/etc/init.d/mdns start") to start the daemon running.
You shouldn't need to reboot unless you really want to.

Once the daemon is running, you can use the dns-sd test tool
to exercise all the major functionality of the daemon. Running
"dns-sd" with no arguments gives a summary of the available options.
This test tool is also described in detail, with several examples,
in Chapter 6 of the O'Reilly "Zero Configuration Networking" book.


How It Works
------------
                                                   +--------------------+
                                                   | Client Application |
   +----------------+                              +--------------------+
   |  uds_daemon.c  | <--- Unix Domain Socket ---> |      libmdns       |
   +----------------+                              +--------------------+
   |    mDNSCore    |
   +----------------+
   |  mDNSPosix.c   |
   +----------------+

mdnsd is divided into three sections.

o mDNSCore is the main protocol engine
o mDNSPosix.c provides the glue it needs to run on a Posix OS
o uds_daemon.c exports a Unix Domain Socket interface to
  the services provided by mDNSCore

Client applications link with the libmdns, which implements the functions
defined in the dns_sd.h header file, and implements the IPC protocol
used to communicate over the Unix Domain Socket interface to the daemon.

Note that, strictly speaking, nss_mdns could be just another client of
mdnsd, linking with libmdns just like any other client. However, because
of its central role in the normal operation of multicast DNS, it is built
and installed along with the other essential system support components.


Clients for Embedded Systems
----------------------------

For small devices with very constrained resources, with a single address
space and (typically) no virtual memory, the uds_daemon.c/UDS/libmdns
layer may be eliminated, and the Client Application may live directly
on top of mDNSCore:

    +--------------------+
    | Client Application |
    +--------------------+
    |      mDNSCore      |
    +--------------------+
    |    mDNSPosix.c     |
    +--------------------+

Programming to this model is more work, so using the daemon and its
library is recommended if your platform is capable of that.

The runtime behaviour when using the embedded model is as follows:

1. The application calls mDNS_Init, which in turns calls the platform
   (mDNSPlatformInit).

2. mDNSPlatformInit gets a list of interfaces (get_ifi_info) and registers
   each one with the core (mDNS_RegisterInterface).  For each interface
   it also creates a multicast socket (SetupSocket).

3. The application then calls select() repeatedly to handle file descriptor
   events. Before calling select() each time, the application calls
   mDNSPosixGetFDSet() to give mDNSPosix.c a chance to add its own file
   descriptors to the set, and then after select() returns, it calls
   mDNSPosixProcessFDSet() to give mDNSPosix.c a chance to receive and
   process any packets that may have arrived.

4. When the core needs to send a UDP packet it calls
   mDNSPlatformSendUDP.  That routines finds the interface that
   corresponds to the source address requested by the core, and
   sends the datagram using the UDP socket created for the
   interface.  If the socket is flow send-side controlled it just
   drops the packet.

5. When SocketDataReady runs it uses a complex routine,
   "recvfrom_flags", to actually receive the packet.  This is required
   because the core needs information about the packet that is
   only available via the "recvmsg" call, and that call is complex
   to implement in a portable way.  I got my implementation of
   "recvfrom_flags" from Stevens' "UNIX Network Programming", but
   I had to modify it further to work with Linux.

One thing to note is that the Posix platform code is very deliberately
not multi-threaded.  I do everything from a main loop that calls
"select()".  This is good because it avoids all the problems that often
accompany multi-threaded code. If you decide to use threads in your
platform, you will have to implement the mDNSPlatformLock() and
mDNSPlatformUnlock() calls which are currently no-ops in mDNSPosix.c.


Once you've built the embedded samples you can test them by first
running the client, as shown below.

  quinn% build/mDNSClientPosix
  Hit ^C when you're bored waiting for responses.

By default the client starts a search for AppleShare servers and then
sits and waits, printing a message when services appear and disappear.

To continue with the test you should start the responder in another
shell window.

  quinn% build/mDNSResponderPosix -n Foo

This will start the responder and tell it to advertise a AppleShare
service "Foo".  In the client window you will see the client print out
the following as the service shows up on the network.

  quinn% build/mDNSClientPosix
  Hit ^C when you're bored waiting for responses.
  *** Found name = 'Foo', type = '_afpovertcp._tcp.', domain = 'local.'

Back in the responder window you can quit the responder cleanly using
SIGINT (typically ^C).

  quinn% build/mDNSResponderPosix -n Foo
  ^C
  quinn%

As the responder quits it will multicast that the "Foo" service is
disappearing and the client will see that notification and print a
message to that effect (shown below).  Finally, when you're done with
the client you can use SIGINT to quit it.

  quinn% build/mDNSClientPosix
  Hit ^C when you're bored waiting for responses.
  *** Found name = 'Foo', type = '_afpovertcp._tcp.', domain = 'local.'
  *** Lost  name = 'Foo', type = '_afpovertcp._tcp.', domain = 'local.'
  ^C
  quinn%

If things don't work, try starting each program in verbose mode (using
the "-v 1" option, or very verbose mode with "-v 2") to see if there's
an obvious cause.

That's it for the core functionality.  Each program supports a variety
of other options.  For example, you can advertise and browse for a
different service type using the "-t type" option.  Use the "-?" option
on each program for more user-level information.


Caveats
-------
Currently the program uses a simple make file.

The Multicast DNS protocol can also operate locally over the loopback
interface, but this exposed some problems with the underlying network
stack in early versions of Mac OS X and may expose problems with other
network stacks too.

o On Mac OS X 10.1.x the code failed to start on the loopback interface
  because the IP_ADD_MEMBERSHIP option returns ENOBUFS.

o On Mac OS X 10.2 the loopback-only case failed because
  "sendto" calls fails with error EHOSTUNREACH. (3016042)

Consequently, the code will attempt service discovery on the loopback
interface only if no other interfaces are available.

I haven't been able to test the loopback-only case on other platforms
because I don't have access to the physical machine.


Licencing
---------
This code is distributed under the Apple Public Source License.
Information about the licence is included at the top of each source file.


Credits and Version History
---------------------------
If you find any problems with this sample, mail <[e-mail]> and I
will try to fix them up.

1.0a1 (Jul 2002) was a prerelease version that was distributed
internally at Apple.

1.0a2 (Jul 2002) was a prerelease version that was distributed
internally at Apple.

1.0a3 (Aug 2002) was the first shipping version.  The core mDNS code is
the code from Mac OS 10.2 (Jaguar) GM.

Share and Enjoy

Apple Developer Technical Support
Networking, Communications, Hardware

6 Aug 2002


To Do List
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• port to a System V that's not Solaris
• use sig_atomic_t for signal to main thread flags