pppd/main.c -> main():

……

new_phase(PHASE_INITIALIZE); //PPPD中的狀態機，目前是初始化階段

    /*

* Initialize magic number generator now so that protocols may

* use magic numbers in initialization.

*/

    magic_init();

    /*

* Initialize each protocol.

*/

    for(i=0;(protp=protocols[i])!= NULL;++i) //protocols[]是全域性變數的協議陣列

        (*protp->init)(0); //初始化協議陣列中所有協議

    /*

* Initialize the default channel.

*/

    tty_init(); //channel初始化，預設就是全域性的tty_channel，裡面包括很多TTY函式指標   

    if(!options_from_file(_PATH_SYSOPTIONS,!privileged,0,1)//解析/etc/ppp/options中的引數

       ||!options_from_user() 

       ||!parse_args(argc-1,argv+1)) //解析PPPD命令列引數

       exit(EXIT_OPTION_ERROR);

    devnam_fixed=1;       /* can no longer change device name */

    /*

* Work out the device name, if it hasn't already been specified,

* and parse the tty's options file.

*/

    if(the_channel->process_extra_options)

       (*the_channel->process_extra_options)(); //實際上是呼叫tty_process_extra_options解析TTY 引數

    if(!ppp_available()){ //檢測/dev/ppp裝置檔案是否有效

       option_error("%s",no_ppp_msg);

       exit(EXIT_NO_KERNEL_SUPPORT);

    }

    /*

* Check that the options given are valid and consistent.

*/

    check_options(); //檢查選項引數

    if(!sys_check_options()) //檢測系統引數，比如核心是否支援Multilink等

       exit(EXIT_OPTION_ERROR);

    auth_check_options(); //檢查認證相關的引數

#ifdef HAVE_MULTILINK

mp_check_options();

#endif

    for(i=0;(protp=protocols[i])!= NULL;++i)

       if(protp->check_options!= NULL)

           (*protp->check_options)(); //檢查每個控制協議的引數配置

    if(the_channel->check_options)

       (*the_channel->check_options)(); //實際上是呼叫tty_check_options檢測TTY引數

……

    /*

* Detach ourselves from the terminal, if required,

* and identify who is running us.

*/

    if(!nodetach&&!updetach) 

       detach(); //預設放在後臺以daemon執行，也可配置/etc/ppp/option中的nodetach引數放在前臺執行

……

    syslog(LOG_NOTICE,"pppd %s started by %s, uid %d",VERSION,p,uid); //熟悉的log，現在準備執行了

    script_setenv("PPPLOGNAME",p,0);

    if(devnam[0])

       script_setenv("DEVICE",devnam,1);

    slprintf(numbuf,sizeof(numbuf),"%d",getpid());

    script_setenv("PPPD_PID",numbuf,1);

    setup_signals(); //設定訊號處理函式

    create_linkpidfile(getpid()); //建立PID檔案

    waiting=0;

    /*

* If we're doing dial-on-demand, set up the interface now.

*/

    if(demand){ //以按需撥號方式執行，可配置

       /*

 * Open the loopback channel and set it up to be the ppp interface.

 */

       fd_loop=open_ppp_loopback(); //詳見下面分析

       set_ifunit(1); //設定IFNAME環境變數為介面名稱如ppp0

       /*

 * Configure the interface and mark it up, etc.

 */

       demand_conf();

}

(第二階段)……

/*

* PPP Data Link Layer "protocol" table.

* One entry per supported protocol.

* The last entry must be NULL.

*/

struct protent*protocols[]={

    &lcp_protent, //LCP協議

    &pap_protent, //PAP協議

    &chap_protent, //CHAP協議

#ifdef CBCP_SUPPORT

&cbcp_protent,

#endif

    &ipcp_protent, //IPCP協議,IPv4

#ifdef INET6

&ipv6cp_protent,//IPCP協議,IPv6

#endif

    &ccp_protent,

    &ecp_protent,

#ifdef IPX_CHANGE

&ipxcp_protent,

#endif

#ifdef AT_CHANGE

&atcp_protent,

#endif

    &eap_protent,

    NULL

};

/*

* The following struct gives the addresses of procedures to call

* for a particular protocol.

*/

struct protent{

    u_short protocol;            /* PPP protocol number */

    /* Initialization procedure */

    void(*init)__P((int unit));  //初始化指標,在main()中被呼叫

    /* Process a received packet */

    void(*input)__P((int unit, u_char *pkt,int len)); //接收報文處理

    /* Process a received protocol-reject */

    void(*protrej)__P((int unit));  //協議錯誤處理

    /* Lower layer has come up */

    void(*lowerup)__P((int unit));  //當下層協議UP起來後的處理

    /* Lower layer has gone down */

    void(*lowerdown)__P((int unit));  //當下層協議DOWN後的處理

    /* Open the protocol */

    void(*open)__P((int unit));  //開啟協議

    /* Close the protocol */

    void(*close)__P((int unit,char*reason)); //關閉協議

    /* Print a packet in readable form */

    int (*printpkt)__P((u_char*pkt,int len,

                       void(*printer)__P((void*,char*,...)),

                       void*arg)); //列印報文資訊，除錯用。

    /* Process a received data packet */

    void(*datainput)__P((int unit, u_char *pkt,int len)); //處理已收到的資料包

    boolenabled_flag;         /* 0 iff protocol is disabled */

    char*name;                  /* Text name of protocol */

    char*data_name;          /* Text name of corresponding data protocol */

    option_t*options;        /* List of command-line options */

    /* Check requested options, assign defaults */

    void(*check_options)__P((void)); //檢測和此協議有關的選項引數

    /* Configure interface for demand-dial */

    int (*demand_conf)__P((int unit));  //將介面配置為按需撥號需要做的 動作

    /* Say whether to bring up link for this pkt */

    int (*active_pkt)__P((u_char*pkt,int len)); //判斷報文型別並激活鏈路

};

pppd/sys_linux.c

main() -> ppp_avaiable():

intppp_available(void)

{

……

    no_ppp_msg=

       "This system lacks kernel support for PPP. This could be because\n"

       "the PPP kernel module could not be loaded, or because PPP was not\n"

       "included in the kernel configuration. If PPP was included as a\n"

       "module, try `/sbin/modprobe -v ppp'. If that fails, check that\n"

       "ppp.o exists in /lib/modules/`uname -r`/net.\n"

       "See README.linux file in the ppp distribution for more details.\n";

    /* get the kernel version now, since we are called before sys_init */

    uname(&utsname);

    osmaj=osmin=ospatch=0;

    sscanf(utsname.release,"%d.%d.%d",&osmaj,&osmin,&ospatch);

kernel_version=KVERSION(osmaj,osmin,ospatch);

    fd=open("/dev/ppp", O_RDWR);

    if(fd>=0){

       new_style_driver=1; //支援PPPK

       /* XXX should get from driver */

       driver_version=2;

       driver_modification=4;

       driver_patch=0;

       close(fd);

       return1;

}

……

}

pppd/sys-linux.c

main() -> open_ppp_loopback():

int

open_ppp_loopback(void)

{

    intflags;

    looped=1; //設定全域性變數looped為1，後面會用到

    if(new_style_driver){

       /* allocate ourselves a ppp unit */

       if(make_ppp_unit()<0) //建立PPP網路介面

           die(1);

       modify_flags(ppp_dev_fd,0, SC_LOOP_TRAFFIC); //通過ioctl設定SC_LOOP_TRAFFIC

       set_kdebugflag(kdebugflag);

       ppp_fd=-1;

       returnppp_dev_fd;

    }

……(下面是old driver，忽略)

}

pppd/sys-linux.c

main() -> open_ppp_loopback() -> make_ppp_unit():

staticintmake_ppp_unit()

{

       intx,flags;

       if(ppp_dev_fd>=0){ //如果已經開啟過，先關閉

              dbglog("in make_ppp_unit, already had /dev/ppp open?");

              close(ppp_dev_fd);

       }

       ppp_dev_fd=open("/dev/ppp", O_RDWR);  //開啟/dev/ppp

       if(ppp_dev_fd<0)

              fatal("Couldn't open /dev/ppp: %m");

       flags=fcntl(ppp_dev_fd, F_GETFL);

       if(flags==-1

           ||fcntl(ppp_dev_fd, F_SETFL,flags| O_NONBLOCK)==-1) //設定為非阻塞

              warn("Couldn't set /dev/ppp to nonblock: %m");

       ifunit=req_unit; //傳入請求的unit number，可通過/etc/ppp/options配置

       x=ioctl(ppp_dev_fd, PPPIOCNEWUNIT,&ifunit); //請求建立一個新unit

       if(x<0&&