这个是源码

int create_telnet_cli(const char *host, const char *service)
{
unsigned char rbuf[LINELEN]; // buffer for one line of text
unsigned char wbuf[LINELEN]; // buffer for one line of text
unsigned char msg[LINELEN];
unsigned char ch;
unsigned char lcOption;
int li_sock, li_count; // li_socket descriptor, read count
int outchars, inchars; // characters sent and received
int i;

printf("entering into telnet_cli\n");
li_sock = connectTCP(host,service);
printf("li_socket is: %d\n", li_sock);
bzero(wbuf, LINELEN);
bzero(rbuf, LINELEN);

sprintf(wbuf, "%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c\n", IAC, DODO, 3, IAC, WILL, 24,IAC, WILL, 31,\
IAC, WILL, 32, IAC, WILL, 33, IAC, WILL, 34, IAC, WILL, 39, IAC, DODO, 5);
write(li_sock, wbuf, strlen(wbuf));

//sprintf(wbuf, "%c%c%c\n", IAC, WONT, 37);
//write(li_sock, wbuf, strlen(wbuf));

while ((li_count = recv(li_sock, rbuf, LINELEN, 0))>=0 )
{
printf("length: %d, %s\n", li_count, rbuf);
//sleep(5);
if (li_count == 0)
{
printf("socket has been closed!\n");
exit(1);
}
//if li_count > 0, it means that the socket has received some characters from the server
for (i = 0; i <= li_count - 1; )
{
printf("rbuf[%d] is : %d\n", i, rbuf[i]);
//sleep(2);
//ch = rbuf[i];
//printf("%c\n",ch);
if ( rbuf[i] == IAC)
{
//printf("rbuf[%d] is IAC\n", i);
//gsSendMsg[i]= rbuf[i];

ch = rbuf[i + 1];
printf("%d\n", ch);
//i++;
lcOption = rbuf[i + 2];
printf("fdsafafsafasfsafsa%d\n", lcOption);
i = i + 3;
switch (ch)
{
case DODO:
if (lcOption == AUTH)
{
printf("RECV DO AUTHENTICATION (37)\n");
//sleep(5);
bzero(gsSendMsg, LINELEN);
sprintf(gsSendMsg, "%c%c%c\n", IAC, WONT, AUTH);
write(li_sock, gsSendMsg, strlen(gsSendMsg));
}

if (lcOption == NAWS)
{
printf("RECV DO NAWS (31)\n");
//sleep(5);
bzero(gsSendMsg, LINELEN);
//////SEND IAC SB NAWS 0 132 (132) 0 52 (52)
//the key point is to find SB syntax.
sprintf(gsSendMsg, "%c%c%c%d%d%d%d%c%c\n", IAC, SB, NAWS, 0,80,0,24,IAC,SE);
write(li_sock, gsSendMsg, strlen(gsSendMsg));
}

if (lcOption == XDISPLOC)
{
printf("RECV DO XDISPLOC (35)\n");
//sleep(5);
bzero(gsSendMsg, LINELEN);
sprintf(gsSendMsg, "%c%c%c\n", IAC, WONT, XDISPLOC);
write(li_sock, gsSendMsg, strlen(gsSendMsg));
}

if (lcOption == OLD_ENVIRON)
{
printf("RECV DO OLD-ENVIRON (36)\n");
//sleep(5);
bzero(gsSendMsg, LINELEN);
sprintf(gsSendMsg, "%c%c%c\n", IAC, WONT, OLD_ENVIRON);
write(li_sock, gsSendMsg, strlen(gsSendMsg));
}
if (lcOption == ECHO_ON)
{
printf("RECV DO ECHO (1)\n");
sleep(5);
bzero(gsSendMsg, LINELEN);
sprintf(gsSendMsg, "%c%c%c\n", IAC, WONT, ECHO_ON);
write(li_sock, gsSendMsg, strlen(gsSendMsg));
}
break;

case DONT:
printf("ch is DONT\n");
break;
case WILL:
if (lcOption == ECHO_ON)
{
printf("RECV WILL ECHO (1)\n");
sleep(5);
bzero(gsSendMsg, LINELEN);
sprintf(gsSendMsg, "%c%c%c\n", IAC, DODO, ECHO_ON);
write(li_sock, gsSendMsg, strlen(gsSendMsg));
}
break;
case WONT:
break;
case SB:
printf("ch is SB\n");
sleep(5);
if (lcOption == TSPEED)
{
printf("RECV IAC SB TERMINAL SPEED SEND (32)\n");
printf("%d%d%d\n", rbuf[i], rbuf[i + 1], rbuf[i + 2]);
//i = i + 3;
sleep(5);
bzero(gsSendMsg, LINELEN);
sprintf(gsSendMsg, "%c%c%c%c%d%d%d%d%c%c\n", IAC, SB, TSPEED, IS, 38400 >> 8, (38400 & 0xFF), 38400 >> 8, (38400 & 0xFF), IAC,SE);
write(li_sock, gsSendMsg, strlen(gsSendMsg));
}
if (lcOption == NEW_ENVIRON)
{
printf("RECV IAC SB NEW-ENVIRON SEND (39)\n");
printf("%d%d%d\n", rbuf[i], rbuf[i + 1], rbuf[i + 2]);
//i = i + 3;
sleep(5);
bzero(gsSendMsg, LINELEN);
sprintf(gsSendMsg, "%c%c%c%c%c%c\n", IAC, SB, NEW_ENVIRON, IS, IAC, SE);
write(li_sock, gsSendMsg, strlen(gsSendMsg));
}

if (lcOption == TERMINAL)
{
printf("RECV IAC SB TERMINAL TYPE SEND (24)\n");
printf("%d%d%d\n", rbuf[i], rbuf[i + 1], rbuf[i + 2]);
//i = i + 3;
sleep(5);
bzero(gsSendMsg, LINELEN);
sprintf(gsSendMsg, "%c%c%c%c%s%c%c\n", IAC, SB, NEW_ENVIRON, IS,"ANSI", IAC, SE);
write(li_sock, gsSendMsg, strlen(gsSendMsg));
printf("okokok\n");
bzero(gsSendMsg, LINELEN);
sprintf(gsSendMsg, "%c%c%c\n", IAC, DODO,ECHO_ON);
write(li_sock, gsSendMsg, strlen(gsSendMsg));
printf("okokok\n");
}
i = i + 3;
break;

default:
printf("ch is other character\n");
break;
}

}
else
{
//printf("ch isn't IAC, there are maybe some errors occur!\n");
i++;
}
}
}
}