This is a simple telnet client written in C. In the negotiation phase of the connection it will respond with WONT to any DO coming from the server, and it will encourage the server to DO anything it offers with WILL. The only exception is the screen size setting, advertised as 24 x 80 by the client.
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <arpa/inet.h>
#include <termios.h>
#include <fcntl.h>
#define DO 0xfd
#define WONT 0xfc
#define WILL 0xfb
#define DONT 0xfe
#define CMD 0xff
#define CMD_ECHO 1
#define CMD_WINDOW_SIZE 31
void negotiate(int sock, unsigned char *buf, int len) {
int i;
if (buf[1] == DO && buf[2] == CMD_WINDOW_SIZE) {
unsigned char tmp1[10] = {255, 251, 31};
if (send(sock, tmp1, 3 , 0) < 0)
exit(1);
unsigned char tmp2[10] = {255, 250, 31, 0, 80, 0, 24, 255, 240};
if (send(sock, tmp2, 9, 0) < 0)
exit(1);
return;
}
for (i = 0; i < len; i++) {
if (buf[i] == DO)
buf[i] = WONT;
else if (buf[i] == WILL)
buf[i] = DO;
}
if (send(sock, buf, len , 0) < 0)
exit(1);
}
static struct termios tin;
static void terminal_set(void) {
// save terminal configuration
tcgetattr(STDIN_FILENO, &tin);
static struct termios tlocal;
memcpy(&tlocal, &tin, sizeof(tin));
cfmakeraw(&tlocal);
tcsetattr(STDIN_FILENO,TCSANOW,&tlocal);
}
static void terminal_reset(void) {
// restore terminal upon exit
tcsetattr(STDIN_FILENO,TCSANOW,&tin);
}
#define BUFLEN 20
int main(int argc , char *argv[]) {
int sock;
struct sockaddr_in server;
unsigned char buf[BUFLEN + 1];
int len;
int i;
if (argc != 2) {
printf("Usage: %s address [port]\n", argv[0]);
return 1;
}
int port = 23;
if (argc == 3)
port = atoi(argv[2]);
//Create socket
sock = socket(AF_INET , SOCK_STREAM , 0);
if (sock == -1) {
perror("Could not create socket. Error");
return 1;
}
server.sin_addr.s_addr = inet_addr(argv[1]);
server.sin_family = AF_INET;
server.sin_port = htons(23);
//Connect to remote server
if (connect(sock , (struct sockaddr *)&server , sizeof(server)) < 0) {
perror("connect failed. Error");
return 1;
}
puts("Connected...\n");
// set terminal
terminal_set();
atexit(terminal_reset);
struct timeval ts;
ts.tv_sec = 1; // 1 second
ts.tv_usec = 0;
while (1) {
// select setup
fd_set fds;
FD_ZERO(&fds);
if (sock != 0)
FD_SET(sock, &fds);
FD_SET(0, &fds);
// wait for data
int nready = select(sock + 1, &fds, (fd_set *) 0, (fd_set *) 0, &ts);
if (nready < 0) {
perror("select. Error");
return 1;
}
else if (nready == 0) {
ts.tv_sec = 1; // 1 second
ts.tv_usec = 0;
}
else if (sock != 0 && FD_ISSET(sock, &fds)) {
// start by reading a single byte
int rv;
if ((rv = recv(sock , buf , 1 , 0)) < 0)
return 1;
else if (rv == 0) {
printf("Connection closed by the remote end\n\r");
return 0;
}
if (buf[0] == CMD) {
// read 2 more bytes
len = recv(sock , buf + 1 , 2 , 0);
if (len < 0)
return 1;
else if (len == 0) {
printf("Connection closed by the remote end\n\r");
return 0;
}
negotiate(sock, buf, 3);
}
else {
len = 1;
buf[len] = '\0';
printf("%s", buf);
fflush(0);
}
}
else if (FD_ISSET(0, &fds)) {
buf[0] = getc(stdin); //fgets(buf, 1, stdin);
if (send(sock, buf, 1, 0) < 0)
return 1;
if (buf[0] == '\n') // with the terminal in raw mode we need to force a LF
putchar('\r');
}
}
close(sock);
return 0;
}
