netcat clone hacked together in 30 minutes

nc.c

#include <stdio.h>
#include <sys/socket.h>
#include <netdb.h>
#include <stdarg.h>
#include <string.h>
#include <arpa/inet.h>
#include <sys/select.h>

#define LG_SEVERE 0
#define LG_ERROR  1
#define LG_WARN   2
#define LG_INFO   3
#define LG_DEBUG  4

#define BUFSIZE 65536

char recvbuf[BUFSIZE];
char sendbuf[BUFSIZE];
size_t rb_size = 0;
size_t sb_size = 0;

int running = 0;
int sock = -1;

void nlog(int level, const char *fmt, ...)
{
	char buf[BUFSIZE];
	va_list va;

	va_start(va, fmt);
	vsnprintf(buf, BUFSIZE, fmt, va);
	va_end(va);

	printf("%s\n", buf);
}

int connect_to(const char *host, unsigned short port)
{
	char buf[INET_ADDRSTRLEN];
	struct hostent *he;
	struct sockaddr_in sin;
	int err = -1;

	if (sock >= 0) {
		nlog(LG_SEVERE, "Already connected!");
		return -1;
	}

	he = gethostbyname(host);
	if (he == NULL) {
		nlog(LG_SEVERE, "Can't find host %s", host);
		return err;
	}

	if (he->h_length <= 0 || he->h_addrtype != AF_INET) {
		nlog(LG_SEVERE, "gethostbyname() was not helpful");
		goto out;
	}

	if (inet_ntop(AF_INET, he->h_addr_list[0], buf, INET_ADDRSTRLEN) < 0) {
		nlog(LG_WARN, "inet_pton() complained! Using 0.0.0.0...");
		strcpy(buf, "0.0.0.0");
	}
	nlog(LG_DEBUG, "Connecting to %s (%s:%u)...", he->h_name, buf, port);

	sin.sin_family = AF_INET;
	sin.sin_port = htons(port);
	memcpy((void*)&sin.sin_addr, he->h_addr_list[0], 4); /* 4... */

	if ((sock = socket(AF_INET, SOCK_STREAM, 0)) < 0) {
		nlog(LG_SEVERE, "socket() failed!");
		goto out;
	}

	if (connect(sock, (struct sockaddr*)&sin, sizeof(sin)) < 0) {
		nlog(LG_SEVERE, "connect() failed!");
		goto out;
	}

	nlog(LG_DEBUG, "Connected!");

	err = 0;
out:
	endhostent();
	return err;
}

void disconnect(void)
{
	if (sock < 0) {
		nlog(LG_SEVERE, "Can't disconnect! (not connected)");
		return;
	}
	close(sock);
	sock = -1;
	nlog(LG_DEBUG, "Disconnected!");
}

void do_read(int fd, char *buf, size_t *size)
{
	ssize_t sz;

	sz = read(fd, buf + *size, BUFSIZE - *size);

	if (sz <= 0) {
		if (sz < 0)
			nlog(LG_INFO, "Read error");
		running = 0;
		return;
	}

	*size += sz;
}

void do_write(int fd, char *buf, size_t *size)
{
	ssize_t sz;

	sz = write(fd, buf, *size);

	if (sz <= 0) {
		nlog(LG_INFO, "Write error");
		running = 0;
		return;
	}

	*size -= sz;
	if (*size > 0)
		memmove(buf, buf + sz, *size);
}

void loop(void)
{
	fd_set r, w, x;
	int err;

	for (running=1; running;) {
		FD_ZERO(&r);
		FD_ZERO(&w);
		FD_ZERO(&x);

		FD_SET(0, &r);
		FD_SET(sock, &r);

		if (rb_size > 0)
			FD_SET(1, &w);
		if (sb_size > 0)
			FD_SET(sock, &w);

		err = select(sock + 1, &r, &w, &x, NULL);

		if (FD_ISSET(0, &r))
			do_read(0, sendbuf, &sb_size);
		if (FD_ISSET(sock, &r))
			do_read(sock, recvbuf, &rb_size);

		if (FD_ISSET(1, &w))
			do_write(1, recvbuf, &rb_size);
		if (FD_ISSET(sock, &w))
			do_write(sock, sendbuf, &sb_size);
	}
}

int main(int argc, char *argv[])
{
	if (argc != 3) {
		fprintf(stderr, "Usage: %s HOST PORT\n", argv[0]);
		return 1;
	}
	if (connect_to(argv[1], atoi(argv[2])) < 0) {
		nlog(LG_SEVERE, "Connecting failed :(");
		return 1;
	}
	loop();
	disconnect();
	return 0;
}