proxy.c

 /****************************************************************************
  program:      proxyd
  module:       proxyd.c
  summary:      provides proxy tcp service for a host on an isolated network.

  programmer:   Carl Harris ([e-mail])
  date:         22 Feb 94

  description:
        This code implements a daemon process which listens for tcp connec-
        tions on a specified port number.  When a connection is established,
        a child is forked to handle the new client.  The child then estab-
        lishes a tcp connection to a port on the isolated host.  The child
        then falls into a loop in which it writes data to the isolated host
        for the client and vice-versa.  Once a child has been forked, the
        parent resumes listening for additional connections.

        The name of the isolated host and the port to serve as proxy for,
        as well as the port number the server listen on are specified as
        command line arguments.

       
 ****************************************************************************/


#include
#include
#include
#include
#include
#include
#include
#include
#include
#include
#include


#define  TCP_PROTO      "tcp"

int proxy_port;                 /* port to listen for proxy connections on */
struct sockaddr_in hostaddr;    /* host addr assembled from gethostbyname() */

extern int errno;               /* defined by libc.a */
extern char *sys_errlist[];     /* defined by libc.a */


void parse_args (int argc, char **argv);
void daemonize (int servfd);
void do_proxy (int usersockfd);
void reap_status (void);
void errorout (char *msg);



/****************************************************************************
  function:      main
  description:   Main level driver.  After daemonizing the process, a socket
                 is opened to listen for connections on the proxy port,
                 connections are accepted and children are spawned to handle
                 each new connection.
  arguments:
    argc,argv    you know what those are.

  return value:  none.
  calls:         parse_args, do_proxy.
  globals:       reads proxy_port.
 ****************************************************************************/

main (argc,argv)
int argc;
char **argv;
{
  int clilen;
  int childpid;
  int sockfd, newsockfd;
  struct sockaddr_in servaddr, cliaddr;

  parse_args(argc,argv);

  /* prepare an address struct to listen for connections */
  bzero((char *) &servaddr, sizeof(servaddr));
  servaddr.sin_family = AF_INET;
  servaddr.sin_addr.s_addr = htonl(INADDR_ANY);
  servaddr.sin_port = proxy_port;

  /* get a socket... */
  if ((sockfd = socket(AF_INET,SOCK_STREAM,0)) < 0) {
    fputs("failed to create server socket\r\n",stderr);
    exit(1);
  }

  /* ...and bind our address and port to it */
  if (bind(sockfd,(struct sockaddr_in *) &servaddr,sizeof(servaddr)) < 0) {
    fputs("faild to bind server socket to specified port\r\n",stderr);
    exit(1);
  }

  /* get ready to accept with at most 5 clients waiting to connect */
  listen(sockfd,5);

  /* turn ourselves into a daemon */
  daemonize(sockfd);

  /* fall into a loop to accept new connections and spawn children */
  while (1) {

    /* accept the next connection */
    clilen = sizeof(cliaddr);
    newsockfd = accept(sockfd, (struct sockaddr_in *) &cliaddr, &clilen);
    if (newsockfd < 0 && errno == EINTR)
      continue;    /* a signal might interrupt our accept() call */
    else if (newsockfd < 0)
      /* something quite amiss -- kill the server */
      errorout("failed to accept connection");

    /* fork a child to handle this connection */
    if ((childpid = fork()) == 0) {
      close(sockfd);
      do_proxy(newsockfd);
      exit(0);
    }

    /* if fork() failed, the connection is silently dropped -- oops! */

    close(newsockfd);
  }
}



/****************************************************************************
  function:      parse_args
  description:   parse the command line args.
  arguments:
    argc,argv    you know what these are.

  return value:  none.
  calls:         none.
  globals:       writes proxy_port, writes hostaddr.
 ****************************************************************************/

void parse_args (argc,argv)
int argc;
char **argv;
{
  int i;
  struct hostent *hostp;
  struct servent *servp;
  unsigned long inaddr;
  struct {
    char proxy_port [16];
    char isolated_host [64];
    char service_name [32];
  } pargs;


  if (argc < 4) {
    printf("usage:  %s \r\n",
           argv[0]);
    exit(1);
  }

  strcpy(pargs.proxy_port,argv[1]);
  strcpy(pargs.isolated_host,argv[2]);
  strcpy(pargs.service_name,argv[3]);

  for (i = 0;  i < strlen(pargs.proxy_port);  i++)
    if (!isdigit(*(pargs.proxy_port + i)))
      break;

  if (i == strlen(pargs.proxy_port))
    proxy_port = htons(atoi(pargs.proxy_port));
  else {
    printf("%s: invalid proxy port\r\n",pargs.proxy_port);
    exit(0);
  }

  bzero(&hostaddr,sizeof(hostaddr));
  hostaddr.sin_family = AF_INET;
  if ((inaddr = inet_addr(pargs.isolated_host)) != INADDR_NONE)
    bcopy(&inaddr,&hostaddr.sin_addr,sizeof(inaddr));
  else if ((hostp = gethostbyname(pargs.isolated_host)) != NULL)
    bcopy(hostp->h_addr,&hostaddr.sin_addr,hostp->h_length);
  else {
    printf("%s: unknown host\r\n",pargs.isolated_host);
    exit(1);
  }

  if ((servp = getservbyname(pargs.service_name,TCP_PROTO)) != NULL)
    hostaddr.sin_port = servp->s_port;
  else if (atoi(pargs.service_name) > 0)
    hostaddr.sin_port = htons(atoi(pargs.service_name));
  else {
    printf("%s: invalid/unknown service name or port number\r\n",
           pargs.service_name);
    exit(1);
  }
}



/****************************************************************************
  function:      daemonize
  description:   detach the server process from the current context,
                 creating a pristine, predictable environment in which it
                 will execute.
  arguments:
    servfd       file descriptor in use by server.

  return value:  none.
  calls:         none.
  globals:       none.
 ****************************************************************************/

void daemonize (servfd)
int servfd;
{
  int childpid, fd, fdtablesize;

  /* ignore terminal I/O, stop signals */
  signal(SIGTTOU,SIG_IGN);
  signal(SIGTTIN,SIG_IGN);
  signal(SIGTSTP,SIG_IGN);

  /* fork to put us in the background (whether or not the user
     specified '&' on the command line */

  if ((childpid = fork()) < 0) {
    fputs("failed to fork first child\r\n",stderr);
    exit(1);
  }
  else if (childpid > 0)
    exit(0);    /* terminate parent, continue in child */

  /* dissociate from process group */
  if (setpgrp(0,getpid()) < 0) {
    fputs("failed to become process group leader\r\n",stderr);
    exit(1);
  }

  /* lose controlling terminal */
  if ((fd = open("/dev/tty",O_RDWR)) >= 0) {
    ioctl(fd,TIOCNOTTY,NULL);
    close(fd);
  }

  /* close any open file descriptors */
  for (fd = 0, fdtablesize = getdtablesize();  fd < fdtablesize;  fd++)
    if (fd != servfd)
      close(fd);

  /* set working directory to / to allow filesystems to be unmounted */
  chdir("/");

  /* clear the inherited umask */
  umask(0);

  /* setup zombie prevention */
  signal(SIGCLD,reap_status);

}



/****************************************************************************
  function:      do_proxy
  description:   does the actual work of virtually connecting a client to
                 the telnet service on the isolated host.
  arguments:
    usersockfd   socket to which the client is connected.

  return value:  none.
  calls:         none.
  globals:       reads hostaddr.
 ****************************************************************************/

void do_proxy (usersockfd)
int usersockfd;
{
  int isosockfd;
  fd_set rdfdset;
  int connstat;
  int iolen;
  char buf [2048];

  /* open a socket to connect to the isolated host */
  if ((isosockfd = socket(AF_INET,SOCK_STREAM,0)) < 0)
    errorout("failed to create socket to host");

  /* attempt a connection */
  connstat = connect(isosockfd,
                     (struct sockaddr *) &hostaddr,
                     sizeof(hostaddr));
  switch (connstat) {
    case 0:
      break;
    case ETIMEDOUT:
    case ECONNREFUSED:
    case ENETUNREACH:
      strcpy(buf,sys_errlist[errno]);
      strcat(buf,"\r\n");
      write(usersockfd,buf,strlen(buf));
      close(usersockfd);
      exit(1);  /* die peacefully if we can't establish a connection */
      break;
    default:
      errorout("failed to connect to host");
  }


  /* now we're connected, serve fall into the data echo loop */
  while (1) {
    /* Select for readability on either of our two sockets */
    FD_ZERO(&rdfdset);
    FD_SET(usersockfd,&rdfdset);
    FD_SET(isosockfd,&rdfdset);
    if (select(FD_SETSIZE,&rdfdset,NULL,NULL,NULL) < 0)
      errorout("select failed");

    /* is the client sending data? */
    if (FD_ISSET(usersockfd,&rdfdset)) {
      if ((iolen = read(usersockfd,buf,sizeof(buf))) <= 0)
        break;   /* zero length means the client disconnected */

      write(isosockfd,buf,iolen);   /* copy to host -- blocking semantics */
    }

    /* is the host sending data? */
    if (FD_ISSET(isosockfd,&rdfdset)) {
      if ((iolen = read(isosockfd,buf,sizeof(buf))) <= 0)
        break;   /* zero length means the host disconnected */

      write(usersockfd,buf,iolen);  /* copy to client -- blocking semantics */
    }
  }

  /* we're done with the sockets */
  close(isosockfd);
  close(usersockfd);
}



/****************************************************************************
  function:      errorout
  description:   displays an error message on the console and kills the
                 current process.
  arguments:
    msg          message to be displayed.

  return value:  none -- does not return.
  calls:         none.
  globals:       none.
 ****************************************************************************/

void errorout (msg)
char *msg;
{
  FILE *console;

  console = fopen("/dev/console","a");
  fprintf(console,"proxyd: %s\r\n",msg);
  fclose(console);
  exit(1);
}



/****************************************************************************
  function:      reap_status
  description:   handle a SIGCLD signal by reaping the exit status of the
                 perished child, and discarding it.
  arguments:     none.
  return value:  none.
  calls:         none.
  globals:       none.
 ****************************************************************************/

void reap_status ()
{
  int pid;
  union wait status;

  while ((pid = wait3(&status,WNOHANG,NULL)) > 0)
    ;  /* loop while there are more dead children */
}

/*Compiled by:
cc -O2  -object -s \
        -arch m68k -arch i386 -arch hppa -arch sparc \
        proxyd.c -o proxyd


usage: proxyd

Which means:

proxyd localport-on-this-machine the-host-this-is-to  port-or-name

EXAMPLE:

Say you have a machine named MYHOST want to bind your port #777 to the TELNET port of the host REMOTEHOST.

Use: proxyd 777 REMOTEHOST telnet

Then if you did 'telnet MYHOST 777' you would actually connect to the TELNET port of the host REMOTEHOST.

As far as the 'service-name/port-number' goes: services can be referred to either by a number or a name.  For example, NNTP requests are usually on port 119, so if you wanted to connect directly to a machine to do NNTP you could do either:

telnet REMOTEHOST nntp
        or
telnet REMOTEHOST 119


NOTE/WARNING: Make sure to use an unused port on your machine!  If you are not sure, do this:

 nidump services .

to see a listing of all the services known on your machine.

This program does NOT always have to be run as root... Some ports are available even to regular users, it appears.
*/