path: root/socket.c

/* -*- c-file-style: "linux" -*-
   
   rsync -- fast file replication program
   
   Copyright (C) 1992-2001 by Andrew Tridgell <tridge@samba.org>
   Copyright (C) 2001, 2002 by Martin Pool <mbp@samba.org>
   
   This program is free software; you can redistribute it and/or modify
   it under the terms of the GNU General Public License as published by
   the Free Software Foundation; either version 2 of the License, or
   (at your option) any later version.
   
   This program is distributed in the hope that it will be useful,
   but WITHOUT ANY WARRANTY; without even the implied warranty of
   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
   GNU General Public License for more details.
   
   You should have received a copy of the GNU General Public License
   along with this program; if not, write to the Free Software
   Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
*/

/**
 * @file socket.c
 * 
 * Socket functions used in rsync.
 *
 * This file is now converted to use the new-style getaddrinfo()
 * interface, which supports IPv6 but is also supported on recent
 * IPv4-only machines.  On systems that don't have that interface, we
 * emulate it using the KAME implementation.
 **/

#include "rsync.h"

static const char default_name[] = "UNKNOWN";


/* Establish a proxy connection on an open socket to a web roxy by
 * using the CONNECT method. */
static int establish_proxy_connection(int fd, char *host, int port)
{
	char buffer[1024];
	char *cp;

	snprintf(buffer, sizeof(buffer), "CONNECT %s:%d HTTP/1.0\r\n\r\n", host, port);
	if (write(fd, buffer, strlen(buffer)) != strlen(buffer)) {
		rprintf(FERROR, "failed to write to proxy: %s\n",
			strerror(errno));
		return -1;
	}

	for (cp = buffer; cp < &buffer[sizeof(buffer) - 1]; cp++) {
		if (read(fd, cp, 1) != 1) {
			rprintf(FERROR, "failed to read from proxy: %s\n",
				strerror(errno));
			return -1;
		}
		if (*cp == '\n')
			break;
	}

	if (*cp != '\n')
		cp++;
	*cp-- = '\0';
	if (*cp == '\r')
		*cp = '\0';
	if (strncmp(buffer, "HTTP/", 5) != 0) {
		rprintf(FERROR, "bad response from proxy - %s\n",
			buffer);
		return -1;
	}
	for (cp = &buffer[5]; isdigit(*cp) || (*cp == '.'); cp++)
		;
	while (*cp == ' ')
		cp++;
	if (*cp != '2') {
		rprintf(FERROR, "bad response from proxy - %s\n",
			buffer);
		return -1;
	}
	/* throw away the rest of the HTTP header */
	while (1) {
		for (cp = buffer; cp < &buffer[sizeof(buffer) - 1];
		     cp++) {
			if (read(fd, cp, 1) != 1) {
				rprintf(FERROR, "failed to read from proxy: %s\n",
					strerror(errno));
				return -1;
			}
			if (*cp == '\n')
				break;
		}
		if ((cp > buffer) && (*cp == '\n'))
			cp--;
		if ((cp == buffer) && ((*cp == '\n') || (*cp == '\r')))
			break;
	}
	return 0;
}


/**
 * Try to set the local address for a newly-created socket.  Return -1
 * if this fails.
 **/
int try_bind_local(int s,
		   int ai_family, int ai_socktype,
		   const char *bind_address)
{
	int error;
	struct addrinfo bhints, *bres_all, *r;

	memset(&bhints, 0, sizeof(bhints));
	bhints.ai_family = ai_family;
	bhints.ai_socktype = ai_socktype;
	bhints.ai_flags = AI_PASSIVE;
	if ((error = getaddrinfo(bind_address, NULL, &bhints, &bres_all))) {
		rprintf(FERROR, RSYNC_NAME ": getaddrinfo %s: %s\n",
			bind_address, gai_strerror(error));
		return -1;
	}

	for (r = bres_all; r; r = r->ai_next) {
		if (bind(s, r->ai_addr, r->ai_addrlen) == -1)
			continue;
		return s;
	}

	/* no error message; there might be some problem that allows
	 * creation of the socket but not binding, perhaps if the
	 * machine has no ipv6 address of this name. */
	return -1;
}


/**
 * Open a socket to a tcp remote host with the specified port .
 *
 * Based on code from Warren.  Proxy support by Stephen Rothwell.
 * getaddrinfo() rewrite contributed by KAME.net.
 *
 * Now that we support IPv6 we need to look up the remote machine's
 * address first, using @p af_hint to set a preference for the type
 * of address.  Then depending on whether it has v4 or v6 addresses we
 * try to open a connection.
 *
 * The loop allows for machines with some addresses which may not be
 * reachable, perhaps because we can't e.g. route ipv6 to that network
 * but we can get ip4 packets through.
 *
 * @param bind_address Local address to use.  Normally NULL to bind
 * the wildcard address.
 *
 * @param af_hint Address family, e.g. AF_INET or AF_INET6.
 **/
int open_socket_out(char *host, int port, const char *bind_address,
		    int af_hint)
{
	int type = SOCK_STREAM;
	int error;
	int s;
	struct addrinfo hints, *res0, *res;
	char portbuf[10];
	char *h;
	int proxied = 0;
	char buffer[1024];
	char *cp;

	/* if we have a RSYNC_PROXY env variable then redirect our
	 * connetcion via a web proxy at the given address. The format
	 * is hostname:port */
	h = getenv("RSYNC_PROXY");
	proxied = (h != NULL) && (*h != '\0');

	if (proxied) {
		strlcpy(buffer, h, sizeof(buffer));
		cp = strchr(buffer, ':');
		if (cp == NULL) {
			rprintf(FERROR,
				"invalid proxy specification: should be HOST:PORT\n");
			return -1;
		}
		*cp++ = '\0';
		strcpy(portbuf, cp);
		h = buffer;
	} else {
		snprintf(portbuf, sizeof(portbuf), "%d", port);
		h = host;
	}

	memset(&hints, 0, sizeof(hints));
	hints.ai_family = af_hint;
	hints.ai_socktype = type;
	error = getaddrinfo(h, portbuf, &hints, &res0);
	if (error) {
		rprintf(FERROR, RSYNC_NAME ": getaddrinfo: %s %s: %s\n",
			h, portbuf, gai_strerror(error));
		return -1;
	}

	s = -1;
	/* Try to connect to all addresses for this machine until we get
	 * through.  It might e.g. be multi-homed, or have both IPv4 and IPv6
	 * addresses.  We need to create a socket for each record, since the
	 * address record tells us what protocol to use to try to connect. */
	for (res = res0; res; res = res->ai_next) {
		s = socket(res->ai_family, res->ai_socktype, res->ai_protocol);
		if (s < 0)
			continue;

		if (bind_address)
			if (try_bind_local(s, res->ai_family, type,
					   bind_address) == -1) {
				close(s);
				s = -1;
				continue;
			}

		if (connect(s, res->ai_addr, res->ai_addrlen) < 0) {
			close(s);
			s = -1;
			continue;
		}
		if (proxied &&
		    establish_proxy_connection(s, host, port) != 0) {
			close(s);
			s = -1;
			continue;
		} else
			break;
	}
	freeaddrinfo(res0);
	if (s < 0) {
		rprintf(FERROR, RSYNC_NAME ": failed to connect to %s: %s\n",
			h, strerror(errno));
		return -1;
	}
	return s;
}


/**
 * Open an outgoing socket, but allow for it to be intercepted by
 * $RSYNC_CONNECT_PROG, which will execute a program across a TCP
 * socketpair rather than really opening a socket.
 *
 * We use this primarily in testing to detect TCP flow bugs, but not
 * cause security problems by really opening remote connections.
 *
 * This is based on the Samba LIBSMB_PROG feature.
 *
 * @param bind_address Local address to use.  Normally NULL to get the stack default.
 **/
int open_socket_out_wrapped (char *host,
			     int port,
			     const char *bind_address,
			     int af_hint)
{
	char *prog;

	if ((prog = getenv ("RSYNC_CONNECT_PROG")) != NULL) 
		return sock_exec (prog);
	else 
		return open_socket_out (host, port, bind_address,
					af_hint);
}



/**
 * Open a socket of the specified type, port and address for incoming data
 *
 * Try to be better about handling the results of getaddrinfo(): when
 * opening an inbound socket, we might get several address results,
 * e.g. for the machine's ipv4 and ipv6 name.  
 * 
 * If binding a wildcard, then any one of them should do.  If an address
 * was specified but it's insufficiently specific then that's not our
 * fault.  
 * 
 * However, some of the advertized addresses may not work because e.g. we
 * don't have IPv6 support in the kernel.  In that case go on and try all
 * addresses until one succeeds.
 * 
 * @param bind_address Local address to bind, or NULL to allow it to
 * default.
 **/
static int open_socket_in(int type, int port, const char *bind_address,
			  int af_hint)
{
	int one=1;
	int s;
	struct addrinfo hints, *all_ai, *resp;
	char portbuf[10];
	int error;

	memset(&hints, 0, sizeof(hints));
	hints.ai_family = af_hint;
	hints.ai_socktype = type;
	hints.ai_flags = AI_PASSIVE;
	snprintf(portbuf, sizeof(portbuf), "%d", port);
	error = getaddrinfo(bind_address, portbuf, &hints, &all_ai);
	if (error) {
		rprintf(FERROR, RSYNC_NAME ": getaddrinfo: bind address %s: %s\n",
			bind_address, gai_strerror(error));
		return -1;
	}

	/* We may not be able to create the socket, if for example the
	 * machine knows about IPv6 in the C library, but not in the
	 * kernel. */
	for (resp = all_ai; resp; resp = resp->ai_next) {
		s = socket(resp->ai_family, resp->ai_socktype,
			   resp->ai_protocol);

		if (s == -1) 
			/* See if there's another address that will work... */
			continue;
		
		setsockopt(s, SOL_SOCKET, SO_REUSEADDR,
			   (char *)&one, sizeof one);
		
		/* now we've got a socket - we need to bind it */
		if (bind(s, all_ai->ai_addr, all_ai->ai_addrlen) < 0) {
			/* Nope, try another */
			close(s);
			continue;
		}
		
		return s;
	}

	rprintf(FERROR, RSYNC_NAME ": open inbound socket on port %d failed: "
		"%s\n",
		port, 
		strerror(errno));

	freeaddrinfo(all_ai);
	return -1; 
}


/*
 * Determine if a file descriptor is in fact a socket
 */
int is_a_socket(int fd)
{
	int v;
	socklen_t l;
	l = sizeof(int);

        /* Parameters to getsockopt, setsockopt etc are very
         * unstandardized across platforms, so don't be surprised if
         * there are compiler warnings on e.g. SCO OpenSwerver or AIX.
         * It seems they all eventually get the right idea.
         *
         * Debian says: ``The fifth argument of getsockopt and
         * setsockopt is in reality an int [*] (and this is what BSD
         * 4.* and libc4 and libc5 have).  Some POSIX confusion
         * resulted in the present socklen_t.  The draft standard has
         * not been adopted yet, but glibc2 already follows it and
         * also has socklen_t [*]. See also accept(2).''
         *
         * We now return to your regularly scheduled programming.  */
	return(getsockopt(fd, SOL_SOCKET, SO_TYPE, (char *)&v, &l) == 0);
}


void start_accept_loop(int port, int (*fn)(int ))
{
	int s;
	extern char *bind_address;
	extern int default_af_hint;

	/* open an incoming socket */
	s = open_socket_in(SOCK_STREAM, port, bind_address, default_af_hint);
	if (s == -1)
		exit_cleanup(RERR_SOCKETIO);

	/* ready to listen */
	if (listen(s, 5) == -1) {
		close(s);
		exit_cleanup(RERR_SOCKETIO);
	}


	/* now accept incoming connections - forking a new process
	   for each incoming connection */
	while (1) {
		fd_set fds;
		int fd;
		struct sockaddr_storage addr;
		socklen_t addrlen = sizeof addr;

		/* close log file before the potentially very long select so
		   file can be trimmed by another process instead of growing
		   forever */
		log_close();

		FD_ZERO(&fds);
		FD_SET(s, &fds);

		if (select(s+1, &fds, NULL, NULL, NULL) != 1) {
			continue;
		}

		if(!FD_ISSET(s, &fds)) continue;

		fd = accept(s,(struct sockaddr *)&addr,&addrlen);

		if (fd == -1) continue;

		signal(SIGCHLD, SIG_IGN);

		/* we shouldn't have any children left hanging around
		   but I have had reports that on Digital Unix zombies
		   are produced, so this ensures that they are reaped */
#ifdef WNOHANG
                while (waitpid(-1, NULL, WNOHANG) > 0);
#endif

		if (fork()==0) {
			close(s);
			/* open log file in child before possibly giving
			   up privileges  */
			log_open();
			_exit(fn(fd));
		}

		close(fd);
	}
}


enum SOCK_OPT_TYPES {OPT_BOOL,OPT_INT,OPT_ON};

struct
{
  char *name;
  int level;
  int option;
  int value;
  int opttype;
} socket_options[] = {
  {"SO_KEEPALIVE",      SOL_SOCKET,    SO_KEEPALIVE,    0,                 OPT_BOOL},
  {"SO_REUSEADDR",      SOL_SOCKET,    SO_REUSEADDR,    0,                 OPT_BOOL},
  {"SO_BROADCAST",      SOL_SOCKET,    SO_BROADCAST,    0,                 OPT_BOOL},
#ifdef TCP_NODELAY
  {"TCP_NODELAY",       IPPROTO_TCP,   TCP_NODELAY,     0,                 OPT_BOOL},
#endif
#ifdef IPTOS_LOWDELAY
  {"IPTOS_LOWDELAY",    IPPROTO_IP,    IP_TOS,          IPTOS_LOWDELAY,    OPT_ON},
#endif
#ifdef IPTOS_THROUGHPUT
  {"IPTOS_THROUGHPUT",  IPPROTO_IP,    IP_TOS,          IPTOS_THROUGHPUT,  OPT_ON},
#endif
#ifdef SO_SNDBUF
  {"SO_SNDBUF",         SOL_SOCKET,    SO_SNDBUF,       0,                 OPT_INT},
#endif
#ifdef SO_RCVBUF
  {"SO_RCVBUF",         SOL_SOCKET,    SO_RCVBUF,       0,                 OPT_INT},
#endif
#ifdef SO_SNDLOWAT
  {"SO_SNDLOWAT",       SOL_SOCKET,    SO_SNDLOWAT,     0,                 OPT_INT},
#endif
#ifdef SO_RCVLOWAT
  {"SO_RCVLOWAT",       SOL_SOCKET,    SO_RCVLOWAT,     0,                 OPT_INT},
#endif
#ifdef SO_SNDTIMEO
  {"SO_SNDTIMEO",       SOL_SOCKET,    SO_SNDTIMEO,     0,                 OPT_INT},
#endif
#ifdef SO_RCVTIMEO
  {"SO_RCVTIMEO",       SOL_SOCKET,    SO_RCVTIMEO,     0,                 OPT_INT},
#endif
  {NULL,0,0,0,0}};

	

/****************************************************************************
set user socket options
****************************************************************************/
void set_socket_options(int fd, char *options)
{
	char *tok;
	if (!options || !*options) return;

	options = strdup(options);
	
	if (!options) out_of_memory("set_socket_options");

	for (tok=strtok(options, " \t,"); tok; tok=strtok(NULL," \t,")) {
		int ret=0,i;
		int value = 1;
		char *p;
		int got_value = 0;

		if ((p = strchr(tok,'='))) {
			*p = 0;
			value = atoi(p+1);
			got_value = 1;
		}

		for (i=0;socket_options[i].name;i++)
			if (strcmp(socket_options[i].name,tok)==0)
				break;

		if (!socket_options[i].name) {
			rprintf(FERROR,"Unknown socket option %s\n",tok);
			continue;
		}

		switch (socket_options[i].opttype) {
		case OPT_BOOL:
		case OPT_INT:
			ret = setsockopt(fd,socket_options[i].level,
					 socket_options[i].option,(char *)&value,sizeof(int));
			break;
			
		case OPT_ON:
			if (got_value)
				rprintf(FERROR,"syntax error - %s does not take a value\n",tok);

			{
				int on = socket_options[i].value;
				ret = setsockopt(fd,socket_options[i].level,
						 socket_options[i].option,(char *)&on,sizeof(int));
			}
			break;	  
		}
		
		if (ret != 0)
			rprintf(FERROR, "failed to set socket option %s: %s\n", tok,
				strerror(errno));
	}

	free(options);
}

/****************************************************************************
become a daemon, discarding the controlling terminal
****************************************************************************/
void become_daemon(void)
{
	int i;

	if (fork()) {
		_exit(0);
	}

	/* detach from the terminal */
#ifdef HAVE_SETSID
	setsid();
#else
#ifdef TIOCNOTTY
	i = open("/dev/tty", O_RDWR);
	if (i >= 0) {
		ioctl(i, (int) TIOCNOTTY, (char *)0);      
		close(i);
	}
#endif /* TIOCNOTTY */
#endif
	/* make sure that stdin, stdout an stderr don't stuff things
           up (library functions, for example) */
	for (i=0;i<3;i++) {
		close(i); 
		open("/dev/null", O_RDWR);
	}
}

/**
 * Return the IP addr of the client as a string 
 **/
char *client_addr(int fd)
{
	struct sockaddr_storage ss;
	socklen_t length = sizeof ss;
	static char addr_buf[100];
	static int initialised;

	if (initialised) return addr_buf;

	initialised = 1;

	if (getpeername(fd, (struct sockaddr *)&ss, &length)) {
		exit_cleanup(RERR_SOCKETIO);
	}

	getnameinfo((struct sockaddr *)&ss, length,
		addr_buf, sizeof(addr_buf), NULL, 0, NI_NUMERICHOST);
	return addr_buf;
}


static int get_sockaddr_family(const struct sockaddr_storage *ss)
{
	return ((struct sockaddr *) ss)->sa_family;
}


/**
 * Return the DNS name of the client.
 *
 * The name is statically cached so that repeated lookups are quick,
 * so there is a limit of one lookup per customer.
 *
 * If anything goes wrong, including the name->addr->name check, then
 * we just use "UNKNOWN", so you can use that value in hosts allow
 * lines.
 **/
char *client_name(int fd)
{
	struct sockaddr_storage ss;
	socklen_t ss_len = sizeof ss;
	static char name_buf[100];
	static char port_buf[100];
	static int initialised;

	if (initialised) return name_buf;

	strcpy(name_buf, default_name);
	initialised = 1;

	if (getpeername(fd, (struct sockaddr *)&ss, &ss_len)) {
		/* FIXME: Can we really not continue? */
		rprintf(FERROR, RSYNC_NAME ": getpeername on fd%d failed: %s\n",
			fd, strerror(errno));
		exit_cleanup(RERR_SOCKETIO);
	}

	if (!lookup_name(fd, &ss, ss_len, name_buf, sizeof name_buf, port_buf, sizeof port_buf))
		check_name(fd, &ss, ss_len, name_buf, port_buf);

	return name_buf;
}


/**
 * Look up a name from @p ss into @p name_buf.
 **/
int lookup_name(int fd, const struct sockaddr_storage *ss,
		socklen_t ss_len,
		char *name_buf, size_t name_buf_len,
		char *port_buf, size_t port_buf_len)
{
	int name_err;
	
#ifdef INET6
        if (get_sockaddr_family(ss) == AF_INET6 && 
	    IN6_IS_ADDR_V4MAPPED(&((struct sockaddr_in6 *)ss)->sin6_addr)) {
		/* OK, so ss is in the IPv6 family, but it is really
		 * an IPv4 address: something like
		 * "::ffff:10.130.1.2".  If we use it as-is, then the
		 * reverse lookup might fail or perhaps something else
		 * bad might happen.  So instead we convert it to an
		 * equivalent address in the IPv4 address family.  */
		struct sockaddr_in6 sin6;
		struct sockaddr_in *sin;

		memcpy(&sin6, ss, sizeof(sin6));
		sin = (struct sockaddr_in *)ss;
		memset(sin, 0, sizeof(*sin));
		sin->sin_family = AF_INET;
		ss_len = sizeof(struct sockaddr_in);
#ifdef HAVE_SOCKADDR_LEN
		sin->sin_len = ss_len;
#endif
		sin->sin_port = sin6.sin6_port;
		/* FIXME: Isn't there a macro we can use here rather
		 * than grovelling through the struct?  It might be
		 * wrong on some systems. */
		memcpy(&sin->sin_addr, &sin6.sin6_addr.s6_addr[12],
			sizeof(sin->sin_addr));
        }
#endif

	/* reverse lookup */
	name_err = getnameinfo((struct sockaddr *) ss, ss_len,
			       name_buf, name_buf_len,
			       port_buf, port_buf_len,
			       NI_NAMEREQD | NI_NUMERICSERV);
	if (name_err != 0) {
		strcpy(name_buf, default_name);
		rprintf(FERROR, RSYNC_NAME ": name lookup failed for %s: %s\n",
			client_addr(fd),
			gai_strerror(name_err));
		return name_err;
	}

	return 0;
}



/* Do a forward lookup on name_buf and make sure it corresponds to ss
 * -- otherwise we may be being spoofed.  If we suspect we are, then
 * we don't abort the connection but just emit a warning. */
int check_name(int fd,
	       const struct sockaddr_storage *ss,
	       socklen_t ss_len,
	       char *name_buf,
	       const char *port_buf)
{
	struct addrinfo hints, *res, *res0;
	int error;

	memset(&hints, 0, sizeof(hints));
	hints.ai_family = PF_UNSPEC;
	hints.ai_flags = AI_CANONNAME;
	hints.ai_socktype = SOCK_STREAM;
	error = getaddrinfo(name_buf, port_buf, &hints, &res0);
	if (error) {
		strcpy(name_buf, default_name);
		rprintf(FERROR,
			RSYNC_NAME ": forward name lookup for %s:%s failed: %s\n",
			name_buf, port_buf,
			gai_strerror(error));
		return error;
	}


	/* We expect that one of the results will be the same as ss. */
	for (res = res0; res; res = res->ai_next) {
		if (res->ai_family != get_sockaddr_family(ss))
			continue;
		if (res->ai_addrlen != ss_len)
			continue;
		if (memcmp(res->ai_addr, ss, res->ai_addrlen) == 0)
			break;
	}

	if (res == NULL) {
		strcpy(name_buf, default_name);
		/* We hit the end of the list without finding an
		 * address that was the same as ss. */
		rprintf(FERROR, RSYNC_NAME
			": no address record for \"%s\" corresponds to address %s: "
			"spoofed address?\n",
			name_buf,
			client_addr(fd));
	}

	freeaddrinfo(res0);
	return 0;
}


/*******************************************************************
this is like socketpair but uses tcp. It is used by the Samba
regression test code
The function guarantees that nobody else can attach to the socket,
or if they do that this function fails and the socket gets closed
returns 0 on success, -1 on failure
the resulting file descriptors are symmetrical
 ******************************************************************/
static int socketpair_tcp(int fd[2])
{
	int listener;
	struct sockaddr_in sock;
	struct sockaddr_in sock2;
	socklen_t socklen = sizeof(sock);
	int connect_done = 0;
	
	fd[0] = fd[1] = listener = -1;

	memset(&sock, 0, sizeof(sock));
	
	if ((listener = socket(PF_INET, SOCK_STREAM, 0)) == -1) goto failed;

        memset(&sock2, 0, sizeof(sock2));
#ifdef HAVE_SOCK_SIN_LEN
        sock2.sin_len = sizeof(sock2);
#endif
        sock2.sin_family = PF_INET;

        bind(listener, (struct sockaddr *)&sock2, sizeof(sock2));

	if (listen(listener, 1) != 0) goto failed;

	if (getsockname(listener, (struct sockaddr *)&sock, &socklen) != 0) goto failed;

	if ((fd[1] = socket(PF_INET, SOCK_STREAM, 0)) == -1) goto failed;

	set_nonblocking(fd[1]);

	sock.sin_addr.s_addr = htonl(INADDR_LOOPBACK);

	if (connect(fd[1],(struct sockaddr *)&sock,sizeof(sock)) == -1) {
		if (errno != EINPROGRESS) goto failed;
	} else {
		connect_done = 1;
	}

	if ((fd[0] = accept(listener, (struct sockaddr *)&sock, &socklen)) == -1) goto failed;

	close(listener);
	if (connect_done == 0) {
		if (connect(fd[1],(struct sockaddr *)&sock,sizeof(sock)) != 0
		    && errno != EISCONN) goto failed;
	}

	set_blocking (fd[1]);

	/* all OK! */
	return 0;

 failed:
	if (fd[0] != -1) close(fd[0]);
	if (fd[1] != -1) close(fd[1]);
	if (listener != -1) close(listener);
	return -1;
}



/**
 * Run a program on a local tcp socket, so that we can talk to it's
 * stdin and stdout.  This is used to fake a connection to a daemon
 * for testing -- not for the normal case of running SSH.
 *
 * @return a socket which is attached to a subprocess running
 * "prog". stdin and stdout are attached. stderr is left attached to
 * the original stderr
 **/
int sock_exec(const char *prog)
{
	int fd[2];
	
	if (socketpair_tcp(fd) != 0) {
		rprintf (FERROR, RSYNC_NAME
			 ": socketpair_tcp failed (%s)\n",
			 strerror(errno));
		return -1;
	}
	if (fork() == 0) {
		close(fd[0]);
		close(0);
		close(1);
		dup(fd[1]);
		dup(fd[1]);
		if (verbose > 3) {
			/* Can't use rprintf because we've forked. */
			fprintf (stderr,
				 RSYNC_NAME ": execute socket program \"%s\"\n",
				 prog);
		}
		exit (system (prog));
	}
	close (fd[1]);
	return fd[0];
}