dev/tcploop/tcploop.c

/*
 * TCP client and server for bug hunting
 *
 * Copyright (C) 2016 Willy Tarreau <w@1wt.eu>
 *
 * Permission is hereby granted, free of charge, to any person obtaining
 * a copy of this software and associated documentation files (the
 * "Software"), to deal in the Software without restriction, including
 * without limitation the rights to use, copy, modify, merge, publish,
 * distribute, sublicense, and/or sell copies of the Software, and to
 * permit persons to whom the Software is furnished to do so, subject to
 * the following conditions:
 *
 * The above copyright notice and this permission notice shall be
 * included in all copies or substantial portions of the Software.
 *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
 * EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES
 * OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
 * NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT
 * HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY,
 * WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
 * FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR
 * OTHER DEALINGS IN THE SOFTWARE.
 */

#define _GNU_SOURCE     // for POLLRDHUP
#include <sys/resource.h>
#include <sys/select.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <sys/stat.h>
#include <sys/time.h>
#include <sys/ioctl.h>
#include <sys/wait.h>
#include <arpa/inet.h>
#include <netinet/in.h>
#include <netinet/tcp.h>

#include <ctype.h>
#include <errno.h>
#include <fcntl.h>
#include <limits.h>
#include <netdb.h>
#include <poll.h>
#include <signal.h>
#include <stdarg.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <time.h>
#include <unistd.h>

/* for OSes which don't have it */
#ifndef POLLRDHUP
#define POLLRDHUP 0
#endif

#ifndef MSG_MORE
#define MSG_MORE 0
#endif

struct err_msg {
	int size;
	int len;
	char msg[0];
};

const int zero = 0;
const int one = 1;
const struct linger nolinger = { .l_onoff = 1, .l_linger = 0 };

#define TRASH_SIZE 65536
static char trash[TRASH_SIZE];

volatile int nbproc = 0;
static struct timeval start_time;
static int showtime;
static int verbose;
static int pid;
static int sock_type = SOCK_STREAM;
static int sock_proto = IPPROTO_TCP;


/* display the message and exit with the code */
__attribute__((noreturn)) void die(int code, const char *format, ...)
{
	va_list args;

	if (format) {
		va_start(args, format);
		vfprintf(stderr, format, args);
		va_end(args);
	}
	exit(code);
}

/* display the usage message and exit with the code */
__attribute__((noreturn)) void usage(int code, const char *arg0)
{
	die(code,
	    "Usage : %s [options]* [<ip>:]port [<action>*]\n"
	    "\n"
	    "options :\n"
	    "  -v           : verbose\n"
	    "  -u           : use UDP instead of TCP (limited)\n"
	    "  -t|-tt|-ttt  : show time (msec / relative / absolute)\n"
	    "actions :\n"
	    "  A[<count>]   : Accepts <count> incoming sockets and closes count-1\n"
	    "                 Note: fd=accept(fd)\n"
	    "  B[[ip]:port] : Bind a new socket to ip:port or default one if unspecified.\n"
	    "                 Note: fd=socket,bind(fd)\n"
	    "  C[[ip]:port] : Connects to ip:port or default ones if unspecified.\n"
	    "                 Note: fd=socket,connect(fd)\n"
	    "  D            : Disconnect (connect to AF_UNSPEC)\n"
	    "  E[<size>]    : Echo this amount of bytes. 0=infinite. unset=any amount.\n"
	    "  F            : FIN : shutdown(SHUT_WR)\n"
	    "  G            : disable lingering\n"
	    "  I            : wait for Input data to be present (POLLIN)\n"
	    "  J            : Jump back to oldest post-fork/post-accept action\n"
	    "  K            : kill the connection and go on with next operation\n"
	    "  L[<backlog>] : Listens to ip:port and optionally sets backlog\n"
	    "                 Note: fd=socket,bind(fd),listen(fd)\n"
	    "  N<max>       : fork New process, limited to <max> concurrent (default 1)\n"
	    "  O            : wait for Output queue to be empty (POLLOUT + TIOCOUTQ)\n"
	    "  P[<time>]    : Pause for <time> ms (100 by default)\n"
	    "  Q            : disable TCP Quick-ack\n"
	    "  R[<size>]    : Read this amount of bytes. 0=infinite. unset=any amount.\n"
	    "  S[<size>]    : Send this amount of bytes. 0=infinite. unset=any amount.\n"
	    "  S:<string>   : Send this exact string. \\r, \\n, \\t, \\\\ supported.\n"
	    "  T            : set TCP_NODELAY\n"
	    "  W[<time>]    : Wait for any event on the socket, maximum <time> ms\n"
	    "  X[i|o|e]* ** : execvp() next args passing socket as stdin/stdout/stderr.\n"
	    "                 If i/o/e present, only stdin/out/err are mapped to socket.\n"
	    "  r            : shutr : shutdown(SHUT_RD) (pauses a listener or ends recv)\n"
	    "\n"
	    "It's important to note that a single FD is used at once and that Accept\n"
	    "replaces the listening FD with the accepted one. Thus always do it after\n"
	    "a fork if other connections have to be accepted.\n"
	    "\n"
	    "After a fork, we loop back to the beginning and silently skip L/C if the\n"
	    "main socket already exists.\n"
	    "\n"
	    "Example dummy HTTP request drain server :\n"
	    "   tcploop 8001 L W N20 A R S10 [ F K ]\n"
	    "\n"
	    "Example large bandwidth HTTP request drain server :\n"
	    "   tcploop 8001 L W N20 A R S0 [ F K ]\n"
	    "\n"
	    "Example TCP client with pauses at each step :\n"
	    "   tcploop 8001 C T W P100 S10 O P100 R S10 O R G K\n"
	    "\n"
	    "Simple chargen server :\n"
	    "   tcploop 8001 L A Xo cat /dev/zero\n"
	    "\n"
	    "Simple telnet server :\n"
	    "   tcploop 8001 L W N A X /usr/sbin/in.telnetd\n"
	    "", arg0);
}

void dolog(const char *format, ...)
{
	struct timeval date, tv;
	int delay;
	va_list args;

	if (!verbose)
		return;

	if (showtime) {
		gettimeofday(&date, NULL);
		switch (showtime) {
		case 1: // [msec] relative
			delay = (date.tv_sec - start_time.tv_sec) * 1000000 + date.tv_usec - start_time.tv_usec;
			fprintf(stderr, "[%d] ", delay / 1000);
			break;
		case 2: // [sec.usec] relative
			tv.tv_usec = date.tv_usec - start_time.tv_usec;
			tv.tv_sec  = date.tv_sec  - start_time.tv_sec;
			if ((signed)tv.tv_sec > 0) {
				if ((signed)tv.tv_usec < 0) {
					tv.tv_usec += 1000000;
					tv.tv_sec--;
				}
			} else if (tv.tv_sec == 0) {
				if ((signed)tv.tv_usec < 0)
					tv.tv_usec = 0;
			} else {
				tv.tv_sec = 0;
				tv.tv_usec = 0;
			}
			fprintf(stderr, "[%d.%06d] ", (int)tv.tv_sec, (int)tv.tv_usec);
			break;
		default: // [sec.usec] absolute
			fprintf(stderr, "[%d.%06d] ", (int)date.tv_sec, (int)date.tv_usec);
			break;
		}
	}

	fprintf(stderr, "%5d ", pid);

	va_start(args, format);
	vfprintf(stderr, format, args);
	va_end(args);
}

/* convert '\n', '\t', '\r', '\\' to their respective characters */
int unescape(char *out, int size, const char *in)
{
	int len;

	for (len = 0; len < size && *in; in++, out++, len++) {
		if (*in == '\\') {
			switch (in[1]) {
			case  'n' : *out = '\n'; in++; continue;
			case  't' : *out = '\t'; in++; continue;
			case  'r' : *out = '\r'; in++; continue;
			case '\\' : *out = '\\'; in++; continue;
			default   : break;
			}
		}
		*out = *in;
	}
	return len;
}

struct err_msg *alloc_err_msg(int size)
{
	struct err_msg *err;

	err = malloc(sizeof(*err) + size);
	if (err) {
		err->len = 0;
		err->size = size;
	}
	return err;
}

void sig_handler(int sig)
{
	if (sig == SIGCHLD) {
		while (waitpid(-1, NULL, WNOHANG) > 0)
			__sync_sub_and_fetch(&nbproc, 1);
	}
}

/* converts str in the form [[<ipv4>|<ipv6>|<hostname>]:]port to struct sockaddr_storage.
 * Returns < 0 with err set in case of error.
 */
int addr_to_ss(const char *str, struct sockaddr_storage *ss, struct err_msg *err)
{
	char *port_str;
	int port;

	memset(ss, 0, sizeof(*ss));

	/* look for the addr/port delimiter, it's the last colon. If there's no
	 * colon, it's 0:<port>.
	 */
	if ((port_str = strrchr(str, ':')) == NULL) {
		port = atoi(str);
		if (port < 0 || port > 65535) {
			err->len = snprintf(err->msg, err->size, "Missing/invalid port number: '%s'\n", str);
			return -1;
		}

		ss->ss_family = AF_INET;
		((struct sockaddr_in *)ss)->sin_port = htons(port);
		((struct sockaddr_in *)ss)->sin_addr.s_addr = INADDR_ANY;
		return 0;
	}

	*port_str++ = 0;

	if (strrchr(str, ':') != NULL) {
		/* IPv6 address contains ':' */
		ss->ss_family = AF_INET6;
		((struct sockaddr_in6 *)ss)->sin6_port = htons(atoi(port_str));

		if (!inet_pton(ss->ss_family, str, &((struct sockaddr_in6 *)ss)->sin6_addr)) {
			err->len = snprintf(err->msg, err->size, "Invalid server address: '%s'\n", str);
			return -1;
		}
	}
	else {
		ss->ss_family = AF_INET;
		((struct sockaddr_in *)ss)->sin_port = htons(atoi(port_str));

		if (*str == '*' || *str == '\0') { /* INADDR_ANY */
			((struct sockaddr_in *)ss)->sin_addr.s_addr = INADDR_ANY;
			return 0;
		}

		if (!inet_pton(ss->ss_family, str, &((struct sockaddr_in *)ss)->sin_addr)) {
			struct hostent *he = gethostbyname(str);

			if (he == NULL) {
				err->len = snprintf(err->msg, err->size, "Invalid server name: '%s'\n", str);
				return -1;
			}
			((struct sockaddr_in *)ss)->sin_addr = *(struct in_addr *) *(he->h_addr_list);
		}
	}

	return 0;
}

/* waits up to <ms> milliseconds on fd <fd> for events <events> (POLLIN|POLLRDHUP|POLLOUT).
 * returns poll's status, or -2 if the poller sets POLLERR.
 */
int wait_on_fd(int fd, int events, int ms)
{
	struct pollfd pollfd;
	int ret;

	do {
		pollfd.fd = fd;
		pollfd.events = events;
		ret = poll(&pollfd, 1, ms);
	} while (ret == -1 && errno == EINTR);

	if (ret == 1 && pollfd.revents & POLLERR)
		ret = -2;
	return ret;
}

int tcp_set_nodelay(int sock, const char *arg)
{
	return setsockopt(sock, IPPROTO_TCP, TCP_NODELAY, &one, sizeof(one));
}

int tcp_set_nolinger(int sock, const char *arg)
{
	return setsockopt(sock, SOL_SOCKET, SO_LINGER, (struct linger *) &nolinger, sizeof(struct linger));
}

int tcp_set_noquickack(int sock, const char *arg)
{
#ifdef TCP_QUICKACK
	/* warning: do not use during connect if nothing is to be sent! */
	return setsockopt(sock, IPPROTO_TCP, TCP_QUICKACK, &zero, sizeof(zero));
#else
	return 0;
#endif
}

/* Create a new TCP socket for either listening or connecting */
int tcp_socket()
{
	int sock;

	sock = socket(AF_INET, sock_type, sock_proto);
	if (sock < 0) {
		perror("socket()");
		return -1;
	}

	return sock;
}

/* Try to bind to local address <sa>. Return the fd or -1 in case of error.
 * Supports being passed NULL for arg if none has to be passed.
 */
int tcp_bind(int sock, const struct sockaddr_storage *sa, const char *arg)
{
	struct sockaddr_storage conn_addr;

	if (arg && arg[1]) {
		struct err_msg err;

		if (addr_to_ss(arg + 1, &conn_addr, &err) < 0)
			die(1, "%s\n", err.msg);
		sa = &conn_addr;
	}


	if (sock < 0) {
		sock = tcp_socket();
		if (sock < 0)
			return sock;
	}

	if (setsockopt(sock, SOL_SOCKET, SO_REUSEADDR, &one, sizeof(one)) == -1) {
		perror("setsockopt(SO_REUSEADDR)");
		goto fail;
	}

#ifdef SO_REUSEPORT
	if (setsockopt(sock, SOL_SOCKET, SO_REUSEPORT, (char *) &one, sizeof(one)) == -1) {
		perror("setsockopt(SO_REUSEPORT)");
		goto fail;
	}
#endif
	if (bind(sock, (struct sockaddr *)sa, sa->ss_family == AF_INET6 ?
		 sizeof(struct sockaddr_in6) : sizeof(struct sockaddr_in)) == -1) {
		perror("bind");
		goto fail;
	}

	return sock;
 fail:
	close(sock);
	return -1;
}

/* Try to listen to address <sa>. Return the fd or -1 in case of error */
int tcp_listen(int sock, const struct sockaddr_storage *sa, const char *arg)
{
	int backlog;

	if (sock < 0) {
		sock = tcp_bind(sock, sa, NULL);
		if (sock < 0)
			return sock;
	}

	if (arg[1])
		backlog = atoi(arg + 1);
	else
		backlog = 1000;

	if (backlog < 0 || backlog > 65535) {
		fprintf(stderr, "backlog must be between 0 and 65535 inclusive (was %d)\n", backlog);
		goto fail;
	}

	if (listen(sock, backlog) == -1) {
		perror("listen");
		goto fail;
	}

	return sock;
 fail:
	close(sock);
	return -1;
}

/* accepts a socket from listening socket <sock>, and returns it (or -1 in case of error) */
int tcp_accept(int sock, const char *arg)
{
	int count;
	int newsock;

	if (arg[1])
		count = atoi(arg + 1);
	else
		count = 1;

	if (count <= 0) {
		fprintf(stderr, "accept count must be > 0 or unset (was %d)\n", count);
		return -1;
	}

	do {
		newsock = accept(sock, NULL, NULL);
		if (newsock < 0) { // TODO: improve error handling
			if (errno == EINTR || errno == EAGAIN || errno == ECONNABORTED)
				continue;
			perror("accept()");
			break;
		}

		if (count > 1)
			close(newsock);
		count--;
	} while (count > 0);

	fcntl(newsock, F_SETFL, O_NONBLOCK);
	return newsock;
}

/* Try to establish a new connection to <sa>. Return the fd or -1 in case of error */
int tcp_connect(int sock, const struct sockaddr_storage *sa, const char *arg)
{
	struct sockaddr_storage conn_addr;

	if (arg[1]) {
		struct err_msg err;

		if (addr_to_ss(arg + 1, &conn_addr, &err) < 0)
			die(1, "%s\n", err.msg);
		sa = &conn_addr;
	}

	if (sock < 0) {
		sock = tcp_socket();
		if (sock < 0)
			return sock;
	}

	if (fcntl(sock, F_SETFL, O_NONBLOCK) == -1)
		goto fail;

	if (setsockopt(sock, SOL_SOCKET, SO_REUSEADDR, &one, sizeof(one)) == -1)
		goto fail;

	if (connect(sock, (const struct sockaddr *)sa, sizeof(struct sockaddr_in)) < 0) {
		if (errno != EINPROGRESS)
			goto fail;
	}

	return sock;
 fail:
	close(sock);
	return -1;
}

/* Try to disconnect by connecting to AF_UNSPEC. Return >=0 on success, -1 in case of error */
int tcp_disconnect(int sock)
{
	const struct sockaddr sa = { .sa_family = AF_UNSPEC };

	return connect(sock, &sa, sizeof(sa));
}

/* receives N bytes from the socket and returns 0 (or -1 in case of a recv
 * error, or -2 in case of an argument error). When no arg is passed, receives
 * anything and stops. Otherwise reads the requested amount of data. 0 means
 * read as much as possible.
 */
int tcp_recv(int sock, const char *arg)
{
	int count = -1; // stop at first read
	int ret;
	int max;

	if (arg[1]) {
		count = atoi(arg + 1);
		if (count < 0) {
			fprintf(stderr, "recv count must be >= 0 or unset (was %d)\n", count);
			return -2;
		}
	}

	while (1) {
		max = (count > 0) ? count : INT_MAX;
		if (max > sizeof(trash))
			max = sizeof(trash);
		ret = recv(sock, trash, max, MSG_NOSIGNAL | MSG_TRUNC);
		if (ret < 0) {
			if (errno == EINTR)
				continue;
			if (errno != EAGAIN) {
				dolog("recv %d\n", ret);
				return -1;
			}
			while (!wait_on_fd(sock, POLLIN | POLLRDHUP, 1000));
			continue;
		}
		dolog("recv %d\n", ret);
		if (!ret)
			break;

		if (!count)
			continue;
		else if (count > 0)
			count -= ret;

		if (count <= 0)
			break;
	}

	return 0;
}

/* Sends N bytes to the socket and returns 0 (or -1 in case of send error, -2
 * in case of an argument error. If the byte count is not set, sends only one
 * block. Sending zero means try to send forever. If the argument starts with
 * ':' then whatever follows is interpreted as the payload to be sent as-is.
 * Escaped characters '\r', '\n', '\t' and '\\' are detected and converted. In
 * this case, blocks must be small so that send() doesn't fragment them, as
 * they will be put into the trash and expected to be sent at once.
 */
int tcp_send(int sock, const char *arg)
{
	int count = -1; // stop after first block
	int ret;

	if (arg[1] == ':') {
		count = unescape(trash, sizeof(trash), arg + 2);
	} else if (arg[1]) {
		count = atoi(arg + 1);
		if (count < 0) {
			fprintf(stderr, "send count must be >= 0 or unset (was %d)\n", count);
			return -2;
		}
	}

	while (1) {
		ret = send(sock, trash,
		           (count > 0) && (count < sizeof(trash)) ? count : sizeof(trash),
		           MSG_NOSIGNAL | ((count > sizeof(trash)) ? MSG_MORE : 0));
		if (ret < 0) {
			if (errno == EINTR)
				continue;
			if (errno != EAGAIN) {
				dolog("send %d\n", ret);
				return -1;
			}
			while (!wait_on_fd(sock, POLLOUT, 1000));
			continue;
		}
		dolog("send %d\n", ret);
		if (!count)
			continue;
		else if (count > 0)
			count -= ret;

		if (count <= 0)
			break;
	}

	return 0;
}

/* echoes N bytes to the socket and returns 0 (or -1 in case of error). If not
 * set, echoes only the first block. Zero means forward forever.
 */
int tcp_echo(int sock, const char *arg)
{
	int count = -1; // echo forever
	int ret;
	int rcvd;

	if (arg[1]) {
		count = atoi(arg + 1);
		if (count < 0) {
			fprintf(stderr, "send count must be >= 0 or unset (was %d)\n", count);
			return -1;
		}
	}

	rcvd = 0;
	while (1) {
		if (rcvd <= 0) {
			/* no data pending */
			rcvd = recv(sock, trash, (count > 0) && (count < sizeof(trash)) ? count : sizeof(trash), MSG_NOSIGNAL);
			if (rcvd < 0) {
				if (errno == EINTR)
					continue;
				if (errno != EAGAIN) {
					dolog("recv %d\n", rcvd);
					return -1;
				}
				while (!wait_on_fd(sock, POLLIN | POLLRDHUP, 1000));
				continue;
			}
			dolog("recv %d\n", rcvd);
			if (!rcvd)
				break;
		}
		else {
			/* some data still pending */
			ret = send(sock, trash, rcvd, MSG_NOSIGNAL | ((count > rcvd) ? MSG_MORE : 0));
			if (ret < 0) {
				if (errno == EINTR)
					continue;
				if (errno != EAGAIN) {
					dolog("send %d\n", ret);
					return -1;
				}
				while (!wait_on_fd(sock, POLLOUT, 1000));
				continue;
			}
			dolog("send %d\n", ret);
			rcvd -= ret;
			if (rcvd)
				continue;

			if (!count)
				continue;
			else if (count > 0)
				count -= ret;

			if (count <= 0)
				break;
		}
	}
	return 0;
}

/* waits for an event on the socket, usually indicates an accept for a
 * listening socket and a connect for an outgoing socket.
 */
int tcp_wait(int sock, const char *arg)
{
	int delay = -1; // wait forever
	int ret;

	if (arg[1]) {
		delay = atoi(arg + 1);
		if (delay < 0) {
			fprintf(stderr, "wait time must be >= 0 or unset (was %d)\n", delay);
			return -1;
		}
	}

	/* FIXME: this doesn't take into account delivered signals */
	ret = wait_on_fd(sock, POLLIN | POLLRDHUP | POLLOUT, delay);
	if (ret < 0)
		return ret;

	return 0;
}

/* waits for the input data to be present */
int tcp_wait_in(int sock, const char *arg)
{
	int ret;

	ret = wait_on_fd(sock, POLLIN | POLLRDHUP, 1000);
	if (ret < 0)
		return ret;

	return 0;
}

/* waits for the output queue to be empty */
int tcp_wait_out(int sock, const char *arg)
{
	int ret;

	ret = wait_on_fd(sock, POLLOUT, 1000);
	if (ret < 0)
		return ret;

	/* Now wait for data to leave the socket */
	do {
		if (ioctl(sock, TIOCOUTQ, &ret) < 0)
			return -1;
	} while (ret > 0);
	return 0;
}

/* delays processing for <time> milliseconds, 100 by default */
int tcp_pause(int sock, const char *arg)
{
	int delay = 100;

	if (arg[1]) {
		delay = atoi(arg + 1);
		if (delay < 0) {
			fprintf(stderr, "wait time must be >= 0 or unset (was %d)\n", delay);
			return -1;
		}
	}

	usleep(delay * 1000);
	return 0;
}

/* forks another process while respecting the limit imposed in argument (1 by
 * default). Will wait for another process to exit before creating a new one.
 * Returns the value of the fork() syscall, ie 0 for the child, non-zero for
 * the parent, -1 for an error.
 */
int tcp_fork(int sock, const char *arg)
{
	int max = 1;
	int ret;

	if (arg[1]) {
		max = atoi(arg + 1);
		if (max <= 0) {
			fprintf(stderr, "max process must be > 0 or unset (was %d)\n", max);
			return -1;
		}
	}

	while (nbproc >= max)
		poll(NULL, 0, 1000);

	ret = fork();
	if (ret > 0)
		__sync_add_and_fetch(&nbproc, 1);
	return ret;
}

int main(int argc, char **argv)
{
	struct sockaddr_storage default_addr;
	struct err_msg err;
	const char *arg0;
	int loop_arg;
	int arg;
	int ret;
	int sock;
	int errfd;

	arg0 = argv[0];

	while (argc > 1 && argv[1][0] == '-') {
		argc--; argv++;
		if (strcmp(argv[0], "-t") == 0)
			showtime++;
		else if (strcmp(argv[0], "-tt") == 0)
			showtime += 2;
		else if (strcmp(argv[0], "-ttt") == 0)
			showtime += 3;
		else if (strcmp(argv[0], "-v") == 0)
			verbose ++;
		else if (strcmp(argv[0], "-u") == 0) {
			sock_type = SOCK_DGRAM;
			sock_proto = IPPROTO_UDP;
		}
		else if (strcmp(argv[0], "--") == 0)
			break;
		else
			usage(1, arg0);
	}

	if (argc < 2)
		usage(1, arg0);

	pid = getpid();
	signal(SIGCHLD, sig_handler);

	if (addr_to_ss(argv[1], &default_addr, &err) < 0)
		die(1, "%s\n", err.msg);

	gettimeofday(&start_time, NULL);

	sock = -1;
	loop_arg = 2;
	for (arg = loop_arg; arg < argc; arg++) {
		switch (argv[arg][0]) {
		case 'L':
			sock = tcp_listen(sock, &default_addr, argv[arg]);
			if (sock < 0)
				die(1, "Fatal: tcp_listen() failed.\n");
			break;

		case 'B':
			/* silently ignore existing connections */
			sock = tcp_bind(sock, &default_addr, argv[arg]);
			if (sock < 0)
				die(1, "Fatal: tcp_connect() failed.\n");
			dolog("connect\n");
			break;

		case 'C':
			sock = tcp_connect(sock, &default_addr, argv[arg]);
			if (sock < 0)
				die(1, "Fatal: tcp_connect() failed.\n");
			dolog("connect\n");
			break;

		case 'D':
			/* silently ignore non-existing connections */
			if (sock >= 0 && tcp_disconnect(sock) < 0)
				die(1, "Fatal: tcp_connect() failed.\n");
			dolog("disconnect\n");
			break;

		case 'A':
			if (sock < 0)
				die(1, "Fatal: tcp_accept() on non-socket.\n");
			sock = tcp_accept(sock, argv[arg]);
			if (sock < 0)
				die(1, "Fatal: tcp_accept() failed.\n");
			dolog("accept\n");
			loop_arg = arg + 1; // cannot loop before accept()
			break;

		case 'T':
			if (sock < 0)
				die(1, "Fatal: tcp_set_nodelay() on non-socket.\n");
			if (tcp_set_nodelay(sock, argv[arg]) < 0)
				die(1, "Fatal: tcp_set_nodelay() failed.\n");
			break;

		case 'G':
			if (sock < 0)
				die(1, "Fatal: tcp_set_nolinger() on non-socket.\n");
			if (tcp_set_nolinger(sock, argv[arg]) < 0)
				die(1, "Fatal: tcp_set_nolinger() failed.\n");
			break;

		case 'Q':
			if (sock < 0)
				die(1, "Fatal: tcp_set_noquickack() on non-socket.\n");
			if (tcp_set_noquickack(sock, argv[arg]) < 0)
				die(1, "Fatal: tcp_set_noquickack() failed.\n");
			break;

		case 'R':
			if (sock < 0)
				die(1, "Fatal: tcp_recv() on non-socket.\n");
			ret = tcp_recv(sock, argv[arg]);
			if (ret < 0) {
				if (ret == -1) // usually ECONNRESET, silently exit
					die(0, NULL);
				die(1, "Fatal: tcp_recv() failed.\n");
			}
			break;

		case 'S':
			if (sock < 0)
				die(1, "Fatal: tcp_send() on non-socket.\n");
			ret = tcp_send(sock, argv[arg]);
			if (ret < 0) {
				if (ret == -1) // usually a broken pipe, silently exit
					die(0, NULL);
				die(1, "Fatal: tcp_send() failed.\n");
			}
			break;

		case 'E':
			if (sock < 0)
				die(1, "Fatal: tcp_echo() on non-socket.\n");
			if (tcp_echo(sock, argv[arg]) < 0)
				die(1, "Fatal: tcp_echo() failed.\n");
			break;

		case 'P':
			if (tcp_pause(sock, argv[arg]) < 0)
				die(1, "Fatal: tcp_pause() failed.\n");
			break;

		case 'W':
			if (sock < 0)
				die(1, "Fatal: tcp_wait() on non-socket.\n");
			if (tcp_wait(sock, argv[arg]) < 0)
				die(1, "Fatal: tcp_wait() failed.\n");
			dolog("ready_any\n");
			break;

		case 'I':
			if (sock < 0)
				die(1, "Fatal: tcp_wait_in() on non-socket.\n");
			if (tcp_wait_in(sock, argv[arg]) < 0)
				die(1, "Fatal: tcp_wait_in() failed.\n");
			dolog("ready_in\n");
			break;

		case 'O':
			if (sock < 0)
				die(1, "Fatal: tcp_wait_out() on non-socket.\n");
			if (tcp_wait_out(sock, argv[arg]) < 0)
				die(1, "Fatal: tcp_wait_out() failed.\n");
			dolog("ready_out\n");
			break;

		case 'K':
			if (sock < 0 || close(sock) < 0)
				die(1, "Fatal: close() on non-socket.\n");
			dolog("close\n");
			sock = -1;
			break;

		case 'F':
			/* ignore errors on shutdown() as they are common */
			if (sock >= 0)
				shutdown(sock, SHUT_WR);
			dolog("shutdown(w)\n");
			break;

		case 'r':
			/* ignore errors on shutdown() as they are common */
			if (sock >= 0)
				shutdown(sock, SHUT_RD);
			dolog("shutdown(r)\n");
			break;

		case 'N':
			ret = tcp_fork(sock, argv[arg]);
			if (ret < 0)
				die(1, "Fatal: fork() failed.\n");
			if (ret > 0) {
				/* loop back to first arg */
				arg = loop_arg - 1;
				continue;
			}
			/* OK we're in the child, let's continue */
			pid = getpid();
			loop_arg = arg + 1;
			break;

		case 'J': // jump back to oldest post-fork action
			arg = loop_arg - 1;
			continue;

		case 'X': // execute command. Optionally supports redirecting only i/o/e
			if (arg + 1 >= argc)
				die(1, "Fatal: missing argument after %s\n", argv[arg]);

			errfd = dup(2);
			fcntl(errfd, F_SETFD, fcntl(errfd, F_GETFD, FD_CLOEXEC) | FD_CLOEXEC);
			fcntl(sock,  F_SETFL, fcntl(sock,  F_GETFL, O_NONBLOCK) & ~O_NONBLOCK);
			if (!argv[arg][1] || strchr(argv[arg], 'i'))
				dup2(sock, 0);
			if (!argv[arg][1] || strchr(argv[arg], 'o'))
				dup2(sock, 1);
			if (!argv[arg][1] || strchr(argv[arg], 'e'))
				dup2(sock, 2);
			argv += arg + 1;
			if (execvp(argv[0], argv) == -1) {
				int e = errno;

				dup2(errfd, 2); // restore original stderr
				close(errfd);
				die(1, "Fatal: execvp(%s) failed : %s\n", argv[0], strerror(e));
			}
			break;
		default:
			usage(1, arg0);
		}
	}
	return 0;
}