networking/tcpudp.c - codeaurora/busybox - Gitiles

 /* Based on ipsvd utilities written by Gerrit Pape <pape@smarden.org>
  * which are released into public domain by the author.
  * Homepage: http://smarden.sunsite.dk/ipsvd/
  *
  * Copyright (C) 2007 Denys Vlasenko.
  *
  * Licensed under GPLv2, see file LICENSE in this source tree.
  */

 /* Based on ipsvd-0.12.1. This tcpsvd accepts all options
  * which are supported by one from ipsvd-0.12.1, but not all are
  * functional. See help text at the end of this file for details.
  *
  * Code inside "#ifdef SSLSVD" is for sslsvd and is currently unused.
  *
  * Busybox version exports TCPLOCALADDR instead of
  * TCPLOCALIP + TCPLOCALPORT pair. ADDR more closely matches reality
  * (which is "struct sockaddr_XXX". Port is not a separate entity,
  * it's just a part of (AF_INET[6]) sockaddr!).
  *
  * TCPORIGDSTADDR is Busybox-specific addition.
  *
  * udp server is hacked up by reusing TCP code. It has the following
  * limitation inherent in Unix DGRAM sockets implementation:
  * - local IP address is retrieved (using recvmsg voodoo) but
  *   child's socket is not bound to it (bind cannot be called on
  *   already bound socket). Thus it still can emit outgoing packets
  *   with wrong source IP...
  * - don't know how to retrieve ORIGDST for udp.
  */

 //usage:#define tcpsvd_trivial_usage
 //usage:       "[-hEv] [-c N] [-C N[:MSG]] [-b N] [-u USER] [-l NAME] IP PORT PROG"
 /* with not-implemented options: */
 /* //usage:    "[-hpEvv] [-c N] [-C N[:MSG]] [-b N] [-u USER] [-l NAME] [-i DIR|-x CDB] [-t SEC] IP PORT PROG" */
 //usage:#define tcpsvd_full_usage "\n\n"
 //usage:       "Create TCP socket, bind to IP:PORT and listen for incoming connections.\n"
 //usage:       "Run PROG for each connection.\n"
 //usage:     "\n	IP PORT		IP:PORT to listen on"
 //usage:     "\n	PROG ARGS	Program to run"
 //usage:     "\n	-u USER[:GRP]	Change to user/group after bind"
 //usage:     "\n	-c N		Up to N connections simultaneously (default 30)"
 //usage:     "\n	-b N		Allow backlog of approximately N TCP SYNs (default 20)"
 //usage:     "\n	-C N[:MSG]	Allow only up to N connections from the same IP:"
 //usage:     "\n			new connections from this IP address are closed"
 //usage:     "\n			immediately, MSG is written to the peer before close"
 //usage:     "\n	-E		Don't set up environment"
 //usage:     "\n	-h		Look up peer's hostname"
 //usage:     "\n	-l NAME		Local hostname (else look up local hostname in DNS)"
 //usage:     "\n	-v		Verbose"
 //usage:     "\n"
 //usage:     "\nEnvironment if no -E:"
 //usage:     "\nPROTO='TCP'"
 //usage:     "\nTCPREMOTEADDR='ip:port'" IF_FEATURE_IPV6(" ('[ip]:port' for IPv6)")
 //usage:     "\nTCPLOCALADDR='ip:port'"
 //usage:     "\nTCPORIGDSTADDR='ip:port' of destination before firewall"
 //usage:     "\n	Useful for REDIRECTed-to-local connections:"
 //usage:     "\n	iptables -t nat -A PREROUTING -p tcp --dport 80 -j REDIRECT --to 8080"
 //usage:     "\nTCPCONCURRENCY=num_of_connects_from_this_ip"
 //usage:     "\nIf -h:"
 //usage:     "\nTCPLOCALHOST='hostname' (-l NAME is used if specified)"
 //usage:     "\nTCPREMOTEHOST='hostname'"

 //usage:
 //usage:#define udpsvd_trivial_usage
 //usage:       "[-hEv] [-c N] [-u USER] [-l NAME] IP PORT PROG"
 //usage:#define udpsvd_full_usage "\n\n"
 //usage:       "Create UDP socket, bind to IP:PORT and wait for incoming packets.\n"
 //usage:       "Run PROG for each packet, redirecting all further packets with same\n"
 //usage:       "peer ip:port to it.\n"
 //usage:     "\n	IP PORT		IP:PORT to listen on"
 //usage:     "\n	PROG ARGS	Program to run"
 //usage:     "\n	-u USER[:GRP]	Change to user/group after bind"
 //usage:     "\n	-c N		Up to N connections simultaneously (default 30)"
 //usage:     "\n	-E		Don't set up environment"
 //usage:     "\n	-h		Look up peer's hostname"
 //usage:     "\n	-l NAME		Local hostname (else look up local hostname in DNS)"
 //usage:     "\n	-v		Verbose"
 //usage:     "\n"
 //usage:     "\nEnvironment if no -E:"
 //usage:     "\nPROTO='UDP'"
 //usage:     "\nUDPREMOTEADDR='ip:port'" IF_FEATURE_IPV6(" ('[ip]:port' for IPv6)")
 //usage:     "\nUDPLOCALADDR='ip:port'"
 //usage:     "\nIf -h:"
 //usage:     "\nUDPLOCALHOST='hostname' (-l NAME is used if specified)"
 //usage:     "\nUDPREMOTEHOST='hostname'"

 #include "libbb.h"
 #include "common_bufsiz.h"

 /* Wants <limits.h> etc, thus included after libbb.h: */
 #ifdef __linux__
 #include <linux/types.h> /* for __be32 etc */
 #include <linux/netfilter_ipv4.h>
 #endif

 // TODO: move into this file:
 #include "tcpudp_perhost.h"

 #ifdef SSLSVD
 #include "matrixSsl.h"
 #include "ssl_io.h"
 #endif

 struct globals {
 	unsigned verbose;
 	unsigned max_per_host;
 	unsigned cur_per_host;
 	unsigned cnum;
 	unsigned cmax;
 	char **env_cur;
 	char *env_var[1]; /* actually bigger */
 } FIX_ALIASING;
 #define G (*(struct globals*)bb_common_bufsiz1)
 #define verbose      (G.verbose     )
 #define max_per_host (G.max_per_host)
 #define cur_per_host (G.cur_per_host)
 #define cnum         (G.cnum        )
 #define cmax         (G.cmax        )
 #define env_cur      (G.env_cur     )
 #define env_var      (G.env_var     )
 #define INIT_G() do { \
 	setup_common_bufsiz(); \
 	cmax = 30; \
 	env_cur = &env_var[0]; \
 } while (0)


 /* We have to be careful about leaking memory in repeated setenv's */
 static void xsetenv_plain(const char *n, const char *v)
 {
 	char *var = xasprintf("%s=%s", n, v);
 	*env_cur++ = var;
 	putenv(var);
 }

 static void xsetenv_proto(const char *proto, const char *n, const char *v)
 {
 	char *var = xasprintf("%s%s=%s", proto, n, v);
 	*env_cur++ = var;
 	putenv(var);
 }

 static void undo_xsetenv(void)
 {
 	char **pp = env_cur = &env_var[0];
 	while (*pp) {
 		char *var = *pp;
 		bb_unsetenv_and_free(var);
 		*pp++ = NULL;
 	}
 }

 static void sig_term_handler(int sig)
 {
 	if (verbose)
 		bb_error_msg("got signal %u, exit", sig);
 	kill_myself_with_sig(sig);
 }

 /* Little bloated, but tries to give accurate info how child exited.
  * Makes easier to spot segfaulting children etc... */
 static void print_waitstat(unsigned pid, int wstat)
 {
 	unsigned e = 0;
 	const char *cause = "?exit";

 	if (WIFEXITED(wstat)) {
 		cause++;
 		e = WEXITSTATUS(wstat);
 	} else if (WIFSIGNALED(wstat)) {
 		cause = "signal";
 		e = WTERMSIG(wstat);
 	}
 	bb_error_msg("end %d %s %d", pid, cause, e);
 }

 /* Must match getopt32 in main! */
 enum {
 	OPT_c = (1 << 0),
 	OPT_C = (1 << 1),
 	OPT_i = (1 << 2),
 	OPT_x = (1 << 3),
 	OPT_u = (1 << 4),
 	OPT_l = (1 << 5),
 	OPT_E = (1 << 6),
 	OPT_b = (1 << 7),
 	OPT_h = (1 << 8),
 	OPT_p = (1 << 9),
 	OPT_t = (1 << 10),
 	OPT_v = (1 << 11),
 	OPT_V = (1 << 12),
 	OPT_U = (1 << 13), /* from here: sslsvd only */
 	OPT_slash = (1 << 14),
 	OPT_Z = (1 << 15),
 	OPT_K = (1 << 16),
 };

 static void connection_status(void)
 {
 	/* "only 1 client max" desn't need this */
 	if (cmax > 1)
 		bb_error_msg("status %u/%u", cnum, cmax);
 }

 static void sig_child_handler(int sig UNUSED_PARAM)
 {
 	int wstat;
 	pid_t pid;

 	while ((pid = wait_any_nohang(&wstat)) > 0) {
 		if (max_per_host)
 			ipsvd_perhost_remove(pid);
 		if (cnum)
 			cnum--;
 		if (verbose)
 			print_waitstat(pid, wstat);
 	}
 	if (verbose)
 		connection_status();
 }

 int tcpudpsvd_main(int argc, char **argv) MAIN_EXTERNALLY_VISIBLE;
 int tcpudpsvd_main(int argc UNUSED_PARAM, char **argv)
 {
 	char *str_C, *str_t;
 	char *user;
 	struct hcc *hccp;
 	const char *instructs;
 	char *msg_per_host = NULL;
 	unsigned len_per_host = len_per_host; /* gcc */
 #ifndef SSLSVD
 	struct bb_uidgid_t ugid;
 #endif
 	bool tcp;
 	uint16_t local_port;
 	char *preset_local_hostname = NULL;
 	char *remote_hostname = remote_hostname; /* for compiler */
 	char *remote_addr = remote_addr; /* for compiler */
 	len_and_sockaddr *lsa;
 	len_and_sockaddr local, remote;
 	socklen_t sa_len;
 	int pid;
 	int sock;
 	int conn;
 	unsigned backlog = 20;
 	unsigned opts;

 	INIT_G();

 	tcp = (applet_name[0] == 't');

 	/* 3+ args, -i at most once, -p implies -h, -v is counter, -b N, -c N */
 	opt_complementary = "-3:i--i:ph:vv";
 #ifdef SSLSVD
 	opts = getopt32(argv, "+c:+C:i:x:u:l:Eb:+hpt:vU:/:Z:K:",
 		&cmax, &str_C, &instructs, &instructs, &user, &preset_local_hostname,
 		&backlog, &str_t, &ssluser, &root, &cert, &key, &verbose
 	);
 #else
 	/* "+": stop on first non-option */
 	opts = getopt32(argv, "+c:+C:i:x:u:l:Eb:hpt:v",
 		&cmax, &str_C, &instructs, &instructs, &user, &preset_local_hostname,
 		&backlog, &str_t, &verbose
 	);
 #endif
 	if (opts & OPT_C) { /* -C n[:message] */
 		max_per_host = bb_strtou(str_C, &str_C, 10);
 		if (str_C[0]) {
 			if (str_C[0] != ':')
 				bb_show_usage();
 			msg_per_host = str_C + 1;
 			len_per_host = strlen(msg_per_host);
 		}
 	}
 	if (max_per_host > cmax)
 		max_per_host = cmax;
 	if (opts & OPT_u) {
 		xget_uidgid(&ugid, user);
 	}
 #ifdef SSLSVD
 	if (opts & OPT_U) ssluser = optarg;
 	if (opts & OPT_slash) root = optarg;
 	if (opts & OPT_Z) cert = optarg;
 	if (opts & OPT_K) key = optarg;
 #endif
 	argv += optind;
 	if (!argv[0][0] || LONE_CHAR(argv[0], '0'))
 		argv[0] = (char*)"0.0.0.0";

 	/* Per-IP flood protection is not thought-out for UDP */
 	if (!tcp)
 		max_per_host = 0;

 	bb_sanitize_stdio(); /* fd# 0,1,2 must be opened */

 #ifdef SSLSVD
 	sslser = user;
 	client = 0;
 	if ((getuid() == 0) && !(opts & OPT_u)) {
 		xfunc_exitcode = 100;
 		bb_error_msg_and_die(bb_msg_you_must_be_root);
 	}
 	if (opts & OPT_u)
 		if (!uidgid_get(&sslugid, ssluser, 1)) {
 			if (errno) {
 				bb_perror_msg_and_die("can't get user/group: %s", ssluser);
 			}
 			bb_error_msg_and_die("unknown user/group %s", ssluser);
 		}
 	if (!cert) cert = "./cert.pem";
 	if (!key) key = cert;
 	if (matrixSslOpen() < 0)
 		fatal("can't initialize ssl");
 	if (matrixSslReadKeys(&keys, cert, key, 0, ca) < 0) {
 		if (client)
 			fatal("can't read cert, key, or ca file");
 		fatal("can't read cert or key file");
 	}
 	if (matrixSslNewSession(&ssl, keys, 0, SSL_FLAGS_SERVER) < 0)
 		fatal("can't create ssl session");
 #endif

 	sig_block(SIGCHLD);
 	signal(SIGCHLD, sig_child_handler);
 	bb_signals(BB_FATAL_SIGS, sig_term_handler);
 	signal(SIGPIPE, SIG_IGN);

 	if (max_per_host)
 		ipsvd_perhost_init(cmax);

 	local_port = bb_lookup_port(argv[1], tcp ? "tcp" : "udp", 0);
 	lsa = xhost2sockaddr(argv[0], local_port);
 	argv += 2;

 	sock = xsocket(lsa->u.sa.sa_family, tcp ? SOCK_STREAM : SOCK_DGRAM, 0);
 	setsockopt_reuseaddr(sock);
 	sa_len = lsa->len; /* I presume sockaddr len stays the same */
 	xbind(sock, &lsa->u.sa, sa_len);
 	if (tcp) {
 		xlisten(sock, backlog);
 		close_on_exec_on(sock);
 	} else { /* udp: needed for recv_from_to to work: */
 		socket_want_pktinfo(sock);
 	}
 	/* ndelay_off(sock); - it is the default I think? */

 #ifndef SSLSVD
 	if (opts & OPT_u) {
 		/* drop permissions */
 		xsetgid(ugid.gid);
 		xsetuid(ugid.uid);
 	}
 #endif

 	if (verbose) {
 		char *addr = xmalloc_sockaddr2dotted(&lsa->u.sa);
 		if (opts & OPT_u)
 			bb_error_msg("listening on %s, starting, uid %u, gid %u", addr,
 				(unsigned)ugid.uid, (unsigned)ugid.gid);
 		else
 			bb_error_msg("listening on %s, starting", addr);
 		free(addr);
 	}

 	/* Main accept() loop */

  again:
 	hccp = NULL;

  again1:
 	close(0);
 	/* It's important to close(0) _before_ wait loop:
 	 * fd#0 can be a shared connection fd.
 	 * If kept open by us, peer can't detect PROG closing it.
 	 */
 	while (cnum >= cmax)
 		wait_for_any_sig(); /* expecting SIGCHLD */

  again2:
 	sig_unblock(SIGCHLD);
 	local.len = remote.len = sa_len;
 	if (tcp) {
 		/* Accept a connection to fd #0 */
 		conn = accept(sock, &remote.u.sa, &remote.len);
 	} else {
 		/* In case recv_from_to won't be able to recover local addr.
 		 * Also sets port - recv_from_to is unable to do it. */
 		local = *lsa;
 		conn = recv_from_to(sock, NULL, 0, MSG_PEEK,
 				&remote.u.sa, &local.u.sa, sa_len);
 	}
 	sig_block(SIGCHLD);
 	if (conn < 0) {
 		if (errno != EINTR)
 			bb_perror_msg(tcp ? "accept" : "recv");
 		goto again2;
 	}
 	xmove_fd(tcp ? conn : sock, 0);

 	if (max_per_host) {
 		/* Drop connection immediately if cur_per_host > max_per_host
 		 * (minimizing load under SYN flood) */
 		remote_addr = xmalloc_sockaddr2dotted_noport(&remote.u.sa);
 		cur_per_host = ipsvd_perhost_add(remote_addr, max_per_host, &hccp);
 		if (cur_per_host > max_per_host) {
 			/* ipsvd_perhost_add detected that max is exceeded
 			 * (and did not store ip in connection table) */
 			free(remote_addr);
 			if (msg_per_host) {
 				/* don't block or test for errors */
 				send(0, msg_per_host, len_per_host, MSG_DONTWAIT);
 			}
 			goto again1;
 		}
 		/* NB: remote_addr is not leaked, it is stored in conn table */
 	}

 	if (!tcp) {
 		/* Voodoo magic: making udp sockets each receive its own
 		 * packets is not trivial, and I still not sure
 		 * I do it 100% right.
 		 * 1) we have to do it before fork()
 		 * 2) order is important - is it right now? */

 		/* Open new non-connected UDP socket for further clients... */
 		sock = xsocket(lsa->u.sa.sa_family, SOCK_DGRAM, 0);
 		setsockopt_reuseaddr(sock);
 		/* Make plain write/send work for old socket by supplying default
 		 * destination address. This also restricts incoming packets
 		 * to ones coming from this remote IP. */
 		xconnect(0, &remote.u.sa, sa_len);
 	/* hole? at this point we have no wildcard udp socket...
 	 * can this cause clients to get "port unreachable" icmp?
 	 * Yup, time window is very small, but it exists (is it?) */
 		/* ..."open new socket", continued */
 		xbind(sock, &lsa->u.sa, sa_len);
 		socket_want_pktinfo(sock);

 		/* Doesn't work:
 		 * we cannot replace fd #0 - we will lose pending packet
 		 * which is already buffered for us! And we cannot use fd #1
 		 * instead - it will "intercept" all following packets, but child
 		 * does not expect data coming *from fd #1*! */
 #if 0
 		/* Make it so that local addr is fixed to localp->u.sa
 		 * and we don't accidentally accept packets to other local IPs. */
 		/* NB: we possibly bind to the _very_ same_ address & port as the one
 		 * already bound in parent! This seems to work in Linux.
 		 * (otherwise we can move socket to fd #0 only if bind succeeds) */
 		close(0);
 		set_nport(&localp->u.sa, htons(local_port));
 		xmove_fd(xsocket(localp->u.sa.sa_family, SOCK_DGRAM, 0), 0);
 		setsockopt_reuseaddr(0); /* crucial */
 		xbind(0, &localp->u.sa, localp->len);
 #endif
 	}

 	pid = vfork();
 	if (pid == -1) {
 		bb_perror_msg("vfork");
 		goto again;
 	}

 	if (pid != 0) {
 		/* Parent */
 		cnum++;
 		if (verbose)
 			connection_status();
 		if (hccp)
 			hccp->pid = pid;
 		/* clean up changes done by vforked child */
 		undo_xsetenv();
 		goto again;
 	}

 	/* Child: prepare env, log, and exec prog */

 	{ /* vfork alert! every xmalloc in this block should be freed! */
 		char *local_hostname = local_hostname; /* for compiler */
 		char *local_addr = NULL;
 		char *free_me0 = NULL;
 		char *free_me1 = NULL;
 		char *free_me2 = NULL;

 		if (verbose || !(opts & OPT_E)) {
 			if (!max_per_host) /* remote_addr is not yet known */
 				free_me0 = remote_addr = xmalloc_sockaddr2dotted(&remote.u.sa);
 			if (opts & OPT_h) {
 				free_me1 = remote_hostname = xmalloc_sockaddr2host_noport(&remote.u.sa);
 				if (!remote_hostname) {
 					bb_error_msg("can't look up hostname for %s", remote_addr);
 					remote_hostname = remote_addr;
 				}
 			}
 			/* Find out local IP peer connected to.
 			 * Errors ignored (I'm not paranoid enough to imagine kernel
 			 * which doesn't know local IP). */
 			if (tcp)
 				getsockname(0, &local.u.sa, &local.len);
 			/* else: for UDP it is done earlier by parent */
 			local_addr = xmalloc_sockaddr2dotted(&local.u.sa);
 			if (opts & OPT_h) {
 				local_hostname = preset_local_hostname;
 				if (!local_hostname) {
 					free_me2 = local_hostname = xmalloc_sockaddr2host_noport(&local.u.sa);
 					if (!local_hostname)
 						bb_error_msg_and_die("can't look up hostname for %s", local_addr);
 				}
 				/* else: local_hostname is not NULL, but is NOT malloced! */
 			}
 		}
 		if (verbose) {
 			pid = getpid();
 			if (max_per_host) {
 				bb_error_msg("concurrency %s %u/%u",
 					remote_addr,
 					cur_per_host, max_per_host);
 			}
 			bb_error_msg((opts & OPT_h)
 				? "start %u %s-%s (%s-%s)"
 				: "start %u %s-%s",
 				pid,
 				local_addr, remote_addr,
 				local_hostname, remote_hostname);
 		}

 		if (!(opts & OPT_E)) {
 			/* setup ucspi env */
 			const char *proto = tcp ? "TCP" : "UDP";

 #ifdef SO_ORIGINAL_DST
 			/* Extract "original" destination addr:port
 			 * from Linux firewall. Useful when you redirect
 			 * an outbond connection to local handler, and it needs
 			 * to know where it originally tried to connect */
 			if (tcp && getsockopt(0, SOL_IP, SO_ORIGINAL_DST, &local.u.sa, &local.len) == 0) {
 				char *addr = xmalloc_sockaddr2dotted(&local.u.sa);
 				xsetenv_plain("TCPORIGDSTADDR", addr);
 				free(addr);
 			}
 #endif
 			xsetenv_plain("PROTO", proto);
 			xsetenv_proto(proto, "LOCALADDR", local_addr);
 			xsetenv_proto(proto, "REMOTEADDR", remote_addr);
 			if (opts & OPT_h) {
 				xsetenv_proto(proto, "LOCALHOST", local_hostname);
 				xsetenv_proto(proto, "REMOTEHOST", remote_hostname);
 			}
 			//compat? xsetenv_proto(proto, "REMOTEINFO", "");
 			/* additional */
 			if (cur_per_host > 0) /* can not be true for udp */
 				xsetenv_plain("TCPCONCURRENCY", utoa(cur_per_host));
 		}
 		free(local_addr);
 		free(free_me0);
 		free(free_me1);
 		free(free_me2);
 	}

 	xdup2(0, 1);

 	signal(SIGPIPE, SIG_DFL); /* this one was SIG_IGNed */
 	/* Non-ignored signals revert to SIG_DFL on exec anyway */
 	/*signal(SIGCHLD, SIG_DFL);*/
 	sig_unblock(SIGCHLD);

 #ifdef SSLSVD
 	strcpy(id, utoa(pid));
 	ssl_io(0, argv);
 	bb_perror_msg_and_die("can't execute '%s'", argv[0]);
 #else
 	BB_EXECVP_or_die(argv);
 #endif
 }

 /*
 tcpsvd [-hpEvv] [-c n] [-C n:msg] [-b n] [-u user] [-l name]
 	[-i dir|-x cdb] [ -t sec] host port prog

 tcpsvd creates a TCP/IP socket, binds it to the address host:port,
 and listens on the socket for incoming connections.

 On each incoming connection, tcpsvd conditionally runs a program,
 with standard input reading from the socket, and standard output
 writing to the socket, to handle this connection. tcpsvd keeps
 listening on the socket for new connections, and can handle
 multiple connections simultaneously.

 tcpsvd optionally checks for special instructions depending
 on the IP address or hostname of the client that initiated
 the connection, see ipsvd-instruct(5).

 host
     host either is a hostname, or a dotted-decimal IP address,
     or 0. If host is 0, tcpsvd accepts connections to any local
     IP address.
     * busybox accepts IPv6 addresses and host:port pairs too
       In this case second parameter is ignored
 port
     tcpsvd accepts connections to host:port. port may be a name
     from /etc/services or a number.
 prog
     prog consists of one or more arguments. For each connection,
     tcpsvd normally runs prog, with file descriptor 0 reading from
     the network, and file descriptor 1 writing to the network.
     By default it also sets up TCP-related environment variables,
     see tcp-environ(5)
 -i dir
     read instructions for handling new connections from the instructions
     directory dir. See ipsvd-instruct(5) for details.
     * ignored by busyboxed version
 -x cdb
     read instructions for handling new connections from the constant database
     cdb. The constant database normally is created from an instructions
     directory by running ipsvd-cdb(8).
     * ignored by busyboxed version
 -t sec
     timeout. This option only takes effect if the -i option is given.
     While checking the instructions directory, check the time of last access
     of the file that matches the clients address or hostname if any, discard
     and remove the file if it wasn't accessed within the last sec seconds;
     tcpsvd does not discard or remove a file if the user's write permission
     is not set, for those files the timeout is disabled. Default is 0,
     which means that the timeout is disabled.
     * ignored by busyboxed version
 -l name
     local hostname. Do not look up the local hostname in DNS, but use name
     as hostname. This option must be set if tcpsvd listens on port 53
     to avoid loops.
 -u user[:group]
     drop permissions. Switch user ID to user's UID, and group ID to user's
     primary GID after creating and binding to the socket. If user is followed
     by a colon and a group name, the group ID is switched to the GID of group
     instead. All supplementary groups are removed.
 -c n
     concurrency. Handle up to n connections simultaneously. Default is 30.
     If there are n connections active, tcpsvd defers acceptance of a new
     connection until an active connection is closed.
 -C n[:msg]
     per host concurrency. Allow only up to n connections from the same IP
     address simultaneously. If there are n active connections from one IP
     address, new incoming connections from this IP address are closed
     immediately. If n is followed by :msg, the message msg is written
     to the client if possible, before closing the connection. By default
     msg is empty. See ipsvd-instruct(5) for supported escape sequences in msg.

     For each accepted connection, the current per host concurrency is
     available through the environment variable TCPCONCURRENCY. n and msg
     can be overwritten by ipsvd(7) instructions, see ipsvd-instruct(5).
     By default tcpsvd doesn't keep track of connections.
 -h
     Look up the client's hostname in DNS.
 -p
     paranoid. After looking up the client's hostname in DNS, look up the IP
     addresses in DNS for that hostname, and forget about the hostname
     if none of the addresses match the client's IP address. You should
     set this option if you use hostname based instructions. The -p option
     implies the -h option.
     * ignored by busyboxed version
 -b n
     backlog. Allow a backlog of approximately n TCP SYNs. On some systems n
     is silently limited. Default is 20.
 -E
     no special environment. Do not set up TCP-related environment variables.
 -v
     verbose. Print verbose messsages to standard output.
 -vv
     more verbose. Print more verbose messages to standard output.
     * no difference between -v and -vv in busyboxed version
 */
	/* Based on ipsvd utilities written by Gerrit Pape <pape@smarden.org>
	* which are released into public domain by the author.
	* Homepage: http://smarden.sunsite.dk/ipsvd/
	*
	* Copyright (C) 2007 Denys Vlasenko.
	*
	* Licensed under GPLv2, see file LICENSE in this source tree.
	*/

	/* Based on ipsvd-0.12.1. This tcpsvd accepts all options
	* which are supported by one from ipsvd-0.12.1, but not all are
	* functional. See help text at the end of this file for details.
	*
	* Code inside "#ifdef SSLSVD" is for sslsvd and is currently unused.
	*
	* Busybox version exports TCPLOCALADDR instead of
	* TCPLOCALIP + TCPLOCALPORT pair. ADDR more closely matches reality
	* (which is "struct sockaddr_XXX". Port is not a separate entity,
	* it's just a part of (AF_INET[6]) sockaddr!).
	*
	* TCPORIGDSTADDR is Busybox-specific addition.
	*
	* udp server is hacked up by reusing TCP code. It has the following
	* limitation inherent in Unix DGRAM sockets implementation:
	* - local IP address is retrieved (using recvmsg voodoo) but
	* child's socket is not bound to it (bind cannot be called on
	* already bound socket). Thus it still can emit outgoing packets
	* with wrong source IP...
	* - don't know how to retrieve ORIGDST for udp.
	*/

	//usage:#define tcpsvd_trivial_usage
	//usage: "[-hEv] [-c N] [-C N[:MSG]] [-b N] [-u USER] [-l NAME] IP PORT PROG"
	/* with not-implemented options: */
	/* //usage: "[-hpEvv] [-c N] [-C N[:MSG]] [-b N] [-u USER] [-l NAME] [-i DIR\|-x CDB] [-t SEC] IP PORT PROG" */
	//usage:#define tcpsvd_full_usage "\n\n"
	//usage: "Create TCP socket, bind to IP:PORT and listen for incoming connections.\n"
	//usage: "Run PROG for each connection.\n"
	//usage: "\n IP PORT IP:PORT to listen on"
	//usage: "\n PROG ARGS Program to run"
	//usage: "\n -u USER[:GRP] Change to user/group after bind"
	//usage: "\n -c N Up to N connections simultaneously (default 30)"
	//usage: "\n -b N Allow backlog of approximately N TCP SYNs (default 20)"
	//usage: "\n -C N[:MSG] Allow only up to N connections from the same IP:"
	//usage: "\n new connections from this IP address are closed"
	//usage: "\n immediately, MSG is written to the peer before close"
	//usage: "\n -E Don't set up environment"
	//usage: "\n -h Look up peer's hostname"
	//usage: "\n -l NAME Local hostname (else look up local hostname in DNS)"
	//usage: "\n -v Verbose"
	//usage: "\n"
	//usage: "\nEnvironment if no -E:"
	//usage: "\nPROTO='TCP'"
	//usage: "\nTCPREMOTEADDR='ip:port'" IF_FEATURE_IPV6(" ('[ip]:port' for IPv6)")
	//usage: "\nTCPLOCALADDR='ip:port'"
	//usage: "\nTCPORIGDSTADDR='ip:port' of destination before firewall"
	//usage: "\n Useful for REDIRECTed-to-local connections:"
	//usage: "\n iptables -t nat -A PREROUTING -p tcp --dport 80 -j REDIRECT --to 8080"
	//usage: "\nTCPCONCURRENCY=num_of_connects_from_this_ip"
	//usage: "\nIf -h:"
	//usage: "\nTCPLOCALHOST='hostname' (-l NAME is used if specified)"
	//usage: "\nTCPREMOTEHOST='hostname'"

	//usage:
	//usage:#define udpsvd_trivial_usage
	//usage: "[-hEv] [-c N] [-u USER] [-l NAME] IP PORT PROG"
	//usage:#define udpsvd_full_usage "\n\n"
	//usage: "Create UDP socket, bind to IP:PORT and wait for incoming packets.\n"
	//usage: "Run PROG for each packet, redirecting all further packets with same\n"
	//usage: "peer ip:port to it.\n"
	//usage: "\n IP PORT IP:PORT to listen on"
	//usage: "\n PROG ARGS Program to run"
	//usage: "\n -u USER[:GRP] Change to user/group after bind"
	//usage: "\n -c N Up to N connections simultaneously (default 30)"
	//usage: "\n -E Don't set up environment"
	//usage: "\n -h Look up peer's hostname"
	//usage: "\n -l NAME Local hostname (else look up local hostname in DNS)"
	//usage: "\n -v Verbose"
	//usage: "\n"
	//usage: "\nEnvironment if no -E:"
	//usage: "\nPROTO='UDP'"
	//usage: "\nUDPREMOTEADDR='ip:port'" IF_FEATURE_IPV6(" ('[ip]:port' for IPv6)")
	//usage: "\nUDPLOCALADDR='ip:port'"
	//usage: "\nIf -h:"
	//usage: "\nUDPLOCALHOST='hostname' (-l NAME is used if specified)"
	//usage: "\nUDPREMOTEHOST='hostname'"

	#include "libbb.h"
	#include "common_bufsiz.h"

	/* Wants <limits.h> etc, thus included after libbb.h: */
	#ifdef __linux__
	#include <linux/types.h> /* for __be32 etc */
	#include <linux/netfilter_ipv4.h>
	#endif

	// TODO: move into this file:
	#include "tcpudp_perhost.h"

	#ifdef SSLSVD
	#include "matrixSsl.h"
	#include "ssl_io.h"
	#endif

	struct globals {
	unsigned verbose;
	unsigned max_per_host;
	unsigned cur_per_host;
	unsigned cnum;
	unsigned cmax;
	char **env_cur;
	char env_var[1]; / actually bigger */
	} FIX_ALIASING;
	#define G ((struct globals)bb_common_bufsiz1)
	#define verbose (G.verbose )
	#define max_per_host (G.max_per_host)
	#define cur_per_host (G.cur_per_host)
	#define cnum (G.cnum )
	#define cmax (G.cmax )
	#define env_cur (G.env_cur )
	#define env_var (G.env_var )
	#define INIT_G() do { \
	setup_common_bufsiz(); \
	cmax = 30; \
	env_cur = &env_var[0]; \
	} while (0)


	/* We have to be careful about leaking memory in repeated setenv's */
	static void xsetenv_plain(const char n, const char v)
	{
	char *var = xasprintf("%s=%s", n, v);
	*env_cur++ = var;
	putenv(var);
	}

	static void xsetenv_proto(const char proto, const char n, const char *v)
	{
	char *var = xasprintf("%s%s=%s", proto, n, v);
	*env_cur++ = var;
	putenv(var);
	}

	static void undo_xsetenv(void)
	{
	char **pp = env_cur = &env_var[0];
	while (*pp) {
	char var = pp;
	bb_unsetenv_and_free(var);
	*pp++ = NULL;
	}
	}

	static void sig_term_handler(int sig)
	{
	if (verbose)
	bb_error_msg("got signal %u, exit", sig);
	kill_myself_with_sig(sig);
	}

	/* Little bloated, but tries to give accurate info how child exited.
	* Makes easier to spot segfaulting children etc... */
	static void print_waitstat(unsigned pid, int wstat)
	{
	unsigned e = 0;
	const char *cause = "?exit";

	if (WIFEXITED(wstat)) {
	cause++;
	e = WEXITSTATUS(wstat);
	} else if (WIFSIGNALED(wstat)) {
	cause = "signal";
	e = WTERMSIG(wstat);
	}
	bb_error_msg("end %d %s %d", pid, cause, e);
	}

	/* Must match getopt32 in main! */
	enum {
	OPT_c = (1 << 0),
	OPT_C = (1 << 1),
	OPT_i = (1 << 2),
	OPT_x = (1 << 3),
	OPT_u = (1 << 4),
	OPT_l = (1 << 5),
	OPT_E = (1 << 6),
	OPT_b = (1 << 7),
	OPT_h = (1 << 8),
	OPT_p = (1 << 9),
	OPT_t = (1 << 10),
	OPT_v = (1 << 11),
	OPT_V = (1 << 12),
	OPT_U = (1 << 13), /* from here: sslsvd only */
	OPT_slash = (1 << 14),
	OPT_Z = (1 << 15),
	OPT_K = (1 << 16),
	};

	static void connection_status(void)
	{
	/* "only 1 client max" desn't need this */
	if (cmax > 1)
	bb_error_msg("status %u/%u", cnum, cmax);
	}

	static void sig_child_handler(int sig UNUSED_PARAM)
	{
	int wstat;
	pid_t pid;

	while ((pid = wait_any_nohang(&wstat)) > 0) {
	if (max_per_host)
	ipsvd_perhost_remove(pid);
	if (cnum)
	cnum--;
	if (verbose)
	print_waitstat(pid, wstat);
	}
	if (verbose)
	connection_status();
	}

	int tcpudpsvd_main(int argc, char **argv) MAIN_EXTERNALLY_VISIBLE;
	int tcpudpsvd_main(int argc UNUSED_PARAM, char **argv)
	{
	char str_C, str_t;
	char *user;
	struct hcc *hccp;
	const char *instructs;
	char *msg_per_host = NULL;
	unsigned len_per_host = len_per_host; /* gcc */
	#ifndef SSLSVD
	struct bb_uidgid_t ugid;
	#endif
	bool tcp;
	uint16_t local_port;
	char *preset_local_hostname = NULL;
	char remote_hostname = remote_hostname; / for compiler */
	char remote_addr = remote_addr; / for compiler */
	len_and_sockaddr *lsa;
	len_and_sockaddr local, remote;
	socklen_t sa_len;
	int pid;
	int sock;
	int conn;
	unsigned backlog = 20;
	unsigned opts;

	INIT_G();

	tcp = (applet_name[0] == 't');

	/* 3+ args, -i at most once, -p implies -h, -v is counter, -b N, -c N */
	opt_complementary = "-3:i--i:ph:vv";
	#ifdef SSLSVD
	opts = getopt32(argv, "+c:+C:i:x:u:l:Eb:+hpt:vU:/:Z:K:",
	&cmax, &str_C, &instructs, &instructs, &user, &preset_local_hostname,
	&backlog, &str_t, &ssluser, &root, &cert, &key, &verbose
	);
	#else
	/* "+": stop on first non-option */
	opts = getopt32(argv, "+c:+C:i:x:u:l:Eb:hpt:v",
	&cmax, &str_C, &instructs, &instructs, &user, &preset_local_hostname,
	&backlog, &str_t, &verbose
	);
	#endif
	if (opts & OPT_C) { /* -C n[:message] */
	max_per_host = bb_strtou(str_C, &str_C, 10);
	if (str_C[0]) {
	if (str_C[0] != ':')
	bb_show_usage();
	msg_per_host = str_C + 1;
	len_per_host = strlen(msg_per_host);
	}
	}
	if (max_per_host > cmax)
	max_per_host = cmax;
	if (opts & OPT_u) {
	xget_uidgid(&ugid, user);
	}
	#ifdef SSLSVD
	if (opts & OPT_U) ssluser = optarg;
	if (opts & OPT_slash) root = optarg;
	if (opts & OPT_Z) cert = optarg;
	if (opts & OPT_K) key = optarg;
	#endif
	argv += optind;
	if (!argv[0][0] \|\| LONE_CHAR(argv[0], '0'))
	argv[0] = (char*)"0.0.0.0";

	/* Per-IP flood protection is not thought-out for UDP */
	if (!tcp)
	max_per_host = 0;

	bb_sanitize_stdio(); /* fd# 0,1,2 must be opened */

	#ifdef SSLSVD
	sslser = user;
	client = 0;
	if ((getuid() == 0) && !(opts & OPT_u)) {
	xfunc_exitcode = 100;
	bb_error_msg_and_die(bb_msg_you_must_be_root);
	}
	if (opts & OPT_u)
	if (!uidgid_get(&sslugid, ssluser, 1)) {
	if (errno) {
	bb_perror_msg_and_die("can't get user/group: %s", ssluser);
	}
	bb_error_msg_and_die("unknown user/group %s", ssluser);
	}
	if (!cert) cert = "./cert.pem";
	if (!key) key = cert;
	if (matrixSslOpen() < 0)
	fatal("can't initialize ssl");
	if (matrixSslReadKeys(&keys, cert, key, 0, ca) < 0) {
	if (client)
	fatal("can't read cert, key, or ca file");
	fatal("can't read cert or key file");
	}
	if (matrixSslNewSession(&ssl, keys, 0, SSL_FLAGS_SERVER) < 0)
	fatal("can't create ssl session");
	#endif

	sig_block(SIGCHLD);
	signal(SIGCHLD, sig_child_handler);
	bb_signals(BB_FATAL_SIGS, sig_term_handler);
	signal(SIGPIPE, SIG_IGN);

	if (max_per_host)
	ipsvd_perhost_init(cmax);

	local_port = bb_lookup_port(argv[1], tcp ? "tcp" : "udp", 0);
	lsa = xhost2sockaddr(argv[0], local_port);
	argv += 2;

	sock = xsocket(lsa->u.sa.sa_family, tcp ? SOCK_STREAM : SOCK_DGRAM, 0);
	setsockopt_reuseaddr(sock);
	sa_len = lsa->len; /* I presume sockaddr len stays the same */
	xbind(sock, &lsa->u.sa, sa_len);
	if (tcp) {
	xlisten(sock, backlog);
	close_on_exec_on(sock);
	} else { /* udp: needed for recv_from_to to work: */
	socket_want_pktinfo(sock);
	}
	/* ndelay_off(sock); - it is the default I think? */

	#ifndef SSLSVD
	if (opts & OPT_u) {
	/* drop permissions */
	xsetgid(ugid.gid);
	xsetuid(ugid.uid);
	}
	#endif

	if (verbose) {
	char *addr = xmalloc_sockaddr2dotted(&lsa->u.sa);
	if (opts & OPT_u)
	bb_error_msg("listening on %s, starting, uid %u, gid %u", addr,
	(unsigned)ugid.uid, (unsigned)ugid.gid);
	else
	bb_error_msg("listening on %s, starting", addr);
	free(addr);
	}

	/* Main accept() loop */

	again:
	hccp = NULL;

	again1:
	close(0);
	/* It's important to close(0) _before_ wait loop:
	* fd#0 can be a shared connection fd.
	* If kept open by us, peer can't detect PROG closing it.
	*/
	while (cnum >= cmax)
	wait_for_any_sig(); /* expecting SIGCHLD */

	again2:
	sig_unblock(SIGCHLD);
	local.len = remote.len = sa_len;
	if (tcp) {
	/* Accept a connection to fd #0 */
	conn = accept(sock, &remote.u.sa, &remote.len);
	} else {
	/* In case recv_from_to won't be able to recover local addr.
	* Also sets port - recv_from_to is unable to do it. */
	local = *lsa;
	conn = recv_from_to(sock, NULL, 0, MSG_PEEK,
	&remote.u.sa, &local.u.sa, sa_len);
	}
	sig_block(SIGCHLD);
	if (conn < 0) {
	if (errno != EINTR)
	bb_perror_msg(tcp ? "accept" : "recv");
	goto again2;
	}
	xmove_fd(tcp ? conn : sock, 0);

	if (max_per_host) {
	/* Drop connection immediately if cur_per_host > max_per_host
	* (minimizing load under SYN flood) */
	remote_addr = xmalloc_sockaddr2dotted_noport(&remote.u.sa);
	cur_per_host = ipsvd_perhost_add(remote_addr, max_per_host, &hccp);
	if (cur_per_host > max_per_host) {
	/* ipsvd_perhost_add detected that max is exceeded
	* (and did not store ip in connection table) */
	free(remote_addr);
	if (msg_per_host) {
	/* don't block or test for errors */
	send(0, msg_per_host, len_per_host, MSG_DONTWAIT);
	}
	goto again1;
	}
	/* NB: remote_addr is not leaked, it is stored in conn table */
	}

	if (!tcp) {
	/* Voodoo magic: making udp sockets each receive its own
	* packets is not trivial, and I still not sure
	* I do it 100% right.
	* 1) we have to do it before fork()
	* 2) order is important - is it right now? */

	/* Open new non-connected UDP socket for further clients... */
	sock = xsocket(lsa->u.sa.sa_family, SOCK_DGRAM, 0);
	setsockopt_reuseaddr(sock);
	/* Make plain write/send work for old socket by supplying default
	* destination address. This also restricts incoming packets
	* to ones coming from this remote IP. */
	xconnect(0, &remote.u.sa, sa_len);
	/* hole? at this point we have no wildcard udp socket...
	* can this cause clients to get "port unreachable" icmp?
	* Yup, time window is very small, but it exists (is it?) */
	/* ..."open new socket", continued */
	xbind(sock, &lsa->u.sa, sa_len);
	socket_want_pktinfo(sock);

	/* Doesn't work:
	* we cannot replace fd #0 - we will lose pending packet
	* which is already buffered for us! And we cannot use fd #1
	* instead - it will "intercept" all following packets, but child
	* does not expect data coming from fd #1! */
	#if 0
	/* Make it so that local addr is fixed to localp->u.sa
	* and we don't accidentally accept packets to other local IPs. */
	/* NB: we possibly bind to the _very_ same_ address & port as the one
	* already bound in parent! This seems to work in Linux.
	* (otherwise we can move socket to fd #0 only if bind succeeds) */
	close(0);
	set_nport(&localp->u.sa, htons(local_port));
	xmove_fd(xsocket(localp->u.sa.sa_family, SOCK_DGRAM, 0), 0);
	setsockopt_reuseaddr(0); /* crucial */
	xbind(0, &localp->u.sa, localp->len);
	#endif
	}

	pid = vfork();
	if (pid == -1) {
	bb_perror_msg("vfork");
	goto again;
	}

	if (pid != 0) {
	/* Parent */
	cnum++;
	if (verbose)
	connection_status();
	if (hccp)
	hccp->pid = pid;
	/* clean up changes done by vforked child */
	undo_xsetenv();
	goto again;
	}

	/* Child: prepare env, log, and exec prog */

	{ /* vfork alert! every xmalloc in this block should be freed! */
	char local_hostname = local_hostname; / for compiler */
	char *local_addr = NULL;
	char *free_me0 = NULL;
	char *free_me1 = NULL;
	char *free_me2 = NULL;

	if (verbose \|\| !(opts & OPT_E)) {
	if (!max_per_host) /* remote_addr is not yet known */
	free_me0 = remote_addr = xmalloc_sockaddr2dotted(&remote.u.sa);
	if (opts & OPT_h) {
	free_me1 = remote_hostname = xmalloc_sockaddr2host_noport(&remote.u.sa);
	if (!remote_hostname) {
	bb_error_msg("can't look up hostname for %s", remote_addr);
	remote_hostname = remote_addr;
	}
	}
	/* Find out local IP peer connected to.
	* Errors ignored (I'm not paranoid enough to imagine kernel
	* which doesn't know local IP). */
	if (tcp)
	getsockname(0, &local.u.sa, &local.len);
	/* else: for UDP it is done earlier by parent */
	local_addr = xmalloc_sockaddr2dotted(&local.u.sa);
	if (opts & OPT_h) {
	local_hostname = preset_local_hostname;
	if (!local_hostname) {
	free_me2 = local_hostname = xmalloc_sockaddr2host_noport(&local.u.sa);
	if (!local_hostname)
	bb_error_msg_and_die("can't look up hostname for %s", local_addr);
	}
	/* else: local_hostname is not NULL, but is NOT malloced! */
	}
	}
	if (verbose) {
	pid = getpid();
	if (max_per_host) {
	bb_error_msg("concurrency %s %u/%u",
	remote_addr,
	cur_per_host, max_per_host);
	}
	bb_error_msg((opts & OPT_h)
	? "start %u %s-%s (%s-%s)"
	: "start %u %s-%s",
	pid,
	local_addr, remote_addr,
	local_hostname, remote_hostname);
	}

	if (!(opts & OPT_E)) {
	/* setup ucspi env */
	const char *proto = tcp ? "TCP" : "UDP";

	#ifdef SO_ORIGINAL_DST
	/* Extract "original" destination addr:port
	* from Linux firewall. Useful when you redirect
	* an outbond connection to local handler, and it needs
	* to know where it originally tried to connect */
	if (tcp && getsockopt(0, SOL_IP, SO_ORIGINAL_DST, &local.u.sa, &local.len) == 0) {
	char *addr = xmalloc_sockaddr2dotted(&local.u.sa);
	xsetenv_plain("TCPORIGDSTADDR", addr);
	free(addr);
	}
	#endif
	xsetenv_plain("PROTO", proto);
	xsetenv_proto(proto, "LOCALADDR", local_addr);
	xsetenv_proto(proto, "REMOTEADDR", remote_addr);
	if (opts & OPT_h) {
	xsetenv_proto(proto, "LOCALHOST", local_hostname);
	xsetenv_proto(proto, "REMOTEHOST", remote_hostname);
	}
	//compat? xsetenv_proto(proto, "REMOTEINFO", "");
	/* additional */
	if (cur_per_host > 0) /* can not be true for udp */
	xsetenv_plain("TCPCONCURRENCY", utoa(cur_per_host));
	}
	free(local_addr);
	free(free_me0);
	free(free_me1);
	free(free_me2);
	}

	xdup2(0, 1);

	signal(SIGPIPE, SIG_DFL); /* this one was SIG_IGNed */
	/* Non-ignored signals revert to SIG_DFL on exec anyway */
	/signal(SIGCHLD, SIG_DFL);/
	sig_unblock(SIGCHLD);

	#ifdef SSLSVD
	strcpy(id, utoa(pid));
	ssl_io(0, argv);
	bb_perror_msg_and_die("can't execute '%s'", argv[0]);
	#else
	BB_EXECVP_or_die(argv);
	#endif
	}

	/*
	tcpsvd [-hpEvv] [-c n] [-C n:msg] [-b n] [-u user] [-l name]
	[-i dir\|-x cdb] [ -t sec] host port prog

	tcpsvd creates a TCP/IP socket, binds it to the address host:port,
	and listens on the socket for incoming connections.

	On each incoming connection, tcpsvd conditionally runs a program,
	with standard input reading from the socket, and standard output
	writing to the socket, to handle this connection. tcpsvd keeps
	listening on the socket for new connections, and can handle
	multiple connections simultaneously.

	tcpsvd optionally checks for special instructions depending
	on the IP address or hostname of the client that initiated
	the connection, see ipsvd-instruct(5).

	host
	host either is a hostname, or a dotted-decimal IP address,
	or 0. If host is 0, tcpsvd accepts connections to any local
	IP address.
	* busybox accepts IPv6 addresses and host:port pairs too
	In this case second parameter is ignored
	port
	tcpsvd accepts connections to host:port. port may be a name
	from /etc/services or a number.
	prog
	prog consists of one or more arguments. For each connection,
	tcpsvd normally runs prog, with file descriptor 0 reading from
	the network, and file descriptor 1 writing to the network.
	By default it also sets up TCP-related environment variables,
	see tcp-environ(5)
	-i dir
	read instructions for handling new connections from the instructions
	directory dir. See ipsvd-instruct(5) for details.
	* ignored by busyboxed version
	-x cdb
	read instructions for handling new connections from the constant database
	cdb. The constant database normally is created from an instructions
	directory by running ipsvd-cdb(8).
	* ignored by busyboxed version
	-t sec
	timeout. This option only takes effect if the -i option is given.
	While checking the instructions directory, check the time of last access
	of the file that matches the clients address or hostname if any, discard
	and remove the file if it wasn't accessed within the last sec seconds;
	tcpsvd does not discard or remove a file if the user's write permission
	is not set, for those files the timeout is disabled. Default is 0,
	which means that the timeout is disabled.
	* ignored by busyboxed version
	-l name
	local hostname. Do not look up the local hostname in DNS, but use name
	as hostname. This option must be set if tcpsvd listens on port 53
	to avoid loops.
	-u user[:group]
	drop permissions. Switch user ID to user's UID, and group ID to user's
	primary GID after creating and binding to the socket. If user is followed
	by a colon and a group name, the group ID is switched to the GID of group
	instead. All supplementary groups are removed.
	-c n
	concurrency. Handle up to n connections simultaneously. Default is 30.
	If there are n connections active, tcpsvd defers acceptance of a new
	connection until an active connection is closed.
	-C n[:msg]
	per host concurrency. Allow only up to n connections from the same IP
	address simultaneously. If there are n active connections from one IP
	address, new incoming connections from this IP address are closed
	immediately. If n is followed by :msg, the message msg is written
	to the client if possible, before closing the connection. By default
	msg is empty. See ipsvd-instruct(5) for supported escape sequences in msg.

	For each accepted connection, the current per host concurrency is
	available through the environment variable TCPCONCURRENCY. n and msg
	can be overwritten by ipsvd(7) instructions, see ipsvd-instruct(5).
	By default tcpsvd doesn't keep track of connections.
	-h
	Look up the client's hostname in DNS.
	-p
	paranoid. After looking up the client's hostname in DNS, look up the IP
	addresses in DNS for that hostname, and forget about the hostname
	if none of the addresses match the client's IP address. You should
	set this option if you use hostname based instructions. The -p option
	implies the -h option.
	* ignored by busyboxed version
	-b n
	backlog. Allow a backlog of approximately n TCP SYNs. On some systems n
	is silently limited. Default is 20.
	-E
	no special environment. Do not set up TCP-related environment variables.
	-v
	verbose. Print verbose messsages to standard output.
	-vv
	more verbose. Print more verbose messages to standard output.
	* no difference between -v and -vv in busyboxed version
	*/