common-session.c - third_party/dropbear - Git at Google

 /*
  * Dropbear - a SSH2 server
  *
  * Copyright (c) 2002,2003 Matt Johnston
  * All rights reserved.
  *
  * 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. */

 #include "includes.h"
 #include "session.h"
 #include "dbutil.h"
 #include "packet.h"
 #include "algo.h"
 #include "buffer.h"
 #include "dss.h"
 #include "ssh.h"
 #include "random.h"
 #include "kex.h"
 #include "channel.h"
 #include "atomicio.h"
 #include "runopts.h"

 static void checktimeouts();
 static long select_timeout();
 static int ident_readln(int fd, char* buf, int count);

 struct sshsession ses; /* GLOBAL */

 /* need to know if the session struct has been initialised, this way isn't the
  * cleanest, but works OK */
 int sessinitdone = 0; /* GLOBAL */

 /* this is set when we get SIGINT or SIGTERM, the handler is in main.c */
 int exitflag = 0; /* GLOBAL */


 /* called only at the start of a session, set up initial state */
 void common_session_init(int sock, char* remotehost) {

 	TRACE(("enter session_init"))

 	ses.remotehost = remotehost;

 	ses.sock = sock;
 	ses.maxfd = sock;

 	ses.connect_time = 0;
 	ses.last_packet_time = 0;

 	if (pipe(ses.signal_pipe) < 0) {
 		dropbear_exit("signal pipe failed");
 	}
 	setnonblocking(ses.signal_pipe[0]);
 	setnonblocking(ses.signal_pipe[1]);

 	kexfirstinitialise(); /* initialise the kex state */

 	ses.writepayload = buf_new(TRANS_MAX_PAYLOAD_LEN);
 	ses.transseq = 0;

 	ses.readbuf = NULL;
 	ses.decryptreadbuf = NULL;
 	ses.payload = NULL;
 	ses.recvseq = 0;

 	initqueue(&ses.writequeue);

 	ses.requirenext = SSH_MSG_KEXINIT;
 	ses.dataallowed = 1; /* we can send data until we actually
 							send the SSH_MSG_KEXINIT */
 	ses.ignorenext = 0;
 	ses.lastpacket = 0;
 	ses.reply_queue_head = NULL;
 	ses.reply_queue_tail = NULL;

 	/* set all the algos to none */
 	ses.keys = (struct key_context*)m_malloc(sizeof(struct key_context));
 	ses.newkeys = NULL;
 	ses.keys->recv_algo_crypt = &dropbear_nocipher;
 	ses.keys->trans_algo_crypt = &dropbear_nocipher;

 	ses.keys->recv_algo_mac = &dropbear_nohash;
 	ses.keys->trans_algo_mac = &dropbear_nohash;

 	ses.keys->algo_kex = -1;
 	ses.keys->algo_hostkey = -1;
 	ses.keys->recv_algo_comp = DROPBEAR_COMP_NONE;
 	ses.keys->trans_algo_comp = DROPBEAR_COMP_NONE;

 #ifndef DISABLE_ZLIB
 	ses.keys->recv_zstream = NULL;
 	ses.keys->trans_zstream = NULL;
 #endif

 	/* key exchange buffers */
 	ses.session_id = NULL;
 	ses.kexhashbuf = NULL;
 	ses.transkexinit = NULL;
 	ses.dh_K = NULL;
 	ses.remoteident = NULL;

 	ses.chantypes = NULL;

 	ses.allowprivport = 0;

 	TRACE(("leave session_init"))
 }

 void session_loop(void(*loophandler)()) {

 	fd_set readfd, writefd;
 	struct timeval timeout;
 	int val;

 	/* main loop, select()s for all sockets in use */
 	for(;;) {

 		timeout.tv_sec = select_timeout();
 		timeout.tv_usec = 0;
 		FD_ZERO(&writefd);
 		FD_ZERO(&readfd);
 		dropbear_assert(ses.payload == NULL);
 		if (ses.sock != -1) {
 			FD_SET(ses.sock, &readfd);
 			if (!isempty(&ses.writequeue)) {
 				FD_SET(ses.sock, &writefd);
 			}
 		}

 		/* We get woken up when signal handlers write to this pipe.
 		   SIGCHLD in svr-chansession is the only one currently. */
 		FD_SET(ses.signal_pipe[0], &readfd);

 		/* set up for channels which require reading/writing */
 		if (ses.dataallowed) {
 			setchannelfds(&readfd, &writefd);
 		}
 		val = select(ses.maxfd+1, &readfd, &writefd, NULL, &timeout);

 		if (exitflag) {
 			dropbear_exit("Terminated by signal");
 		}

 		if (val < 0 && errno != EINTR) {
 			dropbear_exit("Error in select");
 		}

 		if (val <= 0) {
 			/* If we were interrupted or the select timed out, we still
 			 * want to iterate over channels etc for reading, to handle
 			 * server processes exiting etc.
 			 * We don't want to read/write FDs. */
 			FD_ZERO(&writefd);
 			FD_ZERO(&readfd);
 		}

 		/* We'll just empty out the pipe if required. We don't do
 		any thing with the data, since the pipe's purpose is purely to
 		wake up the select() above. */
 		if (FD_ISSET(ses.signal_pipe[0], &readfd)) {
 			char x;
 			while (read(ses.signal_pipe[0], &x, 1) > 0) {}
 		}

 		/* check for auth timeout, rekeying required etc */
 		checktimeouts();

 		/* process session socket's incoming/outgoing data */
 		if (ses.sock != -1) {
 			if (FD_ISSET(ses.sock, &writefd) && !isempty(&ses.writequeue)) {
 				write_packet();
 			}

 			if (FD_ISSET(ses.sock, &readfd)) {
 				read_packet();
 			}

 			/* Process the decrypted packet. After this, the read buffer
 			 * will be ready for a new packet */
 			if (ses.payload != NULL) {
 				process_packet();
 			}
 		}

 		/* if required, flush out any queued reply packets that
 		were being held up during a KEX */
 		maybe_flush_reply_queue();

 		/* process pipes etc for the channels, ses.dataallowed == 0
 		 * during rekeying ) */
 		if (ses.dataallowed) {
 			channelio(&readfd, &writefd);
 		}

 		if (loophandler) {
 			loophandler();
 		}

 	} /* for(;;) */

 	/* Not reached */
 }

 /* clean up a session on exit */
 void common_session_cleanup() {

 	TRACE(("enter session_cleanup"))

 	/* we can't cleanup if we don't know the session state */
 	if (!sessinitdone) {
 		TRACE(("leave session_cleanup: !sessinitdone"))
 		return;
 	}

 	m_free(ses.session_id);
 	m_burn(ses.keys, sizeof(struct key_context));
 	m_free(ses.keys);

 	chancleanup();

 	TRACE(("leave session_cleanup"))
 }


 void session_identification() {

 	/* max length of 255 chars */
 	char linebuf[256];
 	int len = 0;
 	char done = 0;
 	int i;

 	/* write our version string, this blocks */
 	if (atomicio(write, ses.sock, LOCAL_IDENT "\r\n",
 				strlen(LOCAL_IDENT "\r\n")) == DROPBEAR_FAILURE) {
 		ses.remoteclosed();
 	}

     /* If they send more than 50 lines, something is wrong */
 	for (i = 0; i < 50; i++) {
 		len = ident_readln(ses.sock, linebuf, sizeof(linebuf));

 		if (len < 0 && errno != EINTR) {
 			/* It failed */
 			break;
 		}

 		if (len >= 4 && memcmp(linebuf, "SSH-", 4) == 0) {
 			/* start of line matches */
 			done = 1;
 			break;
 		}
 	}

 	if (!done) {
 		TRACE(("err: %s for '%s'\n", strerror(errno), linebuf))
 		ses.remoteclosed();
 	} else {
 		/* linebuf is already null terminated */
 		ses.remoteident = m_malloc(len);
 		memcpy(ses.remoteident, linebuf, len);
 	}

     /* Shall assume that 2.x will be backwards compatible. */
     if (strncmp(ses.remoteident, "SSH-2.", 6) != 0
             && strncmp(ses.remoteident, "SSH-1.99-", 9) != 0) {
         dropbear_exit("Incompatible remote version '%s'", ses.remoteident);
     }

 	TRACE(("remoteident: %s", ses.remoteident))

 }

 /* returns the length including null-terminating zero on success,
  * or -1 on failure */
 static int ident_readln(int fd, char* buf, int count) {

 	char in;
 	int pos = 0;
 	int num = 0;
 	fd_set fds;
 	struct timeval timeout;

 	TRACE(("enter ident_readln"))

 	if (count < 1) {
 		return -1;
 	}

 	FD_ZERO(&fds);

 	/* select since it's a non-blocking fd */

 	/* leave space to null-terminate */
 	while (pos < count-1) {

 		FD_SET(fd, &fds);

 		timeout.tv_sec = 1;
 		timeout.tv_usec = 0;
 		if (select(fd+1, &fds, NULL, NULL, &timeout) < 0) {
 			if (errno == EINTR) {
 				continue;
 			}
 			TRACE(("leave ident_readln: select error"))
 			return -1;
 		}

 		checktimeouts();

 		/* Have to go one byte at a time, since we don't want to read past
 		 * the end, and have to somehow shove bytes back into the normal
 		 * packet reader */
 		if (FD_ISSET(fd, &fds)) {
 			num = read(fd, &in, 1);
 			/* a "\n" is a newline, "\r" we want to read in and keep going
 			 * so that it won't be read as part of the next line */
 			if (num < 0) {
 				/* error */
 				if (errno == EINTR) {
 					continue; /* not a real error */
 				}
 				TRACE(("leave ident_readln: read error"))
 				return -1;
 			}
 			if (num == 0) {
 				/* EOF */
 				TRACE(("leave ident_readln: EOF"))
 				return -1;
 			}
 			if (in == '\n') {
 				/* end of ident string */
 				break;
 			}
 			/* we don't want to include '\r's */
 			if (in != '\r') {
 				buf[pos] = in;
 				pos++;
 			}
 		}
 	}

 	buf[pos] = '\0';
 	TRACE(("leave ident_readln: return %d", pos+1))
 	return pos+1;
 }

 void send_msg_ignore() {
 	CHECKCLEARTOWRITE();
 	buf_putbyte(ses.writepayload, SSH_MSG_IGNORE);
 	buf_putstring(ses.writepayload, "", 0);
 	encrypt_packet();
 }

 /* Check all timeouts which are required. Currently these are the time for
  * user authentication, and the automatic rekeying. */
 static void checktimeouts() {

 	time_t now;

 	now = time(NULL);

 	if (ses.connect_time != 0 && now - ses.connect_time >= AUTH_TIMEOUT) {
 			dropbear_close("Timeout before auth");
 	}

 	/* we can't rekey if we haven't done remote ident exchange yet */
 	if (ses.remoteident == NULL) {
 		return;
 	}

 	if (!ses.kexstate.sentkexinit
 			&& (now - ses.kexstate.lastkextime >= KEX_REKEY_TIMEOUT
 			|| ses.kexstate.datarecv+ses.kexstate.datatrans >= KEX_REKEY_DATA)) {
 		TRACE(("rekeying after timeout or max data reached"))
 		send_msg_kexinit();
 	}

 	if (opts.keepalive_secs > 0
 		&& now - ses.last_packet_time >= opts.keepalive_secs) {
 		send_msg_ignore();
 	}
 }

 static long select_timeout() {
 	/* determine the minimum timeout that might be required, so
 	as to avoid waking when unneccessary */
 	long ret = LONG_MAX;
 	if (KEX_REKEY_TIMEOUT > 0)
 		ret = MIN(KEX_REKEY_TIMEOUT, ret);
 	if (AUTH_TIMEOUT > 0)
 		ret = MIN(AUTH_TIMEOUT, ret);
 	if (opts.keepalive_secs > 0)
 		ret = MIN(opts.keepalive_secs, ret);
 	return ret;
 }
	/*
	* Dropbear - a SSH2 server
	*
	* Copyright (c) 2002,2003 Matt Johnston
	* All rights reserved.
	*
	* 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. */

	#include "includes.h"
	#include "session.h"
	#include "dbutil.h"
	#include "packet.h"
	#include "algo.h"
	#include "buffer.h"
	#include "dss.h"
	#include "ssh.h"
	#include "random.h"
	#include "kex.h"
	#include "channel.h"
	#include "atomicio.h"
	#include "runopts.h"

	static void checktimeouts();
	static long select_timeout();
	static int ident_readln(int fd, char* buf, int count);

	struct sshsession ses; /* GLOBAL */

	/* need to know if the session struct has been initialised, this way isn't the
	* cleanest, but works OK */
	int sessinitdone = 0; /* GLOBAL */

	/* this is set when we get SIGINT or SIGTERM, the handler is in main.c */
	int exitflag = 0; /* GLOBAL */



	/* called only at the start of a session, set up initial state */
	void common_session_init(int sock, char* remotehost) {

	TRACE(("enter session_init"))

	ses.remotehost = remotehost;

	ses.sock = sock;
	ses.maxfd = sock;

	ses.connect_time = 0;
	ses.last_packet_time = 0;

	if (pipe(ses.signal_pipe) < 0) {
	dropbear_exit("signal pipe failed");
	}
	setnonblocking(ses.signal_pipe[0]);
	setnonblocking(ses.signal_pipe[1]);

	kexfirstinitialise(); /* initialise the kex state */

	ses.writepayload = buf_new(TRANS_MAX_PAYLOAD_LEN);
	ses.transseq = 0;

	ses.readbuf = NULL;
	ses.decryptreadbuf = NULL;
	ses.payload = NULL;
	ses.recvseq = 0;

	initqueue(&ses.writequeue);

	ses.requirenext = SSH_MSG_KEXINIT;
	ses.dataallowed = 1; /* we can send data until we actually
	send the SSH_MSG_KEXINIT */
	ses.ignorenext = 0;
	ses.lastpacket = 0;
	ses.reply_queue_head = NULL;
	ses.reply_queue_tail = NULL;

	/* set all the algos to none */
	ses.keys = (struct key_context*)m_malloc(sizeof(struct key_context));
	ses.newkeys = NULL;
	ses.keys->recv_algo_crypt = &dropbear_nocipher;
	ses.keys->trans_algo_crypt = &dropbear_nocipher;

	ses.keys->recv_algo_mac = &dropbear_nohash;
	ses.keys->trans_algo_mac = &dropbear_nohash;

	ses.keys->algo_kex = -1;
	ses.keys->algo_hostkey = -1;
	ses.keys->recv_algo_comp = DROPBEAR_COMP_NONE;
	ses.keys->trans_algo_comp = DROPBEAR_COMP_NONE;

	#ifndef DISABLE_ZLIB
	ses.keys->recv_zstream = NULL;
	ses.keys->trans_zstream = NULL;
	#endif

	/* key exchange buffers */
	ses.session_id = NULL;
	ses.kexhashbuf = NULL;
	ses.transkexinit = NULL;
	ses.dh_K = NULL;
	ses.remoteident = NULL;

	ses.chantypes = NULL;

	ses.allowprivport = 0;

	TRACE(("leave session_init"))
	}

	void session_loop(void(*loophandler)()) {

	fd_set readfd, writefd;
	struct timeval timeout;
	int val;

	/* main loop, select()s for all sockets in use */
	for(;;) {

	timeout.tv_sec = select_timeout();
	timeout.tv_usec = 0;
	FD_ZERO(&writefd);
	FD_ZERO(&readfd);
	dropbear_assert(ses.payload == NULL);
	if (ses.sock != -1) {
	FD_SET(ses.sock, &readfd);
	if (!isempty(&ses.writequeue)) {
	FD_SET(ses.sock, &writefd);
	}
	}

	/* We get woken up when signal handlers write to this pipe.
	SIGCHLD in svr-chansession is the only one currently. */
	FD_SET(ses.signal_pipe[0], &readfd);

	/* set up for channels which require reading/writing */
	if (ses.dataallowed) {
	setchannelfds(&readfd, &writefd);
	}
	val = select(ses.maxfd+1, &readfd, &writefd, NULL, &timeout);

	if (exitflag) {
	dropbear_exit("Terminated by signal");
	}

	if (val < 0 && errno != EINTR) {
	dropbear_exit("Error in select");
	}

	if (val <= 0) {
	/* If we were interrupted or the select timed out, we still
	* want to iterate over channels etc for reading, to handle
	* server processes exiting etc.
	* We don't want to read/write FDs. */
	FD_ZERO(&writefd);
	FD_ZERO(&readfd);
	}

	/* We'll just empty out the pipe if required. We don't do
	any thing with the data, since the pipe's purpose is purely to
	wake up the select() above. */
	if (FD_ISSET(ses.signal_pipe[0], &readfd)) {
	char x;
	while (read(ses.signal_pipe[0], &x, 1) > 0) {}
	}

	/* check for auth timeout, rekeying required etc */
	checktimeouts();

	/* process session socket's incoming/outgoing data */
	if (ses.sock != -1) {
	if (FD_ISSET(ses.sock, &writefd) && !isempty(&ses.writequeue)) {
	write_packet();
	}

	if (FD_ISSET(ses.sock, &readfd)) {
	read_packet();
	}

	/* Process the decrypted packet. After this, the read buffer
	* will be ready for a new packet */
	if (ses.payload != NULL) {
	process_packet();
	}
	}

	/* if required, flush out any queued reply packets that
	were being held up during a KEX */
	maybe_flush_reply_queue();

	/* process pipes etc for the channels, ses.dataallowed == 0
	* during rekeying ) */
	if (ses.dataallowed) {
	channelio(&readfd, &writefd);
	}

	if (loophandler) {
	loophandler();
	}

	} /* for(;;) */

	/* Not reached */
	}

	/* clean up a session on exit */
	void common_session_cleanup() {

	TRACE(("enter session_cleanup"))

	/* we can't cleanup if we don't know the session state */
	if (!sessinitdone) {
	TRACE(("leave session_cleanup: !sessinitdone"))
	return;
	}

	m_free(ses.session_id);
	m_burn(ses.keys, sizeof(struct key_context));
	m_free(ses.keys);

	chancleanup();

	TRACE(("leave session_cleanup"))
	}


	void session_identification() {

	/* max length of 255 chars */
	char linebuf[256];
	int len = 0;
	char done = 0;
	int i;

	/* write our version string, this blocks */
	if (atomicio(write, ses.sock, LOCAL_IDENT "\r\n",
	strlen(LOCAL_IDENT "\r\n")) == DROPBEAR_FAILURE) {
	ses.remoteclosed();
	}

	/* If they send more than 50 lines, something is wrong */
	for (i = 0; i < 50; i++) {
	len = ident_readln(ses.sock, linebuf, sizeof(linebuf));

	if (len < 0 && errno != EINTR) {
	/* It failed */
	break;
	}

	if (len >= 4 && memcmp(linebuf, "SSH-", 4) == 0) {
	/* start of line matches */
	done = 1;
	break;
	}
	}

	if (!done) {
	TRACE(("err: %s for '%s'\n", strerror(errno), linebuf))
	ses.remoteclosed();
	} else {
	/* linebuf is already null terminated */
	ses.remoteident = m_malloc(len);
	memcpy(ses.remoteident, linebuf, len);
	}

	/* Shall assume that 2.x will be backwards compatible. */
	if (strncmp(ses.remoteident, "SSH-2.", 6) != 0
	&& strncmp(ses.remoteident, "SSH-1.99-", 9) != 0) {
	dropbear_exit("Incompatible remote version '%s'", ses.remoteident);
	}

	TRACE(("remoteident: %s", ses.remoteident))

	}

	/* returns the length including null-terminating zero on success,
	* or -1 on failure */
	static int ident_readln(int fd, char* buf, int count) {

	char in;
	int pos = 0;
	int num = 0;
	fd_set fds;
	struct timeval timeout;

	TRACE(("enter ident_readln"))

	if (count < 1) {
	return -1;
	}

	FD_ZERO(&fds);

	/* select since it's a non-blocking fd */

	/* leave space to null-terminate */
	while (pos < count-1) {

	FD_SET(fd, &fds);

	timeout.tv_sec = 1;
	timeout.tv_usec = 0;
	if (select(fd+1, &fds, NULL, NULL, &timeout) < 0) {
	if (errno == EINTR) {
	continue;
	}
	TRACE(("leave ident_readln: select error"))
	return -1;
	}

	checktimeouts();

	/* Have to go one byte at a time, since we don't want to read past
	* the end, and have to somehow shove bytes back into the normal
	* packet reader */
	if (FD_ISSET(fd, &fds)) {
	num = read(fd, &in, 1);
	/* a "\n" is a newline, "\r" we want to read in and keep going
	* so that it won't be read as part of the next line */
	if (num < 0) {
	/* error */
	if (errno == EINTR) {
	continue; /* not a real error */
	}
	TRACE(("leave ident_readln: read error"))
	return -1;
	}
	if (num == 0) {
	/* EOF */
	TRACE(("leave ident_readln: EOF"))
	return -1;
	}
	if (in == '\n') {
	/* end of ident string */
	break;
	}
	/* we don't want to include '\r's */
	if (in != '\r') {
	buf[pos] = in;
	pos++;
	}
	}
	}

	buf[pos] = '\0';
	TRACE(("leave ident_readln: return %d", pos+1))
	return pos+1;
	}

	void send_msg_ignore() {
	CHECKCLEARTOWRITE();
	buf_putbyte(ses.writepayload, SSH_MSG_IGNORE);
	buf_putstring(ses.writepayload, "", 0);
	encrypt_packet();
	}

	/* Check all timeouts which are required. Currently these are the time for
	* user authentication, and the automatic rekeying. */
	static void checktimeouts() {

	time_t now;

	now = time(NULL);

	if (ses.connect_time != 0 && now - ses.connect_time >= AUTH_TIMEOUT) {
	dropbear_close("Timeout before auth");
	}

	/* we can't rekey if we haven't done remote ident exchange yet */
	if (ses.remoteident == NULL) {
	return;
	}

	if (!ses.kexstate.sentkexinit
	&& (now - ses.kexstate.lastkextime >= KEX_REKEY_TIMEOUT
	\|\| ses.kexstate.datarecv+ses.kexstate.datatrans >= KEX_REKEY_DATA)) {
	TRACE(("rekeying after timeout or max data reached"))
	send_msg_kexinit();
	}

	if (opts.keepalive_secs > 0
	&& now - ses.last_packet_time >= opts.keepalive_secs) {
	send_msg_ignore();
	}
	}

	static long select_timeout() {
	/* determine the minimum timeout that might be required, so
	as to avoid waking when unneccessary */
	long ret = LONG_MAX;
	if (KEX_REKEY_TIMEOUT > 0)
	ret = MIN(KEX_REKEY_TIMEOUT, ret);
	if (AUTH_TIMEOUT > 0)
	ret = MIN(AUTH_TIMEOUT, ret);
	if (opts.keepalive_secs > 0)
	ret = MIN(opts.keepalive_secs, ret);
	return ret;
	}