大家在同步时间的时候，是不是一般使用这个工具

freeden

NTP

dataset

也可以使用rdate命令同步时间
对于提供非数据库服务相关的服务器来说，在脚本中使用rdate更加方便

NTP主要是每次调整的变化量可以控制，因此对于数据库相关的服务而言，每次时间调整量非常小，业务可以适应。如果时间突然变化1个小时，你可以想象你的数据库会作何反应。
而rdate使用的就是工作于37端口的timeserver协议(不是13端口的daytime协议)，实现并不知道作为参照的远程主机和本地主机之间的时间差，因此有可能变化量比较大，对于数据库类型的服务来说，大的时间变化量会导致不可知的错误

        rc = getaddrinfo(argv[1], \"time\", &hints, &res);

源代码：rdate.c


CODE:[Copy to clipboard]/*
* CDDL HEADER START
*
* The contents of this file are subject to the terms of the
* Common Development and Distribution License, Version 1.0 only
* (the \"License\").  You may not use this file except in compliance
* with the License.
*
* You can obtain a copy of the license at usr/src/OPENSOLARIS.LICENSE
* or http://www.opensolaris.org/os/licensing.
* See the License for the specific language governing permissions
* and limitations under the License.
*
* When distributing Covered Code, include this CDDL HEADER in each
* file and include the License file at usr/src/OPENSOLARIS.LICENSE.
* If applicable, add the following below this CDDL HEADER, with the
* fields enclosed by brackets \"[]\" replaced with your own identifying
* information: Portions Copyright [yyyy] [name of copyright owner]
*
* CDDL HEADER END
*/
/*
* Copyright 2004 Sun Microsystems, Inc.  All rights reserved.
* Use is subject to license terms.
*/

/*        Copyright (c) 1983, 1984, 1985, 1986, 1987, 1988, 1989 AT&T        */
/*          All Rights Reserved          */

/*
* University Copyright- Copyright (c) 1982, 1986, 1988
* The Regents of the University of California
* All Rights Reserved
*
* University Acknowledgment- Portions of this document are derived from
* software developed by the University of California, Berkeley, and its
* contributors.
*/

#pragma ident        \"@(#)rdate.c        1.14        05/06/08 SMI\"

/*
*  rdate - get date from remote machine
*
*        sets time, obtaining value from host
*        on the tcp/time socket.
*/

#include <signal.h>
#include <sys/types.h>
#include <sys/time.h>
#include <sys/socket.h>
#include <netinet/tcp.h>
#include <strings.h>
#include <unistd.h>
#include <stdlib.h>
#include <netdb.h>
#include <stdio.h>

/*
* The timeserver returns with time of day in seconds since
* Jan 1, 1900. We must subtract 86400(365*70 + 17) to get time
* since Jan 1, 1970, which is what get/settimeofday uses.
*/

#define        TOFFSET        ((unsigned int)86400*(365*70 + 17))

/*
* Before setting the system time, the value returned by the
* timeserver is checked for plausibility. If the returned date
* is before the time this program was written it cannot be
* correct.
*/

#define        WRITTEN 440199955                /* 22:45:55 13/Dec/1983 */
#define        SECONDS_TO_MS        1000

static void timeout(int);

int
main(int argc, char **argv)
{
        int s, i;
        uint32_t time;
        struct timeval timestruct;
        unsigned int connect_timeout;
        /* number of seconds to wait for something to happen. */
        unsigned int rdate_timeout = 30;        /* seconds */
        struct addrinfo hints;
        struct addrinfo *res;
        int rc;

        if (argc != 2) {
                (void) fputs(\"usage: rdate host\\n\", stderr);
                exit(EXIT_FAILURE);
        }

        (void) memset(&hints, 0, sizeof (hints));
        hints.ai_protocol = IPPROTO_TCP;
        res = NULL;

        /*
         * getaddrinfo() may take a long time, because it can involve
         * NIS, DNS, or LDAP lookups. Set an alarm timer. Note that this
         * may still fail, depending on how SIGALRM is handled by the
         * functions invoked by getaddrinfo().
         */

        (void) signal(SIGALRM, timeout);
        (void) alarm(rdate_timeout);

        /*
         * Note: memory not freed due to short lifetime of program.
         */
        rc = getaddrinfo(argv[1], \"time\", &hints, &res);
        (void) alarm(0);

        if (rc != 0) {
                (void) fprintf(stderr, \"Host name %s not found: %s\\n\", argv[1],
                    gai_strerror(rc));
                exit(EXIT_FAILURE);
        }

        connect_timeout = rdate_timeout * SECONDS_TO_MS;
        for (; res != NULL; res = res->ai_next) {
                s = socket(res->ai_addr->sa_family, res->ai_socktype,
                    res->ai_protocol);
                if (s < 0) {
                        perror(\"rdate: socket\");
                        exit(EXIT_FAILURE);
                }

                if (setsockopt(s, IPPROTO_TCP, TCP_CONN_ABORT_THRESHOLD,
                    (char *)&connect_timeout, sizeof (connect_timeout)) == -1) {
                        perror(\"setsockopt TCP_CONN_ABORT_THRESHOLD\");
                }

                if (connect(s, res->ai_addr, res->ai_addrlen) >= 0)
                        break;

                if (res->ai_next == NULL) {
                        perror(\"rdate: connect\");
                        (void) close(s);
                        exit(EXIT_FAILURE);
                }

                (void) close(s);
        }

        (void) alarm(rdate_timeout);
        if (read(s, (char *)&time, sizeof (time)) != sizeof (time)) {
                perror(\"rdate: read\");
                exit(EXIT_FAILURE);
        }
        (void) alarm(0);

        time = ntohl(time) - TOFFSET;
        /* date must be later than when program was written */
        if (time < WRITTEN) {
                (void) fprintf(stderr, \"didn\'t get plausible time from %s\\n\",
                    argv[1]);
                exit(EXIT_FAILURE);
        }
        timestruct.tv_usec = 0;
        timestruct.tv_sec = time;
        i = settimeofday(&timestruct, 0);
        if (i == -1) {
                perror(\"couldn\'t set time of day\");
                exit(EXIT_FAILURE);
        } else {
                (void) printf(\"%s\", ctime(&timestruct.tv_sec));
#if defined(i386)
                (void) system(\"/usr/sbin/rtc -c > /dev/null 2>&1\");
#endif
        }
        return (EXIT_SUCCESS);
}

/*ARGSUSED*/
static void
timeout(int sig)
{
        (void) fputs(\"couldn\'t contact time server\\n\", stderr);
        exit(EXIT_FAILURE);
        /* NOTREACHED */
}

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