Posts Tagged ‘server clock’

How to configure NTP server (ntpd) to synchronize server clock over the Internet on FreeBSD

Friday, February 10th, 2012

Reading Time: 4 minutes

 

FreeBSD ntpd logo / How to configure ntpd to synchronize with internet time servers on FreeBSD

On FreeBSD ntpd , ntpdc , ntpdate , ntpq doesn't need to be installed via a specific package like on GNU/Linux as they're part of the FreeBSD world (binary standardly shipped with FreeBSD basis system).

The FreeBSD handbook has a chapter explaining thoroughfully on ntp on FreeBSD ,however for the lazy ones here is a short few steps tutorial on how to install and configure ntpd on bsd :

1. Copy sample ntp.conf file to /etc/

freebsd# cp -rpf /usr/src/etc/ntp.conf /etc/ntp/

No need for any modifications if you don't want to apply some specific restrictions on whom can access the ntpd server. If you update regularly the FreeBSD system with freebsd-update or directly by rebuilding the FreeBSD kernel / world adding restrictions might be not necessery..

If you check /usr/src/etc/ntp.conf you will notice freebsd project people are running their own ntp servers , by default ntpd will use this servers to fetch timing information. The exact server hosts which as of time of writting are used can be seen in ntp.conf and are:

server 0.freebsd.pool.ntp.org iburst maxpoll 9
server 1.freebsd.pool.ntp.org iburst maxpoll 9
server 2.freebsd.pool.ntp.org iburst maxpoll 9

2. Add ntpd daemon to load on system boot via /etc/rc.conf

By default ntpd is disabled on FreeBSD, you can see if it is disabled or enabled by invoking:

freebsd# /etc/rc.d/ntpd rcvar
# ntpd
ntpd_enable=NO

To Enable ntpd to get loaded each time it boots , following 3 lines has to be added in /etc/rc.conf .

ntpdate_enable="YES"
ntpdate_flags="europe.pool.ntp.org"
ntpd_enable="YES"

Quick way to add them is to use echo :

echo 'ntpdate_enable="YES" >> /etc/rc.conf
echo 'ntpdate_flags="europe.pool.ntp.org" >> /etc/rc.conf
echo 'ntpd_enable="YES" >> /etc/rc.conf

Now as the 3 rc.conf vars are set to "YES", the ntpd can be started. Without having this variables in /etc/rc.conf , "/etc/rc.d/ntpd start" will refuse to start ntpd.

3. Start the ntpd service

freebsd# /etc/rc.d/ntpd start
...

One interesting note to make is ntpd can also operate without specifying any config file (/etc/ntp.conf), the only requirement for the server to start is to have a properly set ntpdate server, like lets say (ntpdate_flags="europe.pool.ntp.org")

4. Permit only certain host or localhost to "talk" to the ntpd server

If you want to imply some ntp server restrictions, the configuration directives are same like on Linux:

To allow only a a host inside a local network with IP 192.168.0.2 as well as localhost, to be able to fetch time information via ntpd server put inside /etc/ntp.conf:

restrict 127.0.0.1
restrict 192.168.0.1 mask 255.255.255.0 nomodify notrap

If you want to prohibit ntpd to serve as a Network Time Server, to any other host except localhost, add in /etc/ntp.conf :

restrict default ignore

Allowing and denying certain hosts can be also done on pf (packet filter) or ipfw firewall level, and in my view is easier (and less confusing), than adding restrictions through ntp.conf. Besides that using directly the server firewall to apply restrictions is more secure. If for instance a remote exploit vulnerability is discovered affecting your ntpd server. this will not affect you externally as access to the UDP port 123 will be disabled on a firewall level.
Something good to mention is NTP servers communicate between each other using the UDP source/destination (port 123). Hence if the NTPD server has to be publicly accessible and there is a firewall already implemented, access to source/dest port 123 should be included in the configured firewall …

5. Check if the ntp server is running properly / ntp server query operations

[root@pcfreak /home/hipo]# ps axuww|grep -i ntp
root 15647 0.0 0.2 4672 1848 ?? Ss 2:49PM 0:00.04 /usr/sbin/ntpd -c /etc/ntp.conf -p /var/run/ntpd.pid -f /var/db/ntpd.drift

To query the now running ntpd server as well as set various configuration options "on the fly" (e.g. without need for ntp.conf edits and init script restart), a tool called ntpdc exists. ntpdc tool could be used to connect to localhost running ntpd as well as to connect and manage remotely a ntpd server.
The most basic use of ntpdc is to check (server peers).:
freebsd# ntpdc localhost
ntpdc> peers
remote local st poll reach delay offset disp
===================================================

kgb.comnet.bg 83.228.93.76 2 64 377 0.00282 -0.050575 0.06059
*billing.easy-la 83.228.93.76 2 64 377 0.01068 -0.057400 0.06770
=ns2.novatelbg.n 83.228.93.76 2 64 377 0.01001 -0.055290 0.06058

ntpdc has also a non-interactive interface, handy if there is a need for requests to a ntpd to be scripted. To check ntpd server peers non-interactively:

freebsd# ntpdc -p localhost
===================================================
kgb.comnet.bg 83.228.93.76 2 64 377 0.00284 -0.043157 0.06184
=billing.easy-la 83.228.93.76 2 64 377 0.01059 -0.042648 0.05811
*ns2.novatelbg.n 83.228.93.76 2 64 377 0.00996 -0.041097 0.06094

ntpdc has plenty of other ntpd query options, e.g. :

ntpdc> help
ntpdc commands:
addpeer controlkey fudge keytype quit timeout
addrefclock ctlstats help listpeers readkeys timerstats
addserver debug host loopinfo requestkey traps
addtrap delay hostnames memstats reset trustedkey
authinfo delrestrict ifreload monlist reslist unconfig
broadcast disable ifstats passwd restrict unrestrict
clkbug dmpeers iostats peers showpeer untrustedkey
clockstat enable kerninfo preset sysinfo version
clrtrap exit keyid pstats sysstats

ntpdc is an advanced query tool for ntpd , servers. Another tool exists called ntpq which syntax is almost identical to ntpdc . The main difference between the two is ntpq is a monitoring tool mostly used just for monitoring purposes, where ntpdc can also change plenty of things in the server configuration.

For people who want to learn more on ntpd the man page is a great reading , containing chapters describing thoroughfully exactly how NTPD time servers operate, etc.

How to install and configure NTP Server (ntpd) to synchronize Linux server clock over the Internet on CentOS, RHEL, Fedora

Thursday, February 9th, 2012

Reading Time: 3 minutes

Every now and then I have to work on servers running CentOS or Fedora Linux. Very typical problem that I observe on many servers which I have to inherit is the previous administrator did not know about the existence of NTP (Network Time Protocol) or forgot to install the ntpd server. As a consequence the many installed server services did not have a correct clock and at some specific cases this caused issues for web applications running on the server or any CMS installed etc.

The NTP Daemon is existing in GNU / linux since the early days of Linux and it served quite well so far. The NTP protocol has been used since the early days of the internet and for centuries is a standard protocol for BSD UNIX.

ntp is available in I believe all Linux distributions directly as a precompiled binary and can be installed on Fedora, CentOS with:

[root@centos ~]# yum install ntp

ntpd synchronizes the server clock with one of the /etc/ntp.conf defined RedHat NTP list

server 0.rhel.pool.ntp.org
server 1.rhel.pool.ntp.org
server 2.rhel.pool.ntp.org

To Synchronize manually the server system clock the ntp CentOS rpm package contains a tool called ntpdate :
Hence its a good practice to use ntpdate to synchronize the local server time with a internet server, the way I prefer to do this is via a government owned ntp server time.nist.gov, e.g.

[root@centos ~]# ntpdate time.nist.gov
8 Feb 14:21:03 ntpdate[9855]: adjust time server 192.43.244.18 offset -0.003770 sec

Alternatively if you prefer to use one of the redhat servers use:

[root@centos ~]# ntpdate 0.rhel.pool.ntp.org
8 Feb 14:20:41 ntpdate[9841]: adjust time server 72.26.198.240 offset 0.005671 sec

Now as the system time is set to a correct time via the ntp server, the ntp server is to be launched:

[root@centos ~]# /etc/init.d/ntpd start
...

To permanently enable the ntpd service to start up in boot time issue also:

[root@centos ~]# chkconfig ntpd on

Using chkconfig and /etc/init.d/ntpd cmds, makes the ntp server to run permanently via the ntpd daemon:

[root@centos ~]# ps ax |grep -i ntp
29861 ? SLs 0:00 ntpd -u ntp:ntp -p /var/run/ntpd.pid -g

If you prefer to synchronize periodically the system clock instead of running permanently a network server listening (for increased security), you should omit the above chkconfig ntpd on and /etc/init.d/ntpd start commands and instead set in root crontab the time to get synchronize lets say every 30 minutes, like so:

[root@centos ~]# echo '30 * * * * root /sbin/ntpd -q -u ntp:ntp' > /etc/cron.d/ntpd

The time synchronization via crontab can be also done using the ntpdate cmd. For example if you want to synchronize the server system clock with a network server every 5 minutes:

[root@centos ~]# crontab -u root -e

And paste inside:

*/5 * * * * /sbin/ntpdate time.nist.gov 2>1 > /dev/null

ntp package is equipped with ntpq Standard NTP Query Program. To get very basic stats for the running ntpd daemon use:

[root@centos ~]# ntpq -p
remote refid st t when poll reach delay offset jitter
======================================================
B1-66ER.matrix. 192.43.244.18 2 u 47 64 17 149.280 41.455 11.297
*ponderosa.piney 209.51.161.238 2 u 27 64 37 126.933 32.149 8.382
www2.bitvector. 132.163.4.103 2 u 1 64 37 202.433 12.994 13.999
LOCAL(0) .LOCL. 10 l 24 64 37 0.000 0.000 0.001

The remote field shows the servers to which currently the ntpd service is connected. This IPs are the servers which ntp uses to synchronize the local system server clock. when field shows when last the system was synchronized by the remote time server and the rest is statistical info about connection quality etc.

If the ntp server is to be run in daemon mode (ntpd to be running in the background). Its a good idea to allow ntp connections from the local network and filter incoming connections to port num 123 in /etc/sysconfig/iptables :

-A INPUT -s 192.168.1.0/24 -m state --state NEW -p udp --dport 123 -j ACCEPT
-A INPUT -s 127.0.0.1 -m state --state NEW -p udp --dport 123 -j ACCEPT
-A INPUT -s 0.0.0.0 -m state --state NEW -p udp --dport 123 -j DROP

Restrictions on which IPs can be connected to the ntp server can also be implied on a ntpd level through /etc/ntp.conf. For example if you would like to add the local network IPs range 192.168.0.1/24 to access ntpd, in ntpd.conf should be added policy:

# Hosts on local network are less restricted.
restrict 192.168.0.1 mask 255.255.255.0 nomodify notrap

To deny all access to any machine to the ntpd server add in /etc/ntp.conf:

restrict default ignore

After making any changes to ntp.conf , a server restart is required to load the new config settings, e.g.:

[root@centos ~]# /sbin/service ntpd restart

In most cases I think it is better to imply restrictions on a iptables (firewall) level instead of bothering change the default ntp.conf

Once ntpd is running as daemon, the server listens for UDP connections on udp port 123, to see it use:

[root@centos ~]# netstat -tulpn|grep -i ntp
udp 0 0 10.10.10.123:123 0.0.0.0:* 29861/ntpd
udp 0 0 80.95.28.179:123 0.0.0.0:* 29861/ntpd
udp 0 0 127.0.0.1:123 0.0.0.0:* 29861/ntpd
udp 0 0 0.0.0.0:123 0.0.0.0:* 29861/ntpd