Archive for February 12th, 2012

How to install OpenNTPD NTP server to synchronize system clock on FreeBSD for better security

Sunday, February 12th, 2012

FreeBSD, OpenBSD, NetBSD and Linux ntpd alternative server to synchronize server system time

Lately I've been researching on ntpd and wrote a two articles on how to install ntpd on CentOS, Fedora and how to install ntpd on FreeBSD and during my research on ntpd, I've come across OpenNTPD and decided to give it a go on my FreeBSD home router.
OpenBSD project is well known for it is high security standards and historically has passed the test of time for being a extraordinary secure UNIX like free operating system.
OpenBSD is developed in parallel with FreeBSD, however the development model of the two free operating systems are way different.

As a part of the OpenBSD to be independant in its basis of software from other free operating systems like GNU / Linux and FreeBSD. They develop the all around free software realm known OpenSSH. Along with OpenSSH, one interesting project developed for the main purpose of OpenBSD is OpenNTPD.

Here is how openntpd.org describes OpenNTPD:

"a FREE, easy to use implementation of the Network Time Protocol. It provides the ability to sync the local clock to remote NTP servers and can act as NTP server itself, redistributing the local clock."

OpenNTPD's accent just like OpenBSD's accent is security and hence for FreeBSD installs which targets security openntpd might be a good choice. Besides that the so popular classical ntpd has been well known for being historically "insecure", remote exploits for it has been released already at numerous times.

Another reason for someone to choose run openntpd instead of ntpd is its great simplicity. openntpd configuration is super simple.

Here are the steps I followed to have openntpd time server synchronize clock on my system using other public accessible openntpd servers on the internet.

1. Install openntpd through pkg_add -vr openntpd or via ports tree

a) For binar install with pkg_add issue:

freebsd# pkg_add -vr openntpd
...

b) if you prefer to compile it from source

freebsd# cd /usr/ports/net/openntpd
freebsd# make install clean
...

2. Enable OpenNTPD to start on system boot:

freebsd# echo 'openntpd_enable="YES"' >> /etc/rc.conf

3. Create openntpd ntpd.conf configuration file

There is a default sample ntpd.conf configuration which can be straight use as a conf basis:

freebsd# cp -rpf /usr/local/share/examples/openntpd/ntpd.conf /usr/local/etc/ntpd.conf

Default ntpd.conf works just fine without any modifications, if however there is a requirement the openntpd server to listen and accept time synchronization requests from only certain hosts add to conf something like:

listen on 192.168.1.2
listen on 192.168.1.3
listen on 2607:f0d0:3001:0009:0000:0000:0000:0001
listen on 127.0.0.1

This configuration will enable only 192.168.1.2 and 192.168.1.3 IPv4 addresses as well as the IPv6 2607:f0d0:3001:0009:0000:0000:0000:0001 IP to communicate with openntpd.

4. Start OpenNTPD service

freebsd# /usr/local/etc/rc.d/openntpd

5. Verify if openntpd is up and running

freebsd# ps axuww|grep -i ntp
root 31695 0.0 0.1 3188 1060 ?? Ss 11:26PM 0:00.00 ntpd: [priv] (ntpd)
_ntp 31696 0.0 0.1 3188 1140 ?? S 11:26PM 0:00.00 ntpd: ntp engine (ntpd)
_ntp 31697 0.0 0.1 3188 1088 ?? S 11:26PM 0:00.00 ntpd: dns engine (ntpd)
root 31700 0.0 0.1 3336 1192 p2 S+ 11:26PM 0:00.00 grep -i ntp

Its also good idea to check if openntpd has succesfully established connection with its peer remote openntpd time servers. This is necessery to make sure pf / ipfw firewall rules are not preventing connection to remote 123 UDP port:

freebsd# sockstat -4 -p 123
USER COMMAND PID FD PROTO LOCAL ADDRESS FOREIGN ADDRESS
_ntp ntpd 31696 4 udp4 83.228.93.76:54555 212.70.148.15:123
_ntp ntpd 31696 6 udp4 83.228.93.76:56666 195.69.120.36:123
_ntp ntpd 31696 8 udp4 83.228.93.76:49976 217.75.140.188:123

By default openntpd is also listening to IPv6 if IPv6 support is enabled in freebsd kernel.

6. Resolve openntpd firewall filtering issues

If there is a pf firewall blocking UDP requests to in/out port 123 within /etc/pf.conf rule like:

block in log on $EXT_NIC proto udp all

Before the blocking rule you will have to add pf rules:

# Ipv4 Open outgoing port TCP 123 (NTP)
pass out on $EXT_NIC proto tcp to any port ntp
# Ipv6 Open outgoing port TCP 123 (NTP)
pass out on $EXT_NIC inet6 proto tcp to any port ntp
# Ipv4 Open outgoing port UDP 123 (NTP)
pass out on $EXT_NIC proto udp to any port ntp
# Ipv6 Open outgoing port UDP 123 (NTP)
pass out on $EXT_NIC inet6 proto udp to any port ntp

where $EXT_NIC is defined to be equal to the external lan NIC interface, for example:
EXT_NIC="ml0"

Afterwards to load the new pf.conf rules firewall has to be flushed and reloaded:

freebsd# /sbin/pfctl -f /etc/pf.conf -d
...
freebsd# /sbin/pfctl -f /etc/pf.conf -e
...

In conclusion openntpd should be more secure than regular ntpd and in many cases is probably a better choice.
Anyhow bear in mind on FreeBSD openntpd is not part of the freebsd world and therefore security updates will not be issued directly by the freebsd dev team, but you will have to regularly update with the latest version provided from the bsd ports to make sure openntpd is 100% secure.

For anyone looking for more precise system clock synchronization and not so focused on security ntpd might be still a better choice. The OpenNTPD's official page states it is designed to reach reasonable time accuracy, but is not after the last microseconds.
 

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