NTP keeps distributed systems in step with reality. This practical introduction explains what NTP is, how its algorithms work, the history from early ARPANET experiments to NTPv4 and NTS, and why accurate time underpins security, logging, replication, and operations—plus how NTP compares with PTP in modern environments.
Need to block am IP address from your server? Here’s an IPTables command: /sbin/iptables -I INPUT -s w.x.y.z -j DROP where w.x.y.z is the IP address to be blocked. Here’s a way to block and address with just the route command: /sbin/route add -host w.x.y.z reject
Here’s how to filter or block ICMP timestamp requests and replies. On Redhat/CentOS, edit /etc/sysconfig/iptables and add the following lines
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-A RH-Firewall-1-INPUT -p ICMP --icmp-type timestamp-request -j DROP -A RH-Firewall-1-INPUT -p ICMP --icmp-type timestamp-reply -j DROP |
and then run:
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/sbin/iptables restart |
Or run the following commands:
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/sbin/iptables -I RH-Firewall-1-INPUT 1 -p ICMP --icmp-type timestamp-request -j DROP /sbin/iptables -I RH-Firewall-1-INPUT 1 -p ICMP --icmp-type timestamp-reply -j DROP /sbin/service iptables save |
Recently on an Ubantu server we just added these lines to /etc/rc.local:
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/sbin/iptables -I INPUT 1 -p ICMP --icmp-type timestamp-request -j DROP /sbin/iptables -I INPUT 1 -p ICMP --icmp-type timestamp-reply -j DROP |
OpenNMS is a great network monitoring tool. Here’s the OpenNMS site: http://www.opennms.org In this case I’m going to do an install on to a RedHat Fedora Core 1 server. Here’s the installation guide: https://sourceforge.net/docman/display_doc.php?docid=23937&group_id=4141 The first step is to install the various prerequisites: – Java 2 SDK 1.4.2 or later from http://java.sun.com We did a stock install of the 1.4.2_07 tar/gz package to /usr/local/. Don’t forget to export the JAVA_HOME environment variable:
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export JAVA_HOME=/usr/local/j2sdk1.4.2_07/ |
– Tomcat 4 from http://jakarta.apache.org/tomcat/index.html We did a standard install of the 4.1.31 tar/gz package following the instructions here: http://jakarta.apache.org/tomcat/tomcat-4.1-doc/RUNNING.txt The install was placed at /usr/local/jakarta-tomcat-4.0. We added the following lines to /etc/rc.local to start Tomcat on bootup and open the appropriate firewall ports:
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# tomcat startup export JAVA_HOME=/usr/local/j2sdk1.4.2_07 export CATALINA_HOME=/usr/local/jakarta-tomcat-4.0 $CATALINA_HOME/bin/startup.sh iptables -A INPUT -p tcp --dport 8080 -j ACCEPT |
At this point Tomcat was accessible at http://{myipaddress}:8080 – RRDTool from http://people.ee.ethz.ch/~oetiker/webtools/rrdtool/ We used the 1.0.49 source tar/gz package and built it using the instructions in the enclosed README file. The package
On redhat, the ETHTOOL_OPTS can provide some control over interface speed. Here’s the description from documentation:
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ETHTOOL_OPTS=... Any device-specific options supported by ethtool. For example, if you wanted to force 100Mb full duplex: ETHTOOL_OPTS="speed 100 duplex full autoneg off" Note that changing speed or duplex settings almost always requires disabling autonegotiation with 'autoneg off'. |
The ethtool utility provides extensive low level control over ethernet interfaces.
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