How To Tail (View) Multiple Files on UNIX / Linux Console

Posted on in Categories data center, Debian Linux, Download of the day, fedora linux, File system, FreeBSD, Gentoo Linux, GNU/Open source, Howto, Linux, Linux Log Management, Monitoring, package management, RedHat/Fedora Linux, Suse Linux, Sys admin, Tip of the day, UNIX last updated February 9, 2009

The tail command is one of the best tool to view log files in a real time using tail -f /path/to/log.file syntax on a Unix-like systems. The program MultiTail lets you view one or multiple files like the original tail program. The difference is that it creates multiple windows on your console (with ncurses). This is one of those dream come true program for UNIX sys admin job. You can browse through several log files at once and do various operations like search for errors and more.

Linux Iptables Firewall: Log IP or TCP Packet Header

Posted on in Categories Howto, Iptables, Linux, Networking, Security last updated January 9, 2008

Iptables provides the option to log both IP and TCP headers in a log file. This is useful to:
=> Detect Attacks

=> Analyze IP / TCP Headers

=> Troubleshoot Problems

=> Intrusion Detection

=> Iptables Log Analysis

=> Use 3rd party application such as PSAD (a tool to detect port scans and other suspicious traffic)

=> Use as education tool to understand TCP / IP header formats etc.

How do I turn on Logging IP Packet Header Options?

Add the following command to your iptables script beo:

iptables -A INPUT -j LOG --log-ip-options
iptables -A INPUT -j DROP

How do I turn on Logging TCP Packet Header Options?

Add the following command to your iptables script:

iptables -A INPUT -j LOG --log-tcp-options
iptables -A INPUT -j DROP

You may need to add additional filtering criteria such as source and destination ports/IP-address and other connection tracking features. To see IP / TCP header use tail -f or grep command:
# tail -f /var/log/messages

Recommended readings:

How to: Upgrade Red Hat Enterprise Linux (RHEL) to latest Releases 5.1

Posted on in Categories CentOS, Howto, Linux distribution, Linux Virtualization, RedHat/Fedora Linux, Sys admin, Tips last updated November 8, 2007

RHEL 5.1 has been released. Redhat announced the availability of Red Hat Enterprise Linux 5.1, with integrated virtualization. This release provides the most compelling platform for customers and software developers ever, with its industry-leading virtualization capabilities complementing Red Hat’s newly announced Linux Automation strategy. It offers the industry’s broadest deployment ecosystem, covering standalone systems, virtualized systems, appliances and web-scale “cloud” computing environments.

Besides supporting Linux virtual machines, RHEL 5.1 will also support Windows XP, Windows 2000, Windows Server 2003 and the forthcoming Windows 2008, Crenshaw said. RHEL 5.1 uses Xen for its virtualization.

How do I upgrade to RHEL 5.1?

Login as the root user and simply type the command to fetch all updates via RHN:
# yum update
Depend upon your network condition and software configuration it may take anywhere between 5-20 minutes. Once completed, just reboot the server:
# reboot
Verify that everything is working fine:
# netstat -tulpn
# netstat -nat
# tail -f /var/log/messages
# egrep -i 'error|warn' /var/log/messages
# egrep -i 'error|warn' /path/to/apps/log

Community driven enterprise CentOS Linux users should expect update soon too. You can apply above commends to upgrade your CentOS box.

Test If Linux Server SCSI / SATA Hard Disk Going Bad

Posted on in Categories CentOS, Debian Linux, File system, Gentoo Linux, Howto, Linux, Linux distribution, Sys admin, Tips, Troubleshooting, Ubuntu Linux last updated June 2, 2017

One of our regular reader sends us a question:
   How can I test if my hard disk is going bad? I see few errors in /var/log/messages file.
I/O errors in /var/log/messages indicates that something is wrong with the hard disk and it may be failing. You can check hard disk for errors using smartctl command, which is control and monitor utility for SMART disks under Linux / UNIX like operating systems.

Force iptables to log messages to a different log file

Posted on in Categories Iptables, Linux, Monitoring, Security last updated October 3, 2006

According to man page:
Iptables is used to set up, maintain, and inspect the tables of IP packet filter rules in the Linux kernel. Several different tables may be defined. Each table contains a number of built-in chains and may also contain user defined chains.

By default, Iptables log message to a /var/log/messages file. However you can change this location. I will show you how to create a new logfile called /var/log/iptables.log. Changing or using a new file allows you to create better statistics and/or allows you to analyze the attacks.

Iptables default log file

For example, if you type the following command, it will display current iptables log from /var/log/messages file:
# tail -f /var/log/messages

Oct  4 00:44:28 debian gconfd (vivek-4435): Resolved address "xml:readonly:/etc/gconf/gconf.xml.defaults" to a read-only configuration source at position 2
Oct  4 01:14:19 debian kernel: IN=ra0 OUT= MAC=00:17:9a:0a:f6:44:00:08:5c:00:00:01:08:00 SRC= DST= LEN=60 TOS=0x00 PREC=0x00 TTL=51 ID=18374 DF PROTO=TCP SPT=46040 DPT=22 WINDOW=5840 RES=0x00 SYN URGP=0
Oct  4 00:13:55 debian kernel: IN=ra0 OUT= MAC=ff:ff:ff:ff:ff:ff:00:18:de:55:0a:56:08:00 SRC= DST= TOS=0x00 PREC=0x00 TTL=128 ID=13461 PROTO=UDP SPT=137 DPT=137 LEN=58

Procedure to log the iptables messages to a different log file

Open your /etc/syslog.conf file:
# vi /etc/syslog.conf
Append following line
kern.warning /var/log/iptables.log
Save and close the file.

Restart the syslogd (Debian / Ubuntu Linux):# /etc/init.d/sysklogd restartOn the other hand, use following command to restart syslogd under Red Hat/Cent OS/Fedora Core Linux:# /etc/init.d/syslog restart

Now make sure you pass the log-level 4 option with log-prefix to iptables. For example:
# DROP everything and Log it
iptables -A INPUT -j LOG --log-level 4
iptables -A INPUT -j DROP

For example, drop and log all connections from IP address to your /var/log/iptables.log file:
iptables -A INPUT -s -m limit --limit 5/m --limit-burst 7 -j LOG --log-prefix '** HACKERS **'--log-level 4
iptables -A INPUT -s -j DROP


  • –log-level 4: Level of logging. The level # 4 is for warning.
  • –log-prefix ‘*** TEXT ***’: Prefix log messages with the specified prefix (TEXT); up to 29 letters long, and useful for distinguishing messages in the logs.

You can now see all iptables message logged to /var/log/iptables.log file:
# tail -f /var/log/iptables.log

Updated for accuracy.