Monday, April 9, 2012
Logrotate in Linux server
Logs are important and usefull in security and administration. If a log grows too much, you compress it , back it up for some months or years and create a new log file. All this is done in a much better way by Logrotate. Logrotate enables you to compress the old logfile, and store it for as long as you want, to run every day, montly, or when you want, and if a file expires, it can send it to you by email or delete it, after for example, 2 years. Old logs can be stored in an alternative directory.
You set up logs configuring /etc/logrotate.conf or adding a new file in /etc/logrotate.d/
For example, a logrotate file for apache looks like this:
File: /etc/logrotate.d/httpd
-------8<------- /usr/local/apache/logs/*log { missingok notifempty sharedscripts postrotate /bin/kill -USR1 `cat /var/run/httpd.pid 2>/dev/null` 2> /dev/null || true
endscript
}
You set up logs configuring /etc/logrotate.conf or adding a new file in /etc/logrotate.d/
For example, a logrotate file for apache looks like this:
File: /etc/logrotate.d/httpd
-------8<------- /usr/local/apache/logs/*log { missingok notifempty sharedscripts postrotate /bin/kill -USR1 `cat /var/run/httpd.pid 2>/dev/null` 2> /dev/null || true
endscript
}
Suggestions to debug Segmentation Fault errors
Usually signal #11 (SIGSEGV) set, which is defined in the header file signal.h file. The default action for a program upon receiving SIGSEGV is abnormal termination. This action will end the process, but may generate a core file (also known as core dump) to aid debugging, or perform some other platform-dependent action. A core dump is the recorded state of the working memory of a computer program at a specific time, generally when the program has terminated abnormally.
Segmentation fault can also occur under following circumstances:
a) A buggy program / command, which can be only fixed by applying patch.
b) It can also appear when you try to access an array beyond the end of an array under C programming.
c) Inside a chrooted jail this can occur when critical shared libs, config file or /dev/ entry missing.
d) Sometime hardware or faulty memory or driver can also create problem.
e) Maintain suggested environment for all computer equipment (overheating can also generate this problem).
Suggestions to debug Segmentation Fault errors
To debug this kind of error try one or all of the following techniques :
Use gdb to track exact source of problem.
Make sure correct hardware installed and configured.
Always apply all patches and use updated system.
Make sure all dependencies installed inside jail.
Turn on core dumping for supported services such as Apache.
Use strace which is a useful diagnostic, instructional, and debugging tool.
Google and find out if there is a solution to problem.
Fix your C program for logical errors such as pointer, null pointer, arrays and so on.
Analyze core dump file generated by your system using gdb
Segmentation fault can also occur under following circumstances:
a) A buggy program / command, which can be only fixed by applying patch.
b) It can also appear when you try to access an array beyond the end of an array under C programming.
c) Inside a chrooted jail this can occur when critical shared libs, config file or /dev/ entry missing.
d) Sometime hardware or faulty memory or driver can also create problem.
e) Maintain suggested environment for all computer equipment (overheating can also generate this problem).
Suggestions to debug Segmentation Fault errors
To debug this kind of error try one or all of the following techniques :
Use gdb to track exact source of problem.
Make sure correct hardware installed and configured.
Always apply all patches and use updated system.
Make sure all dependencies installed inside jail.
Turn on core dumping for supported services such as Apache.
Use strace which is a useful diagnostic, instructional, and debugging tool.
Google and find out if there is a solution to problem.
Fix your C program for logical errors such as pointer, null pointer, arrays and so on.
Analyze core dump file generated by your system using gdb
Fix corrupted RPM database on CentOS 5 / Redhat enterprise Linux 5 / Fedora 7
If rpm / yum command hangs during operations or you see error messages - it means your rpm database corrupted. /var/lib/rpm/ stores rpm database just delete the same and rebuild rpm database:
Command to rebuild rpm database
rm -f /var/lib/rpm/__db*
rpm --rebuilddb
Rebuilding corrupted RPM database
One of our client reported that he is getting an error and RPM database is corrupted. He is using Red Hat Linux
Sometime it is possible to fix RPM database errors. I am surprised that many admins do not make back up of RPM DB (/var/lib/rpm).
Anyways if you ever messed up with RPM database, here is a quick guide to fix it (you must have rpmdb tools installed):
Take system to single user mode to avoid further damage and to make backup/restore process easy:
# init 1
Method # 1
Remove /var/lib/rpm/__db* files to avoid stale locks:
# cd /var/lib
# rm __db*
Rebuild RPM database:
# rpm --rebuilddb
# rpmdb_verify Packages
Method # 2
If you are still getting errors, then try your luck with following commands:
# mv Packages Packages-BAKUP
# db_dump Packages-BAKUP | db_load Packages
# rpm -qa
# rpm --rebuilddb
Segmentation Fault Occur on Linux
Usually signal #11 (SIGSEGV) set, which is defined in the header file signal.h file. The default action for a program upon receiving SIGSEGV is abnormal termination. This action will end the process, but may generate a core file (also known as core dump) to aid debugging, or perform some other platform-dependent action. A core dump is the recorded state of the working memory of a computer program at a specific time, generally when the program has terminated abnormally.
Segmentation fault can also occur under following circumstances:
a) A buggy program / command, which can be only fixed by applying patch.
b) It can also appear when you try to access an array beyond the end of an array under C programming.
c) Inside a chrooted jail this can occur when critical shared libs, config file or /dev/ entry missing.
d) Sometime hardware or faulty memory or driver can also create problem.
e) Maintain suggested environment for all computer equipment (overheating can also generate this problem).
Command to rebuild rpm database
rm -f /var/lib/rpm/__db*
rpm --rebuilddb
Rebuilding corrupted RPM database
One of our client reported that he is getting an error and RPM database is corrupted. He is using Red Hat Linux
Sometime it is possible to fix RPM database errors. I am surprised that many admins do not make back up of RPM DB (/var/lib/rpm).
Anyways if you ever messed up with RPM database, here is a quick guide to fix it (you must have rpmdb tools installed):
Take system to single user mode to avoid further damage and to make backup/restore process easy:
# init 1
Method # 1
Remove /var/lib/rpm/__db* files to avoid stale locks:
# cd /var/lib
# rm __db*
Rebuild RPM database:
# rpm --rebuilddb
# rpmdb_verify Packages
Method # 2
If you are still getting errors, then try your luck with following commands:
# mv Packages Packages-BAKUP
# db_dump Packages-BAKUP | db_load Packages
# rpm -qa
# rpm --rebuilddb
Segmentation Fault Occur on Linux
Usually signal #11 (SIGSEGV) set, which is defined in the header file signal.h file. The default action for a program upon receiving SIGSEGV is abnormal termination. This action will end the process, but may generate a core file (also known as core dump) to aid debugging, or perform some other platform-dependent action. A core dump is the recorded state of the working memory of a computer program at a specific time, generally when the program has terminated abnormally.
Segmentation fault can also occur under following circumstances:
a) A buggy program / command, which can be only fixed by applying patch.
b) It can also appear when you try to access an array beyond the end of an array under C programming.
c) Inside a chrooted jail this can occur when critical shared libs, config file or /dev/ entry missing.
d) Sometime hardware or faulty memory or driver can also create problem.
e) Maintain suggested environment for all computer equipment (overheating can also generate this problem).
Linux: TMOUT To Automatically Log Users Out
How do I auto Logout my shell user in Linux after certain minutes of inactivity?
Linux bash shell allows you to define the TMOUT environment variable. Set TMOUT to automatically log users out after a period of inactivity. The value is defined in seconds. For example,
export TMOUT=120
The above command will implement a 2 minute idle time-out for the default /bin/bash shell. You can edit your ~/.bash_profile or /etc/profile file as follows to define a 5 minute idle time out:
# set a 5 min timeout policy for bash shell
TMOUT=300
readonly TMOUT
export TMOUT
Save and close the file. The readonly command is used to make variables and functions readonly i.e. you user cannot change the value of variable called TMOUT.
How Do I Disable TMOUT?
To disable auto-logout, just set the TMOUT to zero or unset it as follows:
$ export TMOUT=0
or
$ unset TMOUT
Please note that readonly variable can only be disabled by root in /etc/profile or ~/.bash_profile
Linux bash shell allows you to define the TMOUT environment variable. Set TMOUT to automatically log users out after a period of inactivity. The value is defined in seconds. For example,
export TMOUT=120
The above command will implement a 2 minute idle time-out for the default /bin/bash shell. You can edit your ~/.bash_profile or /etc/profile file as follows to define a 5 minute idle time out:
# set a 5 min timeout policy for bash shell
TMOUT=300
readonly TMOUT
export TMOUT
Save and close the file. The readonly command is used to make variables and functions readonly i.e. you user cannot change the value of variable called TMOUT.
How Do I Disable TMOUT?
To disable auto-logout, just set the TMOUT to zero or unset it as follows:
$ export TMOUT=0
or
$ unset TMOUT
Please note that readonly variable can only be disabled by root in /etc/profile or ~/.bash_profile
How to Add a new yum repository to install software under CentOS / Redhat Linux
CentOS / Fedora Core / RHEL 5 uses yum for software management. Yum allows you to add a new repository as a source to install binary software.
Understanding yum repository
yum repository configured using /etc/yum.conf file. Additional configuration files are also read from the directories set by the reposdir option (default is /etc/yum.repos.d and /etc/yum/repos.d.
RPMforge repository
Usually repository carries extra and useful packages. RPMforge is one of such repository. You can easily configure RPMforge repository for RHEL5 just by running following single RPM command:
# rpm -Uhv http://apt.sw.be/packages/rpmforge-release/rpmforge-release-0.3.6-1.el5.rf.i386.rpm
For 64 bit RHEL 5 Linux, enter:
# rpm -Uhv http://apt.sw.be/packages/rpmforge-release/rpmforge-release-0.3.6-1.el5.rf.x86_64.rpm
Now you can install software from RPMforge.
How do I install 3rd party repository manually?
Let us say you would like to install 3rd party repository from foo.nixcraft.com. Create a file called foo:
# cd /etc/yum.repos.d
# vi foo
Append following code:
[foo]
name=Foo for RHEL/ CentOS $releasever - $basearch
baseurl=http://foo.nixcraft.com/centos/$releasever/$basearch/
enabled=1
gpgcheck=1
gpgkey=http://foo.nixcraft.com/RPM-GPG-KEY.txt
Save and close the file.
Where,
• [foo] : Repository name i.e. The [main] section must exist for yum to do anything.
• name=Foo for RHEL/ CentOS $releasever - $basearch : A human readable string describing the repository name
• baseurl=http://foo.nixcraft.com/centos/$releasever/$basearch/ : Must be a URL to the directory where the yum repository’s ‘repodata’ directory lives
• enabled=1 : Enabled or disabled repo. To disable the repository temporarily, set the enabled to 0
• gpgcheck=1 : Security feature, use GPG key
• gpgkey=http://foo.nixcraft.com/RPM-GPG-KEY.txt : GPL file location
Also you need to import the gpg key for the repository as follows:
# rpm --import http://foo.nixcraft.com/RPM-GPG-KEY.txt
Now you are ready to install software from foo repository. For further information refer to yum.conf man page:
$ man yum.conf
$ man yum
Hope this t will help you to configure repository as and when required
Understanding yum repository
yum repository configured using /etc/yum.conf file. Additional configuration files are also read from the directories set by the reposdir option (default is /etc/yum.repos.d and /etc/yum/repos.d.
RPMforge repository
Usually repository carries extra and useful packages. RPMforge is one of such repository. You can easily configure RPMforge repository for RHEL5 just by running following single RPM command:
# rpm -Uhv http://apt.sw.be/packages/rpmforge-release/rpmforge-release-0.3.6-1.el5.rf.i386.rpm
For 64 bit RHEL 5 Linux, enter:
# rpm -Uhv http://apt.sw.be/packages/rpmforge-release/rpmforge-release-0.3.6-1.el5.rf.x86_64.rpm
Now you can install software from RPMforge.
How do I install 3rd party repository manually?
Let us say you would like to install 3rd party repository from foo.nixcraft.com. Create a file called foo:
# cd /etc/yum.repos.d
# vi foo
Append following code:
[foo]
name=Foo for RHEL/ CentOS $releasever - $basearch
baseurl=http://foo.nixcraft.com/centos/$releasever/$basearch/
enabled=1
gpgcheck=1
gpgkey=http://foo.nixcraft.com/RPM-GPG-KEY.txt
Save and close the file.
Where,
• [foo] : Repository name i.e. The [main] section must exist for yum to do anything.
• name=Foo for RHEL/ CentOS $releasever - $basearch : A human readable string describing the repository name
• baseurl=http://foo.nixcraft.com/centos/$releasever/$basearch/ : Must be a URL to the directory where the yum repository’s ‘repodata’ directory lives
• enabled=1 : Enabled or disabled repo. To disable the repository temporarily, set the enabled to 0
• gpgcheck=1 : Security feature, use GPG key
• gpgkey=http://foo.nixcraft.com/RPM-GPG-KEY.txt : GPL file location
Also you need to import the gpg key for the repository as follows:
# rpm --import http://foo.nixcraft.com/RPM-GPG-KEY.txt
Now you are ready to install software from foo repository. For further information refer to yum.conf man page:
$ man yum.conf
$ man yum
Hope this t will help you to configure repository as and when required
ext4 Linux File System
ext4 Linux File System
The ext4 filesystem has more features and generally better performance than ext3, which is showing its age in the Linux filesystem world.
Features include:
Delayed allocation & mballoc allocator for better on-disk allocation
* Sub-second timestamps
* Space preallocation
* Journal checksumming
* Large (>2T) file support
* Large (>16T) filesystem support
* Defragmentation support
The ext4 filesystem has more features and generally better performance than ext3, which is showing its age in the Linux filesystem world.
Features include:
Delayed allocation & mballoc allocator for better on-disk allocation
* Sub-second timestamps
* Space preallocation
* Journal checksumming
* Large (>2T) file support
* Large (>16T) filesystem support
* Defragmentation support
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