fuser: check who is using a tcp/udp port

# fuser 702/tcp
702/tcp:              4336
# ps -ef |grep 4336
rpcuser   4336     1  0 Sep26 ?        00:00:00 rpc.statd
root      7692  7458  0 15:46 pts/0    00:00:00 grep 4336

nmap

To check machine 192.168.0.22, run nmap remotely from another machine.

# nmap -sT -sU -p 1-1023 192.168.0.22
Starting Nmap 4.11 ( http://www.insecure.org/nmap/ ) at 2008-10-05 15:44 MDT
Interesting ports on (192.168.0.22):
Not shown: 2040 filtered ports
PORT    STATE         SERVICE
22/tcp  open          ssh
25/tcp  closed        smtp
80/tcp  closed        http
443/tcp closed        https
631/tcp closed        ipp
631/udp open|filtered unknown

RPM

Querying

Command	Description
rpm -q	query all installed packages
rpm -q firefox	query firefox installed
rpm -ql firefox	query list of files in firefox package
rpm -qid firefox	query package information including documentation files
rpm -qf /usr/bin/ab	which package that owns ab
rpm -qdf /usr/bin/ab	list document comes with package that owns ab
rpm -V package	verify an installed package
rpm -Va	verify all installed package
rpm -Vf /usr/bin/ab	verify the package that owns ab
rpm -qa --qf "%-10{SIZE} %-30{NAME}\n" | sort -n | less	sort all installed packages by size
rpm -qa --qf "%{n}-%{v}-%{r}.%{arch}\n"	:list architecture
rpm -qpR package	list package dependencies
rpm -qp --scripts package	list package pre and post scripts
rpm -qa --queryformat '%{SIZE}\n' | awk '{sum += $1} END {printf("Total size in packages = %4.1f GB\n", sum/1024**3)}'	total size of all rpms installed
rpm -q --changelog package	package change log
rpm -q --provides package	what the package provides
rpm -q --qf '[%{obsoletes}\n]' package	show obsolete tags
rpm -K package.rpm	verify package integrity

1. rpm -qa: query all installed packages
2. rpm -q firefox: query firefox installed
3. rpm -ql firefox: query list of files in firefox package
4. rpm -qpl name.rpm query list files in name.rpm
5. rpm -qid firefox: query package information including documentation files
6. rpm -qf /usr/bin/ab: which package that owns ab
7. rpm -qdf /usr/bin/ab: list document comes with package that owns ab
8. rpm -V package: verify a package
9. rpm -Va: verify all installed package
10. rpm -Vf /usr/bin/ab: verify the package that owns ab.
11. rpm -qa --qf "%-10{SIZE} %-30{NAME}\n" | sort -n | less: sort all installed packages by size.
12. rpm -qa --qf "%{n}-%{v}-%{r}.%{arch}\n"    :list architecture
13. rpm -qpR package:    list package dependencies
14. rpm --test -e package: list what requires this package
15. rpm -qp --scripts package:   list package pre and post scripts
16. rpm -q --changelog package: package change log
17. rpm -qa --queryformat '%{SIZE}\n' | awk '{sum += $1} END {printf("Total size in packages = %4.1f GB\n", sum/1024**3)}':  total size of all rpms installed
18. rpm -q --provides  package:  what the package provides
19. rpm -q --qf '[%{obsoletes}\n]' package: show obsolete tags
20. rpm -K package.rpm: verify package integrity

Installing

1. rpm -ivh package.rpm: installing, verbose and show progress with ###
2. rpm -Uvh package.rpm: updating a package
3. rpm -Fvh *.rpm: updating the packages only if an earlier version is present

Uninstalling

1. rpm -e package
2. rpm -e --nodeps package

Check Dependency

$ rpm --test -ivh package.rpm

Rebuild RPM db (when it is corrupted or yum stuck)

# rm /var/lib/rpm/__db*
# rpm --rebuilddb

Auto Reboot after Kernel Panic

Add panic=5 to the kernel boot parameter in grub. Kernel will reboot 5 secs after kernel panic. For example,

kernel /vmlinuz-2.6.26.5-45.fc9.x86_64 ro root=LABEL=/1 rhgb quiet panic=5

Alternatively, add kernel.panic=5 in /etc/sysctl.conf. This will be loaded after next reboot or you can manually load it sysctl -p. To check,

# sysctl -a |grep kernel.panic
kernel.panic_on_unrecovered_nmi = 0
kernel.panic_on_oops = 1
kernel.panic = 5

.

Monitor and Restart Processes

monit can be used to monitor and restart server processes such as apache. See the link below for more info:

Monitor and restart Apache or lighttpd webserver when daemon is killed

RHEL and CentOS monit rpm can be downloaded here. Install on CentOs:

# rpm -Uvh monit-4.10.1-7.el5.i386.rpm
# vi /etc/monit.conf
# chkconfig --level 345 monit on
# /etc/init.d/monit start