Useful commands for Linux admins

Good LINUX commands

* Finding external IP address fast

wget http://ipinfo.io/ip -qO -

* Test download speed using a file download
- the Infinite file downloader
 wget -O /dev/null http://infinite-download.ma.ttias.be/download.php

or
- Nvidia global driver
 wget http://us.download.nvidia.com/Windows/372.90/372.90-desktop-win8-win7-64bit-international-whql.exe

* Remove it afterwards
rm 372.90-desktop-win8-win7-64bit-international-whql.exe

* Install EPEL

- Latest release For CentOS 6:
rpm -Uvh http://mirror.nonstop.co.il/epel//epel-release-latest-6.noarch.rpm

- Latest release for CentOS 7:
rpm -Uvh http://mirror.nonstop.co.il/epel//epel-release-latest-7.noarch.rpm

* Cancel SELINUX server

 setenforce 0
 sed -i 's/enforcing/disabled/' /etc/selinux/config

* Get Information on an IP address

 ipcalc -4bnp 10.9.81.38/29
 
* Checking traffic problems on the network interface: 

 ifconfig eth5 mtu 576

** network traffic monitor: 

* all traffic on interface eth0
 
 tcpdump -i eth0 -nn
 
* ICMP requests (by protocol)

tcpdump -i eth0 -nn 'proto ICMP'

* DHPC requests (by port)
 
 tcpdump -e -i eth5.2 -nn 'port 67 || port 68'
 
 * one host's traffic on eth0 (by host IP - can also use name)
 
 tcpdump -i eth0 -nn 'host 192.168.0.1'
 
 * remove (delete) e-mail files from mail queue
 
 mailq | grep -v '^ *(' | awk 'BEGIN { RS = "" } { if ($7 == "bill.gates@microsoft.com")print $1 }' | tr -d '*!' | postsuper -d -
 
 * install postfix+spamassasin+clamAV
 yum update && yum install postfix spamassassin clamav clamav-scanner clamav-scanner-systemd clamav-data clamav-update