limit number of connection to a server using IPTABLES

We can limit number of connections to the server using the command below.

iptables -I INPUT -p tcp --syn --dport 22 -m connlimit --conlimit-above 2 -j REJECT

service iptables save

service iptables restart

Useful Linux Commands

Command to find files accessed in last 30 days. will find files that is accessed in last 30 days, under root folder.
# find / type f -atime -30
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List contents of a folder along with contents of its subfolder. But it will traverse only to a depth of one. ie, it will not show the contents of subfolder's subfolder.
# ls *
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To print the iptables rules along with line number.
# iptables -L --line-numbers
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To find a particular rule with rule number #; where # is the rule number you want to list
# iptables -L OUTPUT --line-numbers | grep ^#
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Change permission only for folders
# find . -type d -exec chmod 755 {} \;
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List with 777 permission
#find . -type d -perm 777
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To list all the processes listening to port 80
# lsof -i TCP:80|awk {'print $2'}
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To kill all the process listening to apache port 443/80
# lsof -i TCP:443|awk {'print $2'} | xargs kill -9
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Recursively chmod only directories
find . -type d -exec chmod 755 {} \;
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Recursively set the execute bit on every directory
chmod -R a+X *
The +X flag sets the execute bit on directories only
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Recursively chmod only files
find . -type f -exec chmod 644 {} \;
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Recursively chmod only PHP files (with extension .php)
find . -type f -name '*.php' -exec chmod 644 {} \;
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Find all files in /home/user/demo directory
$ find /home/user/demo -print
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Now find all files in /home/user/demo directory with permission 777
$ find /home/user/demo -perm 777 -print
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Next you need to apply chmod on all these files using -exec option:
$ find /home/user/demo -perm 777 -print -exec chmod 755 {} \;
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Command to find files modified on July 12
ll|grep dr|awk '{print $9}' > 123
for i in `cat 123`;do ls -ld $i;done|grep "Jul 12"
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How to See the SSH password guesses

First, find the PID of the listening SSH daemon process:
# ps axuww | egrep 'PID|ssh'
Now become root and attach to the running daemon with strace:
# strace -f -e 'read,write' -p12345
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