Linux firewall rules Sample Script

The options are truly limitless, but you need to be careful. Below is a basic script you may find useful for beginning your firewall rule settings. When copying this script remember the longer commands are broken into several lines:

#!/bin/bash

#

# iptables firewall settings for linux server

#

### DEFAULT POLICY

iptables -P INPUT DROP

iptables -P OUTPUT ACCEPT

iptables -P FORWARD DROP

### ESSENTIAL RULES

# Allow internal host packets on local interface

iptables -A INPUT -i lo -j ACCEPT

### PROTOCOL LEVEL RULES

# Allow PORT 80 TCP packets on ethernet interface

iptables -A INPUT -i eth0 -p tcp --dport 80 -j ACCEPT

# LOG ALL OTHER PACKETS

# Logging for any failed packets for troubleshooting use

iptables -A INPUT -j LOG --log-prefix "INPUT: "

 Usage: route [-nNvee] [-FC] []           List kernel routing tables

[root@localhost]# route del -net 10.10.10.0 netmask 255.255.255.0 gw 192.168.1.1 wlan0

Linux  Command: crontab

 usage:  crontab [-u user] file

        crontab [-u user] [ -e | -l | -r ]

                (default operation is replace, per 1003.2)

        -e      (edit user's crontab)

        -l      (list user's crontab)

        -r      (delete user's crontab)

        -i      (prompt before deleting user's crontab)

Lines that can be in the crontab file.

minute (0-59),

hour (0-23),

day of the month (1-31),

month of the year (1-12),

day of the week (0-6 with 0=Sunday).

Examples

crontab -e = edits the crontab file to be used.

0 12 14 2 * mailx john%Happy Birthday!%Time for lunch.

#crontab -e

 */1 * * * * php /var/www/html/poller.php > /dev/null 2>&1

 0 1 * * * nice -n 15 /var/www/backup.sh

Linux /Unix : Cat

Usage: cat [OPTION] [FILE]...

Concatenate FILE(s), or standard input, to standard output.

  -A, --show-all           equivalent to -vET

  -b, --number-nonblank    number nonblank output lines

  -e                       equivalent to -vE

  -E, --show-ends          display $ at end of each line

  -n, --number             number all output lines

  -s, --squeeze-blank      never more than one single blank line

  -t                       equivalent to -vT

  -T, --show-tabs          display TAB characters as ^I

  -u                       (ignored)

  -v, --show-nonprinting   use ^ and M- notation, except for LFD and TAB

      --help     display this help and exit

      --version  output version information and exit

With no FILE, or when FILE is -, read standard input.

Examples:

  cat f - g  Output f's contents, then standard input, then g's contents.

  cat        Copy standard input to standard output.

Linux / Unix Command: halt

usage: halt [-n] [-w] [-d] [-f] [-i] [-p]
                  -n: don't sync before halting the system
                  -w: only write a wtmp reboot record and exit.
                  -d: don't write a wtmp record.
                  -f: force halt/reboot, don't call shutdown.
                  -p: power down the system (if possible, otherwise halt)

halt, reboot, poweroff - stop the system. 
SYNOPSIS

/sbin/halt [-n] [-w] [-d] [-f] [-i] [-p] [-h]
/sbin/reboot [-n] [-w] [-d] [-f] [-i]
/sbin/poweroff [-n] [-w] [-d] [-f] [-i] [-h] 
DESCRIPTION

Halt notes that the system is being brought down in the file /var/log/wtmp, and then either tells the kernel to halt, reboot or poweroff the system.
If halt or reboot is called when the system is not in runlevel 0 or 6, in other words when it's running normally, shutdown will be invoked instead (with the -h or -r flag). For more info see the shutdown(8) manpage.

The rest of this manpage describes the behaviour in runlevels 0 and 6, that is when the systems shutdown scripts are being run. 

When halting the system, do a poweroff. This is the default when halt is called as poweroff.

Examples

halt
Set the computer to a mode that enables a user to turn off the computer.

poweroff
Stop the processor (if you have proper permission).

Linux / Unix Command: poweroff

usage: poweroff [-n] [-w] [-d] [-f] [-i] [-p]
                  -n: don't sync before poweroffing the system
                  -w: only write a wtmp reboot record and exit.
                  -d: don't write a wtmp record.
                  -f: force halt/reboot, don't call shutdown.
                  -p: power down the system (if possible, otherwise poweroff)

halt, reboot, poweroff - stop the system. 
SYNOPSIS

/sbin/halt [-n] [-w] [-d] [-f] [-i] [-p] [-h]
/sbin/reboot [-n] [-w] [-d] [-f] [-i]
/sbin/poweroff [-n] [-w] [-d] [-f] [-i] [-h] 
DESCRIPTION

Halt notes that the system is being brought down in the file /var/log/wtmp, and then either tells the kernel to halt, reboot or poweroff the system.
If halt or reboot is called when the system is not in runlevel 0 or 6, in other words when it's running normally, shutdown will be invoked instead (with the -h or -r flag). For more info see the shutdown(8) manpage.

The rest of this manpage describes the behaviour in runlevels 0 and 6, that is when the systems shutdown scripts are being run. 

When halting the system, do a poweroff. This is the default when halt is called as poweroff.

Linux / Unix Command: vi

usage: vim [arguments] [file ..]       edit specified file(s)
   or: vim [arguments] -               read text from stdin
   or: vim [arguments] -t tag          edit file where tag is defined

Arguments:
   --                   Only file names after this
   -v                   Vi mode (like "vi")
   -e                   Ex mode (like "ex")
   -s                   Silent (batch) mode (only for "ex")
   -y                   Easy mode (like "evim", modeless)
   -R                   Readonly mode (like "view")
   -Z                   Restricted mode (like "rvim")
   -m                   Modifications (writing files) not allowed
   -M                   Modifications in text not allowed
   -b                   Binary mode
   -C                   Compatible with Vi: 'compatible'
   -N                   Not fully Vi compatible: 'nocompatible'
   -V[N]                Verbose level
   -D                   Debugging mode
   -n                   No swap file, use memory only
   -r                   List swap files and exit
   -r (with file name)  Recover crashed session
   -L                   Same as -r
   -T         Set terminal type to
   -u            Use instead of any .vimrc
   --noplugin           Don't load plugin scripts
   -o[N]                Open N windows (default: one for each file)
   -O[N]                Like -o but split vertically
   +                    Start at end of file
   +              Start at line
   --cmd       Execute before loading any vimrc file
   -c          Execute after loading the first file
   -S          Source file after loading the first file
   -s         Read Normal mode commands from file
   -w        Append all typed commands to file
   -W        Write all typed commands to file
   -h  or  --help       Print Help (this message) and exit
   --version            Print version information and exit

Examples

#vi config.conf

Edits the file config.conf

Linux / Unix Command: tar

GNU `tar' saves many files together into a single tape or disk archive, and
can restore individual files from the archive.

Usage: tar [OPTION]... [FILE]...

Examples:
  tar -cf archive.tar foo bar  # Create archive.tar from files foo and bar.
  tar -tvf archive.tar         # List all files in archive.tar verbosely.
  tar -xf archive.tar          # Extract all files from archive.tar.

If a long option shows an argument as mandatory, then it is mandatory
for the equivalent short option also.  Similarly for optional arguments.

Main operation mode:
  -t, --list              list the contents of an archive
  -x, --extract, --get    extract files from an archive
  -c, --create            create a new archive
  -d, --diff, --compare   find differences between archive and file system
  -r, --append            append files to the end of an archive
  -u, --update            only append files newer than copy in archive
  -A, --catenate          append tar files to an archive
      --concatenate       same as -A
      --delete            delete from the archive (not on mag tapes!)

Operation modifiers:
  -W, --verify               attempt to verify the archive after writing it
      --remove-files         remove files after adding them to the archive
  -k, --keep-old-files       don't replace existing files when extracting
      --keep-newer-files     don't replace existing files that are newer
                             than their archive copies
      --overwrite            overwrite existing files when extracting
      --no-overwrite-dir     preserve metadata of existing directories
  -U, --unlink-first         remove each file prior to extracting over it
      --recursive-unlink     empty hierarchies prior to extracting directory
  -S, --sparse               handle sparse files efficiently
  -O, --to-stdout            extract files to standard output
  -G, --incremental          handle old GNU-format incremental backup
  -g, --listed-incremental=FILE
                             handle new GNU-format incremental backup
      --ignore-failed-read   do not exit with nonzero on unreadable files
      --occurrence[=NUM]     process only the NUMth occurrence of each file in
                             the archive. This option is valid only in
                             conjunction with one of the subcommands --delete,
                             --diff, --extract or --list and when a list of
                             files is given either on the command line or
                             via -T option.
                             NUM defaults to 1.

Handling of file attributes:
      --owner=NAME             force NAME as owner for added files
      --group=NAME             force NAME as group for added files
      --mode=CHANGES           force (symbolic) mode CHANGES for added files
      --atime-preserve         don't change access times on dumped files
  -m, --modification-time      don't extract file modified time
      --same-owner             try extracting files with the same ownership
      --no-same-owner          extract files as yourself
      --numeric-owner          always use numbers for user/group names
  -p, --same-permissions       extract permissions information
      --no-same-permissions    do not extract permissions information
      --preserve-permissions   same as -p
  -s, --same-order             sort names to extract to match archive
      --preserve-order         same as -s
      --preserve               same as both -p and -s

Device selection and switching:
  -f, --file=ARCHIVE             use archive file or device ARCHIVE
      --force-local              archive file is local even if has a colon
      --rmt-command=COMMAND      use given rmt COMMAND instead of /etc/rmt
      --rsh-command=COMMAND      use remote COMMAND instead of rsh
  -[0-7][lmh]                    specify drive and density
  -M, --multi-volume             create/list/extract multi-volume archive
  -L, --tape-length=NUM          change tape after writing NUM x 1024 bytes
  -F, --info-script=FILE         run script at end of each tape (implies -M)
      --new-volume-script=FILE   same as -F FILE
      --volno-file=FILE          use/update the volume number in FILE

Linux / Unix Command: free

free - display information about free and used memory on the system

usage: free [-b|-k|-m|-g] [-l] [-o] [-t] [-s delay] [-c count] [-V]
  -b,-k,-m,-g show output in bytes, KB, MB, or GB
  -l show detailed low and high memory statistics
  -o use old format (no -/+buffers/cache line)
  -t display total for RAM + swap
  -s update every [delay] seconds
  -c update [count] times
  -V display version information and exit

DESCRIPTION

free displays the total amount of free and used physical memory and swap space in the system, as well as the buffers and cache consumed by the kernel.

[root@localhost man]# free -b

             total       used       free     shared    buffers     cached
Mem:     393904128  202096640  191807488          0   35287040  101294080
-/+ buffers/cache:   65515520  328388608
Swap:    797843456          0  797843456