IPTABLES BASICS

IPtables is vast topic which can not be dealt in one post I will be posting this IPtables in different posts but will link all of them

IPtables basics

Iptables is a securit module in Linux kernel, this Iptables can be used a firewall(filter table),Internet sharing(NAT table),For altering Type Of Service(mangle table) and for non tracking purpose(Raw table)

Firewall definition:

A firewall is a security system consisting of a combination of hardware and software that limits the exposure of a computer or computer network to attack from crackers; commonly used on local area networks that are connected to the internet. This is acheived by Filter table

Internetsharing definition:

An Internet standard that enables a LAN to use one set of IP addresses for internal traffic and a second set of addresses for external traffic. A NAT box located where the LAN meets the Internet makes all necessary IP address translations.

Mangling definition:

 Mangling is defined as changing the Type of service value in IP header packet to get the desired values such as decreasing latency(for example telnet,ssh should required less latency) etc. This table is used for imporving some services performance

Non tracking packets definition:

Most of the system/network activitis are logged in some logs, if we want our packets to be not monitored we can use this table, This raw table(which is introduced in 2.6 version linux kernel for some special feature) will set value on each packet not to track where this packet is coming and going.


IPtables is nothing but a set of tables

A table is set of chains and target values.
A chain is nothing but a rule on a packet wether to forword or drop or Reject etc
A rule is nothing but setting a criteria for packet.

Iptables will work on targeting IP packets and chains
These targets may be

1. ACCEPT
   
2. DROP
3. REJECT
4. SNAT
5. DNAT
6. NOTRACK
7. BALANCE
   
8. CLASSIFY
   
9. CLUSTERIP
   
10. CONNMARK
    
11. DSCP
    
12. ECN
    
13. LOG
    
14. MARK
    
15. MASQUERADE
    
16. MIRROR
    
17. NETMAP
18. REDIRECT
19. ROUTE
    
20. SET
    
21. TCPMSS
    
22. TOS
    
23. TRACE
    
24. TTL
    
25. ULOG

these chains may be

1. INPUT (for packets destined to local sockets),
2. FORWARD (for packets being routed through the box),
3. OUTPUT (for locally-generated packets)
4. PREROUTING (for altering packets as soon as they come in),
5. POSTROUTING (for altering packets as they are about to go out).

in next post I will be giving some explination to this targets and chains with some practical exampls, thanks for reading the blog and commenting.

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Crontab configuration in Linux

CRONTAB

Crontab is the default scheduling Application/program in most of the Linux/Nix boxes.
Crontab is define as scheduling of programs/scripts/applicaions with out administrator interventions
Crontab = (Cron)oligical+(tab)le nothing but time table for executing some scripts at that point of time

So where we requre crontab?
where there is no need for admin intervetions such as taking dialy backups sending mails to perticular users monitoring network.

So coming to scheduling task crontab uses clock daemon (/usr/sbin/cron or perhaps /usr/sbin/crond) that executes commands at specific times. and corn tables for each user is located in /var/spool/cron/ with their names
suppose you want to see cron table for root(cat /var/spool/cron/root)

Crontab file content

The content of the file can be devide in to 6 feilds(first 5 for specifing time and last one for executing scripts/app etc)
The first five fields are separated by either a space or a tab and represent the following units, respectively:

* minutes (0-59)
* hour (0-23)
* day of the month (1-31)
* month of the year (1-12)
* day of the week (0-6, 0=Sunday)


Configuration of crontab for user

Step1:Creating crontab for a user

#crontab -e -u username
-e is for specifing editing and -u for specifing username when you are scheduling crontabs for other users
if users want to schedule their own he can just give (crontab -e)
Example:

I will take one example here to explain crontab,suppose I want to execute one script located in /var/test/script.sh and save the output to /temp/script.out should be executed every 10th of each month at 9:43pm

#crontab -e -u surya

so here it will open a temporary in /tmp folder

43 21 10 * * /var/test/script.sh > /temp/script.out

here * in month field and week field indicates any month and any week execute this script

after entering the values save and exit the file, A file will be created in as /var/spool/cron/surya with the above content

Step2:seeing crontab for a perticular user

#crontab -l -u username

Example

#crontab -l -u surya

If you are a user and want to see your crontab you can just give (crontab -l)

Step3:Removing crontab for perticular user

This can be acheived in two ways
1)removing all the crontab entries for a perticual user

#crontab -r -l username

Example

#crontab -r -l surya

2)removing one task for perticual user,for doing this we have to edit the crontab

#crontab -e -l username

Please see here some of the crontab issues

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Getting mulitple files from ftp server with out any prompt

How to get lot of multiple file from server I am using
FTP command(mget ...), but each file asked "yes/no", every file should be put 'yes'
then 'Enter'?

we can resolve this issue in two ways


1)when you are accessing FTP server use -i option which is nothing but interactive way to get files from FTP server, actually this -i option will disable iteractive download of files from server.

Syntax:ftp -i server-ip/servername
#ftp -i 222.1.89.1

2)This is used when you are middle of the transaction you can use prompt command in ftp mode to get multiple files with out any prompt, here is the example and this is for that session

ftp> ls
200 PORT command successful. Consider using PASV.
150 Here comes the directory listing.
-rw-r--r-- 1 1005 0 47 Apr 11 19:37 file1.txt
-rw-r--r-- 1 1005 0 47 Apr 11 19:37 file2.txt
-rw-r--r-- 1 1005 0 47 Apr 11 19:37 file3.txt
-rw-r--r-- 1 1005 0 47 Apr 11 19:37 file4.txt
226 Directory send OK.

ftp> prompt
Interactive mode off.

ftp> mget *
local: file1.txt remote: file1.txt
200 PORT command successful. Consider using PASV.
150 Opening BINARY mode data connection for file1.txt (47 bytes).
226 File send OK.
47 bytes received in 0.00 secs (72.1 kB/s)
local: file2.txt remote: file2.txt
200 PORT command successful. Consider using PASV.
150 Opening BINARY mode data connection for file2.txt (47 bytes).
226 File send OK.
47 bytes received in 0.00 secs (88.3 kB/s)
local: file3.txt remote: file3.txt
200 PORT command successful. Consider using PASV.
150 Opening BINARY mode data connection for file3.txt (47 bytes).
226 File send OK.
47 bytes received in 0.00 secs (136.2 kB/s)
local: file4.txt remote: file4.txt
200 PORT command successful. Consider using PASV.
150 Opening BINARY mode data connection for file4.txt (47 bytes).
226 File send OK.
47 bytes received in 0.00 secs (136.6 kB/s)

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