ssh command

Common SSH Commands or Linux Shell Commands,
ls : list files/directories in a directory, comparable to dir in windows/dos.
ls -al : shows all files (including ones that start with a period), directories, and details attributes for each file.

cd : change directory � � cd /usr/local/apache : go to /usr/local/apache/ directory
cd ~ : go to your home directory
cd - : go to the last directory you were in
cd .. : go up a directory cat : print file contents to the screen

cat filename.txt : cat the contents of filename.txt to your screen

chmod: changes file access permissions
The set of 3 go in this order from left to right:
USER - GROUP - EVERONE

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0 = --- No permission
1 = --X Execute only
2 = -W- Write only
3 = -WX Write and execute
4 = R-- Read only
5 = R-X Read and execute
6 = RW- Read and write
7 = RWX Read, write and execute

Usage:
chmod numberpermissions filename

chmod 000 : No one can access
chmod 644: Usually for HTML pages
chmod 755: Usually for CGI scripts


chown: changes file ownership permissions
The set of 2 go in this order from left to right:
USER - GROUP

chown root myfile.txt : Changes the owner of the file to root
chown root.root myfile.txt : Changes the owner and group of the file to root


tail : like cat, but only reads the end of the file
tail /var/log/messages : see the last 20 (by default) lines of /var/log/messages
tail -f /var/log/messages : watch the file continuously, while it's being updated
tail -200 /var/log/messages : print the last 200 lines of the file to the screen

more : like cat, but opens the file one screen at a time rather than all at once
more /etc/userdomains : browse through the userdomains file. hit Spaceto go to the next page, q to quit

pico : friendly, easy to use file editor
pico /home/burst/public_html/index.html : edit the index page for the user's website.


File Editing with VI ssh commands
vi : another editor, tons of features, harder to use at first than pico
vi /home/burst/public_html/index.html : edit the index page for the user's website.
Whie in the vi program you can use the following useful commands, you will need to hit SHIFT + : to go into command mode

:q! : This force quits the file without saving and exits vi
:w : This writes the file to disk, saves it
:wq : This saves the file to disk and exists vi
:LINENUMBER : EG :25 : Takes you to line 25 within the file
:$ : Takes you to the last line of the file
:0 : Takes you to the first line of the file

grep : looks for patterns in files
grep root /etc/passwd : shows all matches of root in /etc/passwd
grep -v root /etc/passwd : shows all lines that do not match root

ln : create's "links" between files and directories
ln -s /usr/local/apache/conf/httpd.conf /etc/httpd.conf : Now you can edit /etc/httpd.conf rather than the original. changes will affect the orginal, however you can delete the link and it will not delete the original.


last : shows who logged in and when
last -20 : shows only the last 20 logins
last -20 -a : shows last 20 logins, with the hostname in the last field

w : shows who is currently logged in and where they are logged in from.
who : This also shows who is on the server in an shell.

netstat : shows all current network connections.
netstat -an : shows all connections to the server, the source and destination ips and ports.
netstat -rn : shows routing table for all ips bound to the server.

top : shows live system processes in a nice table, memory information, uptime and other useful info. This is excellent for managing your system processes, resources and ensure everything is working fine and your server isn't bogged down.
top then type Shift + M to sort by memory usage or Shift + P to sort by CPU usage

ps: ps is short for process status, which is similar to the top command. It's used to show currently running processes and their PID.
A process ID is a unique number that identifies a process, with that you can kill or terminate a running program on your server (see kill command).
ps U username : shows processes for a certain user
ps aux : shows all system processes
ps aux --forest : shows all system processes like the above but organizes in a hierarchy that's very useful!

touch : create an empty file
touch /home/burst/public_html/404.html : create an empty file called 404.html in the directory /home/burst/public_html/

file : attempts to guess what type of file a file is by looking at it's content.
file * : prints out a list of all files/directories in a directory

du : shows disk usage.
du -sh : shows a summary, in human-readble form, of total disk space used in the current directory, including subdirectories.
du -sh * : same thing, but for each file and directory. helpful when finding large files taking up space.

wc : word count
wc -l filename.txt : tells how many lines are in filename.txt

cp : copy a file
cp filename filename.backup : copies filename to filename.backup
cp -a /home/burst/new_design/* /home/burst/public_html/ : copies all files, retaining permissions form one directory to another.
cp -av * ../newdir : Copies all files and directories recurrsively in the current directory INTO newdir

mv : Move a file command
mv oldfilename newfilename : Move a file or directory from oldfilename to newfilename

rm : delete a file
rm filename.txt : deletes filename.txt, will more than likely ask if you really want to delete it
rm -f filename.txt : deletes filename.txt, will not ask for confirmation before deleting.
rm -rf tmp/ : recursively deletes the directory tmp, and all files in it, including subdirectories. BE VERY CAREFULL WITH THIS COMMAND!!!

TAR: Creating and Extracting .tar.gz and .tar files
tar -zxvf file.tar.gz : Extracts the file
tar -xvf file.tar : Extracts the file
tar -cf archive.tar contents/ : Takes everything from contents/ and puts it into archive.tar
gzip -d filename.gz : Decompress the file, extract it

ZIP Files: Extracting .zip files shell command
unzip file.zip


Firewall - iptables commands
iptables -I INPUT -s IPADDRESSHERE -j DROP : This command stops any connections from the IP address
iptables -L : List all rules in iptables
iptables -F : Flushes all iptables rules (clears the firewall)
iptables --save : Saves the currenty ruleset in memory to disk
service iptables restart : Restarts iptables

Apache Shell Commands
httpd -v : Outputs the build date and version of the Apache server.
httpd -l : Lists compiled in Apache modules
httpd status : Only works if mod_status is enabled and shows a page of active connections
service httpd restart : Restarted Apache web server

MySQL Shell Commands
mysqladmin processlist : Shows active mysql connections and queries
mysqladmin drop databasenamehere : Drops/deletes the selected database
mysqladmin create databasenamehere : Creates a mysql database

Restore MySQL Database Shell Command
mysql -u username -p password databasename < databasefile.sql : Restores a MySQL database from databasefile.sql

Backup MySQL Database Shell Command
mysqldump -u username -p password databasename > databasefile.sql : Backup MySQL database to databasefile.sql




kill: terminate a system process
kill -9 PID EG: kill -9 431
kill PID EG: kill 10550
Use top or ps ux to get system PIDs (Process IDs)

EG:

PID TTY TIME COMMAND
10550 pts/3 0:01 /bin/csh
10574 pts/4 0:02 /bin/csh
10590 pts/4 0:09 APP

Each line represents one process, with a process being loosely defined as a running instance of a program. The column headed PID (process ID) shows the assigned process numbers of the processes. The heading COMMAND shows the location of the executed process.

Putting commands together
Often you will find you need to use different commands on the same line. Here are some examples. Note that the | character is called a pipe, it takes date from one program and pipes it to another.
> means create a new file, overwriting any content already there.
>> means tp append data to a file, creating a newone if it doesn not already exist.
< send input from a file back into a command.

grep User /usr/local/apache/conf/httpd.conf |more
This will dump all lines that match User from the httpd.conf, then print the results to your screen one page at a time.

last -a > /root/lastlogins.tmp
This will print all the current login history to a file called lastlogins.tmp in /root/

tail -10000 /var/log/exim_mainlog |grep domain.com |more
This will grab the last 10,000 lines from /var/log/exim_mainlog, find all occurances of domain.com (the period represents 'anything',
-- comment it out with a so it will be interpretted literally), then send it to your screen page by page.

netstat -an |grep :80 |wc -l
Show how many active connections there are to apache (httpd runs on port 80)

mysqladmin processlist |wc -l
Show how many current open connections there are to mysql

menghapus hak akses vps

Sistem operasi Linux dari awalnya memang sudah dirancang untuk bekerja dengan banyak user, artinya adalah di Linux Kita bisa melakukan login dengan berbagai nama user, dan tentu saja dengan hak yang berbeda-beda dan hak akses yang berbeda pula untuk file dan direktori.

Tidak hanya membuat user saja, tetapi semua user yang ada dapat dikelompokkan. Contohnya, Kita dapat mengelompokkan user berdasarkan departemen yang ada di perusahaan, contohnya group Marketing, Purchasing, Finance, Accounting, dsb.

Sekarang bagaimana caranya kita membuat user dan group di Linux? Caranya cukup mudah, dan seperti Saya beritahukan di awal thread, Kita akan melakukan semuanya melalui command line atau biasa disebut text based.

* Membuat user, gunakan perintah # useradd NamaUser
Perintah tersebut akan membuat user baru sesuai dengan nama user yang kita masukan. Perhatikan juga bahwa pembuatan user tersebut akan membuatkan 1 direktori sesuai dengan nama user tersebut pada direktori /home. Selain itu, default group user ini adalah nama user itu juga.
* Melihat daftar user yang ada, perintahnya # cat /etc/passwd
* Daftar user tersebut, nantinya akan berbentuk seperti ini

Quote:
guest:x:500:500:guest:/home/guest:/bin/bash

Dari daftar tersebut, terlihat kalau daftar tersebut terbagi menjadi 7 kolom/bagian yang dipisahkan oleh tanda : (titik dua).
o Kolom 1 berisi username
o Kolom 2 berisi password (hanya ditandi dengan tanda ‘x’ yang berarti mempunyai password)
o Kolom 3 berisi UID (user ID), UID ini selalu dimulai dari 500
o Kolom 4 berisi GID (Group ID), GUID ini juga selalu dimulai dari 500
o Kolom 5 berisi Full name user
o Kolom 6 berisi home direktori user
o Kolom 7 berisi shell user tersebut
* Melihat password user dalam bentuk terekripsi, perintahnya # cat /etc/shadow
* Menghapus user, gunakan perintah # userdel namauser
Perintah ini akan menghapus user yang kita masukan, tetapi tidak menghapus direktori user di folder /home.
* Menghapus user dan folder home-nya, gunakan perintah # userdel -r namauser
* Membuat Group, gunakan perintah # groupadd NamaGroup
Perintah tersebut akan membuat suatu group baru. Saat baru dibuat group ini akan kosong, artinya tidak ada anggota pada group ini. Seperti penjelasan di atas, bahwa setiap pembuatan GID ini akan dimulai dari 500. Sekarang bagaimana jika kita ingin memberikan GID ini dengan 700? Gunakan perintah # groupadd -g 700 NamaGroup.
* Melihat daftar group yang ada, gunakan perintah # cat /etc/group
* Menambahkan user ke group tertentu, perintahnya # usermod -G NamaGroup NamaUser
* Menghapus Group, gunakan perintah # groupdel NamaGroup

menghapus hak akses client vpn

edit openssl.cnf di folder easy-rsa/2.0 pada bagian berikut, (beri tanda comment '#'):

#[ pkcs11_section ]
#engine_id = pkcs11
#dynamic_path = /usr/lib/engines/engine_pkcs11.so
#MODULE_PATH = $ENV::PKCS11_MODULE_PATH
#PIN = $ENV::PKCS11_PIN
#init = 0

klo udah, lakukan:
source ./vars
./vars
./revoke-full namaklien

setelah itu, kopi file keys/crl.pem ke folder mana suka, misalnnya /etc/openvpn

lalu edit file server.conf, tambahkan baris berikut:
crl-verify /etc/openvpn/crl.pem

Install Squid Proxy Server on CentOS / Redhat enterprise Linux 5

sumber : http://www.cyberciti.biz/tips/howto-rhel-centos-fedora-squid-installation-configuration.html

# yum install squid



Squid Basic Configuration

# vi /etc/squid/squid.conf

At least you need to define ACL (access control list) to work with squid. The defaults port is TCP 3128. Following example ACL allowing access from your local networks 192.168.1.0/24 and 192.168.2.0/24. Make sure you adapt to list your internal IP networks from where browsing should be allowed:

acl our_networks src 192.168.1.0/24 192.168.2.0/24
http_access allow our_networks

# chkconfig squid on
# /etc/init.d/squid start

# netstat -tulpn | grep 3128



tcp        0      0 0.0.0.0:3128                0.0.0.0:*                   LISTEN      20653/(squid)

Open TCP port 3128

Finally make sure iptables is allowing to access squid proxy server. Just open /etc/sysconfig/iptables file:
# vi /etc/sysconfig/iptables
Append configuration:
-A RH-Firewall-1-INPUT -m state --state NEW,ESTABLISHED,RELATED -m tcp -p tcp --dport 3128 -j ACCEPT
Restart iptables based firewall:
# /etc/init.d/iptables restart
Output:

Flushing firewall rules: [ OK ] Setting chains to policy ACCEPT: filter [ OK ] Unloading iptables modules: [ OK ] Applying iptables firewall rules: [ OK ] Loading additional iptables modules: ip_conntrack_netbios_n[ OK ]

Client configuration

Open a webbrowser > Tools > Internet option > Network settings > and setup Squid server IP address and port # 3128.

perintah perintah di command prompt

ADDUSERS Add or list users to/from a CSV file
ARP Address Resolution Protocol
ASSOC Change file extension associations
ASSOCIAT One step file association
AT Schedule a command to run at a later time
ATTRIB Change file attributes
BOOTCFG Edit Windows boot settings
BROWSTAT Get domain, browser and PDC info
CACLS Change file permissions
CALL Call one batch program from another
CD Change Directory – move to a specific Folder

CHANGE Change Terminal Server Session properties
CHKDSK Check Disk – check and repair disk problems
CHKNTFS Check the NTFS file system
CHOICE Accept keyboard input to a batch file
CIPHER Encrypt or Decrypt files/folders
CleanMgr Automated cleanup of Temp files, recycle bin
CLEARMEM Clear memory leaks
CLIP Copy STDIN to the Windows clipboard.
CLS Clear the screen
CLUSTER Windows Clustering
CMD Start a new CMD shell
COLOR Change colors of the CMD window
COMP Compare the contents of two files or sets of files
COMPACT Compress files or folders on an NTFS partition
COMPRESS Compress individual files on an NTFS partition
CON2PRT Connect or disconnect a Printer
CONVERT Convert a FAT drive to NTFS.
COPY Copy one or more files to another location
CSVDE Import or Export Active Directory data
DATE Display or set the date
Dcomcnfg DCOM Configuration Utility
DEFRAG Defragment hard drive
DEL Delete one or more files
DELPROF Delete NT user profiles
DELTREE Delete a folder and all subfolders
DevCon Device Manager Command Line Utility
DIR Display a list of files and folders
DIRUSE Display disk usage
DISKCOMP Compare the contents of two floppy disks
DISKCOPY Copy the contents of one floppy disk to another
DNSSTAT DNS Statistics
DOSKEY Edit command line, recall commands, and create macros
DSADD Add user (computer, group..) to active directory
DSQUERY List items in active directory
DSMOD Modify user (computer, group..) in active directory
ECHO Display message on screen
ENDLOCAL End localisation of environment changes in a batch file
ERASE Delete one or more files
EXIT Quit the CMD shell
EXPAND Uncompress files
EXTRACT Uncompress CAB files
FC Compare two files
FDISK Disk Format and partition
FIND Search for a text string in a file
FINDSTR Search for strings in files
FOR /F Loop command: against a set of files
FOR /F Loop command: against the results of another command
FOR Loop command: all options Files, Directory, List
FORFILES Batch process multiple files
FORMAT Format a disk
FREEDISK Check free disk space (in bytes)
FSUTIL File and Volume utilities
FTP File Transfer Protocol
FTYPE Display or modify file types used in file extension associations
GLOBAL Display membership of global groups
GOTO Direct a batch program to jump to a labelled line
HELP Online Help
HFNETCHK Network Security Hotfix Checker
IF Conditionally perform a command
IFMEMBER Is the current user in an NT Workgroup
IPCONFIG Configure IP
KILL Remove a program from memory
LABEL Edit a disk label
LOCAL Display membership of local groups
LOGEVENT Write text to the NT event viewer.
LOGOFF Log a user off
LOGTIME Log the date and time in a file
MAPISEND Send email from the command line
MEM Display memory usage
MD Create new folders
MODE Configure a system device
MORE Display output, one screen at a time
MOUNTVOL Manage a volume mount point
MOVE Move files from one folder to another
MOVEUSER Move a user from one domain to another
MSG Send a message
MSIEXEC Microsoft Windows Installer
MSINFO Windows NT diagnostics
MSTSC Terminal Server Connection (Remote Desktop Protocol)
MUNGE Find and Replace text within file(s)
MV Copy in-use files
NET Manage network resources
NETDOM Domain Manager
NETSH Configure network protocols
NETSVC Command-line Service Controller
NBTSTAT Display networking statistics (NetBIOS over TCP/IP)
NETSTAT Display networking statistics (TCP/IP)
NOW Display the current Date and Time
NSLOOKUP Name server lookup
NTBACKUP Backup folders to tape
NTRIGHTS Edit user account rights
PATH Display or set a search path for executable files
PATHPING Trace route plus network latency and packet loss
PAUSE Suspend processing of a batch file and display a message
PERMS Show permissions for a user
PERFMON Performance Monitor
PING Test a network connection
POPD Restore the previous value of the current directory saved by PUSHD
PORTQRY Display the status of ports and services
PRINT Print a text file
PRNCNFG Display, configure or rename a printer
PRNMNGR Add, delete, list printers set the default printer
PROMPT Change the command prompt
PsExec Execute process remotely
PsFile Show files opened remotely
PsGetSid Display the SID of a computer or a user
PsInfo List information about a system
PsKill Kill processes by name or process ID
PsList List detailed information about processes
PsLoggedOn Who’s logged on (locally or via resource sharing)
PsLogList Event log records
PsPasswd Change account password
PsService View and control services
PsShutdown Shutdown or reboot a computer
PsSuspend Suspend processes
PUSHD Save and then change the current directory
QGREP Search file(s) for lines that match a given pattern.
RASDIAL Manage RAS connections
RASPHONE Manage RAS connections
RECOVER Recover a damaged file from a defective disk.
REG Read, Set or Delete registry keys and values
REGEDIT Import or export registry settings
REGSVR32 Register or unregister a DLL
REGINI Change Registry Permissions
REM Record comments (remarks) in a batch file
REN Rename a file or files.
REPLACE Replace or update one file with another
RD Delete folder(s)
RDISK Create a Recovery Disk
RMTSHARE Share a folder or a printer
ROBOCOPY Robust File and Folder Copy
ROUTE Manipulate network routing tables
RUNAS Execute a program under a different user account
RUNDLL32 Run a DLL command (add/remove print connections)
SC Service Control
SCHTASKS Create or Edit Scheduled Tasks
SCLIST Display NT Services
ScriptIt Control GUI applications
SET Display, set, or remove environment variables
SETLOCAL Control the visibility of environment variables
SETX Set environment variables permanently
SHARE List or edit a file share or print share
SHIFT Shift the position of replaceable parameters in a batch file
SHORTCUT Create a windows shortcut (.LNK file)
SHOWGRPS List the NT Workgroups a user has joined
SHOWMBRS List the Users who are members of a Workgroup
SHUTDOWN Shutdown the computer
SLEEP Wait for x seconds
SOON Schedule a command to run in the near future
SORT Sort input
START Start a separate window to run a specified program or command
SU Switch User
SUBINACL Edit file and folder Permissions, Ownership and Domain
SUBST Associate a path with a drive letter
SYSTEMINFO List system configuration
TASKLIST List running applications and services
TIME Display or set the system time
TIMEOUT Delay processing of a batch file
TITLE Set the window title for a CMD.EXE session
TOUCH Change file timestamps
TRACERT Trace route to a remote host
TREE Graphical display of folder structure
TYPE Display the contents of a text file
USRSTAT List domain usernames and last login
VER Display version information
VERIFY Verify that files have been saved
VOL Display a disk label
WHERE Locate and display files in a directory tree
WHOAMI Output the current UserName and domain
WINDIFF Compare the contents of two files or sets of files
WINMSD Windows system diagnostics
WINMSDP Windows system diagnostics II
WMIC WMI Commands
XCACLS Change file permissions
XCOPY Copy files and folders

all about iptables

terus terang masih bingung banget nih..seumur umur lum pernah mainan linux.. giliran pegang server problem yang terjadi sering banget karena ip tablesnya.. dr pada bingung skalian belajar dan sebagai dokumentasi terus terang aku copas dari

http://www.redhat.com/docs/en-US/Red_Hat_Enterprise_Linux/5.2/html/Deployment_Guide/s1-fireall-ipt-act.html

buat bahan bacaan .. moga moga cepet paham lah..

The first step in using iptables is to start the iptables service. Use the following command to start the iptables service:

[root@myServer ~] # service iptables start

The ip6tables service can be turned off if you intend to use the iptables service only. If you deactivate the ip6tables service, remember to deactivate the IPv6 network also. Never leave a network device active without the matching firewall.
To force iptables to start by default when the system is booted, use the following command:

[root@myServer ~] # chkconfig --level 345 iptables on

The following sample iptables command illustrates the basic command syntax:

[root@myServer ~ ] # iptables -A -j

The -A option specifies that the rule be appended to . Each chain is comprised of one or more rules, and is therefore also known as a ruleset.

The three built-in chains are INPUT, OUTPUT, and FORWARD. These chains are permanent and cannot be deleted. The chain specifies the point at which a packet is manipulated.

The -j option specifies the target of the rule; i.e., what to do if the packet matches the rule. Examples of built-in targets are ACCEPT, DROP, and REJECT.

Refer to the iptables man page for more information on the available chains, options, and targets.

Establishing basic firewall policies creates a foundation for building more detailed, user-defined rules.

Each iptables chain is comprised of a default policy, and zero or more rules which work in concert with the default policy to define the overall ruleset for the firewall.

The default policy for a chain can be either DROP or ACCEPT. Security-minded administrators typically implement a default policy of DROP, and only allow specific packets on a case-by-case basis. For example, the following policies block all incoming and outgoing packets on a network gateway:

[root@myServer ~ ] # iptables -P INPUT DROP

Changes to iptables are transitory; if the system is rebooted or if the iptables service is restarted, the rules are automatically flushed and reset. To save the rules so that they are loaded when the iptables service is started, use the following command:

[root@myServer ~ ] # service iptables save

The rules are stored in the file /etc/sysconfig/iptables and are applied whenever the service is started or the machine is rebooted.

Common IPTables Filtering

Preventing remote attackers from accessing a LAN is one of the most important aspects of network security. The integrity of a LAN should be protected from malicious remote users through the use of stringent firewall rules.

However, with a default policy set to block all incoming, outgoing, and forwarded packets, it is impossible for the firewall/gateway and internal LAN users to communicate with each other or with external resources.

To allow users to perform network-related functions and to use networking applications, administrators must open certain ports for communication.

For example, to allow access to port 80 on the firewall, append the following rule:

[root@myServer ~ ] # iptables -A INPUT -p tcp -m tcp --dport 80 -j ACCEPT

This allows users to browse websites that communicate using the standard port 80. To allow access to secure websites (for example, https://www.example.com/), you also need to provide access to port 443, as follows:

[root@myServer ~ ] # iptables -A INPUT -p tcp -m tcp --dport 443 -j ACCEPT

Important

When creating an iptables ruleset, order is important.

If a rule specifies that any packets from the 192.168.100.0/24 subnet be dropped, and this is followed by a rule that allows packets from 192.168.100.13 (which is within the dropped subnet), then the second rule is ignored.

The rule to allow packets from 192.168.100.13 must precede the rule that drops the remainder of the subnet.

To insert a rule in a specific location in an existing chain, use the -I option. For example:

[root@myServer ~ ] # iptables -I INPUT 1 -i lo -p all -j ACCEPT

This rule is inserted as the first rule in the INPUT chain to allow local loopback device traffic.

There may be times when you require remote access to the LAN. Secure services, for example SSH, can be used for encrypted remote connection to LAN services.

Administrators with PPP-based resources (such as modem banks or bulk ISP accounts), dial-up access can be used to securely circumvent firewall barriers. Because they are direct connections, modem connections are typically behind a firewall/gateway.

For remote users with broadband connections, however, special cases can be made. You can configure iptables to accept connections from remote SSH clients. For example, the following rules allow remote SSH access:

[root@myServer ~ ] # iptables -A INPUT -p tcp --dport 22 -j ACCEPT

FORWARD and NAT Rules

Most ISPs provide only a limited number of publicly routable IP addresses to the organizations they serve.

Administrators must, therefore, find alternative ways to share access to Internet services without giving public IP addresses to every node on the LAN. Using private IP addresses is the most common way of allowing all nodes on a LAN to properly access internal and external network services.

Edge routers (such as firewalls) can receive incoming transmissions from the Internet and route the packets to the intended LAN node. At the same time, firewalls/gateways can also route outgoing requests from a LAN node to the remote Internet service.

This forwarding of network traffic can become dangerous at times, especially with the availability of modern cracking tools that can spoof internal IP addresses and make the remote attacker's machine act as a node on your LAN.

To prevent this, iptables provides routing and forwarding policies that can be implemented to prevent abnormal usage of network resources.

The FORWARD chain allows an administrator to control where packets can be routed within a LAN. For example, to allow forwarding for the entire LAN (assuming the firewall/gateway is assigned an internal IP address on eth1), use the following rules:

[root@myServer ~ ] # iptables -A FORWARD -i eth1 -j ACCEPT [root@myServer ~ ] # iptables -A FORWARD -o eth1 -j ACCEPT

This rule gives systems behind the firewall/gateway access to the internal network. The gateway routes packets from one LAN node to its intended destination node, passing all packets through its eth1 device.

Note

By default, the IPv4 policy in Red Hat Enterprise Linux kernels disables support for IP forwarding. This prevents machines that run Red Hat Enterprise Linux from functioning as dedicated edge routers. To enable IP forwarding, use the following command:

[root@myServer ~ ] # sysctl -w net.ipv4.ip_forward=1

This configuration change is only valid for the current session; it does not persist beyond a reboot or network service restart. To permanently set IP forwarding, edit the /etc/sysctl.conf file as follows:

Locate the following line:

net.ipv4.ip_forward = 0

Edit it to read as follows:

net.ipv4.ip_forward = 1

Use the following command to enable the change to the sysctl.conf file:

[root@myServer ~ ] # sysctl -p /etc/sysctl.conf

Postrouting and IP Masquerading

Accepting forwarded packets via the firewall's internal IP device allows LAN nodes to communicate with each other; however they still cannot communicate externally to the Internet.

To allow LAN nodes with private IP addresses to communicate with external public networks, configure the firewall for IP masquerading, which masks requests from LAN nodes with the IP address of the firewall's external device (in this case, eth0):

[root@myServer ~ ] # iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE

This rule uses the NAT packet matching table (-t nat) and specifies the built-in POSTROUTING chain for NAT (-A POSTROUTING) on the firewall's external networking device (-o eth0).

POSTROUTING allows packets to be altered as they are leaving the firewall's external device.

The -j MASQUERADE target is specified to mask the private IP address of a node with the external IP address of the firewall/gateway.

Prerouting

If you have a server on your internal network that you want make available externally, you can use the -j DNAT target of the PREROUTING chain in NAT to specify a destination IP address and port where incoming packets requesting a connection to your internal service can be forwarded.

For example, if you want to forward incoming HTTP requests to your dedicated Apache HTTP Server at 172.31.0.23, use the following command:

[root@myServer ~ ] # iptables -t nat -A PREROUTING -i eth0 -p tcp --dport 80 -j DNAT --to 172.31.0.23:80

This rule specifies that the nat table use the built-in PREROUTING chain to forward incoming HTTP requests exclusively to the listed destination IP address of 172.31.0.23.

[root@myServer ~ ] # iptables -A OUTPUT -p tcp --sport 22 -j ACCEPT

These rules allow incoming and outbound access for an individual system, such as a single PC directly connected to the Internet or a firewall/gateway. However, they do not allow nodes behind the firewall/gateway to access these services. To allow LAN access to these services, you can use Network Address Translation (NAT) with iptables filtering rules.

[root@myServer ~ ] # iptables -P OUTPUT DROP

It is also recommended that any forwarded packets — network traffic that is to be routed from the firewall to its destination node — be denied as well, to restrict internal clients from inadvertent exposure to the Internet. To do this, use the following rule:

[root@myServer ~ ] # iptables -P FORWARD DROP

When you have established the default policies for each chain, you can create and save further rules for your particular network and security requirements.

The following sections describe how to save iptables rules and outline some of the rules you might implement in the course of building your iptables firewall.

Malicious Software and Spoofed IP Addresses

More elaborate rules can be created that control access to specific subnets, or even specific nodes, within a LAN. You can also restrict certain dubious applications or programs such as trojans, worms, and other client/server viruses from contacting their server.

For example, some trojans scan networks for services on ports from 31337 to 31340 (called the elite ports in cracking terminology).

Since there are no legitimate services that communicate via these non-standard ports, blocking them can effectively diminish the chances that potentially infected nodes on your network independently communicate with their remote master servers.

The following rules drop all TCP traffic that attempts to use port 31337:

[root@myServer ~ ] # iptables -A OUTPUT -o eth0 -p tcp --dport 31337 --sport 31337 -j DROP [root@myServer ~ ] # iptables -A FORWARD -o eth0 -p tcp --dport 31337 --sport 31337 -j DROP

You can also block outside connections that attempt to spoof private IP address ranges to infiltrate your LAN.

For example, if your LAN uses the 192.168.1.0/24 range, you can design a rule that instructs the Internet-facing network device (for example, eth0) to drop any packets to that device with an address in your LAN IP range.

Because it is recommended to reject forwarded packets as a default policy, any other spoofed IP address to the external-facing device (eth0) is rejected automatically.

[root@myServer ~ ] # iptables -A FORWARD -s 192.168.1.0/24 -i eth0 -j DROP

Note

There is a distinction between the DROP and REJECT targets when dealing with appended rules.

The REJECT target denies access and returns a connection refused error to users who attempt to connect to the service. The DROP target, as the name implies, drops the packet without any warning.

Administrators can use their own discretion when using these targets. However, to avoid user confusion and attempts to continue connecting, the REJECT target is recommended

IPTables and Connection Tracking

You can inspect and restrict connections to services based on their connection state. A module within iptables uses a method called connection tracking to store information about incoming connections. You can allow or deny access based on the following connection states:

• NEW — A packet requesting a new connection, such as an HTTP request.

• ESTABLISHED — A packet that is part of an existing connection.

• RELATED — A packet that is requesting a new connection but is part of an existing connection. For example, FTP uses port 21 to establish a connection, but data is transferred on a different port (typically port 20).

• INVALID — A packet that is not part of any connections in the connection tracking table.

You can use the stateful functionality of iptables connection tracking with any network protocol, even if the protocol itself is stateless (such as UDP). The following example shows a rule that uses connection tracking to forward only the packets that are associated with an established connection:

[root@myServer ~ ] # iptables -A FORWARD -m state --state ESTABLISHED,RELATED -j ACCEPT

IPv6

The introduction of the next-generation Internet Protocol, called IPv6, expands beyond the 32-bit address limit of IPv4 (or IP). IPv6 supports 128-bit addresses, and carrier networks that are IPv6 aware are therefore able to address a larger number of routable addresses than IPv4.

Red Hat Enterprise Linux supports IPv6 firewall rules using the Netfilter 6 subsystem and the ip6tables command. In Red Hat Enterprise Linux 5, both IPv4 and IPv6 services are enabled by default.

The ip6tables command syntax is identical to iptables in every aspect except that it supports 128-bit addresses. For example, use the following command to enable SSH connections on an IPv6-aware network server:

[root@myServer ~ ] # ip6tables -A INPUT -i eth0 -p tcp -s 3ffe:ffff:100::1/128 --dport 22 -j ACCEPT

For more information about IPv6 networking, refer to the IPv6 Information Page at http://www.ipv6.org/.

tetep aja belum paham nih...

*#@???... hah.. :pusing: