simple Port Scanner

Back in my sixth semester Computer Networks lab, we had to write a simple port scanner (port 0 to 1024). The basic principle was to see whether a connection could be made at a port. If connection can be made, it is open, or else it is closed. I had used the following code then. However, it took 17 minutes to perform the scan.

import java.io.IOException; import java.net.InetAddress; import java.net.Socket; import java.net.UnknownHostException; public class port_scanner { static Socket soc; public static void main(String[] args) throws IOException { for(int i=1;i<1024;i++){ try{ soc= new Socket(InetAddress.getLocalHost(),i); System.out.println("port "+i+" open"); soc.close(); }catch(IOException e){ System.out.println("port "+i+" closed"); } } } }

The better option was to use threading. I was lazy back then and the above program was sufficient to fetch me marks, and so I did not write a multi-threaded code. I needed to brush up threading because placements are going to start soon. Therefore, I implemented the multi-threaded version. It took just 2 seconds to perform the scan.

import java.io.IOException; import java.net.InetAddress; import java.net.Socket; class myThread implements Runnable{ Thread t; Socket soc; boolean state=true; int port; myThread(int port){ this.port=port; t=new Thread(this,"Thread"); t.start(); } boolean getState(){ return state; } @Override public void run(){ try{ soc= new Socket(InetAddress.getLocalHost(),port); soc.close(); }catch(IOException e){ state=false; } } } public class new_prt { static myThread threads[] = new myThread[1024]; public static void main(String args[]){ for(int j=0;j<1024;j++){ threads[j]=new myThread(i); } for(int j=0;j<1024;j++){ try{ threads[j].t.join(); if(threads[j].getState()==true) System.out.println("port "+threads[j].port + " open"); else System.out.println("port "+threads[j].port + " closed"); }catch(InterruptedException e){ } } } }

SCP or how to securely transfer files

scp is a useful command. It works on top of ssh.
To transfer file from local to remote:
scp sourcefile user@remoteserver:/path_target/

To transfer from remote to local:
scp user@remoteserver:/path_source/sourcefile /path_target/
Note: if the target is current directory then '.' can be used as in scp user@remoteserver:/path_source/sourcefile .
if the target is the parent directory current directory then '..' can be used as in scp user@remoteserver:/path_source/sourcefile ..

To transfer files from one remote to another remote:
scp user@remoteserver1:/path_source/sourcefile user@remoteserver2:/path_target/
Note: the server must be able to reach each other because files will be transferred directly between them.

To transfer using some specified port:

scp -P port_number user@remoteserver:/path_source/sourcefile .

To transfer all files in a directory:

scp user@remote:/source_path/* .

To transfer a directory recursively:

scp -r user@remote:/source_path/ .

To limit bandwidth used:

scp -l bandwidth user@remoteserver:/path_source/sourcefile .

To view file in remote machine:
vi scp://user@remote/source_path/remotefile

To compare files:
vimdiff scp://user@remote/source_path/remotefile localfile

Ping FU or (ping for u)

I learnt about these commands in my Computer Network classes. I didn't know about these commands initially. Well, I am still learning. One of the commands is
Ping: Packet Internet Groper. Imagine a sonar. Ping is similar. Also, sonar makes a ping sound. Here is a site dedicated to the ping. An interesting introduction can be found there.

The way this works is the computer or device will generate an ICMP packet that is sent over the local network or internet. The ICMP packet will find its way across the network by having a source and destination IP address. When the device receives this information it then sends a reply saying “yes, I am here”.

Ping depends on ARP and DNS.


ping IP 
ping hostname
It is as simple as that. Press enter and packets are sent out and you find out whether the IP is reachable or not. But first you need to check whether the network is up by pinging yourself. Your machine is "localhost" or "127.0.0.1". Ping any of them or 0. Packets will be sent until Ctrl+C is pressed
ping localhost
ping 127.0.0.1
ping 0


Now the options are like ornaments to the basic ping. They do add an edge when used creatively. 
-c specify the number of echo requests. 
in the following example ping sends 3 packets. Notice that although 58 bytes are sent, 64 bytes are received. The extra 8 bytes are due to ICMP headers

using ping the ip can be found if the host name is given. In the following example, the ip of facebook is found

-i specify the number of seconds between each ping. If the interval is less than 0.2 seconds, superuser privileges are required.

One trick with this option is to simulate sleep. You only need to redirect the output to a bit bucket. In Unix or Linux /dev/null and in windows NUL is the bit bucket.

in the example below, the interval will be of 2 seconds and also the simulation of sleep is shown.

-a gives a beep sound when host is reachable. This thing can help by removing the need to look into the screen to check about successful ping

-f ping flood. By default, the interval between pings is set to zero seconds. It send an echo request immediately after receiving the reply to the previous one. I writes a '.' when a packet is sent and a backspace when reply is received. Super user privileges are required for this. In the following example, 20171 packets were sent and received in around 2.8 seconds.

-q Quiet mode.Prints only ping command statistics.

-s Set packet size.  In the following example, size of ping packet is set to 50. However, 78 bytes of data are sent. The additional 28 bytes belong to IP header and ICMP payload. 

This option was, once upon time, to used to create the ping of death.

-w Set deadline. It is used to set the total amount of time the ping packets should be sent. If it is use with -c then, whichever comes first will terminate the sending of packets. Again, this option can be used as a repalcement for the sleep command by directing the output to the bit bucket (/dev/null). In the example below, deadline is set to 2 seconds.

-p Ping pattern. Upto 16 padding bytes to fill out the packet sent can be specified. This is useful for diagnosing data-dependent problems in a network. In the following example, 2 bytes of alternating 0s and 1s is used.

-R Records route. Turns on route recording for the Echo Request packets, and display the route buffer on returned packets (ignored by many routers).

ping hop1 hop2 hop3 hop4 destination This can be used to set the path for ping.

press Ctrl+| (SIGQUIT) to get short summary as shown below

ShoutOut: using google app engine to ping a number of people at the same time

This is the script i used to message to many of my friends at a time

from google.appengine.api import xmpp from google.appengine.ext import webapp from google.appengine.ext.webapp.util import run_wsgi_app from google.appengine.ext.webapp import xmpp_handlers user1="%s said:" dirc=[] class XMPPHandler(xmpp_handlers.CommandHandler):     def text_message(self, message=None):         global dirc         #message.reply("yo man")         myname=unicode(message.sender.split('.')[0].lower()) + '.com'         for dir in dirc:          if myname in dir:           for x in dir:            if myname != x:             xmpp.send_message(x,user1 % myname)             xmpp.send_message(x, message.body)     def start_command(self, message=None):         global dirc         dir=message.body.split(' ')[1].split(';')         dirc[len(dirc):]=[dir[0:]]         myname=message.sender.split('.')[0]         for x in dirc[len(dirc)-1]:          xmpp.send_message(x,"chat session started by %s" % myname)          xmpp.send_message(x,"now in session are:")          for y in dirc[len(dirc)-1]:  xmpp.send_message(x,y)     def stop_command(self, message=None):      i=-1      myname=unicode(message.sender.split('.')[0].lower()) + '.com'      for dir in dirc:       i=i+1       if myname in dir:           for x in dir:          xmpp.send_message(x,"Session Stopped")         del dirc[i] application = webapp.WSGIApplication([('/_ah/xmpp/message/chat/', XMPPHandler)],                                      debug=True) def main():     run_wsgi_app(application) if __name__ == "__main__":     main()

Here the Google App Engine acts as a middleman. A sort of chatroom is formed between a number of users and message belonging to a group is relayed to all members of the group.
A sends a message which B and C receive.
A------->App Engine----->B
                             |
                             |
                            V
                     C
A group chat is started by using /start username1;username2;usernameN
It is stopped by /stop
I kinda stopped working on it. A lot of improvements are to be done. One reason I stopped working on it is because Gmail has this thing inbuilt.

Will come up with an explaination soon. Bear with me.

XMPP or (How to programatically do stuffs in IM)

Hi.
It's been a while since I worked with the Smack API.... 6 months and so I might have forgotten many things. Anyways, it is a brief introduction to XMPP and more specifically to Smack API. It provides Java libraries for playing around with XMPP.

The above snippet will connect to the gmail server and make an status update.

Presence presence =new Presence(Presence.Type.available,

status,

24,

Presence.Mode.available);

Here Presence.Type is a enum to represent the presecence type. Note that presence type is 
often confused with presence mode. Generally, if a user is signed into a server, they have a 
presence type of available, even if the mode is away, dnd, etc. The presence type is only 
unavailable when the user is signing out of the server

status contains the string which you want to set as your status.

24 is the priority. The highest priory client will receive the packets.

Check this out for better documentation of Presence.

Add

Roster roster = connection.getRoster();
System.out.println("No of contacts: " + roster.getEntryCount());
for (RosterEntry entry : roster.getEntries())     
System.out.println("User: " + entry.getUser());

The above code will show the size of your roster and also show the user list

Message msg=new Message(user@gmail.com",Message.Type.chat);
msg.setBody("ssup?");
connection.sendPacket(msg);

Use this to send IM to some user.

presence.setStatus(status);
connection.sendPacket(presence);

Use this to change status.

There are many fun things that you can do with this. You can make your status rotate, write the lyrics of your favourite song line by line, flood, or change your status from busy to available and vice-versa continuously to get a alternating red-green thing.
Do your thing and please share it. Will love to learn from you.

System Monitoring basics

This is just an overview of few of the system administration tools. Infos about what programs are running, how much memory or how much cpu cycles are being used, who are logged in and for how long. 


top: it provides a lot of information about the processes running, percentatage of the cpu and th ememory being consumed by that process, who owns the process and its Process ID. It also shows the uptime and memory usage. The output is a dynamic full screen.  Using hot key k any process can be killed. Press M to sort by memory, P to sort by %CPU and S to sort by time.
du: it shows the disk usage(in Kb) of each directory and sub-directories. It checks recursively and starts from the current folder by default. By supplying a name you can make it start from there.
Options: (these may vary with version run, and there are more ;)
-a (--all) Prints usage for ALL files, not just the subdirectories
-b (--bytes) Prints usage in bytes rather than Kb.
-s (--summarize) Prints ONLY the total usage of the directory
df: this command tells about the amount of free space on all mounted file system, or the name of a device can be specified. It will show the size, used space, available space, used% and mount information.
ps: this command shows what processes are running with your user id. ps aux command gives additional information about all processes. This is similar to top except that it gives only a snapshot and is not updated. 
vmstat: This command  reports information about processes, memory, paging, block IO, traps and cpu activity. 
-a Gives information about Active/Inactive Memory Pages
-m Displays Memory utilization
who: It displays the users who are logged into the system
uptime: to get the uptime, together with the current time, the number of users and load averages for the past 1, 5 and 15 minute intervals
w: It displays information about users currently logged into the machine and their processes. It is a combination of who, uptime, and ps.
free: shows information about the machine's memory. This includes physical memory (RAM), swap as well as the shared memory and buffers used by the kernal. All measurements are in Kb.
iostat: It report Central Processing Unit (CPU) statistics and input/output statistics for devices, partitions and network file systems (NFS).
mpstat: displays activities for each available processor, processor 0 being the first one. mpstat -P ALL to display average CPU utilization per processor.
man the above commands to learn more

SSH Daemon (or Setting up a SSH server)

The SSH Daemon act like a server and listens for and handles any incoming connection. 


To install ssh daemon in Ubuntu:
sudo apt-get install openssh-server


Assuming the OpenSSH client is also installed already,
ssh localhost
to check whether the daemon is working properly.


To start the server
sudo /etc/init.d/ssh start


To stop the server
sudo /etc/init.d/ssh stop


To restart the server
sudo /etc/init.d/ssh restart


To disable ssh in Ubuntu machine
apt-get remove openssh-server



Now lets change some operational parameters.
Don't forget to keep  backup of the original config file /etc/ssh/sshd_config


By default, ssh server listens at port 22. If you may want to change the port address for safety
purpose. For that open /etc/ssh/sshd_config file and look for line Port 22. Change line to Port <desired number>. Restart sshd server.


It is advisable to not allow root login. To do this, in the above file update with PermitRootLogin no


Make sure Protocol 2 line is in the above file since SSH-1 has man-in-the-middle attacks problem and security vulnerabilities. It's best to avoid using the obsolete SSH-1.


Add AllowUsers user1 user2 user3 to the config file to allow only certain users to use ssh to login to the system. Alternatively, if you want to allow every user except some, you need to DenyUsers user1 user2
AllowGroup and DenyGroup can also be used for control at the group level.


To kick user out after certain time interval of idleness add ClientAliveInterval <time in sec> and ClientAliveCountMax <number of messages to be sent without receiving any message in return>
For example
ClientAliveInterval 600
ClientAliveCountMax 0
Server will wait for 600 seconds and will not send any warning message before kicking out the user.

Update config file with IgnoreRhosts yes to prevent login using the insecure RSH

Using host-based authentication, any user on a trusted host can log into another host (with the same username) on which this feature is enabled. To disable host-based authentication update the file with HostbasedAuthentication no

Update config file with PermitEmptyPasswords no to disallow logins with empty passwords.

To limit ssh port binding, add ListenAddress <ip address> to config file.

Please share other helpful infos in the comments so all of us will be able to learn from each other.

Some Linux Commands

The original page went down around 2007. It belonged to Clark College Linux User Group. Found it in the waybackmachine.org
You can check out this. The link redirects to the archive where the page is stored.
Use  man command for more information on the commands.

Console Commands

Utility or Command	Use/Description
passwd	Changes password
nslookup	Queries Internet domain name servers
quota	Displays disk usage and limits
motd	Message of the Day
finger username	User information lookup program
man or xman command	Displays pages of online manual
xman	Displays System Manual in X
less filename or morefilename	Displays the contents of a file in the terminal one page at a time
info	Displays information and documentation on shells, utilities and programs
clear	Clears the terminal window
ls directory	List contents of a directory
cat filename	Displays the contents of a file in the terminal
rm filename	Removes a file
pico filename or emacsfilename	Opens and edits text files
cp sourcefile detstinationfilename	Copies a file
lpr filename	Sends file to printer
grep string filename	looks through files for strings
head filename	Displays first 10 lines of file
tail filename	Displays last 10 lines of file
mv existingfilename newfilename	Moves or renames file
lpq filename	Displays files in printing queue
lprm filename	Removes file from printing queue
sort filename	Displays and sorts file contents
diff filename1 filename2	Displays differences between files
file filename	Displays information about file contents
echo string	Copies string to terminal
date	Displays current date and time
cal	Displays calendar
gzip filename	Compresses a file
compress filename	Compresses a file
gunzip filename	Decompresses a compressed file
zcat filename	Displays contents of a compressed file
apropos command	Lists all man page titles/headers that contain the command
lynx	Text based web browser
dmesg	Displays kernel ring buffer
which command	Displays path to command
whereis command	Displays paths to locations of commands
who	Lists currently logged on users
finger username@hostname	Obtains detailed information about a user currently using the system
w	Lists currently logged on users with processing usage
mesg y/n	Sets options for letting other users write you messages
write user	Sends message to other users
talk user	Allows two way chat to other users
chmod permissions filename	Changes file access permissions
mkdir directoryname	Makes a directory
rmdir directoryname	Removes an empty directory
ln existingfile new-link	Creates link to an existing file (hard link)
stat filename	Lists information about a file
ln -s existingfile new-link	Creates link to an existing file (soft link)
df	Displays all mounted filesystems
ps	Reports process status
command &	Sends a job to the background (job: one or more commands connected by a pipe "|" or pipes) The operating system assigns a number to the job when you press return. example: [1] 3578
top	Displays updating list of currently running processes
tty	Displays the name of the terminal in which the command was issued
command > filename	Redirects standard output
command < filename	Redirects standard input
cat file1 >> file2	Appends standard output from file1 to file2
cat /dev/null > filename orfilename > /dev/null	Redirects "bit bucket" or null string to file (only superuser has write access to this file)
command1 | command2	Pipe sends standard output of one command to the standard input of another command
tr string1 string2 < inputfile	translates each character in string1 to the corresponding character in string2
command | tee filename |grep string	Sends the output of one command to standard output and a file
bg %job number	Sends job to the background by job number
fg %job number	Brings job to the foreground by job number
kill PID or %job number	Aborts a process by PID (Process Identification Number) or job number
jobs	Displays a list of current jobs
netcfg	Utility to set up PPP and network configurations
xev	Utility used to see information flow from X server to client
echo $DISPLAY	Environment variable that displays the ID string for a window
echo $PATH	Variable that displays executable path
netstat	Displays network connections
viewres	Graphical class browser for X
xbill	Game featuring Bill Gates trying to put windows on Macs and NeXT workstations
xevil	Game similar to Loderunner?
xchomp	Linux's version of PacMan
xcmap	Strange color lookup utility
xedit	Text editor for X
asclock	Clock from AfterStep
xconsole	Strange console for X
xmessage message	Sends message to a dialog box
xgal	XGalaga game
xg3	Image viewing program
xgc	Graphing calculator?
xjewel	Jewel game for Linux
xkbvleds	LEDs?
xkbwatch	LEDs?
xlogo	Displays X logo
xmixer	Opens system sound controls
xsnow	Snowflakes are fallin' on your desktop
xwininfo	Displays info about a window
startx	Starts an X Window System server
ghostview	Starts a text preview application
xv filename	Image viewer
xsetroot -color	Set background color in X
xcalc	Starts a calculator in X
xclipboard	Starts a clipboard in X
traceroute host	Prints the route packets take to the host
hostname	Displays system identity name
rlogin host	Utility to connect to a remote system
telnet host	Utility to connect to a remote system (similar to rlogin but more interactive)
rcp file remotemachine	Used to copy from a remote computer
ftp	Utility to transfer files between systems on a network
rsh command	Utility to run a command on a remote system without logging in
ping host	Utility used to test connection to a remote system
lcd directorypath	Changes local machine directory while logged on to remote machine

Shared Directories

Directory	Description
/	Root - The root directory is present in all Linux system file structures. It's the parent of all files in the system.
/bin	Essential common binaries - Holds files needed to boot the system and run it when it comes up in single user mode.
/boot	Static files of the boot loader.
/dev	Device files - All files that represent peripheral devices.
/etc	Machine-local system configuration - Holds administrative, config, and other system files. Holds /etc/passwd file that contains a list of all users who have permission to the system.
/home	User home directories - Contains each user's or client's home directory
/lib	Shared libraries
/mnt	Mount point for temporary partitions
/proc	Kernel and process information (virtual filesystem)
/root	Home directory for root
/sbin	Essential system binaries - Contains utilities for system administration.
/tmp	Temporary files
/usr	Second major hierarchy - Includes subdirectories that contain information used by the system.
/var	Variable data - Contains files whose contents change as system runs.

vi Commands

Command	Description
vi filename	Starts vi and creates a new file
:q!	Quits vi without saving work
p	Pastes data in buffer below current line
P	Pastes data in buffer above current line
yy	Copies current line
:r !command	Reads in output of the command
i	Puts vi in insert mode
:set autoindent	Sets vi to indent automatically
:set showmatch	Sets vi to show matching parenthesis
:set nu	Sets vi to display line numbers
:set showmode	Sets vi to display the mode you're in
ESCAPE	Sets vi to command mode
Control-U	Erases current line (insert mode)
Control-H	Erases on letter (insert mode)
Control-W	Erases current word (insert mode)
h, j, k, l	Moves cursor left, up, down, right respectively
u	Undoes last action
x	Deletes a single character
dw	Deletes a single word
dd	Deletes a single line
ZZ	Writes work buffer to disk and exits vi
o	inserts line after cursor position
Control-L	Redraws screen
:w filename	Save work as filename and exits

Control Characters

Key	Use
Control-H or BACKSPACE	Erases a character on the command line
Control-U	Deletes an entire command line
Control-W	Erases a word on the command line
Control-C	Aborts program execution
COMMAND-Tab	Switches Programs
Control-L or CONtrOL-R	Refreshes the screen
Control-D, logout or exit	Logs you off the system