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How SSH works With Examples and Samples

Discussion in 'Unix' started by lionaneesh, Feb 1, 2011.

  1. lionaneesh

    lionaneesh Active Member

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    SSH is a abbreviation for Secure Shell is a network protocol that allows data-exchange between devices on the network..This is mainly used in linux OS's to access shell accounts , shell commands etc... SSH was mainly designed as a replacement modification to telnet etc..other insecure remote shells which do not use encryption and send passwords/user-names in simple ascii text...Which makes them vulnerable to many attacks...like : suffering passwords etc etc...

    How does SSH work



    The client connects to the server via a TCP connection...Like FTP , HTTP etc...

    Then they send each other their version information and Protocol information...

    Next the server and client discusses what kind of Encryption , keys , hashes they support..

    Now the client sends the server a initialization message that includes the message about the key exchange..and a challenge message...

    Now all the client does is listens for the server's response about the request which will include the message about the server's key and a challenge value that has been signed by the server's private key...

    This is done to provide a validation that the packet could only come from the server that sent it.. (This makes the ssh secure from man in the middle attacks)

    The client then checks the list of known hosts by searching '~/.ssh/known_hosts' file . If the public key is listed , it automatically assumes that the data is valid and the server is trusted..But if the public is not listed here then the user is displayed with a prompt that asks them to verify the finger print...

    Now both the client and server have enough information needed to create the master key that will encrypt the session and the communication starts....

    Thats quite a bit explanation now lets move on to practical example

    Example



    Installing ssh :-
    Code:
    sudo apt-get install ssh 
    
    Running :-
    Code:
    ssh (server hostname)
    
    Other usage can be seen as :-
    Code:
    aneesh@aneesh-laptop:~$ ssh --help 
    usage: ssh [-1246AaCfgKkMNnqsTtVvXxYy] [-b bind_address] [-c cipher_spec] 
               [-D [bind_address:]port] [-e escape_char] [-F configfile] 
               [-i identity_file] [-L [bind_address:]port:host:hostport] 
               [-l login_name] [-m mac_spec] [-O ctl_cmd] [-o option] [-p port] 
               [-R [bind_address:]port:host:hostport] [-S ctl_path] 
               [-w local_tun[:remote_tun]] [user@]hostname [command] 
    
    Installing ssh-server :-
    Code:
    aneesh@aneesh-laptop:~$ sudo apt-get install openssh-server
    
    Now that we successfully installed the main server... Lets check whether its working or not...

    This can be checked by :-
    Code:
    netstat -tupln
    
    Example output :-
    Code:
    aneesh@aneesh-laptop:~$ sudo netstat -tupln
    
    Active Internet connections (only servers)
    
    Proto Recv-Q Send-Q Local Address           Foreign Address         State       PID/Program name
    
    tcp        0      0 0.0.0.0:22              0.0.0.0:*               LISTEN      2095/sshd       
    
    tcp        0      0 127.0.0.1:631           0.0.0.0:*               LISTEN      1051/cupsd      
    
    tcp        0      0 0.0.0.0:1723            0.0.0.0:*               LISTEN      796/pptpd       
    
    tcp        0      0 127.0.0.1:3306          0.0.0.0:*               LISTEN      812/mysqld      
    
    tcp6       0      0 :::80                   :::*                    LISTEN      1142/apache2    
    
    tcp6       0      0 :::22                   :::*                    LISTEN      2095/sshd       
    
    tcp6       0      0 ::1:631                 :::*                    LISTEN      1051/cupsd      
    
    udp        0      0 0.0.0.0:51810           0.0.0.0:*                           634/avahi-daemon: r
    
    udp        0      0 0.0.0.0:5353            0.0.0.0:*                           634/avahi-daemon: r
    
    aneesh@aneesh-laptop:~$ 
    
    We can see that the sshd server is listening on the port no. 22 on 0.0.0.0 I.e localhost...

    Note : The other output is about other servers running on my machine as I am running apache etc etc..

    Now lets connect to the local ssh-server
    Code:
    aneesh@aneesh-laptop:~$ ssh localhost
    
    The authenticity of host 'localhost (::1)' can't be established.
    
    RSA key fingerprint is 18:ee:8c:7f:4e:bf:0c:3e:7a:e5:78:6f:f7:49:53:b1.
    
    Are you sure you want to continue connecting (yes/no)? yes
    
    Warning: Permanently added 'localhost' (RSA) to the list of known hosts.
    
    aneesh@localhost's password: 
    
    Yes we got the connection .. Now after entering our password we can get the shell
    Code:
    aneesh@localhost's password: 
    
    Linux aneesh-laptop 2.6.32.26+drm33.12-explict-hax0r #2 SMP Fri Jan 7 15:33:24 IST 2011 i686 GNU/Linux
    
    Ubuntu 10.04.1 LTS
    
    Welcome to Ubuntu!
    
     * Documentation:  https://help.ubuntu.com/
    
    Last login: Mon Jan 10 16:30:50 2011
    
    aneesh@aneesh-laptop:~$ 
    
    As we see it just looks like a ordinary shell and provides the same usage...

    Now lets test some commands :-
    Code:
    aneesh@aneesh-laptop:~$ cd /
    
    aneesh@aneesh-laptop:/$ ls
    
    bin      dev         initrd.img.old  mnt   sbin     tmp      vmlinuz.old
    
    boot     etc         lib             opt   selinux  usr
    
    cdrom    home        lost+found      proc  srv      var
    
    desktop  initrd.img  media           root  sys      vmlinuz
    
    aneesh@aneesh-laptop:/$ cd usr
    
    aneesh@aneesh-laptop:/usr$ ls
    
    bin  games  include  lib  lib64  local  man  sbin  share  src
    
    aneesh@aneesh-laptop:/usr$ cd ../
    
    aneesh@aneesh-laptop:/$ ls
    
    bin      dev         initrd.img.old  mnt   sbin     tmp      vmlinuz.old
    
    boot     etc         lib             opt   selinux  usr
    
    cdrom    home        lost+found      proc  srv      var
    
    desktop  initrd.img  media           root  sys      vmlinuz
    
    aneesh@aneesh-laptop:/$ cd home
    
    aneesh@aneesh-laptop:/home$ ls
    
    aneesh
    
    aneesh@aneesh-laptop:/home$ cd aneesh/
    
    aneesh@aneesh-laptop:~$ cd articles/
    
    aneesh@aneesh-laptop:~/articles$ ls
    
    a.out  Bash  crackme    debugMe    hello
    
    ASM    C     crackme.c  debugMe.c  helloWorld.c
    
    aneesh@aneesh-laptop:~/articles$ mkdir SSH
    
    aneesh@aneesh-laptop:~/articles$ ls
    
    a.out  Bash  crackme    debugMe    hello         SSH
    
    ASM    C     crackme.c  debugMe.c  helloWorld.c
    
    aneesh@aneesh-laptop:~/articles$ cd SSH
    
    aneesh@aneesh-laptop:~/articles/SSH$ ls
    
    aneesh@aneesh-laptop:~/articles/SSH$ vi HiIamHere
    
    aneesh@aneesh-laptop:~/articles/SSH$ echo "Hello I am using ssh server on my machine ... and its damn exiting..... woooo!!!!!!" > HiIamHere
    
    echo "Hello I am using ssh server on my machine ... and its damn exiting..... woooovi HiIamHerevi HiIamHerevi HiIamHere" > HiIamHere
    
    aneesh@aneesh-laptop:~/articles/SSH$ ls
    
    HiIamHere
    
    aneesh@aneesh-laptop:~/articles/SSH$ cat HiIamHere 
    
    Hello I am using ssh server on my machine ... and its damn exiting..... woooovi HiIamHerevi HiIamHerevi HiIamHere
    
    aneesh@aneesh-laptop:~/articles/SSH$ 
    
    aneesh@aneesh-laptop:~/articles/SSH$ rm HiIamHere 
    
    aneesh@aneesh-laptop:~/articles/SSH$ ls
    
    aneesh@aneesh-laptop:~/articles/SSH$ 
    
    And remember to close your connection simply use 'exit' as in a normal shell :-
    Code:
    aneesh@aneesh-laptop:~/articles/SSH$ exit
    
    logout
    
    Connection to localhost closed.
    
    
    Thats all about basic ssh you have to know to use it...

    But stay tuned I may be writing some more articles on ssh encryption and some vulnerabilities it met with...
     
  2. lionaneesh

    lionaneesh Active Member

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    Re: How SSH works? Examples and Samples

    Shabbir , Thanks for accepting..
    And guyz please comment..
     
  3. kumarmannu

    kumarmannu Banned

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    Re: How SSH works? Examples and Samples

    Great information thanks.........
     
  4. lionaneesh

    lionaneesh Active Member

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    Re: How SSH works? Examples and Samples

    My Pleasure...
     
  5. seangtz

    seangtz New Member

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    Re: How SSH works? Examples and Samples

    This time also a greattttttttt information!!!!!
     
  6. lionaneesh

    lionaneesh Active Member

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    Re: How SSH works? Examples and Samples

    Thanks..

    But if you really liked it..
    Please press thanks button at the bottom of my article...
     

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