AIT 622 Network Architecture and Protocols Remote Login (TELNET, SSH) 1 Remote Interactive Computing TCP can be used for interactive use of remote machines User establish remote login session Then[.]
Trang 1Remote Login
(TELNET, SSH)
Trang 2Remote Interactive Computing
TCP can be used for interactive use of
remote machines
User establish remote login session
Then executes commands
Remote login may not be simple
Most systems designed to expect direct login
May have to modify the machine’s OS
Building interactive client may be difficult, too
Trang 3TELNET Protocol
TCP/IP suite includes TELNET
Simple remote terminal protocol
Allows user to log into computer over internet
Establishes TCP connection
Passes keystrokes to remote machine
Carries output back to user’s screen
Transparent
Gives appearance that user keyboard and display
is directly connected to the remote machine
Trang 4TELNET is not sophisticated as some
Is widely available
ID remote machine by domain name or IP address
Offers three basic services
Defines network virtual terminal
Clients only have to build to the standard interface
Allows negotiation of some standard options
Treats both ends of connection symmetrically
Does not force keyboard input or screen output
Trang 5Figure 24.1
Trang 6Server is more complex than shown
Must handle multiple, concurrent connections
TELNET server consists of:
Master server that listens for new requests
Slave that handles one particular connection
Pseudo terminal
OS entry point
Allows application to transfer characters to OS
as if they came from a keyboard
Cannot build TELNET server without it
Trang 7Adv of TELNET server as application pgm
Modification & control of server easier
Since code is not part of OS
Disadv
Inefficiency
Each keystroke
internet
Then, Server’s OS Application Program
Output comes back over same path
Each keystroke requires several context switches
Expensive; practical due to slow typing speeds
Trang 8Accommodating Heterogeneity
Computers and OSs are different
Ending lines of text
Some require termination by CR Others require LF
Other require two characters: CR-LF
Key to interrupt running program
Most interactive systems provide a way Specific keystroke is different (Control-C or ESC)
Trang 9Use network virtual terminal
Defines how data and command sequences are sent
Figure 24.2
Trang 10NVT definition fairly straightforward
Communication involves 8 bits
Use USASCII 7-bit code for data
Command sequences have high order bit set
95 printable letters, digits, punctuation marks
33 control codes
Trang 11Secure Shell (SSH)
Popular alternative to TELNET
Uses TCP to connect remotely, like TELNET
Two significant enhancements over TELNET
Provides secure communications
Can perform additional, independent data transfers over same connection used for remote login
Uses public key cryptography
Really a general purpose secure connection
Versus just a secure remote login service
Trang 12Rlogin (BSD UNIX)
BSD UNIX operating systems include
remote login service called rlogin
Not a general purpose protocol like TELNET
Protocol understands computing environments
Exports part of user’s environment to remote
machine
Supports trusted hosts
Administrator can choose set of machines over
Trang 13Virtual Network Computing (VNC)
Provides remote desktop capability
See exact copy of desktop on another computer
Use keyboard, mouse to interact with remote
machine
Runs across multiple platforms
Linux, Windows, etc
Trang 14Remote Desktop Protocol (RDP)
Defined by Microsoft for their OS
Similar to other remote desktop systems
See exact copy of remote desktop Can be used across software platforms
Trang 15Application level protocols can be built
on TCP/IP services & client-server model TELNET
TCP/IP internet standard
Widely available remote access system
Secure Shell
Authenticated, confidential remote login
Multiple applications can share SSH connection