Lecture 7 TELNET Protocol

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Lecture 7TELNET Protocol HyperText Transfer
Protocol

• CPE 401 / 601Computer Network Systems

slides are modified from Dave Hollinger
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TELNET vs. telnet

• TELNET is a protocol that provides a general,
  bi-directional, eight-bit byte oriented
  communications facility.
• telnet is a program that supports the TELNET
  protocol over TCP.
• Many application protocols are built upon the
  TELNET protocol.

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The TELNET Protocol

• Reference RFC 854
• TCP connection
• data and control over the same connection.
• Network Virtual Terminal
• intermediate representation of a generic
  terminal.
• provides a standard language for communication of
  terminal control functions.

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Network Virtual Terminal
Server Process
NVT
NVT
TCP
TCP
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Negotiated Options

• All NVTs support a minimal set of capabilities.
• Some terminals have more capabilities than the
  minimal set.
• The set of options is not part of the TELNET
  protocol,
• so that new terminal features can be incorporated
  without changing the TELNET protocol.
• Two endpoints negotiate a set of mutually
  acceptable options
• Line mode vs. character mode
• echo modes
• character set (EBCDIC vs. ASCII)

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Control Functions

• TELNET includes support for a series of control
  functions commonly supported by servers.
• This provides a uniform mechanism for
  communication of (the supported) control
  functions.

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Control Functions

• Interrupt Process (IP)
• suspend/abort process.
• Abort Output (AO)
• send no more output to users terminal.
• Are You There (AYT)
• check to see if system is still running.
• Erase Character (EC)
• delete last character sent
• Erase Line (EL)
• delete all input in current line.

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Command Structure

• All TELNET commands and data flow through the
  same TCP connection.
• Commands start with a special character called
  the Interpret as Command escape character
• The IAC code is 255.
• If a 255 is sent as data - it must be followed by
  another 255.
• If IAC is found and the next byte is IAC
• a single byte is presented to application/terminal
  
• If IAC is followed by any other code
• the TELNET layer interprets this as a command.

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Playing with TELNET

• You can use the telnet program to play with the
  TELNET protocol.
• telnet is a generic TCP client.
• Sends whatever you type to the TCP socket.
• Prints whatever comes back through the TCP socket
• Useful for testing TCP servers (ASCII based
  protocols).
• Many Unix systems have these servers running (by
  default)
• echo port 7 discard port 9
• daytime port 13 chargen port 19

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telnet hostname port

• gt telnet amele-2.cse.unr.edu 7
• Trying 134.197.40.246...
• Connected to amele-2.cse.unr.edu
  (134.197.40.246).
• Escape character is ''.
• Hi mehmet
• Hi mehmet
• stop it
• stop it
• telnetgt quit
• Connection closed.

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telnet vs. TCP

• Not all TCP servers talk TELNET (most don't)
• You can use the telnet program to play with these
  servers, but the fancy commands won't do
  anything.
• type , then "help" for a list of fancy TELNET
  stuff you can do in telnet.

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(No Transcript)
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HyperText Transfer Protocol (HTTP)

• HTTP is the protocol that supports communication
  between web browsers and web servers.
• A Web Server is a HTTP server
• Most clients/servers today speak version 1.1, but
  1.0 is also in use.
• RFC 1945 (HTTP 1.0)
• RFC 2616 (HTTP 1.1)

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From the RFC

• HTTP is an application-level protocol with the
  lightness and speed necessary for distributed,
  hypermedia information systems.
• Transport Independence
• The HTTP protocol generally takes place over a
  TCP connection,
• but the protocol itself is not dependent on a
  specific transport layer.

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Request - Response

• HTTP has a simple structure
• client sends a request
• server returns a reply.
• HTTP can support multiple request-reply exchanges
  over a single TCP connection.
• The well known TCP port for HTTP servers is
  port 80.
• Other ports can be used as well...

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HTTP 1.0 Request

• Lines of text (ASCII).
• Lines end with CRLF \r\n
• First line is called Request-Line

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Request Line

• Method URI HTTP-Version\r\n
• The request line contains 3 tokens (words).
• space characters separate the tokens.
• Newline (\n) seems to work by itself
• but the protocol requires CRLF

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Request Method

• The Request Method can be
• GET HEAD DELETE
• PUT POST TRACE
• OPTIONS
• future expansion is supported
• GET, HEAD and POST are supported everywhere
  (including Lab 2!).
• HTTP 1.1 servers often support PUT, DELETE,
  OPTIONS TRACE.

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Methods

• GET retrieve information identified by the
  URI.
• Typically used to retrieve an HTML document
• HEAD retrieve meta-information about the
  URI.
• used to find out if a document has changed
• POST send information to a URI and retrieve
  result.
• used to submit a form

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More Methods

• PUT Store information in location named by
  URI.
• DELETE remove entity identified by URI.
• TRACE used to trace HTTP forwarding through
  proxies, tunnels, etc.
• OPTIONS used to determine the capabilities
  of the server, or characteristics of a
  named resource.

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URI Universal Resource Identifier

• URIs defined in RFC 2396.
• Absolute URI
• scheme//hostnameport/path
• http//www.cse.unr.edu80/mgunes/cpe401
• Relative URI
• /path
• /blah/foo

No server mentioned
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URI Usage

• When dealing with a HTTP 1.1 server, only a path
  is used (no scheme or hostname).
• HTTP 1.1 servers are required to be capable of
  handling an absolute URI, but there are still
  some out there that wont
• When dealing with a proxy HTTP server, an
  absolute URI is used.
• client has to tell the proxy where to get the
  document!
• more on proxy servers in a bit.

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HTTP Version Number

• HTTP/1.0 or HTTP/1.1
• Starting with HTTP 1.0 the version number is part
  of every request.
• Client tells the server what version it can talk
  (what options are supported, etc).
• HTTP 0.9 did not include a version number in a
  request line.
• If a server gets a request line with no HTTP
  version number, it assumes 0.9
• HTTP 0.9 was used for many years.

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The Header Lines

• Request Headers provide information to the server
  about the client
• what kind of client
• what kind of content will be accepted
• who is making the request
• Each header line contains
• an attribute name followed by a followed by a
  space and the attribute value.
• There can be 0 headers (HTTP 1.0)
• HTTP 1.1 requires a Host header

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Example HTTP Headers

• Accept text/html
• Host www.cse.unr.edu
• From mgunes_at_cse.unr.edu
• User-Agent Mozilla/4.0
• Referer http//www.unr.edu/

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End of the Headers

• Each header ends with a CRLF ( \r\n )
• The end of the header section is marked with a
  blank line.
• just CRLF
• For GET and HEAD requests, the end of the headers
  is the end of the request!

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POST

• A POST request includes some content (some data)
  after the headers (after the blank line).
• There is no format for the data (just raw bytes).
• A POST request must include a Content-Length line
  in the headers
• Content-length 267

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Example POST Request

• POST /mgunes/cpe401/grades.cgi HTTP/1.1
• Accept /
• Host www.cse.unr.edu
• User-Agent SecretAgent V2.3
• Content-Length 35
• Referer http//www.unr.edu/
• stuid6660182722itemtest1grade99

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Example GET Request

• GET /mgunes/cpe401/lab1.htm HTTP/1.1
• Accept /
• Host www.cse.unr.edu
• User-Agent Internet Explorer
• From mgunes_at_cse.unr.edu
• Referer http//www.unr.edu/

There is a blank line here!
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HTTP Response

• ASCII Status Line
• Headers Section
• Content can be anything (not just text)
• typically an HTML document or some kind of image.

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Response Status Line

• HTTP-Version Status-Code Message
• Status Code is 3 digit number (for computers)
• 1xx Informational
• 2xx Success
• 3xx Redirection
• 4xx Client Error
• 5xx Server Error
• Message is text (for humans)

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Example Status Lines

• HTTP/1.0 200 OK
• HTTP/1.0 301 Moved Permanently
• HTTP/1.0 400 Bad Request
• HTTP/1.0 500 Internal Server Error

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Response Headers

• Provide the client with information about the
  returned entity (document).
• what kind of document
• how big the document is
• how the document is encoded
• when the document was last modified
• Response headers end with blank line

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Response Header Examples

• Date Wed, 30 Jan 2002 124817 EST
• Server Apache/1.17
• Content-Type text/html
• Content-Length 1756
• Content-Encoding gzip

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Content

• Content can be anything (sequence of raw bytes).
• Content-Length header is required for any
  response that includes content.
• Content-Type header also required.

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Single Request/Reply

• The client sends a complete request.
• The server sends back the entire reply.
• The server closes its socket.
• If the client needs another document it must open
  a new connection.

This was the default for HTTP 1.0
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Persistent Connections

• HTTP 1.1 supports persistent connections (this is
  the default).
• Multiple requests can be handled over a single
  TCP connection.
• The Connection header is used to exchange
  information about persistence (HTTP/1.1)
• 1.0 Clients used a Keep-alive header

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Try it with telnet

• gt telnet www.cse.unr.edu 80
• GET / HTTP/1.0
• HTTP/1.0 200 OK
• Server Apache
• ...

Request-line
Blank Line (end of headers)
Response
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Try it with telnet (persistent)

• gt telnet www.cse.unr.edu 80
• GET / HTTP/1.1
• Host www.cse.unr.edu
• HTTP/1.0 200 OK
• Server Apache
• ...

Required!
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HTTP Proxy Server
HTTP Server
Browser
Proxy
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Network Lab 2 HTTP Proxy

• You need to write a proxy server.
• Must be able to handle GET, HEAD and POST
  requests.
• Filtering Your proxy will be given a list of
  domain names on the command line, you should
  refuse to forward requests to any server whose
  name is within a specified domain.
• send back status line 403 Forbidden.

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The code you need

• Proxy is both a client and a server
• Parsing the HTTP request is needed.
• You need to understand HTTP
• You will need to parse headers.
• need to look at Content-length, Connection, etc.

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Testing

• Tell your browser to use a proxy
• Edit preferences/options.
• Interrupt a long transfer (press stop).
• Fill out a form (probably uses POST).
• Test it with a browser.
• Test it with telnet
• Write an abusive client and a rude server!

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What is expected

• We should be able to surf through your proxy!
• Proxy should print some info about each request
  (print the request line).
• No memory leaks!
• Check every system call for errors!
• We should not be able to kill your proxy by
• sending a bad request.
• using a server that sends bad replies.
• No crashes, no matter what kind of nonsense we
  send your proxy.

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HTTP V1.1 Details

• The RFC is 114 pages!
• we dont expect you to read it all or to support
  every nitty-gritty detail.
• work on creating a working proxy (one you can use
  through a browser).
• performance is not a big deal (but it shouldnt
  be horribly worse than without your proxy).
• Dont worry about persistence, pipelining,
  chunking, etc.
• you need to turn off persistence if you don't
  want to handle it.

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HTTP Headers

• You will need to look at the Content-Length
  header in a POST.
• you need to know how many bytes to read after the
  end of the headers.
• You will need to either look at Connection
  (Proxy-Connection) headers or (at a minimum) to
  force Connection close as a request header.

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Stuff you might need to know(that we have not
covered)

• Converting hostnames to IP addresses.
• Handling signals (SIGPIPE)
• Check out section 5.13 in the text
• Providing Concurrency (not required, but not hard
  either).
• just fork the server after calling accept.
• MAKE SURE YOU TAKE CARE OF ZOMBIES!