Lecture 7 TELNET Protocol & HyperText Transfer Protocol CPE 401 / 601 Computer Network Systems slides are modified from Dave Hollinger

2 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. TELNET

3 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. TELNET

4 Network Virtual Terminal NVT Server Process TCP TELNET

5 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) TELNET

6 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. TELNET

7 Control Functions  Interrupt Process (IP)  suspend/abort process.  Abort Output (AO)  send no more output to user’s 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. TELNET

8 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. TELNET

9 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 TELNET

10 telnet hostname port > telnet amele-2.cse.unr.edu 7 Trying Connected to amele-2.cse.unr.edu ( ). Escape character is '^]'. Hi mehmet stop it ^] telnet> quit Connection closed. TELNET

11 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. TELNET

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) HTTP 13

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. HTTP 14

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... HTTP 15

HTTP 1.0+ Request  Lines of text (ASCII).  Lines end with CRLF “\r\n”  First line is called “Request-Line” Request-Line Headers. Content... blank line HTTP 16

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 HTTP 17

Request Method  The Request Method can be: GETHEADDELETE PUT POSTTRACE 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. HTTP 18

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 HTTP 19

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. HTTP 20

URI: Universal Resource Identifier  URIs defined in RFC  Absolute URI:  scheme://hostname[:port]/path   Relative URI:  /path  /blah/foo No server mentioned HTTP 21

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 won’t…  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…. HTTP 22

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. HTTP 23

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 HTTP 24

Example HTTP Headers Accept: text/html Host: From: User-Agent: Mozilla/4.0 Referer: HTTP 25

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! HTTP 26

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 HTTP 27

Example POST Request POST /~mgunes/cpe401/grades.cgi HTTP/1.1 Accept: */* Host: User-Agent: SecretAgent V2.3 Content-Length: 35 Referer: stuid= &item=test1&grade=99 HTTP 28

Example GET Request GET /~mgunes/cpe401/lab1.htm HTTP/1.1 Accept: */* Host: User-Agent: Internet Explorer From: Referer: There is a blank line here! HTTP 29

HTTP Response  ASCII Status Line  Headers Section  Content can be anything (not just text)  typically an HTML document or some kind of image. Status-Line Headers. Content... blank line HTTP 30

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) HTTP 31

Example Status Lines HTTP/ OK HTTP/ Moved Permanently HTTP/ Bad Request HTTP/ Internal Server Error HTTP 32

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 HTTP 33

Response Header Examples Date: Wed, 30 Jan :48:17 EST Server: Apache/1.17 Content-Type: text/html Content-Length: 1756 Content-Encoding: gzip HTTP 34

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. HTTP 35

Single Request/Reply  The client sends a complete request.  The server sends back the entire reply.  The server closes it’s socket.  If the client needs another document it must open a new connection. This was the default for HTTP 1.0 HTTP 36

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 HTTP 37

Try it with telnet > telnet 80 GET / HTTP/1.0 HTTP/ OK Server: Apache... Response Request-line Blank Line (end of headers) HTTP 38

Try it with telnet (persistent) > telnet 80 GET / HTTP/1.1 Host: HTTP/ OK Server: Apache... Required! HTTP 39

HTTP Proxy Server HTTP Server Browser Proxy HTTP 40

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. Lab #2 41

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. 42 Lab #2

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! 43 Lab #2

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. 44 Lab #2

HTTP V1.1 Details  The RFC is 114 pages!  we don’t 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 shouldn’t be horribly worse than without your proxy).  Don’t worry about persistence, pipelining, chunking, etc.  you need to turn off persistence if you don't want to handle it. 45 Lab #2

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. 46 Lab #2

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! 47 Lab #2