full-disclosure - Strange netcat behavior

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From: giovanni at giacobbi.net (Giovanni Giacobbi)
Subject: Strange netcat behavior

On Fri, Oct 17, 2003 at 01:56:02PM +0100, thalm wrote:
> Since netcat is a widely used network tool, this may have been discussed already, but since I wasn't able to found such discussion, here goes:
>  
> When using netcat (Windows and Linux versions) to connect to a web server, and everytime ENTER is pressed in the command line, netcat only sends LF (0x0A) instead of CRLF (0x0D 0x0A).
> ex: GET / HTTP/1.0[LF][LF]
>  
> when using telnet, the behavior is different:
> ex: GET / HTTP/1.0[CRLF][CRLF]
>  
> Although webservers (IIS and probably Apache) don't mind such behavior and accept it (LF) as if it was CRLF, RFC 2616 clearly states that the HTTP Request/Response Line and HTTP Headers *MUST* be separated by a CRLF and not only by a LF.
>  
> Why is there such a difference between netcat and telnet behavior?
>  

Because they are different programs, with different purposes.
Netcat is a stream handler, just like "cat". What it actually does is to take the source input stream 
and forward it to the output stream (a socket, in this case). Everything else is outside its scope.

In your example, the difference is made by your TTY. Your TTY is actually feeding Netcat with a single 
linefeed, while telnet parses the input and converts manually those linefeeds in cr/lf combinations.
Also, Netcat has no clue about what a "line" is, it doesn't make any difference between sent bytes,
as it is binary-safe.

I yet have to find a way to tell linux terminal to convert carriage returns to cr/lf rather than a 
single lf or cr (see stty(1) manpage).

A quick workaround for your problem would be:
 $ echo -en 'GET / HTTP/1.0\r\n\r\n' | nc localhost 80

Regards

-- 
Giovanni Giacobbi