Re: a problem with some code

[Guy Harris <guy () alum mit edu>]

On Feb 5, 2009, at 9:03 AM, Tyler Littlefield wrote:

> I'm in a college course, which is using a program that is not  
> working with my screen reader. I am blind, and am using a reader  
> that takes the text on the screen and reads it in synthasized  
> speech. There are some programs that don't work with this however,  
> and this is one of them.

When you say "There are some programs that don't work with this  
however, and this is one of them.", do you mean that tcpdump doesn't  
work with the screen reader?

If so, what doesn't work?  Is the problem that the output has a lot of  
abbreviations and other "non-words" and the screen reader doesn't  
translate them well?  Or is the problem that command-line programs  
such as tcpdump/WinDump don't work with the screen reader?

> The problem is in getting the size of the IP header, I believe.

You are correct.  Your code is doing

        int ipsize=sizeof(*recv);

where "recv" is a pointer to a structure that defines the fixed-length  
portion of the IP header.  As you might infer from my saying "fixed- 
length portion", the IP header is variable in length; it can include  
options.

RFC 791 describes how that works; you'll need to look at the "header  
length" field from the IP header, and use that as the length of the IP  
header (as RFC 791 indicates, that field is in units of 4-byte words,  
so you'll need to multiply it by 4 to get the length in bytes).

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