Re: IP Header Size is always 5

[Shameem Ahamed <shameem.ahamed () hotmail com>]

Hi All,

Thanks for all the replies, and suggestions.

I got it resolved.


The problem was with a character pointer,i defined earlier.   I forgot the fact that header size is in terms of words.

Thanks fore reminding me.


Regards,
Shameem





> From: guy () alum mit edu
> Subject: Re: [tcpdump-workers] IP Header Size is always 5
> Date: Wed, 1 Apr 2009 11:21:12 -0700
> To: tcpdump-workers () lists tcpdump org
>
>
> On Apr 1, 2009, at 8:32 AM, Shameem Ahamed wrote:
>
> > In that case also, we should be able to get the source and  
> > destination IP address from the below code
> >
> > printf("Source IP: %s \n",inet_ntoa(ipHeader->ip_src));
> >
> > For me it gives me Segmentation Fault.
>
> inet_ntoa() takes a "struct in_addr" as an argument; is the ip_src  
> field in "struct ip" a "struct in_addr"?
>
> If not, then you would need to declare a "struct in_addr" variable:
>
>       struct in_addr addr;
>
> and do
>
>       memcpy(&addr.s_addr, ipHeader->ip_src, sizeof addr.s_addr);
>       printf("Source IP: %s\n", inet_ntoa(addr));
>
> as
>
>       1) there might be an alignment issue, as Sebastien said (admittedly,  
> he works at Sun, who have one of the few lines of processors that  
> don't do unaligned accesses; most people are probably programming on  
> x86 machines these days, and they don't have alignment issues with any  
> OS I know of, *but* they shouldn't assume a lack of alignment issues  
> in the general case);
>
>       2) there is no *guarantee* that a 4-byte structure such as a "struct  
> in_addr" is passed as an argument the same way a 4-byte integral  
> value, for example, is passed (it might be likely, but it's not  
> guaranteed, and code should *NOT* assume it in the general case).
>
> > Also, i am not able to access the tcp header details.
>
>       ...
>
> > tcpHeader=(struct tcphdr *)(packet +ETHER_SIZE+size_ip);
>
> size_ip is set to ipHeader->ip_hl*4, right?
>
> If not, it should be, as per what Sebastien said.
>
> > printf("====================TCP Header Details================\n");
> > size_tcp=tcpHeader->doff;
> > printf("TCP Header Size is: %d \n",size_tcp);
>
> That's also in units of 4-byte words, so the TCP header size is  
> tcpHeader->doff*4.
> -
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