newbie: ethernet, ip header proble

Murat Balaban murat at
Fri Jun 25 05:32:57 PDT 2004

On Fri, Jun 25, 2004 at 03:49:11AM -0700, kamal kc wrote:
> Hi i am new to this mailing list.
> I have written a program to capture packets using pcap library routines. I have a FreeBSD 5.1. The problem I faced was I successfully captured packets and parsed to ethernet header and ip header.
> i present a section of code how i did it.
> --
> char *ptr;
> ptr=pcap_next(.....);
> struct ether_header *eth;
> struct ip *ip;
> eth=(struct ether_header *)ptr;  // datalink type DLT_EN10MB
> ptr+=14; // the size of the ether_header being 14 bytes; 
> ip=(struct ip *)ptr;
> printf("\n %s %s", ether_ntoa(eth->ether_dhost), ether_ntoa(eth->ether_shost));
> printf("\n %s %s", inet_ntoa(ip->ip_src), inet_ntoa(ip->ip_dst));
> ----------------
> Now the problem is that the ethernet destination and sender host is printed the same.
> it is equal to that of the sender MAC address(linux) when ICMP packets (by ping utility)
> is sent to the host(FreeBSD) running the program.
> Also that the ip adresses printed is the same as the sender ip address(ie linux).
> The program is run on host with FreeBSD.
> The ip address of the computers are: 
>  has Linux
>  has FreeBSD
> I couldn't think of a solution as i guess the coding was alright.

ether_ntoa returns a pointer to a static buffer, which means you'll need to save the string 
returned by call to first ether_ntoa [ether_ntoa(eth->ether_dhost)] to a temporary space.

Same thing applies to inet_ntoa.

read the manual pages.

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