what's going on with inet_ntoa() ?

HI !

Preface: This is my first time writing a posting on the usenet. So if this
isn't the right place for this topic please tell me an alternative.


Currently I'm playing a little with raw sockets on linux. But I have a
strange problem.

Watch this simple c-code which compiles fine and runs:

#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <netinet/ip.h>

#include <netinet/ether.h>
#include <net/ethernet.h>

#include <unistd.h>
#include <stdlib.h>
#include <netdb.h>

#include <netinet/tcp.h>
#include <stdio.h>

int main( int argc , char** argv ) {

int packetsize = sizeof( struct ether_header ) + sizeof( struct iphdr )
+ sizeof( struct tcphdr );
char* packet = malloc( packetsize );

struct ether_header* eth = (struct ether_header*) packet;
struct iphdr* ip = (struct iphdr*) (packet + sizeof( struct
ether_header ) );
struct tcphdr* tcp = (struct tcphdr*) (packet + sizeof( struct
ether_header) + sizeof( struct iphdr ) );

int uid = getuid();
if( uid != 0 ) { printf("Your UID is %d. Please log in as root.\n",uid);
exit(1); }

int socket_fd = socket( AF_INET , SOCK_PACKET , htons(0x3) );
if( socket_fd < 0 ) { printf("Error, creating socket.\n"); exit(1); }

while(1) {
recv( socket_fd , packet , packetsize, 0 );
printf("Ethernet destination MAC address: %s\n" ,
ether_ntoa( (struct ether_addr*) eth ) );

struct in_addr* daddr = malloc( sizeof( struct in_addr ) );
daddr->s_addr =ip->daddr;

printf("Source: inet_ntoa( %d ) -> %s\nDest: inet_ntoa( %d ) ->
%s\n" , ip->saddr , inet_ntoa( *(struct in_addr*) &ip->saddr ) ,
ip->daddr , inet_ntoa( *(struct in_addr*) &ip->daddr ) );
}

} //end source code


Okay the line

printf("Source: inet_ntoa( %d ) -> %s\nDest: inet_ntoa( %d ) -> %s\n" ,
ip->saddr , inet_ntoa( *(struct in_addr*) &ip->saddr ) , ip->daddr ,
inet_ntoa( *(struct in_addr*) &ip->daddr ) );

produces a strange output. ip->daddr and ip->saddr are different, but after
appling the method inet_ntoa the results are identical. Here is a sample
output of my xterm (communication between my wlan card and my wlan router):

Ethernet destination MAC address: 0:3:c9:84:af:cd
Source: inet_ntoa( 184723648 ) -> 192.168.2.11
Dest: inet_ntoa( 16951488 ) -> 192.168.2.11
Ethernet destination MAC address: 0:c:f1:35:c2:53
Source: inet_ntoa( 16951488 ) -> 192.168.2.1
Dest: inet_ntoa( 184723648 ) -> 192.168.2.1

I really do not understand how this can happen. I've already read some man
pages and read the apropriate kernel header's source. I would be very
thankful if anybody could help. It's driving me crazy and I don't know
what's going on.

Paul wrote:

> I really do not understand how this can happen. I've already read some man
> pages and read the apropriate kernel header's source. I would be very
> thankful if anybody could help. It's driving me crazy and I don't know
> what's going on.

You did not read the right man pages :-(
inet_ntoa() documentation states:

The inet_ntoa() function converts the Internet host address 'in' given
in network byte order to a string in standard numbers-and-dots
notation. The string is returned in a statically allocated buffer,
which subsequent calls will overwrite.

Ciao
Giovanni
--
A computer is like an air conditioner,
it stops working when you open Windows.
Registered Linux user #337974 <http://counter.li.org/>

> The inet_ntoa() function converts the Internet host address 'in' given
> in network byte order to a string in standard numbers-and-dots
> notation. The string is returned in a statically allocated buffer,
> which subsequent calls will overwrite.

Acutally I do not understand how this solves my problem. I read this man
page and I understand, that a given address in nethwork byte order of
format struct in_addr is transformed into a dot-number string. Could you
please be a little bit more precisly ? Thanks.

Paul wrote:
>> The inet_ntoa() function converts the Internet host address 'in' given
>> in network byte order to a string in standard numbers-and-dots
>> notation. The string is returned in a statically allocated buffer,
>> which subsequent calls will overwrite.
>
> Acutally I do not understand how this solves my problem. I read this man
> page and I understand, that a given address in nethwork byte order of
> format struct in_addr is transformed into a dot-number string. Could you
> please be a little bit more precisly ? Thanks.
>
>

In a single statement like:
printf("Source: inet_ntoa( %d ) -> %s\nDest: inet_ntoa( %d ) -> %s\n",
ip->saddr, inet_ntoa(*(struct in_addr*)&ip->saddr ), ip->daddr,
inet_ntoa(*(struct in_addr*) &ip->daddr ));
you call inet_ntoa() twice. The first call fills the buffer with the
string corresponding to the 'ip->daddr' end returns the pointer to the
buffer. The second call fills the *same* buffer (it is a static buffer)
with the string corresponding to the 'ip->saddr' and returns the pointer
to the buffer. Unfortunately this destroys the results of the previous
call before it is used.

Try with two different statements:
printf("Source: inet_ntoa( %d ) -> %s\n",
ip->saddr, inet_ntoa(*(struct in_addr*)&ip->saddr));
printf("Dest: inet_ntoa( %d ) -> %s\n",
ip->daddr, inet_ntoa(*(struct in_addr*) &ip->daddr));
or better replace also the format specifier %d with %08x which gives a
better view for adresses.

Ciao
Giovanni
--
A computer is like an air conditioner,
it stops working when you open Windows.
Registered Linux user #337974 <http://counter.li.org/>

Oh man, thank you really much ! Now I understand what is meant by a static
buffer. And the program works fine. Thanks for help!