Unix Programming - ARP Request packet??plz reply soon

Author

ARP Request packet??plz reply soon

ANaiveProgrammer

2004-11-23, 6:08 pm

hi there

i have defined the following two structs( Code is attached at the
bottom of this message....) i actually want to send ARP-Request
message to my loopback because im not attached to an Ethernet? Now
what im doing is i have tried to sent my ethernet_frame(See code..) to
my loopback?( Is it a legitimate way to check that ARP-request has
been processed successfully if u r not attached to a LAN, if not then
please tell me how can i check that my ARP-REQUEST has been
successfully processed even if im not attached to a ethernet/LAN?)
Now another problem is that i need to know the correct size of both
the structs i.e. ethernet_frame and arp_request. and when i check that
using sizeof operator then it doesn't show me the correct size.
How will i send a struct over the wire, is the following way correct
write(sockfd,(char *)&ef,sizeof(ef));// ef is theinstance of
ethernet_frame
if above is not correct, thenn who will i send the struct over a
socket?

lastly, i need to be sure about the structs that i have designed i.e.
their structure and datatype and the way i have initialized them in
set_ethernet()
and set_request()..are my initializations correct in these two
functions???
I really appreciate any help, at earliest, as im very new in C under
linux, and this is the first time im programming the link-layer.....
I know this is a long message and will take qutie a bit of ur time.
However i appreciate your effort and time and i hope i will get a
reply soon
Cheers

CODE( in c under RED HAT Linux 8)
--------------------------------
#include <sys/socket.h>
#include <sys/types.h>
#include <netinet/in.h>
#include <string.h>

typedef struct {
//This struct defines Arp--Request.
uint8_t haddr_type;// For Ethernet it is 1
uint8_t proto_type[4];// 0x800
uint8_t hard_len;// 6 Bytes
uint8_t proto_len;// 4 Bytes
uint16_t operation;// 1---> for request
uint8_t sender_hard[6];//Sender hardware Address
uint8_t sender_proto[4];//Sender protocol Address
uint8_t target_ip[4];// Target IP Address
//Target hardware Address, to me, is optional so i didn't mention it.

}arp_request;

typedef struct{
uint8_t destip[4];
uint8_t sourceip[4];
uint8_t frametype[3];// 0x806 identifies ARP message
arp_request *req;
} ethernet_frame;

void set_ethernet(ethernet_frame *ef, arp_request *rep1){
//This function set the values of variables defined in ethernet_frame
struct
uint8_t dest[4] = {127,0,0,1};

int k;
for ( k=0; k < 4; k++)
ef->destip[k] = dest[k];

uint8_t source[4] = {144,5,4,1};
for ( k=0; k < 4; k++)
ef->sourceip[k] = source[k];

uint8_t frame[3]={0x8,0x0,0x6};
for( k=0; k<3;k++)
ef->frametype[k] = frame[k];
ef->req = rep1;

}

void set_request(arp_request *req){
//This function set the values of variables defined in arp_request
struct
req->haddr_type = 1;
uint8_t ptype[4] = {0x0,0x8,0x0,0x0};
int k;
for ( k=0; k < 4; k++)
req->proto_type[k] = ptype[k];
req->operation = 1;
uint8_t sendh[6] = {0xff,0xdd,0xcd,0xfa,0x8f,0xff};
for ( k=0; k < 6; k++)
req->sender_hard[k] = sendh[k];
strcpy(req->sender_hard,sendh);
uint8_t senderp[4]={128,0,0,1};
int i;
for (i=0; i < 4; ++i)
req->sender_proto[i] = senderp[i];

uint8_t tip[4] = {127,0,0,1};
int ii;
for ( ii =0; ii < 4; ++ii)
req->target_ip[ii] = tip[ii];

}

int main(){
arp_request request;
set_request(&request);
ethernet_frame ef;
set_ethernet(&ef,&request);
printf("Size of ARP---%d\n",sizeof(request));//It doesnt print
correct size
printf("Size of Ethernet---%d\n",sizeof(ef));//It doesnt print
correct size
int len=0;
set_ethernet(&ef,&request);

int sockfd;
struct sockaddr_in dest;
sockfd = socket(AF_INET,SOCK_STREAM,0);
dest.sin_family = AF_INET;

char type1[4]="0800";

int i;
for( i=0; i < strlen(type1); ++i)
printf("%c",type1[i]);
printf("\n");

dest.sin_addr.s_addr = inet_addr("127.0.0.1");
bind(sockfd,(struct sockaddr*)&dest ,sizeof(struct sockaddr));
printf("Trying to Connect\n");
connect(sockfd,(struct sockaddr*)&dest,sizeof(struct sockaddr));
printf("Connected...\n");
write(sockfd,(char *)&ef,10);
return 0;
}

Jens.Toerring@physik.fu-berlin.de

2004-11-24, 8:12 am

In comp.unix.programmer ANaiveProgrammer <myhotline@gmail.com> wrote:
> hi there

> i have defined the following two structs( Code is attached at the
> bottom of this message....) i actually want to send ARP-Request
> message to my loopback because im not attached to an Ethernet? Now
> what im doing is i have tried to sent my ethernet_frame(See code..) to
> my loopback?( Is it a legitimate way to check that ARP-request has
> been processed successfully if u r not attached to a LAN, if not then
> please tell me how can i check that my ARP-REQUEST has been
> successfully processed even if im not attached to a ethernet/LAN?)
> Now another problem is that i need to know the correct size of both
> the structs i.e. ethernet_frame and arp_request. and when i check that
> using sizeof operator then it doesn't show me the correct size.
> How will i send a struct over the wire, is the following way correct
> write(sockfd,(char *)&ef,sizeof(ef));// ef is theinstance of
> ethernet_frame
> if above is not correct, thenn who will i send the struct over a
> socket?

> lastly, i need to be sure about the structs that i have designed i.e.
> their structure and datatype and the way i have initialized them in
> set_ethernet()
> and set_request()..are my initializations correct in these two
> functions???
> I really appreciate any help, at earliest, as im very new in C under
> linux, and this is the first time im programming the link-layer.....
> I know this is a long message and will take qutie a bit of ur time.
> However i appreciate your effort and time and i hope i will get a
> reply soon
> Cheers

> CODE( in c under RED HAT Linux 8)
> --------------------------------
> #include <sys/socket.h>
> #include <sys/types.h>
> #include <netinet/in.h>
> #include <string.h>
>
> typedef struct {
> //This struct defines Arp--Request.
> uint8_t haddr_type;// For Ethernet it is 1
> uint8_t proto_type[4];// 0x800
> uint8_t hard_len;// 6 Bytes
> uint8_t proto_len;// 4 Bytes
> uint16_t operation;// 1---> for request
> uint8_t sender_hard[6];//Sender hardware Address
> uint8_t sender_proto[4];//Sender protocol Address
> uint8_t target_ip[4];// Target IP Address
> //Target hardware Address, to me, is optional so i didn't mention it.
> }arp_request;
>
> typedef struct{
> uint8_t destip[4];
> uint8_t sourceip[4];
> uint8_t frametype[3];// 0x806 identifies ARP message
> arp_request *req;
> } ethernet_frame;
>
> void set_ethernet(ethernet_frame *ef, arp_request *rep1){
> //This function set the values of variables defined in ethernet_frame
> struct
> uint8_t dest[4] = {127,0,0,1};
>
> int k;
> for ( k=0; k < 4; k++)
> ef->destip[k] = dest[k];
>
> uint8_t source[4] = {144,5,4,1};
> for ( k=0; k < 4; k++)
> ef->sourceip[k] = source[k];
>
> uint8_t frame[3]={0x8,0x0,0x6};
> for( k=0; k<3;k++)
> ef->frametype[k] = frame[k];
> ef->req = rep1;

> }
>
> void set_request(arp_request *req){
> //This function set the values of variables defined in arp_request
> struct
> req->haddr_type = 1;
> uint8_t ptype[4] = {0x0,0x8,0x0,0x0};
> int k;
> for ( k=0; k < 4; k++)
> req->proto_type[k] = ptype[k];
> req->operation = 1;
> uint8_t sendh[6] = {0xff,0xdd,0xcd,0xfa,0x8f,0xff};
> for ( k=0; k < 6; k++)
> req->sender_hard[k] = sendh[k];
> strcpy(req->sender_hard,sendh);
> uint8_t senderp[4]={128,0,0,1};
> int i;
> for (i=0; i < 4; ++i)
> req->sender_proto[i] = senderp[i];
>
> uint8_t tip[4] = {127,0,0,1};
> int ii;
> for ( ii =0; ii < 4; ++ii)
> req->target_ip[ii] = tip[ii];
> }
>
> int main(){
> arp_request request;
> set_request(&request);
> ethernet_frame ef;
> set_ethernet(&ef,&request);
> printf("Size of ARP---%d\n",sizeof(request));//It doesnt print
> correct size
> printf("Size of Ethernet---%d\n",sizeof(ef));//It doesnt print
> correct size

It prints the correct size - the structure just hasn't the size you
assume it to be. As I already wrote in comp.lang.c the compiler is
free to insert as many "padding bytes" between the members of the
structure as it deems necessary. So don't use structures if you
need to assemble data in a certain binary format. Take instead a
flat array of unsigned chars for the "ethernet_frame" and assemble
the data in it by memcpy()ing to the correct offsets. That's
basically the only way to guarantee that you get the binary
format right (well, modulo big-endian/little-endian issues if
you copy binary data from ints or other types that are longer
than a single byte).

> int len=0;

Do you have a C99 compiler? Otherwise defining variables after the
first executable statement isn't allowed...

> set_ethernet(&ef,&request);

Why do you do that again?

> int sockfd;
> struct sockaddr_in dest;
> sockfd = socket(AF_INET,SOCK_STREAM,0);
> dest.sin_family = AF_INET;

> char type1[4]="0800";

You need one more character in 'type1' if you want to store the
string "0800" - you need room for the termination '\0'.
>
> int i;
> for( i=0; i < strlen(type1); ++i)
> printf("%c",type1[i]);
> printf("\n");
>
> dest.sin_addr.s_addr = inet_addr("127.0.0.1");
> bind(sockfd,(struct sockaddr*)&dest ,sizeof(struct sockaddr));

Why do you call bind() when you obviously try to connect to some
server?

> printf("Trying to Connect\n");
> connect(sockfd,(struct sockaddr*)&dest,sizeof(struct sockaddr));
> printf("Connected...\n");
> write(sockfd,(char *)&ef,10);

Now things get even more weird. Where do you got the idea from that
a structure of type 'ethernet_frame' has 10 bytes? Even if there
are no padding bytes it would have at least 15 bytes. And what's
sending a pointer ('rep') over the socket good for? The other side
won't be able to do anything useful with that.

> return 0;
> }

Sorry, but I really don't understand what you're trying to do here.
From all the 'ethernet_frame' and 'ARP request' stuff in the
prgram I get the feeling that you actually want to assemble your
own packages and send them somewhere else - but then why do you use
the TCP protocol to send that?
Regards, Jens
--
\ Jens Thoms Toerring ___ Jens.Toerring@physik.fu-berlin.de
\__________________________ http://www.toerring.de

ANaiveProgrammer

2004-11-24, 6:28 pm

Jens.Toerring@physik.fu-berlin.de wrote in message news:<30jmpdF30v6taU1@uni-berlin.de>...
> In comp.unix.programmer ANaiveProgrammer <myhotline@gmail.com> wrote:
>

Many thanks for your reply and time . Im actually new to C and network
programming. I actually want to send a ARP request to a Computer and
make sure that it has been processed successfully. My code is the way
i though it might be implemented. Things are still pretty vague in my
mind about sending ARP-Request and to make sure that it's been
processed.
Sometold told me that you can send a Ethernet Struct to your loop back
and using ethereal you can make sure that your arp-request has been
processed. I still don't know whether it'll work or not but i did as
told.
By posting this message i just wanted to be sure about Lay out of my
Structures. I just needed to be sure that my structs should remain the
same at receiving end as they are before sending.

[vbcol=seagreen]
> char type1[4]="0800";

>You need one more character in 'type1' if you want to store the
>string "0800" - you need room for the termination '\0'.

well not for sure, but i think '\0' is not mandatory because if and
only if packet will be received, it will only look out for 800...

>
> Now things get even more weird. Where do you got the idea from that
> a structure of type 'ethernet_frame' has 10 bytes? Even if there
> are no padding bytes it would have at least 15 bytes.
> sending a pointer ('rep') over the socket good for? The other side
> won't be able to do anything useful with that.

Got it!!!it'll be rectified but still question about sending
ethernet_frame struct over socket is popping up in my mind.,...i mean
will my ethernet_frame will go over the wire like following

write(sockfd,(char *)&ef,sizeof(ef));

Bottom line is, please tell me how to send a struct over socket. Will
above work????

>
> Sorry, but I really don't understand what you're trying to do here.
> From all the 'ethernet_frame' and 'ARP request' stuff in the
> prgram I get the feeling that you actually want to assemble your
> own packages and send them somewhere else - but then why do you use
> the TCP protocol to send that?
As i told you i just wanted to send ARP packet to some PC as im not
connected to LAN, so i sent that to my loopback for testing purposes
over TCP. i hope u would now have an idea what im trying to do????
please reply me . "I know this may will take bit of your time, but i
will be highly compelled for your esteemed favour.
cheers
waqas

Tauno Voipio

2004-11-25, 2:50 am

ANaiveProgrammer wrote:
> hi there
>
> i have defined the following two structs( Code is attached at the
> bottom of this message....) i actually want to send ARP-Request
> message to my loopback because im not attached to an Ethernet? Now
> what im doing is i have tried to sent my ethernet_frame(See code..) to
> my loopback?( Is it a legitimate way to check that ARP-request has
> been processed successfully if u r not attached to a LAN, if not then
> please tell me how can i check that my ARP-REQUEST has been
> successfully processed even if im not attached to a ethernet/LAN?)
> Now another problem is that i need to know the correct size of both
> the structs i.e. ethernet_frame and arp_request. and when i check that
> using sizeof operator then it doesn't show me the correct size.
> How will i send a struct over the wire, is the following way correct
> write(sockfd,(char *)&ef,sizeof(ef));// ef is theinstance of
> ethernet_frame
> if above is not correct, thenn who will i send the struct over a
> socket?
>

1. You cannot send a message to any of the local addresses, so
that it creates an ARP request. The IP stack is smart enough to
route such a request via the loopback interface, lo.

2. The loopback interface never needs ARP - it handles its
transport without any link level addresses, so no translation
from IP to link addresses (by ARP) is needed.

3. You attempted to send a link-level frame with a TCP (stream)
socket - it cannot work.

4. Get a book on Unix networking and read about raw sockets.
Please note that due to security considerations, the raw
socket operations are limited to superuser privileges only.

--

Tauno Voipio
tauno voipio (at) iki fi

ANaiveProgrammer

2004-11-25, 7:50 am

Tauno Voipio <tauno.voipio@iki.fi.NOSPAM.invalid> wrote in message >

Many thanks for this Useful reply......These are the answers i was
looking for!!!
> The IP stack is smart enough to
> route such a request via the loopback interface, lo.

would you please elaborate above. I mean how IP stack route such a
request via loopback( please answer or give any pointer) and why?
>
> 2. The loopback interface never needs ARP - it handles its
> transport without any link level addresses,

loopback interface handles its transport without link level addresses,
well im stumped...i mean who lo handles its transport without any link
level address???

thanks for your invaluable time..i hope i will get a reply soon...Any
help will be highly appreciated....

Regards

Nils O. Selåsdal

2004-11-25, 7:50 am

ANaiveProgrammer wrote:
> Tauno Voipio <tauno.voipio@iki.fi.NOSPAM.invalid> wrote in message >
>
>
> Many thanks for this Useful reply......These are the answers i was
> looking for!!!
>
>
>
> would you please elaborate above. I mean how IP stack route such a
> request via loopback( please answer or give any pointer) and why?
It figures it is on the same machine, and usues the loopback
for performance (and a few other issues).
>
>
> loopback interface handles its transport without link level addresses,
> well im stumped...i mean who lo handles its transport without any link
> level address???
Its just buffers in the kernel.. IP doesn't care what it's transported
over.