Sockets & Proof of Concept

[Raja Sambasivan]

Hello,

I am trying to familiarize myself with sockets, etc. for a project I
am working on. In light of this, I tried earlier to write a simple
server and client using a raw socket, and construcing the ip header
myself. Rather unfortunately, I can't get it to work - and I can't
figure out why:( Tcpdump doesn't show any packets being sent out
between the two machines, and the receiving program simply blocks
within the recvfrom call even after I excecute a sendto to the
appropriate port from another machine.

Anyway, the code I'm using is below...any info that will help my
conceptual confusion with this would be greatly appreciated.

The code that sends the data to another machine using sendto is called
in the following manner
./ip_udp_test <destination host name> <destination port> <source
port>

The code that receives the data is called in the following manner
./ip_udp_test <port on which data will be received>

I am of course making sure that destination port = port on which data
will be received.

In the code below, note that I'm padding the iphdr with 12 bytes in
order to make the header 32bytes, I seem to recall reading somewhere
that that was necessary...is this so?

Also, are my ip header values set properly?


Thank you for all your help.
Raja


/* Code to send some random data to another machine using a raw
socket, and udp */

struct ip_udp_packet {
struct iphdr ip_bits;
char iphdr_padding[12];
struct udphdr udp_bits;
char payload[100];
};
typedef struct ip_udp_packet ip_udp_packet_t;

int main(int argc, char **argv) {
int sock;
int hdrincl = 1;
ip_udp_packet_t packet;
char host_name[256];
struct hostent *ret;
long int source_addr;
long int dest_addr;
int source_port;
int dest_port;
struct sockaddr_in to;
int retval;
int blah;

/* Error checking */
if (argc != 4 ) {
printf("Error : Options - desination name, dest port, source
port");
exit(-1);
}

/* Initialization */
memset( &packet, 0, sizeof(ip_udp_packet_t));

/* Get our name */
gethostname( host_name, 256 );
ret = gethostbyname(host_name);
memcpy( &source_addr, *(ret->h_addr_list), 4);
source_addr = ntohl( source_addr );

/* Get name of host we wish to send to */
ret = gethostbyname(argv[1]);
memcpy( &dest_addr, *(ret->h_addr_list), 4);
dest_addr = ntohl( dest_addr );

/* Get the destination port */
sscanf( argv[2], "%d", &dest_port );
sscanf( argv[3], "%d", &source_port );


/* Create a new socket using the raw protocol */
sock = socket(PF_INET, SOCK_RAW, IPPROTO_RAW);
if ( setsockopt(sock, IPPROTO_IP,
IP_HDRINCL, &hdrincl, sizeof(hdrincl)) == -1 ) {
printf("Failed to set socket options.\n");
exit(-1);
}

/* Create the packet */
packet.ip_bits.version = IPVERSION; /* 4 */
packet.ip_bits.ihl = 5; /* Minimum IP header
length */
packet.ip_bits.tos = 0; /* Filled in by kernel */
packet.ip_bits.tot_len = htons(sizeof(ip_udp_packet_t));
packet.ip_bits.id = 0; /* Let Kernel handle this
*/
packet.ip_bits.frag_off = 0; /* No fragmentation */
packet.ip_bits.ttl = IPDEFTTL; /* 64 */
packet.ip_bits.protocol = IPPROTO_UDP; /* 17 */
packet.ip_bits.check = 0; /* Checksum filled in by
kernel! */
packet.ip_bits.saddr = htonl( source_addr );
packet.ip_bits.daddr = htonl( dest_addr );

packet.udp_bits.source = htons( source_port );
packet.udp_bits.dest = htons( dest_port );
packet.udp_bits.len = htons(sizeof(struct udphdr) + 100);

memset( packet.payload, 1, 256 );


/* Send the packet */
to.sin_family = AF_INET;
to.sin_port = htons( source_port);
to.sin_addr.s_addr = htonl(dest_addr);
retval = sendto( sock, &packet , sizeof(ip_udp_packet_t),
0, (struct sockaddr *)&to, sizeof(struct sockaddr));
if ( retval == -1 ) {
printf("sendtofailed\n");
exit(-1);
}



/* Code that receives the packet on another machine */
int main( int argc, char **argv) {
int sock;
char buf[100];
int listen_port;
struct sockaddr_in my;
struct sockaddr_in their;
int their_len;

/* Get input */
sscanf( argv[1], "%d", &listen_port );


/* Create a new socket using the raw protocal */
sock = socket(PF_INET, SOCK_RAW, IPPROTO_RAW);
if ( sock == -1 ) {
printf("Cannot create socket: %. \n", strerror(errno));
}

/* Bind the socket */
my.sin_family = AF_INET;
my.sin_port = htons(listen_port);
my.sin_addr.s_addr = INADDR_ANY;
memset( &(my.sin_zero), '\0', 8);
if( bind( sock, (struct sockaddr *)&my, sizeof(struct sockaddr)) ==
-1) {
printf("Cannot bind socket: %s.\n", strerror(errno));
exit(-1);
}

/* Attempt to receive from the socket */
their_len = sizeof(struct sockaddr);
if( recvfrom(sock, buf, 100, 0,
(struct sockaddr *)&their, &their_len) == -1 ) {
printf("Cannot receive: %s", strerror(errno));
exit(-1);
}

close( sock );
exit(0);
}

[Michael B Allen]

Too long. Isolate the point of failure with a trival program. Then post it
with a description of what happend as well as what you expected to happen.

Mike

[Barry Margolin]

In article <f394e693.0307181442.219d2fa7@posting.google.com >,
Raja Sambasivan <RS1999ent@aol.com> wrote:

>Hello,
>
>I am trying to familiarize myself with sockets, etc. for a project I
>am working on. In light of this, I tried earlier to write a simple
>server and client using a raw socket

Raw sockets are a pretty advanced feature. If this is your first
experience with sockets, I suggest you start with more basic features and
work your way up.

--
Barry Margolin, [email]barry.margolin@level3.com[/email]
Level(3), Woburn, MA
*** DON'T SEND TECHNICAL QUESTIONS DIRECTLY TO ME, post them to newsgroups.
Please DON'T copy followups to me -- I'll assume it wasn't posted to the group.

[Russell Shaw]

Barry Margolin wrote:

> In article <f394e693.0307181442.219d2fa7@posting.google.com >,
> Raja Sambasivan <RS1999ent@aol.com> wrote:
>

>>Hello,
>>
>>I am trying to familiarize myself with sockets, etc. for a project I
>>am working on. In light of this, I tried earlier to write a simple
>>server and client using a raw socket

>
> Raw sockets are a pretty advanced feature. If this is your first
> experience with sockets, I suggest you start with more basic features and
> work your way up.

[url]http://www.gnu.org/manual/glibc-2.2.5/html_node/Sockets.html#Sockets[/url]