Lectures notes for 2010 - KTH

if (!(proto = getprotobyname("raw"))) {perror("getprotobyname(raw)");exit(2);}
if ((s = socket(AF_INET, SOCK_RAW, proto->p_proto)) < 0) {perror("socket");exit(2);}
memset(&addrfrom, 0, sizeof(struct sockaddr));
from = (struct sockaddr_in *)&addrfrom;
from->sin_family = AF_INET;
from->sin_port=htons(srcp);
if (!inet_aton(src, &from->sin_addr)) {fprintf(stderr,"Incorrect address for ’from’: %s\n",src);exit(2); }
memset(&addrto, 0, sizeof(struct sockaddr));
to = (struct sockaddr_in *)&addrto;
to->sin_family = AF_INET;
to->sin_port=htons(destp);
if (!inet_aton(dest, &to->sin_addr)) {fprintf(stderr,"Incorrect address for ’to’: %s\n",dest);exit(2); }
packetsize=0;
/* build a UDP packet from scratch */
ip=(struct iphdr *)outpack;
ip->version=4; /* IPv4 */
ip->ihl=5; /* IP header length: 5 words */
ip->tos=0; /* no special type of service */
ip->id=0; /* no ID */
ip->frag_off=0; /* not a fragment - so there is no offset */
ip->ttl=0x40; /* TTL = 64 */
if (!(proto = getprotobyname("udp"))) {perror("getprotobyname(udp)"); exit(2);}
ip->protocol=proto->p_proto;
ip->check=0; /* null checksum, will be automatically computed by the kernel */
ip->saddr=from->sin_addr.s_addr; /* set source and destination addresses */
ip->daddr=to->sin_addr.s_addr;
/* end of ip header */
Maguire Building a UDP packet from scratch 4: 25 of 74
maguire@kth.se 2010.03.21 Internetworking/Internetteknik