void print_packet(const struct sniff_ethernet *eth,
const struct sniff_ip *ip,
const struct sniff_tcp *tcp)
{
/* Ethernet */
printf("DADDR:"); print_ethernet_addr(eth->dest_host);
printf("SADDR:"); print_ethernet_addr(eth->src_host);
printf("TYPE:%u\n", eth->ether_type);
/* IP */
printf( "\tVERSION: %u\n"
"\tHEADER LENGTH: %u\n"
"\tTOTAL LENGTH: %u\n"
"\tTOS: %u\n"
"\tTTL: %u\n",
IP_VER(ip),
IP_HL(ip),
ip->ip_len,
ip->ip_tos,
ip->ip_ttl);
/* TCP */
printf( "\t\tSPORT: %u\n"
"\t\tDPORT: %u\n"
"\t\tSEQ: %u\n"
"\t\tACK: %u\n",
ntohs(tcp->th_sport),
ntohs(tcp->th_dport),
tcp->th_seq,
tcp->th_ack);
}
void sr_handlepacket(struct sr_instance* sr,
uint8_t * packet/* lent */,
unsigned int len,
char* interface/* lent */)
{
/* REQUIRES */
assert(sr);
assert(packet);
assert(interface);
printf("*** -> Received packet of length %d on interface \"%s\"\n", len, interface);
// Deconstruct the packet's ethernet header
struct sr_ethernet_hdr *header = malloc(sizeof(struct sr_ethernet_hdr));
memcpy(header, packet, sizeof(struct sr_ethernet_hdr));
header->ether_type = htons(header->ether_type);
// Determine proper routing behavior based on ether_type
switch(header->ether_type) {
case ETHERTYPE_ARP:
{
// Unpack the ARP header
struct sr_arphdr *arp_header = malloc(sizeof(struct sr_arphdr));
memcpy(arp_header, packet + 14, sizeof(struct sr_arphdr));
arp_header->ar_op = ntohs(arp_header->ar_op);
// Check the ARP opcode
switch(arp_header->ar_op) {
case ARP_REQUEST:
handle_arp_request(sr, header, arp_header, interface);
break;
case ARP_REPLY:
printf("\tIt's an ARP reply!\n");
add_arp_cache_entry(cache, arp_header->ar_sip, arp_header->ar_sha);
struct in_addr addr = {arp_header->ar_sip};
printf("\tMapping %s to ", inet_ntoa(addr));
print_ethernet_addr(arp_header->ar_sha, stdout);
printf("\n");
// Scan for packets we can retransmit
struct ip_cache_entry *cache_entry = next_packet_with_dest(ip_cache, addr);
struct sr_if *iface = sr_get_interface(sr, interface);
while(cache_entry) {
// Wrap the IP packet in an ethernet header
uint8_t *wrapped_packet = pack_ethernet_packet(arp_header->ar_sha,
iface->addr, ETHERTYPE_IP, cache_entry->packet,
cache_entry->len);
// Send the newly wrapped packet
sr_send_packet(sr, wrapped_packet,
sizeof(struct sr_ethernet_hdr) + cache_entry->len,
iface->name);
// TODO: We're dumb are forgot this
cache_entry = next_packet_with_dest(ip_cache, addr);
}
break;
}
break;
}
case ETHERTYPE_IP:
{
// TODO
printf("\tIt's an IP packet!\n");
route_ip_packet(sr, packet, len, interface);
break;
}
default:
{
printf("\tIt's an unknown packet type (ether_type = 0x%X)\n", header->ether_type);
break;
}
}
} /* end sr_ForwardPacket */
static void route_ip_packet(struct sr_instance *sr, uint8_t *packet, size_t len, char *interface) {
// Copy the IP header
struct ip *ip_header = calloc(1, sizeof(struct ip));
memcpy(ip_header, packet + sizeof(struct sr_ethernet_hdr), sizeof(struct ip));
// The IP header is a dirty liar
int actual_header_length = ip_header->ip_hl * 4;
int ip_packet_len = len - sizeof(struct sr_ethernet_hdr);
// Decrement the TTL and check if it's 0
ip_header->ip_ttl -= 1;
if(ip_header->ip_ttl <= 0) {
return;
}
// Zero out the old checksum
ip_header->ip_sum = 0;
// Create a buffer to calculate the checksum
uint16_t *header_buffer;
// Create a chunk of memory aligned to 16 bits
posix_memalign((void **) &header_buffer, CHECKSUM_ALIGNMENT, actual_header_length);
bzero(header_buffer, actual_header_length);
// Copy required IP header fields
memcpy(header_buffer, ip_header, sizeof(struct ip));
// Copy the original IP header flags
memcpy(header_buffer, packet + sizeof(struct sr_ethernet_hdr) + sizeof(struct ip),
actual_header_length - sizeof(struct ip));
// Calculate the new checksum
ip_header->ip_sum = header_checksum(header_buffer, actual_header_length / 2);
free(header_buffer);
// Get the IP packet's destination
struct in_addr destination_addr = ip_header->ip_dst;
uint32_t destination_ip = destination_addr.s_addr;
// Check if we're the destination
struct sr_if *iface = sr_get_interface(sr, interface);
if(destination_ip == iface->ip) {
printf("Dropping packet bound for router on %s\n", interface);
return;
}
// Check the routing table for the correct gateway to forward the packet through
struct sr_rt *table_entry = search_routing_table(sr, destination_ip);
printf("\tThe nexthop is %s\n", inet_ntoa(table_entry->gw));
if(table_entry) {
// Get the interface for the gateway
struct sr_if *gw_iface = sr_get_interface(sr, table_entry->interface);
// Determine the IP to forward to
struct in_addr nexthop;
// Determine if the IP to forward to is in our network
if(table_entry->gw.s_addr == 0) {
nexthop = destination_addr;
} else {
nexthop = table_entry->gw;
}
// Create an updated IP packet with the correct headers/data
uint8_t *updated_packet = calloc(ip_packet_len, sizeof(uint8_t));
memcpy(updated_packet, ip_header, actual_header_length);
memcpy(updated_packet + actual_header_length,
packet + sizeof(struct sr_ethernet_hdr ) + actual_header_length,
len - sizeof(struct sr_ethernet_hdr ) - actual_header_length);
// Search the ARP cache for the nexthop
uint8_t *gw_addr = search_arp_cache(cache, nexthop.s_addr);
if(gw_addr) {
printf("\tForwarding packet bound for %s through next hop @ ",
inet_ntoa(destination_addr));
printf("%s (", inet_ntoa(table_entry->gw));
print_ethernet_addr(gw_addr, stdout);
printf(")\n");
uint8_t *buffer = pack_ethernet_packet(gw_addr, gw_iface->addr, ETHERTYPE_IP,
updated_packet, ip_packet_len);
sr_send_packet(sr, buffer, len, gw_iface->name);
} else {
// Otherwise we cache the IP packet and make an ARP request
printf("\tSending ARP request to %s (%s), to forward packet bound for ",
inet_ntoa(nexthop), gw_iface->name);
printf("%s\n", inet_ntoa(destination_addr));
add_ip_cache_entry(ip_cache, updated_packet, nexthop, ip_packet_len);
send_arp_request(sr, gw_iface, nexthop);
}
} else {
// TODO: What do we do here?
}
}
