router

Simple Router

This router routes Ethernet packets between a host and clients with a static routing table by performing the following actions:

• Receives raw Ethernet frame

• If frame is an ARP packet

  • If it's an ARP request, construct an ARP reply and send it back
  • If it's an ARP reply, queue it for caching and send outstanding packets

• If frame is an IP packet

  • If it was sent to the router

    • If it's an ICMP echo request, send an echo reply
    • If it's a TCP/UCP packet, send ICMP port unreachable message
    • If it's neither, packet is discarded

  • If it's for someone else

    • Check checksum of the packet (discard if checksum is incorrect)
    • Send ICMP echo request if packet's TTL is 1, ICMP time exceeded message if TTL is less than 1, decrement TTL otherwise
    • Perform longest prefix match on the routing table (send ICMP net unreachable message if match is not found)
    • Check ARP cache for an entry
    • If ARP cache hit found
    • send packet to next hop interface
    • If no ARP cache entry found, send ARP request (if ARP request has already been sent 5 times, send ICMP host unreachable message)

Network Address Translator

Enable NAT functionality with the -n flag. The following timeout intervals are also configurable with flags:

• -I INTEGER: ICMP query timeout interval in seconds
• -E INTEGER: TCP Established Idle Timeout in seconds
• -R INTEGER: TCP Transitory Idle Timeout in seconds

Translates packets by performing the following actions:

• Receives packet on an interface

• Check of it's ICMP or TCP

• If packet is outbound

  • Look up unique mapping
  • If mapping not found, insert mapping

• Else

  • If no mapping and not a SYN, drop packet

• Rewrite IP header for packet

• Rewrite ID for ICMP and port for TCP

• Update checksums

• Route packet

Does not support UDP