Technical Details

We define a BPF filter as “icmp and dst host <bouncer_ip>”, which filters out all none-ICMP packets and packets not destined to the bouncer. The we compile this filter and set the filter to the capture device.

When the capture device captures a packet, a process_pkt function is called to process the packet. And that is where we validate the packets and then, if the packet is valid, update
the packet and then send it out, or write the packet to a dump file.

The process of validating the packets are as follows:

  1. Validate checksum of IP header.
  2. Validate TTL of IP.
  3. Validate IP source address
  4. Validate ICMP header checksum. 
  5. Validate ICMP type and code. 

Since we write the packet to a dump file in a separate function, so a pointer to the dump file handler is passed to the processing function when the process_pkt function is called. The same method is used to pass server IP address and test mode flag.

We keep a linked list of all the ICMP echo requests. When a ICMP echo reply is received, we go through the linked list to find out where
the original echo request comes from.

The code is as follows.

bouncer.h:

process_pkt.c:

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