Send a Raw ICMP Echo Request (Ping)

ICMP Echo Request Anatomy

Ping sends an ICMP Echo Request (type 8, code 0) and waits for an Echo Reply (type 0, code 0). ICMP sits on IP (protocol 1) with no port numbers. For example, a request to 8.8.8.8 might carry identifier 0x1234 and sequence 1 in the echo header fields.

Building the Header

The ICMP header fields you must set:

• Type: 8 (ICMP_ECHO)
• Code: 0
• Checksum: one's-complement sum over the entire ICMP message
• Identifier: 16-bit value, often getpid() & 0xFFFF
• Sequence: incremented per packet

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Raw Socket Setup

socket(AF_INET, SOCK_RAW, IPPROTO_ICMP) lets you craft the ICMP payload while the kernel builds the IP header. Raw sockets require root or CAP_NET_RAW. A wrong checksum means the receiver silently discards the packet.

Why This Exercise

This task asks you to construct and print every ICMP field for a raw echo request. iputils' ping.c does exactly this on Linux, and you will reuse the same checksum logic for traceroute later in the track. nmap's -PE host discovery and many CTF challenges depend on crafting a byte-perfect ICMP echo that a real kernel will accept and reply to.