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

c
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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.

Implement a C program that builds a real ICMP echo request packet from stdin parameters.

Input (stdin, whitespace-separated, all decimal):

identifier (16-bit), sequence (16-bit), payload length, payload fill byte

Requirements:

Build the 8-byte ICMP header: type=8 (echo request), code=0, checksum placeholder, identifier and sequence in network byte order (big-endian)
Fill payload-length payload bytes with the given fill byte
Compute the ICMP checksum over header + payload using RFC 1071: 16-bit one's-complement sum, folding carries, one's-complement of the result; an odd trailing payload byte is padded with a zero octet
Store the checksum in the header, then print the field summary line and the full packet hex dump exactly as specified

Output format:

Line 1: type=8 code=0 id=0xNNNN seq=0xNNNN payload=N checksum=0xNNNN
Line 2: the complete packet bytes as two-digit lowercase hex, space-separated, prefixed with "hex="

Success Criteria:

Identifier and sequence printed in hex with network byte order applied
Checksum matches the RFC 1071 computation (a wrong checksum is silently discarded by real receivers)
Odd-length payloads are handled with the zero-octet pad

Send a Raw ICMP Echo Request (Ping)

Topic Overview

ICMP Echo Request Anatomy

Building the Header

Raw Socket Setup

Why This Exercise

Implementation Task

Progressive Hints

Resources

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