DNS resolution

Problem

What becomes confusing, fragile, or impossible without understanding DNS resolution? This lesson answers that through explanation, a worked example, two runnable exercises, and a reference solution. No teacher-supplied worksheet is required.

Before integrated development environments, the operating system, terminal, files, and text streams were the environment. Their small composable interfaces still underpin modern tooling.

Concept explanation

DNS resolution belongs to “The network beneath the URL”. A URL request crosses DNS, socket, TLS, and HTTP boundaries before response bytes arrive. This lesson turns DNS resolution into a working part of weekly fixture.

For DNS resolution, trace concrete input, state transition, output, and failure through an operating-system boundary made visible through bytes, files, processes, and exit status.

Programs are processes. They read bytes, transform state, write bytes, open files and sockets, then report success or failure through observable boundaries. Apply that model to supplied normal, boundary, and failure cases; each case below names its input and expected evidence.

Example bank

Compare normal, boundary, failure, and cross-layer cases. Predict each observation before revealing the explanation.

Worked example

Start from supplied lab.sh. Focus: Build DNS resolution around this exact fixture: Resolve example.com, connect with curl -v, and save response headers and body separately.

1. Run: chmod +x lab.sh && ./lab.sh
2. Save baseline evidence. Use wc, od, stat, ps, lsof, curl, or Git plumbing as appropriate; save raw output before interpretation.
3. Boundary case: Add DNS resolution support for weekly boundary case: Repeat with redirects disabled, then enabled.
4. Failure case: Make DNS resolution surface—not hide—this failure: Request an unused local port with a two-second timeout.

RESULT
DNS resolution handles baseline and boundary while preserving this observable result: Trace identifies resolved address, TLS/HTTP exchange, status, headers, body size, and connection failure. Starter-level baseline: Command exits 0, prints a byte count, then prints hexadecimal and character views of the same file.

Starter material

Create lab.sh, paste this exact content, then run the command below.

#!/usr/bin/env sh
set -eu
lab="${TMPDIR:-/tmp}/reforging-lab"
mkdir -p "$lab"
printf '%s\n' 'baseline: DNS resolution' > "$lab/input.txt"
printf 'bytes: ' && wc -c < "$lab/input.txt"
od -An -tx1 -c "$lab/input.txt"

STOP / CLEANUPNo background process. Keep generated evidence files for your notebook.

Independent exercise

Add one real case: DNS resolution

1. Create second case from blank file: Add DNS resolution support for weekly boundary case: Repeat with redirects disabled, then enabled.
2. Then create controlled failure: Make DNS resolution surface—not hide—this failure: Request an unused local port with a two-second timeout.
3. Use Ghostty · tmux · hx · shell · curl · Git to prove behavior, then repair controlled failure.
4. Compare result against supplied acceptance checks and reference approach before marking complete.

Expected result

• DNS resolution handles baseline and boundary while preserving this observable result: Trace identifies resolved address, TLS/HTTP exchange, status, headers, body size, and connection failure.
• Command exits 0, prints a byte count, then prints hexadecimal and character views of the same file.
• Controlled DNS resolution failure produces captured evidence; repair restores stated invariant without hiding error.

Hints

Solution

Attempt both exercises before opening reference approach.