Hands-on lab — Day 15: What Happens When You Load a Web Page

#!/usr/bin/env bash
# Day 015 lab — completed reference implementation.
# Trace the first stops of a web page's request journey for one host:
#   1. DNS lookup    (dig +short)      name -> IP address
#   2. Round-trip time (ping -c 2)     how long a back-and-forth takes
#   3. Connection stages (curl -w)     DNS / TCP / TLS / total timings
#
# Usage:  bash examples/trace_page_load.sh [hostname]
# Default host is example.com (a safe, standard test domain).
#
# Requires a network connection. If a step's tool is missing or the
# network is unavailable, the script prints a clear note and continues,
# so it never crashes mid-report.
set -uo pipefail

host="${1:-example.com}"

echo "=== Request Journey Report ==="
echo "Host: ${host}"
echo "Generated on: $(date '+%Y-%m-%d')"
echo

# --- Stop 1: DNS lookup (name -> IP address) ---
echo "--- Stop 1: DNS lookup (name -> IP) ---"
if command -v dig >/dev/null 2>&1; then
  ips="$(dig +short "${host}" 2>/dev/null)"
  if [ -n "${ips}" ]; then
    echo "Resolved IP address(es):"
    echo "${ips}" | sed 's/^/  /'
  else
    echo "DNS lookup returned no address (offline, or name not found)."
  fi
else
  echo "dig not installed — try: nslookup ${host}  or  host ${host}"
fi
echo

# --- Stop 2: Round-trip time (ping) ---
echo "--- Stop 2: Round-trip time (ping -c 2) ---"
if command -v ping >/dev/null 2>&1; then
  if ping_out="$(ping -c 2 "${host}" 2>&1)"; then
    echo "${ping_out}" | sed 's/^/  /'
  else
    echo "  ping failed or was blocked (many networks block ICMP)."
    echo "  A blocked ping does NOT mean the site is down — see the curl step."
  fi
else
  echo "  ping not installed on this system."
fi
echo

# --- Stop 3: Connection stage timings (curl) ---
echo "--- Stop 3: Connection stages (curl -w timings) ---"
if command -v curl >/dev/null 2>&1; then
  fmt='dns=%{time_namelookup}s connect=%{time_connect}s tls=%{time_appconnect}s total=%{time_total}s\n'
  timing="$(curl -sS --max-time 15 -o /dev/null -w "${fmt}" "https://${host}" 2>/dev/null)"
  rc=$?
  if [ "${rc}" -eq 0 ]; then
    echo "  ${timing}"
    echo "  Read each value as the elapsed time from the start at which that"
    echo "  stage finished. Subtract consecutive values to get each stop's cost:"
    echo "    time_namelookup = when DNS resolved"
    echo "    time_connect    = when the TCP connection opened"
    echo "    time_appconnect = when the TLS handshake finished"
    echo "    time_total      = when the full response arrived"
  else
    echo "  curl could not complete the request (offline, or the host refused"
    echo "  the connection). This often just means no network right now —"
    echo "  reconnect and retry. A failed curl does not prove the site is down."
  fi
else
  echo "  curl not installed on this system."
fi
echo

echo "=== End of report ==="

lesson_id: D015
day: 15
kind: command-line-inspection
languages: [bash]
setup_commands:
  - cd labs/sections/computing-foundations/day-015-what-happens-when-you-load-a
run_commands:
  - bash examples/trace_page_load.sh
  - bash examples/trace_page_load.sh en.wikipedia.org
  - bash starter/trace_page_load.sh
test_commands:
  - bash tests/run_tests.sh
cleanup_commands:
  - 'git checkout -- starter/trace_page_load.sh  # optional: reset your work'
requires_network: true
requires_api_key: false
estimated_minutes: 30
last_executed: '2026-07-12'
executed_on: 'macOS (Apple Silicon), bash tests/run_tests.sh → 12 checks, 0 failure(s), 0 skipped (online)'

# Dependencies — Day 015 lab

**A POSIX shell and three standard network tools.** This lab installs
nothing of its own:

- `bash` ≥ 3.2 (preinstalled on macOS and every mainstream Linux distribution).
- `dig` — DNS lookup tool. Preinstalled on macOS. On minimal Linux images it
  may live in a package: `dnsutils` (Debian/Ubuntu) or `bind-utils`
  (Fedora/RHEL). If you cannot install it, the lesson and troubleshooting
  notes give `nslookup` and `host` as drop-in alternatives.
- `ping` — round-trip probe. Preinstalled on macOS and Linux. **Note:** in
  some container images `ping` needs the `CAP_NET_RAW` capability or root, and
  many networks block ping (ICMP) entirely. A blocked or missing `ping` does
  not stop the lab — the script and tests continue and simply skip that value.
- `curl` — HTTP client. Preinstalled on macOS and most Linux systems
  (`apt install curl` / `dnf install curl` if absent).

There is deliberately no `requirements.txt`/`package.json` here; the tools are
part of a normal developer system. The lab needs a network connection for live
values, but its tests are written to run and pass offline as well, skipping the
network-dependent checks.

# Request Journey Worksheet — Day 015

Pick a website you use often and trace its request journey with the three
tools. Fill in every field below with your real numbers.

**Site chosen (host name):** _______________________________________

## 1. DNS lookup — name to IP address

Command: `dig +short <host>`

- Resolved IP address(es): _______________________________________
- How many addresses came back? ________

## 2. Round-trip time — ping

Command: `ping -c 2 <host>`

- Round-trip time, packet 1: ________ ms
- Round-trip time, packet 2: ________ ms
- Were any packets lost, or was ping blocked? ________________________
  (Remember: a blocked ping does NOT mean the site is down.)

## 3. Connection stages — curl timings

Command:

```
curl -sS -o /dev/null \
  -w "dns=%{time_namelookup}s connect=%{time_connect}s tls=%{time_appconnect}s total=%{time_total}s\n" \
  https://<host>
```

Record each timestamp (elapsed time from the start at which the stage finished):

| Field | Value (seconds) |
| --- | --- |
| `time_namelookup` (DNS done) | |
| `time_connect` (TCP open) | |
| `time_appconnect` (TLS done) | |
| `time_total` (full response) | |

## 4. Read the timeline

Subtract consecutive values to get each stop's cost:

- DNS cost = `time_namelookup` = ________ s
- TCP handshake cost = `time_connect` − `time_namelookup` = ________ s
- TLS handshake cost = `time_appconnect` − `time_connect` = ________ s
- Server + transfer cost = `time_total` − `time_appconnect` = ________ s
- **Which stop cost the most?** _______________________________________

## 5. Narrate the journey (5–8 sentences)

Using your real numbers, tell the story of your site's request journey.
Which stop cost the most? Given your round-trip time from step 2, roughly
how many round trips does the setup imply? Did the site feel latency-bound
(far away, high RTT) or fast and nearby?

_____________________________________________________________________

_____________________________________________________________________

_____________________________________________________________________

_____________________________________________________________________

#!/usr/bin/env bash
# Day 015 lab — YOUR working file. Trace the request journey for one host.
#
# The report skeleton is already here. Your job is the four numbered
# exercises below: replace each `PENDING-exercise-N` echo line with the real
# command named in its comment, so the report prints genuine values. The
# completed reference version is examples/trace_page_load.sh — run that first
# to see the finished result, then rebuild it here yourself.
#
# Usage:  bash starter/trace_page_load.sh [hostname]
# Default host is example.com (a safe, standard test domain).
set -uo pipefail

host="${1:-example.com}"

echo "=== Request Journey Report ==="
echo "Host: ${host}"
echo "Generated on: $(date '+%Y-%m-%d')"
echo

# --- Stop 1: DNS lookup (name -> IP address) ---
echo "--- Stop 1: DNS lookup (name -> IP) ---"
# Exercise 1: resolve the host to its IP address(es).
#   Replace the echo line below with:  dig +short "${host}"
echo "PENDING-exercise-1: run dig +short on the host"
echo

# --- Stop 2: Round-trip time (ping) ---
echo "--- Stop 2: Round-trip time (ping -c 2) ---"
# Exercise 2: measure the round-trip time with two ping packets.
#   Replace the echo line below with:  ping -c 2 "${host}"
echo "PENDING-exercise-2: run ping -c 2 on the host"
echo

# --- Stop 3: Connection stage timings (curl) ---
echo "--- Stop 3: Connection stages (curl -w timings) ---"
# Exercise 3: print curl's per-stage timings for https://<host>.
#   Replace the echo line below with a curl command that uses -w with the
#   fields time_namelookup, time_connect, time_appconnect, time_total, e.g.:
#     curl -sS -o /dev/null \
#       -w "dns=%{time_namelookup}s connect=%{time_connect}s tls=%{time_appconnect}s total=%{time_total}s\n" \
#       "https://${host}"
echo "PENDING-exercise-3: run the curl -w timing command for the host"
echo

# --- Stop 4: your reading of the timeline ---
echo "--- Stop 4: which stop cost the most? ---"
# Exercise 4: after running Stop 3, subtract consecutive curl values and
# write, in the string below, which stop (DNS, TCP connect, TLS, or server)
# took the most time for your host. Replace the placeholder text.
echo "PENDING-exercise-4: replace with your finding, e.g. 'TLS took the most, about 0.02s'"
echo

echo "=== End of report ==="

#!/usr/bin/env bash
# Tests for the Day 015 lab. Run from the lab directory:
#   bash tests/run_tests.sh
#
# The lab needs a network, but the tests degrade gracefully offline:
# structural checks (report shape) always run; the network checks
# (real DNS/curl values) run only when the network is reachable, and are
# SKIPPED with a clear message otherwise. The script exits 0 either way,
# as long as the structural checks pass.
set -u

lab_dir="$(cd "$(dirname "${BASH_SOURCE[0]}")/.." && pwd)"
example="${lab_dir}/examples/trace_page_load.sh"
failures=0
checks=0
skips=0

check() {
  local label="$1" ok="$2"
  checks=$((checks + 1))
  if [ "${ok}" = "yes" ]; then
    echo "  ok: ${label}"
  else
    echo "  FAIL: ${label}"
    failures=$((failures + 1))
  fi
}

skip() {
  skips=$((skips + 1))
  echo "  skip: $1"
}

echo "Running ${example} against example.com ..."
output="$(bash "${example}" example.com 2>&1)"
exit_code=$?

# --- Structural checks (always run, offline or online) ---
[ "${exit_code}" -eq 0 ] && check "script exits successfully" "yes" || check "script exits successfully" "no"
echo "${output}" | grep -q '^=== Request Journey Report ===$' && check "prints report header" "yes" || check "prints report header" "no"
echo "${output}" | grep -q '^=== End of report ===$' && check "prints report footer" "yes" || check "prints report footer" "no"
echo "${output}" | grep -q '^Host: example.com$' && check "names the host" "yes" || check "names the host" "no"
echo "${output}" | grep -q -- '--- Stop 1: DNS lookup' && check "has Stop 1 (DNS)" "yes" || check "has Stop 1 (DNS)" "no"
echo "${output}" | grep -q -- '--- Stop 2: Round-trip time' && check "has Stop 2 (ping)" "yes" || check "has Stop 2 (ping)" "no"
echo "${output}" | grep -q -- '--- Stop 3: Connection stages' && check "has Stop 3 (curl)" "yes" || check "has Stop 3 (curl)" "no"

# --- Network detection: probe independently of the report ---
# Try a real request; if it fails (or curl is missing), treat as offline and
# skip the live-value checks instead of failing the suite.
if command -v curl >/dev/null 2>&1 && curl -sS --max-time 15 -o /dev/null https://example.com >/dev/null 2>&1; then
  network="up"
else
  network="down"
fi

if [ "${network}" = "up" ]; then
  echo "Network is reachable — running online checks."
  echo "${output}" | grep -Eq 'Resolved IP address' && check "DNS returned an address" "yes" || check "DNS returned an address" "no"
  # The IP line is an indented dotted quad or a hex IPv6 address.
  echo "${output}" | grep -Eq '^  [0-9a-fA-F:.]+$' && check "prints a resolved IP value" "yes" || check "prints a resolved IP value" "no"
  echo "${output}" | grep -Eq 'connect=[0-9]' && check "curl reports a connect time" "yes" || check "curl reports a connect time" "no"
  echo "${output}" | grep -Eq 'tls=[0-9]' && check "curl reports a TLS time" "yes" || check "curl reports a TLS time" "no"
  echo "${output}" | grep -Eq 'total=[0-9]' && check "curl reports a total time" "yes" || check "curl reports a total time" "no"
else
  echo "Network appears unavailable (curl did not return timings)."
  skip "DNS returned an address (needs network)"
  skip "prints a resolved IP value (needs network)"
  skip "curl connect/TLS/total timings (needs network)"
  echo "  The report structure is correct; re-run with a connection to see live values."
fi

echo
echo "${checks} checks, ${failures} failure(s), ${skips} skipped."
[ "${failures}" -eq 0 ]