Network Diagnostics

~18 min · ping, traceroute, dig, lsof, diagnostics

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Your day-one diagnostic kit

Before we head into SSH, lock in the network diagnostic muscle-memory. When something doesn't work, the right tool in the first 10 seconds saves an hour of guessing. This is the table I keep mental-pinned.

Question	Tool	Command
Is the host alive?	ping	`ping -c 4 host`
Where does the path break?	mtr / traceroute	`mtr host`
Does the name resolve?	dig	`dig +short domain`
What's my config?	ifconfig / ip	`ifconfig en0`
What's using a port?	lsof	`lsof -i :8000`
What ports are listening?	ss / netstat	`ss -tuln`
What's the gateway?	route / netstat	`netstat -rn`
Is a TCP port open?	nc	`nc -zv host port`

The ladder

Walk the ladder bottom-up: gateway, internet, DNS, target, port, app. Each rung either passes or fails — when one fails, you've found the layer. Tracks 5 and 7 dig deeper into each tool; this lesson exists to imprint the order.

Code

The 60-second connectivity ladder·bash

# 1. Interface up?
ifconfig en0 | grep status

# 2. Gateway reachable?
ping -c 3 $(route -n get default | awk '/gateway/ {print $2}')

# 3. Public internet reachable (no DNS)?
ping -c 3 1.1.1.1

# 4. DNS works?
dig +short github.com

# 5. Specific service reachable?
nc -zv github.com 443

# 6. Application healthy?
curl -sI https://github.com | head -1

External links

Julia Evans: How to find what's making your network slow

Exercise

Pick a service you use (GitHub, your bank, a streaming site). Walk the ladder against it: ifconfig → ping gateway → ping 1.1.1.1 → dig the hostname → nc -zv host 443 → curl -sI the URL. Time yourself — under 60 seconds is the goal. When this is muscle memory, troubleshooting becomes "oh, hop 4" instead of "hmm."