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The User Agent

Lesson 44 of 47 in Coddy's Networking with the Terminal course.

Every request announces what made it, in the User-Agent header. curl sends its own name and version; a browser sends a long historical string.

This matters more than it looks. Servers routinely behave differently based on it: serving a mobile layout, blocking unknown clients, or rejecting anything that looks automated. So "it works in my browser but not in curl" is often exactly this header, and nothing deeper.

curl -s -A 'my-monitor/1.0' http://127.0.0.1:9133/

-A sets it, and it is simply a shorthand for -H 'User-Agent: ...'.

If you run automated requests against a service, setting an honest User-Agent that names your tool is good manners: when your traffic causes a problem, whoever is on call can tell what it is and who to ask.

Challenge

Hard

Introduce yourself honestly.

Start the capture listener on port 9133
Make a request identifying itself as my-monitor/1.0
Print the user agent line from req.txt

Expected output:

User-Agent: my-monitor/1.0

Try it yourself

Terminal

Test yourself

This lesson includes a short quiz. Start the lesson to answer it and track your progress.

All lessons in Networking with the Terminal

1How Networks Talk

What Is a Network Clients and Servers IP Addresses Your First Request

4Is It Reachable?

ICMP and ping Reading Ping Output When Ping Lies Refused or Timed Out Recap: Reachability

7Sending Data

POST with -d What -d Actually Sends Form Encoding Sending JSON PUT and DELETE Recap: Writing Data

2Names and DNS

Why Names Exist The Hosts File Looking Up a Name Record Types Recap: Names

5HTTP by Hand

Anatomy of a Request Request Methods Status Codes Response Headers Writing a Response Recap: HTTP Basics

8HTTPS and TLS

Why HTTPS The Scheme Picks the Port Certificates and Trust When TLS Fails Recap: HTTPS

3Ports and Sockets

What Is a Port Well-Known Ports Localhost Listening vs Connecting Recap: Ports

6curl in Depth

Seeing the Headers Saving Responses Custom Headers Status and Timing curl Exit Codes Recap: curl Flags

9Auth and Debugging

API Keys and BearerThe User AgentTimeouts and Retries Reading a Failure Recap: Debugging

Practice on your own: Terminal playground