How to Set and Verify a cURL User-Agent

Overview

Set a cURL User-Agent with curl --user-agent 'MiyaIP-Tutorial/1.0' URL, or use the short option -A. The value identifies the client in an HTTP header. It does not change the connection's IP address or turn curl into a browser.

Use an explicit identifier when documenting an integration, testing how a service handles different client names, or reproducing an application's HTTP request. Verify the value at the receiver before attributing a response difference to it.

Set, replace or remove the User-Agent header

Choose one method for setting the header in a command. Combining multiple User-Agent options makes the intended value harder to audit. The curl command normally sends a curl/version identifier when you do not override it; inspect your actual result rather than assuming a particular version number.

The official curl guide explains the User-Agent option. --user is a different option: it supplies destination authentication credentials, not a User-Agent string.

Compare three requests against a local echo endpoint

Start the included network_demo.py. Its /headers route returns the received HTTP fields. These commands use loopback and send no account credentials:

curl --noproxy '*' 'http://127.0.0.1:8766/headers'

curl --noproxy '*' --user-agent 'MiyaIP-Tutorial/1.0' \
  'http://127.0.0.1:8766/headers'

curl --noproxy '*' --user-agent '' \
  'http://127.0.0.1:8766/headers'

On Windows, call curl.exe explicitly. Empty native arguments can behave differently in older PowerShell versions; --header 'User-Agent:' is an alternative way to suppress curl's generated field. Verify the receiver's result in the shell you actually use.

For a production service, inspect a redacted application log or an endpoint you control. Do not send cookies or API keys to a public header-echo service just to check the User-Agent.

A User-Agent and a proxy change different parts of the request

A User-Agent is metadata sent by the client. A forward proxy changes the route and the exit visible to the destination. Keep these variables separate when comparing results.

For an HTTP proxy, the macOS/Linux template below sends a custom User-Agent to an HTTPS destination. Replace the connection fields; curl prompts for the proxy password when only a username is supplied.

curl --proxy 'http://PROXY_HOST:PROXY_PORT' \
  --proxy-user 'PROXY_USERNAME' --noproxy '' \
  --user-agent 'MiyaIP-Tutorial/1.0' \
  --connect-timeout 10 --max-time 30 \
  'https://example.com/'

Use the MIYAIP protocol examples if your assigned connection is SOCKS5 or requires a different mode. --proxy-header addresses a separate proxy-facing header use case; it is not a substitute for setting the destination's User-Agent.

To compare server behavior, keep the target, method, cookies, proxy session and redirect policy fixed. Change one User-Agent value and record the response status and expected content. A different result is evidence for that test condition, not proof that User-Agent is the only signal the service uses.

Diagnose what the header cannot change

Verify the network path independently

After checking request metadata, confirm the proxy endpoint and use the intended routing mode in the actual application.

Run the local demonstration

Download network_demo.py, or copy the complete source below into a file with that name. Run python network_demo.py using Python 3. The service listens only on 127.0.0.1:8766; stop it with Ctrl+C.

"""Local HTTP demonstration. No external requests, credentials or MIYAIP service calls."""
from http.server import BaseHTTPRequestHandler, ThreadingHTTPServer
from urllib.parse import urlsplit
import json
import time


class DemoHandler(BaseHTTPRequestHandler):
    def log_message(self, *_):
        pass

    def reply(self, code, body=b"", headers=None):
        self.send_response(code)
        for key, value in (headers or {}).items():
            self.send_header(key, value)
        self.send_header("Content-Length", str(len(body)))
        try:
            self.end_headers()
            self.wfile.write(body)
        except (BrokenPipeError, ConnectionResetError, ConnectionAbortedError):
            pass

    def do_CONNECT(self):
        self.reply(407, headers={"Proxy-Authenticate": 'Basic realm="local-demo"'})

    def do_GET(self):
        path = urlsplit(self.path).path
        if path == "/redirect":
            return self.reply(302, headers={"Location": "/headers"})
        if path == "/loop":
            return self.reply(302, headers={"Location": "/loop"})
        if path == "/slow":
            time.sleep(2)
        if path in {"/headers", "/slow"}:
            payload = {"method": self.command, "headers": dict(self.headers)}
            return self.reply(200, json.dumps(payload, indent=2).encode(),
                              {"Content-Type": "application/json"})
        self.reply(404, b"Use /headers, /redirect, /loop or /slow")

    def do_POST(self):
        body = self.rfile.read(int(self.headers.get("Content-Length", "0")))
        if urlsplit(self.path).path == "/redirect307":
            return self.reply(307, headers={"Location": "/headers"})
        if urlsplit(self.path).path == "/redirect302":
            return self.reply(302, headers={"Location": "/headers"})
        payload = {"method": self.command, "headers": dict(self.headers),
                   "body": body.decode("utf-8", errors="replace")}
        self.reply(200, json.dumps(payload, indent=2).encode(),
                   {"Content-Type": "application/json"})


if __name__ == "__main__":
    print("Local demonstration: http://127.0.0.1:8766 (Ctrl+C to stop)", flush=True)
    ThreadingHTTPServer(("127.0.0.1", 8766), DemoHandler).serve_forever()

Sources

• official curl guide
• Everything curl User-agent
• curl manual