How to Hide Your IP with Linux curl: Proxy Types, Settings, and Anonymity Tests

Proxy classifications

Proxy servers sit between a browser and the target HTTP server. The most common proxies that support HTTP(S) are:

HTTP proxy – includes transparent, anonymous, and high‑anonymity (elite) variants.

SOCKS proxy – SOCKS4, SOCKS4A, and SOCKS5, which can forward any TCP/UDP traffic and support authentication.

Linux curl proxy options

The curl command offers several parameters to set proxies and their credentials: -x host:port or --proxy protocol://[user:pwd@]host[:port] – select HTTP, HTTPS, SOCKS4, SOCKS4A, or SOCKS5. --socks4 <host[:port]>, --socks4a <host[:port]>, --socks5 <host[:port]> – explicitly choose a SOCKS version (overrides -x).

Authentication flags: --proxy-basic, --proxy-ntlm, --proxy-digest, --proxy-negotiate, --proxy-anyauth. -U user:password or --proxy-user user:password – provide proxy credentials.

Example: configuring an HTTP proxy with curl

# Basic HTTP proxy
curl -x 113.185.19.192:80 http://aiezu.com/test.php

# Using explicit protocol name
curl --proxy 113.185.19.192:80 http://aiezu.com/test.php

# Proxy with basic authentication
curl -x aiezu:[email protected]:80 http://aiezu.com/test.php

# Proxy with NTLM authentication
curl -x 113.185.19.192:80 -U aiezu:123456 --proxy-ntlm http://aiezu.com/test.php

Example: configuring a SOCKS proxy with curl

# SOCKS4 (no authentication)
curl --socks4 122.192.32.76:7280 http://aiezu.com/test.php

# SOCKS4A (no authentication)
curl --socks4a 122.192.32.76:7280 http://aiezu.com/test.php

# SOCKS5 with basic authentication
curl --socks5 122.192.32.76:7280 -U aiezu:123456 http://aiezu.com/test.php

Testing proxy anonymity

Create a simple test.php script on the web server that prints selected $_SERVER entries, e.g.:

<?php
$keys = array(
    'HTTP_USER_AGENT','HTTP_HOST','HTTP_ACCEPT','PATH','SERVER_SIGNATURE',
    'SERVER_SOFTWARE','SERVER_NAME','SERVER_ADDR','SERVER_PORT','DOCUMENT_ROOT',
    'SERVER_ADMIN','SCRIPT_FILENAME','REMOTE_PORT','GATEWAY_INTERFACE',
    'SERVER_PROTOCOL','REQUEST_METHOD','QUERY_STRING','REQUEST_URI','SCRIPT_NAME',
    'PHP_SELF','REQUEST_TIME'
);
$srv = $_SERVER;
foreach ($keys as $k) { unset($srv[$k]); }
print_r($srv);
?>

Access the script with different proxy settings and observe the output:

No proxy : REMOTE_ADDR shows the client’s real IP.

Transparent HTTP proxy : REMOTE_ADDR is the proxy IP, and headers like HTTP_VIA and HTTP_X_FORWARDED_FOR reveal the original IP.

Anonymous HTTP proxy : REMOTE_ADDR is the proxy IP, but no original IP is disclosed; the presence of HTTP_PROXY_CONNECTION indicates proxy usage.

High‑anonymity (elite) HTTP proxy : Only REMOTE_ADDR shows the proxy IP; no headers expose the client’s IP.

SOCKS5 proxy : Behaves like a high‑anonymity proxy, returning only the proxy’s IP.

These results demonstrate how different proxy types affect the visibility of the client’s IP address to the target server.