OS Spoofing for Proxies: Set Your TCP/IP Fingerprint

OS spoofing is now live on FlashProxy ISP, datacenter, and IPv6 proxies. You can pick the operating system your connection appears to come from by adding a single flag to your proxy username. Four values are supported: windows, macos, ios, and android. Without the flag your fingerprint stays Linux, exactly as before, so nothing changes for your existing setups. There's no extra cost.

What Is OS Spoofing on a Proxy?

OS spoofing (also called OS targeting) means controlling the operating system a target site infers from your TCP/IP fingerprint, which sits at the network layer, underneath TLS and underneath your headers.

That's a different layer from the one most people manage. Changing your user agent tells a site what OS you claim to run. Changing your TLS fingerprint tells it what client or browser you appear to be. Neither one touches the handshake itself. OS spoofing does.

FlashProxy ISP, datacenter, and IPv6 proxies now support the -os- flag, with four supported values:

• windows

• macos

• ios

• android

We tested this in real time and had it independently verified.

Why Your TCP/IP Fingerprint Matters

Before your browser sends a single header, your connection has already given something away.

Every TCP connection carries small details set by the operating system that opened it: the starting TTL value, the initial window size, the MSS, and the order the TCP options are written in. None of these are things your application picks. Your kernel picks them, and different operating systems pick differently.

Detection systems read those values and work out your operating system without asking you anything. It's completely passive. There's no script to block and no request to refuse.

That's what makes this layer awkward. Most fingerprinting surfaces are things you can intercept somewhere: a canvas call, a WebGL query, a client-hints header. Passive TCP fingerprinting happens in the packets themselves, before your application code has any say in it. You can't patch it from inside the browser, because the browser isn't the thing producing it.

Why Proxy Traffic Has Always Looked Like Linux

This is the part that catches people out.

When you use a proxy, the connection the target site sees isn't the one your machine opened. Your client connects to the proxy, and the proxy opens a separate connection onward. So the TCP/IP fingerprint the site reads belongs to the proxy server, not to you.

Proxy servers run Linux. Ours included. That means proxy traffic has always arrived carrying a Linux fingerprint.

Now think about how that looks from the other side. A request comes in from a residential ISP address. The user agent says Chrome on Windows. The TLS fingerprint says Chrome on Windows. And underneath all of it, the handshake says Linux server.

Real Windows machines don't have Linux network stacks. Everything else being configured perfectly doesn't help, because this one layer contradicts the rest.

It's also why a setup can pass every fingerprint checker you throw at it and still get flagged. The checkers you run from inside a browser can't see the layer that's giving you away. From the outside it's one of the cheapest signals there is to collect, with no JavaScript, no challenge and no added latency.

How to Use OS Targeting on ISP, Datacenter, and IPv6 Proxies

Add the flag to your proxy username:

USERNAME-os-windows

It combines with your existing targeting flags:

USERNAME-country-us-session-abc123-os-macos

A full connection string looks like this:

bash

curl -x http://USERNAME-os-windows:[email protected]:777 https://api.ipify.org

Supported values

The flag works per connection, so different requests on the same credentials can each use a different value. It doesn't change your exit IP, its location, or your session behaviour.

Available on ISP proxies, Datacenter proxies, and IPv6 proxies, included at no extra cost.

Keep Your Fingerprints Consistent

This part is worth reading twice.

Spoofing one layer while ignoring the others can make you easier to spot, not harder. Antibot systems don't just check individual values. They check whether all the signals describe one believable device, and two layers that contradict each other stand out more than either value would on its own.

So if you set -os-windows while your TLS fingerprint says you're a Python library and your user agent says Android, you haven't removed a signal. You've added two.

Make these agree:

• TCP/IP fingerprint, set by the -os- flag

• TLS fingerprint, set by your client or antidetect browser

• User agent and client hints, set in your request headers

• Screen size, fonts, and timezone, set by your browser profile

A simple rule: pick the OS your setup already presents, not the one you assume is most common. Be careful with ios and android in particular, since those mean a mobile device. Pairing either with a desktop user agent describes a device that doesn't exist.

More Control, Same Reliability

Fingerprinting keeps moving further down the stack. Headers and cookies are the layers everyone already manages. The network layer has been sitting there quietly saying Linux, and the vendors reading it know that better than most users do.

Closing that gap felt like something that should come with the product rather than cost extra, so it does.