WebRTC IPv6 leak: why a proxy does not stop it

You routed everything through a proxy, the page shows the proxy’s IPv4, and you still got recognised across sessions. The address that gave you away was never on the surface you were watching. It was an IPv6 host candidate emitted by WebRTC, on a transport your SOCKS proxy never carried, and the pref everyone reaches for to turn it off does nothing in current Firefox.

This page is why that happens, how to see it in your own setup, and what actually removes it.

The two transports, and which one your proxy carries

A SOCKS5 proxy tunnels TCP. That is the whole contract. Your HTTP requests, your TLS, your WebSocket traffic all ride TCP and all exit at the proxy, which is why the page reads the proxy’s address and why the server reflexive WebRTC candidate (the one derived from a STUN round trip) shows the proxy’s egress too.

WebRTC does not only report the address a STUN server sees. Before any of that, it enumerates the host’s own network interfaces and emits a host candidate for each usable local address. That enumeration is local. It does not travel through your proxy, because there is nothing to travel: the browser is reading its own interface list off the operating system.

On an IPv4-only host behind a proxy this is harmless. The host candidate is a private LAN address, and a real browser masks it as an <uuid>.local mDNS name anyway, which is the shape a genuine Firefox produces. But a host with working IPv6 has a global IPv6 address on that interface, routable from anywhere, unique to the machine. WebRTC emits it as a host candidate exactly as it emits the LAN one, and now the page can read a stable global address that your proxy never touched.

That is the leak. The IPv4 srflx candidate shows the proxy; the IPv6 host candidate shows the machine. Both are in the same candidate list, and a detector that reads the list sees both.

ICE candidate	Where the address comes from	Rides TCP?	Does the proxy cover it?	What it reveals
`typ srflx` (server reflexive)	A STUN round trip	Yes	Yes	The proxy’s egress IPv4
`typ host`, IPv4	Local interface enumeration	No	No	A private LAN address, masked as `<uuid>.local`
`typ host`, IPv6	Local interface enumeration	No	No	The machine’s global, routable IPv6

Only the first row travels a transport the proxy carries. The two typ host rows are read straight off the operating system’s interface list, which is why the proxy never sees them and the IPv6 one exposes a stable global address.

Why the standard “disable IPv6” pref does nothing now

Setting media.peerconnection.ice.disableIPv6 does not stop the IPv6 WebRTC leak in current Firefox, because the pref was disconnected from the code path that emits host candidates. The advice you will find everywhere is to toggle media.peerconnection.ice.disableIPv6 in about:config, or to flip the matching Playwright/Selenium preference. That advice is stale.

We went looking for where current Firefox reads that pref during ICE gathering. It does not. A search across the WebRTC transport code finds the pref honoured only in two unrelated places: a DNS resolver flag that suppresses AAAA lookups, and a socket-level flag on the TCP path. Neither runs during the interface enumeration that produces host candidates. The pref that is named as if it governs IPv6 in WebRTC no longer reaches the code that emits IPv6 host candidates.

So setting it is not a smaller version of the fix. It is no version of the fix. The candidate is generated on a code path the pref was disconnected from, and the about:config toggle changes a value that nothing downstream reads. This is the same class of trap covered in testing for presence rather than absence: a setting that looks like it is doing something, on a signal that never moves.

What the leak looks like in a report

You do not need a detector to see it. Ask WebRTC for its own candidates and read them. Here is a page that gathers candidates through the proxied browser and prints them, using stock Playwright methods on the real Browser object:

from invisible_playwright import InvisiblePlaywright

PROXY = {"server": "socks5://gate.example.com:1080", "username": "u", "password": "p"}

GATHER = """
() => new Promise((resolve) => {
    const pc = new RTCPeerConnection({
        iceServers: [{ urls: "stun:stun.l.google.com:19302" }],
    });
    const cands = [];
    pc.onicecandidate = (e) => {
        if (e.candidate === null) { resolve(cands); return; }
        cands.push(e.candidate.candidate);
    };
    pc.createDataChannel("x");
    pc.createOffer().then((o) => pc.setLocalDescription(o));
    setTimeout(() => resolve(cands), 4000);
})
"""

with InvisiblePlaywright(seed=42, proxy=PROXY) as browser:
    page = browser.new_page()
    page.goto("https://example.com")
    for line in page.evaluate(GATHER):
        print(line)

Each printed line is one ICE candidate. The typ host lines are the local interface addresses; the typ srflx lines are what STUN reported. On a leaking setup you will see a typ host line carrying a colon-bearing global IPv6 address next to the srflx line that carries the proxy’s IPv4. That IPv6 string is the machine, printed to the page.

Read the same list from a stock browser on the same host and compare the two, which is the only reliable way to tell a real value from a spoofed one. If the IPv6 host line is present in both, it is your machine leaking, not the tool.

What actually stops it, and how we proved it

Because the pref is disconnected, the only place to remove the candidate is the gathering layer itself, below where the pref used to sit. The engine that invisible_playwright drives filters IPv6 addresses out of the interface enumeration before any candidate is built from them, on a code path shared by Windows and Linux, so an IPv6-capable host gathers as if it were IPv4-only. The synthetic IPv4 srflx that shows the proxy’s egress is untouched; only the IPv6 host and the IPv6 srflx derived from it are dropped.

We did not take that on faith, because a signal that is simply absent could be absent for the wrong reason. We ran it as a live A/B on one machine: with the filter active, the candidate list came back as { host IPv4 as <uuid>.local, srflx = proxy egress } and nothing else. Turn the filter off and re-gather, and the real global IPv6 host candidate reappears in the list. Turn it back on and it is gone again. The candidate tracks the filter and only the filter, which is what lets us say the filter is what removes it rather than some unrelated timing.

Run the code block above against the shipped engine and the IPv6 typ host line is not there. The IPv4 srflx still is, still pointing at the proxy, so the surface that should show the proxy shows the proxy and the surface that leaked the machine shows nothing to leak. That last part matters: a WebRTC section that comes back completely empty is itself a tell, so the goal is a candidate list that looks like a real browser behind NAT, not a suppressed one.

Keep it true when you rotate the exit

The IPv6 filter is a property of the engine, so it holds across every session and every seed. The thing that does not hold for free is agreement between the proxy and the rest of the identity. When you rotate to a new exit, the srflx candidate follows the new proxy automatically, but the timezone, locale and geolocation only follow if you let them, and a browser whose WebRTC says one country while its clock says another is its own signal. That coupling is covered in when the timezone does not match the proxy, and it is worth checking on every rotation rather than once at setup.

Conclusion

The IPv6 WebRTC leak survives a proxy for a simple reason: the proxy carries TCP, the leak is on interface enumeration that never uses TCP, and the pref named to stop it was disconnected from that path in current Firefox. Setting media.peerconnection.ice.disableIPv6 changes nothing an ICE candidate is built from. The fix has to live in the gathering layer, and it has to be verified by watching the candidate appear and disappear as the filter flips, not by trusting that an empty result means a solved one. Read your own candidate list, compare it against a stock browser, and confirm the IPv6 host line is gone while the IPv4 srflx still shows the proxy.

Short answers to the questions that lead here

Does a SOCKS5 proxy stop a WebRTC IPv6 leak? No. A SOCKS5 proxy carries TCP, and the IPv6 host candidate comes from local interface enumeration that never touches the proxy, so the machine’s global IPv6 is still emitted.

Does media.peerconnection.ice.disableIPv6 work? Not for ICE candidate gathering in current Firefox. The pref is still read for a DNS resolver flag and a TCP socket flag, but nothing on the host-candidate path reads it, so setting it does not remove the IPv6 candidate.

Why does my IPv4 show the proxy but I still got tracked? Because the srflx (STUN-derived) candidate rides TCP and exits at the proxy, while the IPv6 host candidate is your real machine address on a separate transport. One is disguised, the other is not.

How do I see the leak myself? Open an RTCPeerConnection, collect onicecandidate events, and print them. A typ host line with a colon-bearing global IPv6 address is the leak. The code block above does exactly this.

Is an empty WebRTC result the safe outcome? No. A completely blocked or empty candidate list is itself a tell. A real browser behind NAT emits a masked host candidate and a server reflexive one, so the target is a realistic list, not a silent one.

Does this come back when I rotate proxies? The IPv6 filter holds across rotations, but timezone and locale agreement with the new exit does not follow automatically, so recheck those each time you change the exit.

Sources

This project’s WebRTC transport patches and the live A/B toggle described above, which made the real IPv6 host candidate appear and disappear on demand and so established that the filter, not timing, is what removes it.
A read of the current Firefox WebRTC transport code confirming the legacy disableIPv6 pref is honoured only for DNS resolution and the TCP socket, and nowhere on the host-candidate gathering path.

Written while maintaining invisible_playwright, a Firefox patched at the C++ level driven by stock Playwright. The IPv6 filter on this page exists because a live ICE test found a real global address leaking past a proxy that was carrying everything else.