# Forge / FML and Proxies

Forge (via FML — Forge Mod Loader) changes the handshake in a way that **collides with legacy forwarding**, because both want to use the same field: the handshake **Server Address** string. A proxy in front of modded backends has to understand and preserve the FML markers, or modded clients fail to connect.

## The FML handshake markers

Since FML 1.8, a Forge client **appends a token to the handshake host** so a Forge server can detect that the client is modded. The marker is a `\0`-delimited suffix:

| Marker | Era |
|---|---|
| `\0FML\0` | FML 1.8+ (legacy Forge, ~1.8–1.12) |
| `\0FML2\0` | newer FML (1.13+ "new" Forge networking) |
| `\0FML3\0` | later Forge revisions |

So a Forge client's handshake host looks like `mc.example.com\0FML\0` instead of plain `mc.example.com`. Both BungeeCord and Velocity hard-code the legacy token:

> ```java
> // BungeeCord
> public static final String FML_TAG            = "FML";
> public static final String FML_HANDSHAKE_TAG  = "FML|HS";
> public static final String FML_HANDSHAKE_TOKEN = "\0FML\0";   // "The FML 1.8 handshake token."
> ```
>
> — `BungeeCord/proxy/.../forge/ForgeConstants.java:13,14,20`

> ```java
> // Velocity (legacy Forge)
> public static final String HANDSHAKE_HOSTNAME_TOKEN = "\0FML\0";
> public static final String FORGE_LEGACY_HANDSHAKE_CHANNEL = "FML|HS";
> ```
>
> — `Velocity/proxy/.../forge/legacy/LegacyForgeConstants.java:29,34`

The deeper mod-list negotiation then happens over the `FML|HS` plugin-message channel (legacy) or modern Forge login plugin messages — that's a separate, larger handshake. The `\0FML\0` marker is just the "I am modded" flag riding in the address field.

## The collision with legacy forwarding

[Legacy/BungeeCord forwarding](bungeecord-legacy.md) **also** packs data into the handshake host (`realHost\0clientIP\0uuid\0props`), `\0`-delimited. If a Forge client adds `\0FML\0` and the proxy then naively appends `\0IP\0UUID\0props`, the two collide — the backend can't tell which `\0` segment is which.

Proxies solve this by **splitting the FML tail off first**, doing their own thing, then restoring it. BungeeCord stashes everything from the first `\0` as `extraDataInHandshake`:

> ```java
> // Starting with FML 1.8, a "\0FML\0" token is appended to the handshake. This interferes
> // with Bungee's IP forwarding, so we detect it, and remove it from the host string, for now.
> if ( handshake.getHost().contains( "\0" ) )
> {
>     String[] split = handshake.getHost().split( "\0", 2 );
>     handshake.setHost( split[0] );
>     extraDataInHandshake = "\0" + split[1];
> }
> ```
>
> — `BungeeCord/proxy/.../connection/InitialHandler.java:355-360`

When IP forwarding is **off**, BungeeCord re-appends that saved tail so the modded backend still sees its `\0FML\0`:

> `copiedHandshake.setHost( copiedHandshake.getHost() + user.getExtraDataInHandshake() );`
> — `BungeeCord/proxy/.../ServerConnector.java:124-128`

But when IP forwarding is **on**, the FML tail can't be reattached — the forwarding segments already own the field. BungeeCord's own code marks this as a known gap (`// TODO: Add support for this data with IP forwarding.`, `ServerConnector.java:127`). Velocity is explicit about the same conflict in **legacy** mode and works around it by moving the Forge flag into a profile **property** instead of the hostname:

> ```java
> // We can't forward the FML token to the server when we are running in legacy forwarding mode,
> // since both use the "hostname" field in the handshake. We add a special property to the
> // profile instead, which will be ignored by non-Forge servers and can be intercepted by a
> // Forge coremod, such as SpongeForge.
> if (forwardingType == PlayerInfoForwarding.LEGACY) {
>     return original.addProperty(IS_FORGE_CLIENT_PROPERTY);
> }
> ```
>
> — `Velocity/proxy/.../forge/legacy/LegacyForgeConnectionType.java:41-49`

## Modern Forge + Velocity (modern forwarding)

[Modern/Velocity forwarding](velocity-modern.md) does **not** touch the handshake address field, so the `\0FML\0` host collision goes away — the FML marker and the identity data no longer fight over the same field. The remaining interplay is the **Forge login-phase handshake itself**, which runs as login plugin messages on the same login state where Velocity sends `velocity:player_info`. The two coexist (different channels), but a modded backend's FML handshake and the forwarding handshake must both complete during login.

Practical notes:

- **Modern Forge (1.13+)** uses its own login plugin-message handshake; a Velocity backend running modern forwarding handles both because they're distinct channels. <!-- UNCONFIRMED: the claim that Velocity modern forwarding coexists cleanly with modern Forge's login-plugin handshake is plausible (distinct channels) but depends on the backend mod (e.g. FabricProxy-Lite or a Forge-specific forwarding mod) and was not verified from Velocity or NeoForge source -->
- **ViaForge / client-side shims**: tools like ViaForge let a Forge client speak to a backend across version gaps; they have to reproduce or tolerate the FML handshake markers so the proxy and backend negotiate the modded handshake correctly. <!-- UNCONFIRMED: ViaForge source not available in refs; description is based on the tool's stated purpose, not code inspection -->
- For modded networks, **modern forwarding is preferable** precisely because it sidesteps the address-field collision that makes legacy + Forge brittle.

## Summary

- The `\0FML\0` / `\0FML2\0` / `\0FML3\0` marker is Forge saying "I'm modded," appended to the handshake host.
- It **collides with legacy forwarding** (shared field) → proxies must split it off and restore it, and **can't** restore it while legacy IP-forwarding is on (BungeeCord TODO; Velocity moves the flag to a property).
- **Modern forwarding avoids the field entirely**, so it's the cleaner choice for Forge backends; the Forge mod-list handshake then runs as separate login plugin messages.

---

**Sources**

- `BungeeCord/proxy/.../forge/ForgeConstants.java:13-20` — `FML_TAG`, `FML_HANDSHAKE_TAG = "FML|HS"`, `FML_HANDSHAKE_TOKEN = "\0FML\0"`.
- `BungeeCord/proxy/.../connection/InitialHandler.java:355-360` — split the `\0FML\0` tail off the host, save as `extraDataInHandshake`.
- `BungeeCord/proxy/.../ServerConnector.java:124-128` — restore the FML tail only when IP forwarding is off (`:127` TODO notes the gap when it's on).
- `Velocity/proxy/.../forge/legacy/LegacyForgeConstants.java:29,34` — `HANDSHAKE_HOSTNAME_TOKEN = "\0FML\0"`, `FORGE_LEGACY_HANDSHAKE_CHANNEL = "FML|HS"`.
- `Velocity/proxy/.../forge/legacy/LegacyForgeConnectionType.java:41-49` — legacy forwarding can't carry the FML host token; flag moved to a profile property instead.
- See [bungeecord-legacy.md](bungeecord-legacy.md) (the colliding `\0` transport) and [velocity-modern.md](velocity-modern.md) (the field-free alternative).