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relay-selector.md

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Glossary

  • Relay - a server that provides one or multiple tunnel and bridge endpoints, and has a weight associated with it
  • Endpoint - a combination of a socket address and the transport protocol
  • Transport protocol - TCP or UDP
  • Tunnel protocol - WireGuard or OpenVPN
  • Obfuscation - Putting WireGuard, OpenVPN or API traffic inside a protocol designed to make it harder to fingerprint or block the contained traffic. This is used to circumvent censorship. Mullvad hosts many different obfuscation protocols. Some are hosted directly on the VPN relays, but most are hosted on separate bridge servers. Even if most obfuscation protocols used include encryption, that encryption is not to be treated as secure. We only use the obfuscation protocol for its obfuscating properties, not for any security properties it might have.

Relay selector

The relay selector's main purpose is to pick a single Mullvad relay from a list of relays taking into account certain user-configurable criteria. Relays can be filtered by their location (country, city, hostname), by the protocols and ports they support (transport protocol, tunnel protocol, port), and by other constraints. The constraints are user specified and stored in the settings. The default value for location constraints restricts relay selection to relays from Sweden. The default protocol constraints default to Auto, which implies specific behavior.

Generally, the filtering process consists of going through each relay in our relay list and removing relay and endpoint combinations that do not match the constraints outlined above. The filtering process produces a list of relays that only contain matching endpoints. Of all the relays that match the constraints, one is selected and a random matching endpoint is selected from that relay.

The relay selector selects a tunnel endpoint first, and then uses the selected tunnel endpoint to select a bridge endpoint if necessary - a bridge will only be selected if the bridge state, current retry attempt and the tunnel protocol allow for it.

Tunnel endpoint constraints

Endpoints may be filtered by:

  • tunnel type (WireGuard or OpenVPN for tunnel endpoints)
  • transport protocol (UDP or TCP), not applicable if the tunnel protocol only allows a single one, like WireGuard
  • entry port
  • location (country, city, hostname)
  • provider
  • ownership (Mullvad-owned or rented)

Default constraints for tunnel endpoints

Whilst all user selected constraints are always honored, when the user hasn't selected any specific constraints the following default ones will take effect:

  • The first three connection attempts will use Wireguard
    • The first attempt will connect to a Wireguard relay on a random port
    • The second attempt will connect to a Wireguard relay on port 443
    • The third attempt will connect to a Wireguard relay over IPv6 (if IPv6 is configured on the host) on a random port
  • The fourth-to-seventh attempt will alternate between Wireguard and OpenVPN
    • The fourth attempt will connect to an OpenVPN relay over TCP on port 443
    • The fifth attempt will connect to a Wireguard relay on a random port using UDP2TCP obfuscation
    • The sixth attempt will connect to a Wireguard relay over IPv6 on a random port using UDP2TCP obfuscation (if IPv6 is configured on the host)
    • The seventh attempt will connect to an OpenVPN relay over a bridge on a random port

If no tunnel has been established after exhausting this list of attempts, the relay selector will loop back to the first default constraint and continue its search from there.

Any default constraint that is incompatible with user specified constraints will simply not be considered. Conversely, all default constraints which do not conflict with user specified constraints will be used in the search for a working tunnel endpoint on repeated connection failures.

Selecting tunnel endpoint between filtered relays

To select a single relay from the set of filtered relays, the relay selector uses a roulette wheel selection algorithm using the weights that are assigned to each relay. The higher the weight is relatively to other relays, the higher the likelihood that a given relay will be picked. Once a relay is picked, then a random endpoint that matches the constraints from the relay is picked.

Bridge endpoint constraints

The explicit constraints are:

  • location
  • provider
  • ownership

The transport protocol is supposedly inferred by the selected bridge- but for now, the daemon only supports TCP bridges, so only TCP bridges are being selected. If no location constraint is specified explicitly, then the relay location will be used.

Selecting a bridge endpoint between filtered relays

When filtering bridge endpoints by location, if multiple bridge endpoints match the specified constraints then endpoints which are geographically closer to the selected tunnel relay are more likely to be selected. If bridge state is set to On, then a bridge is always selected and used. If it's set to Auto, a bridge will only be tried after 3 failed attempts at connecting without a bridge and only if the relay constraints allow for a bridge to be selected.

Bridge caveats

Currently, bridges only support TCP tunnels over TCP bridges. This means that if the bridge state is set to On, the daemon will automatically set the tunnel constraints to OpenVPN over TCP. Once we have bridges that support UDP tunnels over TCP bridges, this behavior should be removed. Conversely, changing the tunnel constraints to ones that do not support bridges (WireGuard, OpenVPN over UDP) will indirectly change the bridge state to Auto if it was previously set to On.

Obfuscator caveats

Currently, there is only a single type of obfuscator - udp2tcp, and it's only used if it's mode is set to On or Auto and the user has selected WireGuard to be the only tunnel protocol to be used.