27b75b83de
- Hybrid TCP mode, uses both TCP and UDP - Simplified TCP protocol - Modified obfuscator to support padding - Obfuscation over TCP - Refactor parts of Win32 code to be more similar to BSD
76 lines
3.8 KiB
Text
76 lines
3.8 KiB
Text
WireGuard over TCP
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We hate running one TCP implementation on top of another TCP implementation.
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There's problems with cascading retransmissions and head of line blocking,
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and performance is always much worse than a UDP based tunnel.
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However, we also recognize that several users need to run WireGuard over TCP.
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One reason is that UDP packets are sometimes blocked by the network in
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corporate scenarios or in other types of firewalls. Also, in misconfigured
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networks outside of the user's control, TCP may be more reliable than UDP.
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Additionally, we want TunSafe to be a drop-in replacement for OpenVPN, which
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also supports TCP based tunneling. The feature could also be used to run
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WireGuard tunnels over ssh tunnels, or through socks/https proxies.
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The TunSafe project therefore takes the pragmatic approach of supporting
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WireGuard over TCP, while discouraging its use. We absolutely don't want
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people to start using TCP by default. It's meant to be used only in the
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extreme cases when nothing else is working.
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We've added experimental support for TCP in the latest TunSafe master,
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which means you can try this out on Windows, OSX, Linux, and FreeBSD.
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On the server side, to listen on a TCP port, use ListenPortTCP=1234. (Not
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working on Windows yet). On the clients, use Endpoint=tcp://5.5.5.5:1234.
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The code is still very experimental and untested, and is not recommended
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for general use. Once the code is more well tested, we'll also release
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support for connecting to WireGuard over TCP in our Android and iOS clients.
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To make the impact as small as possible to our WireGuard protocol handling,
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and to minimize the risk of security related issues, the TCP feature has been
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designed to be as self-contained as possible. When a packet comes in over
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TCP, it's sent over to the WireGuard protocol handler and treated as if it
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was a UDP packet, and vice versa. This means TCP support can also be supported
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in existing WireGuard deployments by using a separate process that converts
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TCP connections into UDP packets sent to the WireGuard Linux kernel module.
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Each packet over TCP is prefixed by a 2-byte big endian number, which contains
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the length of the packet's payload. The payload is then the actual WireGuard
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UDP packet.
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TCP has larger overhead than UDP, and we want to support the usual WireGuard
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MTU of 1420 without introducing extra packet "fragmenting". So we implemented
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an optimization to skip sending the 16-byte WireGuard header for every packet.
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TCP is a reliable connection, we know that sequence numbers are always
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monotonically increasing, so we can predict the contents of this header.
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Here's an example:
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A 1420 byte big packet sent over a WireGuard link will have 2 bytes of
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TCP payload length, 16 bytes of WireGuard headers, 16 bytes of WireGuard MAC,
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20 bytes of TCP headers, and 40 bytes of IPv6 headers.
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This is a total of 1420 + 2 + 16 + 16 + 20 + 40 = 1514 bytes, exceeding
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the usual 1500 byte Ethernet MTU by 14 bytes. This means that a single full
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sized packet over WireGuard will result in 2 TCP packets. With our
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optimization, we reduce this to 1498 bytes, so it fits in one TCP packet.
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Protocol specification
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----------------------
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TT LLLLLL LLLLLLLL [Payload LL bytes]
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| \-- Payload length, high byte first.
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\----- Packet type
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The packet types (TT) currently defined are:
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TT = 00 = Normal The payload is a normal unmodified WireGuard packet
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including the regular WireGuard header.
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10 = Data A WireGuard data packet (type 04) without the 16 byte
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header.
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?1 = Reserved
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When parsing an incoming Data (TT=10) packet, the Key ID and Counter from
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the most recently parsed WireGuard data packet (type 04) is prepended to
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the payload, with Counter incremented by 1.
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This happens independently in each of the two TCP directions.
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