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236 lines
8.0 KiB
Go
236 lines
8.0 KiB
Go
// Copyright (c) Tailscale Inc & AUTHORS
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// SPDX-License-Identifier: BSD-3-Clause
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// Package derp implements the Designated Encrypted Relay for Packets (DERP)
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// protocol.
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//
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// DERP routes packets to clients using curve25519 keys as addresses.
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//
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// DERP is used by Tailscale nodes to proxy encrypted WireGuard
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// packets through the Tailscale cloud servers when a direct path
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// cannot be found or opened. DERP is a last resort. Both sides
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// between very aggressive NATs, firewalls, no IPv6, etc? Well, DERP.
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package derp
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import (
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"bufio"
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"encoding/binary"
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"errors"
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"fmt"
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"io"
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"time"
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)
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// MaxPacketSize is the maximum size of a packet sent over DERP.
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// (This only includes the data bytes visible to magicsock, not
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// including its on-wire framing overhead)
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const MaxPacketSize = 64 << 10
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// magic is the DERP magic number, sent in the frameServerKey frame
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// upon initial connection.
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const magic = "DERP🔑" // 8 bytes: 0x44 45 52 50 f0 9f 94 91
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const (
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nonceLen = 24
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frameHeaderLen = 1 + 4 // frameType byte + 4 byte length
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keyLen = 32
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maxInfoLen = 1 << 20
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keepAlive = 60 * time.Second
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)
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// ProtocolVersion is bumped whenever there's a wire-incompatible change.
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// - version 1 (zero on wire): consistent box headers, in use by employee dev nodes a bit
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// - version 2: received packets have src addrs in frameRecvPacket at beginning
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const ProtocolVersion = 2
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// frameType is the one byte frame type at the beginning of the frame
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// header. The second field is a big-endian uint32 describing the
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// length of the remaining frame (not including the initial 5 bytes).
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type frameType byte
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/*
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Protocol flow:
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Login:
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* client connects
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* server sends frameServerKey
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* client sends frameClientInfo
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* server sends frameServerInfo
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Steady state:
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* server occasionally sends frameKeepAlive (or framePing)
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* client responds to any framePing with a framePong
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* client sends frameSendPacket
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* server then sends frameRecvPacket to recipient
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*/
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const (
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frameServerKey = frameType(0x01) // 8B magic + 32B public key + (0+ bytes future use)
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frameClientInfo = frameType(0x02) // 32B pub key + 24B nonce + naclbox(json)
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frameServerInfo = frameType(0x03) // 24B nonce + naclbox(json)
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frameSendPacket = frameType(0x04) // 32B dest pub key + packet bytes
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frameForwardPacket = frameType(0x0a) // 32B src pub key + 32B dst pub key + packet bytes
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frameRecvPacket = frameType(0x05) // v0/1: packet bytes, v2: 32B src pub key + packet bytes
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frameKeepAlive = frameType(0x06) // no payload, no-op (to be replaced with ping/pong)
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frameNotePreferred = frameType(0x07) // 1 byte payload: 0x01 or 0x00 for whether this is client's home node
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// framePeerGone is sent from server to client to signal that
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// a previous sender is no longer connected. That is, if A
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// sent to B, and then if A disconnects, the server sends
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// framePeerGone to B so B can forget that a reverse path
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// exists on that connection to get back to A. It is also sent
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// if A tries to send a CallMeMaybe to B and the server has no
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// record of B (which currently would only happen if there was
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// a bug).
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framePeerGone = frameType(0x08) // 32B pub key of peer that's gone + 1 byte reason
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// framePeerPresent is like framePeerGone, but for other
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// members of the DERP region when they're meshed up together.
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framePeerPresent = frameType(0x09) // 32B pub key of peer that's connected
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// frameWatchConns is how one DERP node in a regional mesh
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// subscribes to the others in the region.
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// There's no payload. If the sender doesn't have permission, the connection
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// is closed. Otherwise, the client is initially flooded with
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// framePeerPresent for all connected nodes, and then a stream of
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// framePeerPresent & framePeerGone has peers connect and disconnect.
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frameWatchConns = frameType(0x10)
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// frameClosePeer is a privileged frame type (requires the
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// mesh key for now) that closes the provided peer's
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// connection. (To be used for cluster load balancing
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// purposes, when clients end up on a non-ideal node)
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frameClosePeer = frameType(0x11) // 32B pub key of peer to close.
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framePing = frameType(0x12) // 8 byte ping payload, to be echoed back in framePong
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framePong = frameType(0x13) // 8 byte payload, the contents of the ping being replied to
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// frameHealth is sent from server to client to tell the client
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// if their connection is unhealthy somehow. Currently the only unhealthy state
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// is whether the connection is detected as a duplicate.
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// The entire frame body is the text of the error message. An empty message
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// clears the error state.
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frameHealth = frameType(0x14)
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// frameRestarting is sent from server to client for the
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// server to declare that it's restarting. Payload is two big
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// endian uint32 durations in milliseconds: when to reconnect,
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// and how long to try total. See ServerRestartingMessage docs for
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// more details on how the client should interpret them.
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frameRestarting = frameType(0x15)
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)
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// PeerGoneReasonType is a one byte reason code explaining why a
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// server does not have a path to the requested destination.
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type PeerGoneReasonType byte
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const (
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PeerGoneReasonDisconnected = PeerGoneReasonType(0x00) // peer disconnected from this server
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PeerGoneReasonNotHere = PeerGoneReasonType(0x01) // server doesn't know about this peer, unexpected
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)
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var bin = binary.BigEndian
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func writeUint32(bw *bufio.Writer, v uint32) error {
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var b [4]byte
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bin.PutUint32(b[:], v)
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// Writing a byte at a time is a bit silly,
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// but it causes b not to escape,
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// which more than pays for the silliness.
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for _, c := range &b {
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err := bw.WriteByte(c)
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if err != nil {
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return err
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}
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}
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return nil
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}
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func readUint32(br *bufio.Reader) (uint32, error) {
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var b [4]byte
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// Reading a byte at a time is a bit silly,
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// but it causes b not to escape,
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// which more than pays for the silliness.
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for i := range &b {
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c, err := br.ReadByte()
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if err != nil {
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return 0, err
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}
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b[i] = c
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}
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return bin.Uint32(b[:]), nil
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}
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func readFrameTypeHeader(br *bufio.Reader, wantType frameType) (frameLen uint32, err error) {
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gotType, frameLen, err := readFrameHeader(br)
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if err == nil && wantType != gotType {
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err = fmt.Errorf("bad frame type 0x%X, want 0x%X", gotType, wantType)
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}
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return frameLen, err
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}
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func readFrameHeader(br *bufio.Reader) (t frameType, frameLen uint32, err error) {
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tb, err := br.ReadByte()
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if err != nil {
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return 0, 0, err
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}
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frameLen, err = readUint32(br)
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if err != nil {
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return 0, 0, err
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}
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return frameType(tb), frameLen, nil
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}
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// readFrame reads a frame header and then reads its payload into
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// b[:frameLen].
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//
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// If the frame header length is greater than maxSize, readFrame returns
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// an error after reading the frame header.
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//
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// If the frame is less than maxSize but greater than len(b), len(b)
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// bytes are read, err will be io.ErrShortBuffer, and frameLen and t
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// will both be set. That is, callers need to explicitly handle when
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// they get more data than expected.
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func readFrame(br *bufio.Reader, maxSize uint32, b []byte) (t frameType, frameLen uint32, err error) {
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t, frameLen, err = readFrameHeader(br)
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if err != nil {
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return 0, 0, err
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}
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if frameLen > maxSize {
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return 0, 0, fmt.Errorf("frame header size %d exceeds reader limit of %d", frameLen, maxSize)
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}
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n, err := io.ReadFull(br, b[:min(frameLen, uint32(len(b)))])
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if err != nil {
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return 0, 0, err
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}
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remain := frameLen - uint32(n)
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if remain > 0 {
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if _, err := io.CopyN(io.Discard, br, int64(remain)); err != nil {
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return 0, 0, err
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}
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err = io.ErrShortBuffer
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}
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return t, frameLen, err
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}
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func writeFrameHeader(bw *bufio.Writer, t frameType, frameLen uint32) error {
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if err := bw.WriteByte(byte(t)); err != nil {
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return err
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}
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return writeUint32(bw, frameLen)
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}
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// writeFrame writes a complete frame & flushes it.
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func writeFrame(bw *bufio.Writer, t frameType, b []byte) error {
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if len(b) > 10<<20 {
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return errors.New("unreasonably large frame write")
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}
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if err := writeFrameHeader(bw, t, uint32(len(b))); err != nil {
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return err
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}
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if _, err := bw.Write(b); err != nil {
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return err
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}
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return bw.Flush()
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}
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