Support the tsize extension for TFTP.

It turns out many PXE firmwares request tsize to size the download
buffer, and will refuse to download at all if the server cannot
provide a tsize.

This unfortunately breaks API compatibility to add a size parameter,
but I think it's an unfortunate necessity to make this work.

tftp/handlers.go

@@ -31,7 +31,7 @@ func FilesystemHandler(root string) (Handler, error) {
 		return nil, err
 	}
 	root = filepath.ToSlash(root)
-	return func(path string, addr net.Addr) (io.ReadCloser, error) {
+	return func(path string, addr net.Addr) (io.ReadCloser, int64, error) {
 		// Join with a root, which gets rid of directory traversal
 		// attempts. Then we join that canonicalized path with the
 		// actual root, which resolves to the actual on-disk file to
@@ -41,18 +41,19 @@ func FilesystemHandler(root string) (Handler, error) {
 
 		st, err := os.Stat(path)
 		if err != nil {
-			return nil, err
+			return nil, 0, err
 		}
 		if !st.Mode().IsRegular() {
-			return nil, fmt.Errorf("requested path %q is not a file", path)
+			return nil, 0, fmt.Errorf("requested path %q is not a file", path)
 		}
-		return os.Open(path)
+		f, err := os.Open(path)
+		return f, st.Size(), err
 	}, nil
 }
 
 // ConstantHandler returns a Handler that serves bs for all requested paths.
 func ConstantHandler(bs []byte) Handler {
-	return func(path string, addr net.Addr) (io.ReadCloser, error) {
-		return ioutil.NopCloser(bytes.NewBuffer(bs)), nil
+	return func(path string, addr net.Addr) (io.ReadCloser, int64, error) {
+		return ioutil.NopCloser(bytes.NewBuffer(bs)), int64(len(bs)), nil
 	}
 }

tftp/tftp.go

@@ -48,7 +48,16 @@ const (
 )
 
 // A Handler provides bytes for a file.
-type Handler func(path string, clientAddr net.Addr) (io.ReadCloser, error)
+//
+// If size is non-zero, it must be equal to the number of bytes in
+// file. The server will offer the "tsize" extension to clients that
+// request it.
+//
+// Note that some clients (particularly firmware TFTP clients) can be
+// very capricious about servers not supporting all the options that
+// they request, so passing a size of 0 may cause TFTP transfers to
+// fail for some clients.
+type Handler func(path string, clientAddr net.Addr) (file io.ReadCloser, size int64, err error)
 
 // A Server defines parameters for running a TFTP server.
 type Server struct {
@@ -156,7 +165,7 @@ func (s *Server) transfer(addr net.Addr, req *rrq) error {
 	}
 	defer conn.Close()
 
-	file, err := s.Handler(req.Filename, addr)
+	file, size, err := s.Handler(req.Filename, addr)
 	if err != nil {
 		conn.Write(tftpError("failed to get file"))
 		return fmt.Errorf("getting file bytes: %s", err)
@@ -164,35 +173,44 @@ func (s *Server) transfer(addr net.Addr, req *rrq) error {
 	defer file.Close()
 
 	var b bytes.Buffer
-	if req.BlockSize == 0 {
-		// Client didn't negotiate, use classic blocksize from RFC.
-		req.BlockSize = 512
-	} else {
-		// Client requested a specific blocksize, need to OACK before
-		// sending data.
-		maxBlockSize := s.MaxBlockSize
-		if maxBlockSize <= 0 {
-			maxBlockSize = DefaultBlockSize
-		}
-		if req.BlockSize > maxBlockSize {
-			s.infoLog("clamping blocksize to %q: %d -> %d", addr, req.BlockSize, maxBlockSize)
-			req.BlockSize = maxBlockSize
-		}
-
+	if req.BlockSize != 0 || (req.WantSize && size != 0) {
+		// Client requested options, need to OACK them before sending
+		// data.
 		b.WriteByte(0)
 		b.WriteByte(6)
-		b.WriteString("blksize")
-		b.WriteByte(0)
-		b.WriteString(strconv.FormatInt(req.BlockSize, 10))
-		b.WriteByte(0)
+
+		if req.BlockSize != 0 {
+			maxBlockSize := s.MaxBlockSize
+			if maxBlockSize <= 0 {
+				maxBlockSize = DefaultBlockSize
+			}
+			if req.BlockSize > maxBlockSize {
+				s.infoLog("clamping blocksize to %q: %d -> %d", addr, req.BlockSize, maxBlockSize)
+				req.BlockSize = maxBlockSize
+			}
+
+			b.WriteString("blksize")
+			b.WriteByte(0)
+			b.WriteString(strconv.FormatInt(req.BlockSize, 10))
+			b.WriteByte(0)
+		}
+
+		if req.WantSize && size != 0 {
+			b.WriteString("tsize")
+			b.WriteByte(0)
+			b.WriteString(strconv.FormatInt(size, 10))
+			b.WriteByte(0)
+		}
+
 		if err := s.send(conn, b.Bytes(), 0); err != nil {
-			// TODO: some PXE roms request a transfer with the tsize
-			// option to try and find out the file length, and abort
-			// if the server doesn't echo tsize.
 			return fmt.Errorf("sending OACK: %s", err)
 		}
 		b.Reset()
 	}
+	if req.BlockSize == 0 {
+		// Client didn't negotiate, use classic blocksize from RFC.
+		req.BlockSize = 512
+	}
 
 	seq := uint16(1)
 	b.Grow(int(req.BlockSize + 4))
@@ -270,6 +288,7 @@ Attempt:
 type rrq struct {
 	Filename  string
 	BlockSize int64
+	WantSize  bool
 }
 
 func parseRRQ(bs []byte) (*rrq, error) {
@@ -310,6 +329,9 @@ func parseRRQ(bs []byte) (*rrq, error) {
 		}
 		bs = rest
 		if opt != "blksize" {
+			if opt == "tsize" {
+				req.WantSize = true
+			}
 			continue
 		}
 		size, err := strconv.ParseInt(val, 10, 64)