Add ability to set a higher maximum block size for TFTP (#93)

## Description

This PR adds the ability to set a higher maximum block size for TFTP transfers.

## Why is this needed

We are troubleshooting some misbehaving NICs that might benefit from support for larger block sizes. They appear to request 1468 which makes sense on a link with MTU of 1500.

## How Has This Been Tested?

Tested with atftp

## How are existing users impacted? What migration steps/scripts do we need?

The default of 512 is preserved and made explicit in this codebase.

cmd.go

@@ -20,6 +20,8 @@ import (
 type Command struct {
 	// TFTPAddr is the TFTP server address:port.
 	TFTPAddr string `validate:"required,hostname_port"`
+	// TFTPBlockSize is the maximum block size for serving individual TFTP requests.
+	TFTPBlockSize int `validate:"required,gte=512"`
 	// TFTPTimeout is the timeout for serving individual TFTP requests.
 	TFTPTimeout time.Duration `validate:"required,gte=1s"`
 	// HTTPAddr is the HTTP server address:port.
@@ -72,12 +74,13 @@ func Execute(ctx context.Context, args []string) error {
 // Run listens and serves the TFTP and HTTP services.
 func (c *Command) Run(ctx context.Context) error {
 	defaults := Command{
-		TFTPAddr:    "0.0.0.0:69",
-		TFTPTimeout: 5 * time.Second,
-		HTTPAddr:    "0.0.0.0:8080",
-		HTTPTimeout: 5 * time.Second,
-		Log:         logr.Discard(),
-		LogLevel:    "info",
+		TFTPAddr:      "0.0.0.0:69",
+		TFTPBlockSize: 512,
+		TFTPTimeout:   5 * time.Second,
+		HTTPAddr:      "0.0.0.0:8080",
+		HTTPTimeout:   5 * time.Second,
+		Log:           logr.Discard(),
+		LogLevel:      "info",
 	}
 
 	err := mergo.Merge(c, defaults)
@@ -94,8 +97,9 @@ func (c *Command) Run(ctx context.Context) error {
 	}
 	srv := Server{
 		TFTP: ServerSpec{
-			Addr:    tAddr,
-			Timeout: c.TFTPTimeout,
+			Addr:      tAddr,
+			BlockSize: c.TFTPBlockSize,
+			Timeout:   c.TFTPTimeout,
 		},
 		HTTP: ServerSpec{
 			Addr:    hAddr,
@@ -110,6 +114,7 @@ func (c *Command) Run(ctx context.Context) error {
 // RegisterFlags registers a flag set for the ipxe command.
 func (c *Command) RegisterFlags(f *flag.FlagSet) {
 	f.StringVar(&c.TFTPAddr, "tftp-addr", "0.0.0.0:69", "TFTP server address")
+	f.IntVar(&c.TFTPBlockSize, "tftp-blocksize", 512, "TFTP server maximum block size")
 	f.DurationVar(&c.TFTPTimeout, "tftp-timeout", time.Second*5, "TFTP server timeout")
 	f.StringVar(&c.HTTPAddr, "http-addr", "0.0.0.0:8080", "HTTP server address")
 	f.DurationVar(&c.HTTPTimeout, "http-timeout", time.Second*5, "HTTP server timeout")

cmd_test.go

@@ -23,6 +23,7 @@ func TestCommand_RegisterFlags(t *testing.T) {
 			c := &Command{}
 			fs := flag.NewFlagSet("ipxe", flag.ExitOnError)
 			fs.StringVar(&c.TFTPAddr, "tftp-addr", "0.0.0.0:69", "TFTP server address")
+			fs.IntVar(&c.TFTPBlockSize, "tftp-blocksize", 512, "TFTP server maximum block size")
 			fs.DurationVar(&c.TFTPTimeout, "tftp-timeout", time.Second*5, "TFTP server timeout")
 			fs.StringVar(&c.HTTPAddr, "http-addr", "0.0.0.0:8080", "HTTP server address")
 			fs.DurationVar(&c.HTTPTimeout, "http-timeout", time.Second*5, "HTTP server timeout")
@@ -87,19 +88,21 @@ func TestCommand_Validate(t *testing.T) {
 		wantErr error
 	}{
 		{"success", &Command{
-			TFTPAddr:    "0.0.0.0:69",
-			TFTPTimeout: 5 * time.Second,
-			HTTPAddr:    "0.0.0.0:8080",
-			HTTPTimeout: 5 * time.Second,
-			Log:         logr.Discard(),
-			LogLevel:    "info",
+			TFTPAddr:      "0.0.0.0:69",
+			TFTPBlockSize: 512,
+			TFTPTimeout:   5 * time.Second,
+			HTTPAddr:      "0.0.0.0:8080",
+			HTTPTimeout:   5 * time.Second,
+			Log:           logr.Discard(),
+			LogLevel:      "info",
 		}, nil},
 		{"fail", &Command{
-			TFTPTimeout: 5 * time.Second,
-			HTTPAddr:    "0.0.0.0:8080",
-			HTTPTimeout: 5 * time.Second,
-			Log:         logr.Discard(),
-			LogLevel:    "info",
+			TFTPBlockSize: 512,
+			TFTPTimeout:   5 * time.Second,
+			HTTPAddr:      "0.0.0.0:8080",
+			HTTPTimeout:   5 * time.Second,
+			Log:           logr.Discard(),
+			LogLevel:      "info",
 		}, fmt.Errorf(`Key: 'Command.TFTPAddr' Error:Field validation for 'TFTPAddr' failed on the 'required' tag`)},
 	}
 	for _, tt := range tests {

ipxedust.go

@@ -49,6 +49,8 @@ type ServerSpec struct {
 	Timeout time.Duration
 	// Disabled allows a server to be disabled. Useful, for example, to disable TFTP.
 	Disabled bool
+	// BlockSize allows setting a larger maximum block size for TFTP
+	BlockSize int
 	// The patch to apply to the iPXE binary.
 	Patch []byte
 }
@@ -59,6 +61,8 @@ var errNilListener = fmt.Errorf("listener must not be nil")
 //
 // Default TFTP listen address is ":69".
 //
+// Default TFTP block size is 512.
+//
 // Default HTTP listen address is ":8080".
 //
 // Default request timeout for both is 5 seconds.
@@ -67,7 +71,7 @@ var errNilListener = fmt.Errorf("listener must not be nil")
 // See binary/binary.go for the iPXE files that are served.
 func (c *Server) ListenAndServe(ctx context.Context) error {
 	defaults := Server{
-		TFTP: ServerSpec{Addr: netip.AddrPortFrom(netip.IPv4Unspecified(), 69), Timeout: 5 * time.Second},
+		TFTP: ServerSpec{Addr: netip.AddrPortFrom(netip.IPv4Unspecified(), 69), Timeout: 5 * time.Second, BlockSize: 512},
 		HTTP: ServerSpec{Addr: netip.AddrPortFrom(netip.IPv4Unspecified(), 8080), Timeout: 5 * time.Second},
 		Log:  logr.Discard(),
 	}
@@ -190,10 +194,11 @@ func (c *Server) listenAndServeTFTP(ctx context.Context) error {
 	h := &itftp.Handler{Log: c.Log, Patch: c.TFTP.Patch}
 	ts := tftp.NewServer(h.HandleRead, h.HandleWrite)
 	ts.SetTimeout(c.TFTP.Timeout)
+	ts.SetBlockSize(c.TFTP.BlockSize)
 	if c.EnableTFTPSinglePort {
 		ts.EnableSinglePort()
 	}
-	c.Log.Info("serving iPXE binaries via TFTP", "addr", c.TFTP.Addr, "timeout", c.TFTP.Timeout, "singlePortEnabled", c.EnableTFTPSinglePort)
+	c.Log.Info("serving iPXE binaries via TFTP", "addr", c.TFTP.Addr, "blocksize", c.TFTP.BlockSize, "timeout", c.TFTP.Timeout, "singlePortEnabled", c.EnableTFTPSinglePort)
 	go func() {
 		<-ctx.Done()
 		conn.Close()
@@ -210,10 +215,11 @@ func (c *Server) serveTFTP(ctx context.Context, conn net.PacketConn) error {
 	h := &itftp.Handler{Log: c.Log, Patch: c.TFTP.Patch}
 	ts := tftp.NewServer(h.HandleRead, h.HandleWrite)
 	ts.SetTimeout(c.TFTP.Timeout)
+	ts.SetBlockSize(c.TFTP.BlockSize)
 	if c.EnableTFTPSinglePort {
 		ts.EnableSinglePort()
 	}
-	c.Log.Info("serving iPXE binaries via TFTP", "addr", conn.LocalAddr().String(), "timeout", c.TFTP.Timeout, "singlePortEnabled", c.EnableTFTPSinglePort)
+	c.Log.Info("serving iPXE binaries via TFTP", "addr", conn.LocalAddr().String(), "blocksize", c.TFTP.BlockSize, "timeout", c.TFTP.Timeout, "singlePortEnabled", c.EnableTFTPSinglePort)
 	go func() {
 		<-ctx.Done()
 		conn.Close()