[client] add MTU option to embed.Options (#5550)

Expose MTU configuration in the embed package so embedded clients
can set the WireGuard tunnel MTU without the config file workaround.
This is needed for protocols like QUIC that require larger datagrams
than the default MTU of 1280.

Validates MTU range via iface.ValidateMTU() at construction time to
prevent invalid values from being persisted to config.

Closes #5549

client/embed/embed.go

@@ -14,6 +14,7 @@ import (
 	"github.com/sirupsen/logrus"
 	wgnetstack "golang.zx2c4.com/wireguard/tun/netstack"
 
+	"github.com/netbirdio/netbird/client/iface"
 	"github.com/netbirdio/netbird/client/iface/netstack"
 	"github.com/netbirdio/netbird/client/internal"
 	"github.com/netbirdio/netbird/client/internal/auth"
@@ -81,6 +82,12 @@ type Options struct {
 	BlockInbound bool
 	// WireguardPort is the port for the WireGuard interface. Use 0 for a random port.
 	WireguardPort *int
+	// MTU is the MTU for the WireGuard interface.
+	// Valid values are in the range 576..8192 bytes.
+	// If non-nil, this value overrides any value stored in the config file.
+	// If nil, the existing config MTU (if non-zero) is preserved; otherwise it defaults to 1280.
+	// Set to a higher value (e.g. 1400) if carrying QUIC or other protocols that require larger datagrams.
+	MTU *uint16
 }
 
 // validateCredentials checks that exactly one credential type is provided
@@ -112,6 +119,12 @@ func New(opts Options) (*Client, error) {
 		return nil, err
 	}
 
+	if opts.MTU != nil {
+		if err := iface.ValidateMTU(*opts.MTU); err != nil {
+			return nil, fmt.Errorf("invalid MTU: %w", err)
+		}
+	}
+
 	if opts.LogOutput != nil {
 		logrus.SetOutput(opts.LogOutput)
 	}
@@ -151,6 +164,7 @@ func New(opts Options) (*Client, error) {
 		DisableClientRoutes: &opts.DisableClientRoutes,
 		BlockInbound:        &opts.BlockInbound,
 		WireguardPort:       opts.WireguardPort,
+		MTU:                 opts.MTU,
 	}
 	if opts.ConfigPath != "" {
 		config, err = profilemanager.UpdateOrCreateConfig(input)