// Copyright (c) Tailscale Inc & AUTHORS
// SPDX-License-Identifier: BSD-3-Clause

package netkernelconf

import (
	"fmt"

	"github.com/safchain/ethtool"
)

const (
	rxWantFeature      = "rx-udp-gro-forwarding"
	rxDoNotWantFeature = "rx-gro-list"
	txFeature          = "tx-udp-segmentation"
)

// CheckUDPGROForwarding checks if the machine is optimally configured to
// forward UDP packets between the default route and Tailscale TUN interfaces.
// It returns a non-nil warn in the case that the configuration is suboptimal.
// It returns a non-nil err in the case that an error is encountered while
// performing the check.
func CheckUDPGROForwarding(tunInterface, defaultRouteInterface string) (warn, err error) {
	const kbLink = "\nSee https://tailscale.com/s/ethtool-config-udp-gro"
	errWithPrefix := func(format string, a ...any) error {
		const errPrefix = "couldn't check system's UDP GRO forwarding configuration, "
		return fmt.Errorf(errPrefix+format, a...)
	}
	e, err := ethtool.NewEthtool()
	if err != nil {
		return nil, errWithPrefix("failed to init ethtool: %v", err)
	}
	defer e.Close()
	tunFeatures, err := e.Features(tunInterface)
	if err != nil {
		return nil, errWithPrefix("failed to retrieve TUN device features: %v", err)
	}
	if !tunFeatures[txFeature] {
		// if txFeature is disabled/nonexistent on the TUN then UDP GRO
		// forwarding doesn't matter, we won't be taking advantage of it.
		return nil, nil
	}
	defaultFeatures, err := e.Features(defaultRouteInterface)
	if err != nil {
		return nil, errWithPrefix("failed to retrieve default route interface features: %v", err)
	}
	defaultHasRxWant, ok := defaultFeatures[rxWantFeature]
	if !ok {
		// unlikely the feature is nonexistant with txFeature in the TUN driver
		// being added to the kernel later than rxWantFeature, but let's be sure
		return nil, nil
	}
	if !defaultHasRxWant || defaultFeatures[rxDoNotWantFeature] {
		return fmt.Errorf("UDP GRO forwarding is suboptimally configured on %s, UDP forwarding throughput capability will increase with a configuration change.%s", defaultRouteInterface, kbLink), nil
	}
	return nil, nil
}

// SetUDPGROForwarding enables UDP GRO forwarding for the provided default
// interface. It validates if the provided tun interface has UDP segmentation
// enabled and, if not, returns an error. See
// https://tailscale.com/kb/1320/performance-best-practices#linux-optimizations-for-subnet-routers-and-exit-nodes
func SetUDPGROForwarding(tunInterface, defaultInterface string) error {
	e, err := ethtool.NewEthtool()
	if err != nil {
		return fmt.Errorf("failed to init ethtool: %w", err)
	}
	defer e.Close()
	tunFeatures, err := e.Features(tunInterface)
	if err != nil {
		return fmt.Errorf("failed to retrieve TUN device features: %w", err)
	}
	if !tunFeatures[txFeature] {
		// if txFeature is disabled/nonexistent on the TUN then UDP GRO
		// forwarding doesn't matter, we won't be taking advantage of it.
		return fmt.Errorf("Not enabling UDP GRO forwarding as UDP segmentation is disabled for Tailscale interface")
	}
	if err := e.Change(defaultInterface, map[string]bool{rxWantFeature: true, rxDoNotWantFeature: false}); err != nil {
		return fmt.Errorf("error enabling UDP GRO forwarding: %w", err)
	}
	return nil
}