DebTik/internal/aerotun/mtu.go

package aerotun

import (
	"fmt"
	"net"

	"github.com/vishvananda/netlink"
)

// Account for our authenticated header/tag plus UDP and the outer IP header.
// Route MTU and link MTU are local upper bounds; a smaller remote path MTU can
// still require an explicitly lower configured MTU.
func packetHeaderSize(version int) int {
	if version == 3 {
		return secureHeaderSize
	}
	return headerSize
}
func underlayMTU(requested int, remote net.IP) (int, error) {
	return underlayMTUForHeader(requested, remote, headerSize)
}
func underlayMTUForHeader(requested int, remote net.IP, header int) (int, error) {
	routes, err := netlink.RouteGet(remote)
	if err != nil || len(routes) == 0 {
		// During system boot or temporary network disconnection, route may not be present yet.
		// Fallback gracefully to requested MTU if valid, or default 1360.
		if requested >= 576 {
			return requested, nil
		}
		return 1360, nil
	}
	link, err := netlink.LinkByIndex(routes[0].LinkIndex)
	if err != nil {
		if requested >= 576 {
			return requested, nil
		}
		return 1360, nil
	}
	limit := link.Attrs().MTU
	if routes[0].MTU > 0 && routes[0].MTU < limit {
		limit = routes[0].MTU
	}
	overhead := header + 16 + 8 + 20
	if remote.To4() == nil {
		overhead += 20
	}
	limit -= overhead
	if limit < 576 {
		return 0, fmt.Errorf("underlay MTU too small for AeroTUN")
	}
	if requested > limit {
		return limit, nil
	}
	return requested, nil
}
func (t *TunnelRunner) adjustMTU(remote net.IP, peerMTU int) error {
	mtu, err := underlayMTUForHeader(int(t.effectiveMTU.Load()), remote, packetHeaderSize(t.cfg.Protocol))
	if err != nil {
		return err
	}
	if peerMTU < mtu {
		mtu = peerMTU
	}
	if mtu == int(t.effectiveMTU.Load()) {
		return nil
	}
	ip, _, _ := net.ParseCIDR(t.cfg.LocalAddress)
	if ip.To4() == nil && mtu < 1280 {
		return fmt.Errorf("underlay cannot carry IPv6 tunnel MTU 1280")
	}
	names := []string{t.dev.Name()}
	if v, ok := t.dev.(*vethTun); ok {
		names = append(names, v.epName)
	}
	for _, name := range names {
		if _, err := runIP("link", "set", name, "mtu", fmt.Sprint(mtu)); err != nil {
			return err
		}
	}
	t.effectiveMTU.Store(int64(mtu))
	return nil
}