tailscale/util/linuxfw/linuxfw.go

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

// Package linuxfw returns the kind of firewall being used by the kernel.

//go:build linux

package linuxfw

import (
	"bytes"
	"errors"
	"fmt"
	"os"
	"os/exec"
	"strconv"
	"strings"

	"github.com/tailscale/netlink"
	"tailscale.com/types/logger"
)

// MatchDecision is the decision made by the firewall for a packet matched by a rule.
// It is used to decide whether to accept or masquerade a packet in addMatchSubnetRouteMarkRule.
type MatchDecision int

const (
	Accept MatchDecision = iota
	Masq
)

// The following bits are added to packet marks for Tailscale use.
//
// We tried to pick bits sufficiently out of the way that it's
// unlikely to collide with existing uses. We have 4 bytes of mark
// bits to play with. We leave the lower byte alone on the assumption
// that sysadmins would use those. Kubernetes uses a few bits in the
// second byte, so we steer clear of that too.
//
// Empirically, most of the documentation on packet marks on the
// internet gives the impression that the marks are 16 bits
// wide. Based on this, we theorize that the upper two bytes are
// relatively unused in the wild, and so we consume bits 16:23 (the
// third byte).
//
// The constants are in the iptables/iproute2 string format for
// matching and setting the bits, so they can be directly embedded in
// commands.
const (
	// The mask for reading/writing the 'firewall mask' bits on a packet.
	// See the comment on the const block on why we only use the third byte.
	//
	// We claim bits 16:23 entirely. For now we only use the lower four
	// bits, leaving the higher 4 bits for future use.
	TailscaleFwmarkMask    = "0xff0000"
	TailscaleFwmarkMaskNum = 0xff0000

	// Packet is from Tailscale and to a subnet route destination, so
	// is allowed to be routed through this machine.
	TailscaleSubnetRouteMark    = "0x40000"
	TailscaleSubnetRouteMarkNum = 0x40000

	// Packet was originated by tailscaled itself, and must not be
	// routed over the Tailscale network.
	TailscaleBypassMark    = "0x80000"
	TailscaleBypassMarkNum = 0x80000
)

// getTailscaleFwmarkMaskNeg returns the negation of TailscaleFwmarkMask in bytes.
func getTailscaleFwmarkMaskNeg() []byte {
	return []byte{0xff, 0x00, 0xff, 0xff}
}

// getTailscaleFwmarkMask returns the TailscaleFwmarkMask in bytes.
func getTailscaleFwmarkMask() []byte {
	return []byte{0x00, 0xff, 0x00, 0x00}
}

// getTailscaleSubnetRouteMark returns the TailscaleSubnetRouteMark in bytes.
func getTailscaleSubnetRouteMark() []byte {
	return []byte{0x00, 0x04, 0x00, 0x00}
}

// errCode extracts and returns the process exit code from err, or
// zero if err is nil.
func errCode(err error) int {
	if err == nil {
		return 0
	}
	var e *exec.ExitError
	if ok := errors.As(err, &e); ok {
		return e.ExitCode()
	}
	s := err.Error()
	if strings.HasPrefix(s, "exitcode:") {
		code, err := strconv.Atoi(s[9:])
		if err == nil {
			return code
		}
	}
	return -42
}

// checkIPv6 checks whether the system appears to have a working IPv6
// network stack. It returns an error explaining what looks wrong or
// missing.  It does not check that IPv6 is currently functional or
// that there's a global address, just that the system would support
// IPv6 if it were on an IPv6 network.
func checkIPv6(logf logger.Logf) error {
	_, err := os.Stat("/proc/sys/net/ipv6")
	if os.IsNotExist(err) {
		return err
	}
	bs, err := os.ReadFile("/proc/sys/net/ipv6/conf/all/disable_ipv6")
	if err != nil {
		// Be conservative if we can't find the IPv6 configuration knob.
		return err
	}
	disabled, err := strconv.ParseBool(strings.TrimSpace(string(bs)))
	if err != nil {
		return errors.New("disable_ipv6 has invalid bool")
	}
	if disabled {
		return errors.New("disable_ipv6 is set")
	}

	// Older kernels don't support IPv6 policy routing. Some kernels
	// support policy routing but don't have this knob, so absence of
	// the knob is not fatal.
	bs, err = os.ReadFile("/proc/sys/net/ipv6/conf/all/disable_policy")
	if err == nil {
		disabled, err = strconv.ParseBool(strings.TrimSpace(string(bs)))
		if err != nil {
			return errors.New("disable_policy has invalid bool")
		}
		if disabled {
			return errors.New("disable_policy is set")
		}
	}

	if err := CheckIPRuleSupportsV6(logf); err != nil {
		return fmt.Errorf("kernel doesn't support IPv6 policy routing: %w", err)
	}

	return nil
}

// checkSupportsV6NAT returns whether the system has a "nat" table in the
// IPv6 netfilter stack.
//
// The nat table was added after the initial release of ipv6
// netfilter, so some older distros ship a kernel that can't NAT IPv6
// traffic.
func checkSupportsV6NAT() bool {
	bs, err := os.ReadFile("/proc/net/ip6_tables_names")
	if err != nil {
		// Can't read the file. Assume SNAT works.
		return true
	}
	if bytes.Contains(bs, []byte("nat\n")) {
		return true
	}
	// In nftables mode, that proc file will be empty. Try another thing:
	if exec.Command("modprobe", "ip6table_nat").Run() == nil {
		return true
	}
	return false
}

func CheckIPRuleSupportsV6(logf logger.Logf) error {
	// First try just a read-only operation to ideally avoid
	// having to modify any state.
	if rules, err := netlink.RuleList(netlink.FAMILY_V6); err != nil {
		return fmt.Errorf("querying IPv6 policy routing rules: %w", err)
	} else {
		if len(rules) > 0 {
			logf("[v1] kernel supports IPv6 policy routing (found %d rules)", len(rules))
			return nil
		}
	}

	// Try to actually create & delete one as a test.
	rule := netlink.NewRule()
	rule.Priority = 1234
	rule.Mark = TailscaleBypassMarkNum
	rule.Table = 52
	rule.Family = netlink.FAMILY_V6
	// First delete the rule unconditionally, and don't check for
	// errors. This is just cleaning up anything that might be already
	// there.
	netlink.RuleDel(rule)
	// And clean up on exit.
	defer netlink.RuleDel(rule)
	return netlink.RuleAdd(rule)
}