go-iptables/iptables/iptables.go

// Copyright 2015 CoreOS, Inc.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

package iptables

import (
	"bytes"
	"fmt"
	"io"
	"os/exec"
	"regexp"
	"strconv"
	"strings"
	"syscall"
)

// Adds the output of stderr to exec.ExitError
type Error struct {
	exec.ExitError
	msg string
}

func (e *Error) ExitStatus() int {
	return e.Sys().(syscall.WaitStatus).ExitStatus()
}

func (e *Error) Error() string {
	return fmt.Sprintf("exit status %v: %v", e.ExitStatus(), e.msg)
}

// Protocol to differentiate between IPv4 and IPv6
type Protocol byte

const (
	ProtocolIPv4 Protocol = iota
	ProtocolIPv6
)

type IPTables struct {
	path     string
	proto    Protocol
	hasCheck bool
	hasWait  bool
}

// New creates a new IPTables.
// For backwards compatibility, this always uses IPv4, i.e. "iptables".
func New() (*IPTables, error) {
	return NewWithProtocol(ProtocolIPv4)
}

// New creates a new IPTables for the given proto.
// The proto will determine which command is used, either "iptables" or "ip6tables".
func NewWithProtocol(proto Protocol) (*IPTables, error) {
	path, err := exec.LookPath(getIptablesCommand(proto))
	if err != nil {
		return nil, err
	}
	checkPresent, waitPresent, err := getIptablesCommandSupport(path)
	if err != nil {
		return nil, fmt.Errorf("error checking iptables version: %v", err)
	}
	ipt := IPTables{
		path:     path,
		proto:    proto,
		hasCheck: checkPresent,
		hasWait:  waitPresent,
	}
	return &ipt, nil
}

// Proto returns the protocol used by this IPTables.
func (ipt *IPTables) Proto() Protocol {
	return ipt.proto
}

// Exists checks if given rulespec in specified table/chain exists
func (ipt *IPTables) Exists(table, chain string, rulespec ...string) (bool, error) {
	if !ipt.hasCheck {
		return ipt.existsForOldIptables(table, chain, rulespec)

	}
	cmd := append([]string{"-t", table, "-C", chain}, rulespec...)
	err := ipt.run(cmd...)
	eerr, eok := err.(*Error)
	switch {
	case err == nil:
		return true, nil
	case eok && eerr.ExitStatus() == 1:
		return false, nil
	default:
		return false, err
	}
}

// Insert inserts rulespec to specified table/chain (in specified pos)
func (ipt *IPTables) Insert(table, chain string, pos int, rulespec ...string) error {
	cmd := append([]string{"-t", table, "-I", chain, strconv.Itoa(pos)}, rulespec...)
	return ipt.run(cmd...)
}

// Append appends rulespec to specified table/chain
func (ipt *IPTables) Append(table, chain string, rulespec ...string) error {
	cmd := append([]string{"-t", table, "-A", chain}, rulespec...)
	return ipt.run(cmd...)
}

// AppendUnique acts like Append except that it won't add a duplicate
func (ipt *IPTables) AppendUnique(table, chain string, rulespec ...string) error {
	exists, err := ipt.Exists(table, chain, rulespec...)
	if err != nil {
		return err
	}

	if !exists {
		return ipt.Append(table, chain, rulespec...)
	}

	return nil
}

// Delete removes rulespec in specified table/chain
func (ipt *IPTables) Delete(table, chain string, rulespec ...string) error {
	cmd := append([]string{"-t", table, "-D", chain}, rulespec...)
	return ipt.run(cmd...)
}

// List rules in specified table/chain
func (ipt *IPTables) List(table, chain string) ([]string, error) {
	args := []string{"-t", table, "-S", chain}
	return ipt.executeList(args)
}

// ListChains returns a slice containing the name of each chain in the specified table.
func (ipt *IPTables) ListChains(table string) ([]string, error) {
	args := []string{"-t", table, "-S"}

	result, err := ipt.executeList(args)
	if err != nil {
		return nil, err
	}

	// Iterate over rules to find all default (-P) and user-specified (-N) chains.
	// Chains definition always come before rules.
	// Format is the following:
	// -P OUTPUT ACCEPT
	// -N Custom
	var chains []string
	for _, val := range result {
		if strings.HasPrefix(val, "-P") || strings.HasPrefix(val, "-N") {
			chains = append(chains, strings.Fields(val)[1])
		} else {
			break
		}
	}
	return chains, nil
}

func (ipt *IPTables) executeList(args []string) ([]string, error) {
	var stdout bytes.Buffer
	if err := ipt.runWithOutput(args, &stdout); err != nil {
		return nil, err
	}

	rules := strings.Split(stdout.String(), "\n")
	if len(rules) > 0 && rules[len(rules)-1] == "" {
		rules = rules[:len(rules)-1]
	}

	return rules, nil
}

// NewChain creates a new chain in the specified table.
// If the chain already exists, it will result in an error.
func (ipt *IPTables) NewChain(table, chain string) error {
	return ipt.run("-t", table, "-N", chain)
}

// ClearChain flushed (deletes all rules) in the specified table/chain.
// If the chain does not exist, a new one will be created
func (ipt *IPTables) ClearChain(table, chain string) error {
	err := ipt.NewChain(table, chain)

	eerr, eok := err.(*Error)
	switch {
	case err == nil:
		return nil
	case eok && eerr.ExitStatus() == 1:
		// chain already exists. Flush (clear) it.
		return ipt.run("-t", table, "-F", chain)
	default:
		return err
	}
}

// RenameChain renames the old chain to the new one.
func (ipt *IPTables) RenameChain(table, oldChain, newChain string) error {
	return ipt.run("-t", table, "-E", oldChain, newChain)
}

// DeleteChain deletes the chain in the specified table.
// The chain must be empty
func (ipt *IPTables) DeleteChain(table, chain string) error {
	return ipt.run("-t", table, "-X", chain)
}

// run runs an iptables command with the given arguments, ignoring
// any stdout output
func (ipt *IPTables) run(args ...string) error {
	return ipt.runWithOutput(args, nil)
}

// runWithOutput runs an iptables command with the given arguments,
// writing any stdout output to the given writer
func (ipt *IPTables) runWithOutput(args []string, stdout io.Writer) error {
	args = append([]string{ipt.path}, args...)
	if ipt.hasWait {
		args = append(args, "--wait")
	} else {
		fmu, err := newXtablesFileLock()
		if err != nil {
			return err
		}
		ul, err := fmu.tryLock()
		if err != nil {
			return err
		}
		defer ul.Unlock()
	}

	var stderr bytes.Buffer
	cmd := exec.Cmd{
		Path:   ipt.path,
		Args:   args,
		Stdout: stdout,
		Stderr: &stderr,
	}

	if err := cmd.Run(); err != nil {
		return &Error{*(err.(*exec.ExitError)), stderr.String()}
	}

	return nil
}

// getIptablesCommand returns the correct command for the given protocol, either "iptables" or "ip6tables".
func getIptablesCommand(proto Protocol) string {
	if proto == ProtocolIPv6 {
		return "ip6tables"
	} else {
		return "iptables"
	}
}

// Checks if iptables has the "-C" and "--wait" flag
func getIptablesCommandSupport(path string) (bool, bool, error) {
	vstring, err := getIptablesVersionString(path)
	if err != nil {
		return false, false, err
	}

	v1, v2, v3, err := extractIptablesVersion(vstring)
	if err != nil {
		return false, false, err
	}

	return iptablesHasCheckCommand(v1, v2, v3), iptablesHasWaitCommand(v1, v2, v3), nil
}

// getIptablesVersion returns the first three components of the iptables version.
// e.g. "iptables v1.3.66" would return (1, 3, 66, nil)
func extractIptablesVersion(str string) (int, int, int, error) {
	versionMatcher := regexp.MustCompile("v([0-9]+)\\.([0-9]+)\\.([0-9]+)")
	result := versionMatcher.FindStringSubmatch(str)
	if result == nil {
		return 0, 0, 0, fmt.Errorf("no iptables version found in string: %s", str)
	}

	v1, err := strconv.Atoi(result[1])
	if err != nil {
		return 0, 0, 0, err
	}

	v2, err := strconv.Atoi(result[2])
	if err != nil {
		return 0, 0, 0, err
	}

	v3, err := strconv.Atoi(result[3])
	if err != nil {
		return 0, 0, 0, err
	}

	return v1, v2, v3, nil
}

// Runs "iptables --version" to get the version string
func getIptablesVersionString(path string) (string, error) {
	cmd := exec.Command(path, "--version")
	var out bytes.Buffer
	cmd.Stdout = &out
	err := cmd.Run()
	if err != nil {
		return "", err
	}
	return out.String(), nil
}

// Checks if an iptables version is after 1.4.11, when --check was added
func iptablesHasCheckCommand(v1 int, v2 int, v3 int) bool {
	if v1 > 1 {
		return true
	}
	if v1 == 1 && v2 > 4 {
		return true
	}
	if v1 == 1 && v2 == 4 && v3 >= 11 {
		return true
	}
	return false
}

// Checks if an iptables version is after 1.4.20, when --wait was added
func iptablesHasWaitCommand(v1 int, v2 int, v3 int) bool {
	if v1 > 1 {
		return true
	}
	if v1 == 1 && v2 > 4 {
		return true
	}
	if v1 == 1 && v2 == 4 && v3 >= 20 {
		return true
	}
	return false
}

// Checks if a rule specification exists for a table
func (ipt *IPTables) existsForOldIptables(table, chain string, rulespec []string) (bool, error) {
	rs := strings.Join(append([]string{"-A", chain}, rulespec...), " ")
	args := []string{"-t", table, "-S"}
	var stdout bytes.Buffer
	err := ipt.runWithOutput(args, &stdout)
	if err != nil {
		return false, err
	}
	return strings.Contains(stdout.String(), rs), nil
}