fix(firewall): iptables support detection

- Add dummy rule to `INPUT` to test for iptables support
- This may resolve #896

internal/firewall/firewall.go

@@ -49,7 +49,12 @@ type Config struct { //nolint:maligned
 func NewConfig(ctx context.Context, logger Logger,
 	runner command.Runner, defaultRoutes []routing.DefaultRoute,
 	localNetworks []routing.LocalNetwork) (config *Config, err error) {
-	iptables, err := findIptablesSupported(ctx, runner)
+	iptables, err := checkIptablesSupport(ctx, runner, "iptables", "iptables-nft")
+	if err != nil {
+		return nil, err
+	}
+
+	ip6tables, err := findIP6tablesSupported(ctx, runner)
 	if err != nil {
 		return nil, err
 	}
@@ -59,7 +64,7 @@ func NewConfig(ctx context.Context, logger Logger,
 		logger:            logger,
 		allowedInputPorts: make(map[uint16]map[string]struct{}),
 		ipTables:          iptables,
-		ip6Tables:         findIP6tablesSupported(ctx, runner),
+		ip6Tables:         ip6tables,
 		customRulesPath:   "/iptables/post-rules.txt",
 		// Obtained from routing
 		defaultRoutes: defaultRoutes,

internal/firewall/ip6tables.go

@@ -14,23 +14,14 @@ import (
 // and returns the iptables path that is supported. If none work, an
 // empty string path is returned.
 func findIP6tablesSupported(ctx context.Context, runner command.Runner) (
-	ip6tablesPath string) {
-	binsToTry := []string{"ip6tables", "ip6tables-nft"}
-
-	var err error
-	for _, ip6tablesPath = range binsToTry {
-		cmd := exec.CommandContext(ctx, ip6tablesPath, "-L")
-		_, err = runner.Run(cmd)
-		if err == nil {
-			break
-		}
+	ip6tablesPath string, err error) {
+	ip6tablesPath, err = checkIptablesSupport(ctx, runner, "ip6tables", "ip6tables-nft")
+	if errors.Is(err, ErrIPTablesNotSupported) {
+		return "", nil
+	} else if err != nil {
+		return "", err
 	}
-
-	if err != nil {
-		return ""
-	}
-
-	return ip6tablesPath
+	return ip6tablesPath, nil
 }
 
 func (c *Config) runIP6tablesInstructions(ctx context.Context, instructions []string) error {

internal/firewall/iptables.go

@@ -15,41 +15,11 @@ import (
 )
 
 var (
-	ErrIPTablesNotSupported    = errors.New("no iptables supported found")
-	ErrNetAdminMissing         = errors.New("NET_ADMIN capability is missing")
 	ErrIPTablesVersionTooShort = errors.New("iptables version string is too short")
 	ErrPolicyUnknown           = errors.New("unknown policy")
 	ErrNeedIP6Tables           = errors.New("ip6tables is required, please upgrade your kernel to support it")
 )
 
-func findIptablesSupported(ctx context.Context, runner command.Runner) (iptablesPath string, err error) {
-	binsToTry := []string{"iptables", "iptables-nft"}
-
-	var errMessage string
-	for _, iptablesPath = range binsToTry {
-		cmd := exec.CommandContext(ctx, iptablesPath, "-L")
-		errMessage, err = runner.Run(cmd)
-		if err == nil {
-			break
-		}
-
-		const permissionDeniedString = "Permission denied (you must be root)"
-		if strings.Contains(errMessage, permissionDeniedString) {
-			return "", fmt.Errorf("%w: %s (%s)", ErrNetAdminMissing, errMessage, err)
-		}
-
-		errMessage = fmt.Sprintf("%s (%s)", errMessage, err)
-	}
-
-	if err != nil {
-		return "", fmt.Errorf("%w: from %s: last error is: %s",
-			ErrIPTablesNotSupported, strings.Join(binsToTry, ", "),
-			errMessage)
-	}
-
-	return iptablesPath, nil
-}
-
 func appendOrDelete(remove bool) string {
 	if remove {
 		return "--delete"

internal/firewall/support.go

@@ -0,0 +1,63 @@
+package firewall
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"math/rand"
+	"os/exec"
+	"strings"
+
+	"github.com/qdm12/golibs/command"
+)
+
+var (
+	ErrNetAdminMissing      = errors.New("NET_ADMIN capability is missing")
+	ErrTestRuleCleanup      = errors.New("failed cleaning up test rule")
+	ErrIPTablesNotSupported = errors.New("no iptables supported found")
+)
+
+func checkIptablesSupport(ctx context.Context, runner command.Runner,
+	iptablesPathsToTry ...string) (iptablesPath string, err error) {
+	var errMessage string
+	testInterfaceName := randomInterfaceName()
+	for _, iptablesPath = range iptablesPathsToTry {
+		cmd := exec.CommandContext(ctx, iptablesPath, "-A", "INPUT", "-i", testInterfaceName, "-j", "DROP")
+		errMessage, err = runner.Run(cmd)
+		if err == nil {
+			break
+		}
+
+		const permissionDeniedString = "Permission denied (you must be root)"
+		if strings.Contains(errMessage, permissionDeniedString) {
+			return "", fmt.Errorf("%w: %s (%s)", ErrNetAdminMissing, errMessage, err)
+		}
+		errMessage = fmt.Sprintf("%s (%s)", errMessage, err)
+	}
+
+	if err != nil { // all iptables to try failed
+		return "", fmt.Errorf("%w: from %s: last error is: %s",
+			ErrIPTablesNotSupported, strings.Join(iptablesPathsToTry, ", "),
+			errMessage)
+	}
+
+	// Cleanup test rule
+	cmd := exec.CommandContext(ctx, iptablesPath, "-D", "INPUT", "-i", testInterfaceName, "-j", "DROP")
+	errMessage, err = runner.Run(cmd)
+	if err != nil {
+		return "", fmt.Errorf("%w: %s (%s)", ErrTestRuleCleanup, errMessage, err)
+	}
+
+	return iptablesPath, nil
+}
+
+func randomInterfaceName() (interfaceName string) {
+	const size = 15
+	letterRunes := []rune("abcdefghijklmnopqrstuvwxyz0123456789")
+	b := make([]rune, size)
+	for i := range b {
+		letterIndex := rand.Intn(len(letterRunes)) //nolint:gosec
+		b[i] = letterRunes[letterIndex]
+	}
+	return string(b)
+}