Only enable bridge netfiltering when needed

Kernel module br_netfilter is loaded when the daemon starts with
either iptables or ip6tables enabled. That automatically sets:
  net.bridge.bridge-nf-call-arptables = 1
  net.bridge.bridge-nf-call-ip6tables = 1
  net.bridge.bridge-nf-call-iptables = 1

So, when:
- docker was running happily with iptables=false, and
- no explicit ip6tables=false, and
- br_netfilter was not loaded
... the change in moby 27.0 to enable ip6tables by default, resulted
in net.bridge.bridge-nf-call-iptables being enabled.

If the host also had a firewall with default-drop on its forward
chain - that resulted in packets getting dropped between containers
on a bridge network.

So, only try to load br_netfilter when it's needed - it's only needed
to implement "--icc=false", which can only be used when iptables or
ip6tables is enabled.

Signed-off-by: Rob Murray <rob.murray@docker.com>
(cherry picked from commit db25b0dcd0)
Signed-off-by: Rob Murray <rob.murray@docker.com>

libnetwork/drivers/bridge/bridge_linux.go

@@ -5,8 +5,6 @@ import (
 	"fmt"
 	"net"
 	"net/netip"
-	"os"
-	"os/exec"
 	"strconv"
 	"sync"
 
@@ -482,14 +480,6 @@ func (d *driver) configure(option map[string]interface{}) error {
 		return &ErrInvalidDriverConfig{}
 	}
 
-	if config.EnableIPTables || config.EnableIP6Tables {
-		if _, err := os.Stat("/proc/sys/net/bridge"); err != nil {
-			if out, err := exec.Command("modprobe", "-va", "bridge", "br_netfilter").CombinedOutput(); err != nil {
-				log.G(context.TODO()).Warnf("Running modprobe bridge br_netfilter failed with message: %s, error: %v", out, err)
-			}
-		}
-	}
-
 	if config.EnableIPTables {
 		removeIPChains(iptables.IPv4)
 

libnetwork/drivers/bridge/setup_bridgenetfiltering.go

@@ -7,6 +7,7 @@ import (
 	"errors"
 	"fmt"
 	"os"
+	"os/exec"
 	"syscall"
 
 	"github.com/containerd/log"
@@ -46,6 +47,12 @@ func setupIPv6BridgeNetFiltering(config *networkConfiguration, _ *bridgeInterfac
 
 // Enable bridge net filtering if not already enabled. See GitHub issue #11404
 func enableBridgeNetFiltering(nfParam string) error {
+	if _, err := os.Stat("/proc/sys/net/bridge"); err != nil {
+		if out, err := exec.Command("modprobe", "-va", "bridge", "br_netfilter").CombinedOutput(); err != nil {
+			log.G(context.TODO()).WithError(err).Errorf("Running modprobe bridge br_netfilter failed with message: %s", out)
+			return fmt.Errorf("cannot restrict inter-container communication: modprobe br_netfilter failed: %w", err)
+		}
+	}
 	enabled, err := getKernelBoolParam(nfParam)
 	if err != nil {
 		var pathErr *os.PathError