client: manually cleanup leaked iptables rules

This PR adds a secondary path for cleaning up iptables created for an allocation
when the normal CNI library fails to do so. This typically happens when the state
of the pause container is unexpected - e.g. deleted out of band from Nomad. Before,
the iptables rules would be leaked which could lead to unexpected nat routing
behavior later on (in addition to leaked resources). With this change, we scan
for the rules created on behalf of the allocation being GC'd and delete them.

Fixes #6385

.changelog/15407.txt

@@ -0,0 +1,3 @@
+```release-note:improvement
+client: detect and cleanup leaked iptables rules
+```

client/allocrunner/networking_cni.go

@@ -12,12 +12,14 @@ import (
 	"math/rand"
 	"os"
 	"path/filepath"
+	"regexp"
 	"sort"
 	"strings"
 	"time"
 
 	cni "github.com/containerd/go-cni"
 	cnilibrary "github.com/containernetworking/cni/libcni"
+	"github.com/coreos/go-iptables/iptables"
 	log "github.com/hashicorp/go-hclog"
 	"github.com/hashicorp/nomad/nomad/structs"
 	"github.com/hashicorp/nomad/plugins/drivers"
@@ -226,7 +228,95 @@ func (c *cniNetworkConfigurator) Teardown(ctx context.Context, alloc *structs.Al
 		return err
 	}
 
-	return c.cni.Remove(ctx, alloc.ID, spec.Path, cni.WithCapabilityPortMap(getPortMapping(alloc, c.ignorePortMappingHostIP)))
+	if err := c.cni.Remove(ctx, alloc.ID, spec.Path, cni.WithCapabilityPortMap(getPortMapping(alloc, c.ignorePortMappingHostIP))); err != nil {
+		// most likely the pause container was removed from underneath nomad
+		return c.forceCleanup(alloc.ID)
+	}
+
+	return nil
+}
+
+var (
+	// ipRuleRe is used to parse a postrouting iptables rule created by nomad, e.g.
+	//   -A POSTROUTING -s 172.26.64.191/32 -m comment --comment "name: \"nomad\" id: \"6b235529-8111-4bbe-520b-d639b1d2a94e\"" -j CNI-50e58ea77dc52e0c731e3799
+	ipRuleRe = regexp.MustCompile(`-A POSTROUTING -s (\S+) -m comment --comment "name: \\"nomad\\" id: \\"([[:xdigit:]-]+)\\"" -j (CNI-[[:xdigit:]]+)`)
+)
+
+// forceCleanup is the backup plan for removing the iptables rule and chain associated with
+// an allocation that was using bridge networking. The cni library refuses to handle a
+// dirty state - e.g. the pause container is removed out of band, and so we must cleanup
+// iptables ourselves to avoid leaking rules.
+func (c *cniNetworkConfigurator) forceCleanup(allocID string) error {
+	const (
+		natTable         = "nat"
+		postRoutingChain = "POSTROUTING"
+		commentFmt       = `--comment "name: \"nomad\" id: \"%s\""`
+	)
+
+	// create a handle to iptables
+	manipulator, err := iptables.New()
+	if err != nil {
+		return fmt.Errorf("failed to detect iptables: %w", err)
+	}
+
+	// list the rules on the POSTROUTING chain of the nat table
+	rules, err := manipulator.List(natTable, postRoutingChain)
+	if err != nil {
+		return fmt.Errorf("failed to list iptables rules: %w", err)
+	}
+
+	// find the POSTROUTING rule associated with our allocation
+	matcher := fmt.Sprintf(commentFmt, allocID)
+	var ruleToPurge string
+	for _, rule := range rules {
+		if strings.Contains(rule, matcher) {
+			ruleToPurge = rule
+			break
+		}
+	}
+
+	// no rule found for our allocation, just give up
+	if ruleToPurge == "" {
+		return fmt.Errorf("failed to find postrouting rule for alloc %s", allocID)
+	}
+
+	// re-create the rule we need to delete, as tokens
+	subs := ipRuleRe.FindStringSubmatch(ruleToPurge)
+	if len(subs) != 4 {
+		return fmt.Errorf("failed to parse postrouting rule for alloc %s", allocID)
+	}
+	cidr := subs[1]
+	id := subs[2]
+	chainID := subs[3]
+	toDel := []string{
+		`-s`,
+		cidr,
+		`-m`,
+		`comment`,
+		`--comment`,
+		`name: "nomad" id: "` + id + `"`,
+		`-j`,
+		chainID,
+	}
+
+	// remove the jump rule
+	ok := true
+	if err = manipulator.Delete(natTable, postRoutingChain, toDel...); err != nil {
+		c.logger.Warn("failed to remove iptables nat.POSTROUTING rule", "alloc_id", allocID, "chain", chainID, "error", err)
+		ok = false
+	}
+
+	// remote the associated chain
+	if err = manipulator.ClearAndDeleteChain(natTable, chainID); err != nil {
+		c.logger.Warn("failed to remove iptables nat chain", "chain", chainID, "error", err)
+		ok = false
+	}
+
+	if !ok {
+		return fmt.Errorf("failed to cleanup iptables rules for alloc %s", allocID)
+	}
+
+	return nil
 }
 
 func (c *cniNetworkConfigurator) ensureCNIInitialized() error {
@@ -240,7 +330,7 @@ func (c *cniNetworkConfigurator) ensureCNIInitialized() error {
 // getPortMapping builds a list of portMapping structs that are used as the
 // portmapping capability arguments for the portmap CNI plugin
 func getPortMapping(alloc *structs.Allocation, ignoreHostIP bool) []cni.PortMapping {
-	ports := []cni.PortMapping{}
+	var ports []cni.PortMapping
 
 	if len(alloc.AllocatedResources.Shared.Ports) == 0 && len(alloc.AllocatedResources.Shared.Networks) > 0 {
 		for _, network := range alloc.AllocatedResources.Shared.Networks {

client/allocrunner/networking_cni_test.go

@@ -1,19 +1,35 @@
 //go:build linux
-// +build linux
 
 package allocrunner
 
 import (
+	"fmt"
 	"net"
 	"testing"
 
 	cni "github.com/containerd/go-cni"
+	"github.com/coreos/go-iptables/iptables"
 	"github.com/hashicorp/nomad/ci"
 	"github.com/hashicorp/nomad/helper/testlog"
+	"github.com/shoenig/test/must"
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
 )
 
+func TestCNI_DeleteRule(t *testing.T) {
+	// rule := ""
+
+	manipulator, err := iptables.New()
+	must.NoError(t, err)
+
+	rules, err := manipulator.List("nat", "POSTROUTING")
+	must.NoError(t, err)
+
+	for i, s := range rules {
+		fmt.Printf("rule[%d]: %s\n", i, s)
+	}
+}
+
 // TestCNI_cniToAllocNet_Fallback asserts if a CNI plugin result lacks an IP on
 // its sandbox interface, the first IP found is used.
 func TestCNI_cniToAllocNet_Fallback(t *testing.T) {