added changelog entry

Fix a regression introduced in Nomad 1.9.0 where CNI bridge networking with
IPv6-only interfaces would fail with 'no interface with an address' error.

The issue was that while the code correctly populated the AddressIPv6 field
for IPv6 addresses, several validation checks only examined the Address field
(IPv4), causing IPv6-only configurations to be rejected.

Changes:
- Update interface selection logic in cniToAllocNet to accept interfaces with
  either IPv4 or IPv6 addresses (not just IPv4)
- Update fallback logic to check both Address and AddressIPv6 fields
- Update error check to only fail when both IPv4 and IPv6 are missing
- Update AllocNetworkStatus.IsZero() to check AddressIPv6 field

This allows CNI configurations with IPv6-only interfaces to work correctly,
restoring functionality from Nomad 1.8.x.

Fixes #26905

.changelog/26910.txt

@@ -0,0 +1,3 @@
+```release-note:bug
+client: Fixed CNI bridge networking failure on IPv6-only interfaces
+```

client/allocrunner/networking_cni.go

@@ -480,8 +480,8 @@ func (c *cniNetworkConfigurator) cniToAllocNet(res *cni.Result) (*structs.AllocN
 				}
 			}
 
-			// found a good interface, so we're done
-			if netStatus.Address != "" {
+			// found a good interface (with either IPv4 or IPv6), so we're done
+			if netStatus.Address != "" || netStatus.AddressIPv6 != "" {
 				netStatus.InterfaceName = name
 				return
 			}
@@ -493,7 +493,7 @@ func (c *cniNetworkConfigurator) cniToAllocNet(res *cni.Result) (*structs.AllocN
 
 	// If no IP address was found, use the first interface with an address
 	// found as a fallback
-	if netStatus.Address == "" {
+	if netStatus.Address == "" && netStatus.AddressIPv6 == "" {
 		setStatus(false)
 		c.logger.Debug("no sandbox interface with an address found CNI result, using first available",
 			"interface", netStatus.InterfaceName,
@@ -501,8 +501,8 @@ func (c *cniNetworkConfigurator) cniToAllocNet(res *cni.Result) (*structs.AllocN
 		)
 	}
 
-	// If no IP address could be found, return an error
-	if netStatus.Address == "" {
+	// If no IP address (IPv4 or IPv6) could be found, return an error
+	if netStatus.Address == "" && netStatus.AddressIPv6 == "" {
 		return nil, fmt.Errorf("failed to configure network: no interface with an address")
 
 	}

client/allocrunner/networking_cni_test.go

@@ -498,6 +498,34 @@ func TestCNI_cniToAllocNet_Dualstack(t *testing.T) {
 	test.Eq(t, "eth0", allocNet.InterfaceName)
 }
 
+// TestCNI_cniToAllocNet_IPv6Only asserts that CNI results containing only IPv6
+// addresses work correctly. This is a regression test for a bug introduced in
+// GH-23882 where IPv6-only interfaces would fail with "no interface with an address".
+func TestCNI_cniToAllocNet_IPv6Only(t *testing.T) {
+	ci.Parallel(t)
+
+	cniResult := &cni.Result{
+		Interfaces: map[string]*cni.Config{
+			"eth0": {
+				Sandbox: "nomad-sandbox",
+				IPConfigs: []*cni.IPConfig{
+					{IP: net.ParseIP("fd00:a110:c8::b")}, // only IPv6
+				},
+			},
+		},
+	}
+
+	c := &cniNetworkConfigurator{
+		logger: testlog.HCLogger(t),
+	}
+	allocNet, err := c.cniToAllocNet(cniResult)
+	must.NoError(t, err)
+	must.NotNil(t, allocNet)
+	test.Eq(t, "", allocNet.Address)
+	test.Eq(t, "fd00:a110:c8::b", allocNet.AddressIPv6)
+	test.Eq(t, "eth0", allocNet.InterfaceName)
+}
+
 func TestCNI_addCustomCNIArgs(t *testing.T) {
 	ci.Parallel(t)
 	cniArgs := map[string]string{

nomad/structs/structs.go

@@ -12292,7 +12292,7 @@ func (a *AllocNetworkStatus) IsZero() bool {
 	if a == nil {
 		return true
 	}
-	if a.InterfaceName != "" || a.Address != "" {
+	if a.InterfaceName != "" || a.Address != "" || a.AddressIPv6 != "" {
 		return false
 	}
 	if !a.DNS.IsZero() {