ExternalName service cannot be used as default-backend

[kibs]

NGINX Ingress controller version:
0.33

Kubernetes version (use kubectl version):
Client Version: version.Info{Major:"1", Minor:"18", GitVersion:"v1.18.3", GitCommit:"2e7996e3e2712684bc73f0dec0200d64eec7fe40", GitTreeState:"clean", BuildDate:"2020-05-21T14:51:23Z", GoVersion:"go1.14.3", Compiler:"gc", Platform:"darwin/amd64"}
Server Version: version.Info{Major:"1", Minor:"16+", GitVersion:"v1.16.6-beta.0", GitCommit:"e7f962ba86f4ce7033828210ca3556393c377bcc", GitTreeState:"clean", BuildDate:"2020-01-15T08:18:29Z", GoVersion:"go1.13.5", Compiler:"gc", Platform:"linux/amd64"}

Environment:
Docker-For-Mac

What happened:

When setting "default-backend" annotation on an ingress to a service which points to an externalName, the ingress won't updated. If editing again, the annotation is gone. If i set it to a ClusterIP service, it works fine.

When setting the default-backend to my externalname service, the controller logs the following

│ controller 2020/07/06 19:34:43 http: panic serving 192.168.65.3:54926: runtime error: index out of range [0] with length 0                                                                                   │
│ controller goroutine 8668 [running]:                                                                                                                                                                         │
│ controller net/http.(*conn).serve.func1(0xc0001392c0)                                                                                                                                                        │
│ controller     /home/ubuntu/.gimme/versions/go1.14.2.linux.amd64/src/net/http/server.go:1772 +0x139                                                                                                          │
│ controller panic(0x176ed60, 0xc000893c60)                                                                                                                                                                    │
│ controller     /home/ubuntu/.gimme/versions/go1.14.2.linux.amd64/src/runtime/panic.go:975 +0x3e3                                                                                                             │
│ controller k8s.io/ingress-nginx/internal/ingress/controller.(*NGINXController).getBackendServers(0xc0003d55e0, 0xc0001b20e0, 0x4, 0x4, 0x187ca2a, 0x1b, 0x185e5a8, 0x0, 0x185e5a8, 0x0)                      │
│ controller     /tmp/go/src/k8s.io/ingress-nginx/internal/ingress/controller/controller.go:645 +0x2e6a                                                                                                        │
│ controller k8s.io/ingress-nginx/internal/ingress/controller.(*NGINXController).getConfiguration(0xc0003d55e0, 0xc0001b20e0, 0x4, 0x4, 0xc000194d00, 0xc00095e401, 0x40c626, 0xc00095e458, 0x118d44b)         │
│ controller     /tmp/go/src/k8s.io/ingress-nginx/internal/ingress/controller/controller.go:404 +0x80                                                                                                          │
│ controller k8s.io/ingress-nginx/internal/ingress/controller.(*NGINXController).CheckIngress(0xc0003d55e0, 0xc000a89b00, 0x4d0, 0x500)                                                                        │
│ controller     /tmp/go/src/k8s.io/ingress-nginx/internal/ingress/controller/controller.go:230 +0x2e2                                                                                                         │
│ controller k8s.io/ingress-nginx/internal/admission/controller.(*IngressAdmission).HandleAdmission(0xc00037aeb0, 0xc0008fd740)                                                                                │
│ controller     /tmp/go/src/k8s.io/ingress-nginx/internal/admission/controller/main.go:100 +0x8fe                                                                                                             │
│ controller k8s.io/ingress-nginx/internal/admission/controller.(*AdmissionControllerServer).ServeHTTP(0xc000117600, 0x1b054a0, 0xc00014c9a0, 0xc0009a3700)                                                    │
│ controller     /tmp/go/src/k8s.io/ingress-nginx/internal/admission/controller/server.go:68 +0x166                                                                                                            │
│ controller net/http.serverHandler.ServeHTTP(0xc0003d40e0, 0x1b054a0, 0xc00014c9a0, 0xc0009a3700)                                                                                                             │
│ controller     /home/ubuntu/.gimme/versions/go1.14.2.linux.amd64/src/net/http/server.go:2807 +0xa3                                                                                                           │
│ controller net/http.(*conn).serve(0xc0001392c0, 0x1b076a0, 0xc00047dac0)                                                                                                                                     │
│ controller     /home/ubuntu/.gimme/versions/go1.14.2.linux.amd64/src/net/http/server.go:1895 +0x86c                                                                                                          │
│ controller created by net/http.(*Server).Serve                                                                                                                                                               │
│ controller     /home/ubuntu/.gimme/versions/go1.14.2.linux.amd64/src/net/http/server.go:2933 +0x35c                                                                                                          │

What you expected to happen:

Would think the externalName service could be used as a default-backend for an Ingress

/kind bug

[aledbf]

Would think the externalName service could be used as a default-backend for an Ingress

Does it seem the service you are creating has no ports defined? Please make sure you have one (without it, the ingress controller does not know where to send the traffic)

[kibs]

Okay that makes sense, but an external name service cannot have ports defined. Thanks for clarifying, and thank you for the pull request

[aledbf]

, but an external name service cannot have ports defined.

Yes, it can have ports.