builder: call Comm.Prepare before resetting host

builder/qemu/comm_config.go

@@ -3,6 +3,7 @@ package qemu
 
 import (
 	"errors"
+	"log"
 
 	"github.com/hashicorp/packer-plugin-sdk/communicator"
 	"github.com/hashicorp/packer-plugin-sdk/template/interpolate"
@@ -46,11 +47,6 @@ func (c *CommConfig) Prepare(ctx *interpolate.Context) (warnings []string, errs
 		c.HostPortMax = c.SSHHostPortMax
 	}
 
-	if c.Comm.Host() == "" && c.SkipNatMapping {
-		c.Comm.SSHHost = "127.0.0.1"
-		c.Comm.WinRMHost = "127.0.0.1"
-	}
-
 	if c.HostPortMin == 0 {
 		c.HostPortMin = 2222
 	}
@@ -60,6 +56,13 @@ func (c *CommConfig) Prepare(ctx *interpolate.Context) (warnings []string, errs
 	}
 
 	errs = c.Comm.Prepare(ctx)
+
+	if c.Comm.Host() == "" && c.SkipNatMapping {
+		log.Printf("Resetting ssh host to 127.0.0.1")
+		c.Comm.SSHHost = "127.0.0.1"
+		c.Comm.WinRMHost = "127.0.0.1"
+	}
+
 	if c.HostPortMin > c.HostPortMax {
 		errs = append(errs,
 			errors.New("host_port_min must be less than host_port_max"))

builder/qemu/comm_config_test.go

@@ -70,6 +70,54 @@ func TestCommConfigPrepare_SSHHostPort(t *testing.T) {
 	if len(warns) != 0 {
 		t.Fatal("should not have any warnings")
 	}
+
+	tc := []struct {
+		name               string
+		host, expectedHost string
+		skipNat            bool
+	}{
+		{
+			name:         "skip_nat_mapping false should not change host",
+			host:         "192.168.1.1",
+			expectedHost: "192.168.1.1",
+		},
+		{
+			name:         "skip_nat_mapping true should not change host",
+			host:         "192.168.1.1",
+			expectedHost: "192.168.1.1",
+			skipNat:      true,
+		},
+		{
+			name:         "skip_nat_mapping true with no set host",
+			expectedHost: "127.0.0.1",
+			skipNat:      true,
+		},
+	}
+
+	for _, tt := range tc {
+		tt := tt
+		t.Run(tt.name, func(t *testing.T) {
+			c := &CommConfig{
+				SkipNatMapping: tt.skipNat,
+				Comm: communicator.Config{
+					SSH: communicator.SSH{
+						SSHUsername: "tester",
+						SSHHost:     tt.host,
+					},
+				},
+			}
+			warns, errs := c.Prepare(interpolate.NewContext())
+			if len(errs) > 0 {
+				t.Fatalf("should not have error: %s", errs)
+			}
+			if len(warns) != 0 {
+				t.Fatal("should not have any warnings")
+			}
+			if c.Comm.SSHHost != tt.expectedHost {
+				t.Errorf("unexpected SSHHost: wanted: %s, got: %s", tt.expectedHost, c.Comm.SSHHost)
+			}
+		})
+	}
 }
 
 func TestCommConfigPrepare_SSHPrivateKey(t *testing.T) {

builder/qemu/step_run.go

@@ -112,7 +112,7 @@ func (s *stepRun) getDefaultArgs(config *Config, state multistep.StateBag) map[s
 	// Configure "-netdev" arguments
 	defaultArgs["-netdev"] = fmt.Sprintf("bridge,id=user.0,br=%s", config.NetBridge)
 	if config.NetBridge == "" {
-		defaultArgs["-netdev"] = fmt.Sprintf("user,id=user.0")
+		defaultArgs["-netdev"] = "user,id=user.0"
 		if config.CommConfig.Comm.Type != "none" {
 			commHostPort := state.Get("commHostPort").(int)
 			defaultArgs["-netdev"] = fmt.Sprintf("user,id=user.0,hostfwd=tcp::%v-:%d", commHostPort, config.CommConfig.Comm.Port())