fix(device): report the configured macOS hostname, not kern.hostname

CHANGELOG.md

@@ -7,6 +7,12 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 See [VERSIONING.md](VERSIONING.md) for why the version starts at 1.8.1.
 
+## [Unreleased]
+
+### Fixed
+
+- **macOS hostname no longer flips with the network.** The agent reported `kern.hostname`, which macOS rewrites from DHCP or reverse DNS whenever `HostName` is unset, so a Mac on a VPN resolving through AWS showed up as `ip-…ec2.internal` on some scans and its real name on others. It now reports the configured `HostName` when set, else `LocalHostName`, read from the SystemConfiguration preferences plist with `scutil --get` as the fallback, and only then the kernel hostname. Existing Macs will show a new name once.
+
 ## [1.17.0] - 2026-09-24
 
 ### Added

internal/device/device.go

@@ -3,16 +3,18 @@ package device
 import (
 	"context"
 	"strings"
+	"time"
 
 	"github.com/step-security/dev-machine-guard/internal/executor"
 	"github.com/step-security/dev-machine-guard/internal/model"
+	"howett.net/plist"
 )
 
 // Gather collects device information (hostname, serial, OS version, user identity).
 func Gather(ctx context.Context, exec executor.Executor) model.Device {
-	hostname, _ := exec.Hostname()
-	userIdentity := getDeveloperIdentity(exec)
 	platform := exec.GOOS()
+	hostname := getHostname(ctx, exec, platform)
+	userIdentity := getDeveloperIdentity(exec)
 
 	var serial, osVersion string
 	switch platform {
@@ -54,6 +56,52 @@ func SerialNumber(ctx context.Context, exec executor.Executor) string {
 	}
 }
 
+// darwinSCPrefsPath is where SystemConfiguration stores the names scutil
+// reports. Reading it skips a subprocess; its layout is not a documented
+// contract, so scutil remains the fallback.
+const darwinSCPrefsPath = "/Library/Preferences/SystemConfiguration/preferences.plist"
+
+type darwinSCPrefs struct {
+	System struct {
+		System struct {
+			HostName string `plist:"HostName"`
+		} `plist:"System"`
+		Network struct {
+			HostNames struct {
+				LocalHostName string `plist:"LocalHostName"`
+			} `plist:"HostNames"`
+		} `plist:"Network"`
+	} `plist:"System"`
+}
+
+// getHostname prefers the macOS configured names over os.Hostname: with no
+// HostName set, macOS rewrites kern.hostname from DHCP or reverse DNS on every
+// network change, so a VPN can turn "dev-mac" into "ip-10-0-1-5.ec2.internal".
+func getHostname(ctx context.Context, exec executor.Executor, platform string) string {
+	if platform == model.PlatformDarwin {
+		if data, err := exec.ReadFile(darwinSCPrefsPath); err == nil {
+			var prefs darwinSCPrefs
+			if _, err := plist.Unmarshal(data, &prefs); err == nil {
+				for _, name := range []string{prefs.System.System.HostName, prefs.System.Network.HostNames.LocalHostName} {
+					if name = strings.TrimSpace(name); name != "" {
+						return name
+					}
+				}
+			}
+		}
+		for _, key := range []string{"HostName", "LocalHostName"} {
+			stdout, _, exitCode, err := exec.RunWithTimeout(ctx, 10*time.Second, "scutil", "--get", key)
+			if err == nil && exitCode == 0 {
+				if name := strings.TrimSpace(stdout); name != "" {
+					return name
+				}
+			}
+		}
+	}
+	hostname, _ := exec.Hostname()
+	return hostname
+}
+
 // getSerialNumberWindows and getOSVersionWindows are implemented in
 // device_windows.go (native API) and device_other.go (stub).
 

internal/device/device_test.go

@@ -6,6 +6,7 @@ import (
 	"testing"
 
 	"github.com/step-security/dev-machine-guard/internal/executor"
+	"howett.net/plist"
 )
 
 func TestGather_BasicFields(t *testing.T) {
@@ -240,3 +241,78 @@ func TestGather_Windows(t *testing.T) {
 		t.Errorf("user_identity: expected testuser, got %s", dev.UserIdentity)
 	}
 }
+
+func TestGetHostname(t *testing.T) {
+	tests := []struct {
+		name      string
+		goos      string
+		hostName  *string // nil = scutil reports "not set"
+		localName *string
+		want      string
+	}{
+		{"darwin prefers explicit HostName", "darwin", ptr("build-box"), ptr("dev-mac"), "build-box"},
+		{"darwin falls back to LocalHostName", "darwin", nil, ptr("dev-mac"), "dev-mac"},
+		{"darwin falls back to kernel hostname", "darwin", nil, nil, "ip-10-0-1-5.ec2.internal"},
+		{"darwin ignores blank scutil output", "darwin", ptr("  \n"), ptr("dev-mac"), "dev-mac"},
+		{"linux uses kernel hostname", "linux", ptr("build-box"), ptr("dev-mac"), "ip-10-0-1-5.ec2.internal"},
+	}
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			mock := executor.NewMock()
+			mock.SetGOOS(tc.goos)
+			mock.SetHostname("ip-10-0-1-5.ec2.internal")
+			for key, val := range map[string]*string{"HostName": tc.hostName, "LocalHostName": tc.localName} {
+				if val == nil {
+					mock.SetCommand("", key+": not set\n", 1, "scutil", "--get", key)
+				} else {
+					mock.SetCommand(*val+"\n", "", 0, "scutil", "--get", key)
+				}
+			}
+			if got := getHostname(context.Background(), mock, tc.goos); got != tc.want {
+				t.Errorf("getHostname() = %q, want %q", got, tc.want)
+			}
+		})
+	}
+}
+
+func TestGetHostname_SCPrefsPlist(t *testing.T) {
+	prefs := func(hostName, localName string, format int) []byte {
+		t.Helper()
+		v := map[string]any{"System": map[string]any{
+			"System":  map[string]any{"HostName": hostName, "ComputerName": "Dev's Mac"},
+			"Network": map[string]any{"HostNames": map[string]any{"LocalHostName": localName}},
+		}}
+		data, err := plist.Marshal(v, format)
+		if err != nil {
+			t.Fatalf("marshal plist: %v", err)
+		}
+		return data
+	}
+	tests := []struct {
+		name string
+		file []byte // nil = file absent
+		want string
+	}{
+		{"xml HostName wins", prefs("build-box", "dev-mac", plist.XMLFormat), "build-box"},
+		{"binary LocalHostName when HostName unset", prefs("", "dev-mac", plist.BinaryFormat), "dev-mac"},
+		{"no names in plist falls back to scutil", prefs("", "", plist.XMLFormat), "scutil-name"},
+		{"garbage plist falls back to scutil", []byte("not a plist"), "scutil-name"},
+		{"missing plist falls back to scutil", nil, "scutil-name"},
+	}
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			mock := executor.NewMock()
+			mock.SetHostname("ip-10-0-1-5.ec2.internal")
+			if tc.file != nil {
+				mock.SetFile(darwinSCPrefsPath, tc.file)
+			}
+			mock.SetCommand("", "HostName: not set\n", 1, "scutil", "--get", "HostName")
+			mock.SetCommand("scutil-name\n", "", 0, "scutil", "--get", "LocalHostName")
+			if got := getHostname(context.Background(), mock, "darwin"); got != tc.want {
+				t.Errorf("getHostname() = %q, want %q", got, tc.want)
+			}
+		})
+	}
+}
+
+func ptr(s string) *string { return &s }