fix: preserve all hostnames per IP and write hosts file atomically

CHANGELOG.md

@@ -6,7 +6,11 @@ adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
 ## [Unreleased]
 
-## [1.20.23] - 2026-06-12
+## [1.20.24] - 2026-06-12
+
+### Fixed
+- **Hosts entries with multiple hostnames per IP are no longer silently lost.** A line like `127.0.0.1 a b c` was read keeping only the first hostname, so editing and saving dropped the rest. Each hostname is now read as its own entry and survives a round trip.
+- **The hosts file is now written atomically.** Saving wrote directly over the file, so a crash mid-write could leave it empty or truncated. It now writes to a temporary file and atomically replaces the target, cleaning up the temp file afterward.
 
 ### Fixed
 - **No more leaked process handles when opening Explorer / links.** Ten places that launch Explorer ("show in folder"/"open file location"), Event Viewer, the browser, or the updater left the returned process handle undisposed. Each now releases it. The launched program is unaffected; only the orphaned handle is cleaned up. Covers Deep Cleanup, Disk Analyzer, Duplicate Finder, Startup Manager, Logs, About, and the Context Menu refresh.

SysManager/SysManager.Tests/HostsFileServiceTests.cs

@@ -93,4 +93,48 @@ public void RestoreBackup_NoBackup_ReturnsFalse()
             try { Directory.Delete(dir, recursive: true); } catch { }
         }
     }
+
+    [Fact]
+    public async Task ReadHostsAsync_MultipleHostnamesPerIp_AllPreserved()
+    {
+        // Regression (data loss): a line mapping one IP to several hostnames
+        // ("127.0.0.1  a  b  c") previously kept only the first hostname, so the
+        // others were dropped on a read→save round trip. Each must survive as its
+        // own entry.
+        var (svc, _, dir) = NewServiceWithTempHosts("127.0.0.1\talpha beta gamma\n");
+        try
+        {
+            var entries = await svc.ReadHostsAsync();
+            var hosts = entries.Where(e => e.IpAddress == "127.0.0.1").Select(e => e.Hostname).ToList();
+            Assert.Contains("alpha", hosts);
+            Assert.Contains("beta", hosts);
+            Assert.Contains("gamma", hosts);
+        }
+        finally
+        {
+            try { Directory.Delete(dir, recursive: true); } catch { }
+        }
+    }
+
+    [Fact]
+    public void SaveHosts_LeavesNoTempFileBehind()
+    {
+        // Regression (atomic write): SaveHosts writes to a temp file then moves it
+        // into place. After a successful save no ".sysmanager.tmp" must remain.
+        var (svc, hosts, dir) = NewServiceWithTempHosts("127.0.0.1 localhost\n");
+        try
+        {
+            svc.SaveHosts(new List<HostsEntry>
+            {
+                new() { IpAddress = "127.0.0.1", Hostname = "localhost", IsEnabled = true }
+            });
+            Assert.False(File.Exists(hosts + ".sysmanager.tmp"), "temp file was left behind after save");
+            Assert.True(File.Exists(hosts));
+            Assert.Contains("localhost", File.ReadAllText(hosts));
+        }
+        finally
+        {
+            try { Directory.Delete(dir, recursive: true); } catch { }
+        }
+    }
 }

SysManager/SysManager/Services/HostsFileService.cs

@@ -70,18 +70,23 @@ public async Task<List<HostsEntry>> ReadHostsAsync(CancellationToken ct = defaul
             if (tokens.Length < 2) continue;
 
             string ip = tokens[0];
-            string hostname = tokens[1];
 
             // Validate IP
             if (!IPAddress.TryParse(ip, out _)) continue;
 
-            entries.Add(new HostsEntry
+            // A single hosts line can map one IP to several hostnames
+            // (e.g. "127.0.0.1  a  b  c"). Emit one entry per hostname so none are
+            // silently dropped on a read→save round trip.
+            for (int t = 1; t < tokens.Length; t++)
             {
-                IpAddress = ip,
-                Hostname = hostname,
-                Comment = comment,
-                IsEnabled = !isDisabled
-            });
+                entries.Add(new HostsEntry
+                {
+                    IpAddress = ip,
+                    Hostname = tokens[t],
+                    Comment = comment,
+                    IsEnabled = !isDisabled
+                });
+            }
         }
 
         return entries;
@@ -122,7 +127,24 @@ public void SaveHosts(List<HostsEntry> entries)
             lines.Add(line);
         }
 
-        File.WriteAllLines(HostsPath, lines);
+        // Write atomically: a crash midway through File.WriteAllLines would otherwise
+        // leave the hosts file truncated or empty. Write to a temp file in the same
+        // directory, then replace the target in one move.
+        var tempPath = HostsPath + ".sysmanager.tmp";
+        try
+        {
+            File.WriteAllLines(tempPath, lines);
+            File.Move(tempPath, HostsPath, overwrite: true);
+        }
+        finally
+        {
+            if (File.Exists(tempPath))
+            {
+                try { File.Delete(tempPath); }
+                catch (IOException) { /* best-effort temp cleanup */ }
+                catch (UnauthorizedAccessException) { /* best-effort temp cleanup */ }
+            }
+        }
     }
 
     /// <summary>

SysManager/SysManager/SysManager.csproj

@@ -10,9 +10,9 @@
     <RootNamespace>SysManager</RootNamespace>
     <AllowUnsafeBlocks>true</AllowUnsafeBlocks>
     <NoWarn>NU1603;NU1701</NoWarn>
-    <Version>1.20.23</Version>
-    <FileVersion>1.20.23.0</FileVersion>
-    <AssemblyVersion>1.20.23.0</AssemblyVersion>
+    <Version>1.20.24</Version>
+    <FileVersion>1.20.24.0</FileVersion>
+    <AssemblyVersion>1.20.24.0</AssemblyVersion>
     <Product>SysManager</Product>
     <Description>SysManager — Windows system monitoring toolkit by laurentiu021. Network, updates, health, logs, safe deep cleanup.</Description>
     <PackageProjectUrl>https://github.com/laurentiu021/SystemManager</PackageProjectUrl>