Fix UTF-8 BOM in dnsmasq config/hosts writes so dnsmasq does not fail

- Add DnsmasqFileEncoding.Utf8NoBom for all config and hosts file writes
- Strip BOM from first line when reading main config and managed config
- Prevents 'bad option at line 1' when dnsmasq loads files written by the app

src/DnsmasqWebUI/Infrastructure/DnsmasqFileEncoding.cs

@@ -0,0 +1,9 @@
+using System.Text;
+
+namespace DnsmasqWebUI.Infrastructure;
+
+/// <summary>Encoding for dnsmasq config and hosts files. UTF-8 without BOM so dnsmasq does not see a BOM as "bad option" on line 1.</summary>
+public static class DnsmasqFileEncoding
+{
+    public static readonly Encoding Utf8NoBom = new UTF8Encoding(false);
+}

src/DnsmasqWebUI/Services/DnsmasqConfigService.cs

@@ -1,4 +1,5 @@
 using System.Text;
+using DnsmasqWebUI.Infrastructure;
 using DnsmasqWebUI.Models.Config;
 using DnsmasqWebUI.Models.Dhcp;
 using DnsmasqWebUI.Models.EffectiveConfig;
@@ -82,7 +83,7 @@ public class DnsmasqConfigService : IDnsmasqConfigService
         var dir = Path.GetDirectoryName(managedHostsPath);
         if (!string.IsNullOrEmpty(dir) && !Directory.Exists(dir))
             Directory.CreateDirectory(dir);
-        File.WriteAllText(managedHostsPath, "");
+        File.WriteAllText(managedHostsPath, "", DnsmasqFileEncoding.Utf8NoBom);
     }
 
     public async Task WriteDhcpHostsAsync(IReadOnlyList<DhcpHostEntry> entries, CancellationToken ct = default)
@@ -110,7 +111,10 @@ public class DnsmasqConfigService : IDnsmasqConfigService
 
         IReadOnlyList<string> rawLines;
         if (File.Exists(path))
-            rawLines = await File.ReadAllLinesAsync(path, Encoding.UTF8, ct);
+        {
+            var read = await File.ReadAllLinesAsync(path, DnsmasqFileEncoding.Utf8NoBom, ct);
+            rawLines = StripBomFromFirstLine(read);
+        }
         else
             rawLines = Array.Empty<string>();
 
@@ -136,7 +140,7 @@ public class DnsmasqConfigService : IDnsmasqConfigService
             output.Add(DnsmasqConfDhcpHostLineParser.ToLine(entry));
 
         var tmpPath = path + ".tmp";
-        await File.WriteAllLinesAsync(tmpPath, output, Encoding.UTF8, ct);
+        await File.WriteAllLinesAsync(tmpPath, output, DnsmasqFileEncoding.Utf8NoBom, ct);
         File.Move(tmpPath, path, overwrite: true);
         EnsureManagedHostsFileExists(set.ManagedHostsFilePath);
         var effectiveHostsPathDhcp = configLines.OfType<AddnHostsLine>().FirstOrDefault()?.AddnHostsPath ?? "";
@@ -152,7 +156,7 @@ public class DnsmasqConfigService : IDnsmasqConfigService
         foreach (var file in set.Files.Where(f => !f.IsManaged && !string.Equals(f.Path, managedPath, StringComparison.Ordinal)))
         {
             if (!File.Exists(file.Path)) continue;
-            var lines = await File.ReadAllLinesAsync(file.Path, Encoding.UTF8, ct);
+            var lines = await File.ReadAllLinesAsync(file.Path, DnsmasqFileEncoding.Utf8NoBom, ct);
             var configLines = DnsmasqConfFileLineParser.ParseFile(lines);
             foreach (var dhcp in configLines.OfType<DhcpHostLine>().Select(c => c.DhcpHost))
             {
@@ -191,11 +195,25 @@ public class DnsmasqConfigService : IDnsmasqConfigService
         var output = list.Select(DnsmasqConfFileLineParser.ToLine).ToList();
 
         var tmpPath = path + ".tmp";
-        await File.WriteAllLinesAsync(tmpPath, output, Encoding.UTF8, ct);
+        await File.WriteAllLinesAsync(tmpPath, output, DnsmasqFileEncoding.Utf8NoBom, ct);
         File.Move(tmpPath, path, overwrite: true);
         EnsureManagedHostsFileExists(set.ManagedHostsFilePath);
         var effectiveHostsPath = list.OfType<AddnHostsLine>().FirstOrDefault()?.AddnHostsPath ?? "";
         _configSetCache.NotifyWeWroteManagedConfig(new ManagedConfigContent(list, effectiveHostsPath));
         _logger.LogInformation("Wrote managed config file: {Path}", path);
     }
+
+    /// <summary>Strip UTF-8 BOM from first line if present.</summary>
+    private static IReadOnlyList<string> StripBomFromFirstLine(IReadOnlyList<string> lines)
+    {
+        if (lines.Count == 0) return lines;
+        var first = lines[0];
+        if (first.Length > 0 && first[0] == '\uFEFF')
+        {
+            var list = lines.ToList();
+            list[0] = first[1..];
+            return list;
+        }
+        return lines;
+    }
 }

src/DnsmasqWebUI/Services/EnsureManagedConfigHostedService.cs

@@ -1,4 +1,5 @@
 using System.Text;
+using DnsmasqWebUI.Infrastructure;
 using DnsmasqWebUI.Models.Config;
 using DnsmasqWebUI.Models.EffectiveConfig;
 using DnsmasqWebUI.Configuration;
@@ -48,8 +49,9 @@ public class EnsureManagedConfigHostedService : IApplicationHostedService
         var confFileLine = "conf-file=" + Path.GetRelativePath(mainDir, managedPath);
 
         var lines = File.Exists(mainFull)
-            ? (await File.ReadAllLinesAsync(mainFull, Encoding.UTF8, cancellationToken)).ToList()
+            ? (await File.ReadAllLinesAsync(mainFull, DnsmasqFileEncoding.Utf8NoBom, cancellationToken)).ToList()
             : new List<string>();
+        StripBomFromFirstLine(lines);
 
         // Remove any conf-file= line that points to our managed path (we will add it as the last line).
         for (var i = lines.Count - 1; i >= 0; i--)
@@ -75,12 +77,12 @@ public class EnsureManagedConfigHostedService : IApplicationHostedService
             if (lines.Count > 0 && !string.IsNullOrWhiteSpace(lines[^1]))
                 lines.Add("");
             lines.Add(confFileLine);
-            await File.WriteAllLinesAsync(mainFull, lines, Encoding.UTF8, cancellationToken);
+            await File.WriteAllLinesAsync(mainFull, lines, DnsmasqFileEncoding.Utf8NoBom, cancellationToken);
             _logger.LogInformation("Set {Line} as the last line of main config {Path} so the managed file is included only by conf-file=.", confFileLine, mainFull);
         }
         else
         {
-            await File.WriteAllLinesAsync(mainFull, lines, Encoding.UTF8, cancellationToken);
+            await File.WriteAllLinesAsync(mainFull, lines, DnsmasqFileEncoding.Utf8NoBom, cancellationToken);
         }
 
         Directory.CreateDirectory(managedDir);
@@ -92,5 +94,14 @@ public class EnsureManagedConfigHostedService : IApplicationHostedService
         await configService.WriteManagedConfigAsync(Array.Empty<DnsmasqConfLine>(), cancellationToken);
     }
 
+    /// <summary>Strip UTF-8 BOM from first line if present, so we do not perpetuate it when writing.</summary>
+    private static void StripBomFromFirstLine(List<string> lines)
+    {
+        if (lines.Count == 0) return;
+        var first = lines[0];
+        if (first.Length > 0 && first[0] == '\uFEFF')
+            lines[0] = first[1..];
+    }
+
     public Task StopAsync(CancellationToken cancellationToken) => Task.CompletedTask;
 }

src/DnsmasqWebUI/Services/HostsFileService.cs

@@ -1,4 +1,4 @@
-using System.Text;
+using DnsmasqWebUI.Infrastructure;
 using DnsmasqWebUI.Models.Hosts;
 using DnsmasqWebUI.Parsers;
 using DnsmasqWebUI.Services.Abstractions;
@@ -36,7 +36,7 @@ public class HostsFileService : IHostsFileService
 
         var tmpPath = path + ".tmp";
         var lines = entries.Select(HostsFileLineParser.ToLine).ToList();
-        await File.WriteAllLinesAsync(tmpPath, lines, Encoding.UTF8, ct);
+        await File.WriteAllLinesAsync(tmpPath, lines, DnsmasqFileEncoding.Utf8NoBom, ct);
         File.Move(tmpPath, path, overwrite: true);
         _hostsCache.NotifyWeWroteManagedHosts(entries);
         _logger.LogInformation("Wrote managed hosts file: {Path}", path);