Check against too long domain name

src/Microsoft.Extensions.ServiceDiscovery.Dns/Resolver/DnsPrimitives.cs

@@ -9,6 +9,8 @@ namespace Microsoft.Extensions.ServiceDiscovery.Dns.Resolver;
 
 internal static class DnsPrimitives
 {
+    internal const int MaxDomainNameLength = 253;
+
     internal static bool TryWriteQName(Span<byte> destination, string name, out int written)
     {
         //
@@ -103,7 +105,15 @@ private static bool TryReadQNameCore(StringBuilder sb, ReadOnlySpan<byte> messag
                     {
                         sb.Append('.');
                     }
+
                     sb.Append(Encoding.ASCII.GetString(messageBuffer.Slice(currentOffset + 1, length)));
+
+                    if (sb.Length > MaxDomainNameLength)
+                    {
+                        // domain name is too long
+                        return false;
+                    }
+
                     currentOffset += 1 + length;
                     bytesRead += 1 + length;
                 }

src/Microsoft.Extensions.ServiceDiscovery.Dns/Resolver/DnsResolver.cs

@@ -4,6 +4,7 @@
 using System.Buffers;
 using System.Buffers.Binary;
 using System.Diagnostics;
+using System.Globalization;
 using System.Net;
 using System.Net.Sockets;
 using System.Runtime.InteropServices;
@@ -14,7 +15,6 @@ namespace Microsoft.Extensions.ServiceDiscovery.Dns.Resolver;
 
 internal partial class DnsResolver : IDnsResolver, IDisposable
 {
-    private const int MaximumNameLength = 253;
     private const int IPv4Length = 4;
     private const int IPv6Length = 16;
 
@@ -66,6 +66,8 @@ public async ValueTask<ServiceResult[]> ResolveServiceAsync(string name, Cancell
         ObjectDisposedException.ThrowIf(_disposed, this);
         cancellationToken.ThrowIfCancellationRequested();
 
+        name = GetNormalizedHostName(name);
+
         NameResolutionActivity activity = Telemetry.StartNameResolution(name, QueryType.SRV, _timeProvider.GetTimestamp());
         SendQueryResult result = await SendQueryWithRetriesAsync(name, QueryType.SRV, cancellationToken).ConfigureAwait(false);
 
@@ -118,7 +120,7 @@ public async ValueTask<ServiceResult[]> ResolveServiceAsync(string name, Cancell
 
     public async ValueTask<AddressResult[]> ResolveIPAddressesAsync(string name, CancellationToken cancellationToken = default)
     {
-        if (name == "localhost")
+        if (string.Equals(name, "localhost", StringComparison.OrdinalIgnoreCase))
         {
             // name localhost exists outside of DNS and can't be resolved by a DNS server
             int len = (Socket.OSSupportsIPv4 ? 1 : 0) + (Socket.OSSupportsIPv6 ? 1 : 0);
@@ -157,7 +159,7 @@ public async ValueTask<AddressResult[]> ResolveIPAddressesAsync(string name, Add
             throw new ArgumentOutOfRangeException(nameof(addressFamily), addressFamily, "Invalid address family");
         }
 
-        if (name == "localhost")
+        if (string.Equals(name, "localhost", StringComparison.OrdinalIgnoreCase))
         {
             // name localhost exists outside of DNS and can't be resolved by a DNS server
             if (addressFamily == AddressFamily.InterNetwork && Socket.OSSupportsIPv4)
@@ -172,10 +174,7 @@ public async ValueTask<AddressResult[]> ResolveIPAddressesAsync(string name, Add
             return Array.Empty<AddressResult>();
         }
 
-        if (name.Length > MaximumNameLength)
-        {
-            throw new ArgumentException("Name is too long", nameof(name));
-        }
+        name = GetNormalizedHostName(name);
 
         var queryType = addressFamily == AddressFamily.InterNetwork ? QueryType.A : QueryType.AAAA;
         NameResolutionActivity activity = Telemetry.StartNameResolution(name, queryType, _timeProvider.GetTimestamp());
@@ -623,4 +622,12 @@ public void Dispose()
 
         return (pendingRequestsCts, DisposeTokenSource: false, pendingRequestsCts);
     }
+
+    private static readonly IdnMapping s_idnMapping = new IdnMapping();
+
+    private static string GetNormalizedHostName(string name)
+    {
+        // TODO: better exception message
+        return s_idnMapping.GetAscii(name);
+    }
 }

tests/Microsoft.Extensions.ServiceDiscovery.Dns.Tests/Resolver/DnsPrimitivesTests.cs

@@ -122,4 +122,32 @@ public void TryReadQName_ReservedBits()
 
         Assert.False(DnsPrimitives.TryReadQName(data, 0, out _, out _));
     }
+
+    [Theory]
+    [InlineData(253)]
+    [InlineData(254)]
+    [InlineData(255)]
+    public void TryReadQName_NameTooLong(int length)
+    {
+        // longest possible label is 63 bytes + 1 byte for length
+        byte[] labelData = new byte[64];
+        Array.Fill(labelData, (byte)'a');
+        labelData[0] = 63;
+
+        int remainder = length - 3 * 64;
+
+        byte[] lastLabelData = new byte[remainder + 1];
+        Array.Fill(lastLabelData, (byte)'a');
+        lastLabelData[0] = (byte)remainder;
+
+        byte[] data = Enumerable.Repeat(labelData, 3).SelectMany(x => x).Concat(lastLabelData).Concat(new byte[1]).ToArray();
+        if (length > 253)
+        {
+            Assert.False(DnsPrimitives.TryReadQName(data, 0, out _, out _));
+        }
+        else
+        {
+            Assert.True(DnsPrimitives.TryReadQName(data, 0, out _, out _));
+        }
+    }
 }