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Implement RFC6761 reserved DNS names handling #6924

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8fc756e
Implement RFC6761 reserved DNS names handling
rzikm Oct 15, 2025
8755be4
Update test/Libraries/Microsoft.Extensions.ServiceDiscovery.Dns.Tests…
rzikm Oct 15, 2025
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114 changes: 85 additions & 29 deletions src/Libraries/Microsoft.Extensions.ServiceDiscovery.Dns/Resolver/DnsResolver.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -63,6 +63,12 @@ public ValueTask<ServiceResult[]> ResolveServiceAsync(string name, CancellationT
ObjectDisposedException.ThrowIf(_disposed, this);
cancellationToken.ThrowIfCancellationRequested();

if (CheckIsReservedDnsName(name) is ReservedNameType type && type != ReservedNameType.None)
{
// RFC 6761 requires that libraries return negative results for all queries except localhost A/AAAA
return ValueTask.FromResult(Array.Empty<ServiceResult>());
}

// dnsSafeName is Disposed by SendQueryWithTelemetry
EncodedDomainName dnsSafeName = GetNormalizedHostName(name);
return SendQueryWithTelemetry(name, dnsSafeName, QueryType.SRV, ProcessResponse, cancellationToken);
Expand Down Expand Up @@ -111,24 +117,9 @@ public ValueTask<ServiceResult[]> ResolveServiceAsync(string name, CancellationT

public async ValueTask<AddressResult[]> ResolveIPAddressesAsync(string name, CancellationToken cancellationToken = default)
{
if (string.Equals(name, "localhost", StringComparison.OrdinalIgnoreCase))
if (CheckIsReservedDnsName(name) is ReservedNameType type && type != ReservedNameType.None)
{
// 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);
AddressResult[] res = new AddressResult[len];

int index = 0;
if (Socket.OSSupportsIPv6) // prefer IPv6
{
res[index] = new AddressResult(DateTime.MaxValue, IPAddress.IPv6Loopback);
index++;
}
if (Socket.OSSupportsIPv4)
{
res[index] = new AddressResult(DateTime.MaxValue, IPAddress.Loopback);
}

return res;
return ResolveDnsReservedNameAddress(type, null);
}

var ipv4AddressesTask = ResolveIPAddressesAsync(name, AddressFamily.InterNetwork, cancellationToken);
Expand All @@ -153,19 +144,9 @@ internal ValueTask<AddressResult[]> ResolveIPAddressesAsync(string name, Address
throw new ArgumentOutOfRangeException(nameof(addressFamily), addressFamily, "Invalid address family");
}

if (string.Equals(name, "localhost", StringComparison.OrdinalIgnoreCase))
if (CheckIsReservedDnsName(name) is ReservedNameType type && type != ReservedNameType.None)
{
// name localhost exists outside of DNS and can't be resolved by a DNS server
if (addressFamily == AddressFamily.InterNetwork && Socket.OSSupportsIPv4)
{
return ValueTask.FromResult<AddressResult[]>([new AddressResult(DateTime.MaxValue, IPAddress.Loopback)]);
}
else if (addressFamily == AddressFamily.InterNetworkV6 && Socket.OSSupportsIPv6)
{
return ValueTask.FromResult<AddressResult[]>([new AddressResult(DateTime.MaxValue, IPAddress.IPv6Loopback)]);
}

return ValueTask.FromResult<AddressResult[]>([]);
return ValueTask.FromResult(ResolveDnsReservedNameAddress(type, addressFamily));
}

// dnsSafeName is Disposed by SendQueryWithTelemetry
Expand Down Expand Up @@ -342,6 +323,41 @@ static bool TryReadAddress(in DnsResourceRecord record, QueryType type, [NotNull
}
}

private static AddressResult[] ResolveDnsReservedNameAddress(ReservedNameType type, AddressFamily? addressFamily)
{
switch (type)
{
case ReservedNameType.Localhost:
// resolve to appropriate loopback address
bool doIpv6 = Socket.OSSupportsIPv6 && (addressFamily == null || addressFamily == AddressFamily.InterNetworkV6);
bool doIpv4 = Socket.OSSupportsIPv4 && (addressFamily == null || addressFamily == AddressFamily.InterNetwork);

int len = (doIpv4 ? 1 : 0) + (doIpv6 ? 1 : 0);
AddressResult[] res = new AddressResult[len];

int count = 0;
if (doIpv6) // put IPv6 first if both are requested
{
res[count] = new AddressResult(DateTime.MaxValue, IPAddress.IPv6Loopback);
count++;
}
if (doIpv4)
{
res[count] = new AddressResult(DateTime.MaxValue, IPAddress.Loopback);
}

return res;

case ReservedNameType.Invalid:
// RFC 6761 requires that libraries return negative results for 'invalid'
return [];

default:
Debug.Fail("Should be unreachable");
throw new ArgumentOutOfRangeException(nameof(type), type, "Invalid reserved name type");
}
}

private async ValueTask<TResult[]> SendQueryWithTelemetry<TResult>(string name, EncodedDomainName dnsSafeName, QueryType queryType, Func<EncodedDomainName, QueryType, DnsResponse, (SendQueryError error, TResult[] result)> processResponseFunc, CancellationToken cancellationToken)
{
NameResolutionActivity activity = Telemetry.StartNameResolution(name, queryType, _timeProvider.GetTimestamp());
Expand Down Expand Up @@ -928,4 +944,44 @@ private static EncodedDomainName GetNormalizedHostName(string name)
}
}
}

//
// See RFC 6761 for reserved DNS names.
//
private static ReservedNameType CheckIsReservedDnsName(string name)
{
ReadOnlySpan<char> nameAsSpan = name;
nameAsSpan = nameAsSpan.TrimEnd('.'); // trim potential explicit root label

if (MatchesReservedName(nameAsSpan, "localhost"))
{
return ReservedNameType.Localhost;
}

if (MatchesReservedName(nameAsSpan, "invalid"))
{
return ReservedNameType.Invalid;
}

return ReservedNameType.None;

static bool MatchesReservedName(ReadOnlySpan<char> name, string reservedName)
{
// check if equal to reserved name or is a subdomain of it
if (name.EndsWith(reservedName, StringComparison.OrdinalIgnoreCase) &&
(name.Length == reservedName.Length || name[name.Length - reservedName.Length - 1] == '.'))
{
return true;
}

return false;
}
}

private enum ReservedNameType
{
None,
Localhost,
Invalid,
}
}
41 changes: 34 additions & 7 deletions ...braries/Microsoft.Extensions.ServiceDiscovery.Dns.Tests/Resolver/ResolveAddressesTests.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -57,6 +57,9 @@ public async Task ResolveIPv4_NoSuchName_Success(bool includeSoa)
[Theory]
[InlineData("www.resolveipv4.com")]
[InlineData("www.resolveipv4.com.")]
[InlineData("notlocalhost")]
[InlineData("notlocalhost.")]
[InlineData("notinvalid.")]
[InlineData("www.ř.com")]
public async Task ResolveIPv4_Simple_Success(string name)
{
Expand Down Expand Up @@ -220,15 +223,39 @@ public async Task ResolveIP_InvalidAddressFamily_Throws()
}

[Theory]
[InlineData(AddressFamily.InterNetwork, "127.0.0.1")]
[InlineData(AddressFamily.InterNetworkV6, "::1")]
public async Task ResolveIP_Localhost_ReturnsLoopback(AddressFamily family, string addressAsString)
[InlineData("localhost", AddressFamily.InterNetwork, "127.0.0.1")]
[InlineData("localhost", AddressFamily.InterNetworkV6, "::1")]
[InlineData("localhost.", AddressFamily.InterNetwork, "127.0.0.1")]
[InlineData("inner.localhost.", AddressFamily.InterNetwork, "127.0.0.1")]
[InlineData("inner.localhost", AddressFamily.InterNetwork, "127.0.0.1")]
[InlineData("invalid", AddressFamily.InterNetwork, null)]
[InlineData("invalid", AddressFamily.InterNetworkV6, null)]
[InlineData("invalid.", AddressFamily.InterNetwork, null)]
[InlineData("inner.invalid.", AddressFamily.InterNetwork, null)]
[InlineData("inner.invalid", AddressFamily.InterNetwork, null)]
public async Task ResolveIP_SpecialName(string localhost, AddressFamily family, string? addressAsString)
{
IPAddress address = IPAddress.Parse(addressAsString);
AddressResult[] results = await Resolver.ResolveIPAddressesAsync("localhost", family);
AddressResult result = Assert.Single(results);
IPAddress? address = addressAsString != null ? IPAddress.Parse(addressAsString) : null;

Assert.Equal(address, result.Address);
bool serverCalled = false;
_ = DnsServer.ProcessUdpRequest(builder =>
{
serverCalled = true;
return Task.CompletedTask;
});

AddressResult[] results = await Resolver.ResolveIPAddressesAsync(localhost, family);
Assert.False(serverCalled, "Special name resolution should not call the DNS server.");

if (address == null)
{
Assert.Empty(results);
}
else
{
AddressResult result = Assert.Single(results);
Assert.Equal(address, result.Address);
}
}

[Fact]
Expand Down
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