IPv6 source address

[IngmarStein]

Hi,

I recently changed my ISP and finally have a working native IPv6 connection, so I'm now reconfiguring my LAN to use IPv6 wherever possible. It is going fine, but DNS is causing me some headaches.

My Technitium DNS instance runs on a server which has multiple interfaces which are assigned
192.168.1.x and 2a02:XXXX:XXXX::x (redacted) where x = 2...10. The DNS server should respond on x = 10 (other services are bound to the other IPs).

When I test the DNS server logged in to the server over ssh (or the DNS Client feature on the web), it all seems fine:

❯ dig +short AAAA www.google.com @192.168.1.10
2a00:1450:400a:804::2004
❯ dig +short AAAA www.google.com @2a02:XXXX:XXXX::10
2a00:1450:400a:804::2004

However, from other machines on the LAN:

❯ dig +short AAAA www.google.com @192.168.1.10
2a00:1450:400a:804::2004
❯ dig +short AAAA www.google.com @2a02:XXXX:XXXX::10
;; reply from unexpected source: 2a02:XXXX:XXXX::3#53, expected 2a02:XXXX:XXXX::10#53

So it seems like, when using IPv6, the server chooses the wrong source address in its response, which are set to the defaults:

DNS Server IPv4 Source address: 0.0.0.0
DNS Server IPv6 Source address: ::
Prefer IPv6: disabled

Changing the DNS Server IPv6 Source address alone doesn't change anything (as is documented).

As soon as I set DNS Server IPv6 Source address to 2a02:XXXX:XXXX::10 and enable Prefer IPv6, DNS resolution stops working and produces a lot of timeouts instead. For example:

[2025-07-13 21:52:44 Local] DNS Server failed to resolve the request 'gcs-download-eu.l.googleusercontent.com. A IN'.^M
TechnitiumLibrary.Net.Dns.DnsClientNoResponseException: DnsClient failed to recursively resolve the request 'gcs-download-eu.l.googleusercontent.com. A IN': no response from name servers [ns4.google.com (216.239.38.10), ns1.google.com (216.239.32.10), ns2.google.com (216.239.34.10), ns3.google.com (216.239.36.10)] at delegation googleusercontent.com.
 ---> TechnitiumLibrary.Net.Dns.DnsClientNoResponseException: DnsClient failed to resolve the request 'gcs-download-eu.l.googleusercontent.com. A IN': request timed out for name servers [ns4.google.com (216.239.38.10), ns1.google.com (216.239.32.10), ns2.google.com (216.239.34.10), ns3.google.com (216.239.36.10)].
   at TechnitiumLibrary.Net.Dns.DnsClient.InternalResolveAsync(DnsDatagram request, Func`3 getValidatedResponseAsync, Boolean doNotReorderNameServers, CancellationToken cancellationToken) in /private/tmp/technitium-library-20250426-6077-w2pvpm/TechnitiumLibrary-dns-server-v13.6.0/TechnitiumLibrary.Net/Dns/DnsClient.cs:line 4826
   at TechnitiumLibrary.Net.Dns.DnsClient.InternalResolveAsync(DnsDatagram request, Func`3 getValidatedResponseAsync, Boolean doNotReorderNameServers, CancellationToken cancellationToken) in /private/tmp/technitium-library-20250426-6077-w2pvpm/TechnitiumLibrary-dns-server-v13.6.0/TechnitiumLibrary.Net/Dns/DnsClient.cs:line 4809
   at TechnitiumLibrary.Net.Dns.DnsClient.RecursiveResolveAsync(DnsQuestionRecord question, IDnsCache cache, NetProxy proxy, Boolean preferIPv6, UInt16 udpPayloadSize, Boolean randomizeName, Boolean qnameMinimization, Boolean dnssecValidation, NetworkAddress eDnsClientSubnet, Int32 retries, Int32 timeout, Int32 concurrency, Int32 maxStackCount, Boolean minimalResponse, Boolean asyncNsResolution, List`1 rawResponses, CancellationToken cancellationToken) in /private/tmp/technitium-library-20250426-6077-w2pvpm/TechnitiumLibrary-dns-server-v13.6.0/TechnitiumLibrary.Net/Dns/DnsClient.cs:line 1068
   --- End of inner exception stack trace ---
   at TechnitiumLibrary.Net.Dns.DnsClient.RecursiveResolveAsync(DnsQuestionRecord question, IDnsCache cache, NetProxy proxy, Boolean preferIPv6, UInt16 udpPayloadSize, Boolean randomizeName, Boolean qnameMinimization, Boolean dnssecValidation, NetworkAddress eDnsClientSubnet, Int32 retries, Int32 timeout, Int32 concurrency, Int32 maxStackCount, Boolean minimalResponse, Boolean asyncNsResolution, List`1 rawResponses, CancellationToken cancellationToken) in /private/tmp/technitium-library-20250426-6077-w2pvpm/TechnitiumLibrary-dns-server-v13.6.0/TechnitiumLibrary.Net/Dns/DnsClient.cs:line 1811
   at TechnitiumLibrary.TaskExtensions.TimeoutAsync[T](Func`2 func, Int32 timeout, CancellationToken cancellationToken)
   at TechnitiumLibrary.TaskExtensions.TimeoutAsync[T](Func`2 func, Int32 timeout, CancellationToken cancellationToken) in /private/tmp/technitium-library-20250426-6077-w2pvpm/TechnitiumLibrary-dns-server-v13.6.0/TechnitiumLibrary/TaskExtensions.cs:line 65
   at DnsServerCore.Dns.DnsServer.RecursiveResolverBackgroundTaskAsync(DnsQuestionRecord question, NetworkAddress eDnsClientSubnet, Boolean advancedForwardingClientSubnet, IReadOnlyList`1 conditionalForwarders, Boolean dnssecValidation, Boolean cachePrefetchOperation, Boolean cacheRefreshOperation, Boolean skipDnsAppAuthoritativeRequestHandlers, TaskCompletionSource`1 taskCompletionSource) in /private/tmp/technitium-dns-20250426-12670-26asim/DnsServer-13.6.0/DnsServerCore/Dns/DnsServer.cs:line 3239
[2025-07-13 21:52:44 Local] DNS Server failed to resolve the request 'gcs-download-eu.l.googleusercontent.com. AAAA IN'.^M
TechnitiumLibrary.Net.Dns.DnsClientNoResponseException: DnsClient failed to recursively resolve the request 'gcs-download-eu.l.googleusercontent.com. AAAA IN': no response from name servers [ns4.google.com (216.239.38.10), ns1.google.com (216.239.32.10), ns2.google.com (216.239.34.10), ns3.google.com (216.239.36.10)] at delegation googleusercontent.com.
 ---> TechnitiumLibrary.Net.Dns.DnsClientNoResponseException: DnsClient failed to resolve the request 'gcs-download-eu.l.googleusercontent.com. AAAA IN': request timed out for name servers [ns4.google.com (216.239.38.10), ns1.google.com (216.239.32.10), ns2.google.com (216.239.34.10), ns3.google.com (216.239.36.10)].
   at TechnitiumLibrary.Net.Dns.DnsClient.InternalResolveAsync(DnsDatagram request, Func`3 getValidatedResponseAsync, Boolean doNotReorderNameServers, CancellationToken cancellationToken) in /private/tmp/technitium-library-20250426-6077-w2pvpm/TechnitiumLibrary-dns-server-v13.6.0/TechnitiumLibrary.Net/Dns/DnsClient.cs:line 4826
   at TechnitiumLibrary.Net.Dns.DnsClient.InternalResolveAsync(DnsDatagram request, Func`3 getValidatedResponseAsync, Boolean doNotReorderNameServers, CancellationToken cancellationToken) in /private/tmp/technitium-library-20250426-6077-w2pvpm/TechnitiumLibrary-dns-server-v13.6.0/TechnitiumLibrary.Net/Dns/DnsClient.cs:line 4809

I noticed that in the above example, all name server were resolved the IPv4 instead of IPv6 addresses. I also have many examples of log entries which resulted in SERVFAIL responses (I think that's when I had also set the IPv4 source address list to just 192.168.1.10):

[2025-07-13 19:15:08 Local] DNS Server failed to resolve the request 'calendars.fe2.apple-dns.net. A IN'.^M
TechnitiumLibrary.Net.Dns.DnsClientNoResponseException: DnsClient failed to recursively resolve the request 'calendars.fe2.apple-dns.net. A IN': no response from name servers [ns-383.awsdns-47.com ([2600:9000:5301:7f00::1]), ns-1919.awsdns-47.co.uk ([2600:9000:5307:7f00::1]), ns-693.awsdns-22.net ([2600:9000:5302:b500::1]), ns-1396.awsdns-46.org ([2600:9000:5305:7400::1]), ns-383.awsdns-47.com (205.251.193.127), ns-1919.awsdns-47.co.uk (205.251.199.127), ns-693.awsdns-22.net (205.251.194.181), ns-1396.awsdns-46.org (205.251.197.116)] at delegation fe2.apple-dns.net.
 ---> System.Net.Sockets.SocketException (22): Invalid argument
   at System.Net.Sockets.Socket.SetRawSocketOption(Int32 optionLevel, Int32 optionName, ReadOnlySpan`1 optionValue)
   at TechnitiumLibrary.Net.Dns.ClientConnection.UdpClientConnection.PooledSocket..ctor(AddressFamily addressFamily, Int32 index, Boolean ignoreSourceBinding) in /private/tmp/technitium-library-20250426-6077-w2pvpm/TechnitiumLibrary-dns-server-v13.6.0/TechnitiumLibrary.Net/Dns/ClientConnection/UdpClientConnection.cs:line 373
   at TechnitiumLibrary.Net.Dns.ClientConnection.UdpClientConnection.GetPooledSocketFor(IPEndPoint serverEP) in /private/tmp/technitium-library-20250426-6077-w2pvpm/TechnitiumLibrary-dns-server-v13.6.0/TechnitiumLibrary.Net/Dns/ClientConnection/UdpClientConnection.cs:line 175
   at TechnitiumLibrary.Net.Dns.ClientConnection.UdpClientConnection.QueryAsync(DnsDatagram request, Int32 timeout, Int32 retries, CancellationToken cancellationToken) in /private/tmp/technitium-library-20250426-6077-w2pvpm/TechnitiumLibrary-dns-server-v13.6.0/TechnitiumLibrary.Net/Dns/ClientConnection/UdpClientConnection.cs:line 269
   at TechnitiumLibrary.Net.Dns.DnsClient.<>c__DisplayClass90_0.<<InternalResolveAsync>g__DoResolveAsync|1>d.MoveNext() in /private/tmp/technitium-library-20250426-6077-w2pvpm/TechnitiumLibrary-dns-server-v13.6.0/TechnitiumLibrary.Net/Dns/DnsClient.cs:line 4499
--- End of stack trace from previous location ---
   at TechnitiumLibrary.Net.Dns.DnsClient.<>c__DisplayClass90_0.<<InternalResolveAsync>g__DoResolveAsync|1>d.MoveNext() in /private/tmp/technitium-library-20250426-6077-w2pvpm/TechnitiumLibrary-dns-server-v13.6.0/TechnitiumLibrary.Net/Dns/DnsClient.cs:line 4718
--- End of stack trace from previous location ---
   at TechnitiumLibrary.Net.Dns.DnsClient.<>c__DisplayClass90_0.<<InternalResolveAsync>g__DoResolveAsync|1>d.MoveNext() in /private/tmp/technitium-library-20250426-6077-w2pvpm/TechnitiumLibrary-dns-server-v13.6.0/TechnitiumLibrary.Net/Dns/DnsClient.cs:line 4415
--- End of stack trace from previous location ---
   at TechnitiumLibrary.Net.Dns.DnsClient.InternalResolveAsync(DnsDatagram request, Func`3 getValidatedResponseAsync, Boolean doNotReorderNameServers, CancellationToken cancellationToken) in /private/tmp/technitium-library-20250426-6077-w2pvpm/TechnitiumLibrary-dns-server-v13.6.0/TechnitiumLibrary.Net/Dns/DnsClient.cs:line 4867
   at TechnitiumLibrary.Net.Dns.DnsClient.InternalResolveAsync(DnsDatagram request, Func`3 getValidatedResponseAsync, Boolean doNotReorderNameServers, CancellationToken cancellationToken) in /private/tmp/technitium-library-20250426-6077-w2pvpm/TechnitiumLibrary-dns-server-v13.6.0/TechnitiumLibrary.Net/Dns/DnsClient.cs:line 4809
   at TechnitiumLibrary.Net.Dns.DnsClient.RecursiveResolveAsync(DnsQuestionRecord question, IDnsCache cache, NetProxy proxy, Boolean preferIPv6, UInt16 udpPayloadSize, Boolean randomizeName, Boolean qnameMinimization, Boolean dnssecValidation, NetworkAddress eDnsClientSubnet, Int32 retries, Int32 timeout, Int32 concurrency, Int32 maxStackCount, Boolean minimalResponse, Boolean asyncNsResolution, List`1 rawResponses, CancellationToken cancellationToken) in /private/tmp/technitium-library-20250426-6077-w2pvpm/TechnitiumLibrary-dns-server-v13.6.0/TechnitiumLibrary.Net/Dns/DnsClient.cs:line 1068
   --- End of inner exception stack trace ---
   at TechnitiumLibrary.Net.Dns.DnsClient.RecursiveResolveAsync(DnsQuestionRecord question, IDnsCache cache, NetProxy proxy, Boolean preferIPv6, UInt16 udpPayloadSize, Boolean randomizeName, Boolean qnameMinimization, Boolean dnssecValidation, NetworkAddress eDnsClientSubnet, Int32 retries, Int32 timeout, Int32 concurrency, Int32 maxStackCount, Boolean minimalResponse, Boolean asyncNsResolution, List`1 rawResponses, CancellationToken cancellationToken) in /private/tmp/technitium-library-20250426-6077-w2pvpm/TechnitiumLibrary-dns-server-v13.6.0/TechnitiumLibrary.Net/Dns/DnsClient.cs:line 1811
   at TechnitiumLibrary.TaskExtensions.TimeoutAsync[T](Func`2 func, Int32 timeout, CancellationToken cancellationToken)
   at TechnitiumLibrary.TaskExtensions.TimeoutAsync[T](Func`2 func, Int32 timeout, CancellationToken cancellationToken) in /private/tmp/technitium-library-20250426-6077-w2pvpm/TechnitiumLibrary-dns-server-v13.6.0/TechnitiumLibrary/TaskExtensions.cs:line 65
   at DnsServerCore.Dns.DnsServer.RecursiveResolverBackgroundTaskAsync(DnsQuestionRecord question, NetworkAddress eDnsClientSubnet, Boolean advancedForwardingClientSubnet, IReadOnlyList`1 conditionalForwarders, Boolean dnssecValidation, Boolean cachePrefetchOperation, Boolean cacheRefreshOperation, Boolean skipDnsAppAuthoritativeRequestHandlers, TaskCompletionSource`1 taskCompletionSource) in /private/tmp/technitium-dns-20250426-12670-26asim/DnsServer-13.6.0/DnsServerCore/Dns/DnsServer.cs:line 3239

I think I'm probably misunderstanding the semantics of these configuration settings.

Could you please help me configuring Technitium DNS so that it can also serve as the DNS server for the LAN clients using IPv6 (in addition to IPv4)?

[IngmarStein]

Some debugging: FindEndPointsFor here saves the interface name in the endpoints tuple because Environment.OSVersion.Platform returns PlatformID.Unix (per
https://github.com/dotnet/runtime/blob/9c0555e89d91149226cfade4a395e02b8d9401ff/src/libraries/System.Private.CoreLib/src/System/Environment.OSVersion.OSX.cs#L12-L14) where I'm running Technitium DNS.

This, in turn, causes _socket.SetRawSocketOption(SOL_SOCKET, SO_BINDTODEVICE, ipv6SourceEP.Item2); to be called.in
PooledSocket.

I found a few places which claim that SO_BINDTODEVICE is not available on macOS (e.g. here), but it is defined in usr/include/sys/socket.h:

#define SO_BINDTODEVICE            0x1134  /* bind socket to a network device (max valid option length IFNAMSIZ) */

In any case, I found some pointers, that SO_BINDTODEVICE requires the process to run as root (which I'm not), so I could try that once I have your feedback. It looks like what's needed is a way to only Bind to the IP, but not to the device, as it's done on non-Unix platforms.

[ShreyasZare]

Thanks for asking. The Source Address feature is used for outbound requests and thus is not related to the issue you have here. The issue is also not related to IPv6 and will come with IPv4 too with multiple interfaces on Linux servers. The issue is just that the DNS responses get routed out of the interface which has the default route on Linux.

The fix is to configure the IP address on the additional interfaces with the "DNS Server Local End Points" option. Once the DNS server binds to those specific IP addresses, the responses will be returned via the correct interface.

Let me know if that worked.

[IngmarStein]

I see. It's interesting that I don't have the same issue with IPv4 where the same interface also has multiple assigned IPs. Am I just lucky that it happens to work there?

Because of #1328, I've configured "DNS Server Local End Points" as "0.0.0.0:53,[::]:53", so the server binds to all IPs. I guess I'll have to wait until the next release so that I can change that to single addresses?

[ShreyasZare]

It happens with IPv4 too so not sure how its not an issue in your setup for IPv4. There should be some reason for it but cannot guess due to limited info.

Ya, it looks like you have to wait for the next update. Trying to get it done as fast as I can.

[IngmarStein]

No rush, take your time!
In the meantime, I got it working by setting the local endpoint to [2a02:XXXX:XXXX::10]:53 as you suggested.
That meant that I have to run the server as root again because of https://developer.apple.com/forums/thread/674179?login=true&page=1#835886022, but I can live with that. Should I ever need to reboot the machine before the next Technitium DNS version, I'll just manually log in and restart the DNS server once all IPs are up.