Compatibility of Low-Level Socket APIs with Mapped IPv6 Addresses

Question

Created Jan ’24

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I am currently working on an application that communicates with an IPv4 node in an IPv6-only network. During DNS resolution for the server node on JIO and T-Mobile networks, I am receiving IPv4 and mapped IPv6 addresses.

In my application, I am using these mapped IPv6 addresses in two different contexts:

For high-level API calls on the app side, I am using URLSession API (in either Objective-C or Swift).
For another target(c/c++), I am making low-level socket API calls(bind etc). These calls use the address passed from the app layer.

As node is resolved to IPV4 and mapped IPv6 addresses

My question is: Are low-level Socket APIs(bind etc) compatible with these mapped IPv6 addresses (example 64:ff9b::103.135.122.10)?

Mapped IPv6 with Well know perfix(64:ff9b)

Please provide the document/rfc reference for the same.

Any guidance or resources on this topic would be greatly appreciated.

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Answer 1

DTS Engineer OP

Apple

Jan ’24

You’re using the sytem’s DNS resolver, right?

During DNS resolution for the server node on JIO and T-Mobile networks, I am receiving IPv4 and mapped IPv6 addresses.

The resolve-then-connect approach is suboptimal on Apple platforms. In general, I recommend that you use a connect-by-name API. I put a bunch of info about this in TN3151 Choosing the right networking API.

For high-level API calls on the app side, I am using URLSession API (in either Objective-C or Swift).

You’re passing IP addresses to URLSession? Why not pass the DNS name in directly?

For another target (c/c++), I am making low-level socket API calls (bind etc). These calls use the address passed from the app layer.

Using the resolve-then-connect approach is your only option with BSD Sockets, but to work in all situations your sockets code must implement Happy Eyeballs. Again, TN3151 talks more about this.

Are low-level Socket APIs (bind etc) compatible with these mapped IPv6 addresses (example 64:ff9b::103.135.122.10)?

Yes.

Although, questions like this are irrelevant once you implement Happy Eyeballs, because if a specific address fails you’ll fall back to one that works.

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Quinn “The Eskimo!” @ Developer Technical Support @ Apple
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Answer 2

KhothAmit OP

Feb ’24

Hi @eskimo ,

My concern is the compatibility of low-level socket APIs with IPv4-mapped IPv6 addresses. Specifically, I'm interested in whether functions like bind, accept, and connect can seamlessly work with a mapped IPv6 address like 64:ff9b::103.135.122.10. I've consulted the following thread for reference and it is stated:

We do not support under-the-sockets bump-in-API (RFC 3338) and we do not support 464XLAT...

https://developer.apple.com/forums/thread/5643.

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Answer 3

DTS Engineer OP

Apple

Feb ’24

Things might move faster if you answer my specific questions, including:

You’re using the sytem’s DNS resolver, right?

and:

You’re passing IP addresses to URLSession? Why not pass the DNS name in directly?

You wrote:

I've consulted the following thread for reference and it is stated:

That thread is 8 years old, so you should be cautious about drawing conclusions from it.

There’s a difference between “supports IPv4-mapped IPv6 addresses” and “supports 464XLAT”. Apple platforms support IPv4-mapping IPv6 addresses just fine, and that support is used regularly on DNS64/NAT64 networks. See the various resources linked to by Supporting IPv6-only Networks.

If you explain more about the background for your questions, I may be able to offer more concrete advice.

Share and Enjoy
—
Quinn “The Eskimo!” @ Developer Technical Support @ Apple
let myEmail = "eskimo" + "1" + "@" + "apple.com"

0