Enable support of scope ID for IPv6 link local addresses

[M-Tarasov]

Try to resolve issue #568

[alexcrichton]

Thanks! Is it possible to add a test for this?

[M-Tarasov]

Thanks! Is it possible to add a test for this?

Unfortunately, I have no idea how it could be tested with unit tests: from rust point of view, it just on more define in build script for a C library, from the other side, the effect of this define could be observed deep in the libcurl library itself. It is possible, to test it in an integration test within specific environment, but it would require much effort to set up. I could just say, that this change is working for my project.

[alexcrichton]

Does connecting with ipv6 not work at all without this? Or could a dummy server be set up with a different link-local ID and then curl tries to connect to it and it would fail if this isn't supported?

[M-Tarasov]

Does connecting with ipv6 not work at all without this? Or could a dummy server be set up with a different link-local ID and then curl tries to connect to it and it would fail if this isn't supported?

The issue is related to the IPv6 link-local addresses (all addresses within fe80::/10 range) only.

Just one example: let suppose you have a local web server running with python3 -m http.server -b "::" 8123. Your eth0 interface has a link local IPv6 address fe80::c4:d164:150d:282c. Then you make standard curl calls like this:

curl http://[fe80::c4:d164:150d:282c%eth0]:8123  # works fine, with scope id eth0
curl http://[fe80::c4:d164:150d:282c]:8123       # will not work without scope id

Without the flag (HAVE_SOCKADDR_IN6_SIN6_SCOPE_ID) enabled, both variants would fail.

In the libcurl itself, the HAVE_SOCKADDR_IN6_SIN6_SCOPE_ID flag is defined whenever the system's struct sockaddr_in6 has the field sin6_scope_id (no idea, where it could be missing, maybe some very old systems). See curl's original CMakeLists.txt: https://github.com/curl/curl/blob/curl-8_9_0/CMakeLists.txt#L291