IPv6 Multicast unable to set send adapter/scope

[BitFis]

Is your feature request related to a problem? Please describe.

Unable to send on a specific adapter in case of multiple adapters available. (tested on linux)

Describe the solution you would like

Provide an option to configure the ipv6 adapter/scope either via source address (in coap_new_client_session call).
Or on send.

Describe alternatives you have considered

using coap_new_client_session, I tryed to configure the scope for the local_if / coapLocalAddress and server / mcastAddress argument.
For both the scope is ignored and the request is sent on the default adapter.
(If required can provide test code)

Only workaround I found by accessing the socket directly and configure:

// hidden setup code ...
auto session = coap_new_client_session(ctx, coapLocalAddress,
                                          mcastAddress,
                                          COAP_PROTO_UDP);
setsockopt(session->sock.fd, SOL_SOCKET, SO_BINDTODEVICE,
                   interface, strlen(interface))

Adapter test setup on linux for reference

# create links
sudo ip link add name veth0 type veth peer name veth1
sudo ip link add name veth2 type veth peer name veth3

# add some ipv6 addresses
sudo ip -6 addr add 2001:db8::1/64 dev veth0
sudo ip -6 addr add 2001:db9::2/64 dev veth2

# bring all up
sudo ip link set veth0 up
sudo ip link set veth1 up
sudo ip link set veth2 up
sudo ip link set veth3 up

# adapter available on veth0 and veth2 for testing

# cleanup
sudo ip link delete veth0
sudo ip link delete veth2

Additional context

Non.

[mrdeep1]

I need to do further testing here to get to the bottom of what is not working as expected.

IP6 addresses specified with a scope_id/interface suffix (e.g. ip6-address%scope_id) get the correct scope_id filled in to sin6.sin6_scope_id by coap_resolve_address_info() which calls getaddrinfo().

The sendmsg() to the multicast address does have the scope_id set. If the local address to bind to is defined using %scope_id, the bind() takes place, but the socket information (using getsockname()) no longer contains the scope_id.

The sendmsg() however fails unless the multicast's scope_id is for the default interface.

[mrdeep1]

Please see #1571 which works for me on Ubuntu 24. Does this work for you?