ipv6_mreq does not allow interface to be set

[hjr3]

The ability to set an interface index value should be exposed in

nix/src/sys/socket/multicast.rs

Line 34 in 5e3c954

pub fn new(group: Ipv6Addr) -> ipv6_mreq {

Related to tokio-rs/mio#377

[kamalmarhubi]

@hjr3 what's the urgency for this issue? Are you or someone else trying to do this right now? There's some refactoring I'd like to see happen to this module before we expose this properly...

[hjr3]

@kamalmarhubi i put a PR up, but we can hold off if there are larger plans

[hjr3]

Oops, I see #324 was already opened. I won't be opening a PR then. We can hold off until refactor too. I just wanted to track the issue.

[simias]

I just stumbled upon this issue because... I wanted to specify the mreq interface.

I was thinking about the best way to solve this problem but then I realized that nix has already decided, the IPV4 counterpart to this constructor looks like:

nix/src/sys/socket/mod.rs

Lines 349 to 359 in 566e2d7

	impl IpMembershipRequest {
	/// Instantiate a new `IpMembershipRequest`
	///
	/// If `interface` is `None`, then `Ipv4Addr::any()` will be used for the interface.
	pub fn new(group: Ipv4Addr, interface: Option<Ipv4Addr>) -> Self {
	IpMembershipRequest(libc::ip_mreq {
	imr_multiaddr: group.0,
	imr_interface: interface.unwrap_or_else(Ipv4Addr::any).0,
	})
	}
	}

I think if the IPV4 version just uses an optional parameter for the interface it would be strange for the IPV6 not to do the same thing.

Therefore I propose:

impl Ipv6MembershipRequest {
    /// Instantiate a new `Ipv6MembershipRequest`
    ///
    /// If `interface` is `None`, then `0` will be used for the interface
    pub fn new(group: Ipv6Addr, interface: Option<c_uint>) -> Self {
        Ipv6MembershipRequest(libc::ipv6_mreq {
            ipv6mr_multiaddr: group.0,
            ipv6mr_interface: interface.unwrap_or(0),
        })
    }
}

Of course, it would be a breaking change but I think it's warranted to make the interface consistent with IpMembershipRequest.

[sgasse]

Just stumbled into this issue as well. What is the recommended way to work around it until it is solved properly? My first quick working hack was this, but I'd really like to avoid so much unsafe code:

let ipv6mr_interface = nix::net::if_::if_nametoindex("my_interface").unwrap();

let join_request = libc::ipv6_mreq {
    ipv6mr_multiaddr: unsafe { std::mem::transmute(*group) },
    // Without the conversion, there is a mismatch between `u32` and `i32` when cross-compiling for ARM
    ipv6mr_interface: ipv6mr_interface.try_into().unwrap(),
};
let join_request: Ipv6MembershipRequest = unsafe { std::mem::transmute(join_request) };