Ipv4RecvDstAddr recvmsg stuff disabled on Linux

[brianmay]

It looks like definitions needed to find the real IP destination of a TPROXY redirected stream are disabled on Linux. Such as:

nix::sys::socket::sockopt::Ipv4RecvDstAddr
ControlMessageOwned::Ipv4RecvDstAddr

And the equivalent IPv6 versions.

https://powerdns.org/tproxydoc/tproxy.md.html

These are valid on Linux and should be enabled.

e.g. the code has:

#[cfg(any(
    target_os = "freebsd",
    target_os = "ios",
    target_os = "macos",
    target_os = "netbsd",
    target_os = "openbsd",
))]
#[cfg(feature = "net")]
sockopt_impl!(
    #[cfg_attr(docsrs, doc(cfg(feature = "net")))]
    /// The `recvmsg(2)` call will return the destination IP address for a UDP
    /// datagram.
    Ipv4RecvDstAddr, Both, libc::IPPROTO_IP, libc::IP_RECVDSTADDR, bool);

I believe this explains why I cannot use the on Linux.

[asomers]

How have you determined that "These are valid on Linux"? They aren't defined by the libc crate. If they really are defined on Linux, you should submit a PR to https://github.com/rust-lang/libc to add them there. Then you can submit a PR to Nix to enable them.

[brianmay]

I use these functions in code I am porting from Python. Like I said, these are required to get TPROXY support working, which I believe is a Linux only thing.

Not sure I understand what needs to change in libc, it already has constants such as IP_RECVDSTADDR defined for all platforms.

[brianmay]

Yuck, I find this so confusing. Linux might have a slightly different API that looks very similar:

   Compiling nix v0.24.1 (/home/brian/tree/3rdparty/nix)
error[E0425]: cannot find value `IP_RECVDSTADDR` in crate `libc`
   --> src/sys/socket/sockopt.rs:577:52
    |
577 |     Ipv4RecvDstAddr, Both, libc::IPPROTO_IP, libc::IP_RECVDSTADDR, bool);
    |                                                    ^^^^^^^^^^^^^^ help: a constant with a similar name exists: `IP_ORIGDSTADDR`
    |
   ::: /home/brian/.cargo/git/checkouts/libc-d1297d136f47e864/8dbd2c9/src/unix/linux_like/mod.rs:804:1
    |
804 | pub const IP_ORIGDSTADDR: ::c_int = 20;
    | --------------------------------------- similarly named constant `IP_ORIGDSTADDR` defined here

For more information about this error, try `rustc --explain E0425`.
error: could not compile `nix` due to previous error
warning: build failed, waiting for other jobs to finish...
error: could not compile `nix` due to previous error

So, yes, I think this is defined already in libc... But I might have got the name wrong.

In the python code I use IP_RECVORIGDSTADDR, libc6 defines an alias for that:

pub const IP_ORIGDSTADDR: ::c_int = 20;
pub const IP_RECVORIGDSTADDR: ::c_int = IP_ORIGDSTADDR;