Is_loopback failing on ipv4 in ipv6, bug or desired behavior?

help
1

While writing some networking code and making it IPv6 ready I encountered some odd behavior in the standard library.
If I bind my server so 0.0.0.0 or [::1] and telnet/curl from localhost, I can easily tell whether the connection came from a local user or not by using the is_loopback call on IpAddr, Ipv4Addr or Ipv6Addr.

Once I bind my server to [::] to work on IPv4 AND IPv6 at the same time and then query it via v4 to 127.0.0.1 the is_loopback call returns false.
I then have to manually try conversion of the Ipv6Addr into an Ipv4Addr and perform a second check with is_loopback.

In my opinion, this behavior should either be clearly documented in the standard library or better yet should happen automatically (at least in IpAddr) since an ipv6 encapsulated ipv4 loopback address is still a perfectly valid loopback address.

The current documentation in Ipv6Addr states that it is a check for [::1] but a clear statement that IPv4 in IPv6 loopback addresses are not covered might help. I also guess that having the current minimal checks in both variants (v4 and v6) makes sense but the general is_loopback in IpAddr itself could provide the convenient conversion as it covers v4 and v6 anyways.

use std::net::{Ipv4Addr, Ipv6Addr};

fn main() {
    let ipv4 = Ipv4Addr::new(127, 0, 0, 1);
    let ipv6 = Ipv6Addr::new(0, 0, 0, 0, 0, 0, 0, 0x1);
    let v4_in_v6 = Ipv6Addr::new(0, 0, 0, 0, 0, 0xffff, 0x7f00, 0x1);
    
    println!("{} is loopback? {} ", ipv4, ipv4.is_loopback());
    println!("{} is loopback? {} ", ipv6, ipv6.is_loopback());
    println!("{} is loopback? {} ", v4_in_v6, v4_in_v6.is_loopback());
    println!("{} is loopback? {} ", v4_in_v6, v4_in_v6.to_ipv4().unwrap().is_loopback());
}

(Playground)

Output:

127.0.0.1 is loopback? true 
::1 is loopback? true 
::ffff:127.0.0.1 is loopback? false 
::ffff:127.0.0.1 is loopback? true

Is this forum the right place for this discussion or should I have used the internals forum or just open an issue in github?

2

I think it makes sense for this to be considered a loopback address.

3

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