std::net::Ipv6Addr::from_str accepts "::f:f:f:f:f:f:f:f" but rejects "f:f:f:f:f:f:f:f::"

[l4AgXc]

When all eight segments are present in an IPv6 address specification, std::net::Ipv6Addr::from_str allows :: anywhere, except at the very end.

Here is a test program:

use std::net;
fn main() {
    let tests = [
        "::f:f:f:f:f:f:f:f",
        "f::f:f:f:f:f:f:f",
        "f:f::f:f:f:f:f:f",
        "f:f:f::f:f:f:f:f",
        "f:f:f:f::f:f:f:f",
        "f:f:f:f:f::f:f:f",
        "f:f:f:f:f:f::f:f",
        "f:f:f:f:f:f:f::f",
        "f:f:f:f:f:f:f:f::",
    ];
    for test in tests.iter() {
        match test.parse::<net::Ipv6Addr>() {
            Ok(addr) => println!("{:?}\t{}", test, addr),
            Err(err) => println!("{:?}\t{}", test, err),
        }
    }
}

The output is:

"::f:f:f:f:f:f:f:f"	f:f:f:f:f:f:f:f
"f::f:f:f:f:f:f:f"	f:f:f:f:f:f:f:f
"f:f::f:f:f:f:f:f"	f:f:f:f:f:f:f:f
"f:f:f::f:f:f:f:f"	f:f:f:f:f:f:f:f
"f:f:f:f::f:f:f:f"	f:f:f:f:f:f:f:f
"f:f:f:f:f::f:f:f"	f:f:f:f:f:f:f:f
"f:f:f:f:f:f::f:f"	f:f:f:f:f:f:f:f
"f:f:f:f:f:f:f::f"	f:f:f:f:f:f:f:f
"f:f:f:f:f:f:f:f::"	invalid IP address syntax

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rustc 1.21.0
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commit-date: unknown
host: x86_64-unknown-linux-gnu
release: 1.21.0
LLVM version: 4.0

[l4AgXc]

The grammar in RFC 3986 (URI syntax) says that all these cases should be rejected; i.e., :: is not allowed to expand to nothing.

IPv6address =                            6( h16 ":" ) ls32
            /                       "::" 5( h16 ":" ) ls32
            / [               h16 ] "::" 4( h16 ":" ) ls32
            / [ *1( h16 ":" ) h16 ] "::" 3( h16 ":" ) ls32
            / [ *2( h16 ":" ) h16 ] "::" 2( h16 ":" ) ls32
            / [ *3( h16 ":" ) h16 ] "::"    h16 ":"   ls32
            / [ *4( h16 ":" ) h16 ] "::"              ls32
            / [ *5( h16 ":" ) h16 ] "::"              h16
            / [ *6( h16 ":" ) h16 ] "::"

ls32        = ( h16 ":" h16 ) / IPv4address
            ; least-significant 32 bits of address

h16         = 1*4HEXDIG
            ; 16 bits of address represented in hexadecimal

[kennytm]

I think all of them should be rejected because according to rfc4291 §2.2,

The use of "::" indicates one or more groups of 16 bits of zeros.

At least both Python3's ipaddress.ip_address function and Go's net.ParseIP function doesn't recognize f:f:f:f::f:f:f:f.