Consider setting use_std3_ascii_rules=false to support underscores

[eminence]

Currently dog will set use_std3_ascii_rules: true when doing IDNA label conversion:

dns/src/strings.rs:25:

#[cfg(feature = "with_idna")]
fn label_to_ascii(label: &str) -> Result<String, unic_idna::Errors> {
    let flags = unic_idna::Flags{use_std3_ascii_rules: true, transitional_processing: false, verify_dns_length: true};
    unic_idna::to_ascii(label, flags)
}

This prevents domain names with an underscore (_) from being resolved. Underscores are common in SRV records.

Here is an example of dog in its current state:

$ cargo run -- _xmpp-client._tcp.gmail.com SRV
    Finished dev [unoptimized] target(s) in 0.28s
     Running `target/debug/dog _xmpp-client._tcp.gmail.com SRV`
dog: Invalid options: Invalid domain "_xmpp-client._tcp.gmail.com"

Here's what dig will return for the same domain:

$ dig _xmpp-client._tcp.gmail.com SRV +nostats +nocomments

; <<>> DiG 9.11.5-P4-5.1+deb10u5-Debian <<>> _xmpp-client._tcp.gmail.com SRV +nostats +nocomments
;; global options: +cmd
;_xmpp-client._tcp.gmail.com.   IN      SRV
_xmpp-client._tcp.gmail.com. 839 IN     SRV     20 0 5222 alt3.xmpp.l.google.com.
_xmpp-client._tcp.gmail.com. 839 IN     SRV     20 0 5222 alt4.xmpp.l.google.com.
_xmpp-client._tcp.gmail.com. 839 IN     SRV     20 0 5222 alt2.xmpp.l.google.com.
_xmpp-client._tcp.gmail.com. 839 IN     SRV     5 0 5222 xmpp.l.google.com.
_xmpp-client._tcp.gmail.com. 839 IN     SRV     20 0 5222 alt1.xmpp.l.google.com.

If dog is modified to not enforce std3 ascii rules, we get the output expected:

$ cargo run -- _xmpp-client._tcp.gmail.com SRV
    Finished dev [unoptimized] target(s) in 0.19s
     Running `target/debug/dog _xmpp-client._tcp.gmail.com SRV`
SRV _xmpp-client._tcp.gmail.com. 13m13s   20 0 "alt1.xmpp.l.google.com.":5222
SRV _xmpp-client._tcp.gmail.com. 13m13s   20 0 "alt3.xmpp.l.google.com.":5222
SRV _xmpp-client._tcp.gmail.com. 13m13s   20 0 "alt4.xmpp.l.google.com.":5222
SRV _xmpp-client._tcp.gmail.com. 13m13s   20 0 "alt2.xmpp.l.google.com.":5222
SRV _xmpp-client._tcp.gmail.com. 13m13s   5 0 "xmpp.l.google.com.":5222

Thanks!

See also: https://www.unicode.org/reports/tr46/#UseSTD3ASCIIRules

[noperator]

Ran into this issue today, and had to resort to using dig to resolve a subdomain containing an underscore.