How to check whether hostname is domain name in Go?

Question

How can I check whether URL.Hostname is a valid domain name? I am searching for the exact same behaviour as URI.CheckHostName in the .NET framework.

Punchy

Currently, there is no solution using standard library only. You need to use third party library/regex. However, if you need to support internationalized domain name, the task may not trivial. See https://mcmap.net/q/378938/-fully-qualified-domain-name-validation, https://mcmap.net/q/209847/-valid-characters-of-a-hostname – Stanislaus

Answer 1

You could use idna, kind of, to check on this.

import  "golang.org/x/net/idna"
h, err := idna.Lookup.ToASCII("example.com")
fmt.Println(h, err)

Also see;

• https://github.com/golang/go/blob/go1.20.5/src/net/dnsclient.go#L72-L75
• https://github.com/golang/go/blob/go1.20.5/src/net/http/cookie.go#L329-L333

Coyne

Why do you say "kind of"? Would you mind explaining the caveat? – Collum

Answer 2

See my code here.

The function checks if the provided domain name is valid relative to the rules. It doesn't check if the domain name exist.

Kirstinkirstyn

Answer 3

The function IsDNSName from the govalidator package should do what you want. For details on installing it, see the README.

Loaves

It allows underscore in domain names which is wrong. See RFC1034 section 3.5. A later change to this RFC is to allow digits as first label character. The TLD label can't start with a digit to avoid confusion between domain names and IPv4 addresses. – Kirstinkirstyn

Answer 4

Here you have. https://pkg.go.dev/github.com/dchest/validator

func ValidateDomainByResolvingIt(domain string) error ValidateDomainByResolvingIt queries DNS for the given domain name, and returns nil if the the name resolves, or an error.

Gritty