func validHostname()

in transport/tlscommon/validhostname.go [117:159]

• 35 lines of code
• 18 McCabe index (conditional complexity)

func validHostname(host string, isPattern bool) bool {
	if !isPattern {
		host = strings.TrimSuffix(host, ".")
	}
	if len(host) == 0 {
		return false
	}

	for i, part := range strings.Split(host, ".") {
		if part == "" {
			// Empty label.
			return false
		}
		if isPattern && i == 0 && part == "*" {
			// Only allow full left-most wildcards, as those are the only ones
			// we match, and matching literal '*' characters is probably never
			// the expected behavior.
			continue
		}
		for j, c := range part {
			if 'a' <= c && c <= 'z' {
				continue
			}
			if '0' <= c && c <= '9' {
				continue
			}
			if 'A' <= c && c <= 'Z' {
				continue
			}
			if c == '-' && j != 0 {
				continue
			}
			if c == '_' {
				// Not a valid character in hostnames, but commonly
				// found in deployments outside the WebPKI.
				continue
			}
			return false
		}
	}

	return true
}