Measure how often DNS hostnames aren't in preferred name form.

net/dns/dns_util.h

Index: net/dns/dns_util.h
diff --git a/net/dns/dns_util.h b/net/dns/dns_util.h
index 06cc3d6f8b731e5fe95f96b218afb339528d3036..81eb1a28ba11dd57a0af00b91c9f69274babe707 100644
--- a/net/dns/dns_util.h
+++ b/net/dns/dns_util.h
@@ -27,6 +27,21 @@ NET_EXPORT_PRIVATE bool DNSDomainFromDot(const base::StringPiece& dotted,
 // Checks that a hostname is valid. Simple wrapper around DNSDomainFromDot.
 NET_EXPORT_PRIVATE bool IsValidDNSDomain(const base::StringPiece& dotted);
 
+// Returns true if the character is valid in a DNS hostname label, whether in
+// the first position or later in the label.
+//
+// This function asserts a looser form of the restrictions in RFC 7719 (section
+// 2; https://tools.ietf.org/html/rfc7719#section-2): hostnames can include
+// characters a-z, A-Z, 0-9, -, and _, and any of those characters (except -)
+// are legal in the first position. The looser rules are necessary to support
+// service records (initial _), and non-compliant but attested hostnames that
+// include _. These looser rules also allow Punycode and hence IDN.
+//
+// TODO(palmer): In the future, when we can remove support for invalid names,
+// this can be a private implementation detail of |DNSDomainFromDot|, and need
+// not be NET_EXPORT_PRIVATE.
+NET_EXPORT_PRIVATE bool IsValidHostLabelCharacter(char c, bool is_first_char);
+
 // DNSDomainToString converts a domain in DNS format to a dotted string.
 // Excludes the dot at the end.
 NET_EXPORT_PRIVATE std::string DNSDomainToString(