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Underscore characters in dnsNames for SSL Certificates
The use of underscore characters in dnsNames is not allowed in Internet standards but has historically been treated as a gray area when used in the SAN field of TLS/SSL certificates. Most CAs are disallowing this issuance following discussion in the CA/Browser Forum.
We have previously issued browser-trusted TLS/SSL certificates that include dnsNames with underscore characters in the SAN fields.
We will cease this practice on January 25, 2019. Customers will no longer be able to renew certificates, nor to request new TLS/SSL certificates that include dnsNames with underscore characters, after this date.
We apologise for the inconvenience and suggest that you begin transition to domains that do not use underscore characters.
Please contact us on certificates@jisc.ac.uk if you have any further questions.
Comments
Underscores was one of the first things I banned (for the UK e-Science CA). Back in the early noughties a university had creatively asked for O=University_of_Place in the DN which bizarrely (and totally incorrectly) got encoded as IA5String by the software (as opposed to printableString). Although the alternative name is encoded as IA5String, we also have the hostname in the CN to support older software (cf RFC 2818, section 3.1), which, until UTF8 came along, had to be printableString or you were asking for trouble...
See also GFD.225 for a long list of experiences learned the hard way. http://www.ogf.org/documents/GFD.225.pdf