IP address as hostname (CN) when creating a certificate? (HTTPS hostname wrong: should be <ipAddress>) [duplicate]

Question

Possible Duplicate:
How are SSL certificate server names resolved/Can I add alternative names using keytool?

I created a certificate and set the CN to the IP address of my server which is in xxx.xxx.xxx.xxx format. But when I try to run my code in Java, I'm getting a HTTPS hostname wrong: should be <xxx.xxx.xxx.xx> error message.

What could be wrong? I'm sure that I'm connecting to the correct IP address. However, I did not specify the port of the server on the certificate. Is the port required when supplying the value for CN? But I'm using the default port for https which is 8443. Also, I tried changing my CN from the IP address of my server to "localhost". It works after that. I'm thinking if the CN part does not accept an IP address as value?

Can you please explain how the CN works and what are the required value for it?

Thanks!

Answer 1

The identity you need to put in the certificate needs to be the one you're looking for via the URL. For example, if you're using https://www.example.net, your certificate needs to be valid for www.example.net; if you're using https://10.0.0.1/, your certificate needs to be valid for 10.0.0.1.

The Common Name RDN in the Subject DN of the certificate is normally only used when (a) there is no Subject Alternative Name DNS entry and (b) it's looking for a host name, not an IP address. This is defined in RFC 2818 Section 3.1:

If a subjectAltName extension of type dNSName is present, that MUST be used as the identity. Otherwise, the (most specific) Common Name field in the Subject field of the certificate MUST be used. Although the use of the Common Name is existing practice, it is deprecated and Certification Authorities are encouraged to use the dNSName instead.

[...]

In some cases, the URI is specified as an IP address rather than a hostname. In this case, the iPAddress subjectAltName must be present in the certificate and must exactly match the IP in the URI.

In general, using IP address in certificates is not recommended (see problems mentioned in RFC 6125. However, if you really need to, you'll need a certificate with the IP address is a SAN entry of IP address type, which you can generate as described in this answer.