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

Question

These may be phrased as separate questions for clarity, but they are all related to the same issue.

How are SSL certificate server names resolved?

Why do browsers seem to use the CN field of the certificate, but Java's mechanism seem to only look at "subject alternative names" only?

Is it possible to add alternative names to a SSL certificate using keytool? If not, is using openSSL instead a good option??

Just a little background: I need to get a main server to communicate with several servers using HTTPS. Obviously, we don't want to buy SSL certificates for every server (there could be many), so I want to use self-signed certificates (I have been using keytool to generate them). After I add the certificates as trusted in the OS, the browsers (IE and Chrome) happily accept the connection as trusted. However, even after adding the certificates to Java's cacerts, Java still won't accept the connection as trusted and throws the following Exception:

Caused by: java.security.cert.CertificateException: No subject alternative names present at sun.security.util.HostnameChecker.matchIP(HostnameChecker.java:142) at sun.security.util.HostnameChecker.match(HostnameChecker.java:75) at com.sun.net.ssl.internal.ssl.X509TrustManagerImpl.checkIdentity(X509T rustManagerImpl.java:264) at com.sun.net.ssl.internal.ssl.X509TrustManagerImpl.checkServerTrusted( X509TrustManagerImpl.java:250) at com.sun.net.ssl.internal.ssl.ClientHandshaker.serverCertificate(Clien tHandshaker.java:1185) ... 14 more

I found that I can make Java trust the certificate implementing my own HostNameVerifier, which I copied from here: com.sun.jbi.internal.security.https.DefaultHostnameVerifier just to test (by the way, the hostname passed as an argument to the HostnameVerifier is correct, so I think it should have been accepted).

I have been using the certificate field CN as the hostname (usually the IP address).

Can anybody please tell me if I am doing something wrong and point me in the right direction?

Answer 1

How host name verification should be done is defined in RFC 6125, which is quite recent and generalises the practice to all protocols, and replaces RFC 2818, which was specific to HTTPS. (I'm not even sure Java 7 uses RFC 6125, which might be too recent for this.)

From 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.

Essentially, the specific problem you have comes from the fact that you're using IP addresses in your CN and not a host name. Some browsers might work because not all tools follow this specification strictly, in particular because "most specific" in RFC 2818 isn't clearly defined (see discussions in RFC 6215).

If you're using keytool, as of Java 7, keytool has an option to include a Subject Alternative Name (see the table in the documentation for -ext): you could use -ext san=dns:www.example.com or -ext san=ip:10.0.0.1.

EDIT:

You can request a SAN in OpenSSL by changing openssl.cnf (it will pick the copy in the current directory if you don't want to edit the global configuration, as far as I remember, or you can choose an explicit location using the OPENSSL_CONF environment variable).

Set the following options (find the appropriate sections within brackets first):

[req]
req_extensions = v3_req

[ v3_req ]
subjectAltName=IP:10.0.0.1
# or subjectAltName=DNS:www.example.com

There's also a nice trick to use an environment variable for this (rather in than fixing it in a configuration file) here: http://www.crsr.net/Notes/SSL.html