To generate a certificate that will be signed by an external CA, issue the following command and input the information requested at the prompts. This input information will be written to the certificate. At the Common Name prompt, input the fully qualified name for the system that will use the certificate. If this name does not match the server, the application verifying the certificate will issue a warning to the user, rendering the verification provided by the certificate as useless.

# openssl req -new -nodes -out req.pem -keyout cert.key -sha256 -newkey rsa:2048
Generating a 2048 bit RSA private key
..................+++
.............................................................+++
writing new private key to 'cert.key'
-----
You are about to be asked to enter information that will be incorporated
into your certificate request.
What you are about to enter is what is called a Distinguished Name or a DN.
There are quite a few fields but you can leave some blank
For some fields there will be a default value,
If you enter '.', the field will be left blank.
-----
Country Name (2 letter code) [AU]:US
State or Province Name (full name) [Some-State]:PA
Locality Name (eg, city) []:Pittsburgh
Organization Name (eg, company) [Internet Widgits Pty Ltd]:My Company
Organizational Unit Name (eg, section) []:Systems Administrator
Common Name (eg, YOUR name) []:localhost.example.org
Email Address []:trhodes@FreeBSD.org

Please enter the following 'extra' attributes
to be sent with your certificate request
A challenge password []:
An optional company name []:Another Name

Other options, such as the expire time and alternate encryption algorithms, are available when creating a certificate. A complete list of options is described in openssl(1).

This command will create two files in the current directory. The certificate request, req.pem, can be sent to a CA who will validate the entered credentials, sign the request, and return the signed certificate. The second file, cert.key, is the private key for the certificate and should be stored in a secure location. If this falls in the hands of others, it can be used to impersonate the user or the server.

Alternately, if a signature from a CA is not required, a self-signed certificate can be created. First, generate the RSA key:

# openssl genrsa -rand -genkey -out cert.key 2048
0 semi-random bytes loaded
Generating RSA private key, 2048 bit long modulus
.............................................+++
.................................................................................................................+++
e is 65537 (0x10001)

Use this key to create a self-signed certificate. Follow the usual prompts for creating a certificate:

# openssl req -new -x509 -days 365 -key cert.key -out cert.crt -sha256
You are about to be asked to enter information that will be incorporated
into your certificate request.
What you are about to enter is what is called a Distinguished Name or a DN.
There are quite a few fields but you can leave some blank
For some fields there will be a default value,
If you enter '.', the field will be left blank.
-----
Country Name (2 letter code) [AU]:US
State or Province Name (full name) [Some-State]:PA
Locality Name (eg, city) []:Pittsburgh
Organization Name (eg, company) [Internet Widgits Pty Ltd]:My Company
Organizational Unit Name (eg, section) []:Systems Administrator
Common Name (e.g. server FQDN or YOUR name) []:localhost.example.org
Email Address []:trhodes@FreeBSD.org

This will create two new files in the current directory: a private key file cert.key, and the certificate itself, cert.crt. These should be placed in a directory, preferably under /etc/ssl/, which is readable only by root. Permissions of 0700 are appropriate for these files and can be set using chmod.

One use for a certificate is to encrypt connections to the Sendmail mail server in order to prevent the use of clear text authentication.

In FreeBSD 10.0-RELEASE and above, it is possible to create a self-signed certificate for Sendmail automatically. To enable this, add the following lines to /etc/rc.conf:

sendmail_enable="YES"
sendmail_cert_create="YES"
sendmail_cert_cn="localhost.example.org"

This will automatically create a self-signed certificate, /etc/mail/certs/host.cert, a signing key, /etc/mail/certs/host.key, and a CA certificate, /etc/mail/certs/cacert.pem. The certificate will use the Common Name specified in sendmail_cert_cn. After saving the edits, restart Sendmail:

# service sendmail restart

If all went well, there will be no error messages in /var/log/maillog. For a simple test, connect to the mail server's listening port using telnet:

# telnet example.com 25
Trying 192.0.34.166...
Connected to example.com.
Escape character is '^]'.
220 example.com ESMTP Sendmail 8.14.7/8.14.7; Fri, 18 Apr 2014 11:50:32 -0400 (EDT)
ehlo example.com
250-example.com Hello example.com [192.0.34.166], pleased to meet you
250-ENHANCEDSTATUSCODES
250-PIPELINING
250-8BITMIME
250-SIZE
250-DSN
250-ETRN
250-AUTH LOGIN PLAIN
250-STARTTLS
250-DELIVERBY
250 HELP
quit
221 2.0.0 example.com closing connection
Connection closed by foreign host.

If the STARTTLS line appears in the output, everything is working correctly.