STARTTLS and stream encryption are discussed in detail in Section 10.10. Among the items you must provide is a directory that contains the certificate of the authority for the server (ServerCertFile) and client (ClientCertFile), as well as other certificates of authority you wish to trust. This directory contains both the certificates of authority and hashes of those certificates (more about this soon). The location of the CA certificate directory is specified with this CACertPath option, with declarations that look like this:

O CACertPath=dir                configuration file (V8.12 and later) 
-OCACertPath=dir                command line (V8.12 and later) 
define(`confCACERT_PATH',`dir') mc configuration (V8.12 and later 

Here, dir is a full path specification of the directory containing the CA certificate files and their hashes. The dir can contain sendmail macros, and if so, those macros will be expanded (their values used) when the configuration file, or command line, is read:

define(`confCACERT_PATH', `${MyCERTPath}')

The dir must be a full pathname (must begin with a slash), or the directory will be rejected and the following error logged:

STARTTLS=server: file dir unsafe: reason 
STARTTLS=client: file dir unsafe: reason 

Here, dir is the directory specified by the CACertPath option (CACertPath), and path is the file specified by this option. The num is the error number returned by the ssl(8) software.

The dir must contain the hashes of each certificate of authority, where each hash is either a file, or a link to the certificate. Symbolic links can be generated with a command such as the following:^[12]

^[12] On your system the command might be ssl instead.

% ln -s cert_file `openssl x509 -noout -hash <  cert_file`.0  

The CACertFile option is not safe. If specified from the command line, it can cause sendmail to relinquish its special privileges.