Setting SMTP to Use TLS

Starting in the Solaris 10 1/06 release, SMTP can use Transport Layer Security (TLS) in version 8.13 of sendmail. This service to SMTP servers and clients provides private, authenticated communications over the Internet, as well as protection from eavesdroppers and attackers. Note that this service is not enabled by default.

How to Set SMTP to Use TLS

The following procedure uses sample data to show you how to set up the certificates that enable sendmail to use TLS. For more information, see Support for Running SMTP With TLS in Version 8.13 of sendmail.

Become superuser or assume an equivalent role.
Roles contain authorizations and privileged commands. For more information about roles, see Configuring RBAC (Task Map) in System Administration Guide: Security Services. To configure a role with the Primary Administrator profile, see Chapter 2, Working With the Solaris Management Console (Tasks), in System Administration Guide: Basic Administration.

Stop sendmail.

# svcadm -t disable network/smtp:sendmail

Set up the certificates that enable sendmail to use TLS.
1. Complete the following:
```
# cd /etc/mail
# mkdir -p certs/CA
# cd certs/CA
# mkdir certs crl newcerts private
# echo "01" > serial
# cp /dev/null index.txt
# cp /etc/sfw/openssl/openssl.cnf .
```
2. Use your preferred text editor to change the dir value in the openssl.cnf file from /etc/sfw/openssl to /etc/mail/certs/CA.
3. Use the openssl command-line tool to implement TLS.
  Note that the following command line generates interactive text.
```
# openssl req -new -x509 -keyout private/cakey.pem -out cacert.pem -days 365 \ -config openssl.cnf
Generating a 1024 bit RSA private key
.....................................++++++
.....................................++++++
writing new private key to 'private/cakey.pem'
Enter PEM pass phrase:
Verifying - Enter PEM pass phrase:
-----
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) []:US
State or Province Name (full name) []:California
Locality Name (eg, city) []:Menlo Park
Organization Name (eg, company) [Unconfigured OpenSSL Installation]:Sun Microsystems
Organizational Unit Name (eg, section) []:Solaris
Common Name (eg, YOUR name) []:somehost.somedomain.example.com
Email Address []:[email protected]
```
  req
  
  This command creates and processes certificate requests.
  
  -new
  
  This req option generates a new certificate request.
  
  -x509
  
  This req option creates a self-signed certificate.
  
  -keyout private/cakey.pem
  
  This req option enables you to assign private/cakey.pem as the file name for your newly created private key.
  
  -out cacert.pem
  
  This req option enables you to assign cacert.pem as your output file.
  
  -days 365
  
  This req option enables you to certify the certificate for 365 days. The default value is 30.
  
  -config openssl.cnf
  
  This req option enables you to specify openssl.cnf as the configuration file.
  
  Note that this command requires that you provide the following:
  - Country Name, such as US.
  - State or Province Name, such as California.
  - Locality Name, such as Menlo Park.
  - Organization Name, such as Sun Microsystems.
  - Organizational Unit Name, such as Solaris.
  - Common Name, which is the machine's fully qualified host name. For more information, see the check-hostname(1M) man page.
  - Email Address, such as [email protected].

(Optional) If you need a new secure connection, make a new certificate and sign the new certificate with the certificate authority.

Make a new certificate.

# cd /etc/mail/certs/CA
# openssl req -nodes -new -x509 -keyout newreq.pem -out newreq.pem -days 365 \ -config openssl.cnf
Generating a 1024 bit RSA private key
..............++++++
..............++++++
writing new private key to 'newreq.pem'
-----
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) []:US
State or Province Name (full name) []:California
Locality Name (eg, city) []:Menlo Park
Organization Name (eg, company) [Unconfigured OpenSSL Installation]:Sun Microsystems
Organizational Unit Name (eg, section) []:Solaris
Common Name (eg, YOUR name) []:somehost.somedomain.example.com
Email Address []:[email protected]

This command requires that you provide the same information that you provided in step 3c.

Note that in this example, the certificate and private key are in the file newreq.pem.

Sign the new certificate with the certificate authority.

# cd /etc/mail/certs/CA
# openssl x509 -x509toreq -in newreq.pem -signkey newreq.pem -out tmp.pem
Getting request Private Key
Generating certificate request
# openssl ca -config openssl.cnf -policy policy_anything -out newcert.pem -infiles tmp.pem
Using configuration from openssl.cnf
Enter pass phrase for /etc/mail/certs/CA/private/cakey.pem:
Check that the request matches the signature
Signature ok
Certificate Details:
        Serial Number: 1 (0x1)
        Validity
            Not Before: Jun 23 18:44:38 2005 GMT
            Not After : Jun 23 18:44:38 2006 GMT
        Subject:
            countryName               = US
            stateOrProvinceName       = California
            localityName              = Menlo Park
            organizationName          = Sun Microsystems
            organizationalUnitName    = Solaris
            commonName                = somehost.somedomain.example.com
            emailAddress              = [email protected]
        X509v3 extensions:
            X509v3 Basic Constraints: 
                CA:FALSE
            Netscape Comment: 
                OpenSSL Generated Certificate
            X509v3 Subject Key Identifier: 
                93:D4:1F:C3:36:50:C5:97:D7:5E:01:E4:E3:4B:5D:0B:1F:96:9C:E2
            X509v3 Authority Key Identifier: 
                keyid:99:47:F7:17:CF:52:2A:74:A2:C0:13:38:20:6B:F1:B3:89:84:CC:68
                DirName:/C=US/ST=California/L=Menlo Park/O=Sun Microsystems/OU=Solaris/\
                [email protected]/[email protected]
                serial:00

Certificate is to be certified until Jun 23 18:44:38 2006 GMT (365 days)
Sign the certificate? [y/n]:y


1 out of 1 certificate requests certified, commit? [y/n]y
Write out database with 1 new entries
Data Base Updated
# rm -f tmp.pem

In this example the file newreq.pem contains the unsigned certificate and private key. The file newcert.pem contains the signed certificate.

x509 utility: Displays certificate information, converts certificates to various forms, and signs certificate requests
ca application: Used to sign certificate requests in a variety of forms and to generate CRLs (certificate revocation lists)

Enable sendmail to use the certificates by adding the following lines to your .mc file.

define(`confCACERT_PATH', `/etc/mail/certs')dnl define(`confCACERT', `/etc/mail/certs/CAcert.pem')dnl define(`confSERVER_CERT', `/etc/mail/certs/MYcert.pem')dnl define(`confSERVER_KEY', `/etc/mail/certs/MYkey.pem')dnl define(`confCLIENT_CERT', `/etc/mail/certs/MYcert.pem')dnl define(`confCLIENT_KEY', `/etc/mail/certs/MYkey.pem')dnl

For more information, see Configuration File Options for Running SMTP With TLS.

Rebuild and install your sendmail.cf file in your /etc/mail directory.
For detailed instructions, see Building the sendmail.cf Configuration File.

Create symbolic links from the files you created with openssl to the files you defined in your .mc file.

# cd /etc/mail/certs
# ln -s CA/cacert.pem CAcert.pem
# ln -s CA/newcert.pem MYcert.pem
# ln -s CA/newreq.pem MYkey.pem

For added security, deny read permission to group and others for MYkey.pem.
```
# chmod go-r MYkey.pem
```
Use a symbolic link to install CA certs in the directory assigned to confCACERT_PATH.
```
# C=CAcert.pem
# ln -s $C `openssl x509 -noout -hash < $C`.0
```
For secure mail with other hosts, install their host certificates.
1. Copy the file defined by the other host's confCACERT option to /etc/mail/certs/host.domain.cert.pem.
  Replace host.domain with the other host's fully qualified host name.
2. Use a symbolic link to install CA certs in the directory assigned to confCACERT_PATH.
```
# C=host.domain.cert.pem
# ln -s $C `openssl x509 -noout -hash < $C`.0
```
  Replace host.domain with the other host's fully qualified host name.
Restart sendmail.
```
# svcadm enable network/smtp:sendmail
```

Example 13-1 Received: Mail Header

The following is an example of a Received: header for secure mail with TLS.

Received: from his.example.com ([IPv6:2001:db8:3c4d:15::1a2f:1a2b])
        by her.example.com (8.13.4+Sun/8.13.4) with ESMTP id j2TNUB8i242496
        (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-SHA bits=256 verify=OK)
        for <[email protected]>; Tue, 29 Mar 2005 15:30:11 -0800 (PST)
Received: from her.example.com (her.city.example.com [192.168.0.0])
        by his.example.com (8.13.4+Sun/8.13.4) with ESMTP id j2TNU7cl571102
        version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-SHA bits=256 verify=OK)
        for <[email protected]>; Tue, 29 Mar 2005 15:30:07 -0800 (PST)

Note that the value for verify is OK, which means that the authentication was successful. For more information, see Macros for Running SMTP With TLS.

Setting SMTP to Use TLS

How to Set SMTP to Use TLS

See Also