First-time Service Setup
Many system services require some amount of initial setup before they can run properly for the first time. Common examples are the generation of private keys and certificates or a unique, system-specific identifier.
Traditionally, this was done by RPM scriptlets as part of the installation or upgrade of a package. This was sensible for a time when the majority of installations were performed by attended or unattended installers (such as anaconda and kickstart).
Today we see an increased reliance on generating virtual machine images for use in both traditional and cloud-computing environments. In those cases, having system-specific data created at package installation time is problematic. It means that the production of such images need to have significant care applied to remove any system-specific information about them and then additional tools written to apply the corrected information post-deployment.
This guideline describes a mechanism that can be used for both traditional and cloud-based deployment styles.
Note: this requirement can be waived if the equivalent functionality is incorporated as part of the service's own standard startup. These guidelines are meant to address services that require setup before the service can be started.
Defining System-Specific Setup
A particular setup task is defined thusly: "Any action that must be performed on the system where the service will be run that is not common to all systems running that service."
Some non-exhaustive examples:
- The SSH daemon generates a public/private host key
- The mod_ssl httpd module creates a self-signed certificate for the machine's hostname
- A remote logging service creates a UUID to represent this machine
Common Guidelines
For all system-specific cases, we will take advantage of systemd's ExecStartPre functionality.
Packagers will create a script in /usr/lib/systemd/system-init/ named after the package that it initializes. So for sshd, the script file would be /usr/lib/systemd/system-init/openssh-server (note that the language of the script is up to the packager, but the file must be executable). This script must implement all of the following steps:
- Perform a test for whether the initialization has already completed. This may be a simple test of file existence or a more complicated examination of the configuration, as needed. If the initialization has already occurred, the script must immediately return zero (success).
- Perform whatever steps are necessary to generate the configuration. If this cannot be accomplished, the script must return with a non-zero error code. This will prevent systemd from attempting to start the actual service. If this completes successfully, it must satisfy any and all requirements for the first step to to pass. On success, it must return zero.
The service's systemd unit file must be modified to include the following line within the [Service] section:
ExecStartPre=/usr/lib/systemd/system-init/<packagename>
Special Case: Self-signed Certificate Generation
OpenSSL and PEM Certificates
This tutorial will provide the steps to produce something akin to a self-signed certificate, except that the resulting service certificate cannot be used to sign further certificates (thus closing a potential security issue). In broad terms, what we are doing is creating a short-lived certificate authority, using that to sign a service certificate and then destroying the key material for the temporary authority.
(Note: for any instance of $package below, substitute the name of your package)
Create Short-Lived Certificate Authority
First, create an OpenSSL configuration file for the CA certificate. It should be similar to this:
[ req ] distinguished_name = req_distinguished_name prompt = no x509_extensions = v3_ca [ req_distinguished_name ] C = -- ST = SomeState L = SomeCity O = The Fedora Project OU = $package CN = $package.<real_fqdn> [ v3_ca ] subjectKeyIdentifier=hash authorityKeyIdentifier=keyid:always,issuer basicConstraints = CA:TRUE
Most of the organizational values above can be changed, but CN must be $package.<real_fqdn>. Create this file with the name $package-ssl-ca.cnf in a location owned by your package. You may retain or destroy this file later as you prefer.
Next, we will generate the private key for this temporary Certificate Authority. Set the umask so that only root/owner can read the files we create. Then create the key file with the openssl command. We will create the temporary key file in /dev/shm so that it is unlikely to ever be written to persistent media.
OLDUMASK=`umask` umask 0077 TMPKEY=`mktemp --tmpdir=/dev/shm XXXXXXXXXXXX` /usr/bin/openssl genrsa -out $TMPKEY 2048
Note: it is *highly* recommended to use a 2048-bit or greater key. Any package that is not compatible with 2048-bit keys should have a bug open to track this shortcoming.
Next, we will use this private key to create the temporary CA Certificate:
/usr/bin/openssl req -new -x509 -days 3650 \
-config /path/to/$package-ssl-ca.cnf \
-key $TMPKEY \
-out /path/to/$package-ssl-ca.crt
You can adjust the value of -days as you prefer. Store the $package-ssl-ca.crt file in the same location as the $package-ssl-ca.cnf. Now we have a certificate authority available to sign our service certificate.
Create and Sign a Service Certificate
Similar to the CA certificate generation, we need to create an OpenSSL configuration file for the service certificate as below:
[ req ] distinguished_name = req_distinguished_name prompt = no req_extensions = v3_req [ req_distinguished_name ] C = -- ST = SomeState L = SomeCity O = The Fedora Project OU = $package CN = <real FQDN> [ v3_req ] basicConstraints = CA:FALSE
Note the specific differences from the CA certificate configuration file: the v3_ca reference and section has been replaced by a v3_req section which asserts that this certificate is not a Certificate Authority (and cannot be used to sign other certificates). Also note that unlike the CA certificate, this configuration requires the CN to match the machine's real fully-qualified hostname. If you do not provide this, clients will be unable to validate this machine's certificate.
Save this file as $package-ssl-service.cnf in the same location as the CA configuration. As with that configuration, this file can be removed after processing is complete.
Next, we will generate a secure private key for the service certificate. This key must be retained and provided in the location that the service expects it for SSL to function properly. Substitute the correct location for your package below:
/usr/bin/openssl genrsa -out /path/to/service/key 2048
As above, this generates a 2048-bit key. This should be considered the recommended minimum key strength. If a package does not support 2048-bit or higher keys, a bug should be opened.
Next, we will create a signing request for this key and sign it with our temporary CA.
/usr/bin/openssl req -new \
-config /path/to/$package-ssl-service.cnf \
-key /path/to/service/key \
-out /path/to/$package-ssl.csr
/usr/bin/openssl x509 -req -days 3650 \
-in /path/to/$package-ssl.csr \
-CA /path/to/$package-ssl-ca.crt \
-CAkey $TMPKEY \
-CAcreateserial \
-out /path/to/service/certificate \
-extfile /path/to/$package-ssl-ca.cnf
Carefully note the "-ca" suffixes there; it can be tricky to see where to use the CA certificate information vs. the service certificate information.
The last step is to clean up the resulting files, as they are all owned by root with restrictive permissions:
# Delete the temporary CA key so nothing else can be signed with it
# We'll also overwrite it with zeros before deleting it, just in case.
dd if=/dev/zero of=$TMPKEY bs=1k count=16
rm -f $TMPKEY
# Optionally delete unneeded CSR and the OpenSSL Configuration Files
rm -f /path/to/$package-ssl.csr \
/path/to/$package-ssl-ca.cnf \
/path/to/$package-ssl-service.cnf
# Restore the original umask
umask $OLDUMASK
chown pkguser:pkguser /path/to/service/certificate \
/path/to/service/key
# Appropriate chmod() calls here
As a final, optional step, the private temporary CA certificate can be added to the local root CA list so that requests from the local machine will be able to connect as trusted clients without having to skip validation (as would be normal for self-signed certificates). Unlike traditional self-signed certificates, since this temporary CA cannot be used to sign anything else, this does not open your system to a risk of the service certificate being used to falsely sign something.
cp /path/to/$package-ssl-ca.crt /etc/pki/ca-trust/source/anchors/ update-ca-trust extract