Managing Services with `systemd`
Photon OS manages services with systemd. By using systemd, Photon OS adopts a contemporary Linux standard to bootstrap the user space and concurrently start services. This is an architecture that differs from traditional Linux systems such as SUSE Linux Enterprise Server.
A traditional Linux system contains an initialization system called SysVinit. With SLES 11, for instance, the SysVinit-style init programs control how the system starts up and shuts down. Init implements system runlevels. A SysVinit runlevel defines a state in which a process or service runs.
In contrast to a SysVinit system, systemd defines no such runlevels. Instead, systemd uses a dependency tree of targets to determine which services to start when. Combined with the declarative nature of systemd commands, systemd targets reduce the amount of code needed to run a command, leaving you with code that is easier to maintain and probably faster to execute. For an overview of systemd, see systemd System and Service Manager and the man page for systemd.
On Photon OS, you must manage services with systemd and systemctl, its command-line utility for inspecting and controlling the system, and not the deprecated commands of init.d.
For more information, see the index of all the systemd man pages, including systemctl, at
https://www.freedesktop.org/software/systemd/man/
1 - Viewing Services
To view a description of all the loaded and active units, run the systemctl command without any options or arguments:
systemctl
To see all the loaded, active, and inactive units and their description, run the following command:
systemctl --all
To see all the unit files and their current status but no description, run thie following command:
systemctl list-unit-files
The grep command filters the services by a search term, a helpful tactic to recall the exact name of a unit file without looking through a long list of names. Example:
systemctl list-unit-files | grep network
org.freedesktop.network1.busname static
dbus-org.freedesktop.network1.service enabled
systemd-networkd-wait-online.service enabled
systemd-networkd.service enabled
systemd-networkd.socket enabled
network-online.target static
network-pre.target static
network.target static
2 - Controlling Services
To control services on Photon OS, use systemctl command.
For example, instead of running the /etc/init.d/ssh script to stop and start the OpenSSH server on a init.d-based Linux system, run the following systemctl commands on Photon OS:
systemctl stop sshd
systemctl start sshd
The systemctl tool includes a range of commands and options for inspecting and controlling the state of systemd and the service manager. For more information, see the systemctl man page.
3 - Creating a Startup Service
Use systemd to create a startup service.
The following example shows you how to create a systemd startup service that changes the maximum transmission unit (MTU) of the default Ethernet connection, eth0.
- Concatenate the following block of code into a file:
cat << EOF >> /lib/systemd/system/eth0.service
[Unit]
Description=Network interface initialization
After=local-fs.target network-online.target network.target
Wants=local-fs.target network-online.target network.target
[Service]
ExecStart=/usr/sbin/ifconfig eth0 mtu 1460 up
Type=oneshot
[Install]
WantedBy=multi-user.target
EOF
- Set the service to auto-start when the system boots:
cd /lib/systemd/system/multi-user.target.wants/
ln -s ../eth0.service eth0.service
4 - Disabling the Photon OS httpd.service
If your application or appliance includes its own HTTP server, you must turn off and disable the HTTP server that comes with Photon OS so that it does not conflict with your own HTTP server.
To stop it and disable it, run the following commands as root:
systemctl stop httpd.service
systemctl disable httpd.service
5 - Installing Sendmail
Before you install Sendmail, you should set the fully qualified domain name (FQDN) of your Photon OS machine.
By default, Sendmail is not installed with either the minimal or full version of Photon OS. When you install Sendmail, it provides Photon OS with a systemd service file that typically enables Sendmail. If the service is not enabled after installation, you must enable it.
Sendmail resides in the Photon extras repository. You can install it with tdnf after setting the machine’s FQDN.
Procedure
- Check whether the FQDN of the machine is set by running the
hostnamectl status command:
hostnamectl status
Static hostname: photon-d9ee400e194e
Icon name: computer-vm
Chassis: vm
Machine ID: a53b414142f944319bd0c8df6d811f36
Boot ID: 1f75baca8cc249f79c3794978bd82977
Virtualization: vmware
Operating System: VMware Photon/Linux
Kernel: Linux 4.4.8
Architecture: x86-64
Note
In the results above, the FQDN is not set. The Photon OS machine only has a short name. If the FQDN were set, the hostname would be in its full form, typically with a domain name.If the machine does not have an FQDN, set one by running hostnamectl set-hostname new-name, replacing new-name with the FQDN that you want. For example:
hostnamectl set-hostname photon-d9ee400e194e.corp.example.com
The hostnamectl status command now shows that the machine has an FQDN:
root@photon-d9ee400e194e [ ~ ]# hostnamectl status
Static hostname: photon-d9ee400e194e.corp.example.com
Icon name: computer-vm
Chassis: vm
Machine ID: a53b414142f944319bd0c8df6d811f36
Boot ID: 1f75baca8cc249f79c3794978bd82977
Virtualization: vmware
Operating System: VMware Photon/Linux
Kernel: Linux 4.4.8
Architecture: x86-64
Install Sendmail:
Verify if Sendmail is enabled:
systemctl status sendmail
Enable Sendmail if it is disabled and then start it:
systemctl enable sendmail
systemctl start sendmail
6 - Auditing System Events with auditd
To manage security on Photon OS, the Linux auditing service auditd is enabled and active by default on the full version of Photon OS.
The following command shows the security status:
systemctl status auditd
* auditd.service - Security Auditing Service
Loaded: loaded (/usr/lib/systemd/system/auditd.service; enabled; vendor preset: enabled)
Active: active (running) since Fri 2016-04-29 15:08:50 UTC; 1 months 9 days ago
Main PID: 250 (auditd)
CGroup: /system.slice/auditd.service
`-250 /sbin/auditd -n
To help improve security, the auditd service can monitor file changes, system calls, executed commands, authentication events, and network access. After you implement an audit rule to monitor an event, the aureport tool generates reports to display information about the events.
You can use the auditctl utility to set a rule that monitors the sudoers file for changes:
auditctl -w /etc/sudoers -p wa -k sudoers_changes
This rule specifies that the auditd service must watch (-w) the /etc/sudoers file to log permissions changes (-p) to the write access (w) or attributes (a) of the file and to identify them in logs as sudoers_changes. The auditing logs appear in /var/log/audit/audit.log. You can list the auditing rules as follows:
auditctl -l
-w /etc/sudoers -p wa -k sudoers_changes
For more information on the Linux Audit Daemon, see the auditd man page:
man auditd
For more information on setting auditing rules and options, see the auditctl man page:
man auditctl
For more information on viewing reports on audited events, see the aureport man page:
man aureport
7 - Analyzing systemd Logs with journalctl
The journalctl tool queries the contents of the systemd journal.
The following command displays the messages that systemd generated the last time the machine started:
journalctl -b
The following command reveals the messages for the systemd service unit specified by the -u option:
journalctl -u auditd
In the above example, auditd is the system service unit.
For more information, see the journalctl man page by running the following command on Photon OS:
man journalctl
8 - Migrating Scripts to systemd
Although systemd maintains compatibility with init.d scripts, as a best practice, you must adapt the scripts that you want to run on Photon OS to systemd to avoid potential problems.
Such a conversion standardizes the scripts, reduces the footprint of your code, makes the scripts easier to read and maintain, and improves their robustness on a systemd system.