Enabling persistent logging in journald
=======================================

To enable persistent logging, create /var/log/journal:

  mkdir -p /var/log/journal
  systemd-tmpfiles --create --prefix /var/log/journal

systemd will make the journal files owned by the "systemd-journal" group and
add an ACL for read permissions for users in the "adm" group.
To grant a user read access to the system journal, add them to one of the two
groups.

This will allow you to look at previous boot logs with e. g.
"journalctl -b -1".

If you enable persistent logging, consider uninstalling rsyslog or any other
system-log-daemon, to avoid logging everything twice.

Debugging boot/shutdown problems
================================

The "debug-shell" service starts a root shell on VT 9 which is available very
early during boot and very late during shutdown. You can temporarily enable
this when booting the system does not get sufficiently far to get a desktop or
even the text console logins (getty), or when shutdown hangs eternally.

For boot problems the recommended way is to append "systemd.debug-shell" to the
kernel command line in the bootloader.
For shutdown problems, run "systemctl start debug-shell" as root, then shut
down.

WARNING: Please avoid "systemctl enable debug-shell" as this will start the
debug shell permanently which is a SECURITY HOLE as it allows unauthenticated
and unrestricted root access to your computer if you forget to disable it!
Please only enable it if you cannot pass "systemd.debug-shell" to the boot
loader for some reason, and then immediately run "systemctl disable debug-shell"
after booting.

Once the boot/shutdown problem happened, switch to VT9 (Ctrl+Alt+F9). There you
can use the usual systemctl or journalctl commands, or any other Linux shell
command to list or kill processes. For example, run "systemctl list-jobs" to
see what's currently being run, or "systemctl" to find units which are not in
the expected state (e. g. "failed" for boot or still "active" during shutdown),
and then get more detailed information with "systemctl status -l foo.service"
to get a service "foo"'s status and recent logging.

In situations where the debug shell is not available, you can generate a
/shutdown-log.txt file instead:
1. Boot with these kernel command line options:
    systemd.log_level=debug systemd.log_target=kmsg log_buf_len=1M
2. Save the following script as /lib/systemd/system-shutdown/debug.sh and make it executable:
    #!/bin/sh
    mount -o remount,rw /
    dmesg > /shutdown-log.txt
    mount -o remount,ro /
3. Reboot

Enable and use networkd
=======================
networkd is a small and lean service to configure network interfaces, designed
mostly for server use cases in a world with hotplugged and virtualized
networking. Its configuration is similar in spirit and abstraction level to
ifupdown, but you don't need any extra packages to configure bridges, bonds,
vlan etc. It is not very suitable for managing WLANs yet; NetworkManager is
still much more appropriate for such Desktop use cases.

networkd is not enabled by default; run

  systemctl enable systemd-networkd

if you want to use it. After that you need to create some *.network
configuration files. In the simplest case you just want to run DHCP on all
available Ethernet interfaces:

--- /etc/systemd/network/all-eth.network ---
[Match]
Name=e*
[Network]
DHCP=yes

This will match on both the kernel "ethN" as well as the predictable interface
names "en*". Please see man systemd.network(5) for all available configuration
options and examples.

You need to make sure that interfaces handled by networkd are not handled by
ifupdown (/etc/network/interfaces) and NetworkManager.

Note that interfaces brought up/down will *not* run hooks in
/etc/network/if-*.d/.

It is recommended to use networkd together with systemd-resolved(8) to
dynamically manage /etc/resolv.conf:

  systemctl enable systemd-resolved
  ln -sf /run/systemd/resolve/resolv.conf /etc/resolv.conf

Debian's networkd has been modified to also work with the resolvconf package.

Going back to SysV init
=======================
If you upgraded from Debian 7 (or earlier) and you need to switch back to SysV
init for one boot, add "init=/lib/sysvinit/init" to the kernel command line.
This will NOT WORK if you installed Debian 8 (or later) and thus do not have no
/etc/inittab -- you need to create one first before you can use sysvinit.

If want to switch to SysV init permanently, install the "sysvinit-core"
package; this will provide a SysV /sbin/init and thus remove the "systemd-sysv"
package.  After that you can do an one-time boot with systemd by adding
"init=/bin/systemd" to the kernel command line option.

Please do not try to uninstall the "systemd" package itself, parts of it
(mostly logind) are needed for desktop environments.
