This is necessary to prevent a race. Udev 197 has a new naming scheme
for network devices, so it will rename (say) eth0 to eno0. This fails
with "error changing net interface name eth0 to eno1: Device or
resource busy" if another process has opened the interface in the
meantime.
During a configuration switch, changed units are stopped in the old
configuration, then started in the new configuration (i.e. after
running the activation script and running "systemctl daemon-reload").
This ensures that services are stopped using the ExecStop/ExecStopPost
commands from the old configuration.
However, for some services it's undesirable to stop them; in
particular dhcpcd, which deconfigures its network interfaces when it
stops. This is dangerous when doing remote upgrades - usually things
go right (especially because the switch script ignores SIGHUP), but
not always (see 9aa69885f04969e5d31dcb8265c327adc908954e). Likewise,
sshd should be kept running for as long as possible to prevent a
lock-out if the switch fails.
So the new option ‘stopIfChanged = false’ causes "systemctl restart"
to be used instead of "systemctl stop" followed by "systemctl start".
This is only proper for services that don't have stop commands. (And
it might not handle dependencies properly in some cases, but I'm not
sure.)
Charon needs this to include the dynamically generated
/root/.vbox-charon-client-key. (We used
users.extraUsers.root.openssh.authorizedKeys.keyFiles for this, but
that no longer works.)
Instead of the somewhat hacky script that inserted public keys
into the users' .ssh/authorized_keys files, use the AuthorizedKeysFile
configuration directive in sshd_config and generate extra key
files for each user (placed in /etc/authorized_keys.d/).
Thus
networking.interfaces = [ { name = "eth0"; ipAddress = "192.168.15.1"; } ];
can now be written as
networking.interfaces.eth0.ipAddress = "192.168.15.1";
The old notation still works though.
My main reason for adding this is the ability to turn off helpers
altogether. If you are not using any of the special protocols, keeping
them turned off is safest, and in case you do want to use them, it's
best to configure them through the new CT target for your network
topology. Perhaps some sane defaults for nixos can be examined in the
future.
This change has no impact if you don't touch the added options, so no
need to adapt.
This is meant to replace /proc/sys/net/ipv4/conf/*/rp_filter, which
only works for ipv4. Furthermore, it's nicer to handle this kind of
filtering in the firewall.
There are some more subtle differences, please see:
https://home.regit.org/netfilter-en/secure-use-of-helpers/
I chose to enable this by default (when the firewall is enabled) as
it's a good idea in general. Only people with advanced routing needs
might not want this, but I guess they don't use the nixos firewall
anyway and use a custom solution. Furthermore, the option only becomes
available in kernel 3.3+, so conservative nixos users that just stick
to the default kernel will not need to act now just yet.
