"nm-device-logging.h" defines logging macros for a NMDevice instance.
It also expects a "self" variable in the call environment, and that
variable had to be in the type of NMDevice or the NMDevice subclass.
Extend the macro foo, so that @self can be either a NMDevice* pointer
or a NMDevice$SUBTYPE.
Of course, that would have always been possible, if we would simply cast
to "(NMDevice *)" where we need it. The trick is that the macro only
works if @self is one of the two expected types, and not some arbitrary
unrelated type.
Run:
./contrib/scripts/nm-code-format.sh -i
./contrib/scripts/nm-code-format.sh -i
Yes, it needs to run twice because the first run doesn't yet produce the
final result.
Signed-off-by: Antonio Cardace <acardace@redhat.com>
The _GET_PRIVATE() macros are all implemented based on
_NM_GET_PRIVATE(). That macro tries to be more type safe and uses
_Generic() to do the right thing. Explicitly casting is not only
unnecessary, it defeats these (static) type checks.
Don't do that.
Introduce a generic function to set a MTU based on parent's one. Also
define a device-specific @mtu_parent_delta value that specifies the
difference from parent MTU that should be set by default. For VLAN it
is zero but other interface types (for example MACsec) require a
positive value due to encapsulation overhead.
NMDevice's act_stage1_prepare() now does nothing. Calling it is not
useful and has no effect.
In general, when a subclass overwrites a virtual function, it must be
defined whether the subclass must, may or must-not call the parents
implementation. Likewise, it must be clear when the parents
implementation should be chained: first, as last, does it matter?
In any case, that very much depends on how the parent is implemented
and this can only be solved by documentation and common conventions.
It's a forgiving approach to have a parents implementation do nothing,
then the subclass may call it at any time (or not call it at all).
This is especially useful if classes don't know their parent class well.
But in NetworkManager code the relationship between classes are known
at compile time, so every of these classes knows it derives directly
from NMDevice.
This forgingin approach was what NMDevice's act_stage1_prepare() was doing.
However, it also adds lines of code resulting in a different kind of complexity.
So, it's not clear that this forgiving approach is really better. Note
that it also has a (tiny) runtime and code-size overhead.
Change the expectation of how NMDevice's act_stage1_prepare() should be
called: it is no longer implemented, and subclasses *MUST* not chain up.
We no longer add these. If you use Emacs, configure it yourself.
Also, due to our "smart-tab" usage the editor anyway does a subpar
job handling our tabs. However, on the upside every user can choose
whatever tab-width he/she prefers. If "smart-tabs" are used properly
(like we do), every tab-width will work.
No manual changes, just ran commands:
F=($(git grep -l -e '-\*-'))
sed '1 { /\/\* *-\*- *[mM]ode.*\*\/$/d }' -i "${F[@]}"
sed '1,4 { /^\(#\|--\|dnl\) *-\*- [mM]ode/d }' -i "${F[@]}"
Check remaining lines with:
git grep -e '-\*-'
The ultimate purpose of this is to cleanup our files and eventually use
SPDX license identifiers. For that, first get rid of the boilerplate lines.
Platform had it's own scheme for reporting errors: NMPlatformError.
Before, NMPlatformError indicated success via zero, negative integer
values are numbers from <errno.h>, and positive integer values are
platform specific codes. This changes now according to nm-error:
success is still zero. Negative values indicate a failure, where the
numeric value is either from <errno.h> or one of our error codes.
The meaning of positive values depends on the functions. Most functions
can only report an error reason (negative) and success (zero). For such
functions, positive values should never be returned (but the caller
should anticipate them).
For some functions, positive values could mean additional information
(but still success). That depends.
This is also what systemd does, except that systemd only returns
(negative) integers from <errno.h>, while we merge our own error codes
into the range of <errno.h>.
The advantage is to get rid of one way how to signal errors. The other
advantage is, that these error codes are compatible with all other
nm-errno values. For example, previously negative values indicated error
codes from <errno.h>, but it did not entail error codes from netlink.
Add a helper function nm_device_parent_find_for_connection() to
unify implementations of setting the parent in update_connection().
There is some change in behavior, in particular for nm-device-vlan.c,
which no longer compares the link information from platform. But
update_connection() is anyway a questionable concept, only used
for external assumed connection (which itself, is questionable). Meaning,
update_connection() is a hack not science, and it's not at all clear
what the correct behavior is.
Also, note how vlan's implementation differs from all others. Why?
Should we always resort to also check the information from platform?
Either way, one of the two approaches should be used consistently and
nm_device_parent_find_for_connection() opts to not consult platform
cache.
Note the special error codes NM_UTILS_ERROR_CONNECTION_AVAILABLE_*.
This will be used to determine, whether the profile is fundamentally
incompatible with the device, or whether just some other properties
mismatch. That information will be importand during a plain `nmcli
connection up`, where NetworkManager searches all devices for a device
to activate. If no device is found (and multiple errors happened),
we want to show the error that is most likely relevant for the user.
Also note, how NMDevice's check_connection_compatible() uses the new
class field "device_class->connection_type_check_compatible" to simplify
checks for compatible profiles.
The error reason is still unused.
It seems to me the NM_DEVICE_CLASS_DECLARE_TYPES() macro confuses more
than helping. Let's explicitly initialize the two fields, albeit with
another helper macro NM_DEVICE_DEFINE_LINK_TYPES() to get the list of
link-types right.
For consistency, also leave nop-lines like
device_class->connection_type_supported = NULL;
device_class->link_types = NM_DEVICE_DEFINE_LINK_TYPES ();
because all NMDevice class init methods should have this same
boiler plate code and to make it explicit that this is intended.
And there are only 3 occurences where this actually comes into play.
The majority of device implementations name their parent-class variable
"device_class". That also makes more sense as it is more consistant.
E.g. "parent" sounds like it's the direct parent, but that is not
the crucial point here. The crucial point at this place, is that we
access the NMDeviceClass typed pointer. Rename.
If commit_mtu() is called multiple times and dev->get_configured_mtu()
returns @is_user_config=FALSE, only the first call changes the
MTU. So, for example, when the parent MTU of a VLAN changes, we apply
the new MTU only the first time.
Rework the handling of MTU in NMDevice, and store the source of the
configured MTU. When commit_mtu() is called again, we ask the subclass
a MTU to configure and apply it only if the source has higher
priority, or when the parent MTU changed.
(cherry picked from commit 2f8917237f)
Instead of returning a boolean @is_user_config value from
get_configured_mtu(), return an mtu-source enum with possible values
NONE,CONNECTION. This enum will be expanded later; for now there is no
change in behavior.
(cherry picked from commit 9f8b0697de)
NMSettings exposes a cached list of all connection. We don't need
to clone it. Note that this is not save against concurrent modification,
meaning, add/remove of connections in NMSettings will invalidate the
list.
However, it wasn't save against that previously either, because
altough we cloned the container (GSList), we didn't take an additional
reference to the elements.
This is purely a performance optimization, we don't need to clone the
list. Also, since the original list is of type "NMConnection *const*",
use that type insistently, instead of dependent API requiring GSList.
IMO, GSList is anyway not a very nice API for many use cases because
it requires an additional slice allocation for each element. It's
slower, and often less convenient to use.
Previously, we used the generated GDBusInterfaceSkeleton types and glued
them via the NMExportedObject base class to our NM types. We also used
GDBusObjectManagerServer.
Don't do that anymore. The resulting code was more complicated despite (or
because?) using generated classes. It was hard to understand, complex, had
ordering-issues, and had a runtime and memory overhead.
This patch refactors this entirely and uses the lower layer API GDBusConnection
directly. It replaces the generated code, GDBusInterfaceSkeleton, and
GDBusObjectManagerServer. All this is now done by NMDbusObject and NMDBusManager
and static descriptor instances of type GDBusInterfaceInfo.
This adds a net plus of more then 1300 lines of hand written code. I claim
that this implementation is easier to understand. Note that previously we
also required extensive and complex glue code to bind our objects to the
generated skeleton objects. Instead, now glue our objects directly to
GDBusConnection. The result is more immediate and gets rid of layers of
code in between.
Now that the D-Bus glue us more under our control, we can address issus and
bottlenecks better, instead of adding code to bend the generated skeletons
to our needs.
Note that the current implementation now only supports one D-Bus connection.
That was effectively the case already, although there were places (and still are)
where the code pretends it could also support connections from a private socket.
We dropped private socket support mainly because it was unused, untested and
buggy, but also because GDBusObjectManagerServer could not export the same
objects on multiple connections. Now, it would be rather straight forward to
fix that and re-introduce ObjectManager on each private connection. But this
commit doesn't do that yet, and the new code intentionally supports only one
D-Bus connection.
Also, the D-Bus startup was simplified. There is no retry, either nm_dbus_manager_start()
succeeds, or it detects the initrd case. In the initrd case, bus manager never tries to
connect to D-Bus. Since the initrd scenario is not yet used/tested, this is good enough
for the moment. It could be easily extended later, for example with polling whether the
system bus appears (like was done previously). Also, restart of D-Bus daemon isn't
supported either -- just like before.
Note how NMDBusManager now implements the ObjectManager D-Bus interface
directly.
Also, this fixes race issues in the server, by no longer delaying
PropertiesChanged signals. NMExportedObject would collect changed
properties and send the signal out in idle_emit_properties_changed()
on idle. This messes up the ordering of change events w.r.t. other
signals and events on the bus. Note that not only NMExportedObject
messed up the ordering. Also the generated code would hook into
notify() and process change events in and idle handle, exhibiting the
same ordering issue too.
No longer do that. PropertiesChanged signals will be sent right away
by hooking into dispatch_properties_changed(). This means, changing
a property in quick succession will no longer be combined and is
guaranteed to emit signals for each individual state. Quite possibly
we emit now more PropertiesChanged signals then before.
However, we are now able to group a set of changes by using standard
g_object_freeze_notify()/g_object_thaw_notify(). We probably should
make more use of that.
Also, now that our signals are all handled in the right order, we
might find places where we still emit them in the wrong order. But that
is then due to the order in which our GObjects emit signals, not due
to an ill behavior of the D-Bus glue. Possibly we need to identify
such ordering issues and fix them.
Numbers (for contrib/rpm --without debug on x86_64):
- the patch changes the code size of NetworkManager by
- 2809360 bytes
+ 2537528 bytes (-9.7%)
- Runtime measurements are harder because there is a large variance
during testing. In other words, the numbers are not reproducible.
Currently, the implementation performs no caching of GVariants at all,
but it would be rather simple to add it, if that turns out to be
useful.
Anyway, without strong claim, it seems that the new form tends to
perform slightly better. That would be no surprise.
$ time (for i in {1..1000}; do nmcli >/dev/null || break; echo -n .; done)
- real 1m39.355s
+ real 1m37.432s
$ time (for i in {1..2000}; do busctl call org.freedesktop.NetworkManager /org/freedesktop org.freedesktop.DBus.ObjectManager GetManagedObjects > /dev/null || break; echo -n .; done)
- real 0m26.843s
+ real 0m25.281s
- Regarding RSS size, just looking at the processes in similar
conditions, doesn't give a large difference. On my system they
consume about 19MB RSS. It seems that the new version has a
slightly smaller RSS size.
- 19356 RSS
+ 18660 RSS
We need to pass more alias-types. Instead of having numbered
versions, use variadic number of macro arguments.
Also, fix build failure with old compiler:
In file included from src/nm-ip6-config.c:24:
./src/nm-ip6-config.h:44:29: error: controlling expression type 'typeof (ipconf_iter->current->obj)' (aka 'const void *const') not compatible with any generic association type
*out_address = has_next ? NMP_OBJECT_CAST_IP6_ADDRESS (ipconf_iter->current->obj) : NULL;
^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Fixes: b1810d7a68
_NM_GET_PRIVATE() used typeof() to propagate constness of the @self
pointer. However, that means, it could only be used with a self pointer
of the exact type. That means, you explicitly had to cast from (GObject *)
or from (void *).
The requirement is cumbersome, and often led us to either create @self
pointer we didn't need:
NMDeviceVlan *self = NM_DEVICE_VLAN (device);
NMDeviceVlanPrivate *priv = NM_DEVICE_VLAN_GET_PRIVATE (self);
or casting:
NMDeviceVlanPrivate *priv = NM_DEVICE_VLAN_GET_PRIVATE ((NMDevice *) device);
In both cases we forcefully cast the source variable, loosing help from
the compiler to detect a bug.
For "nm-linux-platform.c", instead we commonly have a pointer of type
NMPlatform. Hence, we always forcefully cast the type via _NM_GET_PRIVATE_VOID().
Rework the macro to use _Generic(). If compiler supports _Generic(), then we
will get all compile time checks as desired. If the compiler doesn't support
_Generic(), it will still work. You don't get the compile-time checking of course,
but you'd notice that something is wrong once you build with a suitable
compiler.
Kernel does not allow setting the MTU of a VLAN larger
then the MTU of the underlying device. Hence, we might
initially fail to set a large MTU of the VLAN, but we
have to retry when the MTU of the parent changes.
https://bugzilla.redhat.com/show_bug.cgi?id=1414901
At startup the manager tries to create virtual devices without a
specific order and spits warnings when a device can't be realized
because the parent device is not yet created. These failures are not
something the user should worry about because the creation will be
retried when the parent appears.
A better approach is to return an error code from the device's
create_and_realize() telling that it failed because the parent doesn't
exist. In this way, the manager knows that the device isn't ready and
can avoid printing warning messages.
Change the output of nm_platform_error_to_string() to print the numeric value.
Also, accept a string buffer instead of using an alloca() allocated buffer.
There is still a macro to provide the previous functionality, but it
was ill-suited to call from inside a loop.
do a check on parent ifindex before calling "nm_device_supports_vlans"
otherwise if the parent device is a software device and its ifindex
member has not been updated yet we will trigger the g_return_if_fail
statement in "nmp_cache_lookup_entry_link".
This has been osserved in NetworkManager CI test suite, on NetworkManager
boot, during the creation of a vlan on top of a bond interface.
CI test: vlan_update_mac_from_bond
[...]
<info> [1503323670.0229] manager: (bond0): new Bond device (/org/freedesktop/NetworkManager/Devices/23)
<debug> [1503323670.0231] device[0x555555c3e320] (vlan10): constructed (NMDeviceVlan)
<debug> [1503323670.0231] manager: (vlan-vlan10) create virtual device vlan10
<debug> [1503323670.0231] device[0x555555c3e320] (vlan10): unmanaged: flags set to [platform-init,!sleeping=0x10/0x11/unmanaged/unrealized], set-managed [sleeping=0x1])
<trace> [1503323670.0235] exported-object[0x555555c3e320]: export: "/org/freedesktop/NetworkManager/Devices/24"
<trace> [1503323670.0235] properties-changed[0x555555c3e320]: ignoring notification for prop g-object-path on type NMDeviceVlan
<trace> [1503323670.0236] properties-changed[0x555555c3e320]: ignoring notification for prop path on type NMDeviceVlan
<info> [1503323670.0237] manager: (vlan10): new VLAN device (/org/freedesktop/NetworkManager/Devices/24)
<debug> [1503323670.0239] device[0x555555c3e320] (vlan10): create (is nm-owned)
Program received signal SIGTRAP, Trace/breakpoint trap.
g_logv (log_domain=0x5555557c39a9 "NetworkManager", log_level=
G_LOG_LEVEL_CRITICAL, format=<optimized out>,
args=args@entry=0x7fffffffdef0) at gmessages.c:1086
1086 g_private_set (&g_log_depth, GUINT_TO_POINTER (depth));
(gdb) bt
#0 0x00007ffff5ce3643 in g_logv (log_domain=0x5555557c39a9 "NetworkManager", log_level=
G_LOG_LEVEL_CRITICAL, format=<optimized out>, args=args@entry=0x7fffffffdef0) at gmessages.c:1086
#1 0x00007ffff5ce37bf in g_log (log_domain=log_domain@entry=0x5555557c39a9 "NetworkManager", log_level=log_level@entry=G_LOG_LEVEL_CRITICAL, format=format@entry=0x7ffff5d51190 "%s: assertion '%s' failed") at gmessages.c:1119
#2 0x00007ffff5ce37f9 in g_return_if_fail_warning (log_domain=log_domain@entry=0x5555557c39a9 "NetworkManager", pretty_function=pretty_function@entry=0x5555557b2a20 <__func__.32407> "nmp_cache_lookup_entry_link", expression=expression@entry=0x5555557b1037 "ifindex > 0") at gmessages.c:1128
#3 0x000055555566688a in nmp_cache_lookup_entry_link (cache=0x555555a780f0, ifindex=<optimized out>) at src/platform/nmp-object.c:1449
#4 0x00005555556668f9 in nmp_cache_lookup_link (cache=<optimized out>, ifindex=ifindex@entry=0) at src/platform/nmp-object.c:1464
#5 0x00005555556515e9 in nm_platform_link_get_obj (self=self@entry=0x555555a88880 [NMLinuxPlatform], ifindex=ifindex@entry=0, visible_only=visible_only@entry=1) at src/platform/nm-platform.c:618
#6 0x0000555555633e91 in link_supports_vlans (platform=0x555555a88880 [NMLinuxPlatform], ifindex=0) at src/platform/nm-linux-platform.c:4482
#7 0x00005555556d6d41 in create_and_realize (device=0x555555c3e320 [NMDeviceVlan], connection=0x7fffdc007890, parent=0x555555c33560 [NMDeviceBond], out_plink=0x7fffffffe1f8, error=0x7fffffffe358) at src/devices/nm-device-vlan.c:239
#8 0x00005555556b934c in nm_device_create_and_realize (self=self@entry=0x555555c3e320 [NMDeviceVlan], connection=connection@entry=0x7fffdc007890, parent=0x555555c33560 [NMDeviceBond], error=error@entry=0x7fffffffe358)
at src/devices/nm-device.c:2946
#9 0x00005555555b84c7 in connection_changed (connection=0x7fffdc007890, self=0x555555ab1070 [NMManager]) at src/nm-manager.c:1381
#10 0x00005555555b84c7 in connection_changed (self=0x555555ab1070 [NMManager], connection=0x7fffdc007890) at src/nm-manager.c:1431
#11 0x00005555555b9130 in retry_connections_for_parent_device (self=self@entry=0x555555ab1070 [NMManager], device=device@entry=0x555555c33560 [NMDeviceBond])
at src/nm-manager.c:1416
#12 0x00005555555b95c7 in add_device (self=self@entry=0x555555ab1070 [NMManager], device=device@entry=0x555555c33560 [NMDeviceBond], error=error@entry=0x7fffffffe598) at src/nm-manager.c:2238
#13 0x00005555555b83e1 in connection_changed (connection=0x7fffdc007b30, self=0x555555ab1070 [NMManager]) at src/nm-manager.c:1352
#14 0x00005555555b83e1 in connection_changed (self=0x555555ab1070 [NMManager], connection=0x7fffdc007b30) at src/nm-manager.c:1431
#15 0x00005555555be25b in nm_manager_start (self=0x555555ab1070 [NMManager], error=error@entry=0x7fffffffe720) at src/nm-manager.c:5202
#16 0x0000555555586b13 in main (argc=1, argv=0x7fffffffe888) at src/main.c:413
Since commit 2b51d3967 "device: merge branch 'th/device-mtu-bgo777251'",
we always set the MTU for certain device types during activation. Even
if the MTU is neither specified via the connection nor other means, like
DHCP.
Revert that change. On activation, if nothing explicitly configures the
MTU, leave it unchanged. This is like what we do with ethernet's
cloned-mac-address, which has a default value "preserve".
So, as last resort the default value for MTU is now 0 (don't change),
instead of depending on the device type.
Note that you also can override the default value in global
configuration via NetworkManager.conf.
This behavior makes sense, because whenever NM actively resets the MTU,
it remembers the previous value and restores it when deactivating
the connection. That wasn't implemented before 2b51d3967, and the
MTU would depend on which connection was previously active. That
is no longer an issue as the MTU gets reset when deactivating.
https://bugzilla.redhat.com/show_bug.cgi?id=1460760
Reduce the use of NM_PLATFORM_GET / nm_platform_get() to get
the platform singleton instance.
For one, this is a step towards supporting namespaces, where we need
to use different NMNetns/NMPlatform instances depending on in which
namespace the device lives.
Also, we should reduce our use of singletons. They are difficult to
coordinate on shutdown. Instead there should be a clear order of
dependencies, expressed by owning a reference to those singelton
instances. We already own a reference to the platform singelton,
so use it and avoid NM_PLATFORM_GET.
(cherry picked from commit 94d9ee129d)
When deciding whether to touch a device we sometimes look at whether
the active connection is external/assumed. In many cases however,
there is no active connection around (e.g. while moving the device
from state unmanaged to disconnected before assuming).
So in most cases we instead look at the device-state-reason to decide
whether to touch the interface (see nm_device_state_reason_check()).
Often it's desirable to have no state and passing data as function
arguments. However, the state reason has to be passed along several hops
(e.g. a queued state change). Or a change to a master/slave can affect
the slave/master, where we pass on the state reason. Or an intermediate
event might invalidate a previous state reason. Passing the state
whether to touch a device or not as a state-reason is cumbersome
and limited.
Instead, the device should be aware of whats going on. Add a
sys-iface-state with:
- SYS_IFACE_STATE_EXTERNAL: meaning, NM should not touch it
- SYS_IFACE_STATE_ASSUME: meaning, NM is gracefully taking over
- SYS_IFACE_STATE_MANAGED: meaning, the device is managed by NM
- SYS_IFACE_STATE_REMOVED: the device no longer exists
This replaces most checks of nm_device_state_reason_check() and
nm_active_connection_get_activation_type() by instead looking at
the sys-iface-state of the device.
This patch probably has still issues, but the previous behavior was
not very clear either. We will need to identify those issues in future
tests and tweak the behavior. At least, now there is one flag that
describes how to behave.
nm_device_uses_assumed_connection() basically called
nm_active_connection_get_assumed() on the device.
Rename those functions to be closer to the activation-type
flags.
The concepts of "assume", "external", and "assume_or_external"
will make sense with the following commits.
Scenario:
Have a connection with DHCPv4 and a default-route. When externally
removing the default route (`ip route delete 0.0.0.0/0`) and issuing
`nmcli device reapply $IF`, the default route was not restored.
That was because when externally removing the default route,
we would remove the gateway from priv->con_ip4_config (see
update_ip4_config()). Later, when reapplying the connection,
the IP method doesn't actually change. So we would not restart
DHCP and thus there is no gateway around to add the default route.
The default route would only be restored after receiving a DHCP lease
in the far future.
Fix that, by always restarting the IP method.
The state-change of a device has a reason argument, which is mostly for information
only.
There are many places in code that are the source of a state-reason.
Mostly these are calls to:
- nm_device_state_changed()
- nm_device_queue_state()
- nm_device_queue_recheck_available()
- nm_device_set_unmanaged_by_*()
- nm_device_master_release_one_slave()
- nm_device_ip_method_failed()
- nm_modem_emit_prepare_result()
- nm_modem_emit_ppp_failed()
- nm_manager_deactivate_connection()
- NM_SET_OUT (out_failure_reason, NM_DEVICE_STATE_REASON_*);
However, there are a few places in code that look at the reason
to decide how to proceed. I think this is a bad pattern, because
cause and effect are decoupled and it gets hard to understand where
a certain reason is set and what consequences that has.
Add a nop-function nm_device_state_reason_check() to mark all uses
of the device state reason that derive decisions from it. That is,
highlight the "effect" part.
This argument is only relevant when the NMActStageReturn argument
indicates NM_ACT_STAGE_RETURN_FAILURE. In all other cases it is ignored.
Rename the argument to make the meaning clearer. The argument is passed
through several layers of code, it isn't obvious that this argument only
matters for the failure case. Also, the distinct name makes it easier
to distinguish from other uses of the "reason" name.
While at it, do some drive-by cleanup:
- use g_return_*() instead of g_assert() to have a more graceful
assertion.
- functions like dhcp4_start() don't need to return a failure reason.
Most callers don't care, and the caller who does can determine the
proper reason.
- allow omitting the out-argument via NM_SET_OUT().
After commit 22e8af6242 ("device: set a per-device default MTU on
activation") we explicitly set the VLAN MTU to 1500 if not overridden
by user settings. This has the advantage that the MTU is set to a
predictable value, while before it could have different values
depending on when the interface was created (for example, the
interface would get a 1500 MTU if created during boot, or would
inherit the parent's MTU if activated manually).
However, a better default value is the MTU of the parent interface
which is in most cases what the user wants. This value was the default
before commit 22e8af6242 for manually activated connections.
https://bugzilla.redhat.com/show_bug.cgi?id=1414186
(cherry picked from commit 7dde8d8106)
Instead of overwriting ip4_config_pre_commit(), add a new function
get_mtu().
This also adds a default value in case there is no user-configuration.
This will allow us later to reset a default MTU based on the device
type.
