Saturday, January 2, 2016

Processing RA messages in NetworkManager

The goal of this post is to analyze processing of RA messages through the NetworkManager code, starting with the initial reception all the way through the assignment of parameters to a device through which the RA was received. As a special case will also take a look what happens when RS is sent, i.e. what is different in comparison to unsolicited RAs. But first, we'll take a look at the relevant code organization and initialization process.

Code to process RAs and initialization phase

For receiving RA and sending RA NetworkManager uses libndp library. This library is used in class NM_TYPE_LNDP_RDISC (defined in the file src/rdisc/nm-lndp-rdisc.c) which is a platform specific class tailored for the Linux OS. This class inherits from class NM_TYPE_RDISC (defined in the file src/rdisc/nm-rdisc.c) which is a platform independent base class. It contains functionality that is platform independent so theoretically NetworkManager can be more easily ported to, e.g. FreeBSD.

To create a new object of the type NM_TYPE_LNDP_RDISC it is necessary to call function nm_lndp_rdisc_new(). This is, for example, done by the class NM_DEVICE_TYPE in function addrconf6_start() for each device for which IPv6 configuration is started.

Now, if NetworkManager will use RAs or not depends on the configuration setting for IPv6 that the user defines. If you go to configuration dialog for some network interface there is a setting for IPv6 configuration which might be ON or OFF. In case it is OFF, no IPv6 configuration will be done. If IPv6 configuration is enabled (switch placed in ON state) then the specific configuration methods should be selected. The selected option is checked in the function src/devices/nm-device.c:act_stage3_ip6_config_start(), where, depending on the option selected, a specific initialization is started:
  • Automatic (method NM_SETTING_IP6_CONFIG_METHOD_AUTO)

    Start full IPv6 configuration by calling src/devices/nm-device.c:addrconf6_start() function.
  • Automatic, DHCP only (method NM_SETTING_IP6_CONFIG_METHOD_DHCP)

    Only configuration based on DHCP parameters received will be done. This type of configuration is initiated by calling function src/devices/nm-device.c:dhcp_start().

    Manual configuration using parameters specified in the configuration dialog and nothing else. The configuration of this type is initiated by calling function nm_ip6_config_new() which returns appropriate IPv6 configuration object.
  • Link-local Only (method NM_SETTING_IP6_CONFIG_METHOD_LINK_LOCAL)

    Initiate only a link-local address configuration by calling function src/devices/nm-device.c:linklocal6_start().
Since in this post we are concerned with RA processing than we are obviously interested only in Automatic configuration type, the one that calls addrconf6_start() function. This function, in turn, calls function src/nm-device.c:linklocal6_start() to ensure that link local configuration is present. It might happen that link local address isn't configured and so RA configuration must wait, or link local configuration is still present. In either case, when link local configuration is present RA processing can start. RA processing is kicked off by calling src/nm-device.c:addrconf6_start_with_link_ready() which in turn calls src/nm-rdisc.c:nm_rdisc_start() to kick off RA configuration.

nm_rdisc_start() is called with a pointer to NM_LNDP_RDISC class (defined in src/rdisc/nm_lndp_rdisc.c). Note that a method (nm_rdisc_start()) from a base class (NM_RDISC_TYPE, defined in src/rdisc/nm_rdisc.c) is called with a pointer to a subclass of a NM_RDISC_TYPE! Method in a base class does the following:
  1. Checks that there is a subclass which defined virtual method start() (gassert(klass->start)).
  2. Initializes timeout for the configuration process. If timeout fires, then rdisc_ra_timeout_cb() will be called that emits NM_RDISC_RA_TIMEOUT signal.
  3. Invokes a method start() from a subclass. Subclass is, as already said, NM_LNDP_RDISC and the given method registers a callback src/rdisc/nm-lndp-rdisc.c:receive_ra() with libndp library. The callback is called by libndp library whenever RA is received.
  4. Starts solicit process by invoking solicit() method. This method schedules RS to be sent in certain amount of time (variable next) by send_rs() method. This method, actually, invokes send_rs() method from a subclass (src/nm-rdisc/nm-rdisc-linux.c:send_rs()) which sends RS using libndp library. Note that the number of RSes sent is bounded and after certain amount of them sent the process is stopped under the assumption that there is no IPv6 capable router on the network.
  5. After RA has been received and processed the application of configuration parameters is done in src/device/nm-device.c:rdisc_config_changed() method. This callback is achieved by registering to NM_RDISC_CONFIG_CHANGED signal that is emitted by src/rdisc/nm-rdisc.c class whenever IPv6 configuration changes.
So, in conclusion, when link local configuration is finished, RA processing is started. The RA processing consists of waiting for RA in src/rdisc/nm-lndp-rdisc.c:receive_ra(). If RA doesn't arrive is certain amount of time then RS is sent in function src/nm-rdisc/nm-rdisc-linux.c:send_rs().

RA processing

When RA is received it is processed by the function src/rdisc/nm-lndp-rdisc.c:receive_ra(). The following configuration options are processed from RA by the given function:
  1. DHCP level.
  2. Default gateway.
  3. Addresses and routes.
  4. DNS information (RDNSS option).
  5. DNS search list (DNSSL option).
  6. Hop limit.
  7. MTU.
All the options that were parsed are stored (or removed from) a private attributes of the base object (NMRDisc defined in src/rdisc/nm-rdisc.h).

Finally, the method src/nm-rdisc.c:nm_rdisc_ra_received() is called to cancel all the timeouts. It will also emit signal NM_RDISC_CONFIG_CHANGED that will trigger application of received configuration parameters to a networking device.

Processing RS/RA

The RS/RA processing differs only by the fact that RS is sent after certain amount of time has passed and RA wasn't received, as described in the Code to process RAs and initialization phase section. After RS is sent, the RA processing is the same as it would be without RS being sent.

Applying IPv6 configuration data

Application of received IPv6 configuration data is done in the method src/device/nm-device.c:rdisc_config_changed(). IPv6 configuration is stored in IPv6 configuration object NM_TYPE_IP6_CONFIG defined in src/nm-ip6-config.c.

Note that this isn't the real application of configuration data, but only that the configuration data is stored in the appropriate object.

The function that really applies configuration data is src/devices/nm-device.c:ip6_config_merge_and_apply().

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