[Kea-users] Need help assigning subnets by class with host reservations

Thomas Markwalder tmark at isc.org
Fri Nov 11 12:23:29 UTC 2016

On 11/10/16 11:50 PM, mrobti at insiberia.net wrote:
> On 2016-11-10 06:12, Thomas Markwalder wrote:
>> On 11/9/16 2:28 PM, mrobti at insiberia.net wrote:
>>> On 2016-11-09 07:56, Thomas Markwalder wrote:
>>> On 11/9/16 4:20 AM, mrobti at insiberia.net wrote:
>>> On 2016-11-08 15:44, mrobti at insiberia.net wrote:
>>> I want to assign a client-class using hwaddr, from MySQL backend,
>>> and
>>> restrict a subnet for that client-class. In other words, only allow
>>> clients with known MAC addresses to use the subnet they are trying
>>> to
>>> connect to.
>>> DB hosts table has an entry for the client:
>>> dhcp4_subnet_id = 1
>>> dhcp_identifier_type = 0
>>> dhcp_identifier = UNHEX(REPLACE('aa:bb:cc:dd:ee:ff', ':', ''))
>>> hostname = test.local
>>> dhcp4_client_classes = test_class
>>> Config file has:
>>> "client-classes": [ {
>>> "name": "test_class"
>>> } ],
>>> "subnet4": [ {
>>> "id": 1,
>>> "subnet": "",
>>> "pools": [ { "pool": " -" } ],
>>> "client-class": "test_class"
>>> } ],
>>> But Kea says (debug level 50):
>>> : client packet has been assigned to the following class(es):
>>> : failed to select subnet for the client
>>> : no suitable subnet configured for a direct client
>>> It works if I remove "client-class" from the subnet definition, so
>>> something is not synchronizing the class somewhere.
>>> Could it be a problem that the DB hosts entry has no ipv4_address
>>> listed? (that column is NULL)  I don't have any other ideas.
>>> I've found this in the logs:
>>> : HOSTS_CFG_GET_ALL_IDENTIFIER get all hosts with reservations using
>>> identifier: hwaddr=aa:bb:cc:dd:ee:ff
>>> hwaddr=aa:bb:cc:dd:ee:ff, found 0 host(s)
>>> Why would this happen? Yes, I double checked the MAC address. I
>>> enabled MySQL logging, and I can't match up timestamps exactly, but
>>> I
>>> do find a query:
>>> SELECT host_id, dhcp_identifier, dhcp_identifier_type,
>>> dhcp4_subnet_id, dhcp6_subnet_id, ipv4_address, hostname,
>>> dhcp4_client_classes, dhcp6_client_classes FROM hosts WHERE
>>> dhcp4_subnet_id = ? AND dhcp_identifier_type = ?    AND
>>> dhcp_identifier = ?
>>> I don't know if it's possible to see the executed version of this
>>> prepared query(?). Is it possible that the value Kea is placing in
>>> the
>>> query is not the correct binary string?
>>> _______________________________________________
>>> Kea-users mailing list
>>> Kea-users at lists.isc.org
>>> https://lists.isc.org/mailman/listinfo/kea-users
>> Hello:
>> We are looking into this for you.  As you probably know,  Kea will
>> look
>> first for hosts defined its configuration file and then within the
>> hosts
>> database (if it is specified).  Any log statements you see that
>> contain
>> "HOSTS_CFG_" pertain to looking at hosts defined via the configuration
>> file.  In your case, since there are none, you see none found.  When
>> Kea
>> accesses the host database the logs should contain
>> HOSTS_MGR_ALTERNATIVE_.   The following is a snippet from of the log
>> in
>> a setup I am testing with:
>> 2016-11-09 10:18:45.018 DEBUG [kea-dhcp4.hosts/24940]
>> HOSTS_CFG_GET_ONE_SUBNET_ID_ADDRESS4 get one host with reservation for
>> subnet id 1 and IPv4 address
>> 2016-11-09 10:18:45.018 DEBUG [kea-dhcp4.hosts/24940]
>> HOSTS_CFG_GET_ALL_ADDRESS4 get all hosts with reservations for IPv4
>> address
>> 2016-11-09 10:18:45.018 DEBUG [kea-dhcp4.hosts/24940]
>> HOSTS_CFG_GET_ALL_ADDRESS4_COUNT using address, found 0
>> host(s)
>> 2016-11-09 10:18:45.018 DEBUG [kea-dhcp4.hosts/24940]
>> HOSTS_CFG_GET_ONE_SUBNET_ID_ADDRESS4_NULL host not found using subnet
>> id
>> 1 and address
>> 2016-11-09 10:18:45.018 DEBUG [kea-dhcp4.hosts/24940]
>> HOSTS_MGR_ALTERNATE_GET4_SUBNET_ID_ADDRESS4 trying alternate source
>> for
>> host using subnet id 1 and address
>> The last log statement indicates that Kea is looking in MySQL for
>> hosts
>> that have the allocated address.  It just so happens that Kea actually
>> conducted a search in MySQL prior to the last one show above.  This
>> search is done by subnet id and dhcp identifier.   However the
>> function
>> that performs this search happens to be missing a log entry.  This is
>> why you see  the two selects  you cited in the MySQL log but only one
>> log message against the alternate.   The original function we used
>> was deprecated without the new one getting a log message.  Sorry about
>> that.
>> On the surface, it looks like Kea should be matching your host, we're
>> still researching it.  You might try defining your host in the
>> configuration file, for testing purposes.   Do you have a packet
>> capture
>> and what version of Kea are you running?
>> Thank you for your response. I don't have a packet capture at the
>> moment, but I do see the HOSTS_MGR_ALTERNATE_ line just like yours.
>> I'm running the ubuntu package which shows version 1.0.0, the package
>> name is version 1.0.0-1build1. I know that's behind the curve, but
>> sometimes don't these packages include bugfixes from newer versions
>> than they report?
>> I can begrudgingly compile from source, but would not be happy taking
>> it outside the system package manager.
>> I just tried to test by putting the host in the config file, got this
>> startup error:
>> DHCP4_PARSER_FAIL failed to create or run parser for configuration
>> element subnet4: unsupported configuration parameter 'client-classes'
>>  I think you meant client-classes failed to parse as part of the host
>> element (not subnet4) ?
> I think so, you mean the host element inside the subnet4 reservations
> list, right? I put "client-classes" in a "reservations" entry in the
> "subnet4" as such:
> "subnet4": [ {
>   "reservations": [ {
>     "hw-address": "aa:bb:cc:dd:ee:ff",
>     "client-classes": [ "test_class" ]
>   }],
>   "id": 1,
>   "subnet": "",
> ...
> That's the right way to do it, correct? Version 1.0.0 didn't see
> "client-classes" as valid in that context.
>>  But yes, "client-classes" was added in 1.1.0.
>> 1.1.0 added a good deal more functionality to the RDBMS Host
>> Reservations implementations, as well as a lot more with
>> classification expression matching.
>>> Looks like assigning client-classes to host reservations was a
>>> feature only added after version 1.0? Can you please confirm when it
>>> was added?
>>> _______________________________________________
>>> Kea-users mailing list
>>> Kea-users at lists.isc.org
>>> https://lists.isc.org/mailman/listinfo/kea-users
>> When an inbound packet is received things happen in this order:
>> 1. The packet is classified by evaluating it against the test
>> expression for defined classes
>> 2. Subnet matching is conducted based on packet content.  This
>> includes comparing the classes matched to the packet in step 1 against
>> the classes specified by the subnet's "client-class" list.
>> 3. Look for host reservations
>> The problem you have is two fold.   First, your "test-class" does not
>> define a "test" expression and thus matching it against a packet
>> always fails.   This causes the subnet selection to fail and the
>> server drops the packet.  The second issue is that you are assigning a
>> class to the host reservation, but associating the client to a
>> reservation occurs after the subnet selection.  This is something of a
>> chicken-and-the-egg situation.
> This question did come up when I was reading the docs. It does seem
> like the host class assignment needs to happen earlier and in a
> different configuration scope.
>> So while you can specify that a host belongs to one or more classes,
>> this currently only means the host inherits options from those
>> classes.
>> I think it would help to understand what problem you are trying to
>> solve.  Are you trying to ensure that only known clients get
>> addresses?
> Yes, that's the primary goal.
>>  Are you trying to map specific hosts to specific subnets?
> Secondary goal is not mapping hosts to subnets, but to pools within
> the same subnet. (Later we will add subnets and map to those.)
>>  How any hosts in how many subnets do you anticipate having?
> Not many for now, but storing configuration information in a database
> is a requirement in this case.
>> If your network isn't large, you could define a class for each host
>> whose "test" expression matches the host's hardware address. Then add
>> these classes to the desired subnet4 client-class list.   Not ideal
>> but it would work.  This would be akin to ISC DHCP sub-classing,
>> though not quite as neat.   Another alternative would be to write a
>> hook but we would need to understand your problem to offer a more
>> detailed suggestion.  Our example hook library, user-chk,  does
>> something similar to what you're after and is a good starting point
>> for what is possible,
>> http://kea.isc.org/docs/kea-guide.html#idp54000992.   Hooks are
>> described in detail in our developer's guide:
>> https://jenkins.isc.org/job/Kea_doc/doxygen/df/d46/hooksdgDevelopersGuide.html.
> Sounds like Kea doesn't support a "known-clients" type of
> configuration. I'll read about hooks next, but I wouldn't think the
> "known-clients" feature is unusual - is this design issue something
> that will be addressed in a future version without the need for
> external hook libraries?
This is a feature that is likely to be added.  ISC is a small,
non-profit company and we can only do so much at a time.  Of course we
are also an open-source project so contributors are always welcome.   As
you mention, this is likely a feature many users would like to have.

> Now, I read briefly about user-chk. Reading a text file upon each
> packet arrival doesn't sound efficient, regardless our requirement is
> using a database. Would it be possible to:
> 1. remove "client-class" from the subnet so all clients can initially
> be assigned to the subnet just for purposes of working around the
> chicken/egg problem you mentioned
> 2. assign class names to known clients using the existing database
> reservations system (note, class would be defined in config file with
> no "test" expression and our reservations have NULL for the IP address)
> 3. have the user-chk hook library inspect the assigned class and deny
> or reassign if the class is empty (not having been assigned in step 2)
> If this is possible, can I do it in version 1.0 or is 1.1.0 required
> for any of the above?

The user_chk library as it stands is really meant as a learning
tool/starting point, not as something intended for busy production
environment.   You are free to alter it however you like.   You may find
it more expedient to use it as guide or skeleton for your own new hook
lib.  It likely has a fair amount of stuff you don't need.   What you
are proposing is doable, but not with 1.0 as it does not support
client-classes in the host reservations.  

If you look at the lease4_select hook point arguments:

# *Arguments:*

  * name: *query4*, type: isc::dhcp::Pkt4Ptr
    direction: *in*
  * name: *subnet4*, type: isc::dhcp::Subnet4Ptr
    direction: *in*
  * name: *fake_allocation*, type: bool, direction: *in*
  * name: *lease4*, type: isc::dhcp::Lease4Ptr
    direction: *in/out*

All classes matched to the client, including those via host
reservations, are accessible via query4->getClasses() (inherited from

If you want to do something earlier in the process, it would have be in
the subnet4_select hook point, but bear in mind two things:

1. The host matching as not yet been done, so query4->getClasses() would
not contain any classes contributed by a reservation.  You can, however,
look for a host reservation yourself via the HostMgr singleton, which
can be accessed
with this static method:


and the reservation can be searched for with either of these:

    virtual ConstHostPtr
    get4(const SubnetID& subnet_id, const HWAddrPtr& hwaddr,
         const DuidPtr& duid = DuidPtr()) const;

    virtual ConstHostPtr
    get4(const SubnetID& subnet_id, const Host::IdentifierType&
         const uint8_t* identifier_begin, const size_t identifier_len)

    (I would recommend the latter, as the former may eventually be

2. Host reservations are tied to a subnet via subnet ID.  Kea does not
support subnet-less host reservations.  Suppose  you define a host
reservation to subnet id 1, and you override the subnet selection with a
different subnet, say subnet 23.  When
Kea attempts to match hosts to this client, it will use subnet id 23 and
fail to find any, unless you add a second reservation for the host with
subnet id 23.    On the other hand, if you only want to use the host
reservation to attribute hosts to classes,  this may not matter to you,
as you're doing the look explicitly in your hook.

Thomas Markwalder
ISC Software Engineering

> _______________________________________________
> Kea-users mailing list
> Kea-users at lists.isc.org
> https://lists.isc.org/mailman/listinfo/kea-users

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://lists.isc.org/pipermail/kea-users/attachments/20161111/2140d962/attachment-0001.html>

More information about the Kea-users mailing list