Syncing NetBox with a custom Ansible module

Vincent Bernat

The netbox.netbox collection from Ansible Galaxy provides several modules to update NetBox objects:

- name: create a device in NetBox
    netbox_url: http://netbox.local
    netbox_token: s3cret
      device_type: QFX5110-48S
      device_role: Compute Switch
      site: SFO1

However, if NetBox is not your source of truth, you may want to ensure it stays in sync with your configuration management database1 by removing outdated devices or IP addresses. While it should be possible to glue together a playbook with a query, a loop, and some filtering to delete unwanted elements, it feels clunky, inefficient, and abuse of YAML as a programming language. A specific Ansible module solves this issue and is likely more flexible.


I recommend that you read “Writing a custom Ansible module” as an introduction, as well as “Syncing MySQL tables” for a first simpler example.


The module has the following signature and it syncs NetBox with the content of the provided YAML file:

  source: netbox.yaml
  token: s3cret

The synchronized objects are:

  • sites,
  • manufacturers,
  • device types,
  • device roles,
  • devices, and
  • IP addresses.

In our environment, the YAML file is generated from our configuration management database and contains a set of devices and a list of IP addresses:

     datacenter: sfo1
     manufacturer: Cisco
     model: Catalyst 2960G-48TC-L
     role: net_tor_oob_switch
     datacenter: sfo1
     manufacturer: Juniper
     model: QFX5110-48S
     role: net_tor_gpu_switch
# […]
  - device:
    interface: oob
  - device:
    interface: oob
  - device:
    interface: lo0.0
# […]

The network team is not the sole tenant in NetBox. While adding new objects or modifying existing ones should be relatively safe, deleting unwanted objects can be risky. The module only deletes objects it did create or modify. To identify them, it marks them with a specific tag, cmdb. Most objects in NetBox accept tags.

Module definition#

Starting from the skeleton described in the previous article, we define the module:

module_args = dict(
    source=dict(type='path', required=True),
    api=dict(type='str', required=True),
    token=dict(type='str', required=True, no_log=True),
    max_workers=dict(type='int', required=False, default=10)

result = dict(

module = AnsibleModule(

It contains additional optional arguments defining the number of workers to talk to NetBox and query the existing objects in parallel to speed up the execution.

Abstracting synchronization#

We need to synchronize different object types, but once we have a list of objects we want in NetBox, the grunt work is always the same:

  • check if the objects already exist,
  • retrieve them and put them in a form suitable for comparison,
  • retrieve the extra objects we don’t want anymore,
  • compare the two sets, and
  • add missing objects, update existing ones, delete extra ones.

We code these behaviors into a Synchronizer abstract class. For each kind of object, a concrete class is built with the appropriate class attributes to tune its behavior and a wanted() method to provide the objects we want.

I am not explaining the abstract class code here. Have a look at the source if you want.

Synchronizing tags and tenants#

As a starter, here is how we define the class synchronizing the tags:

class SyncTags(Synchronizer):
    app = "extras"
    table = "tags"
    key = "name"

    def wanted(self):
        return {"cmdb": dict(
            description="synced by network CMDB")}

The app and table attributes define the NetBox objects we want to manipulate. The key attribute is used to determine how to lookup for existing objects. In this example, we want to lookup tags using their names.

The wanted() method is expected to return a dictionary mapping object keys to the list of wanted attributes. Here, the keys are tag names and we create only one tag, cmdb, with the provided slug, color, and description. This is the tag we will use to mark the objects we create or modify.

If the tag does not exist, it is created. If it exists, the provided attributes are updated. Other attributes are left untouched.

We also want to create a specific tenant for objects accepting such an attribute (devices and IP addresses):

class SyncTenants(Synchronizer):
    app = "tenancy"
    table = "tenants"
    key = "name"

    def wanted(self):
        return {"Network": dict(slug="network",
                                description="Network team")}

Synchronizing sites#

We also need to synchronize the list of sites. This time, the wanted() method uses the information provided in the YAML file: it walks the devices and builds a set of datacenter names.

class SyncSites(Synchronizer):

    app = "dcim"
    table = "sites"
    key = "name"
    only_on_create = ("status", "slug")

    def wanted(self):
        result = set(details["datacenter"]
                     for details in self.source['devices'].values()
                     if "datacenter" in details)
        return {k: dict(slug=k,
                for k in result}

Thanks to the use of the only_on_create attribute, the specified attributes are not updated if they are different. The goal of this synchronizer is mostly to collect the references to the different sites for other objects.

>>> pprint(SyncSites(**sync_args).wanted())
{'sfo1': {'slug': 'sfo1', 'status': 'planned'},
 'chi1': {'slug': 'chi1', 'status': 'planned'},
 'nyc1': {'slug': 'nyc1', 'status': 'planned'}}

Synchronizing manufacturers, device types, and device roles#

The synchronization of manufacturers is pretty similar, except we do not use the only_on_create attribute:

class SyncManufacturers(Synchronizer):

    app = "dcim"
    table = "manufacturers"
    key = "name"

    def wanted(self):
        result = set(details["manufacturer"]
                     for details in self.source['devices'].values()
                     if "manufacturer" in details)
        return {k: {"slug": slugify(k)}
                for k in result}

Regarding the device types, we use the foreign attribute linking a NetBox attribute to the synchronizer handling it.

class SyncDeviceTypes(Synchronizer):

    app = "dcim"
    table = "device_types"
    key = "model"
    foreign = {"manufacturer": SyncManufacturers}

    def wanted(self):
        result = set((details["manufacturer"], details["model"])
                     for details in self.source['devices'].values()
                     if "model" in details)
        return {k[1]: dict(manufacturer=k[0],
                for k in result}

The wanted() method refers to the manufacturer using its key attribute. In this case, this is the manufacturer name.

>>> pprint(SyncManufacturers(**sync_args).wanted())
{'Cisco': {'slug': 'cisco'},
 'Dell': {'slug': 'dell'},
 'Juniper': {'slug': 'juniper'}}
>>> pprint(SyncDeviceTypes(**sync_args).wanted())
{'ASR 9001': {'manufacturer': 'Cisco', 'slug': 'asr-9001'},
 'Catalyst 2960G-48TC-L': {'manufacturer': 'Cisco',
                           'slug': 'catalyst-2960g-48tc-l'},
 'MX10003': {'manufacturer': 'Juniper', 'slug': 'mx10003'},
 'QFX10002-36Q': {'manufacturer': 'Juniper', 'slug': 'qfx10002-36q'},
 'QFX10002-72Q': {'manufacturer': 'Juniper', 'slug': 'qfx10002-72q'},
 'QFX5110-32Q': {'manufacturer': 'Juniper', 'slug': 'qfx5110-32q'},
 'QFX5110-48S': {'manufacturer': 'Juniper', 'slug': 'qfx5110-48s'},
 'QFX5200-32C': {'manufacturer': 'Juniper', 'slug': 'qfx5200-32c'},
 'S4048-ON': {'manufacturer': 'Dell', 'slug': 's4048-on'},
 'S6010-ON': {'manufacturer': 'Dell', 'slug': 's6010-on'}}

The device roles are defined like this:

class SyncDeviceRoles(Synchronizer):

    app = "dcim"
    table = "device_roles"
    key = "name"

    def wanted(self):
        result = set(details["role"]
                     for details in self.source['devices'].values()
                     if "role" in details)
        return {k: dict(slug=slugify(k),
                for k in result}

Synchronizing devices#

A device is mostly a name with references to a role, a model, a datacenter and a tenant. These references are declared as foreign keys using the synchronizers defined previously.

class SyncDevices(Synchronizer):
    app = "dcim"
    table = "devices"
    key = "name"
    foreign = {"device_role": SyncDeviceRoles,
               "device_type": SyncDeviceTypes,
               "site": SyncSites,
               "tenant": SyncTenants}
    remove_unused = 10

    def wanted(self):
        return {name: dict(device_role=details["role"],
                for name, details in self.source['devices'].items()
                if {"datacenter", "model", "role"} <= set(details.keys())}

The remove_unused attribute is a safety implemented to fail if we have to delete more than 10 devices: this may be the indication there is a bug somewhere, unless one of your datacenter suddenly caught fire.

>>> pprint(SyncDevices(**sync_args).wanted())
{'': {'device_role': 'net_tor_oob_switch',
                             'device_type': 'Catalyst 2960G-48TC-L',
                             'site': 'sfo1',
                             'tenant': 'Network'},
 '': {'device_role': 'net_tor_gpu_switch',
                             'device_type': 'QFX5110-48S',
                             'site': 'sfo1',
                             'tenant': 'Network'},

Synchronizing IP addresses#

The last step is to synchronize IP addresses. We do not attach them to a device.2 Instead, we specify the device names in the description of the IP address:

class SyncIPs(Synchronizer):
    app = "ipam"
    table = "ip-addresses"
    key = "address"
    foreign = {"tenant": SyncTenants}
    remove_unused = 1000

    def wanted(self):
        wanted = {}
        for details in self.source['ips']:
            if details['ip'] in wanted:
                wanted[details['ip']]['description'] = \
                    f"{details['device']} (and others)"
                wanted[details['ip']] = dict(
                    dns_name="",        # information is present in DNS
                    description=f"{details['device']}: {details['interface']}",
        return wanted

There is a slight difficulty: NetBox allows duplicate IP addresses, so a simple lookup is not enough. In case of multiple matches, we choose the best by preferring those tagged with cmdb, then those already attached to an interface:

def get(self, key):
    """Grab IP address from NetBox."""
    # There may be duplicate. We need to grab the "best."
    results = super(Synchronizer, self).get(key)
    if len(results) == 0:
        return None
    if len(results) == 1:
        return results[0]
    scores = [0]*len(results)
    for idx, result in enumerate(results):
        if "cmdb" in result.tags:
            scores[idx] += 10
        if result.interface is not None:
            scores[idx] += 5
    return sorted(zip(scores, results),
                  reverse=True, key=lambda k: k[0])[0][1]

Getting the current and wanted states#

Each synchronizer is initialized with a reference to the Ansible module, a reference to a pynetbox’s API object, the data contained in the provided YAML file and two empty dictionaries for the current and expected states:

source = yaml.safe_load(open(module.params['source']))
netbox = pynetbox.api(module.params['api'],

sync_args = dict(
synchronizers = [synchronizer(**sync_args) for synchronizer in [

Each synchronizer has a prepare() method whose goal is to compute the current and wanted states. It returns True in case of a difference:

# Check what needs to be synchronized
    for synchronizer in synchronizers:
        result['changed'] |= synchronizer.prepare()
except AnsibleError as e:
    result['msg'] = e.message

Applying changes#

Back to the skeleton described in the previous article, the last step is to apply the changes if there is a difference between these states. Each synchronizer registers the current and wanted states in sync_args["before"][table] and sync_args["after"][table] where table is the name of the table for a given NetBox object type. The diff object is a bit elaborate as it is built table by table. This enables Ansible to display the name of each table before the diff representation:

# Compute the diff
if module._diff and result['changed']:
    result['diff'] = [
        for table in sync_args["after"]
        if sync_args["before"][table] != sync_args["after"][table]

# Stop here if check mode is enabled or if no change
if module.check_mode or not result['changed']:

Each synchronizer also exposes a synchronize() method to apply changes and a cleanup() method to delete unwanted objects. Order is important due to the relation between the objects.

# Synchronize
for synchronizer in synchronizers:
for synchronizer in synchronizers[::-1]:

The complete code is available on GitHub. Compared to using netbox.netbox collection, the logic is written in Python instead of trying to glue Ansible tasks together. I believe this is both more flexible and easier to read, notably when trying to delete outdated objects. While I did not test it, it should also be faster. An alternative would have been to reuse code from the netbox.netbox collection, as it contains similar primitives. Unfortunately, I didn’t think of it until now. 😶

  1. In my opinion, a good option for a source of truth is to use YAML files in a Git repository. You get versioning for free and people can get started with a text editor. ↩︎

  2. This limitation is mostly due to laziness: we do not care about this information. Our main motivation for putting IP addresses in NetBox is to keep track of the used IP addresses. However, if an IP address is already attached to an interface, we leave this association untouched. ↩︎

Share this article