Pdns Dnsbackend Unable To Load Module In Backend

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• 11Troubleshooting

Samba provides support for using the BIND DNS server as the DNS back end on a Samba Active Directory (AD) domain controller (DC). The BIND9_DLZ back end is recommended for complex DNS setups that the Samba internal DNS server does not support.

The BIND9_DLZ module is a BIND9 plugin that accesses the Samba Active Directory (AD) database directly for registered zones. For this reason:

• BIND must be installed on the same machine as the Samba AD domain controller (DC).
• BIND must not run in a changed root environment.
• zones are stored and replicated within the directory.

Bind9 operates a threading model with the 'worker threads' concept. Each plugin has an associated mutex, so no two worker threads can call API functions provided by our plugin at once. Database access by the plugin is guarded by a fcntl lock.

For high traffic environments, it is not recommended to use BIND9_DLZ-backed samba as a primary DNS server. Instead, use an external server that only forwards queries to BIND9_DLZ-backed samba DNS installations when the query is addressed to a zone managed by that node.

For details, see Setting up a BIND DNS Server.

During the domain provisioning, join, or classic upgrade, the /usr/local/samba/bind-dns/named.conf file has been created.

To enable the BIND9_DLZ module for your BIND version:

• Add the following include statement to your BIND named.conf file:

• Display the BIND version:

• Edit the /usr/local/samba/bind-dns/named.conf file and uncomment the module for your BIND version. For example:

The following table shows the supported BIND versions and from which version of Samba the support started:

Samba needs to have some options set to allow Kerberos clients to automatically update the Active Directory (AD) zone managed by the BIND9_DLZ back end and improve performance.

To enable dynamic DNS updates using Kerberos and avoid returning NS records in all responses:

• Add the following to the options {} section of your BIND named.conf file. For example:

• If you provisioned or joined an AD forest or run the classic upgrade using a Samba version prior to 4.4.0, the BIND Kerberos key tab file was generated using wrong permissions. To fix, enable read access for the BIND user:

• If you upgrade from a version earlier than 4.8.0, you should check the permissions on the /usr/local/samba/bind-dns directory, these should be:

• Verify that your /etc/krb5.conf Kerberos client configuration file is readable by your BIND user. For example:

• Verify that the nsupdate utility exists on your domain controller (DC):

The nsupdate command is used to update the DNS. If the utility is missing, see you distribution's documentation how to identify the package containing the command and how to install.

For details, see BIND9_DLZ AppArmor and SELinux Integration.

• Before you start the service, verify the BIND configuration:

If no output is shown, the BIND configuration is valid.

• Start the BIND service.

For details, see Testing Dynamic DNS Updates.

When a BIND thread calls one of the BIND9_DLZ plugin API calls, execution can be blocked on database access calls if locks are out on the database at the time. Unfortunately, during that time, BIND will not be able to serve any queries, even external (non-samba) queries. Bind has a '-n' option that can increase the number of worker threads but testing has shown that increasing this number does not fix the problem, indicating that BIND's threading and queueing models are probably a bit broken.In small-scale environments this problem is unlikely to come up, but, in high-traffic environments, it may cause DNS outage. The only solution right now is to use an external DNS server that only forwards queries to BIND9_DLZ-backed samba DNS installations when the query is addressed to a zone managed by that node.

Reconfiguring the BIND9_DLZ Back End

Running the BIND9_DLZ back end setup automatically fixes several problems, such as recreating the Active Directory (AD) BIND DNS account (dns-*) and BIND Kerberos keytab file problems.

To fix the problem:

• Run the auto-reconfiguration:

• Restart the BIND service.

Debugging the BIND9_DLZ Module

To set a log level for the BIND9_DLZ module:

• Append the -d parameter and log level to the module in the /usr/local/samba/bind-dns/named.conf file. For example:

• Stop the BIND service.

• Start BIND manually to display the debug out put and to capture the log output in the /tmp/named.log file:

See the named (8) man page for details about the used parameters.

New DNS Entries Are Not Resolvable

If you create new DNS records in the directory and are not able to resolve them using the nslookup, host or other DNS lookup tools, the database hard links can got lost. This happens, for example, if you move the databases across mount points.

To verify that the domain and forest partition as well as the metadata.tdb database are hard linked in both directories, run

The same files must have the same inode number in the first column of the output in the both directories. If they differ, the hard link got lost and Samba and BIND use separate database files and thus DNS updates in the directory are not resolveable through the BIND DNS server.

To auto-repair the hard linking, see Reconfiguring the BIND9_DLZ Back End.

Updating the DNS Fails: NOTAUTH

If BIND uses incorrect Kerberos settings on the Samba Active Directory (AD) domain controller (DC), dynamic DNS updates fail. For example:

To solve the problem:

• Verify that BIND configuration is set up correctly. For further details, see Setting up Dynamic DNS Updates Using Kerberos.

• Recreate the BIND back end settings. For details, see Reconfiguring the BIND9_DLZ Back End.

Updating the DNS Fails: dns_tkey_negotiategss: TKEY is unacceptable

For details, see dns_tkey_negotiategss: TKEY is unacceptable.

Reloading the Bind9 DNS Server

If you reload Bind9, you are likely to see lines similar to these in the logs:

You cannot reload Bind9 on a Samba AD DC, you must use restart.You should check if logrotate is using reload and change it if it is.

If using systemd this can be disabled or changed to restart.You can do this in a systemd override file or the bind9.service file.If 'systemctl edit' is used, an override file is automatically created:

run:

add:

Ensure that Samba always starts after Bind9:

This creates: /etc/systemd/system/samba-ad-dc.service.d/override.conf

Add:

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Starting Bind9 DNS Server fails with 'unhandled record type 65281' (Windows AD + Samba AD)

If when starting Bind9 DNS Server you see something like:

This is likely caused because you have a Windows Server Active Directory that has WINS entries and you are joining it.To fix it, you have to disable WINS resolving in DNS of Windows Server DC direct search zones, restart Samba AD service, reload DNS config samba_upgradedns --dns-backend=BIND9_DLZ, and then, restart Bind9 service.

I cannot find the Bind9 dns directory

You have searched for the dns folder /usr/local/samba/bind-dns but cannot find it. This directory was introduced at Samba version 4.8.0, but is only created if one of these three things has occurred:

• You provisioned Samba with the '--dns-backend=BIND9_DLZ' option.

• You joined a DC with the '--dns-backend=BIND9_DLZ' option.

• You upgraded to Bind9 with 'samba_upgradedns' and the '--dns-backend=BIND9_DLZ' option.

Azure Cache for Redis has different cache offerings, which provide flexibility in the choice of cache size and features, including Premium tier features such as clustering, persistence, and virtual network support. This article describes how to configure clustering in a premium Azure Cache for Redis instance.

For information on other premium cache features, see Introduction to the Azure Cache for Redis Premium tier.

What is Redis Cluster?

Azure Cache for Redis offers Redis cluster as implemented in Redis. With Redis Cluster, you get the following benefits:

• The ability to automatically split your dataset among multiple nodes.
• The ability to continue operations when a subset of the nodes is experiencing failures or are unable to communicate with the rest of the cluster.
• More throughput: Throughput increases linearly as you increase the number of shards.
• More memory size: Increases linearly as you increase the number of shards.

Clustering does not increase the number of connections available for a clustered cache. For more information about size, throughput, and bandwidth with premium caches, see What Azure Cache for Redis offering and size should I use?

In Azure, Redis cluster is offered as a primary/replica model where each shard has a primary/replica pair with replication where the replication is managed by Azure Cache for Redis service.

Clustering

Clustering is enabled on the New Azure Cache for Redis blade during cache creation.

To create a premium cache, sign in to the Azure portal and click Create a resource > Databases > Azure Cache for Redis.

Note

In addition to creating caches in the Azure portal, you can also create them using Resource Manager templates, PowerShell, or Azure CLI. For more information about creating an Azure Cache for Redis, see Create a cache.

To configure premium features, first select one of the premium pricing tiers in the Pricing tier drop-down list. For more information about each pricing tier, click View full pricing details and select a pricing tier from the Choose your pricing tier blade.

Clustering is configured on the Redis Cluster blade.

You can have up to 10 shards in the cluster. Click Enabled and slide the slider or type a number between 1 and 10 for Shard count and click OK.

Each shard is a primary/replica cache pair managed by Azure, and the total size of the cache is calculated by multiplying the number of shards by the cache size selected in the pricing tier.

Once the cache is created you connect to it and use it just like a non-clustered cache, and Redis distributes the data throughout the Cache shards. If diagnostics is enabled, metrics are captured separately for each shard and can be viewed in the Azure Cache for Redis blade.

Note

There are some minor differences required in your client application when clustering is configured. For more information, see Do I need to make any changes to my client application to use clustering?

For sample code on working with clustering with the StackExchange.Redis client, see the clustering.cs portion of the Hello World sample.

Change the cluster size on a running premium cache

To change the cluster size on a running premium cache with clustering enabled, click Cluster Size from the Resource menu.

To change the cluster size, use the slider or type a number between 1 and 10 in the Shard count text box and click OK to save.

Increasing the cluster size increases max throughput and cache size. Increasing the cluster size doesn't increase the max. connections available to clients.

Note

Scaling a cluster runs the MIGRATE command, which is an expensive command, so for minimal impact, consider running this operation during non-peak hours. During the migration process, you will see a spike in server load. Scaling a cluster is a long running process and the amount of time taken depends on the number of keys and size of the values associated with those keys.

Clustering FAQ

The following list contains answers to commonly asked questions about Azure Cache for Redis clustering.

Do I need to make any changes to my client application to use clustering?

• When clustering is enabled, only database 0 is available. If your client application uses multiple databases and it tries to read or write to a database other than 0, the following exception is thrown. Unhandled Exception: StackExchange.Redis.RedisConnectionException: ProtocolFailure on GET --->StackExchange.Redis.RedisCommandException: Multiple databases are not supported on this server; cannot switch to database: 6

  For more information, see Redis Cluster Specification - Implemented subset.

• If you are using StackExchange.Redis, you must use 1.0.481 or later. You connect to the cache using the same endpoints, ports, and keys that you use when connecting to a cache that does not have clustering enabled. The only difference is that all reads and writes must be done to database 0.

  • Other clients may have different requirements. See Do all Redis clients support clustering?

• If your application uses multiple key operations batched into a single command, all keys must be located in the same shard. To locate keys in the same shard, see How are keys distributed in a cluster?

• If you are using Redis ASP.NET Session State provider you must use 2.0.1 or higher. See Can I use clustering with the Redis ASP.NET Session State and Output Caching providers?

How are keys distributed in a cluster?

Per the Redis Keys distribution model documentation: The key space is split into 16384 slots. Each key is hashed and assigned to one of these slots, which are distributed across the nodes of the cluster. You can configure which part of the key is hashed to ensure that multiple keys are located in the same shard using hash tags.

• Keys with a hash tag - if any part of the key is enclosed in { and }, only that part of the key is hashed for the purposes of determining the hash slot of a key. For example, the following 3 keys would be located in the same shard: {key}1, {key}2, and {key}3 since only the key part of the name is hashed. For a complete list of keys hash tag specifications, see Keys hash tags.
• Keys without a hash tag - the entire key name is used for hashing. This results in a statistically even distribution across the shards of the cache.

For best performance and throughput, we recommend distributing the keys evenly. If you are using keys with a hash tag it is the application's responsibility to ensure the keys are distributed evenly.

For more information, see Keys distribution model, Redis Cluster data sharding, and Keys hash tags.

For sample code on working with clustering and locating keys in the same shard with the StackExchange.Redis client, see the clustering.cs portion of the Hello World sample.

What is the largest cache size I can create?

The largest premium cache size is 120 GB. You can create up to 10 shards giving you a maximum size of 1.2TB GB. If you need a larger size you can request more. For more information, see Azure Cache for Redis Pricing.

Do all Redis clients support clustering?

Not all clients support Redis clustering! Please check the documentation for the library you are using, to verify you are using a library and version which support clustering. StackExchange.Redis is one library that does support clustering, in its newer versions. For more information on other clients, see the Playing with the cluster section of the Redis cluster tutorial.

The Redis clustering protocol requires each client to connect to each shard directly in clustering mode, and also defines new error responses such as 'MOVED' na 'CROSSSLOTS'. Attempting to use a client that doesn't support clustering with a cluster mode cache can result in a lot of MOVED redirection exceptions, or just break your application, if you are doing cross-slot multi-key requests.

Note

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If you are using StackExchange.Redis as your client, ensure you are using the latest version of StackExchange.Redis 1.0.481 or later for clustering to work correctly. If you have any issues with move exceptions, see move exceptions for more information.

How do I connect to my cache when clustering is enabled?

You can connect to your cache using the same endpoints, ports, and keys that you use when connecting to a cache that does not have clustering enabled. Redis manages the clustering on the backend so you don't have to manage it from your client.

Can I directly connect to the individual shards of my cache?

The clustering protocol requires that the client make the correct shard connections. So the client should do this correctly for you. With that said, each shard consists of a primary/replica cache pair, collectively known as a cache instance. You can connect to these cache instances using the redis-cli utility in the unstable branch of the Redis repository at GitHub. This version implements basic support when started with the -c switch. For more information, see Playing with the cluster on https://redis.io in the Redis cluster tutorial.

For non-TLS, use the following commands.

For TLS, replace 1300N with 1500N.

Can I configure clustering for a previously created cache?

Yes. First ensure that your cache is premium, by scaling if is not. Next, you should be able to see the cluster configuration options, including an option to enable cluster. You can change the cluster size after the cache is created, or after you have enabled clustering for the first time.

Important

You can't undo enabling clustering. And a cache with clustering enabled and only one shard behaves differently than a cache of the same size with no clustering.

Can I configure clustering for a basic or standard cache?

Clustering is only available for premium caches.

Can I use clustering with the Redis ASP.NET Session State and Output Caching providers?

• Redis Output Cache provider - no changes required.
• Redis Session State provider - to use clustering, you must use RedisSessionStateProvider 2.0.1 or higher or an exception is thrown. This is a breaking change; for more information, see v2.0.0 Breaking Change Details.

I am getting MOVE exceptions when using StackExchange.Redis and clustering, what should I do?

If you are using StackExchange.Redis and receive MOVE exceptions when using clustering, ensure that you are using StackExchange.Redis 1.1.603 or later. For instructions on configuring your .NET applications to use StackExchange.Redis, see Configure the cache clients.

Next steps

Learn how to use more premium cache features.