XDCR Security and Networking

      Specific requirements must be satisfied in order to ensure the successful creation of XDCR replications over different network configurations.

      Security and Networking Contexts

      This page considers three, specific contexts in which XDCR replications must sometimes be established. These respectively involve:

      • Handling Couchbase Schemes, Using DNS SRV

      • Using Alternate Addresses

      • Using Certificates

      Each is explained below.

      Handling Couchbase Schemes, Using DNS SRV

      Targets for XDCR replication can be specified with the standard port number 8091 or 18091, as applicable. Targets cannot be specified with the Couchbase schemes couchbase:// and couchbases://, which are, instead, used to support the memcached protocol on ports 11210 and 11207, respectively.

      Targets can, however, be specified with a Fully Qualified Domain Name (FQDN) that is either tagged with one of the standard port numbers, or is untagged. In such cases, XDCR performs a LookupSRV on the FQDN, using the local machine’s DNS resolver. LookupSRV obtains the appropriate name from a local DNS SRV record: which is a specification of data in the Domain Name System that defines the location (the hostname and port number) of a server for a particular service. When LookupSRV has returned the name, XDCR concatenates a standard port number to the name, and uses the resulting concatenation as the target-reference.

      Consequently, resources named by means of the Couchbase schemes can be resolved to appropriate XDCR-target names: by first removing the couchbase or couchbases prefix, and then specifying the remaining symbol, which is the FQDN — so that LookupSRV resolves the FQDN to a name that XDCR can include in the ultimate target-reference. Note, however, that this does assume the target’s use of one of the standard port numbers — 8091 or 18091.


      Suppose a resource is provided with the Couchbase scheme as follows: couchbases://7cb51b5b-d9cd-410d-9cc7-1be93e2f31d9.dp.cloud.couchbase.com. The scheme’s prefix couchbases:// can be removed, so as to leave an FQDN as follows: 7cb51b5b-d9cd-410d-9cc7-1be93e2f31d9.dp.cloud.couchbase.com. This FQDN can be specified by the administrator as the hostname of the target cluster for XDCR replication.

      Note that the command nslookup -type=srv can be used manually to determine whether this hostname corresponds to one or more true, DNS SVR records. The format of an SRV record is as follows:

      _service._proto.name. ttl IN SRV priority weight port target

      See SRV record, for a full explanation of this syntax.

      For manual lookup to be performed, values must be provided for the _service._proto.name. arguments. Thus, couchbases can be used for the service argument; tcp for the proto argument; and the hostname 7cb51b5b-d9cd-410d-9cc7-1be93e2f31d9.dp.cloud.couchbase.com for the name argument. The command therefore becomes as follows:

      nslookup -type=srv _couchbases._tcp.7cb51b5b-d9cd-410d-9cc7-1be93e2f31d9.dp.cloud.couchbase.com

      If execution is successful, output is similar to the following:

      Non-authoritative answer:
      _couchbases._tcp.7cb51b5b-d9cd-410d-9cc7-1be93e2f31d9.dp.cloud.couchbase.com	service = 0 0 11207 cb-0001.7cb51b5b-d9cd-410d-9cc7-1be93e2f31d9.dp.cloud.couchbase.com.
      _couchbases._tcp.7cb51b5b-d9cd-410d-9cc7-1be93e2f31d9.dp.cloud.couchbase.com	service = 0 0 11207 cb-0002.7cb51b5b-d9cd-410d-9cc7-1be93e2f31d9.dp.cloud.couchbase.com.
      _couchbases._tcp.7cb51b5b-d9cd-410d-9cc7-1be93e2f31d9.dp.cloud.couchbase.com	service = 0 0 11207 cb-0000.7cb51b5b-d9cd-410d-9cc7-1be93e2f31d9.dp.cloud.couchbase.com.

      This output shows that three SRV records have been found to correspond with the values specified. The final value of each displayed record is the target for the record: each is a name beginning with cb-; and each name corresponds to an individual, server node.

      The process shown manually above is essentially that followed programmatically by LookupSRV. Note that XDCR ignores the value of the port argument returned in the record (which is in each case the secure memcached port, 11207).

      Using the target-reference derived from SRV, XDCR then performs its usual cluster add procedure, to contact the other server nodes.

      Using Alternate Addresses

      Alternate addresses are used to allow a cluster to keep its internally visible addresses private, while using a router or other networked entity to provide externally visible addresses that are to be used by networked clients of the cluster. Instances of Kubernetes and AWS frequently use alternate addresses for their connections with external entities.

      Alternate addresses used in this way are said to constitute an external network. Any cluster that is established within an external network must have its nodes configured to use the corresponding alternate addresses for external communications; and if the cluster is to be used as an XDCR target, these addresses must be referenced by the XDCR source. This involves, on the target, port forwarding, which is described in Alternate Addresses. It also involves, on the source, specification of the external network interface, either as an FQDN, or an IP address.

      When a cluster has been configured in this way for external networking, and status on the cluster is returned by means of Couchbase Web Console, the CLI, or the REST API, both the internal and the external addresses are represented.

      The REST API reference page Managing Alternate Addresses explains how alternate addresses can be established for and removed from nodes. The REST API reference page Listing Node Services explains how to retrieve all internal and external address settings.


      Kubernetes employs a dual network environment, where an external interface is established for the reception of externally generated network traffic. Such traffic includes inbound XDCR mutations: therefore, the source cluster must use the external interface established for Kubernetes.

      A diagram of dual-network setup is provided in Dual Network.

      AWS and Cloud

      If a cluster is established on a public cloud such as AWS, or is a Kubernetes cluster, and is intended to become the target for XDCR, the cluster must be configured with alternate addresses, to support the source cluster’s access. A diagram is provided in Internal Network or Cloud Access.

      Specifying Addresses

      If a target cluster is configured with alternate addresses, the particular address to which a connection should be made can be specified on the source cluster. The following options are available for specifying the target-cluster address:

      • When using Couchbase Web Console, from the XDCR Replications screen, left-clicking the ADD REMOTE tab brings up the Add Remote Cluster dialog, which provides an IP/Hostname interactive field. (See Create an XDCR Reference with the UI.)

      • When using the CLI, the xdcr-setup command provides the --xdcr-hostname field. (See xdcr-setup.)

      • When using the REST API, the POST /pools/default/remoteClusters method and URI provide the hostname field. (See Setting a Reference.)

      In each case:

      • An IP address or a qualified domain name can be specified. The name can optionally be tagged with the port number 8091 or 18091.

      • If a specified IP address corresponds to the external address of the target cluster, the external address is used for the connection. If a specified IP address corresponds to the internal address of the target cluster, the internal address is used for the connection.

        In either case, if the name is not tagged with a port number, the connection defaults to 8091; unless a secure connection is specified in another field, in which case 18091 is used. (Note that the REST API provides a network_type parameter, which can be set to external, so as to enforce a secure connection: see Setting a Reference.)

      • If an FQDN is specified without a port number, LookupSRV is invoked on the FQDN, to match the FQDN to an appropriate target-name. If an FQDN is specified with one of the standard port numbers, 8091 and 18091, the port number is stripped from the FQDN, and LookupSRV is invoked on the resulting symbol, to match the FQDN to an appropriate target-name. In either case, if a match is found, a connection is attempted, using the corresponding target-name. If no match is found, a connection is attempted, using the FQDN’s standard mapping to the internal or external IP address of a non-SRV target cluster.

        Note that if LookupSRV is attempted and fails, XDCR retries the connection, using the FQDN’s standard mapping: if the retry succeeds, the standard mapping continues to be used. Note that connectivity status for all of a source cluster’s defined references can be retrieved by means of the REST API: see Getting a Reference.

      Using Certificates

      To be fully secure, XDCR requires x.509 certificates to have been established on the target cluster. The source cluster can authenticate with the target by means either of a username and password or of its own x.509 certificates.

      Preparing to Configure Secure Replications

      A complete overview of certificate management for Couchbase Server is provided in Certificates; examples of establishing secure connections are provided in Secure a Replication. An overview of Transport Layer Security is provided in On-the-Wire Security; and examples of configuring TLS are provided in Manage On-the-Wire Security. Detailed information on the communication handshake implemented by Transport Layer Security can be found in TLS Handshake.

      Administrators intending to establish secure replications should be familiar with all of the above content. A number of key issues are summarized below.

      Defining Client and Server

      When a fully secure XDCR replication is configured, the source cluster should be considered the client, and the target cluster the server.

      Understanding Root, Intermediate, and Node Certificates

      As described in Certificates, the authority of a networked entity, such as a cluster or an application, is, in a typical production context, represented by a root certificate that has been provided by a known Certificate Authority. This root certificate (or CA) must be included in the trust store of the target cluster. See Using Multiple Root Certificates for information on the trust store, and see Load Root Certificates for information on loading a CA into a trust store.

      Note that when the known authority’s CA has been successfully loaded, it is visible by means of Couchbase Web Console, as shown in the documentation for the Certificates security screen. If, on the target cluster, the CA is not visible here, it has not been loaded, and no fully secure replication will be supported.

      Each node in the target cluster must be represented by its own node certificate. The CA for the target cluster must have been used to digitally sign each node certificate: either directly, or (more likely) indirectly, by means of an intermediate certificate. When signing is complete, each node certificate must be concatenated with however many intermediate certificates have been used, to form a certificate chain, and then appropriately posted on the node it is representing.

      To support fully secure XDCR, each node in a target cluster must be identified by its subjectAlName extension, in its node certificate.

      An overview of node-certificate preparation is provided in Certificate Hierarchies, and examples (including use of subjectAltName) are provided in Manage Certificates.

      Performing Certificate-Based Authentication

      When the XDCR source cluster authenticates with the target cluster, the TLS Handshake is performed. During the course of this, the server (which is the target-cluster node) provides its certificate chain to the client (which is the source cluster). The source cluster validates the chain, as described in Node Certificates.

      Recognizing CAs

      The top certificate in the chain that is provided to the source cluster (the client) points to the CA for the target cluster (the server). As explained above, this CA must have been loaded into the trust store for the target cluster. Additionally, this CA must be recognizable to the source cluster: therefore, XDCR allows the CA to be passed to the source cluster during the set-up of the secure connection. See Enable Fully Secure Replications for examples; covering the UI, the CLI, and the REST API.

      Note that, in Couchbase Server 7.1+, multiple root certificates are supported (see Using Multiple Root Certificates). Therefore, source and target clusters need not rely on the authority of the same CA: however, each must trust the CA of the other, if the client is to perform certificate-based authentication — and consequently, if the CAs are different, the CA of the client must have been loaded into the trust store of the server, for authentication to succeed.

      See Load Root Certificates, for further information.

      Handling Client Certificates

      If the source cluster (the client) wishes to authenticate with the target cluster (the server) by means of client certificates, the administrator must first Enable Client-Certificate Handling, on the server.

      See Specify Root and Client Certificates, and Client Private Key, for an example of making the subsequent connection, from the client.

      XDCR, Certificates, and Containers

      If either a source or a target cluster for an XDCR replication resides within a container, such as a Kubernetes pod, the container’s image must itself contain the trusted CA that is relied on for validating the cluster that is being connected to.

      Note that Couchbase Operator has a Dynamic Admissions Controller (DAC), which performs TLS certificate-generation and assignment, including rotation. Therefore, if DAC is being used, the Root Certificate referred to by the client when setting up a fully secure replication must be the one whose authority, on the server, is relied on for these DAC operations.