Passive Peer

Description — Couchbase Lite’s Peer-to-Peer Synchronization enables edge devices to synchronize securely without consuming centralized cloud-server resources
Abstract — How to set up a Listener to accept a Replicator connection and sync using peer-to-peer
Related Content — API Reference | Passive Peer | Active Peer

iOS Restrictions

iOS 14 Applications: When your application attempts to access the user’s local network, iOS will prompt them to allow (or deny) access. You can customize the message presented to the user by editing the description for the NSLocalNetworkUsageDescription key in the Info.plist.

Code Snippets

All code examples are indicative only. They demonstrate the basic concepts and approaches to using a feature. Use them as inspiration and adapt these examples to best practice when developing applications for your platform.

Introduction

This content provides code and configuration examples covering the implementation of Peer-to-Peer Sync over websockets. Specifically it covers the implementation of a Passive Peer.

This passive peer (also referred to as the server, or listener) will accept a connection from an Active Peer (also referred to as the client or replicator) and participate in the replication of database changes to synchronize both databases.

Subsequent sections provide additional details, and examples for the main configuration options.

Secure Storage

The use of TLS, its associated keys and certificates requires using secure storage to minimize the chances of a security breach. The implementation of this storage differs from platform to platform — see Using secure storage.

Configuration Summary

You should configure and initialize a listener for each Couchbase Lite database instance you want to sync. There is no limit on the number of listeners you may configure — Example 1 shows a simple initialization and configuration process.

Example 1. Listener configuration and initialization


var config = URLEndpointListenerConfiguration(database: otherDB) (1)

/* optionally */ let wsPort: UInt16 = 55991
/* optionally */ let wssPort: UInt16 = 55990
config.port =  wssPort (2)

config.networkInterface = "10.1.1.10"  (3)

config.enableDeltaSync = true (4)

config.disableTLS  = false (5)

// Set the credentials the server presents the client
// Use an anonymous self-signed cert
config.tlsIdentity = nil (6)

// Configure how the client is to be authenticated
// Here, use Basic Authentication
config.authenticator = ListenerPasswordAuthenticator(authenticator: { uname, pword -> Bool in
    return self.isValidCredentials(uname, password: pword)
}) (7)


// Initialize the listener
self.listener = URLEndpointListener(config: config) (8)
if self.listener == nil {
    fatalError("ListenerError Not Initialized")
    // ... take appropriate actions
}

// Start the listener
try self.listener.start() (9)

1	Identify the local database to be used — see: Initialize the Listener Configuration
2	Optionally, choose a port to use. By default the system will automatically assign a port — to over-ride this, see: Set Port and Network Interface
3	Optionally, choose a network-interface to use. By default the system will listen on all network interfaces — to over-ride this see: Set Port and Network Interface
4	Optionally, choose to sync only changes, the default is not to enable delta-sync — see: Delta Sync.
5	Set server security. TLS is always enabled out-of-the-box, so you can usually omit this line. But you can, optionally, disable TLS (not advisable in production) — see: TLS Security
6	Set the credentials this server will present to the client for authentication. Here we show the default TLS authentication, which is by anonymous self-signed certificate. The server must always authenticate itself to the client.
7	Set client security — define the credentials the server expects the client to present for authentication. Here we show how basic authentication is configured to authenticate the client-supplied credentials from the http authentication header against valid credentials — see Authenticating the Client for more options. Note that client authentication is optional.
8	Initialize the listener using the configuration settings.
9	Start Listener

API References

You can find Swift API References here.

Device Discovery

This phase is optional: If the listener is initialized on a well known URL endpoint (for example, a static IP Address or well known DNS address) then you can configure active peers to connect to those.

Prior to initiating the listener you may execute a peer discovery phase. For the passive peer, this involves advertising the service using, for example Bonjour (see: https://developer.apple.com/bonjour/) and waiting for an invite from the active peer. The connection is established once the passive peer has authenticated and accepted an active peer’s invitation.

Initialize the Listener Configuration

Initialize the listener configuration with the local database — see Example 2 All other configuration values take their default setting.

Each Listener instance serves one Couchbase Lite database. Couchbase sets no hard limit on the number of listeners you can initialize.

Example 2. Specify Local Database

var config = URLEndpointListenerConfiguration(database: otherDB) (1)

1	Set the local database using the URLEndpointListenerConfiguration's constructor init(database:). The database must be opened before the listener is started. `thisDB` has previously been declared as an object of type `Database`.

Set Port and Network Interface

Port number

The Listener will automatically select an available port if you do not specify one — see Example 3 for how to specify a port.

Example 3. Specify a port

/* optionally */ let wsPort: UInt16 = 55991
/* optionally */ let wssPort: UInt16 = 55990
config.port =  wssPort (1)

1	To use a canonical port — one known to other applications — specify it explicitly using the port method shown here. Ensure that any port you do specify is not blocked by firewall rules.

Network Interface

The Listener will listen on all network interfaces by default.

Example 4. Specify a Network Interface to Use

config.networkInterface = "10.1.1.10"  (1)

1	To specify an interface — one known to other applications — identify it explicitly, using the networkInterface method shown here. This must be either an IP Address or network interface name such as `en0`.

Where necessary, you can identify the available interfaces at runtime, using appropriate platform tools — see Example 5.

Example 5. Identify available network interfaces

import SystemConfiguration
// . . .

class SomeClass {
    func SomeFunction() {
        for interface in SCNetworkInterfaceCopyAll() as! [SCNetworkInterface] {
            // do something with this `interface`
        }
    }

    // . . .
}

Delta Sync

Delta Sync allows clients to sync only those parts of a document that have changed. This can result in significant savings in bandwidth consumption as well as throughput improvements. Both valuable benefits, especially when network bandwidth is constrained.

Example 6. Enable delta sync

config.enableDeltaSync = true (1)

1	Delta sync replication is not enabled by default. Use URLEndpointListenerConfiguration's enableDeltaSync method to activate or deactivate it.

TLS Security

Enable or Disable TLS

Define whether the connection is to use TLS or clear text.

TLS based encryption is enabled by default and this setting ought to be used in any production environment. However, it can be disabled, for example for development and-or test environments.

When TLS is enabled, Couchbase Lite provides several options on how the Listener may be configured with an appropriate TLS Identity — see Configure TLS Identity for Listener.

You can use URLEndpointListenerConfiguration's disableTLS method to disable TLS communication if necessary

The disableTLS setting must be 'false' when Client Cert Authentication is required.

Basic Authentication can be used with, or without, TLS.

disableTLS works in conjunction with TLSIdentity, to enable developers to define the key and certificate to be used.

If disableTLS is true — TLS communication is disabled and TLS identity is ignored. Active peers will use the ws:// URL scheme used to connect to the listener.
If disableTLS is false or not specified — TLS communication is enabled.

Active peers will use the wss:// URL scheme to connect to the listener.

Configure TLS Identity for Listener

Define the credentials the server will present to the client for authentication. Note that the server must always authenticate itself with the client — see: Authenticate Listener on Active Peer for how the client deals with this.

Use URLEndpointListenerConfiguration's tlsIdentity method to configure the TLS Identity used in TLS communication.

If TLSIdentity is not set, then the listener uses an auto-generated anonymous self-signed identity (unless disableTLS = true). Whilst the client cannot use this to authenticate the server, it will use it to encrypt communication, giving a more secure option than non-TLS communication.

The auto-generated anonymous self-signed identity is saved in secure storage for future use to obviate the need to re-generate it.

Typically, you will configure the listener’s TLS Identity once during initial launch and re-use it (from secure storage on any subsequent starts.

Here are some example code snippets showing:

Importing a TLS identity — see: Example 7
Setting TLS identity to expect self-signed certificate — — see: Example 8
Setting TLS identity to expect anonymous certificate — see: Example 9

Example 7. Import Listener’s TLS identity

Import an identity from a secure key and certificate data source.

config.disableTLS  = false (1)

guard let path = Bundle.main.path(forResource: "cert", ofType: "p12") else {
    /* process error */ return
}

guard let certData = try? NSData(contentsOfFile: path) as Data else {
    /* process error */ return
} (2)

let tlsIdentity = try TLSIdentity.importIdentity(withData: certData,
                                                 password: "123",
                                                 label: "Server-Cert-Label") (3)

// Set the credentials the server presents the client
config.tlsIdentity = tlsIdentity    (4)

1	Ensure TLS is used
2	Get key and certificate data
3	Use the retrieved data to create and store the TLS identity
4	Set this identity as the one presented in response to the client’s prompt

Example 8. Create Self-Signed Cert

Create a TLSIdentity for the server using convenience API. The system generates a self-signed certificate.

config.disableTLS  = false (1)

let attrs = [certAttrCommonName: "Couchbase Inc"] (2)

let identity = try TLSIdentity.createIdentity(forServer: true, /* isServer */
                                              attributes: attrs,
                                              expiration: Date().addingTimeInterval(86400),
                                              label: "Server-Cert-Label") (3)

// Set the credentials the server presents the client
config.tlsIdentity = tlsIdentity    (4)

1	Ensure TLS is used.
2	Map the required certificate attributes, in this case the common name.
3	Create the required TLS identity using the attributes. Add to secure storage as 'couchbase-docs-cert'.
4	Configure the server to present the defined identity credentials when prompted.

Example 9. Use Anonymous Self-Signed Certificate

This example uses an anonymous self signed certificate. Generated certificates are held in secure storage.

config.disableTLS  = false (1)

// Set the credentials the server presents the client
// Use an anonymous self-signed cert
config.tlsIdentity = nil (2)

1	Ensure TLS is used. This is the default setting.
2	Authenticate using an anonymous self-signed certificate. This is the default setting.

Authenticating the Client

In this section: Use Basic Authentication | Using Client Certificate Authentication | Delete Entry | The Impact of TLS Settings

Define how the server (listener) will authenticate the client as one it is prepared to interact with.

Whilst client authentication is optional, Couchbase lite provides the necessary tools to implement it. Use the URLEndpointListenerConfiguration class’s authenticator method to specify how the client-supplied credentials are to be authenticated.

Valid options are:

No authentication — If you do not define an Authenticator then all clients are accepted.
Basic Authentication — uses the ListenerPasswordAuthenticator to authenticate the client using the client-supplied username and password (from the http authentication header).
ListenerCertificateAuthenticator — which authenticates the client using a client supplied chain of one or more certificates. You should initialize the authenticator using one of the following constructors:
- A list of one or more root certificates — the client supplied certificate must end at a certificate in this list if it is to be authenticated
- A block of code that assumes total responsibility for authentication — it must return a boolean response (true for an authenticated client, or false for a failed authentication).

Use Basic Authentication

Define how to authenticate client-supplied username and password credentials. To use client-supplied certificates instead — see: Using Client Certificate Authentication

Example 10. Password authentication

// Configure how the client is to be authenticated
// Here, use Basic Authentication
config.authenticator = ListenerPasswordAuthenticator(authenticator: { uname, pword -> Bool in
    return self.isValidCredentials(uname, password: pword)
}) (1)

1	Where 'username'/'password' are the client-supplied values (from the http-authentication header) and `validUser`/`validPassword` are the values acceptable to the server.

Using Client Certificate Authentication

Define how the server will authenticate client-supplied certificates.

There are two ways to authenticate a client:

A chain of one or more certificates that ends at a certificate in the list of certificates supplied to the constructor for ListenerCertificateAuthenticator — see: Example 11
Application logic: This method assumes complete responsibility for verifying and authenticating the client — see: Example 12

If the parameter supplied to the constructor for ListenerCertificateAuthenticator is of type ListenerCertificateAuthenticatorDelegate, all other forms of authentication are bypassed.

The client response to the certificate request is passed to the method supplied as the constructor parameter. The logic should take the form of function or block (such as, a closure expression) where the platform allows.

Example 11. Set Certificate Authorization

Configure the server (listener) to authenticate the client against a list of one or more certificates provided by the server to the the ListenerCertificateAuthenticator.

// Authenticate using Cert Authority

// cert is a pre-populated object of type:SecCertificate representing a certificate
let rootCertData = SecCertificateCopyData(cert) as Data (1)
let rootCert = SecCertificateCreateWithData(kCFAllocatorDefault, rootCertData as CFData)! //

config.authenticator = ListenerCertificateAuthenticator.init (rootCerts: [rootCert]) (2) (3)

1	Get the identity data to authenticate against. This can be, for example, from a resource file provided with the app, or an identity previously saved in secure storage.
2	Configure the authenticator to authenticate the client supplied certificate(s) using these root certs. A valid client will provide one or more certificates that match a certificate in this list.
3	Add the authenticator to the listener configuration.

Example 12. Application Logic

Configure the server (listener) to authenticate the client using user-supplied logic.

// Authenticate self-signed cert using application logic

config.authenticator = ListenerCertificateAuthenticator { certs -> Bool in (1)
    // Validate the cert
    return self.isValidCertificates(certs)
} (2)

1	Get the identity data to authenticate against. This can be, for example, from a resource file provided with the app, or an identity previously saved in secure storage.
2	Configure the Authenticator to pass the root certificates to a user supplied code block. This code assumes complete responsibility for authenticating the client supplied certificate(s). It must return a boolean value; with `true` denoting the client supplied certificate authentic.
3	Add the authenticator to the listener configuration.

Delete Entry

You can remove unwanted TLS identities from secure storage using the convenience API.

Example 13. Deleting TLS Identities


try TLSIdentity.deleteIdentity(withLabel: "doco-sync-server")

The Impact of TLS Settings

The table in this section shows the expected system behavior (in regards to security) depending on the TLS configuration settings deployed.

Table 1. Expected system behavior
disableTLS	tlsIdentity (corresponding to server)	Expected system behavior
true	Ignored	TLS is disabled; all communication is plain text.
false	set to nil	The system will auto generate an anonymous self signed cert. Active peers (clients) should be configured to accept self-signed certificates. Communication is encrypted
false	Set to server identity generated from a self- or CA-signed certificate On first use — Bring your own certificate and private key; for example, using the TLSIdentity class’s CreateIdentity() method to add it to the secure storage. Each time — Use the server identity from the certificate stored in the secure storage; for example, using the TLSIdentity class’s identity(withLabel:) method with the alias you want to retrieve..	System will use the configured identity. Active peers will validate the server certificate corresponding to the TLSIdentity (as long as they are configured to not skip validation — see TLS Security).

Start Listener

Once you have completed the Listener’s configuration settings you can initialize the Listener instance and start it running — see: Example 14

Example 14. Initialize and start listener

// Initialize the listener
self.listener = URLEndpointListener(config: config) (1)
if self.listener == nil {
    fatalError("ListenerError Not Initialized")
    // ... take appropriate actions
}

// Start the listener
try self.listener.start() (2)

Monitor Listener

Use the Listener’s status property/method to get counts of total and active connections — see: Example 15.

You should note that these counts can be extremely volatile. So, the actual number of active connections may have changed, by the time the ConnectionStatus class returns a result.

Example 15. Get connection counts

let totalConnections = self.listener.status.connectionCount
let activeConnections = self.listener.status.activeConnectionCount

Stop Listener

It is best practice to check tha status of the listener’s connections and stop only when you have confirmed that there are no active connections — see Example 15.

Example 16. Stop listener using stop method

self.listener.stop()

Closing the database will also close the listener.

How to

Concepts

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Getting Started with Peer-to-Peer Synchronization