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Connect
Consul Connect provides service-to-service connection authorization and encryption using mutual TLS. Applications can use sidecar proxies to automatically establish TLS connections for inbound and outbound connections without being aware of Connect at all. Applications may also natively integrate with Connect for optimal performance and security.
Connect enables deployment best-practices with service-to-service encryption everywhere and identity-based authorization. Rather than authorizing host-based access with IP address access rules, Connect uses the registered service identity to enforce access control with intentions. This makes it much easier to reason about access control and also enables services to freely move, such as in a scheduled environment with software such as Kubernetes or Nomad. Additionally, intention enforcement can be done regardless of the underlying network, so Connect works with physical networks, cloud networks, software-defined networks, cross-cloud, and more.
How it Works
The core of Connect is based on mutual TLS.
Connect provides each service with an identity encoded as a TLS certificate. This certificate is used to establish and accept connections to and from other services. The identity is encoded in the TLS certificate in compliance with the SPIFFE X.509 Identity Document. This enables Connect services to establish and accept connections with other SPIFFE-compliant systems.
The client service verifies the destination service certificate against the public CA bundle. This is very similar to a typical HTTPS web browser connection. In addition to this, the client provides its own client certificate to show its identity to the destination service. If the connection handshake succeeds, the connection is encrypted and authorized.
The destination service verifies the client certificate against the public CA bundle. After verifying the certificate, it must also call the authorization API to authorize the connection against the configured set of Consul intentions. If the authorization API responds successfully, the connection is established. Otherwise, the connection is rejected.
To generate and distribute certificates, Consul has a built-in CA that requires no other dependencies, and also ships with built-in support for Vault. The PKI system is pluggable and can be extended to support any system.
All APIs required for Connect typically respond in microseconds and impose minimal overhead to existing services. This is because the Connect-related APIs are all made to the local Consul agent over a loopback interface, and all agent Connect endpoints implement local caching, background updating, and support blocking queries. As a result, most API calls operate on purely local in-memory data and can respond in microseconds.
Agent Caching and Performance
To enable microsecond-speed responses on agent Connect API endpoints, the Consul agent locally caches most Connect-related data and sets up background blocking queries against the server to update the cache in the background. This allows most API calls such as retrieving certificates or authorizing connections to use in-memory data and respond very quickly.
All data cached locally by the agent is populated on demand. Therefore, if Connect is not used at all, the cache does not store any data. On first request, the data is loaded from the server and cached. The set of data cached is: public CA root certificates, leaf certificates, and intentions. For leaf certificates and intentions, only data related to the service requested is cached, not the full set of data.
Further, the cache is partitioned by ACL token and datacenters. This is done to minimize the complexity of the cache and prevent bugs where an ACL token may see data it shouldn't from the cache. This results in higher memory usage for cached data since it is duplicated per ACL token, but with the benefit of simplicity and security.
With Connect enabled, you'll likely see increased memory usage by the local Consul agent. The total memory is dependent on the number of intentions related to the services registered with the agent accepting Connect-based connections. The other data (leaf certificates and public CA certificates) is a relatively fixed size per service. In most cases, the overhead per service should be relatively small: single digit kilobytes at most.
The cache does not evict entries due to memory pressure. If memory capacity is reached, the process will attempt to swap. If swap is disabled, the Consul agent may begin failing and eventually crash. Cache entries do have TTLs associated with them and will evict their entries if they're not used. Given a long period of inactivity (3 days by default), the cache will empty itself.
Multi-Datacenter
Connect currently only works for service-to-service connections wtihin a single Consul datacenter. Connect may be enabled on multiple Consul datacenters, but only services within the same datacenters can establish Connect-based connections. CA configurations and intentions are both local to their respective datacenters; they are not replicated across datacenters.
Multi-datacenter support for Connect is under development and will be released as a feature of Consul Enterprise in late 2018. This feature will facilitate intention replication, datacenter constraints on intentions, CA state replication, multi-datacenter certificate rotations, and more.