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---
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layout: docs
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page_title: How Connect Works
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description: >-
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This page details the internals of Consul Connect: mutual TLS, agent caching
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and performance, intention and certificate authority replication.
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---
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# How Connect Works
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This page details the inner workings of some of Connect's core features.
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Understanding how these features work isn't a prerequisite for using Connect,
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but will help you build a mental model of what's going on under the hood, which
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may help you reason about Connect's behavior in more complex deployment
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scenarios.
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2020-07-08 23:09:00 +00:00
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To try Connect locally, complete the [Getting Started with Consul service
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mesh](https://learn.hashicorp.com/tutorials/consul/service-mesh?utm_source=WEBSITE&utm_medium=WEB_IO&utm_offer=ARTICLE_PAGE&utm_content=DOCS)
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tutorial.
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## Mutual Transport Layer Security (mTLS)
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The core of Connect is based on [mutual TLS](https://en.wikipedia.org/wiki/Mutual_authentication).
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Connect provides each service with an identity encoded as a TLS certificate.
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This certificate is used to establish and accept connections to and from other
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services. The identity is encoded in the TLS certificate in compliance with
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the [SPIFFE X.509 Identity Document](https://github.com/spiffe/spiffe/blob/master/standards/X509-SVID.md).
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This enables Connect services to establish and accept connections with
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other SPIFFE-compliant systems.
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The client service verifies the destination service certificate
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against the [public CA bundle](/api/connect/ca#list-ca-root-certificates).
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This is very similar to a typical HTTPS web browser connection. In addition
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to this, the client provides its own client certificate to show its
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identity to the destination service. If the connection handshake succeeds,
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the connection is encrypted and authorized.
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The destination service verifies the client certificate against the [public CA
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bundle](/api/connect/ca#list-ca-root-certificates). After verifying the
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certificate, the next step depends upon the configured application protocol of
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the destination service. TCP (L4) services must authorize incoming _connections_
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against the configured set of Consul [intentions](/docs/connect/intentions),
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whereas HTTP (L7) services must authorize incoming _requests_ against those same
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intentions. If the intention check responds successfully, the
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connection/request is established. Otherwise the connection/request is
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rejected.
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To generate and distribute certificates, Consul has a built-in CA that
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requires no other dependencies, and
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also ships with built-in support for [Vault](/docs/connect/ca/vault). The PKI system is designed to be pluggable
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and can be extended to support any system by adding additional CA providers.
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All APIs required for Connect typically respond in microseconds and impose
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minimal overhead to existing services. To ensure this, Connect-related API calls
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are all made to the local Consul agent over a loopback interface, and all [agent
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Connect endpoints](/api/agent/connect) implement local caching, background
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updating, and support blocking queries. Most API calls operate on purely local
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in-memory data.
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## Agent Caching and Performance
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To enable fast responses on endpoints such as the [agent Connect
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API](/api/agent/connect), the Consul agent locally caches most Connect-related
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data and sets up background [blocking queries](/api/features/blocking) against
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the server to update the cache in the background. This allows most API calls
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such as retrieving certificates or authorizing connections to use in-memory
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data and respond very quickly.
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All data cached locally by the agent is populated on demand. Therefore, if
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Connect is not used at all, the cache does not store any data. On first request,
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the data is loaded from the server and cached. The set of data cached is: public
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CA root certificates, leaf certificates, intentions, and service discovery
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results for upstreams. For leaf certificates and intentions, only data related
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to the service requested is cached, not the full set of data.
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Further, the cache is partitioned by ACL token and datacenters. This is done
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to minimize the complexity of the cache and prevent bugs where an ACL token
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may see data it shouldn't from the cache. This results in higher memory usage
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for cached data since it is duplicated per ACL token, but with the benefit
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of simplicity and security.
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With Connect enabled, you'll likely see increased memory usage by the
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local Consul agent. The total memory is dependent on the number of intentions
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related to the services registered with the agent accepting Connect-based
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connections. The other data (leaf certificates and public CA certificates)
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is a relatively fixed size per service. In most cases, the overhead per
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service should be relatively small: single digit kilobytes at most.
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The cache does not evict entries due to memory pressure. If memory capacity
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is reached, the process will attempt to swap. If swap is disabled, the Consul
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agent may begin failing and eventually crash. Cache entries do have TTLs
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associated with them and will evict their entries if they're not used. Given
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a long period of inactivity (3 days by default), the cache will empty itself.
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2019-07-08 14:12:42 +00:00
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## Connections Across Datacenters
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A sidecar proxy's [upstream configuration](/docs/connect/registration/service-registration#upstream-configuration-reference)
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may specify an alternative datacenter or a prepared query that can address services
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in multiple datacenters (such as the [geo failover](https://learn.hashicorp.com/tutorials/consul/automate-geo-failover) pattern).
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[Intentions](/docs/connect/intentions) verify connections between services by
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source and destination name seamlessly across datacenters.
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Connections can be made via gateways to enable communicating across network
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topologies, allowing connections between services in each datacenter without
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externally routable IPs at the service level.
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## Intention Replication
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Intention replication happens automatically but requires the
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[`primary_datacenter`](/docs/agent/options#primary_datacenter)
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configuration to be set to specify a datacenter that is authoritative
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for intentions. In production setups with ACLs enabled, the
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[replication token](/docs/agent/options#acl_tokens_replication) must also
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be set in the secondary datacenter server's configuration.
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## Certificate Authority Federation
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The primary datacenter also acts as the root Certificate Authority (CA) for Connect.
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The primary datacenter generates a trust-domain UUID and obtains a root certificate
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from the configured CA provider which defaults to the built-in one.
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Secondary datacenters fetch the root CA public key and trust-domain ID from the
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primary and generate their own key and Certificate Signing Request (CSR) for an
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intermediate CA certificate. This CSR is signed by the root in the primary
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datacenter and the certificate is returned. The secondary datacenter can now use
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this intermediate to sign new Connect certificates in the secondary datacenter
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without WAN communication. CA keys are never replicated between datacenters.
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The secondary maintains watches on the root CA certificate in the primary. If the
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CA root changes for any reason such as rotation or migration to a new CA, the
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secondary automatically generates new keys and has them signed by the primary
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datacenter's new root before initiating an automatic rotation of all issued
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certificates in use throughout the secondary datacenter. This makes CA root key
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rotation fully automatic and with zero downtime across multiple datacenters.
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