---
layout: docs
page_title: Vault Proxy Static Secret Caching
description: |-
  Vault Proxy's static secret caching functionality allows you to cache KVv1 and KVv2 secrets for calling clients.
  The secrets will be automatically updated by Proxy, minimizing requests made to Vault, and offering resiliency.
---

# Vault Proxy static secret caching

@include 'alerts/enterprise-only.mdx'

Use static secret caching with Vault Proxy to cache KVv1 and KVv2 secrets to
minimize requests made to Vault and provide resilient connections for clients.

Vault Proxy utilizes the Enterprise only [Vault event notification system](/vault/docs/concepts/events)
feature for cache freshness. As a result, static secret caching can only be used
with Vault Enterprise installations.

## Step 1: Subscribe Vault Proxy to KV events

Vault Proxy uses Vault events and auto-auth to monitor secret status and make
appropriate cache updates.
1. Enable [auto-auth](/vault/docs/agent-and-proxy/autoauth).
1. Create an auto-auth token with permission to subscribe to KV event updates
with the [Vault event notification system](/vault/docs/concepts/events). For
example, to create a policy that grants access to static secret (KVv1 and KVv2)
events, you need permission to subscribe to the `events` endpoint, as well as
the `list` and `subscribe` permissions on KV secrets you want to get secrets
from:
 ```hcl
  path "sys/events/subscribe/kv*" {
    capabilities = ["read"]
  }

  path "*" {
    capabilities = ["list", "subscribe"]
    subscribe_event_types = ["kv*"]
  }
   ```

Subscribing to KV events means that Proxy receives updates as soon as a secret
changes, which reduces staleness in the cache. Vault Proxy only checks for a
secret update if an event notification indicates that the related secret was
updated.

## Step 2: Ensure tokens have `capabilities-self` access

Tokens require `update` access to the
[`sys/capabilies-self`](/vault/api-docs/system/capabilities-self) endpoint to
request cached secrets. Vault tokens receive `update` permissions
[by default](/vault/docs/concepts/policies#default-policy). If you have modified
or removed the default policy, you must explicitly create a policy with the
appropriate permissions. For example:
```hcl
  path "sys/capabilities-self" {
    capabilities = ["update"]
  }
```

## Step 3: Configure an appropriate refresh interval
By default, Vault Proxy refreshes tokens every five minutes. You can change the
default behavior and configure Proxy to verify and update cached token
capabilities with the `static_secret_token_capability_refresh_interval`
parameter in the `cache` configuration stanza. For example, to set a refresh
interval of one minute:
```hcl
cache {
  cache_static_secrets = true
  static_secret_token_capability_refresh_interval = "1m"
}
```

## Functionality

With static secret caching, Vault Proxy caches `GET` requests for KVv1 and KVv2
endpoints.

When a client sends a `GET` request for a new KV secret, Proxy forwards the
request to Vault but caches the response before forwarding it to the client. If
that client makes subsequent `GET` requests for the same secret, Vault Proxy
serves the cached response rather than forwarding the request to Vault.

Similarly, when a token requests access to a KV secret, it must complete a
success `GET` request. If the request is successful, Proxy caches the fact that
the token was successful in addition to the result. Subsequent requests by the
same token can then access this secret from the cache instead of Vault.

Vault Proxy uses the [event notification system](/vault/docs/concepts/events) to keep the
cache up to date. It monitors the KV event feed for events related to any secret
currently stored in the cache, including modification events like updates and
deletes. When Proxy detects a change in a cached secret, it will update or
evict the cache entry as appropriate.

Vault Proxy also checks and refreshes the access permissions of known tokens
according to the window set with `static_secret_token_capability_refresh_interval`.
By default, the refresh interval is five minutes.

Every interval, Proxy calls [`sys/capabilies-self`](/vault/api-docs/system/capabilities-self) on
behalf of every token in the cache to confirm the token still has permission to
access the cached secret. If the result from Vault indicates that permission (or
the token itself) was revoked, Proxy updates the cache entry so that the affected
token can no longer access the relevant paths from the cache. The refresh interval
is essentially the maximum period after which permission to read a KV secret is
fully revoked for the relevant token.

If the capabilities have been removed, or Proxy receives a `403` response, the
capability is removed from the token, and that token cannot be used to access the
cache. For other kinds of errors, such as Vault being unreachable or sealed,
the `static_secret_token_capability_refresh_behavior` config is consulted.
If set to `optimistic` (the default), the capability will not be removed unless we
receive a `403` or valid response without the capability. If set to `pessimistic`,
the capability will be removed for any error, such as would occur if Vault is sealed.

For token refresh to work, any token that will access the cache also needs
`update` permission for [`sys/capabilies-self`](/vault/api-docs/system/capabilities-self).
Having `update` permission for the token lets Proxy test capabilities for the
token against multiple paths with a single request instead of testing for a `403`
response for each path explicitly.

<Tip title="Refresh is per token, not per secret">

  If Proxy's API proxy is configured to use auto-authentication for tokens, and **all**
  requests that pass through Vault Proxy use the same token, Proxy only
  makes a single request to Vault every refresh interval, no matter how many
  secrets are currently cached.

</Tip>

When static secret caching is enabled, Proxy returns `HIT` or `MISS` in the `X-Cache`
response header for requests so client can tell if the response was served from
the cache or forwarded from Vault. In the event of a hit, Proxy also sets the
`Age` header to indicate, in seconds, how old the cache entry is.

<Tip title="Old does not mean stale">

  The fact that a cache entry is old, does not necessarily mean that the
  information is out of date. Vault Proxy continually monitors KV events for
  updates. A large value for `Age` may simply mean that the secret has not been
  rotated recently.

</Tip>

## Configuration

The top level `cache` block has the following configuration entries relating to static secret caching:

- `cache_static_secrets` `(bool: false)` - Enables static secret caching when
set to `true`. When `cache_static_secrets` and `auth_auth` are both enabled,
Vault Proxy serves KV secrets directly from the cache to clients with
sufficient permission.

- `static_secret_token_capability_refresh_interval` `(duration: "5m", optional)` -
Sets the interval as a [duration format string](/vault/docs/concepts/duration-format)
at which Vault Proxy rechecks the permissions of tokens used to access cached
secrets. The refresh interval is the maximum period after which permission to
read a cached KV secret is fully revoked. Ignored when `cache_static_secrets`
is `false`.

- `static_secret_token_capability_refresh_behavior` `(string: "optimistic", optional)` -
Sets the capability refresh behavior in the case of an error when attempting to
refresh capabilities. In the case of a `403`, capabilities will be removed for the token
with either option. In case of other errors, such as Vault being sealed or Vault being
unavailable, this setting controls the behavior. If set to `optimistic` (the default),
capabilities will be removed for only `403` errors. If set to `pessimistic`, capabilities
will be removed for any error. This essentially allows configuring a preference between
favoring availability (`optimistic`) or access fidelity (`pessimistic`) of cached
static secrets. Ignored when `cache_static_secrets` is `false`.

### Example configuration

The following example Vault Proxy configuration:
- Defines a TCP listener (`listener`) with TLS disabled.
- Forces clients using API proxy (`api_proxy`) to identify with an auto-auth token.
- Configures auto-authentication (`auto-auth`) for `approle`.
- Enables static secret caching with `cache_static_secrets`.
- Sets an explicit token capability refresh window of 1 hour with `static_secret_token_capability_refresh_interval`.

```hcl
# Other Vault Proxy configuration blocks
# ...

cache {
  cache_static_secrets = true
  static_secret_token_capability_refresh_interval = "1h"
}

api_proxy {
  use_auto_auth_token = "force"
}

listener "tcp" {
    address = "127.0.0.1:8100"
    tls_disable = true
}

auto_auth {
  method {
    type = "approle"
    config = {
      role_id_file_path = "roleid"
      secret_id_file_path = "secretid"
      remove_secret_id_file_after_reading = false
    }
  }
```
[event-system]: /vault/docs/concepts/events
