By default, Linkerd automatically enables mutually-authenticated Transport Layer Security (mTLS) for all TCP traffic between meshed pods. This means that Linkerd adds authenticated, encrypted communication to your application with no extra work on your part. (And because the Linkerd control plane also runs on the data plane, this means that communication between Linkerd’s control plane components are also automatically secured via mTLS.)
See Caveats and future work below for some details.
What is mTLS?
mTLS, or mutual TLS, is simply “regular TLS” with the extra stipulation that the client is also authenticated. TLS guarantees authenticity, but by default this only happens in one direction–the client authenticates the server but the server doesn’t authenticate the client. mTLS makes the authenticity symmetric.
mTLS is a large topic. For a broad overview of what mTLS is and how it works in Kuberentes clusters, we suggest reading through A Kubernetes engineer’s guide to mTLS.
Which traffic can Linkerd automatically mTLS?
Linkerd transparently applies mTLS to all TCP communication between meshed pods. However, there are still ways in which you may still have non-mTLS traffic in your system, including:
- Traffic to or from non-meshed pods (e.g. Kubernetes healthchecks)
- Traffic on ports that were marked as skip ports, which bypass the proxy entirely.
Linkerd’s mTLS requires some preparation for production use, especially for long-lived clusters or clusters that expect to have cross-cluster traffic.
The trust anchor generated by the default
linkerd install CLI command expires
after 365 days. After that, it must be manually
non-trivial task. Alternatively, you can provide the trust anchor
yourself and control the expiration date,
e.g. setting it to 10 years rather than one year.
Kubernetes clusters that make use of Linkerd’s multi-cluster
communication must share a trust anchor. Thus, the default
linkerd install setup will not work for this situation and you must provide
an explicit trust anchor.
External systems such as Buoyant Cloud can be used to monitor cluster credentials and to send reminders if they are close to expiration.
How does Linkerd’s mTLS implementation work?
The Linkerd control plane contains a certificate
authority (CA) called
identity. This CA issues TLS certificates to each
Linkerd data plane proxy. Each certificate is bound to the Kubernetes
identity of the containing pod. These TLS certificates expire after 24 hours
and are automatically rotated. The proxies use these certificates to encrypt
and authenticate TCP traffic to other proxies.
On the control plane side, Linkerd maintains a set of credentials in the
cluster: a trust anchor, and an issuer certificate and private key. These
credentials can be generated by Linkerd during install time, or optionally
provided by an external source, e.g. Vault or
cert-manager. The issuer
certificate and private key are stored in a Kubernetes
Secret; this Secret
is placed in the
linkerd namespace and can only be read by the service
account used by the Linkerd control plane’s
On the data plane side, each proxy is passed the trust anchor in an environment
variable. At startup, the proxy generates a private key, stored in a tmpfs
stays in memory and never leaves the pod. The proxy connects to the control
identity component, validating the connection to
identity with the
trust anchor, and issues a certificate signing request
(CSR). The CSR
contains an initial certificate with identity set to the pod’s Kubernetes
and the actual service account token, so that
identity can validate that the
CSR is valid. After validation, the signed trust bundle is returned to the
proxy, which can use it as both a client and server certificate. These
certificates are scoped to 24 hours and dynamically refreshed using the same
Finally, when a proxy receives an outbound connection from the application container within its pod, it looks up that destination with the Linkerd control plane. If it’s in the Kubernetes cluster, the control plane provides the proxy with the destination’s endpoint addresses, along with metadata including an identity name. When the proxy connects to the destination, it initiates a TLS handshake and verifies that that the destination proxy’s certificate is signed by the trust anchor and contains the expected identity.
Caveats and future work
Linkerd does not require mTLS unless authorization policies are configured.
Ideally, the ServiceAccount token that Linkerd uses would not be shared with other potential uses of that token. In future Kubernetes releases, Kubernetes will support audience/time-bound ServiceAccount tokens, and Linkerd will use those instead.