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Protocol Detection and Opaque Ports in Linkerd

Charles Pretzer

Charles Pretzer
February 23, 2021 • 6 min read

Radio Satellite

The upcoming Linkerd 2.10 release adds a new opaque ports feature that further extends Linkerd’s ability to provide zero-config mutual TLS for all TCP traffic. There have been quite a few questions from the Linkerd community in Slack and GitHub about this feature, so this article focuses on one of the most important underlying features that enables Linkerd to perform this feat: Protocol Detection.

Protocol detection, as the name suggests, allows Linkerd to automatically detect the protocol in use in a TCP connection. One of Linkerd’s design principles is to “just work,” and protocol detection is an important part of how Linkerd achieves that goal.

In this article, you’ll learn what protocol detection is, how this subtle feature gives Linkerd such power, and what the upcoming opaque ports feature will bring to Linkerd.

What is protocol detection?

Put simply, protocol detection is the ability to determine the protocol in use on a TCP connection by inspecting the traffic on the connection.

Protocol Detection.

Protocol Detection

Linkerd uses protocol detection to avoid asking the user to specify the protocol. Rather than requiring the user to configure what protocol each port uses, Linkerd’s proxy simply performs protocol detection to answer the question.

Linkerd’s protocol detection works by peeking at the first few bytes of a client connection to get information about the traffic. This implementation has some consequences, which we’ll get into below.

But first, let’s answer the question of why Linkerd cares about any protocol in the first place.

Observability, reliability, and security

We typically categorize Linkerd’s broad swath of features into three buckets: observability, reliability, and security. Understanding which protocol is in use on a connection is fundamental to each of those categories.


Central to Linkerd’s observability features is the instrumentation of traffic. This instrumentation requires understanding the protocol in use, because knowledge of the protocol can provide rich metrics. For example, knowing that a connection is using HTTP allows Linkerd to parse the requests, responses, and response codes, and to report metrics such as response latencies, request volumes, and error rates. These metrics are so valuable that they are part of what is known as the “golden signals” in Google’s SRE book. On the other hand, if all that Linkerd knows is that a connection is TCP, it is limited to recording very basic information like the number of bytes read and written—there is no ability to interpret the bytes further.


Mutual TLS (mTLS) is a core feature of Linkerd. As of Linkerd 2.9, all TCP traffic between meshed endpoints is mTLS’d by the Linkerd proxies by default. (With some caveats—see the section on skip-ports below.)

Here, again, knowing the protocol of the connection is critical. For example, if the connection is already TLS’d (e.g. by the application), there is no reason to re-TLS it. (Strictly speaking, TLS is a transport-layer protocol rather than an application-layer protocol like HTTP, but for the purposes of this article the distinction doesn’t matter.)


Finally, knowing the protocol of the underlying connection allows Linkerd to provide sophisticated reliability features. One example here is load balancing. Without knowing the protocol of the connection, Linkerd is limited to balancing connections: once a TCP connection is established to a server, it has no further ability to manipulate that connection.

However, if Linkerd knows that a connection is HTTP, it can move from connection balancing to request balancing. Linkerd will establish a connection pool across endpoints, and will balance requests across this pool. Since it now has access to requests and responses, Linkerd can be very sophisticated in how it balances requests; in fact, it balances requests based on the recent performance of each possible endpoint (used a metric called “exponentially weighted moving average”, or EWMA) so as to avoid incurring tail latency from slow endpoints.

(Linkerd is also an easy solution to the unique considerations of load balancing gRPC connections within Kubernetes.)

When protocol detection fails

While protocol detection is designed to allow Linkerd “just work”, there are some cases where it cannot: the infamous server-speaks-first protocols. These protocols, which include things like MySQL and SMTP, work by having the client establish a connection and then waiting for the server to respond. This is a perfectly legal behavior from the TCP perspective, but means that Linkerd cannot detect the protocol as the relevant information comes from the server, not the client.

(Why not simply use the server’s bytes to detect the protocol? Because at the time of protocol detection Linkerd hasn’t even established a connection to the server. Choosing which server to speak to is a function of the load balancer, and which load balancer to use is a function of the protocol. It’s a delicious, TCP-flavored chicken-and-egg problem.)

To circumvent this, Linkerd introduced the skip-inbound-ports and skip-outbound-ports configuration options. These options instruct Linkerd to bypass the proxy entirely for certain ports by modifying the iptables rules that Linkerd uses to wire connections for a pod through its sidecar proxy. For example, adding the annotation config.linkerd.io/skip-outbound-ports: 3306 to the PodSpec of a workload instructs Linkerd to create an iptables rule that ensures that the Linkerd proxy never handles any traffic to port 3306 (the MySQL port). Similarly, the annotation config.linkerd.io/skip-inbound-ports: 3306 will write an iptables rule so that the proxy never handles MySQL traffic that is sent to it.

Skip Ports Configuration

Skip Ports Configuration

These options provide a workaround for the inability of protocol detection to handle server-speaks-first protocols. However, they have one significant drawback: because they bypass the Linkerd proxy entirely, Linkerd cannot apply mTLS or capture any metrics for these ports.

Opaque ports and improved protocol detection in Linkerd 2.10

To address the deficiency with skip-ports, in the upcoming 2.10 release, Linkerd will add the concept of opaque ports (and the corresponding opaque-ports annotation). An opaque port is simply one that Linkerd will proxy without performing protocol detection. While this approach still requires configuration, marking a port as opaque allows Linkerd to apply mTLS and report TCP-level metrics—a big improvement over skipping it entirely.

Opaque Ports Configuration

Opaque Ports Configuration

Linkerd 2.10 will also improve how protocol detection works by making it “fail open”: if the protocol detection code sees no client-side bytes after 10 seconds, it will treat the connection as a TCP connection and continue, rather than failing as in 2.9. This means that worst-case behavior of not annotating a server-speaks-first port with opaque-ports (or skip-ports) is a 10-second connect-time latency, rather than a connect failure.


Protocol detection is one of Linkerd’s most powerful features and is fundamental to Linkerd’s “just works” principle. While protocol detection is not a panacea, the introduction of opaque ports in Linkerd 2.10 should address most of the downsides of the earlier skip-ports features, and will allow Linkerd adopters to extend mTLS across their entire Kubernetes environment, regardless of protocol.

(Want to give opaque ports a try? You don’t have to wait for the 2.10 release because the feature is currently available in the edge releases!)

Linkerd is for everyone

Linkerd is a community project and is hosted by the Cloud Native Computing Foundation. Linkerd is committed to open governance. If you have feature requests, questions, or comments, we’d love to have you join our rapidly-growing community! Linkerd is hosted on GitHub, and we have a thriving community on Slack, Twitter, and the mailing lists. Come and join the fun!

Image Credit: Gilles Rolland-Monnet

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