User Guide


The primary aim of the Caddy Telemetry Project is to gain insights into the status and health of the Internet, globally and in near-real-time, from a server-side perspective, without being constrained to a specific network or proprietary source. A secondary goal is to provide server operators with information about their servers and their interactions with clients.


  1. Benefits of Telemetry
  2. The Implementation
  3. The Metrics
  4. Disabling Telemetry

Benefits of Telemetry

Site owners: Telemetry data makes it possible for operators to have a white-box understanding of their web servers. Traditional monitoring tools usually require tedious analysis when anomalies occur because they have only an external perspective of the process. Caddy telemetry, on the other hand, operates from within the process and can give detailed insights when you need answers. And when everything is nominal, it's also just really interesting to see how and what your web server is doing. Telemetry is useful beyond access logging because it gives unique data points over time about the effects your clients have on your servers.

Researchers: While client-side scans of the Internet are not uncommon, for the first time you now have access to a global, server-side perspective from which to observe the behavior and health of the Internet. Caddy telemetry is uniquely positioned to offer anonymized aggregate data about clients on the Internet in conjunction together with an internal view of the web servers which answer them. Our long-term hope is that with your participation and feedback, we can build methods to detect emerging botnets, DDoS attacks, and other threats in real-time and work to automatically mitigate them.

Industry experts: Information provided by Caddy telemetry can certainly be useful when making decisions about new Web standards, building out or monitoring network infrastructure, and developing Internet software.

The Implementation

When telemetry is enabled, Caddy takes various tallies and records certain events in the background while it is running. It sends updates to a collector endpoint on a regular basis, flushing the local buffer of data.

Telemetry is implemented in a way that makes it unobtrusive and non-blocking to your process. Your Caddy instance should not suffer any noticable performance degredation. It has several safety measures built-in to ensure optimal performance, even at the expense of the data, including a limit to the number of data points that can be buffered. The collection endpoint may notice if certain metrics are too expensive and temporarily disable them on a per-instance basis to improve performance. The collection endpoint may also entirely terminate telemetry reporting from any instance. In addition, collection updates are tightly rate-limited, ensuring that telemetry never interferes with network throughput.

Each Caddy instance generates its own unique, random ID called a UUID. It is stored in a file called $CADDYPATH/uuid (the default CADDYPATH is $HOME/.caddy). This UUID is NOT generated in connection with the collection endpoint in any way, and does NOT associate with any individual person. We recommend that each Caddy instance you run have its own CADDYPATH so that your reports are more discernable when you go to look up your instance.

As you would expect, all transmissions are encrypted with HTTPS.

The Metrics

This table lists the metrics that are collected by Caddy core and the standard plugins in alphabetical order; but keep in mind that third-party plugins might add their own which are not documented here; check their documentation instead.

Key Description
arch The microarchitecture compiled for
caddy_version Caddy version
container Whether the process is running in a container
cpu.aes_ni Whether AES-NI is available
cpu.brand_name The brand name of the CPU
cpu.num_logical Number of logical cores
directives The list of directives used (directive name only)
disabled_metrics The list of individual metrics that have been disabled
goroutines Number of goroutines currently running
http_deployment_guess A rough guess as to whether it looks like a dev or production instance
http_mitm Count of whether MITM was detected
http_num_sites Number of sites defined in your HTTP Caddyfile (∑ blocks * number of keys per block)
http_request_count Number of HTTP(S) requests handled
http_user_agent User-Agent request header values
http_user_agent_count Number of requests with the associated User-Agent string
memory.heap_alloc Bytes of allocated heap objects (reachable, or unreachable but not yet freed)
memory.sys Bytes of memory obtained from the OS
instance_id The instance UUID
num_listeners Number of listeners opened
num_server_blocks The number of server blocks defined in your Caddyfile
os The OS compiled for
server_type The server type plugin being run (HTTP, DNS, etc.)
sigtrap Name and count of signal (or interrupt) trapped
timestamp The timestamp of the telemetry update
tls_acme_certs_obtained Number of certificates automatically obtained with ACME
tls_acme_certs_renewed Number of certificates automatically renewed with ACME
tls_acme_certs_revoked Number of certificates revoked using ACME
tls_client_hello.cipher_suites Cipher suites advertised in the TLS ClientHello
tls_client_hello.compression Compression methods advertised in the TLS ClientHello
tls_client_hello.curves Curves advertised in the TLS ClientHello
tls_client_hello.extensions Extensions advertised in the TLS ClientHello
tls_client_hello.points Points advertised in the TLS ClientHello
tls_client_hello.version Supported version advertised in the TLS ClientHello
tls_client_hello_ua Counts of HTTPS requests with the given User-Agent strings that connected with the associated TLS ClientHello
tls_handshake_count Number of TLS handshakes completed
tls_managed_cert_count How many certificates are being managed
tls_manual_cert_count How many certificates are manually provided
tls_on_demand_count Number of sites configured for On-Demand TLS
tls_self_signed_count Number of sites configured to use integrated self-signed certificates

Disabling Telemetry

Telemetry is enabled by default in the source code and disabled by default on our download page. In order to better know how representative the aggregate data is, telemetry may be toggled at compile-time or customized at run-time.

Note that telemetry does NOT target personal information. Telemetry is programmed to report only technical data about machines, connections, and Caddy instances; NOT end users, session IDs, cookies, etc. If you are considering turning off telemetry because of applicable laws, make sure the laws actually apply to you.

The recommended way to disable telemetry is to turn off only the metrics that you do not want to report. You can do this with the -disabled-metrics CLI flag. (The disabled_metrics, timestamp, and instance_id metrics cannot be disabled individually.) This will prevent Caddy from collecting the specified information throughout the lifetime of the process, and is useful if, for example, you discover that a particular metric is causing your telemetry buffer to fill up too quickly under heavy load (there is a limit to how many items can be buffered for emission).

However, if you wish to disable telemetry entirely, it can be done at compile-time. When you download Caddy from the website, you can choose to have telemetry disabled. If building from source, you can set enableTelemetry to false to turn it off. Note that if you disable telemetry you will not have the ability to look up your instance and view its metrics. It also does not contribute to the research efforts that are otherwise made possible, and it makes it difficult to diagnose problems and improve Caddy. We recommend leaving telemetry on to gain its benefits and to improve the Web overall.