The Go Blog

Module Mirror and Checksum Database Launched

Katie Hockman
29 August 2019

We are excited to share that our module mirror, index, and checksum database are now production ready! The go command will use the module mirror and checksum database by default for Go 1.13 module users. See for privacy information about these services and the go command documentation for configuration details, including how to disable the use of these servers or use different ones. If you depend on non-public modules, see the documentation for configuring your environment.

This post will describe these services and the benefits of using them, and summarizes some of the points from the Go Module Proxy: Life of a Query talk at Gophercon 2019. See the recording if you are interested in the full talk.

Module Mirror

Modules are sets of Go packages that are versioned together, and the contents of each version are immutable. That immutability provides new opportunities for caching and authentication. When go get runs in module mode, it must fetch the module containing the requested packages, as well as any new dependencies introduced by that module, updating your go.mod and go.sum files as needed. Fetching modules from version control can be expensive in terms of latency and storage in your system: the go command may be forced to pull down the full commit history of a repository containing a transitive dependency, even one that isn’t being built, just to resolve its version.

The solution is to use a module proxy, which speaks an API that is better suited to the go command’s needs (see go help goproxy). When go get runs in module mode with a proxy, it will work faster by only asking for the specific module metadata or source code it needs, and not worrying about the rest. Below is an example of how the go command may use a proxy with go get by requesting the list of versions, then the info, mod, and zip file for the latest tagged version.

A module mirror is a special kind of module proxy that caches metadata and source code in its own storage system, allowing the mirror to continue to serve source code that is no longer available from the original locations. This can speed up downloads and protect you from disappearing dependencies. See Go Modules in 2019 for more information.

The Go team maintains a module mirror, served at, which the go command will use by default for module users as of Go 1.13. If you are running an earlier version of the go command, then you can use this service by setting GOPROXY= in your local environment.

Checksum Database

Modules introduced the go.sum file, which is a list of SHA-256 hashes of the source code and go.mod files of each dependency when it was first downloaded. The go command can use the hashes to detect misbehavior by an origin server or proxy that gives you different code for the same version.

The limitation of this go.sum file is that it works entirely by trust on your first use. When you add a version of a dependency that you’ve never seen before to your module (possibly by upgrading an existing dependency), the go command fetches the code and adds lines to the go.sum file on the fly. The problem is that those go.sum lines aren’t being checked against anyone else’s: they might be different from the go.sum lines that the go command just generated for someone else, perhaps because a proxy intentionally served malicious code targeted to you.

Go’s solution is a global source of go.sum lines, called a checksum database, which ensures that the go command always adds the same lines to everyone’s go.sum file. Whenever the go command receives new source code, it can verify the hash of that code against this global database to make sure the hashes match, ensuring that everyone is using the same code for a given version.

The checksum database is served by, and is built on a Transparent Log (or “Merkle tree”) of hashes backed by Trillian. The main advantage of a Merkle tree is that it is tamper proof and has properties that don’t allow for misbehavior to go undetected, which makes it more trustworthy than a simple database. The go command uses this tree to check “inclusion” proofs (that a specific record exists in the log) and “consistency” proofs (that the tree hasn’t been tampered with) before adding new go.sum lines to your module’s go.sum file. Below is an example of such a tree.

The checksum database supports a set of endpoints used by the go command to request and verify go.sum lines. The /lookup endpoint provides a “signed tree head” (STH) and the requested go.sum lines. The /tile endpoint provides chunks of the tree called tiles which the go command can use for proofs. Below is an example of how the go command may interact with the checksum database by doing a /lookup of a module version, then requesting the tiles required for the proofs.

This checksum database allows the go command to safely use an otherwise untrusted proxy. Because there is an auditable security layer sitting on top of it, a proxy or origin server can’t intentionally, arbitrarily, or accidentally start giving you the wrong code without getting caught. Even the author of a module can’t move their tags around or otherwise change the bits associated with a specific version from one day to the next without the change being detected.

If you are using Go 1.12 or earlier, you can manually check a go.sum file against the checksum database with gosumcheck:

$ go get
$ gosumcheck /path/to/go.sum

In addition to verification done by the go command, third-party auditors can hold the checksum database accountable by iterating over the log looking for bad entries. They can work together and gossip about the state of the tree as it grows to ensure that it remains uncompromised, and we hope that the Go community will run them.

Module Index

The module index is served by, and is a public feed of new module versions that become available through This is particularly useful for tool developers that want to keep their own cache of what’s available in, or keep up-to-date on some of the newest modules that people are using.

Feedback or bugs

We hope these services improve your experience with modules, and encourage you to file issues if you run into problems or have feedback!

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