Original post

We are excited to share that our module mirror,

index, and

checksum database are now production ready! The command

will use the module mirror and checksum database by default for

Go 1.13 module users. See

proxy.golang.org/privacy 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


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

proxy.golang.org, 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=https://proxy..org 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 sum.golang.org, 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


go get golang.org/x/exp/sumdb/gosumcheck 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 index.golang.org, and

is a public feed of new module versions that become available through

proxy.golang.org. This is particularly useful for

tool developers that want to keep their own cache of what’s available in

proxy.golang.org, 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!