29 August 2019
We are excited to share that our module mirror,
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
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.
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,
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.
go help goproxy
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.golang.org in your local environment.
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
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/mod/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.
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.
We hope these services improve your experience with modules, and encourage you
to file issues if you run into
problems or have feedback!