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+Module Mirror and Checksum Database Launched
+29 Aug 2019
+Tags: tools, versioning
+
+Katie Hockman
+
+* Introduction
+
+We are excited to share that our module [[https://proxy.golang.org][mirror]],
+[[https://index.golang.org][index]], and
+[[https://sum.golang.org][checksum database]] are now production ready! The `go` command
+will use the module mirror and checksum database by default for
+[[https://tip.golang.org/doc/go1.13#introduction][Go 1.13 module users]].  See
+[[https://proxy.golang.org/privacy][proxy.golang.org/privacy]] for privacy
+information about these services and the
+[[https://tip.golang.org/cmd/go/#hdr-Module_downloading_and_verification][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
+[[https://tip.golang.org/cmd/go/#hdr-Module_configuration_for_non_public_modules][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
+[[https://youtu.be/KqTySYYhPUE][Go Module Proxy: Life of a Query]] talk at Gophercon 2019.
+See the [[https://youtu.be/KqTySYYhPUE][recording]] if you are interested in the full talk.
+
+** Module Mirror
+
+[[https://blog.golang.org/versioning-proposal][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
+[[https://golang.org/cmd/go/#hdr-The_go_mod_file][go.mod]] and
+[[https://golang.org/cmd/go/#hdr-Module_downloading_and_verification][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.
+
+.image module-mirror-launch/proxy-protocol.png _ 800
+
+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
+[[https://blog.golang.org/modules2019][Go Modules in 2019]] for more information.
+
+The Go team maintains a module mirror, served at
+[[https://proxy.golang.org][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.
+
+** 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
+[[https://go.googlesource.com/proposal/+/master/design/25530-sumdb.md#checksum-database][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 [[https://sum.golang.org][sum.golang.org]], and
+is built on a [[https://research.swtch.com/tlog][Transparent Log]] (or “Merkle
+tree”) of hashes backed by [[https://github.com/google/trillian][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.
+
+.image module-mirror-launch/tree.png _ 800
+
+The checksum database supports
+[[https://go.googlesource.com/proposal/+/master/design/25530-sumdb.md#checksum-database][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.
+
+.image module-mirror-launch/sumdb-protocol.png _ 800
+
+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
+[[https://godoc.org/golang.org/x/exp/sumdb/gosumcheck][gosumcheck]]:
+
+`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 [[https://index.golang.org][index.golang.org]], and
+is a public feed of new module versions that become available through
+[[https://proxy.golang.org][proxy.golang.org]]. This is particularly useful for
+tool developers that want to keep their own cache of what’s available in
+[[https://proxy.golang.org][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 [[https://github.com/golang/go/issues/new?title=proxy.golang.org][file issues]] if you run into
+problems or have feedback!
\ No newline at end of file
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