go/doc/godebug.md

title	layout
Go, Backwards Compatibility, and GODEBUG	article

Introduction

Go's emphasis on backwards compatibility is one of its key strengths. There are, however, times when we cannot maintain complete compatibility. If code depends on buggy (including insecure) behavior, then fixing the bug will break that code. New features can also have similar impacts: enabling the HTTP/2 use by the HTTP client broke programs connecting to servers with buggy HTTP/2 implementations. These kinds of changes are unavoidable and permitted by the Go 1 compatibility rules. Even so, Go provides a mechanism called GODEBUG to reduce the impact such changes have on Go developers using newer toolchains to compile old code.

A GODEBUG setting is a key=value pair that controls the execution of certain parts of a Go program. The environment variable GODEBUG can hold a comma-separated list of these settings. For example, if a Go program is running in an environment that contains

GODEBUG=http2client=0,http2server=0

then that Go program will disable the use of HTTP/2 by default in both the HTTP client and the HTTP server. It is also possible to set the default GODEBUG for a given program (discussed below).

When preparing any change that is permitted by Go 1 compatibility but may nonetheless break some existing programs, we first engineer the change to keep as many existing programs working as possible. For the remaining programs, we define a new GODEBUG setting that allows individual programs to opt back in to the old behavior. A GODEBUG setting may not be added if doing so is infeasible, but that should be extremely rare.

GODEBUG settings added for compatibility will be maintained for a minimum of two years (four Go releases). Some, such as http2client and http2server, will be maintained much longer, even indefinitely.

When possible, each GODEBUG setting has an associated runtime/metrics counter named /godebug/non-default-behavior/<name>:events that counts the number of times a particular program's behavior has changed based on a non-default value for that setting. For example, when GODEBUG=http2client=0 is set, /godebug/non-default-behavior/http2client:events counts the number of HTTP transports that the program has configured without HTTP/2 support.

Default GODEBUG Values

When a GODEBUG setting is not listed in the environment variable, its value is derived from three sources: the defaults for the Go toolchain used to build the program, amended to match the Go version listed in go.mod, and then overridden by explicit //go:debug lines in the program.

The GODEBUG History gives the exact defaults for each Go toolchain version. For example, Go 1.21 introduces the panicnil setting, controlling whether panic(nil) is allowed; it defaults to panicnil=0, making panic(nil) a run-time error. Using panicnil=1 restores the behavior of Go 1.20 and earlier.

When compiling a work module or workspace that declares an older Go version, the Go toolchain amends its defaults to match that older Go version as closely as possible. For example, when a Go 1.21 toolchain compiles a program, if the work module's go.mod or the workspace's go.work says go 1.20, then the program defaults to panicnil=1, matching Go 1.20 instead of Go 1.21.

Because this method of setting GODEBUG defaults was introduced only in Go 1.21, programs listing versions of Go earlier than Go 1.20 are configured to match Go 1.20, not the older version.

To override these defaults, starting in Go 1.23, the work module's go.mod or the workspace's go.work can list one or more godebug lines:

godebug (
	default=go1.21
	panicnil=1
	asynctimerchan=0
)

The special key default indicates a Go version to take unspecified settings from. This allows setting the GODEBUG defaults separately from the Go language version in the module. In this example, the program is asking for Go 1.21 semantics and then asking for the old pre-Go 1.21 panic(nil) behavior and the new Go 1.23 asynctimerchan=0 behavior.

Only the work module's go.mod is consulted for godebug directives. Any directives in required dependency modules are ignored. It is an error to list a godebug with an unrecognized setting. (Toolchains older than Go 1.23 reject all godebug lines, since they do not understand godebug at all.)

The defaults from the go and godebug lines apply to all main packages that are built. For more fine-grained control, starting in Go 1.21, a main package's source files can include one or more //go:debug directives at the top of the file (preceding the package statement). The godebug lines in the previous example would be written:

//go:debug default=go1.21
//go:debug panicnil=1
//go:debug asynctimerchan=0

Starting in Go 1.21, the Go toolchain treats a //go:debug directive with an unrecognized GODEBUG setting as an invalid program. Programs with more than one //go:debug line for a given setting are also treated as invalid. (Older toolchains ignore //go:debug directives entirely.)

The defaults that will be compiled into a main package are reported by the command:

{{raw go list -f '{{.DefaultGODEBUG}}' my/main/package}}

Only differences from the base Go toolchain defaults are reported.

When testing a package, //go:debug lines in the *_test.go files are treated as directives for the test's main package. In any other context, //go:debug lines are ignored by the toolchain; go vet reports such lines as misplaced.

GODEBUG History

This section documents the GODEBUG settings introduced and removed in each major Go release for compatibility reasons. Packages or programs may define additional settings for internal debugging purposes; for example, see the runtime documentation and the go command documentation.

Go 1.24

Go 1.24 changed the global math/rand.Seed to be a no-op. This behavior is controlled by the randseednop setting. For Go 1.24 it defaults to randseednop=1. Using randseednop=0 reverts to the pre-Go 1.24 behavior.

Go 1.24 added new values for the multipathtcp setting. The possible values for multipathtcp are now:

• "0": disable MPTCP on dialers and listeners by default
• "1": enable MPTCP on dialers and listeners by default
• "2": enable MPTCP on listeners only by default
• "3": enable MPTCP on dialers only by default

For Go 1.24, it now defaults to multipathtcp="2", thus enabled by default on listerners. Using multipathtcp="0" reverts to the pre-Go 1.24 behavior.

Go 1.23

Go 1.23 changed the channels created by package time to be unbuffered (synchronous), which makes correct use of the Timer.Stop and Timer.Reset method results much easier. The asynctimerchan setting disables this change. There are no runtime metrics for this change, This setting may be removed in a future release, Go 1.27 at the earliest.

Go 1.23 changed the mode bits reported by os.Lstat and os.Stat for reparse points, which can be controlled with the winsymlink setting. As of Go 1.23 (winsymlink=1), mount points no longer have os.ModeSymlink set, and reparse points that are not symlinks, Unix sockets, or dedup files now always have os.ModeIrregular set. As a result of these changes, filepath.EvalSymlinks no longer evaluates mount points, which was a source of many inconsistencies and bugs. At previous versions (winsymlink=0), mount points are treated as symlinks, and other reparse points with non-default os.ModeType bits (such as os.ModeDir) do not have the ModeIrregular bit set.

Go 1.23 changed os.Readlink and filepath.EvalSymlinks to avoid trying to normalize volumes to drive letters, which was not always even possible. This behavior is controlled by the winreadlinkvolume setting. For Go 1.23, it defaults to winreadlinkvolume=1. Previous versions default to winreadlinkvolume=0.

Go 1.23 enabled the experimental post-quantum key exchange mechanism X25519Kyber768Draft00 by default. The default can be reverted using the tlskyber setting.

Go 1.23 changed the behavior of crypto/x509.ParseCertificate to reject serial numbers that are negative. This change can be reverted with the x509negativeserial setting.

Go 1.23 re-enabled support in html/template for ECMAScript 6 template literals by default. The jstmpllitinterp setting no longer has any effect.

Go 1.23 changed the default TLS cipher suites used by clients and servers when not explicitly configured, removing 3DES cipher suites. The default can be reverted using the tls3des setting.

Go 1.23 changed the behavior of tls.X509KeyPair and tls.LoadX509KeyPair to populate the Leaf field of the returned tls.Certificate. This behavior is controlled by the x509keypairleaf setting. For Go 1.23, it defaults to x509keypairleaf=1. Previous versions default to x509keypairleaf=0.

Go 1.23 changed net/http.ServeContent, net/http.ServeFile, and net/http.ServeFS to remove Cache-Control, Content-Encoding, Etag, and Last-Modified headers when serving an error. This behavior is controlled by the httpservecontentkeepheaders setting. Using httpservecontentkeepheaders=1 restores the pre-Go 1.23 behavior.

Go 1.22

Go 1.22 adds a configurable limit to control the maximum acceptable RSA key size that can be used in TLS handshakes, controlled by the tlsmaxrsasize setting. The default is tlsmaxrsasize=8192, limiting RSA to 8192-bit keys. To avoid denial of service attacks, this setting and default was backported to Go 1.19.13, Go 1.20.8, and Go 1.21.1.

Go 1.22 made it an error for a request or response read by a net/http client or server to have an empty Content-Length header. This behavior is controlled by the httplaxcontentlength setting.

Go 1.22 changed the behavior of ServeMux to accept extended patterns and unescape both patterns and request paths by segment. This behavior can be controlled by the httpmuxgo121 setting.

Go 1.22 added the Alias type to go/types for the explicit representation of type aliases. Whether the type checker produces Alias types or not is controlled by the gotypesalias setting. For Go 1.22 it defaults to gotypesalias=0. For Go 1.23, gotypesalias=1 will become the default. This setting will be removed in a future release, Go 1.27 at the earliest.

Go 1.22 changed the default minimum TLS version supported by both servers and clients to TLS 1.2. The default can be reverted to TLS 1.0 using the tls10server setting.

Go 1.22 changed the default TLS cipher suites used by clients and servers when not explicitly configured, removing the cipher suites which used RSA based key exchange. The default can be reverted using the tlsrsakex setting.

Go 1.22 disabled ConnectionState.ExportKeyingMaterial when the connection supports neither TLS 1.3 nor Extended Master Secret (implemented in Go 1.21). It can be reenabled with the tlsunsafeekm setting.

Go 1.22 changed how the runtime interacts with transparent huge pages on Linux. In particular, a common default Linux kernel configuration can result in significant memory overheads, and Go 1.22 no longer works around this default. To work around this issue without adjusting kernel settings, transparent huge pages can be disabled for Go memory with the disablethp setting. This behavior was backported to Go 1.21.1, but the setting is only available starting with Go 1.21.6. This setting may be removed in a future release, and users impacted by this issue should adjust their Linux configuration according to the recommendations in the GC guide, or switch to a Linux distribution that disables transparent huge pages altogether.

Go 1.22 added contention on runtime-internal locks to the mutex profile. Contention on these locks is always reported at runtime._LostContendedRuntimeLock. Complete stack traces of runtime locks can be enabled with the runtimecontentionstacks setting. These stack traces have non-standard semantics, see setting documentation for details.

Go 1.22 added a new crypto/x509.Certificate field, Policies, which supports certificate policy OIDs with components larger than 31 bits. By default this field is only used during parsing, when it is populated with policy OIDs, but not used during marshaling. It can be used to marshal these larger OIDs, instead of the existing PolicyIdentifiers field, by using the x509usepolicies setting..

Go 1.21

Go 1.21 made it a run-time error to call panic with a nil interface value, controlled by the panicnil setting.

Go 1.21 made it an error for html/template actions to appear inside of an ECMAScript 6 template literal, controlled by the jstmpllitinterp setting. This behavior was backported to Go 1.19.8+ and Go 1.20.3+.

Go 1.21 introduced a limit on the maximum number of MIME headers and multipart forms, controlled by the multipartmaxheaders and multipartmaxparts settings respectively. This behavior was backported to Go 1.19.8+ and Go 1.20.3+.

Go 1.21 adds the support of Multipath TCP but it is only used if the application explicitly asked for it. This behavior can be controlled by the multipathtcp setting.

There is no plan to remove any of these settings.

Go 1.20

Go 1.20 introduced support for rejecting insecure paths in tar and zip archives, controlled by the tarinsecurepath setting and the zipinsecurepath setting. These default to tarinsecurepath=1 and zipinsecurepath=1, preserving the behavior of earlier versions of Go. A future version of Go may change the defaults to tarinsecurepath=0 and zipinsecurepath=0.

Go 1.20 introduced automatic seeding of the math/rand global random number generator, controlled by the randautoseed setting.

Go 1.20 introduced the concept of fallback roots for use during certificate verification, controlled by the x509usefallbackroots setting.

Go 1.20 removed the preinstalled .a files for the standard library from the Go distribution. Installations now build and cache the standard library like packages in other modules. The installgoroot setting restores the installation and use of preinstalled .a files.

There is no plan to remove any of these settings.

Go 1.19

Go 1.19 made it an error for path lookups to resolve to binaries in the current directory, controlled by the execerrdot setting. There is no plan to remove this setting.

Go 1.19 started sending EDNS0 additional headers on DNS requests. This can reportedly break the DNS server provided on some routers, such as CenturyLink Zyxel C3000Z. This can be changed by the netedns0 setting. This setting is available in Go 1.21.12, Go 1.22.5, Go 1.23, and later. There is no plan to remove this setting.

Go 1.18

Go 1.18 removed support for SHA1 in most X.509 certificates, controlled by the x509sha1 setting. This setting will be removed in a future release, Go 1.22 at the earliest.

Go 1.10

Go 1.10 changed how build caching worked and added test caching, along with the gocacheverify, gocachehash, and gocachetest settings. There is no plan to remove these settings.

Go 1.6

Go 1.6 introduced transparent support for HTTP/2, controlled by the http2client, http2server, and http2debug settings. There is no plan to remove these settings.

Go 1.5

Go 1.5 introduced a pure Go DNS resolver, controlled by the netdns setting. There is no plan to remove this setting.