crypto/tls: ConnectionState isn't properly initialized until read or write from *tls.Conn

[flrdv]

Go version

go1.26.3-X:nodwarf5 linux/amd64

Output of go env in your module/workspace:

AR='ar'
CC='gcc'
CGO_CFLAGS='-O2 -g'
CGO_CPPFLAGS=''
CGO_CXXFLAGS='-O2 -g'
CGO_ENABLED='1'
CGO_FFLAGS='-O2 -g'
CGO_LDFLAGS='-O2 -g'
CXX='g++'
GCCGO='gccgo'
GO111MODULE=''
GOAMD64='v1'
GOARCH='amd64'
GOAUTH='netrc'
GOBIN=''
GOCACHE='/home/pavlo/.cache/go-build'
GOCACHEPROG=''
GODEBUG=''
GOENV='/home/pavlo/.config/go/env'
GOEXE=''
GOEXPERIMENT='nodwarf5'
GOFIPS140='off'
GOFLAGS=''
GOGCCFLAGS='-fPIC -m64 -pthread -Wl,--no-gc-sections -fmessage-length=0 -ffile-prefix-map=/tmp/go-build805846040=/tmp/go-build -gno-record-gcc-switches'
GOHOSTARCH='amd64'
GOHOSTOS='linux'
GOINSECURE=''
GOMOD='/home/pavlo/go/indigo/go.mod'
GOMODCACHE='/home/pavlo/go/pkg/mod'
GONOPROXY=''
GONOSUMDB=''
GOOS='linux'
GOPATH='/home/pavlo/go'
GOPRIVATE=''
GOPROXY='https://proxy.golang.org,direct'
GOROOT='/usr/lib/golang'
GOSUMDB='sum.golang.org'
GOTELEMETRY='local'
GOTELEMETRYDIR='/home/pavlo/.config/go/telemetry'
GOTMPDIR=''
GOTOOLCHAIN='local'
GOTOOLDIR='/usr/lib/golang/pkg/tool/linux_amd64'
GOVCS=''
GOVERSION='go1.26.3-X:nodwarf5'
GOWORK=''
PKG_CONFIG='pkg-config'

What did you do?

I tried calling *tls.Conn.ConnectionState() immediately after accepting a connection (using standard tls.NewListener(...))

I tested my assumption with the following server-example: https://go.dev/play/p/p_lxvj1RcTv
I used curl to connect. In fact, I only needed anything that can TLS.

What did you see happen?

ConnectionState is zeroed (except for NegotiatedProtocolIsMutual and ekm fields). Also enabling ALPN results in empty NegotiatedProtocol. The connection state is properly initialized only after the first read or write. Read([]byte(nil)) makes the job as a workaround.

What did you expect to see?

Completely initialized and not zeroed ConnectionState immediately after Accept() on tls.NewListener().

[flrdv]

Accept() on the listener provided by tls package directly returns net.Conn, constructed by the Server() function from tls/tls.go:46:

// Server returns a new TLS server side connection
// using conn as the underlying transport.
// The configuration config must be non-nil and must include
// at least one certificate or else set GetCertificate.
func Server(conn net.Conn, config *Config) *Conn {
	c := &Conn{
		conn:   conn,
		config: config,
	}
	c.handshakeFn = c.serverHandshake
	return c
}

where *Conn is *tls.Conn. What I can't see here is setting ConnectionState.

[seankhliao]

I believe you can also call crypto/tls.Conn.Handshake yourself.
Perhaps this just needs some more documentation.

[flrdv]

I believe you can also call crypto/tls.Conn.Handshake yourself. Perhaps this just needs some more documentation.

I somehow lost this method out of my sight. Thank you.

I agree, this should be noted in documentation more explicitly. Should it go to tls.Conn.ConnectionState() a-la "ensure handshake first, otherwise the connection state is gonna be zero"?

[dmitshur]

CC @golang/security.

[rolandshoemaker]

The Conn.ConnectionState has no meaning until a (TLS) connection is established, it inherently cannot provide any information before the handshake completes. Perhaps we should explicitly document this in the Conn.ConnectionState docs.

[gopherbot]

Change https://go.dev/cl/792720 mentions this issue: crypto/tls: document that ConnectionState is valid only after handshake