x/crypto/ssh/knownhosts: fails to find key when port = "ssh"

[jackc]

What version of Go are you using (go version)?

go version go1.16 darwin/amd64

Does this issue reproduce with the latest release?

Yes

What operating system and processor architecture are you using (go env)?

go env Output

$ go env
GO111MODULE=""
GOARCH="amd64"
GOBIN=""
GOCACHE="/Users/jack/Library/Caches/go-build"
GOENV="/Users/jack/Library/Application Support/go/env"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOINSECURE=""
GOMODCACHE="/Users/jack/go/pkg/mod"
GONOPROXY=""
GONOSUMDB=""
GOOS="darwin"
GOPATH="/Users/jack/go"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/usr/local/Cellar/go/1.16/libexec"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/usr/local/Cellar/go/1.16/libexec/pkg/tool/darwin_amd64"
GOVCS=""
GOVERSION="go1.16"
GCCGO="gccgo"
AR="ar"
CC="clang"
CXX="clang++"
CGO_ENABLED="1"
GOMOD="/Users/jack/dev/ccsalespro/isoamp/go.mod"
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -arch x86_64 -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/49/sq09r9hs3mlgyzk8r18zswfw0000gn/T/go-build464070472=/tmp/go-build -gno-record-gcc-switches -fno-common"

What did you do?

net.Dial and ssh.Dial allow specifying ports by name. e.g. host.example.com:ssh. This fails in combination with knownhosts.

	sshConfig := &ssh.ClientConfig{
		// ...
	}

	if homeDir, err := os.UserHomeDir(); err == nil {
		if hostKeyCallback, err := knownhosts.New(fmt.Sprintf("%s/.ssh/known_hosts", homeDir)); err == nil {
			sshConfig.HostKeyCallback = hostKeyCallback
		}
	}

	return ssh.Dial("tcp", "host.example.com:ssh", sshConfig)

What did you expect to see?

I expect for the knownhost entry for host.example.com to be found.

What did you see instead?

ssh: handshake failed: knownhosts: key is unknown

If the port is specified numerically it succeeds. e.g. host.example.com:22