net/http/httputil: NewSingleHostReverseProxy - omit X-Forwarded-For not working

[firefart]

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

$ go version
go version go1.18 linux/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="/home/firefart/.cache/go-build"
GOENV="/home/firefart/.config/go/env"
GOEXE=""
GOEXPERIMENT=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOINSECURE=""
GOMODCACHE="/home/firefart/go/pkg/mod"
GONOPROXY=""
GONOSUMDB=""
GOOS="linux"
GOPATH="/home/firefart/go"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/usr/local/go"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/usr/local/go/pkg/tool/linux_amd64"
GOVCS=""
GOVERSION="go1.18"
GCCGO="gccgo"
GOAMD64="v1"
AR="ar"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
GOMOD="/home/firefart/code/zwiebelproxy/go.mod"
GOWORK=""
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -m64 -pthread -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build534953705=/tmp/go-build -gno-record-gcc-switches"

What did you do?

In #38079 a check was added to not leak the IP if the X-Forwarded-For header is nil.

Doing this does not seem to work as it always return an empty string slice

r.Header["X-Forwarded-For"] = nil
fmt.Printf("%T", r.Header["X-Forwarded-For"])

Output:

[]string

What did you expect to see?

The header is not added

What did you see instead?

The IP header is always added as it seems impossible to set the value to nil

go/src/net/http/httputil/reverseproxy.go

Line 301 in 9068c68

omit := ok && prior == nil // Issue 38079: nil now means don't populate the header

[firefart]

The actual issue lies in the Header.Clone method.

ServeHTTP calls req.Clone(ctx) over here:

go/src/net/http/httputil/reverseproxy.go

Line 246 in 9068c68

outreq := req.Clone(ctx)

Clone then calls the Clone method on the Headers map:

go/src/net/http/request.go

Line 380 in 9068c68

r2.Header = r.Header.Clone()

I added a debug output before and after the r.Header.Clone() call and this is the output:

http.Header{"X-Forwarded-For":[]string(nil)}
-- Clone()
http.Header{"X-Forwarded-For":[]string{}}

So it looks like the Header.Clone() method does not preserve nil values in headers which makes the check for nil invalid and thus the client ip is always added.

This can be a privacy issue because currently golang is leaking the original client ip to the end targets even if the header is set to nil to prevent that

[seankhliao]

Seems to be working? https://go.dev/play/p/ME0dFW6gT1g

[firefart]

Nope. Here is a short play to demonstrate the underlying issue:
https://go.dev/play/p/c8Oc19Te76u

http.Header{"X-Forwarded-For":[]string(nil)}
http.Header{"X-Forwarded-For":[]string{}}

The Clone converts the nil value to an empty slice

Here is also some pseudocode for an http handler that forwards the request using httputil.NewSingleHostReverseProxy.

func (app *application) proxyHandler(w http.ResponseWriter, r *http.Request) {
	host, port, err := net.SplitHostPort(r.Host)
	if err != nil {
		// no port present
		host = r.Host
		port = r.URL.Port()
	}

	ctx, cancel := context.WithTimeout(r.Context(), app.timeout)
	defer cancel()
	r = r.WithContext(ctx)

	if port != "" && port != "80" && port != "443" {
		host = net.JoinHostPort(host, port)
	}
	r.URL.Host = host
	r.Host = host

	if r.URL.Scheme == "" {
		switch port {
		case "":
			r.URL.Scheme = "http"
		case "80":
			r.URL.Scheme = "http"
		case "443":
			r.URL.Scheme = "https"
		default:
			r.URL.Scheme = "http"
		}
	}

	// needed so the ip will not be leaked
	r.Header["X-Forwarded-For"] = nil

	app.logger.Debugf("port: %+v", port)
	app.logger.Debugf("r.URL: %+v", r.URL)
	app.logger.Debugf("r.RequestURI: %+v", r.RequestURI)
	app.logger.Debugf("r.Host: %+v", r.Host)
	app.logger.Debugf("r.Header: %+v", r.Header)

	proxy := httputil.NewSingleHostReverseProxy(r.URL)
	proxy.FlushInterval = -1
	proxy.ModifyResponse = app.modifyResponse
	proxy.Transport = app.httpClient.tr.Clone()

	app.logger.Debugf("sending request %+v", r)

	proxy.ServeHTTP(w, r)
}

If you add the debug statements on the clone method mentioned in the comment, you can see that the header value is not nil after cloning

[seankhliao]

I understood #38079 to mean allowing control over X-Forwarded-For from within the proxy, ie using your own Director, not from the outside.

cc @neild

[firefart]

Maybe, but the comment does not say this exactly, it only mentions the header value needs to be nil

// If an X-Forwarded-For header already exists, the client IP is
// appended to the existing values. As a special case, if the header
// exists in the Request.Header map but has a nil value (such as when
// set by the Director func), the X-Forwarded-For header is
// not modified.

I also think the Clone() function not handling nil values is a bit inconsistent as it modifies the original request.

[neild]

Given that ReverseProxy assigns meaning to nil header values, Header.Clone should probably preserve nil-ness.

[gopherbot]

Change https://go.dev/cl/412857 mentions this issue: net/http: preserve nil values in Header.Clone