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

$ go version
go version go1.13.8 linux/386

Does this issue reproduce with the latest release?

Yes

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

$ go env
GO111MODULE="on"
GOARCH="386"
GOBIN="~/go/bin/"
GOCACHE="~/.cache/go-build"
GOENV="~/.config/go/env"
GOEXE=""
GOFLAGS=" -mod=vendor"
GOHOSTARCH="386"
GOHOSTOS="linux"
GONOPROXY=""
GONOSUMDB=""
GOOS="linux"
GOPATH="~/go"
GOPRIVATE=""
GOPROXY="direct"
GOROOT="/usr/local/go1.13"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/usr/local/go1.13/pkg/tool/linux_386"
GCCGO="gccgo"
GO386="sse2"
AR="ar"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
GOMOD="~/git/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 -m32 -pthread -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build275301714=/tmp/go-build -gno-record-gcc-switches"

What did you do?

I'm simply using the ReverseProxy (https://github.com/golang/go/blob/master/src/net/http/httputil/reverseproxy.go) to implement an HTTP proxy. By default, and as documented, ReverseProxy automatically sets the client IP as the value of the X-Forwarded-For header. If an X-Forwarded-For header already exists, the client IP is appended to the existing values.. In our situation, the proxy should be transparent and invisible to the servers, so we'd like the proxy not to add any headers to the HTTP requests he forwards. Unfortunately, there is no option to disable the addition of the X-Forwarded-For header. Unless I'm mistaken, there is also no way to remove this header manually before messages are forwarded (like through a hook or something that is called at some point).

What did you expect to see?

An option in the ReverseProxy struct to disable the X-Forwarded-For header. For example, something like disableForwardedForHeader bool, which would be false per default, that means that the behavior stays like now. If true, the proxy wouldn't set the X-Forwarded-For header, through, e.g., if !p.disableForwardedForHeader at line 246 of https://github.com/golang/go/blob/master/src/net/http/httputil/reverseproxy.go.

Another option would be to add a hook that can modify the request just before the proxy sends/forwards it. That would actually be a more elegant and general solution. That could be the current Director hook, but then it should be called later in the ServeHTTP function, i.e., just before transport.RoundTrip(outreq). Not sure if that would have any other bad impact. In any case, another hook is possible.

What did you see instead?

No option and the proxy always sets the X-Forwarded-For header.