Add support for unix socket as upstream (#1866)

* Add support for unix socket as upstream

* Add CHANGELOG.md entry

* Add Unix socket documentation

* Don't export unixRoundTripper, switch from string prefix to Scheme match

* Add basic unix server mock

* Add some tests and comments

pkg/upstream/http.go

@@ -1,6 +1,8 @@
 package upstream
 
 import (
+	"context"
+	"net"
 	"net/http"
 	"net/http/httputil"
 	"net/url"
@@ -18,6 +20,7 @@ const (
 
 	httpScheme  = "http"
 	httpsScheme = "https"
+	unixScheme  = "unix"
 )
 
 // SignatureHeaders contains the headers to be signed by the hmac algorithm
@@ -40,7 +43,10 @@ var SignatureHeaders = []string{
 // to a single upstream host.
 func newHTTPUpstreamProxy(upstream options.Upstream, u *url.URL, sigData *options.SignatureData, errorHandler ProxyErrorHandler) http.Handler {
 	// Set path to empty so that request paths start at the server root
-	u.Path = ""
+	// Unix scheme need the path to find the socket
+	if u.Scheme != "unix" {
+		u.Path = ""
+	}
 
 	// Create a ReverseProxy
 	proxy := newReverseProxy(u, upstream, errorHandler)
@@ -92,6 +98,25 @@ func (h *httpUpstreamProxy) ServeHTTP(rw http.ResponseWriter, req *http.Request)
 	}
 }
 
+// Unix implementation of http.RoundTripper, required to register unix protocol in reverse proxy
+type unixRoundTripper struct {
+	Transport *http.Transport
+}
+
+// Implementation of https://pkg.go.dev/net/http#RoundTripper interface to support http protocol over unix socket
+func (t *unixRoundTripper) RoundTrip(req *http.Request) (*http.Response, error) {
+	// Inspired by https://github.com/tv42/httpunix
+	// Not having a Host, even if not used, makes the reverseproxy fail with a "no Host in request URL"
+	if req.Host == "" {
+		req.Host = "localhost"
+	}
+	req.URL.Host = req.Host
+	tt := t.Transport
+	req = req.Clone(req.Context())
+	req.URL.Scheme = "http"
+	return tt.RoundTrip(req)
+}
+
 // newReverseProxy creates a new reverse proxy for proxying requests to upstream
 // servers based on the upstream configuration provided.
 // The proxy should render an error page if there are failures connecting to the
@@ -102,6 +127,14 @@ func newReverseProxy(target *url.URL, upstream options.Upstream, errorHandler Pr
 	// Inherit default transport options from Go's stdlib
 	transport := http.DefaultTransport.(*http.Transport).Clone()
 
+	if target.Scheme == "unix" {
+		transport.DialContext = func(ctx context.Context, _, _ string) (net.Conn, error) {
+			dialer := net.Dialer{}
+			return dialer.DialContext(ctx, target.Scheme, target.Path)
+		}
+		transport.RegisterProtocol(target.Scheme, &unixRoundTripper{Transport: transport})
+	}
+
 	// Change default duration for waiting for an upstream response
 	if upstream.Timeout != nil {
 		transport.ResponseHeaderTimeout = upstream.Timeout.Duration()