Add reverse proxy configuration support for remote IP address (#14959)

* Add reverse proxy configuration support for remote IP address validation

* Trust all IP addresses in containerized environments by default

* Use single option to specify networks and proxy IP addresses. By default trust all loopback IPs

Co-authored-by: techknowlogick <techknowlogick@gitea.io>

vendor/github.com/chi-middleware/proxy/middleware.go

@@ -0,0 +1,77 @@
+// Copyright 2020 Lauris BH. All rights reserved.
+// Use of this source code is governed by a MIT-style
+// license that can be found in the LICENSE file.
+
+package proxy
+
+// Ported from Goji's middleware, source:
+// https://github.com/zenazn/goji/tree/master/web/middleware
+
+import (
+	"net"
+	"net/http"
+	"strings"
+)
+
+var xForwardedFor = http.CanonicalHeaderKey("X-Forwarded-For")
+var xRealIP = http.CanonicalHeaderKey("X-Real-IP")
+
+// ForwardedHeaders is a middleware that sets a http.Request's RemoteAddr to the results
+// of parsing either the X-Real-IP header or the X-Forwarded-For header (in that
+// order).
+func ForwardedHeaders(options ...*ForwardedHeadersOptions) func(h http.Handler) http.Handler {
+	opt := defaultOptions
+	if len(options) > 0 {
+		opt = options[0]
+	}
+	return func(h http.Handler) http.Handler {
+		fn := func(w http.ResponseWriter, r *http.Request) {
+			// Treat unix socket as 127.0.0.1
+			if r.RemoteAddr == "@" {
+				r.RemoteAddr = "127.0.0.1:0"
+			}
+			if rip := realIP(r, opt); len(rip) > 0 {
+				r.RemoteAddr = net.JoinHostPort(rip, "0")
+			}
+
+			h.ServeHTTP(w, r)
+		}
+
+		return http.HandlerFunc(fn)
+	}
+}
+
+func realIP(r *http.Request, options *ForwardedHeadersOptions) string {
+	host, _, err := net.SplitHostPort(r.RemoteAddr)
+	if err != nil {
+		return ""
+	}
+
+	if !options.isTrustedProxy(net.ParseIP(host)) {
+		return ""
+	}
+
+	var ip string
+
+	if xrip := r.Header.Get(xRealIP); xrip != "" {
+		ip = xrip
+	} else if xff := r.Header.Get(xForwardedFor); xff != "" {
+		p := 0
+		for i := options.ForwardLimit; i > 0; i-- {
+			if p > 0 {
+				xff = xff[:p-2]
+			}
+			p = strings.LastIndex(xff, ", ")
+			if p < 0 {
+				p = 0
+				break
+			} else {
+				p += 2
+			}
+		}
+
+		ip = xff[p:]
+	}
+
+	return ip
+}