Bugfix/1834 Fix X-Real-IP bug (#2007)

* Fix incorrect return ip value for RealIpHeader
* Improve test file to compare correct real IPs to each other and have better comments
* Refactor ip extractor tests to be more readable (longer but readable)

Co-authored-by: toimtoimtoim <desinformatsioon@gmail.com>

Author: yeyisan

echo.go

@@ -214,9 +214,9 @@ const (
 	HeaderXForwardedSsl       = "X-Forwarded-Ssl"
 	HeaderXUrlScheme          = "X-Url-Scheme"
 	HeaderXHTTPMethodOverride = "X-HTTP-Method-Override"
-	HeaderXRealIP             = "X-Real-IP"
-	HeaderXRequestID          = "X-Request-ID"
-	HeaderXCorrelationID      = "X-Correlation-ID"
+	HeaderXRealIP             = "X-Real-Ip"
+	HeaderXRequestID          = "X-Request-Id"
+	HeaderXCorrelationID      = "X-Correlation-Id"
 	HeaderXRequestedWith      = "X-Requested-With"
 	HeaderServer              = "Server"
 	HeaderOrigin              = "Origin"

ip.go

@@ -6,6 +6,130 @@ import (
 	"strings"
 )
 
+/**
+By: https://github.com/tmshn (See: https://github.com/labstack/echo/pull/1478 , https://github.com/labstack/echox/pull/134 )
+Source: https://echo.labstack.com/guide/ip-address/
+
+IP address plays fundamental role in HTTP; it's used for access control, auditing, geo-based access analysis and more.
+Echo provides handy method [`Context#RealIP()`](https://godoc.org/github.com/labstack/echo#Context) for that.
+
+However, it is not trivial to retrieve the _real_ IP address from requests especially when you put L7 proxies before the application.
+In such situation, _real_ IP needs to be relayed on HTTP layer from proxies to your app, but you must not trust HTTP headers unconditionally.
+Otherwise, you might give someone a chance of deceiving you. **A security risk!**
+
+To retrieve IP address reliably/securely, you must let your application be aware of the entire architecture of your infrastructure.
+In Echo, this can be done by configuring `Echo#IPExtractor` appropriately.
+This guides show you why and how.
+
+> Note: if you dont' set `Echo#IPExtractor` explicitly, Echo fallback to legacy behavior, which is not a good choice.
+
+Let's start from two questions to know the right direction:
+
+1. Do you put any HTTP (L7) proxy in front of the application?
+    - It includes both cloud solutions (such as AWS ALB or GCP HTTP LB) and OSS ones (such as Nginx, Envoy or Istio ingress gateway).
+2. If yes, what HTTP header do your proxies use to pass client IP to the application?
+
+## Case 1. With no proxy
+
+If you put no proxy (e.g.: directory facing to the internet), all you need to (and have to) see is IP address from network layer.
+Any HTTP header is untrustable because the clients have full control what headers to be set.
+
+In this case, use `echo.ExtractIPDirect()`.
+
+```go
+e.IPExtractor = echo.ExtractIPDirect()
+```
+
+## Case 2. With proxies using `X-Forwarded-For` header
+
+[`X-Forwared-For` (XFF)](https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/X-Forwarded-For) is the popular header
+to relay clients' IP addresses.
+At each hop on the proxies, they append the request IP address at the end of the header.
+
+Following example diagram illustrates this behavior.
+
+```text
+┌──────────┐            ┌──────────┐            ┌──────────┐            ┌──────────┐
+│ "Origin" │───────────>│ Proxy 1  │───────────>│ Proxy 2  │───────────>│ Your app │
+│ (IP: a)  │            │ (IP: b)  │            │ (IP: c)  │            │          │
+└──────────┘            └──────────┘            └──────────┘            └──────────┘
+
+Case 1.
+XFF:  ""                    "a"                     "a, b"
+                                                    ~~~~~~
+Case 2.
+XFF:  "x"                   "x, a"                  "x, a, b"
+                                                    ~~~~~~~~~
+                                                    ↑ What your app will see
+```
+
+In this case, use **first _untrustable_ IP reading from right**. Never use first one reading from left, as it is
+configurable by client. Here "trustable" means "you are sure the IP address belongs to your infrastructre".
+In above example, if `b` and `c` are trustable, the IP address of the client is `a` for both cases, never be `x`.
+
+In Echo, use `ExtractIPFromXFFHeader(...TrustOption)`.
+
+```go
+e.IPExtractor = echo.ExtractIPFromXFFHeader()
+```
+
+By default, it trusts internal IP addresses (loopback, link-local unicast, private-use and unique local address
+from [RFC6890](https://tools.ietf.org/html/rfc6890), [RFC4291](https://tools.ietf.org/html/rfc4291) and
+[RFC4193](https://tools.ietf.org/html/rfc4193)).
+To control this behavior, use [`TrustOption`](https://godoc.org/github.com/labstack/echo#TrustOption)s.
+
+E.g.:
+
+```go
+e.IPExtractor = echo.ExtractIPFromXFFHeader(
+	TrustLinkLocal(false),
+	TrustIPRanges(lbIPRange),
+)
+```
+
+- Ref: https://godoc.org/github.com/labstack/echo#TrustOption
+
+## Case 3. With proxies using `X-Real-IP` header
+
+`X-Real-IP` is another HTTP header to relay clients' IP addresses, but it carries only one address unlike XFF.
+
+If your proxies set this header, use `ExtractIPFromRealIPHeader(...TrustOption)`.
+
+```go
+e.IPExtractor = echo.ExtractIPFromRealIPHeader()
+```
+
+Again, it trusts internal IP addresses by default (loopback, link-local unicast, private-use and unique local address
+from [RFC6890](https://tools.ietf.org/html/rfc6890), [RFC4291](https://tools.ietf.org/html/rfc4291) and
+[RFC4193](https://tools.ietf.org/html/rfc4193)).
+To control this behavior, use [`TrustOption`](https://godoc.org/github.com/labstack/echo#TrustOption)s.
+
+- Ref: https://godoc.org/github.com/labstack/echo#TrustOption
+
+> **Never forget** to configure the outermost proxy (i.e.; at the edge of your infrastructure) **not to pass through incoming headers**.
+> Otherwise there is a chance of fraud, as it is what clients can control.
+
+## About default behavior
+
+In default behavior, Echo sees all of first XFF header, X-Real-IP header and IP from network layer.
+
+As you might already notice, after reading this article, this is not good.
+Sole reason this is default is just backward compatibility.
+
+## Private IP ranges
+
+See: https://en.wikipedia.org/wiki/Private_network
+
+Private IPv4 address ranges (RFC 1918):
+* 10.0.0.0 – 10.255.255.255 (24-bit block)
+* 172.16.0.0 – 172.31.255.255 (20-bit block)
+* 192.168.0.0 – 192.168.255.255 (16-bit block)
+
+Private IPv6 address ranges:
+* fc00::/7 address block = RFC 4193 Unique Local Addresses (ULA)
+
+*/
+
 type ipChecker struct {
 	trustLoopback    bool
 	trustLinkLocal   bool
@@ -52,6 +176,7 @@ func newIPChecker(configs []TrustOption) *ipChecker {
 	return checker
 }
 
+// Go1.16+ added `ip.IsPrivate()` but until that use this implementation
 func isPrivateIPRange(ip net.IP) bool {
 	if ip4 := ip.To4(); ip4 != nil {
 		return ip4[0] == 10 ||
@@ -87,25 +212,26 @@ type IPExtractor func(*http.Request) string
 // ExtractIPDirect extracts IP address using actual IP address.
 // Use this if your server faces to internet directory (i.e.: uses no proxy).
 func ExtractIPDirect() IPExtractor {
-	return func(req *http.Request) string {
-		ra, _, _ := net.SplitHostPort(req.RemoteAddr)
-		return ra
-	}
+	return extractIP
+}
+
+func extractIP(req *http.Request) string {
+	ra, _, _ := net.SplitHostPort(req.RemoteAddr)
+	return ra
 }
 
 // ExtractIPFromRealIPHeader extracts IP address using x-real-ip header.
 // Use this if you put proxy which uses this header.
 func ExtractIPFromRealIPHeader(options ...TrustOption) IPExtractor {
 	checker := newIPChecker(options)
 	return func(req *http.Request) string {
-		directIP := ExtractIPDirect()(req)
 		realIP := req.Header.Get(HeaderXRealIP)
 		if realIP != "" {
-			if ip := net.ParseIP(directIP); ip != nil && checker.trust(ip) {
+			if ip := net.ParseIP(realIP); ip != nil && checker.trust(ip) {
 				return realIP
 			}
 		}
-		return directIP
+		return extractIP(req)
 	}
 }
 
@@ -115,7 +241,7 @@ func ExtractIPFromRealIPHeader(options ...TrustOption) IPExtractor {
 func ExtractIPFromXFFHeader(options ...TrustOption) IPExtractor {
 	checker := newIPChecker(options)
 	return func(req *http.Request) string {
-		directIP := ExtractIPDirect()(req)
+		directIP := extractIP(req)
 		xffs := req.Header[HeaderXForwardedFor]
 		if len(xffs) == 0 {
 			return directIP