Get client IP when using Cloudflare (gin-gonic#2723)

Co-authored-by: thinkerou <thinkerou@gmail.com>
(cherry picked from commit 6703dea)

context.go

@@ -731,10 +731,15 @@ func (c *Context) ShouldBindBodyWith(obj interface{}, bb binding.BindingBody) (e
 // If the headers are nots syntactically valid OR the remote IP does not correspong to a trusted proxy,
 // the remote IP (coming form Request.RemoteAddr) is returned.
 func (c *Context) ClientIP() string {
-	if c.engine.AppEngine {
+	switch {
+	case c.engine.AppEngine:
 		if addr := c.requestHeader("X-Appengine-Remote-Addr"); addr != "" {
 			return addr
 		}
+	case c.engine.CloudflareProxy:
+		if addr := c.requestHeader("CF-Connecting-IP"); addr != "" {
+			return addr
+		}
 	}
 
 	remoteIP, trusted := c.RemoteIP()

context_test.go

@@ -1472,6 +1472,13 @@ func TestContextClientIP(t *testing.T) {
 	c.Request.Header.Del("X-Appengine-Remote-Addr")
 	assert.Equal(t, "40.40.40.40", c.ClientIP())
 
+	c.engine.AppEngine = false
+	c.engine.CloudflareProxy = true
+	assert.Equal(t, "60.60.60.60", c.ClientIP())
+
+	c.Request.Header.Del("CF-Connecting-IP")
+	assert.Equal(t, "40.40.40.40", c.ClientIP())
+
 	// no port
 	c.Request.RemoteAddr = "50.50.50.50"
 	assert.Empty(t, c.ClientIP())
@@ -1481,6 +1488,7 @@ func resetContextForClientIPTests(c *Context) {
 	c.Request.Header.Set("X-Real-IP", " 10.10.10.10  ")
 	c.Request.Header.Set("X-Forwarded-For", "  20.20.20.20, 30.30.30.30")
 	c.Request.Header.Set("X-Appengine-Remote-Addr", "50.50.50.50")
+	c.Request.Header.Set("CF-Connecting-IP", "60.60.60.60")
 	c.Request.RemoteAddr = "  40.40.40.40:42123 "
 	c.engine.AppEngine = false
 }

gin.go

@@ -105,6 +105,10 @@ type Engine struct {
 	// 'X-AppEngine...' for better integration with that PaaS.
 	AppEngine bool
 
+	// If enabled, it will trust the CF-Connecting-IP header to determine the
+	// IP of the client.
+	CloudflareProxy bool
+
 	// If enabled, the url.RawPath will be used to find parameters.
 	UseRawPath bool