chore(IP): add TrustedPlatform constant for Fly.io. (#3839)

Also add some more detail to the docs for how to use TrustedPlatform.

https://fly.io/docs/reference/runtime-environment/#fly-client-ip

context_test.go

@@ -1569,6 +1569,12 @@ func TestContextClientIP(t *testing.T) {
 	c.Request.Header.Del("CF-Connecting-IP")
 	assert.Equal(t, "40.40.40.40", c.ClientIP())
 
+	c.engine.TrustedPlatform = PlatformFlyIO
+	assert.Equal(t, "70.70.70.70", c.ClientIP())
+
+	c.Request.Header.Del("Fly-Client-IP")
+	assert.Equal(t, "40.40.40.40", c.ClientIP())
+
 	c.engine.TrustedPlatform = ""
 
 	// no port
@@ -1581,6 +1587,7 @@ func resetContextForClientIPTests(c *Context) {
 	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.Header.Set("Fly-Client-IP", "70.70.70.70")
 	c.Request.RemoteAddr = "  40.40.40.40:42123 "
 	c.engine.TrustedPlatform = ""
 	c.engine.trustedCIDRs = defaultTrustedCIDRs

docs/doc.md

@@ -2214,10 +2214,16 @@ import (
 func main() {
   router := gin.Default()
   // Use predefined header gin.PlatformXXX
+  // Google App Engine
   router.TrustedPlatform = gin.PlatformGoogleAppEngine
-  // Or set your own trusted request header for another trusted proxy service
-  // Don't set it to any suspect request header, it's unsafe
-  router.TrustedPlatform = "X-CDN-IP"
+  // Cloudflare
+  router.TrustedPlatform = gin.PlatformCloudflare
+  // Fly.io
+  router.TrustedPlatform = gin.PlatformFlyIO
+  // Or, you can set your own trusted request header. But be sure your CDN
+  // prevents users from passing this header! For example, if your CDN puts
+  // the client IP in X-CDN-Client-IP:
+  router.TrustedPlatform = "X-CDN-Client-IP"
 
   router.GET("/", func(c *gin.Context) {
     // If you set TrustedPlatform, ClientIP() will resolve the

gin.go

@@ -77,6 +77,8 @@ const (
 	// PlatformCloudflare when using Cloudflare's CDN. Trust CF-Connecting-IP for determining
 	// the client's IP
 	PlatformCloudflare = "CF-Connecting-IP"
+	// PlatformFlyIO when running on Fly.io. Trust Fly-Client-IP for determining the client's IP
+	PlatformFlyIO = "Fly-Client-IP"
 )
 
 // Engine is the framework's instance, it contains the muxer, middleware and configuration settings.