Add guide about IP address (#134)

Author: tmshn

website/content/guide/ip-address.md

@@ -0,0 +1,104 @@
++++
+title = "IP Address"
+description = "IP address handling in Echo"
+[menu.main]
+  name = "IP Address"
+  parent = "guide"
++++
+
+IP address plays fundamental role in HTTP; it's used for access control, auditting, 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 infrastructrure.
+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
+            ┌──────────┐            ┌──────────┐            ┌──────────┐
+───────────>│ Proxy 1  │───────────>│ Proxy 2  │───────────>│ Your app │
+            │ (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.