add flag for trusted proxies

if you run ipecho behind a reverse proxy, you need to add the IP of
that reverse proxy as seen by ipecho with the -trusted-proxies flag:

ipecho -proxy="10.0.0.1"

README.md

@@ -21,11 +21,29 @@ This will compile to the binary file `ipecho`.
 Port 80 requires root permissions, so you need to run the binary as root. For example:
 
 ```
-sudo ./ipecho
+# ./ipecho
 ```
 
 Logging is done to STDOUT.
 
+If you expect a reverse proxy to contact this application with headers
+that indicate the actual origin IP, such as `X-Real-IP` or
+`X-Forwarded-For`, you must indicate with a flag what the IP of this
+reverse proxy will be to mark it as trusted.
+
+For example, if ipecho runs on `10.0.0.20` and a reverse proxy on
+`10.0.0.1`, on the node that runs ipecho you would run:
+
+```
+# ./ipecho -proxy='10.0.0.1'
+```
+
+This flag can be specified multiple times to trust multiple proxies:
+
+```
+# ./ipecho -proxy='10.0.0.1' -proxy='10.0.0.2' (...)
+```
+
 ## Step 3: Examples
 
 ```

main.go

@@ -1,18 +1,41 @@
 package main
 
 import (
+	"flag"
 	"fmt"
+	"log"
 	"net/http"
 	"strings"
 
 	"github.com/gin-gonic/gin"
 )
 
+type stringSlice []string
+
+func (i *stringSlice) String() string {
+	return strings.Join(*i, ",")
+}
+
+func (i *stringSlice) Set(value string) error {
+	*i = append(*i, value)
+	return nil
+}
+
 func main() {
+	var trustedProxies stringSlice
+	flag.Var(&trustedProxies, "proxy", "IP address of reverse proxy that will contact us, and we trust to provide correct headers (this flag can be repeated)")
+	flag.Parse()
+
 	gin.SetMode(gin.ReleaseMode)
 	r := gin.Default()
-	// do not trust proxies to provide "real ip" via header
-	r.SetTrustedProxies(nil)
+
+	if len(trustedProxies) > 0 {
+		if err := r.SetTrustedProxies(trustedProxies); err != nil {
+			log.Fatalf("Tried and failed to set trusted proxies: %v", err)
+		}
+	} else {
+		r.SetTrustedProxies(nil)
+	}
 
 	r.GET("/json", func(c *gin.Context) {
 		// c.Request.Header returns []string, use 'strings' to convert to string
@@ -24,8 +47,7 @@ func main() {
 	})
 
 	r.GET("/", func(c *gin.Context) {
-		ip := c.ClientIP() + "\n"
-		c.String(http.StatusOK, ip)
+		c.String(http.StatusOK, c.ClientIP()+"\n")
 	})
 
 	r.NoRoute(func(c *gin.Context) {