From e62b6121d12b847d625e15f9f8129fc485f71358 Mon Sep 17 00:00:00 2001
From: Patrik Lundin <patrik@sigterm.se>
Date: Thu, 26 Dec 2019 11:17:26 +0100
Subject: [PATCH] Only enable IP forwarding if it is disabled

Any value other than 0 is considered enabled:
https://www.kernel.org/doc/Documentation/networking/ip-sysctl.txt

Fixes #30
---
 server.go | 14 ++++++++++++--
 1 file changed, 12 insertions(+), 2 deletions(-)

diff --git a/server.go b/server.go
index 88c34a7..c811560 100644
--- a/server.go
+++ b/server.go
@@ -112,9 +112,19 @@ func NewServer() *Server {
 }
 
 func (s *Server) enableIPForward() error {
-	log.Info("Enabling sys.net.ipv4.ip_forward")
 	p := "/proc/sys/net/ipv4/ip_forward"
-	return ioutil.WriteFile(p, []byte("1"), 0640)
+
+	content, err := ioutil.ReadFile(p)
+	if err != nil {
+		return err
+	}
+
+	if string(content) == "0\n" {
+		log.Info("Enabling sys.net.ipv4.ip_forward")
+		return ioutil.WriteFile(p, []byte("1"), 0640)
+	}
+
+	return nil
 }
 
 func (s *Server) initInterface() error {