From 8a8c4bdddd81518db6f561279ca33a078281dd8a Mon Sep 17 00:00:00 2001
From: Zoltan Papp <zoltan.pmail@gmail.com>
Date: Thu, 18 May 2023 19:31:54 +0200
Subject: [PATCH] Fix issue 872 (#873)

Read and check ip_forward from proc before write
---
 client/internal/routemanager/systemops_linux.go | 14 ++++++++++++--
 1 file changed, 12 insertions(+), 2 deletions(-)

diff --git a/client/internal/routemanager/systemops_linux.go b/client/internal/routemanager/systemops_linux.go
index 4f56e8299..67c59be2d 100644
--- a/client/internal/routemanager/systemops_linux.go
+++ b/client/internal/routemanager/systemops_linux.go
@@ -62,6 +62,16 @@ func removeFromRouteTable(prefix netip.Prefix) error {
 }
 
 func enableIPForwarding() error {
-	err := os.WriteFile(ipv4ForwardingPath, []byte("1"), 0644)
-	return err
+	bytes, err := os.ReadFile(ipv4ForwardingPath)
+	if err != nil {
+		return err
+	}
+
+	// check if it is already enabled
+	// see more: https://github.com/netbirdio/netbird/issues/872
+	if len(bytes) > 0 && bytes[0] == 49 {
+		return nil
+	}
+
+	return os.WriteFile(ipv4ForwardingPath, []byte("1"), 0644)
 }