assume ip_forwarding = 1 by default

api.go

@@ -526,7 +526,7 @@ func postContainersCreate(srv *Server, version float64, w http.ResponseWriter, r
 		out.Warnings = append(out.Warnings, "Your kernel does not support memory swap capabilities. Limitation discarded.")
 	}
 
-	if !srv.runtime.capabilities.IPv4Forwarding {
+	if srv.runtime.capabilities.IPv4ForwardingDisabled {
 		log.Println("Warning: IPv4 forwarding is disabled.")
 		out.Warnings = append(out.Warnings, "IPv4 forwarding is disabled.")
 	}

container.go

@@ -548,7 +548,7 @@ func (container *Container) Start(hostConfig *HostConfig) error {
 		container.Config.MemorySwap = -1
 	}
 
-	if !container.runtime.capabilities.IPv4Forwarding {
+	if container.runtime.capabilities.IPv4ForwardingDisabled {
 		log.Printf("WARNING: IPv4 forwarding is disabled. Networking will not work")
 	}
 

runtime.go

@@ -15,9 +15,9 @@ import (
 )
 
 type Capabilities struct {
-	MemoryLimit    bool
-	SwapLimit      bool
-	IPv4Forwarding bool
+	MemoryLimit            bool
+	SwapLimit              bool
+	IPv4ForwardingDisabled bool
 }
 
 type Runtime struct {
@@ -244,8 +244,8 @@ func (runtime *Runtime) UpdateCapabilities(quiet bool) {
 	}
 
 	content, err3 := ioutil.ReadFile("/proc/sys/net/ipv4/ip_forward")
-	runtime.capabilities.IPv4Forwarding = err3 == nil && len(content) > 0 && content[0] == '1'
-	if !runtime.capabilities.IPv4Forwarding && !quiet {
+	runtime.capabilities.IPv4ForwardingDisabled = err3 != nil || len(content) == 0 || content[0] != '1'
+	if runtime.capabilities.IPv4ForwardingDisabled && !quiet {
 		log.Printf("WARNING: IPv4 forwarding is disabled.")
 	}
 }

server.go

@@ -269,7 +269,7 @@ func (srv *Server) DockerInfo() *APIInfo {
 		Images:             imgcount,
 		MemoryLimit:        srv.runtime.capabilities.MemoryLimit,
 		SwapLimit:          srv.runtime.capabilities.SwapLimit,
-		IPv4Forwarding:     srv.runtime.capabilities.IPv4Forwarding,
+		IPv4Forwarding:     !srv.runtime.capabilities.IPv4ForwardingDisabled,
 		Debug:              os.Getenv("DEBUG") != "",
 		NFd:                utils.GetTotalUsedFds(),
 		NGoroutines:        runtime.NumGoroutine(),