pkg/ip: don't write to /proc/sys if ipforward enabled

This enables setup in a container env like systemd nspawn
where /proc/sys is mouted as read only.

Signed-off-by: Shengjing Zhu <i@zhsj.me>

pkg/ip/ipforward_linux_test.go

@@ -0,0 +1,31 @@
+package ip
+
+import (
+	"io/ioutil"
+	"os"
+	"time"
+
+	. "github.com/onsi/ginkgo"
+	. "github.com/onsi/gomega"
+)
+
+var _ = Describe("IpforwardLinux", func() {
+	It("echo1 must not write the file if content is 1", func() {
+		file, err := ioutil.TempFile(os.TempDir(), "containernetworking")
+		defer os.Remove(file.Name())
+		err = echo1(file.Name())
+		Expect(err).NotTo(HaveOccurred())
+		statBefore, err := file.Stat()
+		Expect(err).NotTo(HaveOccurred())
+
+		// take a duration here, otherwise next file modification operation time
+		// will be same as previous one.
+		time.Sleep(100 * time.Millisecond)
+
+		err = echo1(file.Name())
+		Expect(err).NotTo(HaveOccurred())
+		statAfter, err := file.Stat()
+		Expect(err).NotTo(HaveOccurred())
+		Expect(statBefore.ModTime()).To(Equal(statAfter.ModTime()))
+	})
+})