WIP netlink stuff

go.mod

@@ -7,7 +7,11 @@ require (
 	nullprogram.com/x/rng v1.1.0
 )
 
-require golang.org/x/sys v0.13.0 // indirect
+require (
+	github.com/vishvananda/netlink v1.1.0 // indirect
+	github.com/vishvananda/netns v0.0.0-20191106174202-0a2b9b5464df // indirect
+	golang.org/x/sys v0.13.0 // indirect
+)
 
 retract (
 	v0.9.1 // premature (race condition)

go.sum

@@ -1,5 +1,10 @@
+github.com/vishvananda/netlink v1.1.0 h1:1iyaYNBLmP6L0220aDnYQpo1QEV4t4hJ+xEEhhJH8j0=
+github.com/vishvananda/netlink v1.1.0/go.mod h1:cTgwzPIzzgDAYoQrMm0EdrjRUBkTqKYppBueQtXaqoE=
+github.com/vishvananda/netns v0.0.0-20191106174202-0a2b9b5464df h1:OviZH7qLw/7ZovXvuNyL3XQl8UFofeikI1NW1Gypu7k=
+github.com/vishvananda/netns v0.0.0-20191106174202-0a2b9b5464df/go.mod h1:JP3t17pCcGlemwknint6hfoeCVQrEMVwxRLRjXpq+BU=
 golang.org/x/crypto v0.14.0 h1:wBqGXzWJW6m1XrIKlAH0Hs1JJ7+9KBwnIO8v66Q9cHc=
 golang.org/x/crypto v0.14.0/go.mod h1:MVFd36DqK4CsrnJYDkBA3VC4m2GkXAM0PvzMCn4JQf4=
+golang.org/x/sys v0.0.0-20190606203320-7fc4e5ec1444/go.mod h1:h1NjWce9XRLGQEsW7wpKNCjG9DtNlClVuFLEZdDNbEs=
 golang.org/x/sys v0.13.0 h1:Af8nKPmuFypiUBjVoU9V20FiaFXOcuZI21p0ycVYYGE=
 golang.org/x/sys v0.13.0/go.mod h1:oPkhp1MJrh7nUepCBck5+mAzfO9JrbApNNgaTdGDITg=
 nullprogram.com/x/rng v1.1.0 h1:SMU7DHaQSWtKJNTpNFIFt8Wd/KSmOuSDPXrMFp/UMro=

linux/net/netlink.go

@@ -0,0 +1,26 @@
+//go:build linux
+
+package net
+
+import (
+	"errors"
+	"fmt"
+	"syscall"
+
+	"github.com/vishvananda/netlink"
+)
+
+var ErrNoDefaultRoute = errors.New("no default route")
+
+func getDefaultIPv4RouteString() (string, error) {
+	routes, err := netlink.RouteListFiltered(syscall.AF_INET, &netlink.Route{Dst: nil}, netlink.RT_FILTER_DST)
+	if err != nil {
+		return "", fmt.Errorf("failed to get default route with error: %w", err)
+	}
+	for _, route := range routes {
+		if route.Dst == nil {
+			return route.Gw.String(), nil
+		}
+	}
+	return "", ErrNoDefaultRoute
+}

linux/net/netlink_test.go

@@ -0,0 +1,43 @@
+//go:build linux
+
+package net
+
+import (
+	"bytes"
+	"os"
+	"os/exec"
+	"strings"
+	"testing"
+)
+
+func Test_getDefaultIPv4Route(t *testing.T) {
+	if os.Getuid() != 0 {
+		t.Skip("skipping test; must be root to run")
+	}
+	// use cmd exec to parse and check `ip route show	 default` for a control value
+	cmd := exec.Command("ip", "route", "show", "default")
+	out, err := cmd.Output()
+	if err != nil {
+		t.Skipf("failed to get default route with error: %e", err)
+	}
+	if len(out) < 1 {
+		t.Skip("failed to get default route control value")
+	}
+	fields := bytes.Fields(out)
+	if len(fields) < 3 {
+		t.Skip("failed to get default route control value")
+	}
+	value := fields[2]
+	if len(value) < 1 {
+		t.Skip("failed to get default route control value")
+	}
+	t.Logf("control value: %s", string(value))
+	var res string
+	if res, err = getDefaultIPv4RouteString(); err != nil {
+		t.Fatalf("failed to get default route with error: %e", err)
+	}
+	if !strings.EqualFold(res, string(value)) {
+		t.Fatalf("failed to get default route with error: %e", err)
+	}
+	t.Logf("default route: %s", res)
+}