all: illumos/solaris userspace only support

Updates #14565

Change-Id: I743148144938794db0a224873ce76c10dbe6fa5f
Signed-off-by: Nahum Shalman <nahamu@gmail.com>

net/netutil/ip_forward.go

@@ -63,6 +63,11 @@ func CheckIPForwarding(routes []netip.Prefix, state *netmon.State) (warn, err er
 		switch runtime.GOOS {
 		case "dragonfly", "freebsd", "netbsd", "openbsd":
 			return fmt.Errorf("Subnet routing and exit nodes only work with additional manual configuration on %v, and is not currently officially supported.", runtime.GOOS), nil
+		case "illumos", "solaris":
+			_, err := ipForwardingEnabledSunOS(ipv4, "")
+			if err != nil {
+				return nil, fmt.Errorf("Couldn't check system's IP forwarding configuration, subnet routing/exit nodes may not work: %w%s", err, "")
+			}
 		}
 		return nil, nil
 	}
@@ -325,3 +330,24 @@ func reversePathFilterValueLinux(iface string) (int, error) {
 	}
 	return v, nil
 }
+
+func ipForwardingEnabledSunOS(p protocol, iface string) (bool, error) {
+	var proto string
+	if p == ipv4 {
+		proto = "ipv4"
+	} else if p == ipv6 {
+		proto = "ipv6"
+	} else {
+		return false, fmt.Errorf("unknown protocol")
+	}
+
+	ipadmCmd := "\"ipadm show-prop " + proto + " -p forwarding -o CURRENT -c\""
+	bs, err := exec.Command("ipadm", "show-prop", proto, "-p", "forwarding", "-o", "CURRENT", "-c").Output()
+	if err != nil {
+		return false, fmt.Errorf("couldn't check %s (%v).\nSubnet routes won't work without IP forwarding.", ipadmCmd, err)
+	}
+	if string(bs) != "on\n" {
+		return false, fmt.Errorf("IP forwarding is set to off. Subnet routes won't work. Try 'routeadm -u -e %s-forwarding'", proto)
+	}
+	return true, nil
+}