IP_STATUS_FILE and routing improvements (#130)

- Obtains VPN public IP address from routing table
- Logs and writes VPN Public IP address to `/ip` as soon as VPN is up
- Obtain port forward, logs it and writes it as soon as VPN is up
- Routing fully refactored and tested
- Routing reads from `/proc/net/route`
- Routing mutates the routes using `ip route ...`

internal/routing/entry.go

@@ -0,0 +1,93 @@
+package routing
+
+import (
+	"encoding/hex"
+	"fmt"
+	"net"
+	"strconv"
+
+	"strings"
+)
+
+type routingEntry struct {
+	iface       string
+	destination net.IP
+	gateway     net.IP
+	flags       string
+	refCount    int
+	use         int
+	metric      int
+	mask        net.IPMask
+	mtu         int
+	window      int
+	irtt        int
+}
+
+func parseRoutingEntry(s string) (r routingEntry, err error) {
+	wrapError := func(err error) error {
+		return fmt.Errorf("line %q: %w", s, err)
+	}
+	fields := strings.Fields(s)
+	if len(fields) < 11 {
+		return r, wrapError(fmt.Errorf("not enough fields"))
+	}
+	r.iface = fields[0]
+	r.destination, err = reversedHexToIPv4(fields[1])
+	if err != nil {
+		return r, wrapError(err)
+	}
+	r.gateway, err = reversedHexToIPv4(fields[2])
+	if err != nil {
+		return r, wrapError(err)
+	}
+	r.flags = fields[3]
+	r.refCount, err = strconv.Atoi(fields[4])
+	if err != nil {
+		return r, wrapError(err)
+	}
+	r.use, err = strconv.Atoi(fields[5])
+	if err != nil {
+		return r, wrapError(err)
+	}
+	r.metric, err = strconv.Atoi(fields[6])
+	if err != nil {
+		return r, wrapError(err)
+	}
+	r.mask, err = hexToIPv4Mask(fields[7])
+	if err != nil {
+		return r, wrapError(err)
+	}
+	r.mtu, err = strconv.Atoi(fields[8])
+	if err != nil {
+		return r, wrapError(err)
+	}
+	r.window, err = strconv.Atoi(fields[9])
+	if err != nil {
+		return r, wrapError(err)
+	}
+	r.irtt, err = strconv.Atoi(fields[10])
+	if err != nil {
+		return r, wrapError(err)
+	}
+	return r, nil
+}
+
+func reversedHexToIPv4(reversedHex string) (IP net.IP, err error) {
+	bytes, err := hex.DecodeString(reversedHex)
+	if err != nil {
+		return nil, fmt.Errorf("cannot parse reversed IP hex %q: %s", reversedHex, err)
+	} else if len(bytes) != 4 {
+		return nil, fmt.Errorf("hex string contains %d bytes instead of 4", len(bytes))
+	}
+	return []byte{bytes[3], bytes[2], bytes[1], bytes[0]}, nil
+}
+
+func hexToIPv4Mask(hexString string) (mask net.IPMask, err error) {
+	bytes, err := hex.DecodeString(hexString)
+	if err != nil {
+		return nil, fmt.Errorf("cannot parse hex mask %q: %s", hexString, err)
+	} else if len(bytes) != 4 {
+		return nil, fmt.Errorf("hex string contains %d bytes instead of 4", len(bytes))
+	}
+	return []byte{bytes[3], bytes[2], bytes[1], bytes[0]}, nil
+}