Maint: refactor internal/routing

- Split Go files better
- Reduce public API for exported errors

internal/routing/ip.go

@@ -0,0 +1,39 @@
+package routing
+
+import (
+	"errors"
+	"fmt"
+	"net"
+)
+
+func IPIsPrivate(ip net.IP) bool {
+	return ip.IsPrivate() || ip.IsLoopback() ||
+		ip.IsLinkLocalUnicast() || ip.IsLinkLocalMulticast()
+}
+
+var (
+	errInterfaceIPNotFound = errors.New("IP address not found for interface")
+	errInterfaceListAddr   = errors.New("cannot list interface addresses")
+	errInterfaceNotFound   = errors.New("network interface not found")
+)
+
+func (r *Routing) assignedIP(interfaceName string) (ip net.IP, err error) {
+	iface, err := net.InterfaceByName(interfaceName)
+	if err != nil {
+		return nil, fmt.Errorf("%w: %s: %s", errInterfaceNotFound, interfaceName, err)
+	}
+	addresses, err := iface.Addrs()
+	if err != nil {
+		return nil, fmt.Errorf("%w: %s: %s", errInterfaceListAddr, interfaceName, err)
+	}
+	for _, address := range addresses {
+		switch value := address.(type) {
+		case *net.IPAddr:
+			return value.IP, nil
+		case *net.IPNet:
+			return value.IP, nil
+		}
+	}
+	return nil, fmt.Errorf("%w: interface %s in %d addresses",
+		errInterfaceIPNotFound, interfaceName, len(addresses))
+}