refactor: remove dependency on ip CLI tool and abstract network logic

Eliminate the external dependency on the `ip` (iproute2) command-line tool by centralizing network configuration and diagnostics within a new `internal/network` package using the `netlink` library.

Changes:
- Introduced `internal/network` package to handle network interface listing and configuration.
- Replaced `exec.Command("ip", "link")` in `internal/namespace.VerifyIsolation` with `network.ListInterfaces()`.
- Improved `VerifyIsolation` to explicitly ensure only the loopback interface is present in a fresh network namespace.
- Moved interface and routing configuration logic from `internal/wireguard` to `internal/network`.
- Removed unnecessary `os/exec` imports from network-related files.

This change increases the tool's portability by removing the requirement for `iproute2` to be installed in the target environment.

1 files changed, 3 insertions, 50 deletions

diff --git a/internal/wireguard/wireguard.go b/internal/wireguard/wireguard.go
index 5db588e..8ffe794 100644
--- a/internal/wireguard/wireguard.go
+++ b/internal/wireguard/wireguard.go
@@ -33,9 +33,9 @@ import (
 	"strings"
 
 	"git.theodohertyfamily.com/wg-wrap/internal/namespace"
+	"git.theodohertyfamily.com/wg-wrap/internal/network"
 	"git.theodohertyfamily.com/wg-wrap/internal/paths"
 	"git.theodohertyfamily.com/wg-wrap/pkg/wgconf"
-	"github.com/vishvananda/netlink"
 	"golang.org/x/sys/unix"
 	"golang.zx2c4.com/wireguard/conn"
 	"golang.zx2c4.com/wireguard/device"
@@ -117,8 +117,8 @@ func StartTunnel(pm *paths.PathManager, profile string, cfg *wgconf.Config, dnsS
 		return nil, fmt.Errorf("failed to bring up WireGuard device: %w", err)
 	}
 
-	// 4. Configure network interface using netlink
-	if err := configureInterface(tunName, cfg.Address, mtu); err != nil {
+	// 4. Configure network interface
+	if err := network.ConfigureInterface(tunName, cfg.Address, mtu); err != nil {
 		return nil, fmt.Errorf("failed to configure network interface %s: %w", tunName, err)
 	}
 
@@ -201,53 +201,6 @@ func buildUAPIConfig(cfg *wgconf.Config) (string, error) {
 	return sb.String(), nil
 }
 
-// configureInterface uses netlink to set address, MTU, and default routing table.
-func configureInterface(name, address string, mtu int) error {
-	link, err := netlink.LinkByName(name)
-	if err != nil {
-		return fmt.Errorf("failed to find link %s: %w", name, err)
-	}
-
-	if err := netlink.LinkSetMTU(link, mtu); err != nil {
-		return fmt.Errorf("failed to set MTU %d on link %s: %w", mtu, name, err)
-	}
-
-	if err := netlink.LinkSetUp(link); err != nil {
-		return fmt.Errorf("failed to bring up link %s: %w", name, err)
-	}
-
-	addr, err := netlink.ParseAddr(address)
-	if err != nil {
-		return fmt.Errorf("invalid IP address %s: %w", address, err)
-	}
-	if err := netlink.AddrAdd(link, addr); err != nil {
-		if !strings.Contains(err.Error(), "file exists") {
-			return fmt.Errorf("failed to add address %s to link %s: %w", address, name, err)
-		}
-	}
-
-	var dst *net.IPNet
-	if addr.IP.To4() != nil {
-		_, dst, _ = net.ParseCIDR("0.0.0.0/0")
-	} else {
-		_, dst, _ = net.ParseCIDR("::/0")
-	}
-
-	route := &netlink.Route{
-		Scope:     netlink.SCOPE_UNIVERSE,
-		LinkIndex: link.Attrs().Index,
-		Dst:       dst,
-	}
-
-	if err := netlink.RouteAdd(route); err != nil {
-		if err := netlink.RouteReplace(route); err != nil {
-			return fmt.Errorf("failed to configure default route via %s: %w", name, err)
-		}
-	}
-
-	return nil
-}
-
 // GetTunnelLocalIP extracts the local IP address (without CIDR) from the config.
 func GetTunnelLocalIP(cfg *wgconf.Config) (string, error) {
 	if cfg.Address == "" {