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, 82 insertions, 0 deletions

diff --git a/internal/network/network.go b/internal/network/network.go
new file mode 100644
index 0000000..6afcf5e
--- /dev/null
+++ b/internal/network/network.go
@@ -0,0 +1,82 @@
+//go:build linux
+
+package network
+
+import (
+	"fmt"
+	"net"
+	"strings"
+
+	"github.com/vishvananda/netlink"
+)
+
+// InterfaceInfo contains basic information about a network interface.
+type InterfaceInfo struct {
+	Name  string
+	Index int
+}
+
+// ListInterfaces returns a list of all network interfaces present in the current namespace.
+func ListInterfaces() ([]InterfaceInfo, error) {
+	links, err := netlink.LinkList()
+	if err != nil {
+		return nil, fmt.Errorf("failed to list interfaces: %w", err)
+	}
+
+	var interfaces []InterfaceInfo
+	for _, link := range links {
+		interfaces = append(interfaces, InterfaceInfo{
+			Name:  link.Attrs().Name,
+			Index: link.Attrs().Index,
+		})
+	}
+	return interfaces, nil
+}
+
+// ConfigureInterface sets the MTU, brings the interface up, assigns an IP address,
+// and configures the default route.
+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
+}