Added some extra utilities for working with bridge and interfaces

README.md

@@ -182,6 +182,51 @@ func main() {
 }
 ```
 
+### Working with existing bridges and interfaces
+
+The following examples show how to retrieve exisiting interfaces as a tenus link and bridge
+
+```
+package main
+
+import (
+	"fmt"
+	"log"
+	"net"
+
+	"github.com/milosgajdos83/tenus"
+)
+
+func main() {
+	// RETRIEVE EXISTING BRIDGE
+	br, err := tenus.BridgeFromName("bridge0")
+	if err != nil {
+		log.Fatal(err)
+	}
+
+	// REMOVING AN IP FROM A BRIDGE INTERFACE (BEFORE RECONFIGURATION)
+	brIp, brIpNet, err := net.ParseCIDR("10.0.41.1/16")
+	if err != nil {
+		log.Fatal(err)
+	}
+	if err := br.UnsetLinkIp(brIp, brIpNet); err != nil {
+		log.Fatal(err)
+	}
+
+	// RETRIEVE EXISTING INTERFACE
+	dl, err := tenus.NewLinkFrom("eth0")
+	if err != nil {
+		log.Fatal(err)
+	}
+
+	// RENAMING AN INTERFACE BY NAME
+	if err := tenus.RenameInterfaceByName("vethPSQSEl", "vethNEWNAME"); err != nil {
+		log.Fatal(err)
+	}
+
+}
+```
+
 ### VLAN and MAC VLAN interfaces
 
 You can check out [VLAN](https://gist.github.com/milosgajdos83/9f68b1818dca886e9ae8) and [Mac VLAN](https://gist.github.com/milosgajdos83/296fb90d076f259a5b0a) examples, too.

bridge_linux.go

@@ -85,6 +85,25 @@ func NewBridgeWithName(ifcName string) (Bridger, error) {
 	}, nil
 }
 
+// BridgeFromName returns a tenus network bridge from an existing bridge of given name on the Linux host.
+// It returns error if the bridge of the given name cannot be found.
+func BridgeFromName(ifcName string) (Bridger, error) {
+	if ok, err := NetInterfaceNameValid(ifcName); !ok {
+		return nil, err
+	}
+
+	newIfc, err := net.InterfaceByName(ifcName)
+	if err != nil {
+		return nil, fmt.Errorf("Could not find the new interface: %s", err)
+	}
+
+	return &Bridge{
+		Link: Link{
+			ifc: newIfc,
+		},
+	}, nil
+}
+
 // AddToBridge adds network interfaces to network bridge.
 // It is equivalent of running: ip link set ${netIfc name} master ${netBridge name}
 // It returns error when it fails to add the network interface to bridge.

helpers_linux.go

@@ -33,6 +33,10 @@ func randomString(size int) string {
 	return string(bytes)
 }
 
+func MakeNetInterfaceName(base string) string {
+	return makeNetInterfaceName(base)
+}
+
 // generates new unused network interfaces name with given prefix
 func makeNetInterfaceName(base string) string {
 	for {

link_linux.go

@@ -38,6 +38,8 @@ type Linker interface {
 	SetLinkDown() error
 	// SetLinkIp configures the link's IP address
 	SetLinkIp(net.IP, *net.IPNet) error
+	// UnsetLinkIp remove and IP address from the link
+	UnsetLinkIp(net.IP, *net.IPNet) error
 	// SetLinkDefaultGw configures the link's default gateway
 	SetLinkDefaultGw(*net.IP) error
 	// SetLinkNetNsPid moves the link to network namespace specified by PID
@@ -80,6 +82,22 @@ func NewLink(ifcName string) (Linker, error) {
 	}, nil
 }
 
+// NewLinkFrom creates new tenus link on Linux host from an existing interface of given name
+func NewLinkFrom(ifcName string) (Linker, error) {
+	if ok, err := NetInterfaceNameValid(ifcName); !ok {
+		return nil, err
+	}
+
+	newIfc, err := net.InterfaceByName(ifcName)
+	if err != nil {
+		return nil, fmt.Errorf("Could not find the new interface: %s", err)
+	}
+
+	return &Link{
+		ifc: newIfc,
+	}, nil
+}
+
 // NewLinkWithOptions creates new network link on Linux host and sets some of its network
 // parameters passed in as LinkOptions
 //
@@ -174,6 +192,12 @@ func (l *Link) SetLinkIp(ip net.IP, network *net.IPNet) error {
 	return netlink.NetworkLinkAddIp(l.NetInterface(), ip, network)
 }
 
+// UnsetLinkIp configures the link's IP address.
+// It is equivalent of running: ip address del ${address}/${mask} dev ${interface name}
+func (l *Link) UnsetLinkIp(ip net.IP, network *net.IPNet) error {
+	return netlink.NetworkLinkDelIp(l.NetInterface(), ip, network)
+}
+
 // SetLinkDefaultGw configures the link's default Gateway.
 // It is equivalent of running: ip route add default via ${ip address}
 func (l *Link) SetLinkDefaultGw(gw *net.IP) error {
@@ -232,6 +256,15 @@ func (l *Link) SetLinkNsToDocker(name string, dockerHost string) error {
 	return l.SetLinkNetNsPid(pid)
 }
 
+// RenameInterfaceByName renames an interface of given name.
+func RenameInterfaceByName(old string, newName string) error {
+	iface, err := net.InterfaceByName(old)
+	if err != nil {
+		return err
+	}
+	return netlink.NetworkChangeName(iface, newName)
+}
+
 // setLinkOptions validates and sets link's various options passed in as LinkOptions.
 func setLinkOptions(ifc *net.Interface, opts LinkOptions) error {
 	macaddr, mtu, flags, ns := opts.MacAddr, opts.MTU, opts.Flags, opts.Ns