Kernel dummy module

[yaxattax]

Hi,

I would like to use the dummy module to create a fake ethernet interface with a name and HWaddr of my choosing that is present on every boot. What is the best way of achieving this?

I know modprobe dummy will load the kernel module and create a dummy0 interface, but how do I rename it and set its HWaddr? I would like to know how to perform these tasks in a fashion that is persistent across reboots.

Kind regards,

Yax

[DBelton]

You can set it up just like a "normal" ethernet interface

modprobe and set up network address of dummy interface:

[root@tower20 etc]# modprobe dummy
[root@tower20 etc]# ifconfig dummy0 192.168.2.10

add routing, check network binding and route commands:

[root@tower20 etc]# route add 192.168.2.10 dummy0
[root@tower20 etc]# ifconfig dummy0
dummy0: flags=195<UP,BROADCAST,RUNNING,NOARP>  mtu 1500
        inet 192.168.2.10  netmask 255.255.255.0  broadcast 192.168.2.255
        inet6 fe80::6c21:16ff:feac:cc09  prefixlen 64  scopeid 0x20<link>
        ether 6e:21:16:ac:cc:09  txqueuelen 0  (Ethernet)
        RX packets 0  bytes 0 (0.0 B)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 3  bytes 210 (210.0 B)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0

[root@tower20 etc]# route
Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
default         dslrouter       0.0.0.0         UG    0      0        0 p33p2
192.168.1.0     *               255.255.255.0   U     0      0        0 p33p2
192.168.2.0     *               255.255.255.0   U     0      0        0 dummy0
tower20.home    *               255.255.255.255 UH    0      0        0 dummy0

You would probably also wish to add the address to your /etc/hosts file as well.

[yaxattax]

Cheers for the response, but principally I want to know how to rename the interface and fix its HWaddr

[root@myhost ~]# modprobe dummy; ifconfig -a | grep dummy0; rmmod dummy
dummy0    Link encap:Ethernet  HWaddr 26:3E:8D:8D:9D:AC
[root@myhost ~]# modprobe dummy; ifconfig -a | grep dummy0; rmmod dummy
dummy0    Link encap:Ethernet  HWaddr 82:15:35:6C:7C:F1
[root@myhost ~]# modprobe dummy; ifconfig -a | grep dummy0; rmmod dummy
dummy0    Link encap:Ethernet  HWaddr 26:7A:77:57:A7:7C
[root@myhost ~]# modprobe dummy; ifconfig -a | grep dummy0; rmmod dummy
dummy0    Link encap:Ethernet  HWaddr FE:AD:E2:B8:8F:43
[root@myhost ~]# modprobe dummy; ifconfig -a | grep dummy0; rmmod dummy
dummy0    Link encap:Ethernet  HWaddr A6:AB:66:C1:79:85
[root@myhost ~]# modprobe dummy; ifconfig -a | grep dummy0; rmmod dummy
dummy0    Link encap:Ethernet  HWaddr BE:FC:14:74:DD:EA
[root@myhost ~]#

I need a fixed HWaddr for it, and I need it to appear as a real ethernet via its name, i.e I want to rename it to eth0.

Cheers,

Yax

[jpollard]

Umm.. Why do you need it to have the same hardware address?

The TCP/IP packets get the mac address of the last network interface. In the case of the dummy interfaces that should be the hardware MAC address of the p33p2 interface (as seen from your routing table).

[yaxattax]

Originally Posted by jpollard

Umm.. Why do you need it to have the same hardware address?

The TCP/IP packets get the mac address of the last network interface. In the case of the dummy interfaces that should be the hardware MAC address of the p33p2 interface (as seen from your routing table).

TCP/IP packets don't get any MAC address (unless the application specifically puts this into the TCP payload). Ethernet frames which may (or may not) encapsulate TCP/IP packets will have MAC addresses in the frame header.

I want the dummy interface to behave like a real ethernet interface. What ethernet card do you have that changes its MAC address every time the driver is loaded into the kernel?

How can I request this thread to be moved to the networking forum?

Cheers

Yax

[jpollard]

You can always try the ethtool utility to set the mac address. I'm not sure it will work with the dummy driver though.

As far as I know, any dummy ethernet device gets a random MAC address simply because it is required as part of the ethernet protocol. The MAC address is useless for anything but DHCP.

[yaxattax]

I've managed to do it by using ifrename and macchanger, but I was hoping to avoid adding stuff to rc.local ... unless I can find a way to do this persistently I don't have much choice

[jpollard]

Note: doing it in rc.local will not start network services properly. Things like NFS can fail as they depend on the network, but are only signaled by the systemd event handling...

[yaxattax]

Cheers for the tip. Fortunately, nothing explicitly depends on this interface.