how does same IP on two interfaces work? / TINC

coreybrett

When using recommended settings for TINC the LAN and TUN interfaces are configuered with the same IP address but different masks.

How does that work (which it does) and not cause a conflict for the LAN address?

alt text

em0: flags=8843<UP,BROADCAST,RUNNING,SIMPLEX,MULTICAST> metric 0 mtu 1500
	options=4209b<RXCSUM,TXCSUM,VLAN_MTU,VLAN_HWTAGGING,VLAN_HWCSUM,WOL_MAGIC,VLAN_HWTSO>
	ether 00:0f:fe:73:5f:26
	hwaddr 00:0f:fe:73:5f:26
	inet6 fe80::20f:feff:fe73:5f26%em0 prefixlen 64 scopeid 0x1
	inet 192.168.117.1 netmask 0xffffff00 broadcast 192.168.117.255
	nd6 options=21<PERFORMNUD,AUTO_LINKLOCAL>
	media: Ethernet autoselect (100baseTX <full-duplex>)
	status: active
tun0: flags=8043<UP,BROADCAST,RUNNING,MULTICAST> metric 0 mtu 1500
	options=80000<LINKSTATE>
	inet 192.168.117.1 netmask 0xffff0000 broadcast 192.168.255.255
	inet6 fe80::20f:feff:fe73:5f26%tun0 prefixlen 64 scopeid 0x9
	nd6 options=21<PERFORMNUD,AUTO_LINKLOCAL>
	groups: tun pkg_tinc
	Opened by PID 55135

stephenw10

Interesting, I'd not noticed that before.

It works in FreeBSD because it can accept a more specific route. So in this case LAN traffic is still sent to em0 because the /24 is more specific than the /16 used for the tun interface.

Steve

coreybrett

thanks for the input, I see that the routing table is the key to this method

it just seems so "wrong" to have the same address on two different interfaces

it also doesn't work out if you have other subnets not covered by the same mask

after exchanging emails on the tinc mailing list, I've decided to put a different IP on the tunnel interface and use the "subnet up" script to handle adding routes

stephenw10

Yes I agree, I seems unnatural to do that.

However I guess that by doing that you can add new nodes to the mesh and as long as they are in that subnet the system routing table does not have to change to reach them. Only the internal routing in the daemon.

Steve