What is the "destination address" for a TAP/TUN device? - tun

Question

What is the purpose of the "destination address" for a TAP/TUN device?
Pytun lets you easily set parameters of a tap/tun device:
tun = TapTunDevice(name='mytun')
tun.addr = '10.66.66.1'
tun.dstaddr = '10.66.66.2'
tun.netmask = '255.255.255.0'
tun.up()
Doing this will result in a device configured as such:
$ ifconfig mytun
mytun: flags=4305<UP,POINTOPOINT,RUNNING,NOARP,MULTICAST> mtu 1500
inet 10.66.66.1 netmask 255.255.255.0 destination 10.66.66.2
unspec 00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00 txqueuelen 500 (UNSPEC)
RX packets 0 bytes 0 (0.0 B)
RX errors 0 dropped 0 overruns 0 frame 0
TX packets 0 bytes 0 (0.0 B)
TX errors 0 dropped 0 overruns 0 carrier 0 collisions 0
I understand that the system now has a virtual interface with IP 10.66.66.1. And it's presumable that in this scenario, the TUN device would be "connected" to a (e.g. VPN gateway) device whose IP address is 10.66.66.2.
But what purpose specifically, does it serve for the kernel to know that this is a "point-to-point" interface, and the IP address of the destination? Does it impact routing in some way that simply configuring the route table would not achieve?
Setting the dstaddr property results in a SIOCSIFDSTADDR ioctl.
The netdevice(7) man page simply says:
SIOCGIFDSTADDR, SIOCSIFDSTADDR
Get or set the destination address of a point-to-point device
using ifr_dstaddr. For compatibility, only AF_INET addresses
are accepted or returned. Setting the destination address is
a privileged operation.

Answer 1

I don't care about all this I want to configure my interface
You don't need to set a destination address. If you want to configure 10.66.66.1/24 on the interface, you can do:
tun = TapTunDevice(name='mytun')
tun.addr = '10.66.66.1'
tun.netmask = '255.255.255.0'
tun.up()
This interface only connects two hosts, so you don't actually need a whole /24. You can only say that 10.66.66.1 is connected to 10.66.66.2 (10.66.66.1 peer 10.66.66.2):
tun = TapTunDevice(name='mytun')
tun.addr = '10.66.66.1'
tun.dstaddr = '10.66.66.2'
tun.netmask = '255.255.255.255'
tun.up()
In this setup, the two IP addresses do not need to be in the same range at all.
Alternatively, you could use a /31, RFC3021:
tun = TapTunDevice(name='mytun')
tun.addr = '10.66.66.2'
tun.dstaddr = '10.66.66.3'
tun.netmask = '255.255.255.254'
tun.up()
Notice, how I had to change the IP addresses in order for them to be in the same /31.
What is a POINTOPOINT device?
The POINTOPOINT means that on this interface there is no Layer 2 addressing (no MAC address) on this interface:
no ARP requests (IPv4);
no NDP requests (IPv6);
the neighbour table is useless for this interface (ip neighbour);
in routing table entries for this interface the via directive is ignored;
packets on this interface are always send to the same (only) next-hop.
Examples of POINTOPOINT devices
PPP interfaces: There is not Layer 2 address for PPP as this type of interface connects a single host to another host (hence the name "point-to-point protocol")
TUN interfaces: They are IP only interfaces without a Layer 2.
POINTOPOINT means that this is a point-to-point interface (surprise!) which means that there can be only one peer connected at the other-side of the interface: you have one neighbor on this interface and you do not need to use ARP/NDP for mapping IP address to link-layer address (and you do not have link layer address at all).
In contrast, an Ethernet device is not a point-to-point interface because multiple hosts can be directly reacheable via this interface. When you send an IP packet to such a device, the network stack has to find a layer 2 identifier (using ARP, NDP) for the intended IP address and send the message to this link-layer address.
Say this, is your routing table (in Ethernet):
default via 192.0.2.1 dev eth0 proto static metric 100
192.0.2.0/24 dev eth0 proto kernel scope link src 192.0.2.2 metric 100
Multiple hosts can be directly connected to you via the eth0 interface. If you want to send a packet to 198.51.100.1, this route is selected:
default via 192.0.2.1 dev eth0 proto static metric 100
which means that among all your neighbors on the eth0 device, you have to send the packet to 192.0.2.1. In order to to that, your network stack has to find the MAC address of 192.0.2.1 by using ARP.
On a POINTOPOINT device, there is always only one neighbor so you don't need to do ARP, you only need to send the packet.
TUN and PPP interfaces are POINTOPOINT devices. Ethernet, Ethernet TAP devices and Wifi interfaces are not POINTOPOINT.
What is the destination (peer) address?
Usually the IP configuration of an interface is in the form: 192.0.2.1/24. This means that the ip address of this interface is 192.0.2.1 and that all IP in the 192.0.2.0/24 subnet are directly reachable via this interface: this adds a routing rule 192.0.2.0/24 dev tun0.
The Linux kernel supports another type of configuration when the local IP address and the peer address does not belong to the same IP subnet: 192.0.2.1 peer 198.51.100.1. This means that the IP address of this interface is 192.0.2.1 and that the IP address of the peer is 198.51.100.1: this adds a routing rule 198.51.100.1 dev tun0. A more general form can be used: 192.0.2.1 peer 198.51.100.1/24.
$ ip address show tun0
14: tun0: mtu 1500 qdisc pfifo_fast state UNKNOWN group default qlen 500
link/none
inet 192.0.2.1 198.51.100.1/24 scope global tun0
valid_lft forever preferred_lft forever
The dstaddr parameter (and the SIOCSIFDSTADDR) can be used to set such as destination address.
This is useful if you don't want to allocate a common subnet for the two peers. You don't have to use a special destination address with point to point interface. You could use a standard IP subnet. Or you could allocate a /31. Using the destination address/peer configuration, you can avoid allocating a subnet for this point-to-point link.
What is the relation between the peer/destination address and POINTOPOINT devices?
These are independant. You don't have top set a destination address on a POINTOPOINT interface. You can set a destination address on a POINTOPOINT and you can do it on a normal one as well.
However, using a peer destination address is especially/mostly useful for POINTOPOINT interfaces.

Answer 2

If you add an interface with
inet 10.66.66.1 netmask 255.255.255.0
No matter if you create it as point to point, or not- a new routing entry will be added to the kernel for 10.66.66.1/24 with destination of the new interface.
So I don't think that there is a difference there.