Unable to create /32 subnet with Quantum

[Hyunsun Moon]

Quantum does not support /32 subnet properly.
When I create /32 subnet it successes but with wrong settings.

root@devstack:~/devstack# quantum subnet-create private-net 192.168.0.1/32
Created a new subnet:
+------------------+--------------------------------------+
| Field | Value |
+------------------+--------------------------------------+
| allocation_pools | |
| cidr | 192.168.0.1/32 |
| dns_nameservers | |
| enable_dhcp | True |
| gateway_ip | 192.168.0.2 |
| host_routes | |
| id | 9f4dee10-7852-4b21-be32-e5bec6f6e691 |
| ip_version | 4 |
| name | |
| network_id | 6dd423d3-2fa8-4c6f-8f7d-46b2c74c8eb8 |
| tenant_id | 543146e3aae04aedb4394c4c61479723 |
+------------------+--------------------------------------+

/32 subnet could be useful for some cases and in that case the IP would be in allocation_pools and disable dhcp with no gateway_ip.

[yong sheng gong (gongysh)]

what cases will need this kind of cidr?

[Hyunsun Moon (hyunsun-moon)]

Our network service model uses /32 subnet for floating IP. (mostly to acheive floating ip mobility)
I found that floating ip is assigned with /32 mask so no problem, but the problem is quantum router's default gateway, it is assigned with actual mask.

See the example.

28: qg-fc9eb6a0-a6: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP qlen 1000
    link/ether fa:16:3e:77:dd:5b brd ff:ff:ff:ff:ff:ff
    inet 172.16.0.101/24 brd 172.16.0.255 scope global qg-fc9eb6a0-a6 # external gateway
    inet 172.16.0.102/32 brd 172.16.0.102 scope global qg-fc9eb6a0-a6 # floating IP
    inet6 fe80::f816:3eff:fe77:dd5b/64 scope link
       valid_lft forever preferred_lft forever

Except that, it looks like a bug for me.
Um...what do you think so? We're using fixed version of this issue on our deploy, by the way.

[Armando Migliaccio (armando-migliaccio)]

There might be better ways to achieve what you are trying to do instead of using a /32 subnet.

One can disagree with specifications but it's pretty clear from IETF rfc that /32 are used to indicate a host route, not subnets. I would argue that this bug should be rephrased as "Quantum does not raise an error when creating networks with /32 CIDR's".

If I understood your requirement correctly, how about using the --allocation-pool option to constraint the size of your floating ip pool to one?

[Mark McClain (markmcclain)]

I agree with Armando. The bug is that Quantum allows you to complete this operation in the first place. A /32 should be an invalid subnet. Unless there is objection, I'm going to file a bug to specifically raise an error when a /32 subnet is requested.

[Armando Migliaccio (armando-migliaccio)]

Mark, feel free to assign the bug to me.

Cheers,
Armando

[Aaron Rosen (arosen)]

Creating a subnet with a /32 cidr is valid. I also agree that we should disable dhcp and set no gateway_ip.. But I don't understand the issue you're are having.

[Hyunsun Moon (hyunsun-moon)]

Sorry for my poor explanation.
I was trying to say that we want to use /32 cidr for public IPs(for floating IPs and router's gateway IPs) and I found that it is impossible with current quantum.

Anyway, it will be great if quantum allows /32 cidr for subnet-create.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/28539

[Brian Haley (brian-haley)]

Even a /31 is pretty unusable - one address is the network, the other the broadcast, no space for a host.

[Armando Migliaccio (armando-migliaccio)]

a subnet with /32 is a valid cidr, but it's not a subnet, it's a host; I don't think that discussing naming conventions can get us anywhere :)

[Aaron Rosen (arosen)]

Can you point me to anything that says a /32 isn't a valid subnet ?

@Brian it doesn't actually reserve a bcast address for /31, /32

root@arosen-laptop:/home/arosen# ifconfig eth0 192.168.3.1/32
root@arosen-laptop:/home/arosen# ifconfig eth0
eth0 Link encap:Ethernet HWaddr 40:6c:8f:1a:a9:31
          inet addr:192.168.3.1 Bcast:255.255.255.255 Mask:0.0.0.0

root@arosen-laptop:/home/arosen# ifconfig eth0 192.168.3.1/31
root@arosen-laptop:/home/arosen# ifconfig eth0
eth0 Link encap:Ethernet HWaddr 40:6c:8f:1a:a9:31
          inet addr:192.168.3.1 Bcast:255.255.255.255 Mask:255.255.255.254

I agree though this isn't very useful but I don't see why we shouldn't support it over not supporting it.

[Armando Migliaccio (armando-migliaccio)]

I am looking at the IETF Request for Comment:

http://tools.ietf.org/html/rfc4632

/32 is a "host route", /31 a "p2p link"

[Hyunsun Moon (hyunsun-moon)]

We are using /32 subnet mask IP by making the host holding the IP to advertise routing announce.
I know this is very special use case and understand that quantum cannot cover all the special use cases, but it is valid.

If there is another way to register /32 IP pool on quantum DB, please share, it would be very helpful.

To Armando, I hvave tried --allocation-pool option(if I understood your suggestion correctly) but it failed to pass validation.
stack@devstack:~/devstack$ quantum subnet-create private 10.0.0.1/32 --no-gateway --disable-dhcp --allocation-pool start=10.0.0.1,end=10.0.0.1
The allocation pool {u'start': u'10.0.0.1', u'end': u'10.0.0.1'} spans beyond the subnet cidr 10.0.0.1/32.

[Akihiro Motoki (amotoki)]

Hi Hyunsun,

According to your previous comment, you are trying to say that we want to use /32 cidr for public IPs(for floating IPs and router's gateway IPs) and I found that it is impossible with current quantum.

In my understanding what you want to do are:
- want to specify an IP address to a gateway interface of a router
- create a floating IP pool with several IP addresses which can not be represented by netmask
- IP addresses of the gateway interface and the pool are not consecutive.
For example, IP address of a gateway interface is 10.0.0.254 and a pool consists of 10.0.0.2 (and 10.0.0.20).
Is my understanding correct?

If my understanding is correct, the current quantum does not support such case since we don't have a way to specify an IP address of a gateway interface (connected to the external network).

P.S.
I also think a subnet for an external network should be a valid "subnet", since the subnet needs a default gateway to the Internet or somewhere even when an floating IP has only one IP address.

[Armando Migliaccio (armando-migliaccio)]

Hyunsun Moon,

Are you still facing the issue and thinking to propose a fix? If so, please update the bug report. Otherwise, I think there's ground to mark this bug as invalid.