[Dnsmasq-discuss] Confusion about "no address range available for DHCPv6 request via ..."

Mon May 1 23:14:27 UTC 2023

Thanks for your response, it helps me understand things better.

On Mon, May 1, 2023 at 1:39 PM Simon Kelley <simon at thekelleys.org.uk> wrote:

> I'm not sure there's much incentive to change this: it's not as if
> addresses are scarce in the IPv6 world.
>

Well, it may not be the most necessary change, but I don't think the reason
has as much to do with the plentiful address space. I think it makes the
interface complex.

The most clear complication is having to advise the recipient of a prefix
that there's a little bite taken out of it, I have to say, "yeah it's
::2/64, but not ::1," where the latter is in a manual somewhere. I have
received networks with little bites taken out of them here and there, and
each such scheme I have to account for, because I can't re-delegate slices
of it freely, if there is a conflict and other layers do things with the
traffic. For example, I recently was working with a vendor, and they
punched out three holes in their static route for my prefix: two for VARP,
one for a gateway. It's not that this can't be sustained, but I have to
itemize the holes out-of-band for each such strategy. Rinse and repeat for
other vendors or software stacks.

Thankfully, there are norms suggesting IPv6 networks shouldn't use :: or
::1 if they can help it, and although I use link local addressing for the
gateway, I suppose having dnsmasq's recursive resolver and DHCP server at
::1 is "close enough" to the intended norm. So I suppose I could use that.

The second problem, related to the first, but more subtle, is how an
interface with an address assigned within the network affects Linux.
Here's an example of a network situation that works with, static
configuration, for example, in a virtual machine's network namespace:

# ip -n vm57nhjf -6 a
2: vethivm57nhjf at if5: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 state UP
qlen 1000
    inet6 2a01:4f9:2b:35a:c6dd::/80 scope global
       valid_lft forever preferred_lft forever
    inet6 fe80::b068:43ff:fe48:50e1/64 scope link
       valid_lft forever preferred_lft forever
3: tapvm57nhjf: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 state UP qlen
1000
    inet6 fe80::d487:b2ff:fe66:40cd/64 scope link
       valid_lft forever preferred_lft forever

# ip -n vm57nhjf -6 r
2a01:4f9:2b:35a:c6dc::/80 via fe80::d028:12ff:feb8:a206 dev tapvm57nhjf
metric 1024 pref medium
2a01:4f9:2b:35a:c6dd::/80 dev vethivm57nhjf proto kernel metric 256 pref
medium
2000::/3 via fe80::2ce1:51ff:feb6:c56f dev vethivm57nhjf metric 1024 pref
medium
fd34:f6d7:d9b1:6c85::/64 via fe80::d028:12ff:feb8:a206 dev tapvm57nhjf
metric 1024 pref medium
fe80::/64 dev vethivm57nhjf proto kernel metric 256 pref medium
fe80::/64 dev tapvm57nhjf proto kernel metric 256 pref medium

# ping 2a01:4f9:2b:35a:c6dc::
PING 2a01:4f9:2b:35a:c6dc::(2a01:4f9:2b:35a:c6dc::) 56 data bytes
< Hangs, which is expected >
^C
--- 2a01:4f9:2b:35a:c6dc:: ping statistics ---
8 packets transmitted, 0 received, 100% packet loss, time 7151ms

Here what you see is a namespace that receives a /79 (not directly
visible), and divides it into two adjacent /80s. The lower /80 is sent into
the VM tap device, the higher /80 is processed by the veth on the host
(where I, contra norms, use the :: address for now). Pinging the
2a01:4f9:2b:35a:c6dc::, at the bottom of the VM's /80 doesn't get any
response  because the VM does not have that address configured (it
configures ::2, which returns ping as expected). When pinging the ::
address, tcpdump shows ICMP packets going in, but no responses coming out,
since the VM doesn't happen to have the address configured. Everything
works as expected.

Now, let's adapt for dnsmasq, and allocate an address in-network on the
host, say, ::1:

ip -n vm57nhjf addr add 2a01:4f9:2b:35a:c6dc::1/80 dev tapvm57nhjf

Now we get this:

# ping 2a01:4f9:2b:35a:c6dc::
PING 2a01:4f9:2b:35a:c6dc::(2a01:4f9:2b:35a:c6dc::) 56 data bytes
64 bytes from 2a01:4f9:2b:35a:c6dd::: icmp_seq=1 ttl=64 time=0.059 ms
(DIFFERENT ADDRESS!)
64 bytes from 2a01:4f9:2b:35a:c6dd::: icmp_seq=2 ttl=64 time=0.080 ms
(DIFFERENT ADDRESS!)
[...etc...]

Which is not so good: rather than forwarding the ping to the VM to then get
no response, we get the reply from the veth, which has a different source
address IP entirely. I could slather on some more Linux features to fix
this, I imagine, but: complexity.

Pinging the bite of the address space works, though it's handled by the
host of course, as does the address the VM listens to:

root at Ubuntu-2204-jammy-amd64-base ~ # ping 2a01:4f9:2b:35a:c6dc::1
PING 2a01:4f9:2b:35a:c6dc::1(2a01:4f9:2b:35a:c6dc::1) 56 data bytes
64 bytes from 2a01:4f9:2b:35a:c6dc::1: icmp_seq=1 ttl=64 time=0.051 ms
64 bytes from 2a01:4f9:2b:35a:c6dc::1: icmp_seq=2 ttl=64 time=0.075 ms
^C
--- 2a01:4f9:2b:35a:c6dc::1 ping statistics ---
2 packets transmitted, 2 received, 0% packet loss, time 1018ms
rtt min/avg/max/mdev = 0.051/0.063/0.075/0.012 ms
root at Ubuntu-2204-jammy-amd64-base ~ # ping 2a01:4f9:2b:35a:c6dc::2
PING 2a01:4f9:2b:35a:c6dc::2(2a01:4f9:2b:35a:c6dc::2) 56 data bytes
64 bytes from 2a01:4f9:2b:35a:c6dc::2: icmp_seq=1 ttl=63 time=0.494 ms
64 bytes from 2a01:4f9:2b:35a:c6dc::2: icmp_seq=2 ttl=63 time=0.504 ms
64 bytes from 2a01:4f9:2b:35a:c6dc::2: icmp_seq=3 ttl=63 time=0.516 ms
^C
--- 2a01:4f9:2b:35a:c6dc::2 ping statistics ---
3 packets transmitted, 3 received, 0% packet loss,

Small note here, you can see the overhead of virtualization here, which
tries to batch network traffic together before toggling in and out of the
VM, as I understand it.

But, moving onto the address ::3 or any other higher address the VM is not
listening for, we get another change in behavior:

root at Ubuntu-2204-jammy-amd64-base ~ # ping 2a01:4f9:2b:35a:c6dc::3
PING 2a01:4f9:2b:35a:c6dc::3(2a01:4f9:2b:35a:c6dc::3) 56 data bytes
>From 2a01:4f9:2b:35a:c6dd:: icmp_seq=1 Destination unreachable: Address
unreachable
>From 2a01:4f9:2b:35a:c6dd:: icmp_seq=2 Destination unreachable: Address
unreachable
>From 2a01:4f9:2b:35a:c6dd:: icmp_seq=3 Destination unreachable: Address
unreachable

It's not an entirely offensive change in behavior, but it is /different/,
and somewhat more complex than the former scenario (where packets are
forwarded without anything interesting going on in the host) because the
tap device has to have a footprint in the guest VM's network and start
processing traffic on it.

These are the interesting transitions I noticed making this change for
dnsmasq, and they're somewhat orthogonal to there being a lot of address
space in IPv6.
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