Re: [PATCH v2 2/2] fwd: Broaden what we consider for DNS specific forwarding rules

[Paul Holzinger <pholzing@redhat.com>]

On 25/07/2024 06:44, David Gibson wrote:
> On Wed, Jul 24, 2024 at 04:30:50PM +0200, Stefano Brivio wrote:
>> On Wed, 24 Jul 2024 11:41:44 +0200
>> Paul Holzinger <pholzing@redhat.com> wrote:
>>
>>> Hi,
>>>
>>> On 24/07/2024 09:51, David Gibson wrote:
>>>> passt/pasta has options to redirect DNS requests from the guest to a
>>>> different server address on the host side.  Currently, however, only UDP
>>>> packets to port 53 are considered "DNS requests".  This ignores DNS
>>>> requests over TCP - less common, but certainly possible.  It also ignores
>>>> encrypted DNS requests on port 853.
>>>>
>>>> Extend the DNS forwarding logic to handle both of those cases.
>>> The question here is if it handles DoT should it handle DoH as well,
>>> i.e. https (443)?
> My first inclination was, no, because for traffic to port 443 we can't
> be confident it's actually DNS.  But, then again, maybe going to an
> address marked as a DNS server address is good enough?  I'm not sure.
>
>> We don't have a flexible interface, yet, to finely configure outbound
>> traffic redirections, so the user couldn't enable or disable this at
>> will. So I'm wondering if there's any use case that we risk breaking
>> with that.
>>
>> The most confusing case I can think of is a host with a local resolver
>> with a loopback address (for example, the usual 127.0.0.53 from
>> systemd-resolved). Without --no-map-gw (or with Podman's --map-gw), we
>> will, by default, use the address of the default gateway (which maps to
>> the host) as implied --dns-forward option.
>>
>> If we now match on HTTPS as well, HTTPS traffic that's supposed to
>> reach the host (because there's an HTTPS server there) will anyway reach
>> the host, even if we mishandle it as DNS traffic somehow.
>>
>> So I don't actually see an issue with that, but given that users can't
>> disable just HTTPS (this should be easier to implement with the flow
>> table, but it will surely be a while before we get to that), we should
>> think quite hard if there's any possibility of breakage before going
>> ahead with it.
> Yeah, that argument inclines me back towards "no" for DoH, at least
> for the time being.
Ok, I agree.
>
>>>> Link: https://github.com/containers/podman/issues/23239
>>>>
>>>> Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
>>> Tested-by: Paul Holzinger <pholzing@redhat.com>
>>>
>>> I tested both dns over tcp and dns over tls with dig.
>> Thanks!
>>
-- 
Paul Holzinger