[Dnsmasq-discuss] CNAME trouble with no AAAA

Petr Mensik pemensik at redhat.com
Tue Oct 29 19:41:00 GMT 2019


Hi Dominick,

more below

On 10/17/19 3:41 AM, Dominick C. Pastore wrote:
> Hello,
> 
> I'm having a bit of a problem with the "cname" option in Dnsmasq. I have some configuration options like these in dnsmasq.conf, where "host1" and "host2" have IPv4 addresses from DHCP:
> 
> domain=philadelphia.example.com
> local=/philadelphia.example.com/
> cname=git.example.com,host1.philadelphia.example.com
> cname=nas.example.com,host2.philadelphia.example.com
> 
> This works well for A lookups on git.example.com and nas.example.com, but the cname options are ignored for AAAA lookups. I think this is by design, since the man page says the target of a cname must be known or it will be ignored. (Although, maybe this is unintentional in this case? It does seem like a bit of a bug for a name to *sometimes* be a CNAME, depending on the request type.) Unfortunately, it's causing problems when the AAAA queries are forwarded upstream, but I'm not sure how to fix it since these servers don't have IPv6 addresses.
> 
> For some background:
> 
> The goal is to provide something like split-horizon DNS. Host1 and host2 reside behind NAT. On public DNS, philadelphia.example.com resolves to their public address, with git.example.com and nas.example.com both being CNAMEs to that name. But within the LAN, git.example.com and nas.example.com should be CNAMEs to their local names.
Is there any reason you want CNAME used? I think they unnecessary 
complicate the setup without obvious advantage. Just provide directly 
either local or public addresses for those names. Why is CNAME used?
> 
> The problem is, some clients cache their DNS requests. When these clients send a AAAA request, it gets forwarded upstream and they end up caching the public CNAME record. Then, they use the (incorrect) cached CNAME for A requests, too.
> 
> Is there a good way to solve this?
Just make sure client's DNS cache is flushed after connection type 
change. That is important for split-horizon.
> 
> Thanks,
> Nick
> 
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-- 
Petr Menšík
Software Engineer
Red Hat, http://www.redhat.com/
email: pemensik at redhat.com  PGP: 65C6C973




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