[PATCH 1/1] dns64: Fall back to plain AAAA query with synthall but no A records

[Daniel Gröber]

Networks which only have tunneled IPv6 access but still want to go
IPv6-only internally can use unbound's DNS64 module together with the
dns64-synthall or dns64-ignore-aaaa options to direct most traffic (any
dualstack domain) to their NAT64.

There is only one problem with this setup, currently domains with only AAAA
records will fail to resolve.

To allow for this use-case arrange for the A sub-query to make the AAAA
super query advance along the module stack when no records are returned.

Signed-off-by: Daniel Gröber <dxld at darkboxed.org>
---
I kept this initial hakcy version as simple as possible, but IMO the
whole DNS64 module needs a bit of a refactoring to do this cleanly.

I'm not entirely sure if the fallback behaviour should be controlled
by a config option or not. I'm open to suggestions.

 dns64/dns64.c | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/dns64/dns64.c b/dns64/dns64.c
index b856a86c..3cb60881 100644
--- a/dns64/dns64.c
+++ b/dns64/dns64.c
@@ -981,6 +981,17 @@ dns64_inform_super(struct module_qstate* qstate, int id,
 		return;
 	}
 
+	/* When no A record is found for synthesis fall back to AAAA again. */
+	if (qstate->qinfo.qtype == LDNS_RR_TYPE_A &&
+	    qstate->return_rcode == LDNS_RCODE_NOERROR &&
+	    !( qstate->return_msg &&
+	       qstate->return_msg->rep &&
+	       reply_find_answer_rrset(&qstate->qinfo, qstate->return_msg->rep)))
+	{
+		super_dq->state = DNS64_INTERNAL_QUERY;
+		return;
+	}
+
 	/* Use return code from A query in response to client. */
 	if (super->return_rcode != LDNS_RCODE_NOERROR)
 		super->return_rcode = qstate->return_rcode;
-- 
2.39.2