I am working again on moving the DNS name resolution outside of the input thread of those sources where this is potentially time-consuming and affecting message acceptance rates. As it turned out, currently imudp seems to be the only case.
While this is potentially easy to do, a problem is ACLs ($AllowedSender) which use system names rather than ip addresses. In order to check these ACLs, we need to do a DNS lookup. Especially in the case of UDP, such a lookup may actually case message loss and thus may be abused by an attacker to cause a certain degree of denial of service (what also points out that these types of ACLs are not really a good idea, even though requested by practice).
In the light of this, I will now do something that sounds strange at first: I will always accept messages that require DNS lookups and enqueue these into the main queue and do the name resolution AND the final name-based ACL check only on the queue consumer part. Please note that it will be done BEFORE message content is parsed, so there is no chance that buffer overlow attacks can be carried out from non-authenticated hosts. The core idea is to move the lengthy, potentially message-loss causing code, away from the input thread. The only questionable effect I can currently see is that queue space is potentially taken up by messages which will immediately be discarded and should not be there in the first place. At the extreme end, that could lead to loss of valid messages. But on the other hand valid messages are more likely to be lost by the DNS name query overhead if I do the ACL check directly in the input thread.
If anyone has an argument against this approach please let me know.