Thank you for the question. If you use any kind of Real Time Blacklisting (RBL) technology (such as in spamassassin, or RBL rules, etc.) you should always run a local DNS. In fact, you should
always run a local DNS no matter what you are doing, theres just no reason not to - a local DNS will be so much much faster than a remote DNS server its like night and day. If you are using Plesk you should already have a local DNS server, so just make sure you add 127.0.0.1 to the first line in /etc/resolv.conf like this:
As for ASL, this does not have anything special to do with running ASL (or not running it). So, for ASL no you dont need
a local DNS.
With that said, you will need
a local DNS if you use any king of RBL technology, including spamassassin, other email antispam tools, web log analyzers, and so. If you use the WAF RBL rules, for example (which are disabled by default), you will want to have a local DNS. RBLs (again, like the ones in spamassassin) perform DNS lookups, and a local DNS will be several orders of magnitude faster than a remote DNS, so much so that you really need to have a local DNS. You will also experience full time outs with a remote DNS given the volume of traffic a local system generates these days (again, this is not specific to ASL, this includes ALL computers). And these elays can be quite large with a remote DNS server to the point that lookups will fail. No matter what you are are doing, a remote DNS server will always be slower than a local one, even for just plain old look ups. You will always see a huge performance gain if you have a local DNS server when doing DNS lookups, and as other things rely on DNS you'll see performance gains all over the system with a local DNS.
So, moral of the story: You should
always have a local DNS server, no matter what you are doing. You need
a local DNS server if you do DNS lookups to make decisions in realtime and block an action until the lookup completes. Again, this has nothing to do with ASL. Remote DNS servers, in any form, will always always always be slower than a local DNS. Did I mention that they are much slower than a local DNS?