DNS Configuration

The DNS configuration of each MailCleaner node is configered in Configuration->Base System->DNS settings.

All of the values configured there, except the HELO option, are written and read directly from /etc/resolv.conf.

Use local DNS caching

Primary DNS server :  127.0.0.1

It is recommended that you set this to 127.0.0.1 (localhost). This will cause queries to be sent to the bind service running locally on MailClenaer. bind supports DNS caching, so although it will not know the answer the first time that it is asked, it will cache the result that is discovered downstream and reuse that result for as long as it remains in the cache. This will greatly speed up DNS lookups which can potentially cut down significantly on processing times.

If you already have a caching DNS server within your local network, then it would also be acceptable to use this instead of 127.0.0.1.

Secondary and Tertiary DNS servers

If you only have your local DNS server configured then any results that it does not currently have cached will require it to perform a hierarchical search from the root DNS servers down to the domain's authoritative server. This process can be slow, so having a secondary host, which is likely to do it's own caching of a much larger number of domains can speed things up.

Of course you can setup MailCleaner to use a DNS resolver within your network, if you have one, especially if you need to resolve a local, private zone. But be warned that MailCleaner use it intensively and it has to be reliable.

You can choose to use a public DNS server such as those provided by your ISP, or a large vendor like Google or CloudFlare. Note that ISP DNS servers are often slow, because it is not a product that they generally have much reward to optimize. Public DNS vendors are generally much faster, but they sometimes have usage limits, so be careful.

Caution

MailCleaner has many fields which accept a hostname or list of hostnames. It is important that these fields resolve correctly.

For example in Configuration->Domains->[select domain]->Delivery->Destination servers all the FQDNs that you define here must to be resolved. If you configure a hostname or MX record which has a different definition on a high priority nameserver within your network, than the public DNS for that hostname, you need to be sure that this is intended.

You must also be sure that, if MailCleaner is behind a firewall and you are using DNS other than one within that firewall, there are no limits on MailCleaner's access on port 53 so that it can perform external lookups.