On Tue, Dec 27, 2011 at 06:12:12PM +0100, Michael Maymann wrote: > Hi all, > > Thanks Peter, for you kind reply - some setup you have there... sounds very > nice indeed...:-) ! > - If i have a lower budget, can this then be achieved without the > loadbalancers and still have same redundancy/flexibility (using e.g. "DNS > RR"/"MX with equal value") - if so what is for/against/preferred ?:
I looked over the rest of the thread and I suspect people are talking about different things. If I understand correctly, you want a relay. You have a lot of servers with a primary function that is not sending mail, but which do send mail, and you want to relay all the mail out through a set of controlled dedicated mail servers. Am I right? If so, the basic question is *how* the servers send mail. Either the applications send mail directly to a hostname (Java Mail or PHP for example), or they use the local mailer, which would be postfix, I suppose, with a default smarthost configuration pointing to your dedicated mail servers. Pros and Cons: - Not using local mailer wil permit loadbalancing mail sent from a single host over several postfix instances. - Using local mailer will always work for all applications (since applications that send to a hostname can send to 127.0.0.1) - Using local mailer forces you to monitor the daemon and the queues on all the machines, and takes up (probable negligable) system resources - Using local mailers will give you the UID of the sending process in the headers - Using local mailer protects you from a short outage of the dedicated servers or some part of the network. Mail will be spooled locally until the dedicated machines come back on line. - Conversely, not using a local mailer will protect you from local failures such as full disks or postfix not running, but expose you more to network problems and availability problems. That will cause you to look at redundant load balancers. - Using a load balancer will probably require you to mask source IPs. That doesn't matter if you trust your servers or if you run local firewalls forcing mail to run through the local mailer. If you worry about client-written forms being exploited to send spam you need to think about that. > DNS RR: so just have like load-sharing (mail1->postfix1, mail2->postfix2, > mail3->postfix1, etc.). But if one postfix servers goes down, will all DNS > replies then be only for alive-postfix - or will there also be dead-postfix > replies that needs to timeout, before it retries (and for how many times?) > and potentially end up dropping the mail if it is so unlucky to get replies > for dead-postfix on all retries ? > "MX with equal value": is this handling differently? does a request load > all MX records for the domain, and then sort them by value and then > alphabetically, ending up with: if one postfix is down it will > automatically try the next one in the sorted list...? If you use a redundant load balancer, it will take care of all that and "always" reply. Unless the network goes down, of course. If you do not, then there will be timeouts if something goes down. You can specify relayhosts with or without brackets; the brackets stop MX lookups. I seem to remember that in postfix a relayhost that resolves to several IP addresses will be handled more or less the same as a relayhost the has several MX records. I think that wondering about which is more efficient is not very useful since the difference is certainly vanishingly small. Using MX permits you to specify main servers and backup servers, but that's about it. However, non-mail applications that send mail directly will probably not be able to handle anything else than a single host/IP correctly. So . . . is there a unique answer . . . probably not, need more info on your situation and needs :-)
