Re: Transport: Multiple routes to internal domain
On Tue, 7 Feb 2012, Lorens Kockum wrote: On Tue, Feb 07, 2012 at 11:42:37AM +1100, Greg Wilson wrote: I use this technique, DNS round robin to evenly spread rdp connections to our terminal servers. My understanding is that a device does a DNS lookup and the server hands out each different IP address sequentially. Each device uses (caches) the first IP address it recieves until it's rebooted. That might be true for rdp connections to your terminal servers, but (luckily!) it's a bit more complicated. The DNS server sends a reply containing all the IP addresses. The order varies. If the code asking the question only wants one address, then it takes only one, and the next time it asks the question it might get another answer. Postfix, however, understands that there are multiple addresses and tries them all, one after the other in a random order until one replies or until it has tried them all and decides to wait before trying again. After waiting Postfix will make another DNS lookup. The response to that lookup may be cached since the previous lookup, but the cache is valid only until the record's time to live (TTL) expires. The TTL is set in the DNS server. It can be anything from zero (do not cache) to a week or more. A common default value is 24 hours. Of course, in the case of a long-lived TCP connection you may have the impression that the IP address is used until the device is restarted, and you might even be correct (for an RDP client running on Windows, for example), but that is not the case when sending mails. Is it possible to setup multiple transport records wth different costs to a domain? This may be a solution. You can do unequal load balancing by creating multiple records with the same IP. That could work using /etc/hosts, you'd have to test. You can do priorities (first try X and if it doesn't work try Y then Z) by using different-priority MX records. This needs access to your DNS server, and cannot be done with /etc/hosts. Hope this helps. As an alternative, it guess it would suffice to put an MX with lower priority in your dns to the backup server, so if mastern in your cluster is down postfix would then try next one. But what Loren wrote before creating loadbalancing with round-robin is probably what you want.
Re: Transport: Multiple routes to internal domain
On Tue, Feb 07, 2012 at 04:32:14PM +1100, Greg Wilson wrote: I've been testing Postifx for some solutions... One attempt was to make 2 entries with the same host name in /etc/hosts e.g 10.222.100.1 exchange.mydomain.local exchange 10.333.200.2 exchange.mydomain.local exchange IPv5? I haven't seen many of those quads255. :) Then changed the transport map to mydomain.local smtp:[exchange.mydomain.local] My info is that the square brackets stop Postifix doing mx record lookups. And instead, do a DNS A lookup. To use hosts(5) resolution specify smtp_host_lookup to include native: http://www.postfix.org/postconf.5.html#smtp_host_lookup This didn't work and I don't know why. It works fine with an IP address in the square brackets and the Linux box can resolve exchange.mydoamin.local to an IP. By default smtp_host_lookup is dns only. Any suggestions? Also, please don't top-post your replies. Your MUA badly mangled quoting in the rest of the thread so I didn't bother to try to fix that. Thanks. -- http://rob0.nodns4.us/ -- system administration and consulting Offlist GMX mail is seen only if /dev/rob0 is in the Subject:
Re: Transport: Multiple routes to internal domain
Greg Wilson: One attempt was to make 2 entries with the same host name in /etc/hosts e.g 10.222.100.1 exchange.mydomain.local exchange 10.333.200.2 exchange.mydomain.local exchange Then changed the transport map to mydomain.local smtp:[exchange.mydomain.local] My info is that the square brackets stop Postifix doing mx record lookups. This didn't work and I don't know why. It works fine with an That 's because LINUX does not support multiple /etc/hosts records per name. Use a better OS, use DNS, or use my smtp_fallback_relay solution. Wietse
RE: Transport: Multiple routes to internal domain
-Original Message- From: owner-postfix-us...@postfix.org [mailto:owner-postfix-us...@postfix.org] On Behalf Of Wietse Venema Sent: 07 February 2012 13:33 To: Greg Wilson Cc: Postfix users Subject: Re: Transport: Multiple routes to internal domain Greg Wilson: One attempt was to make 2 entries with the same host name in /etc/hosts e.g 10.222.100.1 exchange.mydomain.local exchange 10.333.200.2 exchange.mydomain.local exchange Then changed the transport map to mydomain.local smtp:[exchange.mydomain.local] My info is that the square brackets stop Postifix doing mx record lookups. This didn't work and I don't know why. It works fine with an That 's because LINUX does not support multiple /etc/hosts records per name. Use a better OS, use DNS, or use my smtp_fallback_relay solution. Wietse Hi Wietse, When you say a better OS, I'd be interested to know what your preference is. Kind regards, James Day (IT Engineer)
Re: Transport: Multiple routes to internal domain
On Tue, Feb 7, 2012 at 9:03 AM, Wietse Venema wie...@porcupine.org wrote: Greg Wilson: One attempt was to make 2 entries with the same host name in /etc/hosts e.g 10.222.100.1 exchange.mydomain.local exchange 10.333.200.2 exchange.mydomain.local exchange Then changed the transport map to mydomain.local smtp:[exchange.mydomain.local] My info is that the square brackets stop Postifix doing mx record lookups. This didn't work and I don't know why. It works fine with an That 's because LINUX does not support multiple /etc/hosts records per name. Use a better OS, use DNS, or use my smtp_fallback_relay solution. Wietse Eh, Linux *does* support that, from man host.conf (/etc/host.conf): multi Valid values are on and off. If set to on, the resolv+ library will return all valid addresses for a host that appears in the /etc/hosts file, instead of only the first. This is off by default, as it may cause a substantial performance loss at sites with large hosts files.
Re: Transport: Multiple routes to internal domain
Lorens, Thank you for the detailed reply. I'm interested to hear that some of the DNS smarts reside in postfix itself. As I had no luck with multiple records in the /etc/hosts file, I've gone to the DNS option and setup multiple A records for exchange.xxx.local, using that name in the Postfix's transport file. I can now see that mail is coming in via our backup MS Exchange server, which is fine. As long as Postfix is smart enough to send email to the other IP address for this record if one of the Exchange servers is down, all is well. Thanks again, Greg. On Tue, Feb 07, 2012 at 11:42:37AM +1100, Greg Wilson wrote: I use this technique, DNS round robin to evenly spread rdp connections to our terminal servers. My understanding is that a device does a DNS lookup and the server hands out each different IP address sequentially. Each device uses (caches) the first IP address it recieves until it's rebooted. That might be true for rdp connections to your terminal servers, but (luckily!) it's a bit more complicated. The DNS server sends a reply containing all the IP addresses. The order varies. If the code asking the question only wants one address, then it takes only one, and the next time it asks the question it might get another answer. Postfix, however, understands that there are multiple addresses and tries them all, one after the other in a random order until one replies or until it has tried them all and decides to wait before trying again. After waiting Postfix will make another DNS lookup. The response to that lookup may be cached since the previous lookup, but the cache is valid only until the record's time to live (TTL) expires. The TTL is set in the DNS server. It can be anything from zero (do not cache) to a week or more. A common default value is 24 hours. Of course, in the case of a long-lived TCP connection you may have the impression that the IP address is used until the device is restarted, and you might even be correct (for an RDP client running on Windows, for example), but that is not the case when sending mails. Is it possible to setup multiple transport records wth different costs to a domain? This may be a solution. You can do unequal load balancing by creating multiple records with the same IP. That could work using /etc/hosts, you'd have to test. You can do priorities (first try X and if it doesn't work try Y then Z) by using different-priority MX records. This needs access to your DNS server, and cannot be done with /etc/hosts. Hope this helps.
Transport: Multiple routes to internal domain
I'm using Postfix\Amavis\Spamassassin\Clamav to scan incoming emails for virus and spam and forward to our internal MS Exchange email system. Postifx shares the Internet domain with MS Exchange, each forwarding messages to unknown accounts to the other system. MS Exchange uses clustering to duplicate it's mailbox databases to 2 different servers. How do I setup Postfix to automatically forward messages to one of the MS Exchange servers if the other one goes offline? I currently use an IP number in square brackets in Postfix's transport map to route to one of the MS exchange servers. Thanks, Greg. I'm using Postfix\Amavis\Spamassassin\Clamav to scan incoming emails for virus and spam and forward to our internal MS Exchange email system.Postifx shares the Internet domain with MS Exchange, each forwarding messages to unknown accounts to the other system.MS Exchange uses clustering to duplicate it's mailbox databases to 2 different servers. How do I setup Postfix to automatically forward messages to one of the MS Exchange servers if the other one goes offline?I currently use an IP number in square brackets in Postfix's transport map to route to one of the MS exchange servers.Thanks,Greg.
Re: Transport: Multiple routes to internal domain
Hi! I would answer you with another question: In the event of a server failure, how are users directed to the other server? I mean, there are several ways of doing that, and knowing your current one would be helpful. Sincerely, Ildefonso Camargo. On Mon, Feb 6, 2012 at 6:57 PM, Greg Wilson gr...@actionforms.com.au wrote: I'm using Postfix\Amavis\Spamassassin\Clamav to scan incoming emails for virus and spam and forward to our internal MS Exchange email system. Postifx shares the Internet domain with MS Exchange, each forwarding messages to unknown accounts to the other system. MS Exchange uses clustering to duplicate it's mailbox databases to 2 different servers. How do I setup Postfix to automatically forward messages to one of the MS Exchange servers if the other one goes offline? I currently use an IP number in square brackets in Postfix's transport map to route to one of the MS exchange servers. Thanks, Greg.
Re: Transport: Multiple routes to internal domain
Greg Wilson: 2 different servers. How do I setup Postfix to automatically forward messages to one of the MS Exchange servers if the other one? goes offline? Two options: A) Assuming that you use the relay transport in master.cf (which you should if relaying mail from outside): /etc/postfix/master.cf: relay unix - - n - - smtp -o smtp_fallback_relay=[1.2.3.4] B) In the transport map, specify a hostname that resolves to TWO IP addresses. Make up your own DNS or /etc/hosts entry if you need to. Wietse
Re: Transport: Multiple routes to internal domain
Thanks for the advice. I don't use relay maps for the domain as the Internet doamin is shared betweent he MS Exchange server and the Postfix server. i.e I use MySql address lookups onthe Postfix server so it's easy to create addresses to forward email to different or multiple accounts. e.g sa...@xxx.com I don't think that multiple DNS records will work. (Although I may be wrong.) I use this technique, DNS round robin to evenly spread rdp connections to our terminal servers. My understanding is that a device does a DNS lookup and the server hands out each different IP address sequentially. Each device uses (caches) the first IP address it recieves until it's rebooted. i.e When the primary MS Exchange server went offline, Postifx wouldn't necessarally do another DNS looup and find the second record. Is it possible to setup multiple transport records wth different costs to a domain? This may be a solution. Cheers, Greg. Greg Wilson: 2 different servers. How do I setup Postfix to automatically forward messages to one of the MS Exchange servers if the other one goes offline? Two options: A) Assuming that you use the relay transport in master.cf (which you should if relaying mail from outside): /etc/postfix/master.cf: relay unix - - n - - smtp -o smtp_fallback_relay=[1.2.3.4] B) In the transport map, specify a hostname that resolves to TWO IP addresses. Make up your own DNS or /etc/hosts entry if you need to. Wietse
Re: Transport: Multiple routes to internal domain
I've been testing Postifx for some solutions... One attempt was to make 2 entries with the same host name in /etc/hosts e.g 10.222.100.1 exchange.mydomain.local exchange 10.333.200.2 exchange.mydomain.local exchange Then changed the transport map to mydomain.local smtp:[exchange.mydomain.local] My info is that the square brackets stop Postifix doing mx record lookups. This didn't work and I don't know why. It works fine with an IP address inthe square brackets and the Linux box can resolve exchange.mydoamin.local to an IP. Any suggestions? Thanks for the advice. I don't use relay maps for the domain as the Internet doamin is shared betweent he MS Exchange server and the Postfix server. i.e I use MySql address lookups onthe Postfix server so it's easy to create addresses to forward email to different or multiple accounts. e.g sa...@xxx.com I don't think that multiple DNS records will work. (Although I may be wrong.) I use this technique, DNS round robin to evenly spread rdp connections to our terminal servers. My understanding is that a device does a DNS lookup and the server hands out each different IP address sequentially. Each device uses (caches) the first IP address it recieves until it's rebooted. i.e When the primary MS Exchange server went offline, Postifx wouldn't necessarally do another DNS looup and find the second record. Is it possible to setup multiple transport records wth different costs to a domain? This may be a solution. Cheers, Greg. Greg Wilson: 2 different servers. How do I setup Postfix to automatically forward messages to one of the MS Exchange servers if the other one goes offline? Two options: A) Assuming that you use the relay transport in master.cf (which you should if relaying mail from outside): /etc/postfix/master.cf: relay unix - - n - - smtp -o smtp_fallback_relay=[1.2.3.4] B) In the transport map, specify a hostname that resolves to TWO IP addresses. Make up your own DNS or /etc/hosts entry if you need to. Wietse
Re: Transport: Multiple routes to internal domain
On Tue, Feb 07, 2012 at 11:42:37AM +1100, Greg Wilson wrote: I use this technique, DNS round robin to evenly spread rdp connections to our terminal servers. My understanding is that a device does a DNS lookup and the server hands out each different IP address sequentially. Each device uses (caches) the first IP address it recieves until it's rebooted. That might be true for rdp connections to your terminal servers, but (luckily!) it's a bit more complicated. The DNS server sends a reply containing all the IP addresses. The order varies. If the code asking the question only wants one address, then it takes only one, and the next time it asks the question it might get another answer. Postfix, however, understands that there are multiple addresses and tries them all, one after the other in a random order until one replies or until it has tried them all and decides to wait before trying again. After waiting Postfix will make another DNS lookup. The response to that lookup may be cached since the previous lookup, but the cache is valid only until the record's time to live (TTL) expires. The TTL is set in the DNS server. It can be anything from zero (do not cache) to a week or more. A common default value is 24 hours. Of course, in the case of a long-lived TCP connection you may have the impression that the IP address is used until the device is restarted, and you might even be correct (for an RDP client running on Windows, for example), but that is not the case when sending mails. Is it possible to setup multiple transport records wth different costs to a domain? This may be a solution. You can do unequal load balancing by creating multiple records with the same IP. That could work using /etc/hosts, you'd have to test. You can do priorities (first try X and if it doesn't work try Y then Z) by using different-priority MX records. This needs access to your DNS server, and cannot be done with /etc/hosts. Hope this helps.
