Split DNS
Overview
Installations of Zimbra behind a firewall often require the creation of some form of split DNS, also called split-horizon or dual-horizon DNS. This is a DNS installation where machines receive different IP address answers to queries depending on whether they are (commonly) inside or outside a firewall.
This is because the Postfix mail system used by Zimbra performs a DNS lookup when attempting to route email to the back-end message store. Frequently, this is the same physical host as Postfix. The DNS server frequently returns the external address of the mail host, not the internal address. Depending on how the firewall and network are configured, the external address may not even be reachable from the mail host, and mail will not be delivered.
Split DNS avoids this problem by providing an internal DNS server (using bind) that can be used to resolve the internal address of the server. This guide will detail how to set up a very specific, single-host DNS server (i.e. bind) that can be installed on the Zimbra host itself so that it can resolve its own address. This should not be used for a multi-node Zimbra installation, and should not be used as the DNS server for any other hosts on your network.
It is possible to use a generalized split-horizon DNS server to perform this function, but it will need to be set up differently, and many people recommend against it because even a couple ms of delay can be too much on a heavily loaded system.
Configuring Bind on the Zimbra Server
Install Bind on Red Hat Enterprise Linux
Use up2date to download bind from Red Hat Network.
Install bind9 on Ubuntu/Kubuntu Hardy Heron
apt-get install bind9
You could also make sure it is installed from Synpatic Package Manager or Adept.
Edit the named.conf file
- Substitute your fully-qualified server name for server.example.com
- If named runs in a chroot'ed directory (i.e. /var/named/chroot), named.conf should be placed in
/etc/named/chroot/etc/named.conf and you should create a symbolic link to /etc/named.conf,
- i.e. ln -s /etc/named.conf /etc/named/chroot/etc/named.conf
- or ln -s /etc/bind/named.conf /etc/bind/named/chroot/etc/named.conf
- For Red Hat, edit: /etc/named.conf
- For Ubuntu/Kubuntu, edit: /etc/bind/named.conf.options
// Default named.conf generated by install of bind-9.2.4-2 options { directory "/var/named"; dump-file "/var/named/data/cache_dump.db"; statistics-file "/var/named/data/named_stats.txt"; forwarders { <address of current DNS server> ; }; }; include "/etc/rndc.key"; // We are the master server for server.example.com zone "server.example.com" { type master; file "db.server.example.com"; };
Make sure to set the forwarders to match the DNS servers currently in use on your system. The forwarders setting allows the server to query those DNS servers for any addresses for which it is not authoritative.
Create a /var/named/db.server.example.com zone file
- If named runs in a chroot'ed directory /var/named/chroot, db.server.example.com should be placed in /etc/named/chroot/var/named/db.server.example.com and you should create a symbolic link to /var/named/db.server.example.com
; ; Addresses and other host information. ; @ IN SOA server.example.com. hostmaster.server.example.com. ( 10118 ; Serial 43200 ; Refresh 3600 ; Retry 3600000 ; Expire 2592000 ) ; Minimum ; Define the nameservers and the mail servers IN NS <internal address of server> IN A <internal address of server> IN MX 10 server.example.com.
Change /etc/resolv.conf
- Change /etc/resolv.conf to use the Zimbra server as the primary DNS address.
- Also remember to change the search path to be the name of the Zimbra server.
Start named on the zimbra server
/etc/init.d/named start
Enable autostart of named on boot
chkconfig named on
Verify that this is working
To verify that your configuration of DNS is correct you should run the following commands on the Zimbra server itself (the expected output is in the boxes below the commands):
dig yourdomain.com mx
; <<>> DiG 9.3.6-P1-RedHat-9.3.6-4.P1.el5_4.2 <<>> yourdomain.com mx ;; global options: printcmd ;; Got answer: ;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 20907 ;; flags: qr aa rd ra; QUERY: 1, ANSWER: 3, AUTHORITY: 0, ADDITIONAL: 2 ;; QUESTION SECTION: ;yourdomain.com. IN MX ;; ANSWER SECTION: yourdomain.com. 7200 IN MX 30 mail.yourdomain.com. ;; ADDITIONAL SECTION: mail.yourdomain.com. 7200 IN A 192.168.1.30 ;; Query time: 4 msec ;; SERVER: 192.168.1.10#53(192.168.1.10) ;; WHEN: Thu Jul 15 14:38:48 2010 ;; MSG SIZE rcvd: 140
dig yourdomain.com any
; <<>> DiG 9.3.6-P1-RedHat-9.3.6-4.P1.el5_4.2 <<>> yourdomain.com any ;; global options: printcmd ;; Got answer: ;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 36845 ;; flags: qr aa rd ra; QUERY: 1, ANSWER: 8, AUTHORITY: 0, ADDITIONAL: 4 ;; QUESTION SECTION: ;yourdomain.com. IN ANY ;; ANSWER SECTION: yourdomain.com. 7200 IN NS ns1.yourdomain.com. yourdomain.com. 7200 IN A 192.168.1.30 yourdomain.com. 7200 IN SOA yourdomain.com. admin. 2010051304 10800 3600 1814400 7200 yourdomain.com. 7200 IN MX 10 mail.yourdomain.com. yourdomain.com. 7200 IN NS ns2.yourdomain.com. ;; ADDITIONAL SECTION: mail.yourdomain.com. 7200 IN A 192.168.1.30 ns2.yourdomain.com. 7200 IN A 192.168.1.11 ns1.yourdomain.com. 7200 IN A 192.168.1.10 ;; Query time: 11 msec ;; SERVER: 192.168.1.10#53(192.168.1.10) ;; WHEN: Thu Jul 15 14:38:52 2010 ;; MSG SIZE rcvd: 367
host `hostname`
mail.yourdomain.com has address 192.168.1.30
NOTE: The host `hostname' command should be typed exactly as you see it using backticks not single quotes and don't change the word "hostname" to anything else.
You should also note that the output on your system may be slightly different than above examples but there should be an A record that points to the LAN IP address of your Zimbra server and an MX record that contains the FQDN (Fully Qualified Domain Name - that's the hostname plus the domain name and it's mail.yourdomain.com in the examples) of your Zimbra server.
You should also make sure that the DNS server that is responding to your dig commands is the one you have configured on your LAN and it's the one that has your Zimbra server DNS records. If you see any IP that is not the correct LAN IP or the correct DNS server then you have entered the wrong information in your DNS configuration files.
If you're asked in the forums to provide the information to confirm your DNS is correct then you should also supply the output of the following commands (run on your Zimbra server):
cat /etc/resolv.conf cat /etc/hosts
If you have a number of servers inside the firewall that need to use internal addresses to communicate to each other, you should consider setting up a full internal DNS server that can be authoritative for the whole domain. This example is not suitable for this task.
For information on performing the same task w/ TinyDNS / DJBDNS: http://www.fefe.de/djbdns/#splithorizon
Additional Information: Zimbra Power Tip: http://www.zimbra.com/blog/archives/2007/06/making_zimbra_bind_work_together_1.html