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What is Server Failover and how does it work?

Server Failover is a feature that works alongside Authoritative DNS to ensure the availability of critical services prior to answering DNS queries. The Server Failover feature can load balance and automatically failover inbound traffic to multiple servers - whether the servers are onsite or offsite. Any server combination is possible including all onsite, all offsite, or a mixture of both.

Each server is tested for a Layer 4 connection as the Ecessa appliance will attempt to connect to the specified service on the defined TCP port. If the connection is successful, the server is considered responsive and the service available.

The operational server with the highest priority will have incoming traffic routed to it via DNS, and if necessary port forwarding rules. If multiple servers or sites are selected with a PRIMARY priority level, the traffic will be load balanced. All other operational servers with lower priority are considered spares. If the service fails connectivity testing, the server is considered down. Priority levels include Disabled, Primary, Backup, 2nd Backup, and 3rd Backup.

To add a service, enter the name of the service then click on the Add Service button. Enter the name, IP address, service port, and priority level for each server that will be used for the defined service. If the server is located offsite, select the Offsite check box.

For servers that are onsite, enter the WAN IP addresses and port that will be used to answer DNS requests. Specify the fully qualified domain name for the service that will be used for DNS. The Redundancy Only check box determines how DNS requests will be answered. If not selected, the Ecessa appliance will respond with all specified WAN IP addresses. If selected, the Ecessa appliance will respond with the WAN IP addresses in sequential order depending on WAN status (WAN IP #1 will be used unless the WAN line is down, then WAN IP #2 will be used, and so on).

For servers that are offsite, the Ecessa will answer DNS requests with the IP address of the highest priority server.

The following diagram shows a basic Server Failover setup:

 Server Failover_Diagram.png

Here is a screenshot showing the status of the three servers:

Server_Failover-Example.png

As the screenshot shows, the server at 192.168.10.10 is responding on port 80 while the server 192.168.10.11 is not. Since the Primary server is available, the Ecessa will respond to DNS requests with the specified WAN IP addresses:

C:\>dig @198.51.100.130 www.example.com

; <<>> DiG 9.9.1-P3 <<>> @192.168.3.201 www.example.com
; (1 server found)
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 23141
;; flags: qr aa rd; QUERY: 1, ANSWER: 2, AUTHORITY: 2, ADDITIONAL: 3
;; WARNING: recursion requested but not available

;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 4096
;; QUESTION SECTION:
;www.example.com. IN A

;; ANSWER SECTION:
www.example.com. 30 IN A 198.51.100.134
www.example.com. 30 IN A 203.0.113.70

;; AUTHORITY SECTION:
example.com. 900 IN NS ns1.example.com.
example.com. 900 IN NS ns2.example.com.

;; ADDITIONAL SECTION:
ns1.example.com. 900 IN A 198.51.100.130
ns2.example.com. 900 IN A 203.0.113.66

;; Query time: 42 msec
;; SERVER: 198.51.100.130#53(198.51.100.130)
;; WHEN: Fri Apr 26 12:30:10 2013
;; MSG SIZE rcvd: 159

In the event both the Primary and Backup servers are offline, the Ecessa will respond with the offsite server’s IP address:

C:\>dig @198.51.100.130 www.example.com

; <<>> DiG 9.9.1-P3 <<>> @192.168.3.204 www.example.com
; (1 server found)
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 3849
;; flags: qr aa rd; QUERY: 1, ANSWER: 1, AUTHORITY: 2, ADDITIONAL: 3
;; WARNING: recursion requested but not available

;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 4096
;; QUESTION SECTION:
;www.example.com. IN A

;; ANSWER SECTION:
www.example.com. 30 IN A 192.0.2.200

;; AUTHORITY SECTION:
example.com. 900 IN NS ns1.example.com.
example.com. 900 IN NS ns2.example.com.

;; ADDITIONAL SECTION:
ns1.example.com. 900 IN A 198.51.100.130
ns2.example.com. 900 IN A 203.0.113.66

;; Query time: 3 msec
;; SERVER: 198.51.100.130#53(198.51.100.130)
;; WHEN: Fri Apr 26 12:11:03 2013
;; MSG SIZE rcvd: 143

 

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