voip pabx technical faq

networking

Can I reduce the PABX's reliance on DNS in case my DNS server dies?

NOTE: This applies only to older PABX releases. Newer systems will automatically use an IP address where prudent to reduce the reliance of the system on DNS.
The PABX has a simple built-in nameserver, which will always serve the PABX's own IP address for the hostname that it is configured to use. It is not possible to add any other hosts or addresses, which means it is not possible to use this facility as your main DNS server, but any unresolvable queries will be forwarded to the DNS servers that the PABX is configured to use.
Given this, there are a number of ways that this can be used as set out below. The "best" way is number 3) as it makes an exception only in the case of a failure, but the other methods are valid in some environments.
1) Manually configure all phone IP/DNS settings.
If it is not possible to change the DHCP or DNS server's settings at-all, then it is possible to manually configure all of the VoIP phone settings to use static IP addresses. This allows a DNS server to be specified manually for each phone, which can be set to point at the PABX instead of the usual DNS server. The PABX will always serve its own name/IP address automatically.
To manually configure phone IP/DNS values, change the settings on the "System -> Phone Hardware" screen of the PABX web interface. There is a significant overhead in managin this, so it is not recommended as a first choice.
WARNING: Ensure that there is no risk of duplicate IP addresses, do not use IP addresses that will be served by your DHCP server!
2) Use the PABX as the primary nameserver.
To use the PABX as a primary, it is necessary that the PABX hostname is put into a sub-domain of the main office domain name. For example, if the office domain is:
office.co.uk
then DO NOT name the PABX:
pabx.office.co.uk
instead, perhaps name it:
pabx.phones.office.co.uk
which is in a subdomain that will not interfere with the operation of the existing office DNS. Configure the PABX to use the office DNS as its own DNS server, and it should then be safe to set DHCP to refer to the PABX as the primary nameserver.
This solution is probably most suitable for environments where there is no internal DNS server at-all, as it removes the reliance on an ISP's DNS server, and the Internet link.
3) Use the PABX as a secondary nameserver.
This is an extension of option 2) above. Configure the PABX using a sub-domain as described, but instead of using the PABX as a primary nameserver, configure it as a secondary nameserver.
It will additionally be necessary to add the PABX subdomain to the existing primary nameserver as a "secondary" or "slave" from the PABX. This will cause the DNS data on the PABX to be transferred to the primary, and made available there.
If the primary nameserver dies, there may be an occasional pause when dialling calls (while the phone determinsed that the primary server is down), but the system will be largely unaffected.
An extension of the above in environments where a primary and secondary nameserver already exist, is to simply transfer/slave the subdomain from the PABX to both of the existing office DNS servers.