If the configuration of the DNS used with a Mediatrix Gateway is not done properly, it may cause delays. Therefore, understanding how DNS servers are used by the system will help to properly configure your DNS..
Up to four DNS servers can be used to resolve network names. The DNS servers can be specified statically or obtained automatically (for example through DHCP or PPP). DNS query results are cached on the system to optimise name resolution time. When the SOA record is processed, the Primary DNS server is never blacklisted when the authority server times out or fails, so the device will continue with SRV/A/AAA requests using the primary DNS server. By default, the device starts first with an NAPTR request and continues with SRV and A-Record requests. This behavior can be configured for the resolution of SIP server addresses via the SipEp.SupportedDnsQueries parameter (SIP/Misc/ DNS/Supported DNS Queries in the DGW Web interface).
In the eventuality of a DNS query timing out because of a server going down or an invalid server address provided by the user, the faulty server is marked as down. It will no longer be used to issue queries for a specified period of time. The amount of time increases depending on the number of valid DNS servers which is by default: 30 seconds times the total number of valid servers. Before sending a query, the downtime for each server is checked and the blacklisting flag is removed if the time interval has been reached, making it available to send the query.
When the device is restarted, several services start before the SIP service (e.g. NTP and CWMP) which may result in the blacklisting of one more DNS servers. A blacklisted DNS server might have records for the SIP service, which are not available (for any reason) on the other listed DNS servers. In this case, the blacklisted DNS server will not be requested for SIP URLs and therefore the SIP registration might not work, until the server is removed from the blacklist. Refer to DNS Server Configuration Best Practices .
To avoid delays when using DNS servers with a Mediatrix gateway, follow these simple best practices:
Make sure all SIP related URLs are available on the secondary DNS servers as
When the device is restarted, several services start before the SIP service
(eg. NTP, CWMP). A blacklisted DNS server might have records for the SIP service,
which are not available (for any reason) on the other listed DNS servers. In this
case, the blacklisted DNS server will be requested for SIP URLs and therefore the
SIP registration might not work, until the server is unlisted.
If handling SIP in IPv4 only, configure the
parameter (not in the Web page). For
more details, refer to the
published on our
Even if we set the DNS request to Address (A) only, delays can occur because
the device also does, by default, an AAAA request (IPv6).
If your DNS server does NOT use NAPTR or DNS SRV, you should consider using
in your configuration.
Each NAPTR and SRV request can take up to a few seconds on busy networks.
This delay can be doubled if using registration with authentication.
If you maintain your own DNS infrastructure, avoid CNAME records.
The best practice is to use the servers supplied by your Internet Service Provider (usually the primary and secondary DNS), then complement with publicly accessible DNS servers from a different network.
For example, when using IPv4: Google (126.96.36.199 and 188.8.131.52), CloudFlare (184.108.40.206 and 220.127.116.11), OpenDNS (18.104.22.168 and 22.214.171.124), Level3 (126.96.36.199 and 188.8.131.52), etc.Or when Using IPv6: Google (2001:4860:4860::8888 and 2001:4860:4860::8844), CloudFlare (2606:4700:4700::1111 and 2606:4700:4700::1001), etc.
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