DNS SRV Support (SIP Mode)

Zenitel devices operating in SIP mode can use DNS SRV service discovery to automatically locate SIP servers, transport protocols, and port numbers. This eliminates the need for fixed SIP server addresses and ports, while enabling redundancy, simplified administration, and compatibility with cloud-based SIP services such as Cisco Webex Calling.

Benefits

Using DNS SRV records offers several advantages:

No need to configure a fixed SIP server port.
Support for multiple SIP servers and automatic failover.
Easier migration and maintenance for SIP service providers.
Automatic adaptation when providers add, remove, or move SIP servers.
Compatibility with cloud-based SIP services that depend on DNS SRV records.

Requirements

To use DNS SRV discovery, the following requirements must be met:

Station software version 9.6.3.0 or later.
A DNS server capable of resolving public SRV records.
When Outbound Transport is set to ANY, the DNS server must also support NAPTR record lookups.
The Outbound Proxy Port should be left empty or set to 0, unless the SIP provider specifically requires a fixed port.

DNS servers that only resolve local hostnames or standard A-records may prevent successful SIP registration when SRV discovery is required.

Configuration

To enable DNS SRV discovery:

Upgrade the station to version 9.6.3.0 or later.
Configure a DNS server under Main > Main Settings.
Enter the SIP provider domain name or outbound proxy hostname.
Leave the Outbound Proxy Port field empty or set it to 0.
Select the required transport protocol, or choose ANY to allow automatic transport selection.

How DNS SRV Discovery Works

A DNS SRV record tells the station:

Which SIP server to use
Which port to connect to
The preferred server priority
How traffic should be distributed between available servers

Example SRV record:

_sips._tcp.example.com. 300 IN SRV 10 60 5061 sip1.example.com.

When a station performs a DNS SRV lookup, it receives one or more available SIP servers and attempts to connect according to the priorities returned by DNS.

When DNS Discovery Is Used

The station resolves the configured Outbound Proxy if one is defined. Otherwise, it resolves the Server Domain.

Outbound Proxy Port Empty or 0: The station performs DNS-based service discovery using SRV records.
Outbound Proxy Port Configured: The station skips SRV discovery and performs a standard DNS lookup of the configured hostname. The configured port and transport are used.
No Outbound Proxy Configured: The station always performs DNS discovery using the configured Server Domain.
Existing Configurations: Existing SIP configurations that use a fixed server address and port continue to operate unchanged. DNS SRV discovery is only used when the configuration permits service discovery.

Transportation Selection

The DNS records used depend on the selected Outbound Transport setting.

Outbound Transport	DNS Query
UDP	_sip._udp.<domain>
TCP	_sip._tcp.<domain>
TLS	_sips._tcp.<domain>
ANY	NAPTR on <domain>, lookup followed by the referenced SRV records

Using ANY

When ANY is selected, the SIP provider determines which transport protocol should be used.

The station first checks for a NAPTR record. If no NAPTR records are available, the station attempts lookups in the following order:

_sips._tcp
_sip._tcp
_sip._udp

This means TLS is preferred whenever the provider offers it.

Failover and DNS Refresh

If multiple servers are returned by DNS:

The station maintains the list in priority order.
If the current server becomes unavailable, the station automatically attempts the next available server.
DNS information is refreshed according to the DNS record TTL.
Changes made by the SIP provider are applied automatically without requiring a station restart.

Backup SIP Servers

Backup domains and backup outbound proxies follow the same DNS discovery rules as the primary SIP server.

Each configured backup receives its own DNS server list. Whether the station switches to a backup server depends on the configured Registration Method, not the DNS system itself.

Cisco Webex Calling Example

DNS SRV support is required when connecting a Zenitel station directly to Cisco Webex Calling.

Webex publishes its SIP servers using SRV records and does not provide traditional SIP server A-records for registration purposes. As a result:

Outbound Transport must be set to TLS or ANY.
The Outbound Proxy Port must be left empty.
The configured DNS server must support public SRV record lookups.

For detailed instructions, see the Webex Calling Setup Guide.