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1 DNS SRV Usage
This configuration note will help you to add some configuration on the Mediatrix gateways to
contact a backup server in case of failure with the first one or to do load balancing.
If any of the SIP server parameters
corresponds to a FQDN that is bound to a SRV record, the corresponding port must be set to
for the unit to perform DNS requests of type SRV (as per RFC 3263). Otherwise, the
unit will not use DNS SRV requests, but will rather use type A requests because it does not
need to have a specified port. We now look at the two types of DNS queries.
The Mediatrix unit will keep the DNS responses it has received in cache for the remainder of the TTL field specified in the DNS response. If you make modifications to your DNS server configuration and want the Mediatrix unit to reissue DNS requests before the end of the TTL, you will need to enter the following command in the CLI or SNMP: Hoc.ClearDnsCache
2 DNS SRV ( RFC 2782)
DNS SRV is an extension of the standard DNS server specification (independent from SIP, as per
RFC 2782). SRV (Service Record) is a type of entry a network administrator may put into the DNS
server. A DNS SRV request is used to get one or more IP addresses of servers, each one having its
own weight, priority and possible port.
Each entry received when using DNS SRV, depending on its weight and priority, can be used as a
primary or backup server or can be part of a load balancing system.
For instance, the client requests the SRV for SIP servers in some domain. The DNS server may
return the A, B, and C addresses, which are all SIP servers. Each address has a weight and the
client must choose one of those three addresses by using an algorithm that considers the
To use DNS SRV, an administrator must set a service records (SRV) into the DNS servers
available on the network.
3 Type A Query
If you specify a SIP port for the registrar and proxy, the Mediatrix unit will issue a type A
query. In this example the requests are sent to server1.media5berlin.com for both the Registrar
and Proxy, with the SIP port being 5060 for both.
Wireshark displays the answer to the query as a “type A” answer, which contains the IP address
for server1.media5berlin.com. The Mediatrix unit then attempts to register itself to that IP
4 Type A Query to a SRV Record
In the following example, the Mediatrix unit administrator is told to use “media5berlin.com” as
FQDN for his registrar and proxy, but is unaware that he should use SRV for his DNS queries.
Consequently he does not configure his registrar and proxy ports to 0.
The Wireshark capture shows no additional SRV query and no registration, why?
By specifying the SIP port to 5060, the unit makes a standard A query, and since
media5berlin.com is configured as a SRV record, no address is returned. The symptom will be a
failed registration with the message “Registrar Unreachable”.
5 Type SRV Query
As previously stated, setting proxy and registrar ports to 0 will make the Mediatrix unit issue
a DNS request of type SRV.
The response contains 2 available SIP servers with the FQDN, IP addresses, priorities, weight
(for equal priority) and SIP ports.
At the bottom of the window you can see “Additional records” with server1.media5berlin.com and
server2.media5berlin.com. Those are 2 valid type A FQDNs which are offered in the SRV response.
If you wished to do so, you could also explicitly enter those FQDNs directly into your Mediatrix
proxy configuration field (as done in Scenario #1).
Please note that a NAPTR query is done before the SRV query. NAPTR is used to find Transport
method, UPD – TCP – TLS. The establishment of persistent (TLS) connections will not send NAPTR
since the transport is already known.
A NAPTR query is made if:
- The host is not an IP address
- And, the port is not explicity specified in the SIP URI (the port is not present or equal to
- And, the SIP URI does not contain a "maddr" with an IP address
- And, the SIP URI does not specify explicity the transport (transport parameter)
6 The Effects of Priority and Weight
In some rare cases you may have a SRV response where some servers are configured with equal
priority. In that case, the clients will use the weight values to determine which host to use. If
the weights are also identical, then 50% of the packets will go to host 1 and the rest to host 2
(in a 2 server scenario). In this example, both proxy1 and proxy2 have the same priority, but
different weights. 51% of the packets will go to proxy1 and 49% to proxy2.
This may cause an issue where the unit REGISTER is sent to host1 and, after the authentication
challenge is sent by the registrar, the answer is sent to host 2 as shown in these screenshots.
The initial REGISTER is sent to 192.168.120.11, and the response to the challenge sent to
192.168.120.10. If your hosts are not synchronized, you will get REGISTER or INVITE failures.
7 Additional Interop Variables
- DNS SRV implementation should imply a shared database between servers since a Register
and an INVITE can be sent to any server, not necessarily the same one;
- For those who do not share their database, this must be enabled, allowing INVITEs to
be sent to the same Registrar host, thus use the same SRV record
This variable can be used to get around the above-mentioned issue. Setting this
variable to “enable” makes the Mediatrix unit “stick” to the IP address associated with the
initial Call-Id of the REGISTER or INVITE.
If using DNS SRV and multiple entries are present, this value is the time spent waiting for
answers from each entry when one server is unreachable or unresponsive. The default value of
this variable is 32 seconds. It has a dramatic effect should a server time out, since a
default 32 seconds delay would be introduced at every call.
A maximum value of 2-3 seconds is recommended when using DNS SRV.
- The penalty box feature is used when a given host address times out. When the address
times out, it is put into the penalty box for a given amount of time. During that time,
the address in question is considered as 'non-responding' for all requests.
- A “timed out” server is considered “not responding” for this amount of time;
- Can be seen as the time it will take to retry a server that failed to respond.
8 Available Documentation
9 Copyright Notice
Copyright © 2019 Media5 Corporation.
This document contains information that is proprietary to Media5 Corporation.
Media5 Corporation reserves all rights to this document as well as to the Intellectual Property
of the document and the technology and know-how that it includes and represents.
This publication cannot be reproduced, neither in whole nor in part, in any form whatsoever,
without written prior approval by Media5 Corporation.
Media5 Corporation reserves the right to revise this publication and make changes at any time
and without the obligation to notify any person and/or entity of such revisions and/or