[Asterisk-Users] DNS SRV records
tvo at enterzone.net
Wed Jun 2 11:16:25 MST 2004
> John Fraizer wrote:
>> Spoken like a true n00b13.
> If the current SIP bug isn't annoying enough to push people away from
> asterisk you just have to chip in your 2 cents worth to push things that
> little bit more...
>> You can *sometimes* get away with not having MX records. You can
>> *sometimes* get away with not having SVR records. Both record types
>> exist for a reason though.
> Oh so that's why SRV lookups are commented out in the default asterisk
> config, so you can't get anything?
> sip://username@***HOSTNAME*** works perfectly well... Before you berate
> others indescriminately remove your foot from your mouth next time so
> you don't look like as big of an ass next time...
Since you decided to keep this on the list, I will too.
MX and SRV records are used to determine the appropriate handler(s) for:
For example, if you had the following in the DNS zone for "domain.name":
DOMAIN IN NS your.name.server.
IN NS your.other.name.server.
IN A 10.1.1.1
IN MX 10 your.mail.exchanger.
IN MX 20 your.other.mail.exchanger.
IN SOA your.name.server. dnsadmin.your.domain.(
_sip._udp SRV 10 10 5060 1.your.sipproxy.domain.name.
_sip._udp SRV 10 20 5060 2.your.sipproxy.domain.name.
_sip._udp SRV 20 10 5060 3.your.sipproxy.domain.name.
1.your.sipproxy IN A 192.168.1.1
2.your.sipproxy IN A 192.168.1.2
3.your.sipproxy IN A 192.168.1.3
Email to you at domain.name would be processed by "your.mail.exchanger" and
not sent to "domain.name" at it's A record. The same goes for email to
you at even.more.fully.qualified.domain.name. In the event that
your.mail.exchanger was down, mail would be handled by
The purpose of the MX record is to be able to point the SMTP service
responsibility at a host OTHER than the FQDN that is being referenced in
The same works for SRV records. In the above example, if someone dialed
the URL "you at domain.name", the SIP INVITE would first be sent to either
"1.your.sipproxy.domain.name" at 192.168.1.1 or
"2.your.sipproxy.domain.name" at 192.168.1.2 (with the first one getting
the larger percentage of use based on its weight entry) and not to
domain.name at 10.1.1.1. If a sipproxy can't be reached at 192.168.1.1
or 192.168.1.3, the SIP INVITE will be sent to
"2.your.sipproxy.domain.name" at 192.168.1.3 based on it's priority as a
backup to the first two entries.
These records exist so that services can be hosted on machines OTHER
than the machine that matches "domain.name". They also allow for fail
over in the event of failure of a server or service.
It also allows you to point a service at a machine or group of machines
without having to give every person a new email address or SIP URL to use.
Next benefit: Say you have "big-fat-server.your.domain.name" and every
person on the planet uses "you at big-fat-server.your.domain.name" as your
email address and SIP URL. Now, you decide that you want to split the
tasks of email and SIP proxy off of "big-fat-server.your.domain.name"
and on to their own dedicated servers.
With MX and SRV records, this is a matter if simply updating the record
appropriate record. Without those records, you now have to tell people
that to send you email, they have to use
"you at your-new-email-server.your.domain.name" and to call you over VOIP,
they have to use "you at your-new-sipproxy.your.domain.name". Kinda
hokey... Huh. That's why we have MX records and SRV records. With
these records set up properly, people can both email and call you over
VOIP at the same place, "you at domain.name". Hrm. That seems a bunch
easier than remembering what the name to your mailserver and/or sipproxy
are, doesn't it.
So, decide for yourself folks. Do you want to do it the n00b1e way or
do you want to do it right the first time and be done with it?
Now. On the matter of who made an ass of them self, I think you should
take a close look in the mirror because it was most certainly not I.
John "not a n00b13" Fraizer
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