[AusNOG] SRV Records

[Mark Andrews]

In message <53C5B462.3040407 at bleeter.id.au>, Peter Lawler writes:
> On 15/07/14 18:41, ANSA SERVERS wrote:
> 
> > So what exactly are these SRV records and what are they used for
> 
> One thing they're used for: http://wiki.xmpp.org/web/SRV_Records
> 
> iirc from my XMPP hacking days (some time ago now so things are a little 
> scratchy), blocking SRV was a cheap and nasty yet effective way (to 

With lots of collateral damage.

> attempt) to block a number of Instant Messaging services without having 
> to bother with all that fiddly stuff 'lower' down. As I say, iirc, a 
> number of people with 'your software sucks' errors turned out to be 
> campuses where SRV wasn't passed.
>
> (random bonus thought via bugger all coffee) No idea if blocking SRV 
> will kill things evolved out of those protocols that required them which 
> may be of some business benefit over 'basic' IM (eg Google Talk->Google 
> Hangouts). I guess YMMV depending on what you use your network for.

SRV is used for lots of protocols.  There aren't enough ports for
every protocol to get one reserved for it.  SRV is used to get
around this limitation without adding a new DNS type for every
protocol.

SRV is a protocol agnostic record which performs basically the same
function that MX records do for SMTP.  It tells the client were the
server for this <protocol,name> lives (address and port).  Blocking
SRV record queries is as stupid as blocking A record queries.

> Pete.
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-- 
Mark Andrews, ISC
1 Seymour St., Dundas Valley, NSW 2117, Australia
PHONE: +61 2 9871 4742                 INTERNET: marka at isc.org