Locating SIP Services in DNS

RFC3263推荐使用DNS，接续过程为NAPTR-> SRV -> A or AAAA，逐一尝试。

详见http://mit.edu/sip/sip.edu/dns.shtml，还提供了很好的实践。

关于Naming Authority Pointer (NAPTR) ：

ThisdocumentdescribesaDomainNameSystem(DNS)resourcerecordwhichspecifiesaregularexpressionbasedrewriterulethat,whenappliedtoanexistingstring,willproduceanewdomainlabelorUniformResourceIdentifier(URI).Dependingonthevalueoftheflagsfieldoftheresourcerecord,theresultingdomainlabelorURImaybeusedinsubsequentqueriesfortheNamingAuthorityPointer(NAPTR)resourcerecords(todelegatethenamelookup)orastheoutputoftheentireprocessforwhichthissystemisused(aresolutionserverforURIresolution,aserviceURIforENUMstylee.164numbertoURImapping,etc).

ThisallowstheDNStobeusedtolookupservicesforawidevarietyofresourcenames(includingURIs)whicharenotindomainnamesyntax.ReasonsfordoingthisrangefromURNResourceDiscoverySystemstomovingout-of-dateservicestonewdomains.

ThisdocumentupdatestheportionsofRFC2168specificallydealingwiththedefinitionoftheNAPTRrecordsandhowother,non-URIspecificapplications,mightuseNAPTR.

其中ENUM - The bridge between the switched telephony network and the Internet 很是有用

详见http://www.voip-info.org/wiki/view/ENUM

还有一篇中文的http://www.cnnic.net.cn/html/Dir/2004/02/17/2159.htm