RCS and DNS: The NAPTR Record - SANS Internet Storm Center --> Internet Storm Center Sign In Sign Up Handler on Duty: Johannes Ullrich Threat Level: green previous Click HERE to learn more about classes Johannes is teaching for SANS RCS and DNS: The NAPTR Record Published : 2026-07-06. Last Updated : 2026-07-06 13:35:58 UTC by Johannes Ullrich (Version: 1) 0 comment(s) Over the last year, with recent updates to iOS and Android, RCS (Rich Communication Services) has become an increasingly used protocol [1]. RCS is supposed to eventually replace SMS, and in addition to richer formatting, provides added (but optional) security. RCS messages may be end-to-end encrypted and digitally signed. Unlike SMS, which was "bolted on" to existing voice-focused phone standards. The SMS standard was based on old-fashioned pagers and allowed for limited clear-text communications. RCS is built from the ground up around modern IP-based network infrastructure and behaves more like IP chat services (think iMessage, WhatsApp...). RCS defines the message format, while protocols like SIP are used to establish connections and transport messages. "Do as you say", I do from time to time take a look at odd DNS traffic on my network. An activity I recommend when teaching SEC503. Recently, I noticed more "NAPTR" queries, a record type I had not seen before. The record type is defined in RFC 2915 [2], which was ratified in 2000. It is not a new record. But so far, at least in my network, it has not really shown up before. The description of the record sounds rather ominous: "a Resource Record that included a regular expression that would be used by a client program to rewrite a string into a domain name." Wow. Regular expressions to rewrite resource records? What could possibly go wrong? However, right now, I just want to talk about how it "goes right" and how these records are currently being used for RCS. Below is the relevant part of the t-shark decode of a record typical for what I have seen in my network:     Queries         fp-us-verizon.rcs.telephony.goog: type NAPTR, class IN             Name: fp-us-verizon.rcs.telephony.goog             [Name Length: 32]             [Label Count: 4]             Type: NAPTR (35) (Naming Authority Pointer)             Class: IN (0x0001)     Answers         fp-us-verizon.rcs.telephony.goog: type NAPTR, class IN, order 100, preference 100, flags s             Name: fp-us-verizon.rcs.telephony.goog             Type: NAPTR (35) (Naming Authority Pointer)             Class: IN (0x0001)             Time to live: 295 (4 minutes, 55 seconds)             Data length: 61             Order: 100             Preference: 100             Flags Length: 1             Flags: s             Service Length: 8             Service: SIPS+D2T             Regex Length: 0             Regex:             [Replacement Length: 43]             Replacement: _sips._tcp.fp-us-verizon.rcs.telephony.goog   This was the only applicable NAPTR record, so order and preference do not matter in this case. The "S" flag indicates that the next lookup should be a SRV record. And indeed, we do have a SRV query (see below). Only a "U" flag would result in a URI.  Remember that this record is about URIs, not IP addresses? The "Service" field indicates what service we may find at the to-be-determined URI. In this case, it is SIPS+D2T. SIPS+D2T is a transport protocol defined in the SIP standard (RFC 3263). SIPS+D2T stands for "Secure SIP Direct to TCP". So we will be using SIP over TLS with TCP as the transport protocol. The SIP standard specifically calls for NAPTR records to find SIP servers. The reason for the NAPTR record is to allow URIs to be returned, not just IP addresses/hostnames (as an SRV record would). Lucky for us (and the DNS server), the regular expression is empty. And this appears to be normal for this use case. Instead, we just get a "SRV" record to request:     Queries         _sips._tcp.fp-us-verizon.rcs.telephony.goog: type SRV, class IN             Name: _sips._tcp.fp-us-verizon.rcs.telephony.goog             [Name Length: 43]             [Label Count: 6]             Type: SRV (33) (Server Selection)             Class: IN (0x0001)     Answers         _sips._tcp.fp-us-verizon.rcs.telephony.goog: type SRV, class IN, priority 20, weight 0, port 5223, target fp-us-verizon.rcs.telephony.goog             Service: _sips             Protocol: _tcp             Name: fp-us-verizon.rcs.telephony.goog             Type: SRV (33) (Server Selection)             Class: IN (0x0001)             Time to live: 300 (5 minutes)             Data length: 40             Priority: 20             Weight: 0             Port: 5223             Target: fp-us-verizon.rcs.telephony.goog         _sips._tcp.fp-us-verizon.rcs.telephony.goog: type SRV, class IN, priority 30, weight 0, port 443, target fp-us-verizon.rcs.telephony.goog             Service: _sips             Protocol: _tcp             Name: fp-us-verizon.rcs.telephony.goog             Type: SRV (33) (Server Selection)             Class: IN (0x0001)             Time to live: 300 (5 minutes)             Data length: 40             Priority: 30             Weight: 0             Port: 443             Target: fp-us-verizon.rcs.telephony.goog ??????? And yes, in the end, there is a "normal" A and AAAA lookup for fp-us-verizon.rcs.telephony.goog. So far, NAPTR records do not appear to be used to their full potential. I am sure that the use of regular expressions will be of interest to bug hunters and penetration testers.  [1] https://support.google.com/messages/answer/13508703?hl=en [2] https://www.ietf.org/rfc/rfc2915.txt -- Johannes B. Ullrich, Ph.D. , Dean of Research, SANS.edu Twitter | Keywords: dns naptr rcs sip sms 0 comment(s) Click HERE to learn more about classes Johannes is teaching for SANS previous Comments Login here to join the discussion. 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📌 来源: SANS ISC | 📅 2026-07-06