| DPRIVE Working Group | D. Wing |
| Internet-Draft | T. Reddy |
| Intended status: Standards Track | Cisco |
| Expires: March 19, 2016 | September 16, 2015 |
DPRIVE TLS/DTLS Profile and Message Flows
draft-wing-dprive-profile-and-msg-flows-00
This document defines a Transport Layer Security (TLS) and Datagram TLS (DTLS) 1.2 profile for providing DNS privacy. Message flows for DNS-over-TLS and DNS-over-DTLS are discussed and compared.
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The DPRIVE working group has two active documents that provide DNS confidentiality, DNS over DTLS [I-D.ietf-dprive-dnsodtls] and DNS over TLS [I-D.ietf-dprive-start-tls-for-dns].
This document defines a profile of DTLS 1.2 [RFC6347] and TLS 1.2 [RFC5246] that offers DNS privacy. This profile defines the configuration options and protocol extensions to perform connection handshakes with the fewest round trips while maintaining privacy. This document also shows message flows for those two documents.
This section defines the profile of DNS over DTLS and of DNS over TLS. Implementations compliant with this profile MUST implement all of the following items:
With DTLS, the client is immediately informed of the server state loss with a DTLS Alert, as shown in the diagram below. This costs one round trip.
client server
| |
|<-----------DPRIVE communications------------->|
| |
| ... |
| |
| (state lost)
| |
1. |-----------DPRIVE query----------------------->|
2. |<----------DTLS Alert--------------------------|
3. |-DLTS ClientHello w/resumption---------------->|
| ... |
Figure 1: Server State Loss, DTLS
An optimization of the above flow is possible, if we consider that the server is more likely to have lost state if the most recent DNS query was sent a "long time ago" (exact value of "long time" is debatable). In that situation, the DNS-over-DTLS client can send a TLS handshake with TLS resumption -- effectively, it sends the TLS handshake identical to packet (3) of Figure 1 (avoiding packets 1 and 2). This packet is larger, though, as it contains the TLS session resumption information. Thus, it is a trade-off of a larger message versus a (possible) round trip savings. This message flow is shown below.
client server | | |<----------DPRIVE communications------------------>| | | | ... | | | | (state lost) | | |--DTLS ClientHello w/resumption ------------------>| |<-DTLS ServerHello, ChangeCipherSpec, Finished-----| |--DTLS ChangeCipherSpec, Finished, DNS query------>| |<-DNS response-------------------------------------| | ... |
Figure 2: Server State Loss, DTLS False Start
With TLS, the client is immediately informed of server state loss with a TCP RST, as shown in the diagram below. This costs one round trip , and is unavoidable.
client server | | |<-----------------DPRIVE communications----------------->| | | | ... | | | | (state lost) | | |-DNS-over-TLS------------------------------------------->| |<------TCP RST-------------------------------------------| |--TCP SYN----------------------------------------------->| |<-TCP SYNACK---------------------------------------------| |--TCP ACK, TLS ClientHello w/Resumption ---------------->| |<-TLS ServerHello, ChangeCipherSpec, Finished -----------| |--TLS ChangeCipherSpec, Finished, DNS query------------->| |<-DNS response-------------------------------------------| | |
Figure 3: Server State Loss, TLS
If the client and server TCP stacks both support TCP Fast Open (TFO) [RFC7413], the TCP 3-way handshake can be done without a round trip, as shown below. Currently, TFO is supported in Linux 3.7 (TCP client and server), iOS 9, and OS X 10.11.
client server | | |<-------------------DPRIVE communications-------------------->| | | | ... | | | | (state lost) | | |-DNS-over-TLS------------------------------------------------>| |<------TCP RST------------------------------------------------| |--TCP SYN, TLS ClientHello w/Resumption --------------------->| |<-TCP SYNACK, TLS ServerHello, ChangeCipherSpec, Finished-----| |--TLS ChangeCipherSpec, Finished, DNS query------------------>| |<-DNS response------------------------------------------------|
Figure 4: Server State Loss, TLS with TCP FastOpen
In between normal DNS traffic while the communication to the DNS server is quiescent, the DNS client may want to probe the server to ensure it has maintained cryptographic state. Such probes can also keep alive firewall or NAT bindings. This probing reduces the frequency of needing a new handshake when a DNS query needs to be resolved, improving the user experience at the cost of bandwidth and processing time; cellular devices could even send the probes while in power-save state [I-D.isomaki-rtcweb-mobile].
If the server has lost state, a DTLS (or TLS) handshake needs to be initiated with the server.
A DTLS heartbeat [RFC6520] verifies the server still has DTLS state by returning a DTLS message. If the server has lost state, it returns a DTLS Alert.
TLS runs over TCP, so a simple probe is a 0-length TCP packet (a "window probe"). This verifies the TCP connection is still working, which is also sufficient to prove the server has retained TLS state, because if the server loses TLS state it abandons the TCP connection. If the server has lost state, a TCP RST is returned immediately.
A NAT or Firewall, on the path between the DPRIVE client and DPRIVE server, lose state -- either due to timing out the pinhole, exhausting its resources (and needing to prematurely close the pinhole), or crashing. This differs from the server losing state.
client NAT or firewall server
| | |
|<-----------DPRIVE communications------------->|
| | |
| (state loss) |
| | |
|-----------DPRIVE query----------------------->|
| (new state created in NAT/firewall) |
| | |
. |<----------DPRIVE response---------------------|
| ... |
Figure 5: NAT/Firewall State Loss, DTLS
With a TCP connection when the NAT or firewall has lost state and sees a TCP packet without the SYN bit set, there are several possible reactions by the NAT or firewall:
client NAT or firewall server
| | |
|<-----------DPRIVE communications---------------------------->|
| | |
| (state loss) |
| | |
|----DPRIVE query---->X |
| (no state exists for TCP flow) |
| | |
|<---TCP RST----------| |
| | |
(client does | |
TLS re-establishment with TCP FastOpen) |
| | |
|--TCP SYN, TLS ClientHello w/Resumption --------------------->|
|<-TCP SYNACK, TLS ServerHello, ChangeCipherSpec, Finished-----|
|--TLS ChangeCipherSpec, Finished, DNS query------------------>|
|<-DNS response------------------------------------------------|
| | |
Figure 6: NAT/Firewall State Loss, TLS with TCP FastOpen
client NAT or firewall server
| | |
|<-----------DPRIVE communications-------------->|
| | |
| (state loss) |
| | |
|----DPRIVE query---->X |
| (no state exists for TCP flow) |
| | |
|<---TCP RST----------| |
| | |
(client does normal | |
TLS re-establishment) | |
| | |
|--TCP SYN-------------------------------------->|
|<-TCP SYNACK------------------------------------|
|--TCP ACK, TLS ClientHello w/Resumption ------->|
|<-TLS ServerHello, ChangeCipherSpec, Finished --|
|--TLS ChangeCipherSpec, Finished, DNS query---->|
|<-DNS response----------------------------------|
| | |
Figure 7: NAT/Firewall State Loss, TLS
DNS-over-DTLS mandates the EDNS0 option defined in [RFC6891] to indicate the maximum DNS response size. This maximum DNS response size will be smaller than the path MTU, due to DTLS overhead, which the client needs to consider in its indicated EDNS0 value. For example, if the client can receive DNS responses of 1024 bytes and is using the cipher suites from [RFC5246]) and null compression, the record-layer output can be at most 1317 bytes: 5 bytes of headers, 1024 bytes of DNS data, 256 bytes of padding, and 32 bytes of MAC. If the DNS sever's response exceeds the EDNS0 value, the DNS server sets the TC (truncated) bit. On receiving a response with the TC bit set, the client establishes a DNS-over-TLS connection to the same server, and sends a new DNS request for the same resource record.
There is concern, especially with DNSSEC responses, that a high frequency of responses will exceed the normal 512 byte DNS limit before the TC bit is set. However, EDNS0 and common network MTUs allow for larger responses.
None.
Authors would like to thank Mankin Allison for comments and review.
| [I-D.ietf-dprive-dnsodtls] | Reddy, T., Wing, D. and P. Patil, "DNS over DTLS (DNSoD)", Internet-Draft draft-ietf-dprive-dnsodtls-01, June 2015. |
| [I-D.ietf-dprive-start-tls-for-dns] | Zi, Z., Zhu, L., Heidemann, J., Mankin, A., Wessels, D. and P. Hoffman, "TLS for DNS: Initiation and Performance Considerations", Internet-Draft draft-ietf-dprive-start-tls-for-dns-01, July 2015. |
| [RFC5246] | Dierks, T. and E. Rescorla, "The Transport Layer Security (TLS) Protocol Version 1.2", RFC 5246, DOI 10.17487/RFC5246, August 2008. |
| [RFC6347] | Rescorla, E. and N. Modadugu, "Datagram Transport Layer Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347, January 2012. |
| [RFC7525] | Sheffer, Y., Holz, R. and P. Saint-Andre, "Recommendations for Secure Use of Transport Layer Security (TLS) and Datagram Transport Layer Security (DTLS)", BCP 195, RFC 7525, DOI 10.17487/RFC7525, May 2015. |