--- 1/draft-ietf-radext-dynamic-discovery-07.txt 2013-10-16 07:14:32.375023987 -0700 +++ 2/draft-ietf-radext-dynamic-discovery-08.txt 2013-10-16 07:14:32.423025258 -0700 @@ -1,19 +1,19 @@ RADIUS Extensions Working Group S. Winter Internet-Draft RESTENA Intended status: Experimental M. McCauley -Expires: January 05, 2014 OSC - July 04, 2013 +Expires: April 19, 2014 OSC + October 16, 2013 NAI-based Dynamic Peer Discovery for RADIUS/TLS and RADIUS/DTLS - draft-ietf-radext-dynamic-discovery-07 + draft-ietf-radext-dynamic-discovery-08 Abstract This document specifies a means to find authoritative RADIUS servers for a given realm. It is used in conjunction with either RADIUS/TLS and RADIUS/DTLS. Status of This Memo This Internet-Draft is submitted in full conformance with the @@ -22,21 +22,21 @@ Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at http://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." - This Internet-Draft will expire on January 05, 2014. + This Internet-Draft will expire on April 19, 2014. Copyright Notice Copyright (c) 2013 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents @@ -54,32 +54,33 @@ 2. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 3 2.1. DNS RR definition . . . . . . . . . . . . . . . . . . . . 3 2.1.1. S-NAPTR . . . . . . . . . . . . . . . . . . . . . . . 3 2.1.2. SRV . . . . . . . . . . . . . . . . . . . . . . . . . 8 2.1.3. Remarks . . . . . . . . . . . . . . . . . . . . . . . 8 2.2. Definition of the X.509 certificate property SubjectAltName:otherName:NAIRealm . . . . . . . . . . . . 10 3. DNS-based NAPTR/SRV Peer Discovery . . . . . . . . . . . . . 11 3.1. Applicability . . . . . . . . . . . . . . . . . . . . . . 11 3.2. Configuration Variables . . . . . . . . . . . . . . . . . 11 - 3.3. Terms . . . . . . . . . . . . . . . . . . . . . . . . . . 11 + 3.3. Terms . . . . . . . . . . . . . . . . . . . . . . . . . . 12 3.4. Realm to RADIUS server resolution algorithm . . . . . . . 12 3.4.1. Input . . . . . . . . . . . . . . . . . . . . . . . . 12 3.4.2. Output . . . . . . . . . . . . . . . . . . . . . . . 13 3.4.3. Algorithm . . . . . . . . . . . . . . . . . . . . . . 13 3.4.4. Validity of results . . . . . . . . . . . . . . . . . 15 3.4.5. Delay considerations . . . . . . . . . . . . . . . . 16 3.4.6. Example . . . . . . . . . . . . . . . . . . . . . . . 16 4. Security Considerations . . . . . . . . . . . . . . . . . . . 19 - 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 19 - 6. Normative References . . . . . . . . . . . . . . . . . . . . 20 - Appendix A. Appendix A: ASN.1 Syntax of NAIRealm . . . . . . . . 21 + 5. Privacy Considerations . . . . . . . . . . . . . . . . . . . 20 + 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 21 + 7. Normative References . . . . . . . . . . . . . . . . . . . . 22 + Appendix A. Appendix A: ASN.1 Syntax of NAIRealm . . . . . . . . 23 1. Introduction RADIUS in all its current transport variants (RADIUS/UDP, RADIUS/TLS, RADIUS/DTLS) requires manual configuration of all peers (clients, servers). Where RADIUS forwarding servers are in use, the number of realms to be forwarded and the corresponding number of servers to configure may be significant. Where new realms with new servers are added or @@ -198,20 +199,28 @@ failure; so even if a discovered target returns a wrong credential instantly, it is not eligible for retry. Furthermore, the contacted RADIUS/TLS server verifies during connection setup whether or not it finds the connecting RADIUS/TLS client authorized or not. If the connecting RADIUS/TLS client is not found acceptable, the server will close the TLS connection immediately with an appropriate alert. Such TLS handshake failures are permanently fatal and not eligible for retry. + If the TLS session setup to a discovered target does not succeed, + that target (as identified by IP address and port number) SHOULD be + ignored from the result set of any subsequent executions of the + discovery algorithm at least until the target's Effective TTL has + expired or until the entity which executes the algorithm changes its + TLS context to either send a new client certificate or expect a + different server certificate. + 2.1.1.3. Server Identification and Handshake After the algorithm in this document has been executed, a RADIUS/TLS session as per [RFC6614] is established. Since the algorithm does not allow to derive confidential keying material between the RADIUS/ TLS client (i.e. the server which executes the discovery algorithm) and the RADIUS/TLS server which was discovered, TLS-PSK ciphersuites can not be used for the subsequent TLS handshake in the RADIUS/TLS conversation. Only TLS ciphersuites using X.509 certificates can be used with this algorithm. @@ -240,31 +248,31 @@ "subjectAlternativeName:otherName:NAIRealm" as defined in Section 2.2. The comparison is a byte-by-byte comparison, except for dot-separated parts of the value whose content is a single "*" character; such labels match all strings in the same part of the NAI realm. If at least one of the sAN:otherName:NAIRealm values matches the NAI realm, the server is considered authorized; if none matches, the server is considered unauthorized. Examples: - +-----------------+---------------------------------------------+ - | NAI realm | sAN:otherName:NAIRealm | MATCH? | - +-----------------+---------------------------------------------+ + +-----------------+-----------------------------------------------+ + | NAI realm | NAIRealm | MATCH? | + +-----------------+-----------------------------------------------+ | foo.example | foo.example | YES | | foo.example | *.example | YES | | bar.foo.example | *.example | NO | - | bar.foo.example | bar.*.example | YES | - | bar.foo.example | *.*.example | YES | - | sub.bar.foo.example | *.*.example | NO | - | sub.bar.foo.example | sub.bar.foo.example | YES | - +-----------------+---------------------------------------------+ + | bar.foo.example | bar.*.example | NO (NAIRealm invalid) | + | bar.foo.example | *.*.example | NO (NAIRealm invalid) | + | sub.bar.foo.example | *.*.example | NO (NAIRealm invalid) | + | sub.bar.foo.example | *.bar.foo.example | YES | + +-----------------+-----------------------------------------------+ Figure 3: Examples for NAI realm vs. certificate matching 2.1.1.3.2. Other mechanism: Trust Roots + policyOID Verification of authority to provide AAA services over RADIUS/TLS is a two-step process. Step 1 is the verification of certificate wellformedness and validity as per [RFC5280] and whether it was issued from a root certificate @@ -458,29 +465,28 @@ a trusted comparison item. Further to this, this specification's NAPTR entries may be of type "A" which do not involve resolution of any SRV records, which again makes subjectAltName:otherName:sRVName unsuited for this purpose. This section defines the NAIRealm name as a form of otherName from the GeneralName structure in SubjectAltName defined in [RFC5280]. id-on-nai OBJECT IDENTIFIER ::= { id-on XXX } - NAIRealm ::= UTF8String (SIZE (1..MAX)) The NAIRealm, if present, MUST contain an NAI realm as defined in - [I-D.ietf-radext-nai]. It MAY substitute labels on all dot-separated - parts of the NAI with the single character "*" to indicate a wildcard - match for "all labels in this part". Further features of regular - expressions, such as a number of characters followed by a * to - indicate a common prefix inside the part, are not permitted. + [I-D.ietf-radext-nai]. It MAY substitute labels on the leftmost dot- + separated part of the NAI with the single character "*" to indicate a + wildcard match for "all labels in this part". Further features of + regular expressions, such as a number of characters followed by a * + to indicate a common prefix inside the part, are not permitted. This subjectAltName MAY occur more than once in a certificate. Appendix A contains the ASN.1 definition of the above objects. 3. DNS-based NAPTR/SRV Peer Discovery 3.1. Applicability Dynamic server discovery as defined in this document is only @@ -528,20 +535,29 @@ Effective TTL: The validity period for discovered RADIUS/TLS target hosts. Calculated as: Effective TTL (set of DNS TTL values) = max { MIN_EFF_TTL, min { DNS TTL values } } SRV lookup: for the purpose of this specification, SRV lookup procedures are defined as per [RFC2782], but excluding that RFCs "A" fallback as defined in its section "Usage Rules", final "else" clause. + Greedy result evaluation: The NAPTR to SRV/A/AAAA resolution may lead + to a tree of results, whose leafs are the IP addresses to contact. + The branches of the tree are ordered according to their order/ + preference DNS properties. An implementation is executing greedy + result evaluation if it uses a depth-first search in the tree along + the highest order results, attempts to connect to the corresponding + resulting IP addresses, and only backtracks to other branches if the + higher ordered results did not end in successful connection attempts. + 3.4. Realm to RADIUS server resolution algorithm 3.4.1. Input For RADIUS Authentication and RADIUS Accounting server discovery, input I to the algorithm is the RADIUS User-Name attribute with content of the form "user@realm"; the literal @ sign being the separator between a local user identifier within a realm and its realm. The use of multiple literal @ signs in a User-Name is strongly discouraged; but if present, the last @ sign is to be @@ -614,25 +629,25 @@ returns with error, O-1 = { empty set }, O-2 = BACKOFF_TIME and terminate. 7. Extract NAPTR records with service tag "aaa+auth", "aaa+acct", "aaa+dynauth" as appropriate. Keep note of the remaining TTL of each of the discovered NAPTR records. 8. If no records found, continue at step 13. 9. For the extracted NAPTRs, perform successive resolution as - defined in [RFC3958], section 2.2.4, with the additional - reservation that all records are to be immediately pursued - through terminal lookup, i.e. have resulted in hostnames. - Failure to achieve terminal lookup for individual records is - non-fatal. + defined in [RFC3958], section 2.2. An implementation MAY use + greedy result evaluation according to the NAPTR order/preference + fields (i.e. can execute the subsequent steps of this algorithm + for the highest-order entry in the set of results, and only + lookup the remainder of the set if necessary). 10. If the set of hostnames is empty, O-1 = { empty set }, O-2 = BACKOFF_TIME and terminate. 11. O' = (set of {hostname; port; order/preference; Effective TTL ( all DNS TTLs that led to this hostname ) } for all terminal lookup results). 12. Proceed with step 18. @@ -675,21 +690,27 @@ the server would forward the request to itself, triggering dynamic discovery again in a perpetual loop), or lead to a potential loop with intermediate hops in between (the server could forward to another host with a higher priority, which might use DNS itself and forward the packet back to the first server). The underlying reason that enables these loops is that the server executing the discovery algorithm is seriously misconfigured in that it does not recognise the request as one that is to be processed by itself. RADIUS has no built-in loop detection, so any such loops would remain undetected. So, if step 18 of the algorithm discovers such a possible-loop - situation, the algorithm should be aborted and an error logged. + situation, the algorithm should be aborted and an error logged. Note + that this safeguard does not provide perfect protection against + routing loops: other reasons include the possiblity that a subsequent + hop has a statically configured next-hop which leads to an earlier + host in the loop; or the algorithm execution was executed with greedy + result evaluation, and the own address was in a lower-priority branch + of the result set which was not retrieved from DNS at all. After executing the above algorithm, the RADIUS server establishes a connection to a home server from the result set. This connection can potentially remain open for an indefinite amount of time. This conflicts with the possibility of changing device and network configurations on the receiving end. Typically, TTL values for records in the name resolution system are used to indicate how long it is safe to rely on the results of the name resolution. If these TTLs are very low, thrashing of connections becomes possible; the Effective TTL mitigates that risk. When a connection is open and the @@ -872,21 +892,105 @@ responses to track, or at least the number of pending DNS queries. Implementations MAY choose lower values than the default for DNS_TIMEOUT to limit the impact of DoS attacks via that vector. They MAY also continue their attempt to resolve DNS records even after DNS_TIMEOUT has passed; a subsequent request for the same realm might benefit from retrieving the results anyway. The amount of time to spent waiting for a result will influence the impact of a possible DoS attack; the waiting time value is implementation dependent and outside the scope of this specification. -5. IANA Considerations + With Dynamic Discovery being enabled for a RADIUS Server, and + depending on the deployment scenario, the server may need to open up + its target IP address and port for the entire internet, because + arbitrary clients may discover it as a target for their + authentication requests. If such clients are not part of the roaming + consortium, the RADIUS/TLS connection setup phase will fail (which is + intended) but the computational cost for the connection attempt is + significant. With the port for a TLS-based service open, the RADIUS + server shares all the typical attack vectors for services based on + TLS (such as HTTPS, SMTPS, ...). Deployments of RADIUS/TLS with + Dynamic Discovery should consider these attack vectors and take + appropriate counter-measures (e.g. blacklisting known-bad IPs on a + firewall, rate-limiting new connection attempts, etc.). + +5. Privacy Considerations + + The classic RADIUS operational model (known, pre-configured peers, + shared secret security, mostly plaintext communication) and this new + RADIUS dynamic discovery model (peer discovery with DNS, PKI security + and packet confidentiality) differ significantly in their impact on + the privacy of end users trying to authenticate to a RADIUS server. + + With classic RADIUS, traffic in large environments gets aggregated by + statically configured clearinghouses. The packets sent to those + clearinghouses and their responses are mostly unprotected. As a + consequence, + + o All intermediate IP hops can inspect most of the packet payload in + clear text, including the User-Name and Calling-Station-Id + attributes, and can observe which client sent the packet to which + clearinghouse. This allows the creation of mobility profiles for + any passive observer on the IP path. + + o The existence of a central clearinghouse creates an opportunity + for the clearinghouse to trivially create the same mobility + profiles. The clearinghouse may or may not be trusted not to do + this, e.g. by sufficiently threatening contractual obligations. + + o In addition to that, with the clearinghouse being a RADIUS + intermediate in possession of a valid shared secret, the + clearinghouse can observe and record even the security-critical + RADIUS attributes such as User-Password. This risk may be + mitigated by choosing authentication payloads which are + cryptographically secured and do not use the attribute User- + Password - such as certain EAP types. + + o There is no additional information disclosure to parties outside + the IP path between the RADIUS client and server (in aprticular, + no DNS servers learn about realms of current ongoing + authentications). + + With RADIUS and dynamic discovery, + + o Passive observers on the IP path cannot inspect any part of the + RADIUS payload. They can observe source and destination of the + traffic flow, but can not easily use this information to create + mobility profiles because the user who tries to authenticate is + not identifiable due to the encrypted payload. + + o Clearinghouses can be eliminated by RADIUS clients directly + contacting the RADIUS home server, if this is desired. The + possibility of aggregation of user information in the + clearinghouse thus does not manifest. Note that despite the + technical possibility of avoid clearinghouses, they may still + remain in operation for other reasons. + + o RADIUS clients which make use of dynamic discovery will need to + query the Domain Name System, and use a user's realm name as the + query label. A passive observer on the IP path between the RADIUS + client and the DNS server(s) being queried can learn that a user + of that specific realm was trying to authenticate at that RADIUS + client at a certain point in time. This may or may not be + sufficient for the passive observer to create a mobility profile. + During the recursive DNS resolution, a fair number of DNS servers + and the IP hops in between those get to learn that information. + Not every single authentication triggers DNS lookups, so there is + no one-to-one relation of leaked realm information and the number + of authentications for that realm. + + In summary, with classic RADIUS, few intermediate entities learn very + detailed data about every ongoing authentications, while with dynamic + discovery, many entities learn only very little about recently + authenticated realms. + +6. IANA Considerations This document requests IANA registration of the following entries in existing registries: o S-NAPTR Application Service Tags registry * aaa+auth * aaa+acct * aaa+dynauth @@ -901,21 +1005,30 @@ Service labels. This document requests the creation of a new IANA registry named "RADIUS/TLS SRV Protocol Registry" with the following initial entries: o _tcp o _udp -6. Normative References + This specification allocates a X.509 certificate property "NAIRealm" + as per section Section 2.2 above, see placeholders "XXX". There is + currently no IANA registry for the subjectAltName:otherName + namespace. The authority for this namespace appears to be the PKIX + working group. Before issuing the RFC, IANA should liaise with PKIX + to ensure that a value for NAIRealm is issued; IANA should + subsequently, prior to issuing the RFC, update the placeholders in + said section. + +7. Normative References [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC2782] Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for specifying the location of services (DNS SRV)", RFC 2782, February 2000. [RFC2865] Rigney, C., Willens, S., Rubens, A., and W. Simpson, "Remote Authentication Dial In User Service (RADIUS)", RFC