One document matched: draft-ietf-radext-dynamic-discovery-07.txt
Differences from draft-ietf-radext-dynamic-discovery-06.txt
RADIUS Extensions Working Group S. Winter
Internet-Draft RESTENA
Intended status: Experimental M. McCauley
Expires: January 05, 2014 OSC
July 04, 2013
NAI-based Dynamic Peer Discovery for RADIUS/TLS and RADIUS/DTLS
draft-ietf-radext-dynamic-discovery-07
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
provisions of BCP 78 and BCP 79.
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.
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
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3
1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
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
3.4. 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
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
details of existing servers change on a regular basis, maintaining a
single monolithic configuration file for all these details may prove
too cumbersome to be useful.
Furthermore, in cases where a roaming consortium consists of
independently working branches, each with their own forwarding
servers, and who add or change their realm lists at their own
discretion, there is additional complexity in synchronising the
changed data across all branches.
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These situations can benefit significantly from a distributed
mechanism for storing realm and server reachability information.
This document describes one such mechanism: storage of realm-to-
server mappings in DNS.
This document also specifies various approaches for verifying that
server information which was retrieved from DNS was from an
authorised party; e.g. an organisation which is not at all part of a
given roaming consortium may alter its own DNS records to yield a
result for its own realm.
1.1. Requirements Language
In this document, several words are used to signify the requirements
of the specification. The key words "MUST", "MUST NOT", "REQUIRED",
"SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY",
and "OPTIONAL" in this document are to be interpreted as described in
RFC 2119. [RFC2119]
1.2. Terminology
RADIUS/TLS Client: a RADIUS/TLS [RFC6614] instance which initiates a
new connection.
RADIUS/TLS Server: a RADIUS/TLS [RFC6614] instance which listens on a
RADIUS/TLS port and accepts new connections
RADIUS/TLS node: a RADIUS/TLS client or server
2. Definitions
2.1. DNS RR definition
DNS definitions of RADIUS/TLS servers can be either S-NAPTR records
(see [RFC3958]) or SRV records. When both are defined, the
resolution algorithm prefers S-NAPTR results (see Section 3.4 below).
2.1.1. S-NAPTR
2.1.1.1. Registration of Application Service and Protocol Tags
This specification defines three S-NAPTR service tags:
+-----------------+-----------------------------------------+
| Service Tag | Use |
+-----------------+-----------------------------------------+
| aaa+auth | RADIUS Authentication, i.e. traffic as |
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| | defined in [RFC2865] |
| - - - - - - - - | - - - - - - - - - - - - - - - - - - - - |
| aaa+acct | RADIUS Accounting, i.e. traffic as |
| | defined in [RFC2866] |
| - - - - - - - - | - - - - - - - - - - - - - - - - - - - - |
| aaa+dynauth | RADIUS Dynamic Authorisation, i.e. |
| | traffic as defined in [RFC5176] |
+--------------- --+-----------------------------------------+
Figure 1: List of Service Tags
This specification defines two S-NAPTR protocol tags:
+-----------------+-----------------------------------------+
| Protocol Tag | Use |
+-----------------+-----------------------------------------+
| radius.tls | RADIUS transported over TLS as defined |
| | in [RFC6614] |
| - - - - - - - - | - - - - - - - - - - - - - - - - - - - - |
| radius.dtls | RADIUS transported over DTLS as defined |
| | in [I-D.ietf-radext-dtls] |
+-----------------+-----------------------------------------+
Figure 2: List of Protocol Tags
Note well:
The S-NAPTR service and protocols are unrelated to the IANA
Service Name and Transport Protocol Number registry
The delimiter '.' in the protocol tags is only a separator for
human reading convenience - not for structure or namespacing; it
MUST NOT be parsed in any way by the querying application or
resolver.
The use of the separator '.' is common also in other protocols'
protocol tags. This is coincidence and does not imply a shared
semantics with such protocols.
2.1.1.2. Definition of Conditions for Retry/Failure
RADIUS is a time-critical protocol; RADIUS clients which do not
receive an answer after a configurable, but short, amount of time,
will consider the request failed. Due to this, there is little
leeway for extensive retries.
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As a general rule, only error conditions which generate an immediate
response from the other end are eligible for a retry of a discovered
target. Any error condition involving time-outs, or the absence of a
reply for more than one second during the connection setup phase is
to be considered a failure; the next target in the set of discovered
NAPTR targets is to be tried.
Note that [RFC3958] already defines that a failure to identify the
server as being authoritative for the realm is always considered a
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.
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.
There are numerous ways to define which certificates are acceptable
for use in this context. This document defines one mandatory-to-
implement mechanism which allows to verify whether the contacted host
is authoritative for a NAI realm or not. It also gives one example
of another mechanism which is currently in wide-spread deployment,
and one possible approach based on DNSSEC which is yet unimplemented.
2.1.1.3.1. Mandatory-to-implement mechanism: Trust Roots + NAIRealm
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
which is deemed trustworthy by the RADIUS/TLS client.
Step 2 is: compare the value of algorithm's variable "R" after the
execution of step 3 of the discovery algorithm in Section 3.4.3 below
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(i.e. after a consortium name mangling, but before conversion to a
form usable by the name resolution library) to all values of the
contacted RADIUS/TLS server's X.509 certificate property
"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? |
+-----------------+---------------------------------------------+
| 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 |
+-----------------+---------------------------------------------+
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
which is deemed trustworthy by the RADIUS/TLS client.
Step 2 is: compare the values of the contacted RADIUS/TLS server's
X.509 certificate's extensions of type "Policy OID" to a list of
configured acceptable Policy OIDs for the roaming consortium. If one
of the configured OIDs is found in the certificate's Policy OID
extensions, then the server is considered authorized; if there is no
match, the server is considered unauthorized.
This mechanism is inferior to the mandatory-to-implement mechanism in
the previous section because all authorized servers are validated by
the same OID value; the mechanism is not fine-grained enough to
express authority for one specific realm inside the consortium. If
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the consortium contains members which are hostile against other
members, this weakness can be exploited by one RADIUS/TLS server
impersonating another if DNS responses can be spoofed by the hostile
member.
It should be noted that these shortcomings can be mitigated by using
the RADIUS infrastructure only with authentication payloads which
provide mutual authentication; that way, the final EAP server that
was reached can be validated by the EAP peer, and any improper
redirections to a different server will be detected.
2.1.1.3.3. Other mechanism: DNSSEC / DANE
Where DNSSEC is used, the results of the algorithm can be trusted;
i.e. the entity which executes the algorithm can be certain that the
realm that triggered the discovery is actually served by the server
that was discovered via DNS. However, this does not guarantee that
the server is also authorized (i.e. a recognised member of the
roaming consortium).
The authorization can be sketched using DNSSEC+DANE as follows: if
DANE/TLSA records of all authorized servers are put into a DNSSEC
zone with a common, consortium-specific branch of the DNS tree, then
the entity executing the algorithm can retrieve TLSA RRs for the
label "realm.commonroot" and verify that the presented server
certificate during the RADIUS/TLS handshake matches the information
in the TLSA record.
Example:
Realm = "example.com"
Common Branch = "idp.roaming-consortium.example.
label for TLSA query = "example.com.idp.roaming-
consortium.example.
result of discovery algorithm for realm "example.com" =
192.0.2.1:2083
( TLS certificate of 192.0.2.1:2083 matches TLSA RR ? "PASS" :
"FAIL" )
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2.1.1.3.4. Remark
Note that RADIUS/TLS connections always mutually authenticate the
RADIUS server and the RADIUS client. This specification provides an
algorithm for a RADIUS client to contact and verify authorization of
a RADIUS server only. During connection setup, the RADIUS server
also needs to verify whether it considers the connecting RADIUS
client authorized; this is outside the scope of this specification.
2.1.2. SRV
This specification defines two SRV prefixes (i.e. two values for the
"_service._proto" part of an SRV RR as per [RFC2782]):
+-----------------+-----------------------------------------+
| SRV Label | Use |
+-----------------+-----------------------------------------+
| _radiustls._tcp | RADIUS transported over TLS as defined |
| | in [RFC6614] |
| - - - - - - - - | - - - - - - - - - - - - - - - - - - - - |
| _radiustls._udp | RADIUS transported over DTLS as defined |
| | in [I-D.ietf-radext-dtls] |
+-----------------+-----------------------------------------+
Figure 4: List of SRV Labels
Just like NAPTR records, the lookup and subsequent follow-up of SRV
records may yield more than one server to contact in a prioritised
list. [RFC2782] does not specify rules regarding "Definition of
Conditions for Retry/Failure", nor "Server Identification and
Handshake". This specification defines that the rules for these two
topics as defined in Section 2.1.1.2 and Section 2.1.1.3 SHALL be
used both for targets retrieved via an initial NAPTR RR as well as
for targets retrieved via an initial SRV RR (i.e. in the absence of
NAPTR RRs).
2.1.3. Remarks
It is expected that in most cases, the SRV and/or NAPTR label used
for the records is the DNS A-label representation of the literal
realm name for which the server is the authoritative RADIUS server
(i.e. the realm name after conversion according to section 5 of
[RFC5891]).
However, arbitrary other labels or service tags may be used if, for
example, a roaming consortium uses realm names which are not
associated to DNS names or special-purpose consortia where a globally
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valid discovery is not a use case. Such other labels require a
consortium-wide agreement about the transformation from realm name to
lookup label, and/or which service tag to use.
Examples:
a. A general-purpose RADIUS server for realm example.com might have
DNS entries as follows:
example.com. IN NAPTR 50 50 "s" "aaa+auth:radius.tls" ""
_radiustls._tcp.foobar.example.com.
_radiustls._tcp.foobar.example.com. IN SRV 0 10 2083
radsec.example.com.
b. The consortium "foo" provides roaming services for its members
only. The realms used are of the form enterprise-name.example.
The consortium operates a special purpose DNS server for the
(private) TLD "example" which all RADIUS servers use to resolve
realm names. "Bad, Inc." is part of the consortium. On the
consortium's DNS server, realm bad.example might have the
following DNS entries:
bad.example IN NAPTR 50 50 "a" "aaa+auth:radius.dtls" ""
very.bad.example
c. The eduroam consortium uses realms based on DNS, but provides its
services to a closed community only. However, a AAA domain
participating in eduroam may also want to expose AAA services to
other, general-purpose, applications (on the same or other RADIUS
servers). Due to that, the eduroam consortium uses the service
tag "x-eduroam" for authentication purposes and eduroam RADIUS
servers use this tag to look up other eduroam servers. An
eduroam participant example.org which also provides general-
purpose AAA on a different server uses the general "aaa+auth"
tag:
example.org. IN NAPTR 50 50 "s" "x-eduroam:radius.tls" ""
_radiustls._tcp.eduroam.example.org.
example.org. IN NAPTR 50 50 "s" "aaa+auth:radius.tls" ""
_radiustls._tcp.aaa.example.org
_radiustls._tcp.eduroam.example.org. IN SRV 0 10 2083 aaa-
eduroam.example.org.
_radiustls._tcp.aaa.example.org. IN SRV 0 10 2083 aaa-
default.example.org.
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2.2. Definition of the X.509 certificate property
SubjectAltName:otherName:NAIRealm
This specification retrieves IP addresses and port numbers from the
Domain Name System which are subsequently used to authenticate users
via the RADIUS/TLS protocol. Since the Domain Name System is not
necessarily trustworthy (e.g. if DNSSEC is not deployed for the
queried domain name), it is important to verify that the server which
was contacted is authorized to service requests for the user which
triggered the discovery process.
The input to the algorithm is a NAI realm as specified in
Section 3.4.1. As a consequence, the X.509 certificate of the server
which is ultimately contacted for user authentication needs to be
able to express that it is authorized to handle requests for that
realm.
Current subjectAltName fields do not semantically allow to express an
NAI realm; the field subjectAltName:dNSName is syntactically a good
match but would inappropriately conflate DNS names and NAI realm
names. Thus, this specification defines a new subjectAltName field
to hold either a single NAI realm name or a wildcard name matching a
set of NAI realms.
The subjectAltName:otherName:sRVName field certifies that a
certificate holder is authorized to provide a service; this can be
compared to the target of DNS label's SRV resource record. If the
Domain Name System is insecure, it is required that the label of the
SRV record itself is known-correct. In this specification, that
label is not known-correct; it is potentially derived from a
(potentially untrusted) NAPTR resource record of another label. If
DNS is not secured with DNSSEC, the NAPTR resource record may have
been altered by an attacker with access to the Domain Name System
resolution, and thus the label to lookup the SRV record for may
already be tainted. This makes subjectAltName:otherName:sRVName not
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))
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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.
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
applicable for AAA transactions where a RADIUS entity which acts as a
forwarding server for one or more realms receives a request with a
realm for which it is not authoritative, and which no explicit next
hop is configured. It is only applicable for
a. new user sessions, i.e. for the initial Access-Request.
Subsequent messages concerning this session, for example Access-
Challenges and Access-Accepts use the previously-established
communication channel between client and server.
b. RADIUS DynAuth server discovery
3.2. Configuration Variables
The algorithm contains various variables for timeouts. These
variables are named here and reasonable default values are provided.
Implementations wishing to deviate from these defaults should make
they understand the implications of changes.
DNS_TIMEOUT: maximum amount of time to wait for the complete set
of all DNS queries to complete: Default = 3 seconds
MIN_EFF_TTL: minimum DNS TTL of discovered targets: Default = 60
seconds
BACKOFF_TIME: if no conclusive DNS response was retrieved after
DNS_TIMEOUT, do not attempt dynamic discovery before BACKOFF_TIME
has elapsed. Default = 600 seconds
3.3. Terms
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Positive DNS response: a response which contains the RR that was
queried for.
Negative DNS response: a response which does not contain the RR that
was queried for, but contains an SOA record along with a TTL
indicating cache duration for this negative result.
DNS Error: Where the algorithm states "name resolution returns with
an error", this shall mean that either the DNS request timed out, or
a DNS response which is neither a positive nor a negative response
(e.g. SERVFAIL).
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.
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
considered the separator. All previous instances of the @ sign are
to be considered part of the local user identifier.
For RADIUS DynAuth Server discovery, input I to the algorithm is the
domain name of the operator of a RADIUS realm as was communicated
during user authentication using the Operator-Name attribute
([RFC5580], section 4.1). Only Operator-Name values with the
namespace "1" are supported by this algorithm - the input to the
algorithm is the actual domain name, preceeded with an "@" (but
without the "1" namespace identifier byte of that attribute).
Note well: The attribute User-Name is defined to contain UTF-8 text.
In practice, the content may or may not be UTF-8. Even if UTF-8, it
may or may not map to a domain name in the realm part. Implementors
MUST take possible conversion error paths into consideration when
parsing incoming User-Name attributes. This document describes
server discovery only for well-formed realms mapping to DNS domain
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names in UTF-8 encoding. The result of all other possible contents
of User-Name is unspecified; this includes, but is not limited to:
Usage of separators other than @
Encoding of User-Name in local encodings
UTF-8 realms which fail the conversion rules as per [RFC5891]
UTF-8 realms which end with a . ("dot") character.
For the last bullet point, "trailing dot", special precautions should
be taken to avoid problems when resolving servers with the algorithm
below: they may resolve to a RADIUS server even if the peer RADIUS
server only is configured to handle the realm without the trailing
dot. If that RADIUS server again uses NAI discovery to determine the
authoritative server, the server will forward the request to
localhost, resulting in a tight endless loop.
3.4.2. Output
Output O of the algorithm is a two-tuple consisting of: O-1) a set of
tuples {hostname; port; order/preference; Effective TTL} - the set
can be empty; and O-2) an integer: if the set in the first part of
the tuple is empty, the integer contains the Effective TTL for
backoff timeout, if the set is not empty, the integer is set to 0
(and not used).
3.4.3. Algorithm
The algorithm to determine the RADIUS server to contact is as
follows:
1. Determine P = (position of last "@" character) in I.
2. generate R = (substring from P+1 to end of I)
3. modify R according to agreed consortium procedures if applicable
4. convert R to a representation usable by the name resolution
library if needed
5. Initialize TIMER = 0; start TIMER. If TIMER reaches
DNS_TIMEOUT, continue at step 20.
6. Using the host's name resolution library, perform a NAPTR query
for R (see "Delay considerations" below). If the result is a
negative DNS response, O-2 = Effective TTL ( TTL value of the
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SOA record ) and continue at step 13. If name resolution
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.
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.
13. Generate R' = (prefix R with "_radiustls._tcp." or
"_radiustls._udp.")
14. Using the host's name resolution library, perform SRV lookup
with R' as label (see "Delay considerations" below).
15. If name resolution returns with error, O-1 = { empty set }, O-2
= BACKOFF_TIME and terminate.
16. If the result is a negative DNS response, O-1 = { empty set },
O-2 = min { O-2, Effective TTL ( TTL value of the SOA record ) }
and terminate.
17. O' = (set of {hostname; port; order/preference; Effective TTL (
all DNS TTLs that led to this result ) } for all hostnames).
18. Generate O-1 by resolving hostnames in O' into corresponding A
and/or AAAA addresses: O-1 = (set of {IP address; port; order/
preference; Effective TTL ( all DNS TTLs that led to this result
) } for all hostnames ), O-2 = 0.
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19. For each element in O-1, test if the original request which
triggered dynamic discovery was received on {IP address; port}.
If yes, O-1 = { empty set }, O-2 = BACKOFF_TIME, log error,
Terminate (see next section for a rationale). If no, O is the
result of dynamic discovery. Terminate.
20. O-1 = { empty set }, O-2 = BACKOFF_TIME, log error, Terminate.
3.4.4. Validity of results
The dynamic discovery algorithm is used by servers which do not have
sufficient configuration information to process an incoming request
on their own. If the discovery algorithm result contains the
server's own listening address (IP address and port), then this will
either lead to a tight loop (if that DNS entry has topmost priority,
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.
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
smallest of the Effective TTL value which was learned during
discovering the server has not expired, subsequent new user sessions
for the realm which corresponds to that open connection SHOULD re-use
the existing connection and SHOULD NOT re-execute the dynamic
discovery algorithm nor open a new connection. To allow for a change
of configuration, a RADIUS server SHOULD re-execute the dynamic
discovery algorithm after the Effective TTL that is associated with
this connection has expired. The server MAY keep the session open
during this re-assessment to avoid closure and immediate re-opening
of the connection should the result not have changed.
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Should the algorithm above terminate with O-1 = empty set, the RADIUS
server SHOULD NOT attempt another execution of this algorithm for the
same target realm before the timeout O-2 has passed.
3.4.5. Delay considerations
The host's name resolution library may need to contact outside
entities to perform the name resolution (e.g. authoritative name
servers for a domain), and since the NAI discovery algorithm is based
on uncontrollable user input, the destination of the lookups is out
of control of the server that performs NAI discovery. If such
outside entities are misconfigured or unreachable, the algorithm
above may need an unacceptably long time to terminate. Many RADIUS
implementations time out after five seconds of delay between Request
and Response. It is not useful to wait until the host name
resolution library signals a time-out of its name resolution
algorithms. The algorithm therefore control execution time with
TIMER. Execution of the NAI discovery algorithm SHOULD be non-
blocking (i.e. allow other requests to be processed in parallel to
the execution of the algorithm).
3.4.6. Example
Assume
a user from the Technical University of Munich, Germany, has a
RADIUS User-Name of "foobar@tu-m[U+00FC]nchen.example".
The name resolution library on the RADIUS forwarding server does
not have the realm tu-m[U+00FC]nchen.example in its forwarding
configuration, but uses DNS for name resolution and has configured
the use of Dynamic Discovery to discover RADIUS servers.
It is IPv6-enabled and prefers AAAA records over A records.
It is listening for incoming RADIUS/TLS requests on 192.0.2.1, TCP
/2083.
May the configuration variables be
DNS_TIMEOUT = 3 seconds
MIN_EFF_TTL = 60 seconds
BACKOFF_TIME = 3600 seconds
If DNS contains the following records:
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xn--tu-mnchen-t9a.example. IN NAPTR 50 50 "s"
"aaa+auth:radius.tls" "" _myradius._tcp.xn--tu-mnchen-t9a.example.
xn--tu-mnchen-t9a.example. IN NAPTR 50 50 "s"
"fooservice:bar.dccp" "" _abc123._def.xn--tu-mnchen-t9a.example.
_myradius._tcp.xn--tu-mnchen-t9a.example. IN SRV 0 10 2083
radsecserver.xn--tu-mnchen-t9a.example.
_myradius._tcp.xn--tu-mnchen-t9a.example. IN SRV 0 20 2083
backupserver.xn--tu-mnchen-t9a.example.
radsecserver.xn--tu-mnchen-t9a.example. IN AAAA
2001:0DB8::202:44ff:fe0a:f704
radsecserver.xn--tu-mnchen-t9a.example. IN A 192.0.2.3
backupserver.xn--tu-mnchen-t9a.example. IN A 192.0.2.7
Then the algorithm executes as follows, with I =
"foobar@tu-m[U+00FC]nchen.example", and no consortium name mangling
in use:
1. P = 7
2. R = "tu-m[U+00FC]nchen.example"
3. NOOP
4. name resolution library converts R to xn--tu-mnchen-t9a.example
5. TIMER starts.
6. Result:
(TTL = 47) 50 50 "s" "aaa+auth:radius.tls" ""
_myradius._tcp.xn--tu-mnchen-t9a.example.
(TTL = 522) 50 50 "s" "fooservice:bar.dccp" ""
_abc123._def.xn--tu-mnchen-t9a.example.
7. Result:
(TTL = 47) 50 50 "s" "aaa+auth:radius.tls" ""
_myradius._tcp.xn--tu-mnchen-t9a.example.
8. NOOP
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9. Successive resolution performs SRV query for label
_myradius._tcp.xn--tu-mnchen-t9a.example, which results in
(TTL 499) 0 10 2083 radsec.xn--tu-mnchen-t9a.example.
(TTL 2200) 0 20 2083 backup.xn--tu-mnchen-t9a.example.
10. NOOP
11. O' = {
(radsec.xn--tu-mnchen-t9a.example.; 2083; 10; 60),
(backup.xn--tu-mnchen-t9a.example.; 2083; 20; 60)
} // minimum TTL is 47, up'ed to MIN_EFF_TTL
12. Continuing at 18.
13. (not executed)
14. (not executed)
15. (not executed)
16. (not executed)
17. (not executed)
18. O-1 = {
(2001:0DB8::202:44ff:fe0a:f704; 2083; 10; 60),
(192.0.2.7; 2083; 20; 60)
}; O-2 = 0
19. No match with own listening address; terminate with tuple (O-1,
O-2) from previous step.
The implementation will then attempt to connect to two servers, with
preference to [2001:0DB8::202:44ff:fe0a:f704]:2083.
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4. Security Considerations
The results from the execution of this algorithm are only trustworthy
if each of the lookup steps by the name resolution library were
cryptographically secured; i.e. if DNSSEC validation was turned on
during the resolution AND all of the records were in a DNSSEC signed
zone AND validation of all those records was successful.
When using DNS without DNSSEC security extensions for at least one of
the replies to NAPTR, SRV and A/AAAA requests as described in section
Section 3, the result O can not be trusted. Even if it can be
trusted (i.e. DNSSEC is in use), actual authorization of the
discovered server to provide service for the given realm needs to be
verified. A mechanism from section Section 2.1.1.3 or equivalent
MUST be used to verify authorization.
The algorithm has a configurable completion time-out DNS_TIMEOUT
defaulting to three seconds for RADIUS' operational reasons. The
lookup of DNS resource records based on unverified user input is an
attack vector for DoS attacks: an attacker might intentionally craft
bogus DNS zones which take a very long time to reply (e.g. due to a
particularly byzantine tree structure, or artificial delays in
responses).
To mitigate this DoS vector, implementations SHOULD consider rate-
limiting either their amount of new executions of the dynamic
discovery algorithm as a whole, or the amount of intermediate
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
This document requests IANA registration of the following entries in
existing registries:
o S-NAPTR Application Service Tags registry
* aaa+auth
* aaa+acct
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* aaa+dynauth
o S-NAPTR Application Protocol Tags registry
* radius.tls
* radius.dtls
This document reserves the use of the "_radiustls" and "_radiusdtls"
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
[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
2865, June 2000.
[RFC2866] Rigney, C., "RADIUS Accounting", RFC 2866, June 2000.
[RFC3958] Daigle, L. and A. Newton, "Domain-Based Application
Service Location Using SRV RRs and the Dynamic Delegation
Discovery Service (DDDS)", RFC 3958, January 2005.
[RFC5176] Chiba, M., Dommety, G., Eklund, M., Mitton, D., and B.
Aboba, "Dynamic Authorization Extensions to Remote
Authentication Dial In User Service (RADIUS)", RFC 5176,
January 2008.
[RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
Housley, R., and W. Polk, "Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List
(CRL) Profile", RFC 5280, May 2008.
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[RFC5580] Tschofenig, H., Adrangi, F., Jones, M., Lior, A., and B.
Aboba, "Carrying Location Objects in RADIUS and Diameter",
RFC 5580, August 2009.
[RFC5891] Klensin, J., "Internationalized Domain Names in
Applications (IDNA): Protocol", RFC 5891, August 2010.
[I-D.ietf-radext-dtls]
DeKok, A., "DTLS as a Transport Layer for RADIUS", draft-
ietf-radext-dtls-05 (work in progress), April 2013.
[RFC6614] Winter, S., McCauley, M., Venaas, S., and K. Wierenga,
"Transport Layer Security (TLS) Encryption for RADIUS",
RFC 6614, May 2012.
[I-D.ietf-radext-nai]
DeKok, A., "The Network Access Identifier", draft-ietf-
radext-nai-03 (work in progress), May 2013.
Appendix A. Appendix A: ASN.1 Syntax of NAIRealm
PKIXServiceNameSAN93 {iso(1) identified-organization(3) dod(6)
internet(1) security(5) mechanisms(5) pkix(7) id-mod(0)
id-mod-dns-srv-name-93(40) }
DEFINITIONS EXPLICIT TAGS ::=
BEGIN
-- EXPORTS ALL --
IMPORTS
id-pkix
FROM PKIX1Explicit88 { iso(1) identified-organization(3)
dod(6) internet(1) security(5) mechanisms(5) pkix(7)
id-mod(0) id-pkix1-explicit(18) } ;
-- from RFC 5280
-- In the GeneralName definition using the 1993 ASN.1 syntax
-- includes:
OTHER-NAME ::= TYPE-IDENTIFIER
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-- Service Name Object Identifier
id-on OBJECT IDENTIFIER ::= { id-pkix 8 }
id-on-nai OBJECT IDENTIFIER ::= { id-on XXX }
-- Service Name
naiRealm OTHER-NAME ::= { NAIRealm IDENTIFIED BY { id-on-nai }}
NAIRealm ::= UTF8String (SIZE (1..MAX))
END
Authors' Addresses
Stefan Winter
Fondation RESTENA
6, rue Richard Coudenhove-Kalergi
Luxembourg 1359
LUXEMBOURG
Phone: +352 424409 1
Fax: +352 422473
EMail: stefan.winter@restena.lu
URI: http://www.restena.lu.
Mike McCauley
Open Systems Consultants
9 Bulbul Place
Currumbin Waters QLD 4223
AUSTRALIA
Phone: +61 7 5598 7474
Fax: +61 7 5598 7070
EMail: mikem@open.com.au
URI: http://www.open.com.au.
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