Internet DRAFT - draft-wierenga-ietf-eduroam
draft-wierenga-ietf-eduroam
Network Working Group K. Wierenga
Internet-Draft Cisco Systems
Intended status: Informational S. Winter
Expires: September 10, 2015 RESTENA
T. Wolniewicz
Nicolaus Copernicus University
March 9, 2015
The eduroam architecture for network roaming
draft-wierenga-ietf-eduroam-05.txt
Abstract
This document describes the architecture of the eduroam service for
federated (wireless) network access in academia. The combination of
IEEE 802.1X, EAP and RADIUS that is used in eduroam provides a
secure, scalable and deployable service for roaming network access.
The successful deployment of eduroam over the last decade in the
educational sector may serve as an example for other sectors, hence
this document. In particular the initial architectural and standards
choices are described, along with the changes that were prompted by
operational experience.
Status of This Memo
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This Internet-Draft will expire on September 10, 2015.
Copyright Notice
Copyright (c) 2015 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
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
1.2. Notational Conventions . . . . . . . . . . . . . . . . . 4
1.3. Design Goals . . . . . . . . . . . . . . . . . . . . . . 4
1.4. Solutions that were considered . . . . . . . . . . . . . 5
2. Classic Architecture . . . . . . . . . . . . . . . . . . . . 5
2.1. Authentication . . . . . . . . . . . . . . . . . . . . . 6
2.1.1. IEEE 802.1X . . . . . . . . . . . . . . . . . . . . . 6
2.1.2. EAP . . . . . . . . . . . . . . . . . . . . . . . . . 7
2.2. Federation Trust Fabric . . . . . . . . . . . . . . . . . 9
2.2.1. RADIUS . . . . . . . . . . . . . . . . . . . . . . . 9
3. Issues with initial Trust Fabric . . . . . . . . . . . . . . 11
3.1. Server Failure Handling . . . . . . . . . . . . . . . . . 12
3.2. No error condition signalling . . . . . . . . . . . . . . 13
3.3. Routing table complexity . . . . . . . . . . . . . . . . 14
3.4. UDP Issues . . . . . . . . . . . . . . . . . . . . . . . 15
3.5. Insufficient payload encryption and EAP server validation 16
4. New Trust Fabric . . . . . . . . . . . . . . . . . . . . . . 17
4.1. RADIUS with TLS . . . . . . . . . . . . . . . . . . . . . 18
4.2. Dynamic Discovery . . . . . . . . . . . . . . . . . . . . 19
4.2.1. Discovery of responsible server . . . . . . . . . . . 19
4.2.2. Verifying server authorisation . . . . . . . . . . . 20
4.2.3. Operational Experience . . . . . . . . . . . . . . . 21
4.2.4. Possible Alternatives . . . . . . . . . . . . . . . . 21
5. Abuse prevention and incident handling . . . . . . . . . . . 21
5.1. Incident Handling . . . . . . . . . . . . . . . . . . . . 22
5.1.1. Blocking users on the SP side . . . . . . . . . . . . 23
5.1.2. Blocking users on the IdP side . . . . . . . . . . . 24
5.1.3. Communicating account blocking to the end user . . . 25
5.2. Operator Name . . . . . . . . . . . . . . . . . . . . . . 25
5.3. Chargeable User Identity . . . . . . . . . . . . . . . . 26
6. Privacy Considerations . . . . . . . . . . . . . . . . . . . 27
6.1. Collusion of Service Providers . . . . . . . . . . . . . 27
6.2. Exposing user credentials . . . . . . . . . . . . . . . . 28
6.3. Track location of users . . . . . . . . . . . . . . . . . 28
7. Security Considerations . . . . . . . . . . . . . . . . . . . 28
7.1. Man in the middle and Tunneling Attacks . . . . . . . . . 28
7.1.1. Verification of Server Name not supported . . . . . . 29
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7.1.2. Neither Specification of CA nor Server Name checks
during bootstrap . . . . . . . . . . . . . . . . . . 29
7.1.3. User does not configure CA or Server Name checks . . 29
7.1.4. Tunneling authentication traffic to obfuscate user
origin . . . . . . . . . . . . . . . . . . . . . . . 30
7.2. Denial of Service Attacks . . . . . . . . . . . . . . . . 30
7.2.1. Intentional DoS by malign individuals . . . . . . . . 31
7.2.2. DoS as a side-effect of expired credentials . . . . . 31
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 32
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 32
9.1. Normative References . . . . . . . . . . . . . . . . . . 32
9.2. Informative References . . . . . . . . . . . . . . . . . 34
Appendix A. Acknowledgments . . . . . . . . . . . . . . . . . . 37
Appendix B. Changes . . . . . . . . . . . . . . . . . . . . . . 37
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 37
1. Introduction
In 2002 the European Research and Education community set out to
create a network roaming service for students and employees in
academia [eduroam-start]. Now over 10 years later this service has
grown to more than 10,000 service locations, serving millions of
users on all continents with the exception of Antarctica.
This memo serves to explain the considerations for the design of
eduroam as well as to document operational experience and resulting
changes that led to IETF standardization effort such as RADIUS over
TCP [RFC6613] and RADIUS with TLS [RFC6614] and that promoted
alternative uses of RADIUS like in ABFAB [I-D.ietf-abfab-arch].
Whereas the eduroam service is limited to academia, the eduroam
architecture can easily be reused in other environments.
First this memo describes the original architecture of eduroam. Then
a number of operational problems are presented that surfaced when
eduroam gained wide-scale deployment. Lastly, enhancements to the
eduroam architecture that mitigate the aforementioned issues are
discussed.
1.1. Terminology
This document uses identity management and privacy terminology from
[RFC6973]. In particular, this document uses the terms Identity
Provider, Service Provider and identity management.
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1.2. Notational Conventions
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].
Note: Also the policy that eduroam participants subscribe to,
expresses the requirements for participation in RFC 2119 language.
1.3. Design Goals
The guiding design considerations for eduroam were as follows:
- Unique identification of users at the edge of the network
The access Service Provider (SP) needs to be able to determine
whether a user is authorized to use the network resources.
Furthermore, in case of abuse of the resources, there is a
requirement to be able to identify the user uniquely (with the
cooperation of the user's Identity Provider (IdP) operator).
- Enable (trusted) guest use
In order to enable roaming it should be possible for users of
participating institutions to get seamless access to the networks of
other institutions.
Note: traffic separation between guest users and normal users is
possible (for example through the use of VLANs), and indeed widely
used in eduroam.
- Scalable
The infrastructure that is created should scale to a large number of
users and organizations without requiring a lot of coordination and
other administrative procedures (possibly with the exception of an
initial set up). Specifically, it should not be necessary for a user
that visits another organization to go through an administrative
process.
- Easy to install and use
It should be easy for both organizations and users to participate in
the roaming infrastructure as that may otherwise inhibit wide scale
adoption. In particular, there should be no or easy client
installation and only one-time configuration.
- Secure
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An important design criterion has been that there needs to be a
security association between the end-user and their Identity
Provider, eliminating the possibility of credentials theft. The
minimal requirements for security are specified in the eduroam policy
and subject to change over time. As an additional protection against
user errors and negligence, it should be possible for participating
Identity Providers add their own requirements for the quality of
authentication of their own users without the need for the
infrastructure as a whole to implement the same standard.
- Privacy preserving
The design of the system should provide for user anonymization, i.e.
a possibility to hide the user's identity from any third parties,
including Service Providers.
- Standards based
In an infrastructure in which many thousands of organizations
participate it is obvious that it should be possible to use equipment
from different vendors, therefore it is important to build the
infrastructure using open standards.
1.4. Solutions that were considered
Three architectures were trialed: one based on the use of VPN-
technology (deemed secure but not-scalable), one Web captive-portal
based (scalable but not secure) and IEEE 802.1X-based, the latter
being the basis of what is now the eduroam architecture. An overview
of the candidate architectures and their relative merits can be found
in [nrenroaming-select].
The chosen architecture is based on:
o IEEE 802.1X ([dot1X-standard]) as port based authentication
framework using
o EAP ([RFC3748]) for integrity and confidentially protected
transport of credentials and a
o RADIUS ([RFC2865]) hierarchy as trust fabric.
2. Classic Architecture
Federations, like eduroam, implement essentially two types of direct
trust relations (and one indirect). The trust relation between an
end-user and the IdP (operated by the home organization of the user)
and between the IdP and the SP (in eduroam the operator of the
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network at the visited location). In eduroam the trust relation
between user and IdP is through mutual authentication. IdPs and SP
establish trust through the use of a RADIUS hierarchy.
These two forms of trust relations in turn provide the transitive
trust relation that makes the SP trust the user to use its network
resources.
2.1. Authentication
Authentication in eduroam is achieved by using a combination of IEEE
802.1X [dot1X-standard] and EAP [RFC4372] (the latter carried over
RADIUS for guest access, see below).
2.1.1. IEEE 802.1X
By using the IEEE 802.1X [dot1X-standard] framework for port-based
network authentication, organizations that offer network access (SPs)
for visiting (and local) eduroam users can make sure that only
authorized users get access. The user (or rather the user's
supplicant) sends an access request to the authenticator (wireless
access point or switch) at the SP, the authenticator forwards the
access request to the authentication server of the SP which in turn
proxies the request through the RADIUS hierarchy to the
authentication server of the user's home organization (the IdP, see
below).
Note: The security of the connections between local wireless
infrastructure and local RADIUS servers is a part of the local
network of each SP, therefore it is out of scope for this document.
For completeness it should be stated that security between access
points and their controllers is vendor specific, security between
controllers (or standalone access points) and local RADIUS servers is
based on the typical RADIUS shared secret mechanism.
In order for users to be aware of the availability of the eduroam
service, an SP that offers wireless network access MUST broadcast the
SSID 'eduroam', unless that conflicts with the SSID of another
eduroam SP, in which case an SSID starting with "eduroam-" MAY be
used. The downside of the latter is that clients will not
automatically connect to that SSID, thus losing the seamless
connection experience.
Note: A direct implication of the common eduroam SSID is that the
users cannot distinguish between a connection to the home network and
a guest network at another eduroam institution (IEEE 802.11-2012 does
have the so-called "Interworking" extensions to make that
distinction, but these are not widely implemented yet). Furthermore,
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without proper server verification users may even be tricked into
joining a rogue eduroam network. Therefore, users should be made
aware that they should not assume data confidentiality in the eduroam
infrastructure.
To protect over-the-air user data confidentiality, IEEE 802.11
wireless networks of eduroam SP's MUST deploy WPA2+AES, and MAY
additionally support WPA/TKIP as a courtesy to users of legacy
hardware.
2.1.2. EAP
The use of the Extensible Authentication Protocol (EAP) [RFC4372]
serves 2 purposes. In the first place a properly chosen EAP-method
allows for integrity and confidentiality protected transport of the
user credentials to the home organization. Secondly, by having all
RADIUS servers transparently proxy access requests regardless of the
EAP-method inside the RADIUS packet, the choice of EAP-method is
between the 'home' organization of the user and the user, in other
words, in principle every authentication form that can be carried
inside EAP can be used in eduroam, as long as they adhere to minimal
requirements as set forth in the eduroam Service Definition
[eduroam-service-definition].
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+-----+
/ \
/ \
/ \
/ \
,----------\ | | ,---------\
| SP | | eduroam | | IdP |
| +----+ trust fabric +---+ |
`------+---' | | '-----+---'
| | | |
| \ / |
| \ / |
| \ / |
| \ / |
+----+ +-----+ +----+
| |
| |
+---+--+ +--+---+
| | | |
+-+------+-+ ___________________________ | |
| | O__________________________ ) +------+
+----------+
Host (supplicant) EAP tunnel Authentication server
Figure 1: Tunneled EAP
Proxying of access requests is based on the outer identity in the
EAP-message. Those outer identities MUST be a valid user identifier
with a mandatory realm as per [I-D.ietf-radext-nai], i.e. be of the
form something@realm or just @realm, where the realm part is the
domain name of the institution that the IdP belongs to. In order to
preserve credentials protection, participating organizations MUST
deploy EAP-methods that provide mutual authentication. For EAP
methods that support outer identity, anonymous outer identities are
recommended. Most commonly used in eduroam are the so-called
tunneled EAP-methods, that first create a server authenticated TLS
tunnel through which the user credentials are transmitted. As
depicted in Figure 1, the use of a tunneled EAP-method creates a
direct logical connection between the supplicant and the
authentication server, even though the actual traffic flows through
the RADIUS-hierarchy.
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2.2. Federation Trust Fabric
The eduroam federation trust fabric is based on RADIUS. RADIUS trust
is based on shared secrets between RADIUS peers. In eduroam any
RADIUS message originating from a trusted peer is implicitly assumed
to originate from a member of the roaming consortium.
Note: See also the security considerations for a discussion on RADIUS
securitythat motivated the work on RADIUS with TLS (RFC6614 [RFC6614]
2.2.1. RADIUS
The eduroam trust fabric consists of a proxy hierarchy of RADIUS
servers (organizational, national, global), loosely based on the DNS
hierarchy. That is, typically an organizational RADIUS server agrees
on a shared secret with a national server and the national server in
turn agrees on a shared secret with the root server. Access requests
are routed through a chain of RADIUS proxies towards the Identity
Provider of the user, and the access accept (or reject) follows the
same path back.
Note: In some circumstances there are more levels of RADIUS servers,
like for example regional or continental servers, but that doesn't
change the general model. Also, the packet exchange that is
described below requires in reality several round-trips.
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+-------+
| |
| . |
| |
+---+---+
/ | \
+----------------/ | \---------------------+
| | |
| | |
| | |
+--+---+ +--+--+ +----+---+
| | | | | |
| .edu | . . . | .nl | . . . | .ac.uk |
| | | | | |
+--+---+ +--+--+ +----+---+
/ | \ | \ |
/ | \ | \ |
/ | \ | \ |
+-----+ | +-----+ | +------+ |
| | | | | |
| | | | | |
+---+---+ +----+---+ +----+---+ +--+---+ +-----+----+ +-----+-----+
| | | | | | | | | | | |
|utk.edu| |utah.edu| |case.edu| |hva.nl| |surfnet.nl| |soton.ac.uk|
| | | | | | | | | | | |
+----+--+ +--------+ +--------+ +------+ +----+-----+ +-----------+
| |
| |
+--+--+ +--+--+
| | | |
+-+-----+-+ | |
| | +-----+
+---------+
user: paul@surfnet.nl surfnet.nl Authentication server
Figure 2: eduroam RADIUS hierarchy
Routing of access requests to the home IdP is done based on the realm
part of the outer identity. For example (see: Figure 2), when user
paul@surfnet.nl of SURFnet (surfnet.nl) tries to gain wireless
network access at the University of Tennessee at Knoxville (utk.edu)
the following happens:
o Paul's supplicant transmits an EAP access request to the Access
Point (Authenticator) at UTK with outer identity say
anonymous@surfnet.nl
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o The Access Point forwards the EAP message to its Authentication
Server (the UTK RADIUS server)
o The UTK RADIUS server checks the realm to see if it is a local
realm, since it isn't the request is proxied to the .edu RADIUS
server
o The .edu RADIUS server verifies the realm, and since it is not a
in a .edu subdomain it proxies the request to the root server
o The root RADIUS server proxies the request to the .nl RADIUS
server, since the ".nl" domain is known to the root server.
o The .nl RADIUS server proxies the request to the surfnet.nl
server, since it knows the SURFnet server.
o The surfnet.nl RADIUS server decapsulates the EAP message and
verifies the user credentials, since the user is known to SURFnet.
o The surfnet.nl RADIUS server informs the utk.edu server of the
outcome of the authentication request (Access-Accept or Access-
Reject) by proxying the outcome through the RADIUS hierarchy in
reverse order.
o The UTK RADIUS server instructs the UTK Access Point to either
accept or reject access based on the outcome of the
authentication.
Note: The depiction of the root RADIUS server is a simplification.
In reality the root server is distributed over 3 continents and each
maintains a list of the top level realms that a specific root server
is responsible for. This also means that, for intercontinental
roaming, there is an extra proxy step from one root server to the
other. Also, the physical distribution of nodes doesn't need to
mirror the logical distribution of nodes. This helps with stability
and scalability.
3. Issues with initial Trust Fabric
While the hierarchical RADIUS architecture described in the previous
section has served as the basis for eduroam operations for an entire
decade, the exponential growth of authentications is expected to lead
to, and has in fact in some cases already led to, performance and
operations bottlenecks on the aggregation proxies. The following
sections describe some of the shortcomings, and the resulting
remedies.
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3.1. Server Failure Handling
In eduroam, authentication requests for roaming users are statically
routed through pre-configured proxies. The number of proxies varies:
in a national roaming case, the number of proxies is typically 1 or 2
(some countries deploy regional proxies, which are in turn aggregated
by a national proxy); in international roaming, 3 or 4 proxy servers
are typically involved (the number may be higher along some routes).
RFC2865 [RFC2865] does not define a failover algorithm. In
particular, the failure of a server needs to be deduced from the
absence of a reply. Operational experience has shown that this has
detrimental effects on the infrastructure and end user experience:
1. Authentication failure: the first user whose authentication path
is along a newly-failed server will experience a long delay and
possibly timeout
2. Wrongly deduced states: since the proxy chain is longer than 1
hop, a failure further along in the authentication path is
indistinguishable from a failure in the next hop.
3. Inability to determine recovery of a server: only a "live"
authentication request sent to a server which is believed
inoperable can lead to the discovery that the server is in
working order again. This issue has been resolved with RFC5997
[RFC5997].
The second point can have significant impact on the operational state
of the system in a worst-case scenario: Imagine one realm's home
server being inoperable. A user from that realm is trying to roam
internationally and tries to authenticate. The RADIUS server on the
hotspot location may assume its own national proxy is down, because
it does not reply. That national server, being perfectly alive, in
turn will assume that the international aggregation proxy is down;
which in turn will believe the home country proxy national server is
down. None of these assumptions are true. Worse yet: should any of
these servers trigger a failover to a redundant backup RADIUS server,
it will still not receive a reply, because the request will still be
routed to the same defunct home server. Within a short time, all
redundant aggregation proxies might be considered defunct by their
preceding hop.
In the absence of proper next-hop state derivation, some interesting
concepts have been introduced by eduroam participants; the most
noteworthy being a failover logic which considers up/down states not
per next-hop RADIUS peer, but instead per realm (See [dead-realm] for
details). As of recent, RFC5997 [RFC5997] implementations and
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cautious failover parameters make false "downs" unlikely to happen,
as long as every hop implements RFC5997. Dead realm detection in
that case serves mainly to prevent proxying of large numbers of
requests to known dead realms.
3.2. No error condition signalling
The RADIUS protocol lacks signalling of error conditions, and the
IEEE 802.1X protocol does not allow to convey extended failure
reasons to the end-user's device. For eduroam, this creates issues
in a twofold way:
o The home server may have an operational problem, for example its
authentication decisions may depend on an external data source
such as ActiveDirectory or an SQL server, and the external data
source is unavailable. If the RADIUS interface is still
functional, there are two options how to reply to an Access-
Request which can't be serviced due to such error conditions:
1. Do Not Reply: the inability to reach a conclusion can be
treated by not replying to the request. The upside of this
approach is that the end-user's software doesn't come to wrong
conclusions and won't give unhelpful hints such as "maybe your
password is wrong". The downside is that intermediate proxies
may come to wrong conclusions because their downstream RADIUS
server isn't responding.
2. Reply with Reject: in this option, the inability to reach a
conclusion is treated like an authentication failure. The
upside of this approach is that intermediate proxies maintain
a correct view on the reachability state of their RADIUS peer.
The downside is that EAP supplicants on end-user devices often
react with either false advice ("your password is wrong") or
even trigger permanent configuration changes (e.g. the Windows
built-in supplicant will delete the credential set from its
registry, prompting the user for their password on the next
connection attempt). The latter case of Windows is a source
of significant helpdesk activity; users may have forgotten
their password after initially storing it, but are suddenly
prompted again.
There have been epic discussions in the eduroam community as well as
in the IETF RADEXT Working Group as to which of the two approaches is
more appropriate; but they were not conclusive.
o Similar considerations as above apply when an intermediate proxy
does not receive a reply from a downstream RADIUS server. The
proxy may either choose not to reply to the original request,
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leading to retries and its upstream peers coming to wrong
conclusions about its own availability; or it may decide to reply
with Access-Reject to indicate its own liveliness, but again with
implications for the end user.
The ability to send Status-Server watchdog requests is only of use
after the fact, in case a downstream server doesn't reply (or hasn't
been contacted in a long while, so that it's previous working state
is stale). The active link-state monitoring of the TCP connection
with e.g. RADIUS/TLS (see below) gives a clearer indication whether
there is an alive RADIUS peer, but does not solve the defunct backend
problem. An explicit ability to send Error-Replies, on the RADIUS
(for other RADIUS peer information) and EAP level (for end-user
supplicant information), would alleviate these problems but is
currently not available.
3.3. Routing table complexity
The aggregation of RADIUS requests based on the structure of the
user's realm implies that realms ending with the same top-level
domain are routed to the same server; i.e. to a common administrative
domain. While this is true for country code Top Level Domains
(ccTLDs), which map into national eduroam federations, it is not true
for realms residing in generic Top Level Domains (gTLDs). Realms in
gTLDs were historically discouraged because the automatic mapping
"realm ending" -> "eduroam federation's server" could not be applied.
However, with growing demand from eduroam realm administrators, it
became necessary to create exception entries in the forwarding rules;
such realms need to be mapped on a realm-by-realm basis to their
eduroam federations. Example: "kit.edu" (Karsruher Institut fuer
Technologie) needs to be routed to the German federation server,
whereas "iu.edu" (Indiana University) needs to be routed to the USA
federation server.
While the ccTLDs occupy only approx. 50 routing entries in total (and
have a upper bound of approx. 200), the potential size of the routing
table becomes virtually unlimited if it needs to accomodate all
individual entries in .edu, .org, etc.
In addition to that, all these routes need to be synchronised between
three international root servers, and the updates need to be applied
manually to RADIUS server configuration files. The frequency of the
required updates makes this approach fragile and error-prone as the
number of entries grows.
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3.4. UDP Issues
RADIUS is based on UDP, which was a reasonable choice when its main
use was with simple PAP requests which required only exactly one
packet exchange in each direction.
When transporting EAP over RADIUS, the EAP conversations requires
multiple round-trips; depending on the total payload size, 8-10
round-trips are not uncommon. The loss of a single UDP packet will
lead to user-visible delays and might result in servers being marked
as dead due to the absence of a reply. The proxy path in eduroam
consists of several proxies, all of which introduce a very small
packet loss probability; i.e. the more proxies are needed, the higher
the failure rate is going to be.
For some EAP types, depending on the exact payload size they carry,
RADIUS servers and/or supplicants may choose to fill as much EAP data
into a single RADIUS packet as the supplicant's layer 2 medium allows
for, typically 1500 Bytes. In that case, the RADIUS encapsulation
around the EAP-Message will add more bytes to the overall RADIUS
payload size and in the end exceed the 1500 Byte limit, leading to
fragmentation of the UDP datagram on the IP layer. While this is not
a problem in theory, practice has shown evidence of misbehaving
firewalls which erroneously discard non-first UDP fragments, which
ultimately leads to a denial of service for users with such EAP types
and that specific configuration.
One EAP type proved to be particularly problematic: EAP-TLS. While
it is possible to configure the EAP server to send smaller chunks of
EAP payload to the supplicant (e.g. 1200 Bytes, to allow for another
300 Bytes of RADIUS overhead without fragmentation), very often the
supplicants which send the client certificate do not expose such a
configuration detail to the user. Consequently, when the client
certificate is beyond 1500 Bytes in size, the EAP-Message will always
make use of the maximum possible layer-2 chunk size, which introduces
the fragmentation on the path from EAP peer to EAP server.
Both of the previously mentioned sources of errors (packet loss,
fragment discard) lead to significant frustration for the affected
users. Operational experience of eduroam shows that such cases are
hard to debug since they require coordinated cooperation of all
eduroam administrators on the authentication path. For that reason
the eduroam community is developing monitoring tools that help to
locate fragmentation problems.
Note: For more detailed discussion of these issues please refer to
section 1.1 of [RFC6613].
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3.5. Insufficient payload encryption and EAP server validation
The RADIUS protocol's design foresaw only the encryption of select
RADIUS attributes, most notably User-Password. With EAP methods
conforming to the requirements of [RFC4017], the user's credential is
not transmitted using the User-Password attribute, and stronger
encryption than the one for RADIUS' User-Password is in use
(typically TLS).
Still, the use of EAP does not encrypt all personally identifiable
details of the user session as some are carried inside clear-text
RADIUS attributes. In particular, the user's device can be
identified by inspecting the Calling-Station-ID attribute; and the
user's location may be derived from observing NAS-IP-Address, NAS-
Identifier or Operator-Name attributes. Since these attributes are
not encrypted, even IP-layer third parties can harvest the
corresponding data. In a worst-case scenario, this enables the
creation of mobility profiles. Pervasive passive surveillance using
this connection metadata such as the recently uncovered NSA/GCHQ
incidents becomes possible by tapping RADIUS traffic from an IP hop
near a RADIUS aggregation proxy. While this is possible, the authors
are not aware whether this has actually been done.
These profiles are not necessarily linkable to an actual user because
EAP allows for the use of anonymous outer identities and protected
credential exchanges. However, practical experience has shown that
many users neglect to configure their supplicants in a privacy-
preserving way or their supplicant doesn't support that. In
particular, for EAP-TLS users, the use of EAP-TLS identity protection
is not usually implemented and cannot be used. In eduroam, concerned
individuals and IdPs which use EAP-TLS are using pseudonymous client
certificates to provide for better privacy.
One way out, at least for EAP types involving a username, is to
pursue the creation and deployment of pre-configured supplicant
configurations which makes all the required settings in user devices
prior to their first connection attempt; this depends heavily on the
remote configuration possibilities of the supplicants though.
A further threat involves the verification of the EAP server's
identity. Even though the cryptographic foundation, TLS tunnels, is
sound, there is a weakness in the supplicant configuration: many
users do not understand or are willing to invest time into the
inspection of server certificates or the installation of a trusted
CA. As a result, users may easily be tricked into connecting to an
unauthorized EAP server, ultimately leading to a leak of their
credentials to that unauthorized third party.
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Again, one way out of this particular threat is to pursue the
creation and deployment of pre-configured supplicant configurations
which makes all the required settings in user devices prior to their
first connection attempt.
Note: there are many different and vendor-proprietary ways to pre-
configure a device with the necessary EAP parameters (examples
include Apple, Inc's "mobileconfig" and Microsoft's "EAPHost" XML
schema). Some manufacturers even completely lack any means to
distribute EAP configuration data. We believe there is value in
defining a common EAP configuration metadata format which could be
used across manufacturers, ideally leading to a situation where IEEE
802.1X network end-users merely need to apply this configuration file
to configure any of their devices securely with the required
connection properties.
Another possible privacy threat involves transport of user-specific
attributes in a Reply-Message. If, for example, a RADIUS server
sends back a hypothetical RADIUS Vendor-Specific-Attribute "User-Role
= Student of Computer Science" (e.g. for consumption of an SP RADIUS
server and subsequent assignment into a "student" VLAN), this
information would also be visible for third parties and could be
added to the mobility profile.
The only way out to mitigate all information leakage to third parties
is by protecting the entire RADIUS packet payload so that IP-layer
third parties cannot extract privacy-relevant information. RFC2865
RADIUS does not offer this possibility though. This motivated
[RFC6614], see below.
4. New Trust Fabric
The operational difficulties with an ever increasing number of
participants, as documented in the previous section, have led to a
number of changes to the eduroam architecture that in turn have, as
mentioned in the introduction, led to standardization effort.
Note: The enhanced architecture components are fully backwards
compatible with the existing installed base, and are in fact
gradually replacing those parts of it where problems may arise.
Whereas the user authentication using IEEE 802.1X and EAP has
remained unchanged (i.e. no need for end-users to change any
configurations), the issues as reported above have resulted in a
major overhaul of the way EAP messages are transported from the
RADIUS server of the SP to that of the IdP and back. The two
fundamental changes are the use of TCP instead of UDP and reliance on
TLS instead of shared secrets between RADIUS peers.
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4.1. RADIUS with TLS
The deficiencies of RADIUS over UDP as described in Section 3.4
warranted a search for a replacement of RFC2865 [RFC2865] for the
transport of EAP. By the time this need was understood, the
designated successor protocol to RADIUS, Diameter [RFC3588], was
already specified by the IETF. However, within the operational
constraints of eduroam:
o reasonably cheap to deploy on many administrative domains
o supporting NASREQ Application
o supporting EAP Application
o supporting Diameter Redirect
o supporting validation of authentication requests of the most
popular EAP types (EAP-TTLS, PEAP, and EAP-TLS)
o possibility to retrieve these credentials from popular backends
such as Microsoft ActiveDirectory, MySQL
no single combination of software could be found. In addition to
that, no Wireless Access Points at the disposal of eduroam
participants supported Diameter, nor did any of the manufacturers
have a roadmap towards Diameter support (that is believed to still be
true, more than 10 years later). This led to the open question of
lossless translation from RADIUS to Diameter and vice versa; a
question not satisfactorily answered by NASREQ.
After monitoring the Diameter implementation landscape for a while,
it became clear that a solution with better compatibility and a
plausible upgrade path from the existing RADIUS hierarchy was needed.
The eduroam community actively engaged in the IETF towards the
specification of several enhancements to RADIUS to overcome the
limitations mentioned in Section 3. The outcome of this process was
[RFC6614] and [I-D.ietf-radext-dynamic-discovery].
With its use of TCP instead of UDP, and with its full packet
encryption, while maintaining full packet format compatibility with
RADIUS/UDP, RADIUS/TLS [RFC6614] allows to upgrade any given RADIUS
link in eduroam without the need of a "flag day".
In a first upgrade phase, the classic eduroam hierarchy (forwarding
decision taken by inspecting the realm) remains intact. That way,
RADIUS/TLS merely enhances the underlying transport of the RADIUS
datagrams. But this already provides some key advantages:
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o explicit peer reachability detection using long-lived TCP sessions
o protection of user credentials and all privacy-relevant RADIUS
attributes
RADIUS/TLS connections for the static hierarchy could be realised
with the TLS-PSK operation mode (which effectively provides a 1:1
replacement for RADIUS/UDP's "shared secrets"), but since this
operation mode is not widely supported as of yet, all RADIUS/TLS
links in eduroam are secured by TLS with X.509 certificates from a
set of accredited CAs.
This first deployment phase does not yet solve the routing table
complexity problem (see (Section 3.3); this aspect is covered by
introducing dynamic discovery for RADIUS/TLS servers.
4.2. Dynamic Discovery
When introducing peer discovery, two separate issues had to be
addressed:
1. How to find the network address of a responsible RADIUS server
for a given realm?
2. How to verify that this realm is an authorized eduroam
participant?
4.2.1. Discovery of responsible server
Issue 1 can relatively simply be addressed by putting eduroam-
specific service discovery information into the global DNS tree. In
eduroam this is done by using Naming Authority Pointer (NAPTR)
records as per the S-NAPTR specification [RFC3958] with a private-use
NAPTR service tag ("x-eduroam:radius.tls"). The usage profile of
that NAPTR resource record is that exclusively "S" type delegations
are allowed, and that no regular expressions are allowed.
A subsequent lookup of the resulting SRV records will eventually
yield hostnames and IP addresses of the authoritative server(s) of a
given realm.
Example (wrapped for readability):
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> dig -t naptr education.example.
;; ANSWER SECTION:
education.example. 43200 IN NAPTR 100 10 "s"
"x-eduroam:radius.tls" ""
_radsec._tcp.eduroam.example.
> dig -t srv _radsec._tcp.eduroam.example.
;; ANSWER SECTION:
_radsec._tcp.eduroam.example. 43200 IN SRV 0 0 2083
tld1.eduroam.example.
> dig -t aaaa tld1.eduroam.example.
;; ANSWER SECTION:
tld1.eduroam.example. 21751 IN AAAA 2001:db8:1::2
Figure 3: SRV record lookup
From the operational experience with this mode of operation, eduroam
is pursuing standardisation of this approach for generic AAA use
cases. The current radext working group document for this is
[I-D.ietf-radext-dynamic-discovery].
Note: It is worth mentioning that this move to a more complex,
flexible sytem may make the system as a whole more fragile, as
compared to the static set up.
4.2.2. Verifying server authorisation
Any organisation can put "x-eduroam" NAPTR entries into their Domain
Name Server, pretending to be eduroam Identity Provider for the
corresponding realm. Since eduroam is a service for a heterogeneous,
but closed, user group, additional sources of information need to be
consulted to verify that a realm with its discovered server is
actually an eduroam participant.
The eduroam consortium has chosen to deploy a separate PKI
infrastructure which issues certificates only to authorised eduroam
Identity Providers and eduroam Service Providers. Since certificates
are needed for RADIUS/TLS anyway, it was a straightforward solution
to reuse the PKI for that. The PKI fabric allows multiple CAs as
trust roots (overseen by a Policy Management Authority), and requires
that certificates which were issued to verified eduroam participants
are marked with corresponding "X509v3 Policy OID" fields; eduroam
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RADIUS servers and clients need to verify the existence of these OIDs
in the incoming certificates.
The policies and OIDs can be retrieved from the "eduPKI Trust Profile
for eduroam Certificates" ([edupki]).
4.2.3. Operational Experience
The discovery model as described above is currently deployed in
approximately 10 countries that participate in eduroam, making more
than 100 realms discoverable via their NAPTR records. Experience has
shown that the model works and scales as expected; the only drawback
being that the additional burden of operating a PKI which is not
local to the national eduroam administrators creates significant
administrative complexities. Also, the presence of multiple CAs and
regular updates of Certificate Revocation Lists makes the operation
of RADIUS servers more complex.
4.2.4. Possible Alternatives
There are two alternatives to the above approach which are monitored
by the eduroam community:
1. DNSSEC + DANE TLSA records
2. ABFAB Trust Router
For DNSSEC+DANE TLSA, its biggest advantage is that the certificate
data itself can be stored in the DNS - possibly obsoleting the PKI
infrastructure *if* a new place for the server authorization checks
can be found. Its most significant downside is that the DANE
specifications only include client-to-server certificate checks,
while RADIUS/TLS requires also server-to-client verification.
For the ABFAB Trust Router, the biggest advantage is that it would
work without certificates altogether (by negotiating TLS-PSK keys ad-
hoc). The downside is that it is currently not formally specified
and not as thoroughly understood as any of the other solutions.
5. Abuse prevention and incident handling
Since the eduroam service is a confederation of autonomous networks,
there is little justification for transferring accounting information
from the Service Provider to any other in general, or in particular
to the Identity Provider of the user. Accounting in eduroam is
therefore considered to be a local matter of the Service Provider.
The eduroam compliance statement ([eduroam-compliance]) in fact
specifies that accounting traffic SHOULD NOT be forwarded.
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The static routing infrastructure of eduroam acts as a filtering
system blocking accounting traffic from misconfigured local RADIUS
servers. Proxy servers are configured to terminate accounting
request traffic by answering to Accounting-Requests with an
Accounting-Response in order to prevent the retransmission of
orphaned Accounting-Request messages. With dynamic discovery,
Identity Providers which are discoverable via DNS will need to apply
these filtering measures themselves. This is an increase in
complexity of the Identity Provider RADIUS configuration.
Roaming creates accountability problems, as identified by [RFC4372]
(Chargeable User Identity). Since the NAS can only see the (likely
anonymous) outer identity of the user, it is impossible to correlate
usage with a specific user (who may use multiple devices). A NAS
that supports this can request the Chargeable-User-Identity and, if
supplied by the authenticating RADIUS server in the Access-Accept
message, add this value to corresponding Access-Request packets.
While eduroam does not have any charging mechanisms, it may still be
desirable to identify traffic originating from one particular user.
One of the reasons is to prevent abuse of guest access by users
living nearby university campuses. Chargeable User Identity (see
below) supplies the perfect answer to this problem, however at the
moment of writing, to our knowledge only one hardware vendor (Meru
Networks) implements RFC4372 on their Access Points. For all other
vendors, requesting the Chargeable-User-Identity attribute needs to
happen on the RADIUS server to which the Access Point is connected
to. FreeRADIUS supports this ability in the latest distribution, and
Radiator can be retrofitted to do the same.
5.1. Incident Handling
10 years of experience with eduroam have not exposed any serious
incident. This may be taken as evidence for proper security design
as well as suggest that awareness of users that they are
identifiable, acts as an effective deterrent. It could of course
also mean that eduroam operations lack the proper tools or insight
into the actual use and potential abuse of the service. In any case,
many of the attack vectors that exist in open networks or networks
where access control is based on shared secrets are not present,
arguably leading to a much more secure system.
Below a discussion of countermeasures that are taken in eduroam.
The European eduroam policy Service Definition
[eduroam-service-definition], as an example, describes incident
scenarios and actions to be taken, in this document we present the
relevant technical issues.
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The initial implementation has been lacking reliable tools for an SP
to make it's own decision or for an IdP to introduce a conditional
rule applying only to a given SP. The introduction of support for
Operator-Name and Chargeable-User-Identity (see below) to eduroam
makes both of these scenarios possible.
5.1.1. Blocking users on the SP side
The first action in the case of an incident is to block the user's
access to eduroam at the Service Provider. Since the roaming user's
true identity is likely hidden behind an anonymous/fake outer
identity, the Service Provider can only rely on the realm of the user
and his MAC address; if the Identity Provider has already sent a
Chargeable-User-Identity (see Section 5.3 for details), then this
extra information can be used to identify the user more reliably.
A first attempt at the SP side may be to block based on the MAC
address or outer identity. This blocking can be executed before the
EAP authentication occurs - typically in the first datagram, acting
on the RADIUS attributes EAP-Message/EAP-Response/Identity and
Calling-Station-ID. The datagram can either be dropped (supplicant
notices a time-out) or replied-to with a RADIUS Access-Reject
containing an EAP-Failure. Since malicious users can change both
their MAC addresses and the local part of their outer identity
between connection attempts, this first attempt is not sufficient to
lock out a determined user.
As a second measure, the SP can let the EAP authentication proceed as
normal, and verify whether the final Access-Accept response from the
RADIUS server contains a Chargeable-User-Identity (CUI). If so, the
SP RADIUS server can be configured to turn all future Access- Accepts
for this CUI into an Access-Reject/EAP-Failure. This measure is
effective and efficient: it locks out exactly the one user which is
supposed to be locked out, and has no side-effects on other users,
even from the same realm.
If the EAP authentication does not reveal a CUI, the SP can not
reliably determine the user in question. The only reliable
information to act upon is then the realm portion of the outer
identity of the user. The SP will need to resort to blocking the
entire realm that the offending user belongs to. This can be done at
the EAP-Message/EAP-Repsonse/Identity stage as described above).
This is effective, but not efficient: it locks out the user in
question, but has a DoS side-effect on all other visiting users from
the same realm.
In the absence of a CUI handle, SPs which are not willing to take the
drastic step of blocking an entire realm will be forced to contact
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the Identity Provider in question and demand that the user be blocked
at the IdP side. This involves human interaction between SP and IdP
is not possible in real-time.
5.1.2. Blocking users on the IdP side
The IdP has the power to disable a user account altogether, thus
blocking this user from accessing eduroam in all sites. If blocking
the user is done due a request of an SP (as per the previous
section), there may be a more fine-grained possibility to block
access to a specific SP - if the SP in question sends the Operator-
Name attribute along with his Access-Requests (see Section 5.2 for
details).
If the IdP decides to block the user globally, this is typically done
by treating the login attempt as if the credentials were wrong: the
entire EAP conversation needs to be executed to the point where the
true inner identity is revealed (before that, the IdP does not know
yet which user is attempting to authenticate). This typically
coincides with the point in time where credentials are exchanged.
Instead, or in addition to, checking the credential for validity, the
Identity Provider also checks whether the user's account is (still)
eligible for eduroam use and will return an Access- Reject/EAP-
Failure if not.
There may well be cases where opinions between the SP desiring a user
lockout and the IdP of the user differ. E.g. an SP might consider
massive amounts of up-/downloads with file sharing protocols
unacceptable as per local policy, and desire blocking of users that
create too much traffic - but the IdP does not take offense on such
actions and would not want to lock his user out of eduroam globally
because of this one SP's opinion.
In the absence of the Operator-Name attribute, there is no way to
apply a login restriction only for a given SP and not eduroam as a
whole; eduroam eligibility is an all-or-nothing decision for the IdP.
If the Operator-Name attribute is present, the IdP can use this
information to fail the authentication attempt only if the attempt
originated from SPs which desire such blocking. Even though the
Operator-Name attribute is available from the first RADIUS Access-
Request datagram onwards, the EAP authentication needs to be carried
out until the true inner identity is known just as in the global
blocking case above. The Operator-Name is simply an additional piece
of information which the IdP can use to make its decision.
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5.1.3. Communicating account blocking to the end user
All the measures above alter the EAP conversation. They either
create a premature rejection or timeout at the start of the
conversation, or change the outcome of the authentication attempt at
the very end of the conversation.
On the supplicant side, these alterations are undistinguishable from
an infrastructure failure: a premature rejection or timeout could be
due to a RADIUS server being unresponsive, and a rejection at the end
of the conversation could be either user error (wrong password) or
server error (credential lookup failed). For the supplicant, it is
thus difficult to communicate a meaningful error to the user. The
newly specified EAP type TEAP, "Tunnel Extensible Authentication
Protocol" [RFC7170] has a means to transport fine-grained error
reason codes to the supplicant; this has the potential to improve the
situation in the future.
The EAP protocol foresees one mechanism to provide such user-
interactive communication: the EAP state machine contains states
which allow user-visible communication: an extra round of EAP-
Request/Notification and the corresponding acknowledgement can be
injected before the final EAP-Failure.
However, anecdotal evidence suggests that supplicants typically do
not implement this part of the EAP state machine at all. One
supplicant is reported to support it, but only logs the contents of
the notification in a log file - which is not at all helpful for the
end user.
The discovery of reasons and scope of account blocking are thus left
as an out-of-band action. The eduroam user support process requires
that users with authentication problems contact their Identity
Provider as a first measure (via unspecified means, e.g. they could
phone their IdP or send an email via a 3G backup link). If the
Identity Provider is the one which blocked their access, the user
will immediately be informed by them. If the reason for blocking is
at the SP side, the Identity Provider will instead inform the user
that the account is in working order and that the user needs to
contact the SP IT support to get further insight. In that case, that
SP-side IT support will notify the users about the reasons for
blocking.
5.2. Operator Name
The Operator-Name attribute is defined in [RFC5580] as a means of
unique identification of the access site.
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The Proxy infrastructure of eduroam makes it impossible for home
sites to tell where their users roam to. While this may be seen as a
positive aspect enhancing user's privacy, it also makes user support,
roaming statistics and blocking offending user's access to eduroam
significantly harder.
Sites participating in eduroam are encouraged to add the Operator-
Name attribute using the REALM namespace, i.e. sending a realm name
under control of the given site.
The introduction of Operator-Name in eduroam has identified one
operational problem - the identifier 126 assigned to this attribute
has been previously used by some vendors for their specific purposes
and has been included in attribute dictionaries of several RADIUS
server distributions. Since the syntax of this hijacked attribute
had been set to Integer, this introduces a syntax clash with the the
RFC definition (Text). Operational tests in eduroam have shown that
servers using the Integer syntax for attribute 126 may either
truncate the value to 4 octets or even drop the entire RADIUS packet
(thus making authentication impossible). The eduroam monitoring and
eduroam test tools try to locate problematic sites. [RFC6929]
clarifies in Section 2.8 the handling of these packets.
When a Service Provider sends its Operator-Name value, it creates a
possibility for the home sites to set up conditional blocking rules,
depriving certain users of access to selected sites. Such action
will cause much less concern than blocking users from all of eduroam.
In eduroam the Operator Name is also used for the generation of
Chargeable User Identity values.
The addition of Operator-Name is a straightforward configuration of
the RADIUS server and may be easily introduced on a large scale.
5.3. Chargeable User Identity
The Chargeable-User-Identity (CUI) attribute is defined by RFC4372
[RFC4372] as an answer to accounting problems caused by the use of
anonymous identity in some EAP methods. In eduroam the primary use
of CUI is in incident handling, but it can also enhance local
accounting.
The eduroam policy requires that a given user's CUI generated for
requests originating from different sites should be different (to
prevent collusion attacks). The eduroam policy thus mandates that a
CUI request be accompanied by the Operator-Name attribute, which is
used as one of the inputs for the CUI generation algorithm. The
Operator-Name requirement is considered to be the "business
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requirement" described in Section 2.1 of RFC4372 [RFC4372] and hence
conforms to the RFC.
When eduroam started considering using CUI, there were no NAS
implementations, therefore the only solution was moving all CUI
support to the RADIUS server.
CUI request generation requires only the addition of NUL CUI
attributes to outgoing Access-Requests, however the real strength of
CUI comes with accounting. Implementation of CUI based accounting in
the server requires that the authentication and accounting RADIUS
servers used directly by the NAS are actually the same or at least
have access to a common source of information. Upon processing of an
Access-Accept the authenticating RADIUS server must store the
received CUI value together with the device's Calling-Station-Id in a
temporary database. Upon receipt of an Accounting-Request, the
server needs to update the packet with the CUI value read from the
database.
A wide introduction of CUI support in eduroam will significantly
simplify incident handling at Service Providers. Introducing local,
per-user access restriction will be possible. Visited sites will
also be able to notify the home site about the introduction of such a
restriction, pointing to the CUI value and thus making it possible
for the home site to identify the user. When the user reports the
problem at his home support, the reason will be already known.
6. Privacy Considerations
The eduroam architecture has been designed with protection of user
credentials in mind as may be clear from the discussion above.
However, operational experience has revealed some more subtle points
with regards to privacy.
6.1. Collusion of Service Providers
If users use anonymous outer identities, SPs cannot easily collude by
linking outer identities to users that are visiting their campus.
This poses however problems with remediation of abuse or
misconfiguration. It is impossible to find the user that exhibits
unwanted behaviour or whose system has been compromised.
For that reason the Chargeable-User-Identity has been introduced in
eduroam, constructed so that only the IdP of the user can uniquely
identify the user. In order to prevent collusion attacks that CUI is
required to be unique per user per Service Provider.
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6.2. Exposing user credentials
Through the use of EAP, user credentials are not visible to anyone
but the IdP of the user. That is, if a sufficiently secure EAP-
method is chosen and EAP is not terminated prematurely.
There is one privacy sensitive user attribute that is necessarily
exposed to third parties and that is the realm the user belongs to.
Routing in eduroam is based on the realm part of the user identifier,
so even though the outer identity in a tunneled EAP-method may be set
to an anonymous identifier it MUST contain the realm of the user, and
may thus lead to identifying the user if the realm in question
contains few users. This is considered a reasonable trade-of between
user privacy and usability.
6.3. Track location of users
Due to the fact that access requests (potentially) travel through a
number of proxy RADIUS servers, the home IdP of the user typically
cannot tell where a user roams to.
The introduction of Operator-Name and dynamic lookups (i.e. direct
connections between IdP and SP) however, give the home IdP insight
into the location of the user.
7. Security Considerations
This section addresses only security considerations associated with
the use of eduroam. For considerations relating to IEEE 802.1X,
RADIUS and EAP in general, the reader is referred to the respective
specification and to other literature.
7.1. Man in the middle and Tunneling Attacks
The security of user credentials in eduroam ultimately lies within
the EAP server verification during the EAP conversation. Therefore,
the eduroam policy mandates that only EAP types capable of mutual
authentication are allowed in the infrastructure, and requires that
IdPs publish all information that is required to uniquely identify
the server (i.e. usually the EAP server's CA certificate and its
Common Name or subjectAltName:dNSName).
While this in principle makes Man-in-the-middle attacks impossible,
practice has shown that several attack vectors exist nonetheless.
Most of these deficiencies are due to implementation shortcomings in
EAP supplicants. Examples:
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7.1.1. Verification of Server Name not supported
Some supplicants only allow to specify which CA issues the EAP server
certificate; it's name is not checked. As a result, any entity that
is able to get a server certificate from the same CA can create its
own EAP server and trick the end user to submit his credentials to
that fake server.
As a mitigation to that problem, eduroam Operations suggests the use
of a private CA which exclusively issues certificates to the
organisation's EAP servers. In that case, no other entity will get a
certificate from the CA and the above supplicant shortcoming does not
present a security threat any more.
7.1.2. Neither Specification of CA nor Server Name checks during
bootstrap
Some supplicants allow for insecure bootstrapping in that they allow
to simply select a network and accept the incoming server
certificate, identified by its fingerprint. The certificate is then
saved as trusted for later re-connection attempts. If users are near
a fake hotspot during initial provisioning, they may be tricked to
submit their credentials to a fake server; and furthermore will be
branded to trust only that fake server in the future.
eduroam Identity Providers are advised to provide their users with
complete documentation for setup of their supplicants without the
shortcut of insecure bootstrapping. In addition, eduroam Operations
has created a tool which makes correct, complete and secure settings
on many supplicants: eduroam CAT ([eduroam-cat] ).
7.1.3. User does not configure CA or Server Name checks
Unless automatic provisioning tools such as eduroam CAT are used, it
is cumbersome for users to initially configure an EAP supplicant
securely. User Inferfaces of supplicants often invite the users to
take shortcuts ("Don't check server certificate") which are easier to
setup or hide important security settings in badly accessible sub-
menus. Such shortcuts or security parameter ommissions make the user
subject to man-in-the-middle attacks.
eduroam IdPs are advised to educate their users regarding the
necessary steps towards a secure setup. eduroam Research and
Development is in touch with supplicant developers to improve their
User Interfaces.
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7.1.4. Tunneling authentication traffic to obfuscate user origin
There is no link between the EAP outer ("anonymous") identity and the
EAP inner ("real") identity. In particular, they can both contain a
realm name, and the realms need not be identical. It is possible to
craft packets with an outer identity of user@RealmB, and an inner
identity of user@realmA. With the eduroam request routing, a Service
Provider would assume that the user is from realmB and send the
request there. The server at realm B inspects the inner user name,
and if proxying is not explicitly disabled for tunneled request
content, may decide to send the tunneled EAP payload to realmA, where
the user authenticates. A CUI value would likely be generated by the
server at realmB, even though this is not its user.
Users can craft such packets to make their identification harder;
usually, the eduroam SP would assume the troublesome user to
originate from realmB and demand there that the user be blocked. The
operator of realmB however has no control over the user, and can only
trace back the user to his real origin if logging of proxied requests
is also enabled for EAP tunnel data.
eduroam Identity Providers are advised to explicitly disable proxying
on the parts of their RADIUS server configuration which processes EAP
tunnel data.
7.2. Denial of Service Attacks
Since eduroam's roaming infrastructure is based on IP and RADIUS, it
suffers from the usual DoS attack vectors that apply to these
protocols.
The eduroam hotspots are susceptible to typical attacks on consumer
edge networks, such as rogue RA, rogue DHCP servers, and others.
Notably, eduroam hotspots are more robust against malign users' DHCP
pool exhaustion than typical open or "captive portal" hotspots,
because a DHCP address is only leased after a successful
authentication, which reduces the pool of possible attackers to
eduroam account holders (as opposed to the general public).
Furthermore, attacks involving ARP spoofing or ARP flooding are also
reduced to authenticated users, because an attacker needs to be in
possession of a valid WPA2 session key to be able to send traffic on
the network.
This section does not discuss standard threats to consumer edge
networks and IP networks in general. The following sections describe
attack vectors specific to eduroam.
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7.2.1. Intentional DoS by malign individuals
The eduroam infrastructure is more robust against Distributed DoS
attacks than typical services which are reachable on the internet
because triggering authentication traffic can only be done when
physically being in proximity of an eduroam hotspot (be it a wired
IEEE 802.1X enabled socket or a Wi-Fi Access Point).
However, when being in the vicinity, it is easy to craft
authentication attempts that traverse the entire international
eduroam infrastructure; an attacker merely needs to choose a realm
from another world region than his physical location to trigger
Access-Requests which need to be processed by the SP, then SP-side
national, then world region, then target world region, then target
national, then target IdP server. So long as the realm actually
exists, this will be followed by an entire EAP conversation on that
path. Not having actual credentials, the request will ultimately be
rejected; but it consumed processing power and bandwidth across the
entire infrastructure, possibly affecting all international
authentication traffic.
EAP is a lock-step protocol. A single attacker at an eduroam hotspot
can only execute one EAP conversation after another, and is thus
rate-limited by round-trip times of the RADIUS chain.
Currently eduroam processes several hundred thousands of successful
international roaming authentications per day (and, incidentally,
approximately 1.5 times as many Access-Rejects). With the
requirement of physical proximity, and the rate-limiting induced by
EAP's lock-step nature, it requires a significant amount of attackers
and a time-coordinated attack to produce significant load. So far
eduroam Operations has not yet observed critical load conditions
which could reasonably be attributed to such an attack.
The introduction of dynamic discovery further eases this problem, as
authentications will then not traverse all infrastructure servers,
removing the world-region aggregation servers as obvious bottlenecks.
Any attack would then be limited between an SP and IdP directly.
7.2.2. DoS as a side-effect of expired credentials
In eduroam Operations it is observed that a significant portion of
(failed) eduroam authentications is due to user accounts which were
once valid, but have in the meantime been de-provisioned (e.g. if a
student has left the university after graduation). Configured
eduroam accounts are often retained on the user devices, and when in
the vicinity of an eduroam hotspot, the user device's operating
system will attempt to connect to this network.
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As operation of eduroam continues, the amount of devices with left-
over configurations is growing, effectively creating a pool of
devices which produce unwanted network traffic whenever they can.
Up until recently, this problem did not emerge with much prominence,
because there is also a natural shrinking of that pool of devices due
to users finally de-commissioning their old computing hardware.
As of recent, particularly smartphones are programmed to make use of
cloud storage and online backup mechanisms which save most, or all,
configuration details of the device with a third-party. When
renewing their personal computing hardware, users can restore the old
settings onto the new device. It has been observed that expired
eduroam accounts can survive perpetually on user devices that way.
If this trend continues, it can be pictured that an always-growing
pool of devices will clog up eduroam infrastructure with doomed-to-
fail authentication requests.
There is not currently a useful remedy to this problem, other than
instructing users to manually delete their configuration in due time.
Possible approaches to this problem are:
o Creating a culture of device provisioning where the provisioning
profile contains a "ValidUntil" property, after which the
configuration needs to be re-validated or disabled. This requires
a data format for provisioning as well as implementation support.
o Improvements to supplicant software so that it maintains state
over failed authentications. E.g. if a previously known-working
configuration failed to authenticate consistently for 30 calendar
days, it should be considered stale and be disabled.
8. IANA Considerations
There are no IANA Considerations
9. References
9.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[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.
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[RFC3748] Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and H.
Levkowetz, "Extensible Authentication Protocol (EAP)", RFC
3748, June 2004.
[RFC4279] Eronen, P. and H. Tschofenig, "Pre-Shared Key Ciphersuites
for Transport Layer Security (TLS)", RFC 4279, December
2005.
[RFC4372] Adrangi, F., Lior, A., Korhonen, J., and J. Loughney,
"Chargeable User Identity", RFC 4372, January 2006.
[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.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008.
[RFC5247] Aboba, B., Simon, D., and P. Eronen, "Extensible
Authentication Protocol (EAP) Key Management Framework",
RFC 5247, August 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.
[RFC5580] Tschofenig, H., Adrangi, F., Jones, M., Lior, A., and B.
Aboba, "Carrying Location Objects in RADIUS and Diameter",
RFC 5580, August 2009.
[RFC5997] DeKok, A., "Use of Status-Server Packets in the Remote
Authentication Dial In User Service (RADIUS) Protocol",
RFC 5997, August 2010.
[RFC6066] Eastlake, D., "Transport Layer Security (TLS) Extensions:
Extension Definitions", RFC 6066, January 2011.
[RFC6613] DeKok, A., "RADIUS over TCP", RFC 6613, May 2012.
[RFC6614] Winter, S., McCauley, M., Venaas, S., and K. Wierenga,
"Transport Layer Security (TLS) Encryption for RADIUS",
RFC 6614, May 2012.
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[RFC6973] Cooper, A., Tschofenig, H., Aboba, B., Peterson, J.,
Morris, J., Hansen, M., and R. Smith, "Privacy
Considerations for Internet Protocols", RFC 6973, July
2013.
9.2. Informative References
[I-D.ietf-abfab-arch]
Howlett, J., Hartman, S., Tschofenig, H., Lear, E., and J.
Schaad, "Application Bridging for Federated Access Beyond
Web (ABFAB) Architecture", draft-ietf-abfab-arch-13 (work
in progress), July 2014.
[I-D.ietf-radext-dtls]
DeKok, A., "DTLS as a Transport Layer for RADIUS", draft-
ietf-radext-dtls-13 (work in progress), July 2014.
[I-D.ietf-radext-dynamic-discovery]
Winter, S. and M. McCauley, "NAI-based Dynamic Peer
Discovery for RADIUS/TLS and RADIUS/DTLS", draft-ietf-
radext-dynamic-discovery-13 (work in progress), March
2015.
[I-D.ietf-radext-nai]
DeKok, A., "The Network Access Identifier", draft-ietf-
radext-nai-15 (work in progress), December 2014.
[MD5-attacks]
Black, J., Cochran, M., and T. Highland, "A Study of the
MD5 Attacks: Insights and Improvements", October 2006,
<http://www.springerlink.com/content/40867l85727r7084/>.
[RFC3539] Aboba, B. and J. Wood, "Authentication, Authorization and
Accounting (AAA) Transport Profile", RFC 3539, June 2003.
[RFC3588] Calhoun, P., Loughney, J., Guttman, E., Zorn, G., and J.
Arkko, "Diameter Base Protocol", RFC 3588, September 2003.
[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.
[RFC4017] Stanley, D., Walker, J., and B. Aboba, "Extensible
Authentication Protocol (EAP) Method Requirements for
Wireless LANs", RFC 4017, March 2005.
[RFC4107] Bellovin, S. and R. Housley, "Guidelines for Cryptographic
Key Management", BCP 107, RFC 4107, June 2005.
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[RFC4346] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.1", RFC 4346, April 2006.
[RFC4953] Touch, J., "Defending TCP Against Spoofing Attacks", RFC
4953, July 2007.
[RFC6125] Saint-Andre, P. and J. Hodges, "Representation and
Verification of Domain-Based Application Service Identity
within Internet Public Key Infrastructure Using X.509
(PKIX) Certificates in the Context of Transport Layer
Security (TLS)", RFC 6125, March 2011.
[RFC6421] Nelson, D., "Crypto-Agility Requirements for Remote
Authentication Dial-In User Service (RADIUS)", RFC 6421,
November 2011.
[RFC6929] DeKok, A. and A. Lior, "Remote Authentication Dial In User
Service (RADIUS) Protocol Extensions", RFC 6929, April
2013.
[RFC7170] Zhou, H., Cam-Winget, N., Salowey, J., and S. Hanna,
"Tunnel Extensible Authentication Protocol (TEAP) Version
1", RFC 7170, May 2014.
[dead-realm]
Tomasek, J., "Dead-realm marking feature for Radiator
RADIUS servers", 2006,
<http://wiki.eduroam.cz/dead-realm/docs/dead-realm.html>.
[dot1X-standard]
IEEE, "IEEE std 802.1X-2010", February 2010,
<http://standards.ieee.org/getieee802/
download/802.1X-2010.pdf>.
[edupki] Delivery of Advanced Network Technology to Europe,
"eduPKI", 2012, <https://www.edupki.org/edupki-pma/edupki-
trust-profiles/>.
[eduroam-cat]
Delivery of Advanced Network Technology to Europe,
"eduroam CAT", 2012, <https://cat.eduroam.org>.
[eduroam-compliance]
Trans-European Research and Education Networking
Association, "eduroam compliance statement", 2011,
<http://www.eduroam.org/downloads/docs/
eduroam_Compliance_Statement_v1_0.pdf>.
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[eduroam-homepage]
Trans-European Research and Education Networking
Association, "eduroam Homepage", 2007,
<http://www.eduroam.org/>.
[eduroam-policy]
Delivery of Advanced Network Technology to Europe,
"European Confederation eduroam policy", 2011,
<http://www.eduroam.org/downloads/docs/GN3-12-194_eduroam-
policy-%20for-signing_ver2%204_18052012.pdf>.
[eduroam-service-definition]
Delivery of Advanced Network Technology to Europe,
"European eduroam policy Service Definition", 2011,
<https://www.eduroam.org/downloads/docs/GN3-12-
192_eduroam-policy-service-definition_ver28_26072012.pdf>.
[eduroam-start]
Wierenga, K., "Initial proposal for what is now called
eduroam", 2002, <http://www.terena.org/activities/tf-
mobility/start-of-eduroam.pdf>.
[geant] Geant Association, "Geant Association", 2008,
<http://www.terena.org/>.
[geant2] Delivery of Advanced Network Technology to Europe,
"European Commission Information Society and Media:
GEANT2", 2008, <http://www.geant2.net/>.
[nrenroaming-select]
Trans-European Research and Education Networking
Association, "Preliminary selection for inter-NREN
roaming", 2003, <http://www.terena.org/activities/tf-
mobility/deliverables/delG/DelG-final.pdf/>.
[radsec-whitepaper]
Open System Consultants, "RadSec - a secure, reliable
RADIUS Protocol", May 2005,
<http://www.open.com.au/radiator/radsec-whitepaper.pdf>.
[radsecproxy-impl]
Venaas, S., "radsecproxy Project Homepage", 2007,
<http://software.uninett.no/radsecproxy/>.
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Appendix A. Acknowledgments
The authors would like to thank the participants in the Geant
Association Task Force on Mobility and Network Middleware as well as
the Geant project for their reviews and contributions. Special
thanks go to Jim Schaad for doing an excellent review of the first
version and to him and Alan de Kok for additional reviews.
The eduroam trademark is registered by TERENA.
Appendix B. Changes
This section to be removed prior to publication.
o 00 Initial Revision
o 01 Added Dynamic Discovery, addressed review comments
o 02 Cosmetic changes
o 03 Even More Cosmetic Changes
o 04 Included review comments from Jim Schaad
o 05 Included review comments Jim Schaad and Alan deKok
Authors' Addresses
Klaas Wierenga
Cisco Systems
Haarlerbergweg 13-19
Amsterdam 1101 CH
The Netherlands
Phone: +31 20 357 1752
Email: klaas@cisco.com
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.
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Tomasz Wolniewicz
Nicolaus Copernicus University
pl. Rapackiego 1
Torun
Poland
Phone: +48-56-611-2750
Fax: +48-56-622-1850
Email: twoln@umk.pl
URI: http://www.home.umk.pl/~twoln/
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