| Rfc | 7593 |
|---|
| Title | The eduroam Architecture for Network Roaming |
|---|
| Author | K. Wierenga, S.
Winter, T. Wolniewicz |
|---|
| Date | September 2015 |
|---|
| Format: | TXT, HTML |
|---|
| Status: | INFORMATIONAL |
|---|
|
Independent Submission K. Wierenga
Request for Comments: 7593 Cisco Systems
Category: Informational S. Winter
ISSN: 2070-1721 RESTENA
T. Wolniewicz
Nicolaus Copernicus University
September 2015
The eduroam Architecture for Network Roaming
Abstract
This document describes the architecture of the eduroam service for
federated (wireless) network access in academia. The combination of
IEEE 802.1X, the Extensible Authentication Protocol (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 choices and selection of standards are
described, along with the changes that were prompted by operational
experience.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for informational purposes.
This is a contribution to the RFC Series, independently of any other
RFC stream. The RFC Editor has chosen to publish this document at
its discretion and makes no statement about its value for
implementation or deployment. Documents approved for publication by
the RFC Editor are not a candidate for any level of Internet
Standard; see Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc7593.
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
(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.
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 . . . . . . . . . . . . . . . . . . . . 6
2.1. Authentication . . . . . . . . . . . . . . . . . . . . . 6
2.1.1. IEEE 802.1X . . . . . . . . . . . . . . . . . . . . . 6
2.1.2. EAP . . . . . . . . . . . . . . . . . . . . . . . . . 7
2.2. Federation Trust Fabric . . . . . . . . . . . . . . . . . 8
2.2.1. RADIUS . . . . . . . . . . . . . . . . . . . . . . . 9
3. Issues with Initial Trust Fabric . . . . . . . . . . . . . . 11
3.1. Server Failure Handling . . . . . . . . . . . . . . . . . 12
3.2. No Signaling of Error Conditions . . . . . . . . . . . . 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 Authorization . . . . . . . . . . . 20
4.2.3. Operational Experience . . . . . . . . . . . . . . . 21
4.2.4. Possible Alternatives . . . . . . . . . . . . . . . . 21
5. Abuse Prevention and Incident Handling . . . . . . . . . . . 22
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 . . . . . . . . . . . . . . . . . . . . . . 26
5.3. Chargeable User Identity . . . . . . . . . . . . . . . . 27
6. Privacy Considerations . . . . . . . . . . . . . . . . . . . 28
6.1. Collusion of Service Providers . . . . . . . . . . . . . 28
6.2. Exposing User Credentials . . . . . . . . . . . . . . . . 28
6.3. Track Location of Users . . . . . . . . . . . . . . . . . 28
7. Security Considerations . . . . . . . . . . . . . . . . . . . 29
7.1. Man-in-the-Middle and Tunneling Attacks . . . . . . . . . 29
7.1.1. Verification of Server Name Not Supported . . . . . . 29
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 . . 30
7.1.4. Tunneling Authentication Traffic to Obfuscate User
Origin . . . . . . . . . . . . . . . . . . . . . . . 30
7.2. Denial-of-Service Attacks . . . . . . . . . . . . . . . . 31
7.2.1. Intentional DoS by Malign Individuals . . . . . . . . 31
7.2.2. DoS as a Side-Effect of Expired Credentials . . . . . 32
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 33
8.1. Normative References . . . . . . . . . . . . . . . . . . 33
8.2. Informative References . . . . . . . . . . . . . . . . . 34
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 36
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 specifications such as RADIUS over TCP
[RFC6613] and RADIUS with TLS [RFC6614] and that promoted alternative
uses of RADIUS like in Application Bridging for Federated Access
Beyond web (ABFAB) [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
[eduroam-homepage]. 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".
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 to which eduroam participants subscribe
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 setup). 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; otherwise, it may inhibit wide-
scale adoption. In particular, there should be no client
installation (or it should be easy) and only one-time
configuration.
- Secure
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 credential 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 to 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 requirements.
- 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 based on Web
captive-portals (scalable but not secure), and one based on IEEE
802.1X, 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 [IEEE.802.1X] as the port-based authentication
framework using
o EAP [RFC3748] for integrity-protected and confidential transport
of credentials and
o a RADIUS [RFC2865] hierarchy as the 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
network at the visited location). In eduroam, the trust relation
between the user and IdP is through mutual authentication. IdPs and
the 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 [IEEE.802.1X] and EAP [RFC4372] (the latter carried over
RADIUS for guest access; see Section 2.2).
2.1.1. IEEE 802.1X
By using the IEEE 802.1X [IEEE.802.1X] 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 (Wi-Fi
Access Point or switch) at the SP, the authenticator forwards the
access request to the authentication server of the SP, that in turn
proxies the request through the RADIUS hierarchy to the
authentication server of the user's home organization (the IdP).
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, and 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
Service Set Identifier (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" to make that distinction, but it is
not widely implemented yet). Furthermore, 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 confidentiality of user data, IEEE 802.11
wireless networks of eduroam SPs MUST deploy WPA2+AES, and they MAY
additionally support Wi-Fi Protected Access with the Temporal Key
Integrity Protocol (WPA/TKIP) as a courtesy to users of legacy
hardware.
2.1.2. EAP
The use of the Extensible Authentication Protocol (EAP) [RFC4372]
serves two purposes. In the first place, a properly chosen EAP
method allows for integrity-protected and confidential 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 Policy Service Definition
[eduroam-service-definition].
+-----+
/ \
/ \
/ \
/ \
,----------\ | | ,---------\
| 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 [RFC7542], 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 credential 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
[RFC5246] 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.
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
security that motivated the work on RADIUS with TLS [RFC6614].
2.2.1. RADIUS
The eduroam trust fabric consists of a proxy hierarchy of RADIUS
servers (organizational, national, global) that is 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
(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.
+-------+
| |
| . |
| |
+---+---+
/ | \
+----------------/ | \---------------------+
| | |
| | |
| | |
+--+---+ +--+--+ +----+---+
| | | | | |
| .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 (as in 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 of
anonymous@surfnet.nl.
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; since it is not 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 three 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.
3.1. Server Failure Handling
In eduroam, authentication requests for roaming users are statically
routed through preconfigured 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).
RFC 2865 [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 one
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 that is believed to be
inoperable can lead to the discovery that the server is in
working order again. This issue has been resolved with RFC 5997
[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: in case of failover
to a back-up next-hop RADIUS server, also that server will be marked
as being defunct, since through that server no reply will be received
from the defunct home server either. 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 that considers up/down states not
per next-hop RADIUS peer, but instead per realm (See [dead-realm] for
details). Recently, implementations of RFC 5997 [RFC5997] and
cautious failover parameters make false "downs" unlikely to happen,
as long as every hop implements RFC 5997. In that case, dead realm
detection serves mainly to prevent proxying of large numbers of
requests to known dead realms.
3.2. No Signaling of Error Conditions
The RADIUS protocol lacks signaling of error conditions, and the IEEE
802.1X standard does not allow conveying of extended failure reasons
to the end user's device. For eduroam, this creates two issues:
o The home server may have an operational problem, for example, its
authentication decisions may depend on an external data source
such as a SQL server or Microsoft's Active Directory, and the
external data source is unavailable. If the RADIUS interface is
still functional, there are two options for how to reply to an
Access-Request that can't be serviced due to such error
conditions:
1. Do Not Reply: The inability to reach a conclusion can be
handled 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 help-desk 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.
Similar considerations 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, 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 its previous working state is
stale). The active link-state monitoring of the TCP connection with,
e.g., RADIUS/TLS (see Section 4.1), gives a clearer indication
whether there is an alive RADIUS peer, but it does not solve the
defunct back-end problem. An explicit ability to send Error-Replies,
on the RADIUS level (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"
(Karlsruher 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 approximately 50 routing entries in
total (and have an upper bound of approximately 200), the potential
size of the routing table becomes virtually unlimited if it needs to
accommodate all individual entries in .edu, .org, etc.
In addition to that, all these routes need to be synchronized 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.
3.4. UDP Issues
RADIUS is based on UDP, which was a reasonable choice when its main
use was with simple Password Authentication Protocol (PAP) requests
that required only exactly one packet exchange in each direction.
When transporting EAP over RADIUS, the EAP conversations require
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; that is, the more proxies 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 put as much EAP data
into a single RADIUS packet as the supplicant's Layer 2 medium allows
-- 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 in theory
this is not a problem, in practice there is evidence of misbehaving
firewalls that erroneously discard non-first UDP fragments; this
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 that send the client certificate do not expose such a
configuration detail to the user. Consequently, when the client
certificate is over 1500 bytes in size, the EAP-Message will always
make use of the maximum possible Layer 2 chunk size, and this
introduces fragmentation on the path from EAP peer to EAP server.
Both of the previously mentioned sources of errors (packet loss and
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].
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 cleartext
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 incidents in
the US National Security Agency (NSA) and the UK Government
Communications Headquarters (GCHQ) 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 supplicants don'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 that 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 preconfigured supplicant
configurations that make 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 not willing to invest time into the
inspection of server certificates or the installation of a trusted
certification authority (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.
Again, one way out of this particular threat is to pursue the
creation and deployment of preconfigured supplicant configurations
that make all the required settings in user devices prior to their
first connection attempt.
Note: There are many different and vendor-proprietary ways to
preconfigure 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 that 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 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. RADIUS as
specified in RFC 2865 does not offer this possibility though. This
motivated [RFC6614]; see Section 4.1.
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 led
to IETF specifications (as mentioned in the introduction).
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 in Section 3 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, as outlined in
[radsec-whitepaper].
4.1. RADIUS with TLS
The deficiencies of RADIUS over UDP as described in Section 3.4
warranted a search for a replacement of RFC 2865 [RFC2865] for the
transport of EAP. By the time this need was understood, the
designated successor protocol to RADIUS, Diameter, was already
specified by the IETF in its intial version [RFC3588]. However,
within the operational constraints of eduroam (listed below), no
single combination of software could be found (and that is believed
to still be true, more than ten years and one revision of Diameter
[RFC6733] later). The constraints are:
o reasonably cheap to deploy on many administrative domains
o supporting the application of Network Access Server Requirements
(NASREQ)
o supporting EAP application
o supporting Diameter Redirect
o supporting validation of authentication requests of the most
popular EAP types (EAP Tunneled Transport Layer Security
(EAP-TTLS), Protected EAP (PEAP), and EAP-TLS)
o possibility to retrieve these credentials from popular back-ends
such as MySQL or Microsoft's Active Directory.
In addition, no Wi-Fi Access Points at the disposal of eduroam
participants supported Diameter, nor did any of the manufacturers
have a roadmap towards Diameter support (and 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 [DYN-DISC].
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 any given RADIUS link in
eduroam to be upgraded without the need of a "flag day".
In a first upgrade phase, the classic eduroam hierarchy (forwarding
decision made 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:
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 realized
with the TLS-PSK [RFC4279] 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 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):
> 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 standardization of this approach for generic AAA use
cases. The current RADEXT working group document for this is
[DYN-DISC].
Note: It is worth mentioning that this move to a more complex,
flexible system may make the system as a whole more fragile, as
compared to the static set up.
4.2.2. Verifying Server Authorization
Any organization can put "x-eduroam" NAPTR entries into their Domain
Name Server, pretending to be the 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 that
issues certificates only to authorized 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 that
were issued to verified eduroam participants are marked with
corresponding "X509v3 Policy OID" fields; eduroam 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 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 that 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 this approach to dynamic server
discovery that are monitored by the eduroam community:
1. DNSSEC + DNS-Based Authentication of Named Entities (DANE) TLSA
records
2. ABFAB Trust Router
For DNSSEC+DANE TLSA, the 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 to the
Identity Provider of the user (in particular). 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 [RFC5280] SHOULD NOT be forwarded.
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 that 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 [RFC4372] 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 near university campuses. Chargeable User Identity (see
Section 5.3) supplies the perfect answer to this problem; however, at
the time of writing, to our knowledge, only one hardware vendor (Meru
Networks) implements RFC 4372 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 users' awareness 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 is 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.
The initial implementation has been lacking reliable tools for an SP
to make its 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 Section 5.3) 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 that is
supposed to be locked out, and it has no side-effects on other users,
even from the same realm.
If the EAP authentication does not reveal a CUI, the SP cannot
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 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 that are not willing to take the
drastic step of blocking an entire realm will be forced to contact
the Identity Provider in question and demand that the user be blocked
at the IdP side. This involves human interaction between SP and IdP
and 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 of, 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. For example, 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 that 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 that the IdP can use to make its decision.
5.1.3. Communicating Account Blocking to the End User
The measures described in Sections 5.1.1 and 5.1.2 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 indistinguishable 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 that
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 that 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.
The Proxy infrastructure of eduroam makes it impossible for home
sites to tell where their users roam. 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 led to the
identification of 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 RFC definition (which defines it
as 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. Section 2.8 of [RFC6929]
clarifies 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 RFC 4372
[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
requirement" described in Section 2.1 of RFC 4372 [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 reading this far. 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.
However, this poses 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 and per Service Provider.
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-off
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.
However, the introduction of Operator-Name and dynamic lookups (i.e.,
direct connections between IdP and SP) gives 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 in principle this makes man-in-the-middle attacks impossible,
in practice several attack vectors exist nonetheless. Most of these
deficiencies are due to implementation shortcomings in EAP
supplicants. Examples:
7.1.1. Verification of Server Name Not Supported
Some supplicants only allow specifying which CA issues the EAP server
certificate; its 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 that exclusively issues certificates to the
organization's EAP servers. In that case, no other entity will get a
certificate from the CA and this 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
the simple selection of a network the acceptance of the incoming
server certificate, identified by its fingerprint. The certificate
is then saved as trusted for later reconnection attempts. If users
are near a fake hotspot during initial provisioning, they may be
tricked to submit their credentials to a fake server; furthermore,
they 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 that 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 interfaces of supplicants often invite the users to
take shortcuts ("Don't check server certificate") that are easier to
set up or hide important security settings in badly accessible sub-
menus. Such shortcuts or security parameter omissions 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.
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 realmB 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 that the troublesome user
originates from realmB and demand there that the user be blocked.
However, the operator of realmB 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 that process 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 edge
networks, such as rogue Router Advertisements (RAs), 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, thereby reducing 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 edge networks and
IP networks in general. The following sections describe attack
vectors specific to eduroam.
7.2.1. Intentional DoS by Malign Individuals
The eduroam infrastructure is more robust against Distributed DoS
attacks than typical services that are reachable on the Internet
because triggering authentication traffic can only be done when
physically in proximity of an eduroam hotspot (be it a wired socket
that is IEEE 802.1X enabled or a Wi-Fi Access Point).
However, when in the vicinity, an attacker can easily 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 that 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 that
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 that 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.
As operation of eduroam continues, the amount of devices with
leftover configurations is growing, effectively creating a pool of
devices that produce unwanted network traffic whenever they can.
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 decommissioning their old computing hardware.
Recently, smartphones are programmed to make use of cloud storage and
online backup mechanisms that 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. For example, if a previously known
working configuration failed to authenticate consistently for 30
calendar days, it should be considered stale and be disabled.
8. References
8.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC2865] Rigney, C., Willens, S., Rubens, A., and W. Simpson,
"Remote Authentication Dial In User Service (RADIUS)",
RFC 2865, DOI 10.17487/RFC2865, June 2000,
<http://www.rfc-editor.org/info/rfc2865>.
[RFC3748] Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and
H. Levkowetz, Ed., "Extensible Authentication Protocol
(EAP)", RFC 3748, DOI 10.17487/RFC3748, June 2004,
<http://www.rfc-editor.org/info/rfc3748>.
[RFC4279] Eronen, P., Ed. and H. Tschofenig, Ed., "Pre-Shared Key
Ciphersuites for Transport Layer Security (TLS)",
RFC 4279, DOI 10.17487/RFC4279, December 2005,
<http://www.rfc-editor.org/info/rfc4279>.
[RFC4372] Adrangi, F., Lior, A., Korhonen, J., and J. Loughney,
"Chargeable User Identity", RFC 4372,
DOI 10.17487/RFC4372, January 2006,
<http://www.rfc-editor.org/info/rfc4372>.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246,
DOI 10.17487/RFC5246, August 2008,
<http://www.rfc-editor.org/info/rfc5246>.
[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, DOI 10.17487/RFC5280, May 2008,
<http://www.rfc-editor.org/info/rfc5280>.
[RFC5580] Tschofenig, H., Ed., Adrangi, F., Jones, M., Lior, A., and
B. Aboba, "Carrying Location Objects in RADIUS and
Diameter", RFC 5580, DOI 10.17487/RFC5580, August 2009,
<http://www.rfc-editor.org/info/rfc5580>.
[RFC5997] DeKok, A., "Use of Status-Server Packets in the Remote
Authentication Dial In User Service (RADIUS) Protocol",
RFC 5997, DOI 10.17487/RFC5997, August 2010,
<http://www.rfc-editor.org/info/rfc5997>.
[RFC6613] DeKok, A., "RADIUS over TCP", RFC 6613,
DOI 10.17487/RFC6613, May 2012,
<http://www.rfc-editor.org/info/rfc6613>.
[RFC6614] Winter, S., McCauley, M., Venaas, S., and K. Wierenga,
"Transport Layer Security (TLS) Encryption for RADIUS",
RFC 6614, DOI 10.17487/RFC6614, May 2012,
<http://www.rfc-editor.org/info/rfc6614>.
[RFC6973] Cooper, A., Tschofenig, H., Aboba, B., Peterson, J.,
Morris, J., Hansen, M., and R. Smith, "Privacy
Considerations for Internet Protocols", RFC 6973,
DOI 10.17487/RFC6973, July 2013,
<http://www.rfc-editor.org/info/rfc6973>.
8.2. Informative References
[ABFAB-ARCH]
Howlett, J., Hartman, S., Tschofenig, H., Lear, E., and
J. Schaad, "Application Bridging for Federated Access
Beyond Web (ABFAB) Architecture", Work in Progress,
draft-ietf-abfab-arch-13, July 2014.
[dead-realm]
Tomasek, J., "Dead-realm marking feature for Radiator
RADIUS servers", 2006,
<http://www.eduroam.cz/dead-realm/docs/dead-realm.html>.
[DYN-DISC] Winter, S. and M. McCauley, "NAI-based Dynamic Peer
Discovery for RADIUS/TLS and RADIUS/DTLS", Work in
Progress, draft-ietf-radext-dynamic-discovery-15, April
2015.
[eduPKI] Delivery of Advanced Network Technology to Europe, "eduPKI
Trust Profiles", 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", October 2011,
<http://www.eduroam.org/downloads/docs/
eduroam_Compliance_Statement_v1_0.pdf>.
[eduroam-homepage]
Trans-European Research and Education Networking
Association, "eduroam Homepage", 2007,
<http://www.eduroam.org/>.
[eduroam-service-definition]
GEANT, "eduroam Policy Service Definition", Version 2.8,
July 2012, <https://www.eduroam.org/downloads/docs/GN3-12-
192_eduroam-policy-service-definition_ver28_26072012.pdf>.
[eduroam-start]
Wierenga, K., "Subject: proposal for inter NREN roaming",
message to the mobility@terena.nl mailing list, initial
proposal for what is now called eduroam, 30 May 2002,
<http://www.terena.org/activities/tf-mobility/
start-of-eduroam.pdf>.
[IEEE.802.1X]
IEEE, "IEEE Standard for Local and metropolitan area
networks - Port-Based Network Access Control", IEEE
802.1X-2010, DOI 10.1109/ieeestd.2010.5409813,
<http://ieeexplore.ieee.org/servlet/
opac?punumber=5409757>.
[nrenroaming-select]
Trans-European Research and Education Networking
Association, "Preliminary selection for inter-NREN
roaming", December 2003,
<http://www.terena.org/activities/tf-mobility/
deliverables/delG/DelG-final.pdf>.
[radsec-whitepaper]
Open System Consultants, "RadSec: a secure, reliable
RADIUS Protocol", October 2012,
<http://www.open.com.au/radiator/radsec-whitepaper.pdf>.
[RFC3588] Calhoun, P., Loughney, J., Guttman, E., Zorn, G., and
J. Arkko, "Diameter Base Protocol", RFC 3588,
DOI 10.17487/RFC3588, September 2003,
<http://www.rfc-editor.org/info/rfc3588>.
[RFC3958] Daigle, L. and A. Newton, "Domain-Based Application
Service Location Using SRV RRs and the Dynamic Delegation
Discovery Service (DDDS)", RFC 3958, DOI 10.17487/RFC3958,
January 2005, <http://www.rfc-editor.org/info/rfc3958>.
[RFC4017] Stanley, D., Walker, J., and B. Aboba, "Extensible
Authentication Protocol (EAP) Method Requirements for
Wireless LANs", RFC 4017, DOI 10.17487/RFC4017, March
2005, <http://www.rfc-editor.org/info/rfc4017>.
[RFC6733] Fajardo, V., Ed., Arkko, J., Loughney, J., and G. Zorn,
Ed., "Diameter Base Protocol", RFC 6733,
DOI 10.17487/RFC6733, October 2012,
<http://www.rfc-editor.org/info/rfc6733>.
[RFC6929] DeKok, A. and A. Lior, "Remote Authentication Dial In User
Service (RADIUS) Protocol Extensions", RFC 6929,
DOI 10.17487/RFC6929, April 2013,
<http://www.rfc-editor.org/info/rfc6929>.
[RFC7170] Zhou, H., Cam-Winget, N., Salowey, J., and S. Hanna,
"Tunnel Extensible Authentication Protocol (TEAP) Version
1", RFC 7170, DOI 10.17487/RFC7170, May 2014,
<http://www.rfc-editor.org/info/rfc7170>.
[RFC7542] DeKok, A., "The Network Access Identifier", RFC 7542,
DOI 10.17487/RFC7542, May 2015,
<http://www.rfc-editor.org/info/rfc7542>.
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 DeKok for additional reviews.
The eduroam trademark is registered by TERENA.
Authors' Addresses
Klaas Wierenga
Cisco Systems
Haarlerbergweg 13-17
Amsterdam 1101 CH
The Netherlands
Phone: +31 20 357 1752
Email: klaas@cisco.com
Stefan Winter
Fondation RESTENA
Maison du Savoir
2, avenue de l'Universite
L-4365 Esch-sur-Alzette
Luxembourg
Phone: +352 424409 1
Fax: +352 422473
Email: stefan.winter@restena.lu
URI: http://www.restena.lu.
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/