Internet DRAFT - draft-ietf-intarea-hostname-practice
draft-ietf-intarea-hostname-practice
Network Working Group C. Huitema
Internet-Draft Private Octopus Inc.
Intended status: Informational D. Thaler
Expires: August 7, 2017 Microsoft
R. Winter
University of Applied Sciences Augsburg
February 3, 2017
Current Hostname Practice Considered Harmful
draft-ietf-intarea-hostname-practice-05.txt
Abstract
Giving a hostname to your computer and publishing it as you roam from
one network to another is the Internet equivalent of walking around
with a name tag affixed to your lapel. This current practice can
significantly compromise your privacy, and something should change in
order to mitigate these privacy threats.
There are several possible remedies, such as fixing a variety of
protocols or avoiding disclosing a hostname at all. This document
describes some of the protocols that reveal hostnames today and
sketches another possible remedy, which is to replace static
hostnames by frequently changing randomized values.
Status of This Memo
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This Internet-Draft will expire on August 7, 2017.
Copyright Notice
Copyright (c) 2017 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Naming Practices . . . . . . . . . . . . . . . . . . . . . . 3
3. Partial Identifiers . . . . . . . . . . . . . . . . . . . . . 4
4. Protocols that leak Hostnames . . . . . . . . . . . . . . . . 4
4.1. DHCP . . . . . . . . . . . . . . . . . . . . . . . . . . 5
4.2. DNS Address to Name Resolution . . . . . . . . . . . . . 5
4.3. Multicast DNS . . . . . . . . . . . . . . . . . . . . . . 5
4.4. Link-local Multicast Name Resolution . . . . . . . . . . 6
4.5. DNS-Based Service Discovery . . . . . . . . . . . . . . . 6
4.6. NetBIOS-over-TCP . . . . . . . . . . . . . . . . . . . . 7
5. Randomized Hostnames as Remedy . . . . . . . . . . . . . . . 7
6. Security Considerations . . . . . . . . . . . . . . . . . . . 8
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 9
9. Informative References . . . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 11
1. Introduction
There is a long established practice of giving names to computers.
In the Internet protocols, these names are referred to as "hostnames"
[RFC7719] . Hostnames are normally used in conjunction with a domain
name suffix to build the "Fully Qualified Domain Name" (FQDN) of a
host [RFC1983]. However, it is common practice to use the hostname
without further qualification in a variety of applications from file
sharing to network management. Hostnames are typically published as
part of domain names, and can be obtained through a variety of name
lookup and discovery protocols.
Hostnames have to be unique within the domain in which they are
created and used. They do not have to be globally unique
identifiers, but they will always be at least partial identifiers, as
discussed in Section 3.
The disclosure of information through hostnames creates a problem for
mobile devices. Adversaries that monitor a remote network such as a
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Wi-Fi hot spot can obtain the hostname through passive monitoring or
active probing of a variety of Internet protocols, such as for
example DHCP, or multicast DNS (mDNS). They can correlate the
hostname with various other information extracted from traffic
analysis and other information sources, and can potentially identify
the device, device properties and its user [TRAC2016].
2. Naming Practices
There are many reasons to give names to computers. This is
particularly true when computers operate on a network. Operating
systems like Microsoft Windows or Unix assume that computers have a
"hostname." This enables users and administrators to do things such
as ping a computer, add its name to an access control list, remotely
mount a computer disk, or connect to the computer through tools such
as telnet or remote desktop. Other operating systems maintain
multiple hostnames for different purposes, e.g. for use with certain
protocols such as mDNS.
In most consumer networks, naming is pretty much left to the fancy of
the user. Some will pick names of planets or stars, other names of
fruits or flowers, and other will pick whatever suits their mood when
they unwrap the device. As long as users are careful to not pick a
name already in use on the same network, anything goes. Very often
however, the operating system is suggesting a hostname at install
time, which can contain the user name, the login name and information
learned from the device itself such as the brand, model or maker of
the device [TRAC2016].
In large organizations, collisions are more likely and a more
structured approach is necessary. In theory, organizations could use
multiple DNS subdomains to ease the pressure on uniqueness, but in
practice many don't and insist on unique flat names, if only to
simplify network management. To ensure unique names, organizations
will set naming guidelines and enforce some kind of structured
naming. For example, within the Microsoft corporate network,
computer names are derived from the login name of the main user,
leading to names like "huitema-test2" for a machine that one of the
authors used to test software.
There is less pressure to assign names to small devices, including
for example smart phones, as these devices typically do not enable
sharing of their disks or remote login. As a consequence, these
devices often have manufacturer assigned names, which vary from very
generic like "Windows Phone" to completely unique like "BrandX-
123456-7890-abcdef" and often contain the name of the device owner,
the device's brand name, and often also a hint as to which language
the device owner speaks [TRAC2016].
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3. Partial Identifiers
Suppose an adversary wants to track the people connecting to a
specific Wi-Fi hot spot, for example in a railroad station. Assume
that the adversary is able to retrieve the hostname used by a
specific laptop. That, in itself, might not be enough to identify
the laptop's owner. Suppose however that the adversary observes that
the laptop name is "dthaler-laptop" and that the laptop has
established a VPN connection to the Microsoft corporate network. The
two pieces of information, put together, firmly point to Dave Thaler,
employed by Microsoft. The identification is successful.
In the example, we saw a login name inside the hostname, and that
certainly helped identification. But generic names like "jupiter" or
"rosebud" also provide partial identification, especially if the
adversary is capable of maintaining a database recording, among other
information, the hostnames of devices used by specific users.
Generic names are picked from vocabularies that include thousands of
potential choices. Finding the name reduces the scope of the search
significantly. Other information such as the visited sites will
quickly complement that data and can lead to user identification.
Also the special circumstances of the network can play a role.
Experiments on operational networks such as the IETF meeting network
have shown that with the help of external data such as the publicly
available IETF attendees list or other data sources such as LDAP
servers on the network [TRAC2016], the identification of the device
owner can become trivial given only partial identifiers in a
hostname.
Unique names assigned by manufacturers do not directly encode a user
identifier, but they have the property of being stable and unique to
the device in a large context. A unique name like "BrandX-
123456-7890-abcdef" allows efficient tracking across multiple
domains. In theory, this only allows tracking of the device but not
of the user. However, an adversary could correlate the device to the
user through other means, for example the one-time capture of some
clear text traffic. Adversaries could then maintain databases
linking unique host name to user identity. This will allow efficient
tracking of both the user and the device.
4. Protocols that leak Hostnames
Many IETF protocols can leak the "hostname" of a computer. A non
exhaustive list includes DHCP, DNS address to name resolution,
Multicast DNS, Link-local Multicast Name Resolution, and DNS service
discovery.
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4.1. DHCP
Shortly after connecting to a new network, a host can use DHCP
[RFC2131] to acquire an IPv4 address and other parameters [RFC2132].
A DHCP query can disclose the "hostname." DHCP traffic is sent to
the broadcast address and can be easily monitored, enabling
adversaries to discover the hostname associated with a computer
visiting a particular network. DHCPv6 [RFC3315] shares similar
issues.
The problems with the hostname and FQDN parameters in DHCP are
analyzed in [RFC7819] and [RFC7824]. Possible mitigations are
described in [RFC7844].
4.2. DNS Address to Name Resolution
The domain name service design [RFC1035] includes the specification
of the special domain "in-addr.arpa" for resolving the name of the
computer using a particular IPv4 address, using the PTR format
defined in [RFC1033]. A similar domain, "ip6.arpa", is defined in
[RFC3596] for finding the name of a computer using a specific IPv6
address.
Adversaries who observe a particular address in use on a specific
network can try to retrieve the PTR record associated with that
address, and thus the hostname of the computer, or even the fully
qualified domain name of that computer. The retrieval may not be
useful in many IPv4 networks due to the prevalence of NAT, but it
could work in IPv6 networks. Other name lookup mechanisms, such as
[RFC4620], share similar issues.
4.3. Multicast DNS
Multicast DNS (mDNS) is defined in [RFC6762]. It enables hosts to
send DNS queries over multicast, and to elicit responses from hosts
participating in the service.
If an adversary suspects that a particular host is present on a
network, the adversary can send mDNS requests to find, for example,
the A or AAAA records associated with the hostname in the ".local"
domain. A positive reply will confirm the presence of the host.
When a new responder starts, it must send a set of multicast queries
to verify that the name that it advertises is unique on the network,
and also to populate the caches of other mDNS hosts. Adversaries can
monitor this traffic and discover the hostname of computers as they
join the monitored network.
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mDNS further allows to send queries via unicast to port 5353. An
adversary might decide to use unicast instead of multicast in order
to hide from e.g. intrusion detection systems.
4.4. Link-local Multicast Name Resolution
Link-local Multicast Name Resolution (LLMNR) is defined in [RFC4795].
The specification did not achieve consensus as an IETF standard, but
it is widely deployed. Like mDNS, it enables hosts to send DNS
queries over multicast, and to elicit responses from computers
implementing the LLMNR service.
Like mDNS, LLMNR can be used by adversaries to confirm the presence
of a specific host on a network, by issuing a multicast request to
find the A or AAAA records associated with the hostname in the
".local" domain.
When an LLMNR responder starts, it sends a set of multicast queries
to verify that the name that it advertises is unique on the network.
Adversaries can monitor this traffic and discover the hostname of
computers as they join the monitored network.
4.5. DNS-Based Service Discovery
DNS-Based Service Discovery (DNS-SD) is described in [RFC6763]. It
enables participating hosts to retrieve the location of services
proposed by other hosts. It can be used with DNS servers, or in
conjunction with mDNS in a server-less environment.
Participating hosts publish a service described by an "instance
name", typically chosen by the user responsible for the publication.
While this is obviously an active disclosure of information, privacy
aspects can be mitigated by user control. Services should only be
published when deciding to do so, and the information disclosed in
the service name should be well under the control of the device's
owner.
In theory there should not be any privacy issue, but in practice the
publication of a service also forces the publication of the hostname,
due to a chain of dependencies. The service name is used to publish
a PTR record announcing the service. The PTR record typically points
to the service name in the local domain. The service names, in turn,
are used to publish TXT records describing service parameters, and
SRV records describing the service location.
SRV records are described in [RFC2782]. Each record contains 4
parameters: priority, weight, port number and hostname. While the
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service name published in the PTR record is chosen by the user, the
"hostname" in the SRV record is indeed the hostname of the device.
Adversaries can monitor the mDNS traffic associated with DNS-SD and
retrieve the hostname of computers advertising any service with DNS-
SD.
4.6. NetBIOS-over-TCP
Amongst other things, NetBIOS-over-TCP ([RFC1002]) implements a name
registration and resolution mechanism called the NetBIOS Name
Service. In practice, NetBIOS resource names are often based on
hostnames.
NetBIOS allows an application to register resource names and to
resolve such names to IP addresses. In environments without an
NetBIOS Name Server, the protocol makes extensive use of broadcasts
from which resource names can be easily extracted. NetBIOS also
allows querying for the names registered by a node directly (node
status).
5. Randomized Hostnames as Remedy
There are several ways to remedy the hostname practices. We could
instruct people to just turn off any protocol that leaks hostnames,
at least when they visit some "insecure" place. We could also
examine each particular standard that publishes hostnames, and
somehow fix the corresponding protocols. Or, we could attempt to
revise the way devices manage the hostname parameter.
There is a lot of merit in "turning off unneeded protocols when
visiting insecure places." This amounts to attack surface reduction,
and is clearly beneficial -- this is an advantage of the stealth mode
defined in [RFC7288]. However, there are two issues with this
advice. First, it relies on recognizing which networks are secure or
insecure. This is hard to automate, but relying on end-user judgment
may not always provide good results. Second, some protocols such as
DHCP cannot be turned off without losing connectivity, which limits
the value of this option. Also, the services that rely on protocols
that leak hostnames such as mDNS will not be available when switched
off. In addition, not always are hostname-leaking protocols well-
known as they might be proprietary and come with an installed
application instead of being provided by the operating system.
It may be possible in many cases to examine a protocol and prevent it
from leaking hostnames. This is for example what is attempted for
DHCP in [RFC7844]. However, it is unclear that we can identify,
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revisit and fix all the protocols that publish hostnames. In
particular, this is impossible for proprietary protocols.
We may be able to mitigate most of the effects of hostname leakage by
revisiting the way platforms handle hostnames. This is in a way
similar to the approach of MAC address randomization described in
[RFC7844]. Let's assume that the operating system, at the time of
connecting to a new network, picks a random hostname and starts
publicizing that random name in protocols such as DHCP or mDNS,
instead of the static value. This will render monitoring and
identification of users by adversaries much more difficult, without
preventing protocols such as DNS-SD from operating as expected. This
has of course implications on the applications making use of such
protocols e.g. when the hostname is being displayed to users of the
application. They will not as easily be able to identify e.g.
network shares or services based on the hostname carried in the
underlying protocols. Also, the generation of new hostnames should
be synchronized with the change of other tokens used in network
protocols such as the MAC or IP address to prevent correlation of
this information. E.g. if the IP address changes but the hostname
stays the same, the new IP address can be correlated to belong to the
same device based on a leaked hostname.
Some operating systems, including Windows, support "per network"
hostnames, but some other operating systems only support "global"
hostnames. In that case, changing the hostname may be difficult if
the host is multi-homed, as the same name will be used on several
networks. Other operating systems already use potentially different
hostnames for different purposes, which might be a good model to
combine both static hostnames and randomized hostnames based on their
potential use and threat to a user's privacy.
Obviously, further studies are required before the idea of randomized
hostnames can be implemented.
6. Security Considerations
This draft does not introduce any new protocol. It does point to
potential privacy issues in a set of existing protocols.
There are obvious privacy gains to changing to randomized hostnames
and also to change these names frequently. Wide deployment might
however affect security functions or current practices. For example,
incident response using hostnames to track the source of traffic
might be affected. It is common practice to include hostnames and
reverse lookup information at various times during an investigation.
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7. IANA Considerations
This draft does not require any IANA action.
8. Acknowledgments
Thanks to the members of the INTAREA Working Group for discussions
and reviews.
9. Informative References
[RFC1002] NetBIOS Working Group in the Defense Advanced Research
Projects Agency, Internet Activities Board, and End-to-End
Services Task Force, "Protocol standard for a NetBIOS
service on a TCP/UDP transport: Detailed specifications",
STD 19, RFC 1002, DOI 10.17487/RFC1002, March 1987,
<http://www.rfc-editor.org/info/rfc1002>.
[RFC1033] Lottor, M., "Domain Administrators Operations Guide",
RFC 1033, DOI 10.17487/RFC1033, November 1987,
<http://www.rfc-editor.org/info/rfc1033>.
[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
November 1987, <http://www.rfc-editor.org/info/rfc1035>.
[RFC1983] Malkin, G., Ed., "Internet Users' Glossary", FYI 18,
RFC 1983, DOI 10.17487/RFC1983, August 1996,
<http://www.rfc-editor.org/info/rfc1983>.
[RFC2131] Droms, R., "Dynamic Host Configuration Protocol",
RFC 2131, DOI 10.17487/RFC2131, March 1997,
<http://www.rfc-editor.org/info/rfc2131>.
[RFC2132] Alexander, S. and R. Droms, "DHCP Options and BOOTP Vendor
Extensions", RFC 2132, DOI 10.17487/RFC2132, March 1997,
<http://www.rfc-editor.org/info/rfc2132>.
[RFC2782] Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
specifying the location of services (DNS SRV)", RFC 2782,
DOI 10.17487/RFC2782, February 2000,
<http://www.rfc-editor.org/info/rfc2782>.
[RFC3315] Droms, R., Ed., Bound, J., Volz, B., Lemon, T., Perkins,
C., and M. Carney, "Dynamic Host Configuration Protocol
for IPv6 (DHCPv6)", RFC 3315, DOI 10.17487/RFC3315, July
2003, <http://www.rfc-editor.org/info/rfc3315>.
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[RFC3596] Thomson, S., Huitema, C., Ksinant, V., and M. Souissi,
"DNS Extensions to Support IP Version 6", RFC 3596,
DOI 10.17487/RFC3596, October 2003,
<http://www.rfc-editor.org/info/rfc3596>.
[RFC4620] Crawford, M. and B. Haberman, Ed., "IPv6 Node Information
Queries", RFC 4620, DOI 10.17487/RFC4620, August 2006,
<http://www.rfc-editor.org/info/rfc4620>.
[RFC4795] Aboba, B., Thaler, D., and L. Esibov, "Link-local
Multicast Name Resolution (LLMNR)", RFC 4795,
DOI 10.17487/RFC4795, January 2007,
<http://www.rfc-editor.org/info/rfc4795>.
[RFC6762] Cheshire, S. and M. Krochmal, "Multicast DNS", RFC 6762,
DOI 10.17487/RFC6762, February 2013,
<http://www.rfc-editor.org/info/rfc6762>.
[RFC6763] Cheshire, S. and M. Krochmal, "DNS-Based Service
Discovery", RFC 6763, DOI 10.17487/RFC6763, February 2013,
<http://www.rfc-editor.org/info/rfc6763>.
[RFC7288] Thaler, D., "Reflections on Host Firewalls", RFC 7288,
DOI 10.17487/RFC7288, June 2014,
<http://www.rfc-editor.org/info/rfc7288>.
[RFC7719] Hoffman, P., Sullivan, A., and K. Fujiwara, "DNS
Terminology", RFC 7719, DOI 10.17487/RFC7719, December
2015, <http://www.rfc-editor.org/info/rfc7719>.
[RFC7819] Jiang, S., Krishnan, S., and T. Mrugalski, "Privacy
Considerations for DHCP", RFC 7819, DOI 10.17487/RFC7819,
April 2016, <http://www.rfc-editor.org/info/rfc7819>.
[RFC7824] Krishnan, S., Mrugalski, T., and S. Jiang, "Privacy
Considerations for DHCPv6", RFC 7824,
DOI 10.17487/RFC7824, May 2016,
<http://www.rfc-editor.org/info/rfc7824>.
[RFC7844] Huitema, C., Mrugalski, T., and S. Krishnan, "Anonymity
Profiles for DHCP Clients", RFC 7844,
DOI 10.17487/RFC7844, May 2016,
<http://www.rfc-editor.org/info/rfc7844>.
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[TRAC2016]
Faath, M., Weisshaar, F., and R. Winter, "How Broadcast
Data Reveals Your Identity and Social Graph", 7th
International Workshop on TRaffic Analysis and
Characterization IEEE TRAC 2016, September 2016.
Authors' Addresses
Christian Huitema
Private Octopus Inc.
Friday Harbor, WA 98250
U.S.A.
Email: huitema@huitema.net
Dave Thaler
Microsoft
Redmond, WA 98052
U.S.A.
Email: dthaler@microsoft.com
Rolf Winter
University of Applied Sciences Augsburg
Augsburg
DE
Email: rolf.winter@hs-augsburg.de
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