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IETF RFC 8117
Last modified on Friday, March 10th, 2017
 Permanent link to RFC 8117
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Internet Engineering Task Force (IETF) C. Huitema
Request for Comments: 8117 Private Octopus Inc.
Category: Informational D. Thaler
ISSN: 2070-1721 Microsoft
R. Winter
University of Applied Sciences Augsburg
March 2017
Current Hostname Practice Considered Harmful
 Abstract
Giving a hostname to your computer and publishing it as you roam from
one network to another is the Internet's 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
This document is not an Internet Standards Track specification; it is
published for informational purposes.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Not all documents
approved by the IESG are a candidate for any level of Internet
Standard; see Section 2 of RFC 7841.
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/RFC 8117.
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RFC 8117 Harmful Hostname Practice March 2017
Copyright Notice
Copyright (c) 2017 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Naming Practices . . . . . . . . . . . . . . . . . . . . . . 3
3. Partial Identifiers . . . . . . . . . . . . . . . . . . . . . 4
4. Protocols That Leak Hostnames . . . . . . . . . . . . . . . . 5
4.1. DHCP . . . . . . . . . . . . . . . . . . . . . . . . . . 5
4.2. DNS Address to Name Resolution . . . . . . . . . . . . . 5
4.3. Multicast DNS . . . . . . . . . . . . . . . . . . . . . . 6
4.4. Link-Local Multicast Name Resolution . . . . . . . . . . 6
4.5. DNS-Based Service Discovery . . . . . . . . . . . . . . . 7
4.6. NetBIOS-over-TCP . . . . . . . . . . . . . . . . . . . . 7
5. Randomized Hostnames as a Remedy . . . . . . . . . . . . . . 8
6. Security Considerations . . . . . . . . . . . . . . . . . . . 9
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
8. Informative References . . . . . . . . . . . . . . . . . . . 9
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 12
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RFC 8117 Harmful Hostname Practice March 2017
1. Introduction
There is a long established practice of giving names to computers.
In the Internet protocols, these names are referred to as "hostnames"
[RFC 7719]. Hostnames are normally used in conjunction with a domain
name suffix to build the Fully Qualified Domain Name (FQDN) of a host
[RFC 1983]. 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
Wi-Fi hot spot can obtain the hostname through passive monitoring or
active probing of a variety of Internet protocols, such as DHCP or
Multicast DNS (mDNS). They can correlate the hostname with various
other information extracted from traffic analysis and other
information sources, and they 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
discretion of the user. Some will pick names of planets or stars,
others will pick names of fruits or flowers, and still others 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
suggests a hostname at the time of installation, 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].
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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,
which leads 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
generic names like "Windows Phone" to completely unique names 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].
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
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RFC 8117 Harmful Hostname Practice March 2017
Lightweight Directory Access Protocol (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
cleartext traffic. Adversaries could then maintain databases linking
a unique hostname to a 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.
4.1. DHCP
Shortly after connecting to a new network, a host can use DHCP
[RFC 2131] to acquire an IPv4 address and other parameters [RFC 2132].
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 [RFC 3315] shares similar
issues.
The problems with the hostname and FQDN parameters in DHCP are
analyzed in [RFC 7819] and [RFC 7824]. Possible mitigations are
described in [RFC 7844].
4.2. DNS Address to Name Resolution
The domain name service design [RFC 1035] 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 [RFC 1033]. A similar domain, "ip6.arpa", is defined in
[RFC 3596] 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 FQDN of
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RFC 8117 Harmful Hostname Practice March 2017
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 [RFC 4620], share similar
issues.
4.3. Multicast DNS
Multicast DNS (mDNS) is defined in [RFC 6762]. 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 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.
mDNS further allows queries to be sent 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 [RFC 4795].
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.
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4.5. DNS-Based Service Discovery
DNS-based Service Discovery (DNS-SD) is described in [RFC 6763]. 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 serverless environment.
Participating hosts publish a service described by an "instance
name", which is 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 [RFC 2782]. Each record contains four
parameters: priority, weight, port number, and hostname. While the
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 [RFC 1002] 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 a
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).
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RFC 8117 Harmful Hostname Practice March 2017
5. Randomized Hostnames as a 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 [RFC 7288]. 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 [RFC 7844]. However, it is unclear that we can identify,
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. In a way, this is
similar to the approach of Media Access Control (MAC) address
randomization described in [RFC 7844]. 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, of course, has 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. For example, if the IP
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RFC 8117 Harmful Hostname Practice March 2017
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 multihomed, 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 document 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 changing these names frequently. However, wide
deployment might 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.
7. IANA Considerations
This document does not require any IANA actions.
8. Informative References
[RFC 1002] 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/RFC 1002, March 1987,
<http://www.rfc-editor.org/info/RFC 1002>.
[RFC 1033] Lottor, M., "Domain Administrators Operations Guide",
RFC 1033, DOI 10.17487/RFC 1033, November 1987,
<http://www.rfc-editor.org/info/RFC 1033>.
Huitema, et al. Informational PAGE 9
RFC 8117 Harmful Hostname Practice March 2017
[RFC 1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, DOI 10.17487/RFC 1035,
November 1987, <http://www.rfc-editor.org/info/RFC 1035>.
[RFC 1983] Malkin, G., Ed., "Internet Users' Glossary", FYI 18,
RFC 1983, DOI 10.17487/RFC 1983, August 1996,
<http://www.rfc-editor.org/info/RFC 1983>.
[RFC 2131] Droms, R., "Dynamic Host Configuration Protocol",
RFC 2131, DOI 10.17487/RFC 2131, March 1997,
<http://www.rfc-editor.org/info/RFC 2131>.
[RFC 2132] Alexander, S. and R. Droms, "DHCP Options and BOOTP Vendor
Extensions", RFC 2132, DOI 10.17487/RFC 2132, March 1997,
<http://www.rfc-editor.org/info/RFC 2132>.
[RFC 2782] Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
specifying the location of services (DNS SRV)", RFC 2782,
DOI 10.17487/RFC 2782, February 2000,
<http://www.rfc-editor.org/info/RFC 2782>.
[RFC 3315] 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/RFC 3315, July
2003, <http://www.rfc-editor.org/info/RFC 3315>.
[RFC 3596] Thomson, S., Huitema, C., Ksinant, V., and M. Souissi,
"DNS Extensions to Support IP Version 6", RFC 3596,
DOI 10.17487/RFC 3596, October 2003,
<http://www.rfc-editor.org/info/RFC 3596>.
[RFC 4620] Crawford, M. and B. Haberman, Ed., "IPv6 Node Information
Queries", RFC 4620, DOI 10.17487/RFC 4620, August 2006,
<http://www.rfc-editor.org/info/RFC 4620>.
[RFC 4795] Aboba, B., Thaler, D., and L. Esibov, "Link-local
Multicast Name Resolution (LLMNR)", RFC 4795,
DOI 10.17487/RFC 4795, January 2007,
<http://www.rfc-editor.org/info/RFC 4795>.
[RFC 6762] Cheshire, S. and M. Krochmal, "Multicast DNS", RFC 6762,
DOI 10.17487/RFC 6762, February 2013,
<http://www.rfc-editor.org/info/RFC 6762>.
[RFC 6763] Cheshire, S. and M. Krochmal, "DNS-Based Service
Discovery", RFC 6763, DOI 10.17487/RFC 6763, February 2013,
<http://www.rfc-editor.org/info/RFC 6763>.
Huitema, et al. Informational PAGE 10
RFC 8117 Harmful Hostname Practice March 2017
[RFC 7288] Thaler, D., "Reflections on Host Firewalls", RFC 7288,
DOI 10.17487/RFC 7288, June 2014,
<http://www.rfc-editor.org/info/RFC 7288>.
[RFC 7719] Hoffman, P., Sullivan, A., and K. Fujiwara, "DNS
Terminology", RFC 7719, DOI 10.17487/RFC 7719, December
2015, <http://www.rfc-editor.org/info/RFC 7719>.
[RFC 7819] Jiang, S., Krishnan, S., and T. Mrugalski, "Privacy
Considerations for DHCP", RFC 7819, DOI 10.17487/RFC 7819,
April 2016, <http://www.rfc-editor.org/info/RFC 7819>.
[RFC 7824] Krishnan, S., Mrugalski, T., and S. Jiang, "Privacy
Considerations for DHCPv6", RFC 7824,
DOI 10.17487/RFC 7824, May 2016,
<http://www.rfc-editor.org/info/RFC 7824>.
[RFC 7844] Huitema, C., Mrugalski, T., and S. Krishnan, "Anonymity
Profiles for DHCP Clients", RFC 7844,
DOI 10.17487/RFC 7844, May 2016,
<http://www.rfc-editor.org/info/RFC 7844>.
[TRAC2016] Faath, M., Winter, R., and F. Weisshaar, "How Broadcast
Data Reveals Your Identity and Social Graph", IEEE,
Wireless Communications and Mobile Computing Conference
(IWCMC), 2016 International,
DOI 10.1109/IWCMC.2016.7577084, September 2016.
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RFC 8117 Harmful Hostname Practice March 2017
Acknowledgments
Thanks to the members of the INTAREA Working Group for discussions
and reviews.
Authors' Addresses
Christian Huitema
Private Octopus Inc.
Friday Harbor, WA 98250
United States of America
Email: huitema@huitema.net
Dave Thaler
Microsoft
Redmond, WA 98052
United States of America
Email: dthaler@microsoft.com
Rolf Winter
University of Applied Sciences Augsburg
Augsburg
DE
Email: rolf.winter@hs-augsburg.de
Huitema, et al. Informational PAGE 12
RFC TOTAL SIZE: 26750 bytes
PUBLICATION DATE: Friday, March 10th, 2017
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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