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Current Hostname Practice Considered Harmful

The information below is for an old version of the document.
Document Type
This is an older version of an Internet-Draft that was ultimately published as RFC 8117.
Authors Christian Huitema , Dave Thaler , Rolf Winter
Last updated 2017-02-02 (Latest revision 2017-01-23)
RFC stream Internet Engineering Task Force (IETF)
Additional resources Mailing list discussion
Stream WG state Submitted to IESG for Publication
Document shepherd Wassim Haddad
Shepherd write-up Show Last changed 2016-08-24
IESG IESG state Became RFC 8117 (Informational)
Consensus boilerplate Yes
Telechat date (None)
Responsible AD Suresh Krishnan
Send notices to "Wassim Haddad" <>,
IANA IANA review state IANA OK - No Actions Needed
Network Working Group                                         C. Huitema
Internet-Draft                                      Private Octopus Inc.
Intended status: Informational                                 D. Thaler
Expires: July 27, 2017                                         Microsoft
                                                               R. Winter
                                 University of Applied Sciences Augsburg
                                                        January 23, 2017

              Current Hostname Practice Considered Harmful


   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

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
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   Drafts is at

   Internet-Drafts are draft documents valid for a maximum of six months
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   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on July 27, 2017.

Copyright Notice

   Copyright (c) 2017 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   ( 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  . . . . . . . . . . . . . . . . . . . . . . . .   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 . . . . . . . . . . . . . . . . . . . . .   8
   8.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .   8
   9.  Informative References  . . . . . . . . . . . . . . . . . . .   9
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  10

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.  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 "huitema-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 Christian
   Huitema, 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

   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

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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

   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 "" for resolving the name of the
   computer using a particular IPv4 address, using the PTR format
   defined in [RFC1033].  A similar domain, "", is defined in
   [RFC3596] for finding the name of a computer using a specific IPv6

   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

   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-

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

   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

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

6.  Security Considerations

   This draft does not introduce any new protocol.  It does point to
   potential privacy issues in a set of existing protocols.

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.

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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,

   [RFC1033]  Lottor, M., "Domain Administrators Operations Guide",
              RFC 1033, DOI 10.17487/RFC1033, November 1987,

   [RFC1035]  Mockapetris, P., "Domain names - implementation and
              specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
              November 1987, <>.

   [RFC2131]  Droms, R., "Dynamic Host Configuration Protocol",
              RFC 2131, DOI 10.17487/RFC2131, March 1997,

   [RFC2132]  Alexander, S. and R. Droms, "DHCP Options and BOOTP Vendor
              Extensions", RFC 2132, DOI 10.17487/RFC2132, March 1997,

   [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,

   [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, <>.

   [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,

   [RFC4620]  Crawford, M. and B. Haberman, Ed., "IPv6 Node Information
              Queries", RFC 4620, DOI 10.17487/RFC4620, August 2006,

   [RFC4795]  Aboba, B., Thaler, D., and L. Esibov, "Link-local
              Multicast Name Resolution (LLMNR)", RFC 4795,
              DOI 10.17487/RFC4795, January 2007,

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   [RFC6762]  Cheshire, S. and M. Krochmal, "Multicast DNS", RFC 6762,
              DOI 10.17487/RFC6762, February 2013,

   [RFC6763]  Cheshire, S. and M. Krochmal, "DNS-Based Service
              Discovery", RFC 6763, DOI 10.17487/RFC6763, February 2013,

   [RFC7288]  Thaler, D., "Reflections on Host Firewalls", RFC 7288,
              DOI 10.17487/RFC7288, June 2014,

   [RFC7719]  Hoffman, P., Sullivan, A., and K. Fujiwara, "DNS
              Terminology", RFC 7719, DOI 10.17487/RFC7719, December
              2015, <>.

   [RFC7819]  Jiang, S., Krishnan, S., and T. Mrugalski, "Privacy
              Considerations for DHCP", RFC 7819, DOI 10.17487/RFC7819,
              April 2016, <>.

   [RFC7824]  Krishnan, S., Mrugalski, T., and S. Jiang, "Privacy
              Considerations for DHCPv6", RFC 7824,
              DOI 10.17487/RFC7824, May 2016,

   [RFC7844]  Huitema, C., Mrugalski, T., and S. Krishnan, "Anonymity
              Profiles for DHCP Clients", RFC 7844,
              DOI 10.17487/RFC7844, May 2016,

              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


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   Dave Thaler
   Redmond, WA  98052


   Rolf Winter
   University of Applied Sciences Augsburg


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