Network Working Group                                         C. Huitema
Internet-Draft                                                 D. Thaler
Intended status: Informational                                 Microsoft
Expires: April 15, 2016                                 October 13, 2015

              Current Hostname Practice Considered Harmful


   Giving a hostname to your computer and publishing it as you roam from
   network to hot spot is the Internet equivalent of walking around with
   a name tag affixed to your lapel.  The practice can significantly
   compromise your privacy, and should stop.

   There are several possible remedies, such as fixing a variety of
   protocols or avoiding disclosing a hostname at all.  This document
   studies another possible remedy, which is to replace the static
   hostnames by frequently changing randomized values.  This idea
   obviously needs more work.

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
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   This Internet-Draft will expire on April 15, 2016.

Copyright Notice

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   document authors.  All rights reserved.

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   ( in effect on the date of
   publication of this document.  Please review these documents

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   carefully, as they describe your rights and restrictions with respect
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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Naming practices  . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Partial identifiers . . . . . . . . . . . . . . . . . . . . .   3
   4.  Protocols that leak hostnames . . . . . . . . . . . . . . . .   4
     4.1.  DHCP  . . . . . . . . . . . . . . . . . . . . . . . . . .   4
     4.2.  DNS address to name resolution  . . . . . . . . . . . . .   4
     4.3.  Multicast DNS . . . . . . . . . . . . . . . . . . . . . .   5
     4.4.  Link-local Multicast Name Resolution  . . . . . . . . . .   5
     4.5.  DNS service discovery . . . . . . . . . . . . . . . . . .   5
   5.  Randomized Host Names as Remedy . . . . . . . . . . . . . . .   6
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
   8.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .   7
   9.  Informative References  . . . . . . . . . . . . . . . . . . .   7
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   9

1.  Introduction

   There is a long established practice of giving names to computers.
   In the Internet protocols, these names are referred to as
   "hostnames." hostnames are normally used in conjunction with a domain
   name prefix 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 lookups
   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 or active
   monitoring of a variety of Internet protocols, such as for example
   DHCP, or multicast DNS.  They can correlate the hostname with various
   other information extracted from traffic analysis, and identify the
   device and its user.

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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 enable 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.

   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.

   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 uses 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-

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, is not 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

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   "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
   by maybe a factor of a thousand.  Other information such as the
   visited sites will quickly complement that data and lead to user

   Of course, unique names assigned by manufacturers are even more
   interesting for such adversaries capable of maintaining a database
   recording the hostnames of devices used by specific user.  With a
   unique name like "BrandX-123456-7890-abcdef" identification can be
   pretty much immediate.

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

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
   multicast addresses 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 hostnames and FQDN parameters in DHCP are
   analyzed in [I-D.ietf-dhc-dhcp-privacy] and
   [I-D.ietf-dhc-dhcpv6-privacy].  Possible mitigations are described in

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

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   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.

4.3.  Multicast DNS

   Multicast DNS (MDNS) is defined in [RFC6762].  It enables hosts to
   send DNS queries over a multicast port, 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.

4.4.  Link-local Multicast Name Resolution

   The Link-local Multicast Name Resolution (LLMNR) is defined in
   [RFC4795].  The specification did not achieve consensus as an IETF
   standard, but is widely deployed.  Like MDNS, it enables hosts to
   send DNS queries over a multicast port, and to elicit responses from
   computers implementing the LLMNR service.

   Like MDNS, LLMNR can be used by adversaries to confirm the presence
   on a network of a specific host, by issuing a multicast requests 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 service discovery

   DNS-Based Service discovery (DNS-SD) is described in [RFC6763].  It
   enables participating host 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.

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   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
   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 host name of computers advertising any service with DNS-

5.  Randomized Host Names 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 our 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.

   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 [I-D.ietf-dhc-anonymity-profile].  However, it is unclear
   that we can identify, revisit an fix all the protocols that publish

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   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
   [I-D.ietf-dhc-anonymity-profile].  Let's assume that the operating
   system, at the time of connecting to a new network, picks a random
   hostname and start publicizing that random name in protocols such as
   DHCP or MDNS, instead of the static value.  This will frustrate
   monitoring by adversaries, without preventing protocols such as DNS
   SD from operating as expected.

   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.  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.

7.  IANA Considerations

   This draft does not require any IANA action.

8.  Acknowledgments

   Contributions will be gladly acknowledged.

9.  Informative References

              Huitema, C., Mrugalski, T., and S. Krishnan, "Anonymity
              profile for DHCP clients", draft-ietf-dhc-anonymity-
              profile-04 (work in progress), October 2015.

              Jiang, S., Krishnan, S., and T. Mrugalski, "Privacy
              considerations for DHCPv4", draft-ietf-dhc-dhcp-privacy-01
              (work in progress), August 2015.

              Krishnan, S., Mrugalski, T., and S. Jiang, "Privacy
              considerations for DHCPv6", draft-ietf-dhc-
              dhcpv6-privacy-01 (work in progress), August 2015.

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   [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,

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

   [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,

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Authors' Addresses

   Christian Huitema
   Redmond, WA  98052


   Dave Thaler
   Redmond, WA  98052


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