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Checking references for intended status: Informational ---------------------------------------------------------------------------- -- Obsolete informational reference (is this intentional?): RFC 3315 (Obsoleted by RFC 8415) -- Obsolete informational reference (is this intentional?): RFC 7719 (Obsoleted by RFC 8499) Summary: 0 errors (**), 0 flaws (~~), 1 warning (==), 3 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Network Working Group C. Huitema 3 Internet-Draft Private Octopus Inc. 4 Intended status: Informational D. Thaler 5 Expires: July 27, 2017 Microsoft 6 R. Winter 7 University of Applied Sciences Augsburg 8 January 23, 2017 10 Current Hostname Practice Considered Harmful 11 draft-ietf-intarea-hostname-practice-04.txt 13 Abstract 15 Giving a hostname to your computer and publishing it as you roam from 16 one network to another is the Internet equivalent of walking around 17 with a name tag affixed to your lapel. This current practice can 18 significantly compromise your privacy, and something should change in 19 order to mitigate these privacy threats. 21 There are several possible remedies, such as fixing a variety of 22 protocols or avoiding disclosing a hostname at all. This document 23 describes some of the protocols that reveal hostnames today and 24 sketches another possible remedy, which is to replace static 25 hostnames by frequently changing randomized values. 27 Status of This Memo 29 This Internet-Draft is submitted in full conformance with the 30 provisions of BCP 78 and BCP 79. 32 Internet-Drafts are working documents of the Internet Engineering 33 Task Force (IETF). Note that other groups may also distribute 34 working documents as Internet-Drafts. The list of current Internet- 35 Drafts is at http://datatracker.ietf.org/drafts/current/. 37 Internet-Drafts are draft documents valid for a maximum of six months 38 and may be updated, replaced, or obsoleted by other documents at any 39 time. It is inappropriate to use Internet-Drafts as reference 40 material or to cite them other than as "work in progress." 42 This Internet-Draft will expire on July 27, 2017. 44 Copyright Notice 46 Copyright (c) 2017 IETF Trust and the persons identified as the 47 document authors. All rights reserved. 49 This document is subject to BCP 78 and the IETF Trust's Legal 50 Provisions Relating to IETF Documents 51 (http://trustee.ietf.org/license-info) in effect on the date of 52 publication of this document. Please review these documents 53 carefully, as they describe your rights and restrictions with respect 54 to this document. Code Components extracted from this document must 55 include Simplified BSD License text as described in Section 4.e of 56 the Trust Legal Provisions and are provided without warranty as 57 described in the Simplified BSD License. 59 Table of Contents 61 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 62 2. Naming Practices . . . . . . . . . . . . . . . . . . . . . . 3 63 3. Partial Identifiers . . . . . . . . . . . . . . . . . . . . . 4 64 4. Protocols that leak Hostnames . . . . . . . . . . . . . . . . 4 65 4.1. DHCP . . . . . . . . . . . . . . . . . . . . . . . . . . 5 66 4.2. DNS Address to Name Resolution . . . . . . . . . . . . . 5 67 4.3. Multicast DNS . . . . . . . . . . . . . . . . . . . . . . 5 68 4.4. Link-local Multicast Name Resolution . . . . . . . . . . 6 69 4.5. DNS-Based Service Discovery . . . . . . . . . . . . . . . 6 70 4.6. NetBIOS-over-TCP . . . . . . . . . . . . . . . . . . . . 7 71 5. Randomized Hostnames as Remedy . . . . . . . . . . . . . . . 7 72 6. Security Considerations . . . . . . . . . . . . . . . . . . . 8 73 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8 74 8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 8 75 9. Informative References . . . . . . . . . . . . . . . . . . . 9 76 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 10 78 1. Introduction 80 There is a long established practice of giving names to computers. 81 In the Internet protocols, these names are referred to as "hostnames" 82 [RFC7719] . Hostnames are normally used in conjunction with a domain 83 name suffix to build the "Fully Qualified Domain Name" (FQDN) of a 84 host. However, it is common practice to use the hostname without 85 further qualification in a variety of applications from file sharing 86 to network management. Hostnames are typically published as part of 87 domain names, and can be obtained through a variety of name lookup 88 and discovery protocols. 90 Hostnames have to be unique within the domain in which they are 91 created and used. They do not have to be globally unique 92 identifiers, but they will always be at least partial identifiers, as 93 discussed in Section 3. 95 The disclosure of information through hostnames creates a problem for 96 mobile devices. Adversaries that monitor a remote network such as a 97 Wi-Fi hot spot can obtain the hostname through passive monitoring or 98 active probing of a variety of Internet protocols, such as for 99 example DHCP, or multicast DNS (mDNS). They can correlate the 100 hostname with various other information extracted from traffic 101 analysis and other information sources, and can potentially identify 102 the device, device properties and its user [TRAC2016]. 104 2. Naming Practices 106 There are many reasons to give names to computers. This is 107 particularly true when computers operate on a network. Operating 108 systems like Microsoft Windows or Unix assume that computers have a 109 "hostname." This enables users and administrators to do things such 110 as ping a computer, add its name to an access control list, remotely 111 mount a computer disk, or connect to the computer through tools such 112 as telnet or remote desktop. Other operating systems maintain 113 multiple hostnames for different purposes, e.g. for use with certain 114 protocols such as mDNS. 116 In most consumer networks, naming is pretty much left to the fancy of 117 the user. Some will pick names of planets or stars, other names of 118 fruits or flowers, and other will pick whatever suits their mood when 119 they unwrap the device. As long as users are careful to not pick a 120 name already in use on the same network, anything goes. Very often 121 however, the operating system is suggesting a hostname at install 122 time, which can contain the user name, the login name and information 123 learned from the device itself such as the brand, model or maker of 124 the device [TRAC2016]. 126 In large organizations, collisions are more likely and a more 127 structured approach is necessary. In theory, organizations could use 128 multiple DNS subdomains to ease the pressure on uniqueness, but in 129 practice many don't and insist on unique flat names, if only to 130 simplify network management. To ensure unique names, organizations 131 will set naming guidelines and enforce some kind of structured 132 naming. For example, within the Microsoft corporate network, 133 computer names are derived from the login name of the main user, 134 leading to names like "huitema-test2" for a machine that one of the 135 authors used to test software. 137 There is less pressure to assign names to small devices, including 138 for example smart phones, as these devices typically do not enable 139 sharing of their disks or remote login. As a consequence, these 140 devices often have manufacturer assigned names, which vary from very 141 generic like "Windows Phone" to completely unique like "BrandX- 142 123456-7890-abcdef" and often contain the name of the device owner, 143 the device's brand name, and often also a hint as to which language 144 the device owner speaks [TRAC2016]. 146 3. Partial Identifiers 148 Suppose an adversary wants to track the people connecting to a 149 specific Wi-Fi hot spot, for example in a railroad station. Assume 150 that the adversary is able to retrieve the hostname used by a 151 specific laptop. That, in itself, might not be enough to identify 152 the laptop's owner. Suppose however that the adversary observes that 153 the laptop name is "huitema-laptop" and that the laptop has 154 established a VPN connection to the Microsoft corporate network. The 155 two pieces of information, put together, firmly point to Christian 156 Huitema, employed by Microsoft. The identification is successful. 158 In the example, we saw a login name inside the hostname, and that 159 certainly helped identification. But generic names like "jupiter" or 160 "rosebud" also provide partial identification, especially if the 161 adversary is capable of maintaining a database recording, among other 162 information, the hostnames of devices used by specific users. 163 Generic names are picked from vocabularies that include thousands of 164 potential choices. Finding the name reduces the scope of the search 165 significantly. Other information such as the visited sites will 166 quickly complement that data and can lead to user identification. 168 Also the special circumstances of the network can play a role. 169 Experiments on operational networks such as the IETF meeting network 170 have shown that with the help of external data such as the publicly 171 available IETF attendees list or other data sources such as LDAP 172 servers on the network [TRAC2016], the identification of the device 173 owner can become trivial given only partial identifiers in a 174 hostname. 176 Unique names assigned by manufacturers do not directly encode a user 177 identifier, but they have the property of being stable and unique to 178 the device in a large context. A unique name like "BrandX- 179 123456-7890-abcdef" allows efficient tracking across multiple 180 domains. In theory, this only allows tracking of the device but not 181 of the user. However, an adversary could correlate the device to the 182 user through other means, for example the one-time capture of some 183 clear text traffic. Adversaries could then maintain databases 184 linking unique host name to user identity. This will allow efficient 185 tracking of both the user and the device. 187 4. Protocols that leak Hostnames 189 Many IETF protocols can leak the "hostname" of a computer. A non 190 exhaustive list includes DHCP, DNS address to name resolution, 191 Multicast DNS, Link-local Multicast Name Resolution, and DNS service 192 discovery. 194 4.1. DHCP 196 Shortly after connecting to a new network, a host can use DHCP 197 [RFC2131] to acquire an IPv4 address and other parameters [RFC2132]. 198 A DHCP query can disclose the "hostname." DHCP traffic is sent to 199 the broadcast address and can be easily monitored, enabling 200 adversaries to discover the hostname associated with a computer 201 visiting a particular network. DHCPv6 [RFC3315] shares similar 202 issues. 204 The problems with the hostname and FQDN parameters in DHCP are 205 analyzed in [RFC7819] and [RFC7824]. Possible mitigations are 206 described in [RFC7844]. 208 4.2. DNS Address to Name Resolution 210 The domain name service design [RFC1035] includes the specification 211 of the special domain "in-addr.arpa" for resolving the name of the 212 computer using a particular IPv4 address, using the PTR format 213 defined in [RFC1033]. A similar domain, "ip6.arpa", is defined in 214 [RFC3596] for finding the name of a computer using a specific IPv6 215 address. 217 Adversaries who observe a particular address in use on a specific 218 network can try to retrieve the PTR record associated with that 219 address, and thus the hostname of the computer, or even the fully 220 qualified domain name of that computer. The retrieval may not be 221 useful in many IPv4 networks due to the prevalence of NAT, but it 222 could work in IPv6 networks. Other name lookup mechanisms, such as 223 [RFC4620], share similar issues. 225 4.3. Multicast DNS 227 Multicast DNS (mDNS) is defined in [RFC6762]. It enables hosts to 228 send DNS queries over multicast, and to elicit responses from hosts 229 participating in the service. 231 If an adversary suspects that a particular host is present on a 232 network, the adversary can send mDNS requests to find, for example, 233 the A or AAAA records associated with the hostname in the ".local" 234 domain. A positive reply will confirm the presence of the host. 236 When a new responder starts, it must send a set of multicast queries 237 to verify that the name that it advertises is unique on the network, 238 and also to populate the caches of other mDNS hosts. Adversaries can 239 monitor this traffic and discover the hostname of computers as they 240 join the monitored network. 242 mDNS further allows to send queries via unicast to port 5353. An 243 adversary might decide to use unicast instead of multicast in order 244 to hide from e.g. intrusion detection systems. 246 4.4. Link-local Multicast Name Resolution 248 Link-local Multicast Name Resolution (LLMNR) is defined in [RFC4795]. 249 The specification did not achieve consensus as an IETF standard, but 250 it is widely deployed. Like mDNS, it enables hosts to send DNS 251 queries over multicast, and to elicit responses from computers 252 implementing the LLMNR service. 254 Like mDNS, LLMNR can be used by adversaries to confirm the presence 255 of a specific host on a network, by issuing a multicast request to 256 find the A or AAAA records associated with the hostname in the 257 ".local" domain. 259 When an LLMNR responder starts, it sends a set of multicast queries 260 to verify that the name that it advertises is unique on the network. 261 Adversaries can monitor this traffic and discover the hostname of 262 computers as they join the monitored network. 264 4.5. DNS-Based Service Discovery 266 DNS-Based Service Discovery (DNS-SD) is described in [RFC6763]. It 267 enables participating hosts to retrieve the location of services 268 proposed by other hosts. It can be used with DNS servers, or in 269 conjunction with mDNS in a server-less environment. 271 Participating hosts publish a service described by an "instance 272 name," typically chosen by the user responsible for the publication. 273 While this is obviously an active disclosure of information, privacy 274 aspects can be mitigated by user control. Services should only be 275 published when deciding to do so, and the information disclosed in 276 the service name should be well under the control of the device's 277 owner. 279 In theory there should not be any privacy issue, but in practice the 280 publication of a service also forces the publication of the hostname, 281 due to a chain of dependencies. The service name is used to publish 282 a PTR record announcing the service. The PTR record typically points 283 to the service name in the local domain. The service names, in turn, 284 are used to publish TXT records describing service parameters, and 285 SRV records describing the service location. 287 SRV records are described in [RFC2782]. Each record contains 4 288 parameters: priority, weight, port number and hostname. While the 289 service name published in the PTR record is chosen by the user, the 290 "hostname" in the SRV record is indeed the hostname of the device. 292 Adversaries can monitor the mDNS traffic associated with DNS-SD and 293 retrieve the hostname of computers advertising any service with DNS- 294 SD. 296 4.6. NetBIOS-over-TCP 298 Amongst other things, NetBIOS-over-TCP ([RFC1002]) implements a name 299 registration and resolution mechanism called the NetBIOS Name 300 Service. In practice, NetBIOS resource names are often based on 301 hostnames. 303 NetBIOS allows an application to register resource names and to 304 resolve such names to IP addresses. In environments without an 305 NetBIOS Name Server, the protocol makes extensive use of broadcasts 306 from which resource names can be easily extracted. NetBIOS also 307 allows querying for the names registered by a node directly (node 308 status). 310 5. Randomized Hostnames as Remedy 312 There are several ways to remedy the hostname practices. We could 313 instruct people to just turn off any protocol that leaks hostnames, 314 at least when they visit some "insecure" place. We could also 315 examine each particular standard that publishes hostnames, and 316 somehow fix the corresponding protocols. Or, we could attempt to 317 revise the way devices manage the hostname parameter. 319 There is a lot of merit in "turning off unneeded protocols when 320 visiting insecure places." This amounts to attack surface reduction, 321 and is clearly beneficial -- this is an advantage of the stealth mode 322 defined in [RFC7288]. However, there are two issues with this 323 advice. First, it relies on recognizing which networks are secure or 324 insecure. This is hard to automate, but relying on end-user judgment 325 may not always provide good results. Second, some protocols such as 326 DHCP cannot be turned off without losing connectivity, which limits 327 the value of this option. Also, the services that rely on protocols 328 that leak hostnames such as mDNS will not be available when switched 329 off. In addition, not always are hostname-leaking protocols well- 330 known as they might be proprietary and come with an installed 331 application instead of being provided by the operating system. 333 It may be possible in many cases to examine a protocol and prevent it 334 from leaking hostnames. This is for example what is attempted for 335 DHCP in [RFC7844]. However, it is unclear that we can identify, 336 revisit and fix all the protocols that publish hostnames. In 337 particular, this is impossible for proprietary protocols. 339 We may be able to mitigate most of the effects of hostname leakage by 340 revisiting the way platforms handle hostnames. This is in a way 341 similar to the approach of MAC address randomization described in 342 [RFC7844]. Let's assume that the operating system, at the time of 343 connecting to a new network, picks a random hostname and starts 344 publicizing that random name in protocols such as DHCP or mDNS, 345 instead of the static value. This will render monitoring and 346 identification of users by adversaries much more difficult, without 347 preventing protocols such as DNS-SD from operating as expected. This 348 has of course implications on the applications making use of such 349 protocols e.g. when the hostname is being displayed to users of the 350 application. They will not as easily be able to identify e.g. 351 network shares or services based on the hostname carried in the 352 underlying protocols. Also, the generation of new hostnames should 353 be synchronized with the change of other tokens used in network 354 protocols such as the MAC or IP address to prevent correlation of 355 this information. E.g. if the IP address changes but the hostname 356 stays the same, the new IP address can be correlated to belong to the 357 same device based on a leaked hostname. 359 Some operating systems, including Windows, support "per network" 360 hostnames, but some other operating systems only support "global" 361 hostnames. In that case, changing the hostname may be difficult if 362 the host is multi-homed, as the same name will be used on several 363 networks. Other operating systems already use potentially different 364 hostnames for different purposes, which might be a good model to 365 combine both static hostnames and randomized hostnames based on their 366 potential use and threat to a user's privacy. Obviously, further 367 studies are required before the idea of randomized hostnames can be 368 implemented. 370 6. Security Considerations 372 This draft does not introduce any new protocol. It does point to 373 potential privacy issues in a set of existing protocols. 375 7. IANA Considerations 377 This draft does not require any IANA action. 379 8. Acknowledgments 381 Thanks to the members of the INTAREA Working Group for discussions 382 and reviews. 384 9. Informative References 386 [RFC1002] NetBIOS Working Group in the Defense Advanced Research 387 Projects Agency, Internet Activities Board, and End-to-End 388 Services Task Force, "Protocol standard for a NetBIOS 389 service on a TCP/UDP transport: Detailed specifications", 390 STD 19, RFC 1002, DOI 10.17487/RFC1002, March 1987, 391 . 393 [RFC1033] Lottor, M., "Domain Administrators Operations Guide", 394 RFC 1033, DOI 10.17487/RFC1033, November 1987, 395 . 397 [RFC1035] Mockapetris, P., "Domain names - implementation and 398 specification", STD 13, RFC 1035, DOI 10.17487/RFC1035, 399 November 1987, . 401 [RFC2131] Droms, R., "Dynamic Host Configuration Protocol", 402 RFC 2131, DOI 10.17487/RFC2131, March 1997, 403 . 405 [RFC2132] Alexander, S. and R. Droms, "DHCP Options and BOOTP Vendor 406 Extensions", RFC 2132, DOI 10.17487/RFC2132, March 1997, 407 . 409 [RFC2782] Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for 410 specifying the location of services (DNS SRV)", RFC 2782, 411 DOI 10.17487/RFC2782, February 2000, 412 . 414 [RFC3315] Droms, R., Ed., Bound, J., Volz, B., Lemon, T., Perkins, 415 C., and M. Carney, "Dynamic Host Configuration Protocol 416 for IPv6 (DHCPv6)", RFC 3315, DOI 10.17487/RFC3315, July 417 2003, . 419 [RFC3596] Thomson, S., Huitema, C., Ksinant, V., and M. Souissi, 420 "DNS Extensions to Support IP Version 6", RFC 3596, 421 DOI 10.17487/RFC3596, October 2003, 422 . 424 [RFC4620] Crawford, M. and B. Haberman, Ed., "IPv6 Node Information 425 Queries", RFC 4620, DOI 10.17487/RFC4620, August 2006, 426 . 428 [RFC4795] Aboba, B., Thaler, D., and L. Esibov, "Link-local 429 Multicast Name Resolution (LLMNR)", RFC 4795, 430 DOI 10.17487/RFC4795, January 2007, 431 . 433 [RFC6762] Cheshire, S. and M. Krochmal, "Multicast DNS", RFC 6762, 434 DOI 10.17487/RFC6762, February 2013, 435 . 437 [RFC6763] Cheshire, S. and M. Krochmal, "DNS-Based Service 438 Discovery", RFC 6763, DOI 10.17487/RFC6763, February 2013, 439 . 441 [RFC7288] Thaler, D., "Reflections on Host Firewalls", RFC 7288, 442 DOI 10.17487/RFC7288, June 2014, 443 . 445 [RFC7719] Hoffman, P., Sullivan, A., and K. Fujiwara, "DNS 446 Terminology", RFC 7719, DOI 10.17487/RFC7719, December 447 2015, . 449 [RFC7819] Jiang, S., Krishnan, S., and T. Mrugalski, "Privacy 450 Considerations for DHCP", RFC 7819, DOI 10.17487/RFC7819, 451 April 2016, . 453 [RFC7824] Krishnan, S., Mrugalski, T., and S. Jiang, "Privacy 454 Considerations for DHCPv6", RFC 7824, 455 DOI 10.17487/RFC7824, May 2016, 456 . 458 [RFC7844] Huitema, C., Mrugalski, T., and S. Krishnan, "Anonymity 459 Profiles for DHCP Clients", RFC 7844, 460 DOI 10.17487/RFC7844, May 2016, 461 . 463 [TRAC2016] 464 Faath, M., Weisshaar, F., and R. Winter, "How Broadcast 465 Data Reveals Your Identity and Social Graph", 7th 466 International Workshop on TRaffic Analysis and 467 Characterization IEEE TRAC 2016, September 2016. 469 Authors' Addresses 471 Christian Huitema 472 Private Octopus Inc. 473 Friday Harbor, WA 98250 474 U.S.A. 476 Email: huitema@huitema.net 477 Dave Thaler 478 Microsoft 479 Redmond, WA 98052 480 U.S.A. 482 Email: dthaler@microsoft.com 484 Rolf Winter 485 University of Applied Sciences Augsburg 486 Augsburg 487 DE 489 Email: rolf.winter@hs-augsburg.de