| INTAREA WG | M. Boucadair |
| Internet-Draft | France Telecom |
| Intended status: Informational | J. Touch |
| Expires: February 20, 2013 | USC/ISI |
| P. Levis | |
| France Telecom | |
| R. Penno | |
| Cisco | |
| August 21, 2012 |
Analysis of Solution Candidates to Reveal a Host Identifier (HOST_ID) in Shared Address Deployments
draft-ietf-intarea-nat-reveal-analysis-04
This document analyzes a set of solution candidates to mitigate some of the issues encountered when address sharing is used. In particular, this document focuses on means to reveal a host identifier (HOST_ID) when a Carrier Grade NAT (CGN) or application proxies are involved in the path. This host identifier must be unique to each host under the same shared IP address.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 [RFC2119].
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As reported in [RFC6269], several issues are encountered when an IP address is shared among several subscribers. These issues are encountered in various deployment contexts: e.g., Carrier Grade NAT (CGN), application proxies or A+P [RFC6346]. Examples of such issues are listed below:
The sole use of the IPv4 address is not sufficient to uniquely distinguish a host. As a mitigation, it is tempting to investigate means which would help in disclosing an information to be used by the remote server as a means to uniquely disambiguate packets of hosts using the same IPv4 address.
The risk of not mitigating these issues are: OPEX (Operational Expenditure) increase for IP connectivity service providers (costs induced by calls to a hotline), revenue loss for content providers (loss of users audience), customers unsatisfaction (low quality of experience, service segregation, etc.).
The purpose of this document is to analyze a set of alternative channels to convey a host identifier and to assess to what extent they solve the problem described in Section 2. Below are listed the candidates analyzed in the document:
Section 5 while the detailed analysis is elaborated in Section 4.
Section 3 discusses privacy issues common to all HOST_ID solutions. It is out of scope of this document to elaborate on privacy issues specific to each solution.
For all solutions analyzed, we provide answers to the following questions:
IP address sharing is motivated by a number of different factors. For years, many network operators have conserved the use of public IPv4 addresses by making use of Customer Premises Equipment (CPE) that assigns a single public IPv4 address to all hosts within the customer's local area network and uses NAT [RFC3022] to translate between locally unique private IPv4 addresses and the CPE's public address. With the exhaustion of IPv4 address space, address sharing between customers on a much larger scale is likely to become much more prevalent. While many individual users are unaware of and uninvolved in decisions about whether their unique IPv4 addresses get revealed when they send data via IP, some users realize privacy benefits associated with IP address sharing, and some may even take steps to ensure that NAT functionality sits between them and the public Internet. IP address sharing makes the actions of all users behind the NAT function unattributable to any single host, creating room for abuse but also providing some identity protection for non-abusive users who wish to transmit data with reduced risk of being uniquely identified.
The proposals considered in this document add a measure of uniqueness back to hosts that share a public IP address. The extent of that uniqueness depends on which information is included in the HOST_ID.
The volatility of the HOST_ID information is similar to the source IP address: a distinct HOST_ID may be used by the address sharing function when the host reboots or gets a new internal IP address. As with persistent IP addresses, persistent HOST_IDs facilitate user tracking over time.
As a general matter, the HOST_ID proposals do not seek to make hosts any more identifiable than they would be if they were using a public, non-shared IP address. However, depending on the solution proposal, the addition of HOST_ID information may allow a device to be fingerprinted more easily than it otherwise would be. Should multiple solutions be combined (e.g., TCP Option and XFF) that include different pieces of information in the HOST_ID, fingerprinting may become even easier.
A HOST_ID can be spoofed as this is also the case for spoofing an IP address. Furthermore, users of network-based anonymity services (like Tor) may be capable of stripping HOST_ID information before it reaches its destination.
HOST_ID specification document(s) SHOULD explain the privacy impact of the solutions they specify, including the extent of HOST_ID uniqueness and persistence, assumptions made about the lifetime of the HOST_ID, whether and how the HOST_ID can be obfuscated or recycled, and the impact of the use of the HOST_ID on device or implementation fingerprinting. [I-D.iab-privacy-considerations] provides further guidance.
For more discussion about privacy, refer to [RFC6462].
Whatever the channel used to convey the HOST_ID, the following requirements are to be met:
IP-ID (Identification field of IP header) can be used to insert an information which uniquely distinguishes a host among those sharing the same IPv4 address. An address sharing function can re-write the IP-ID field to insert a value unique to the host (16 bits are sufficient to uniquely disambiguate hosts sharing the same IP address). Note that this field is not altered by some NATs; hence some side effects such as counting hosts behind a NAT as reported in [Count].
A variant of this approach relies upon the format of certain packets, such as TCP SYN, where the IP-ID can be modified to contain a 16 bit HOST_ID.
Address sharing devices performing this function would require to indicate they are performing this function out of band, possibly using a special DNS record.
This usage is not compliant with what is recommended in [I-D.ietf-intarea-ipv4-id-update].
A solution alternative to convey the HOST_ID is to define an IP option [RFC0791]. HOST_ID IP option can be inserted by the address sharing function to uniquely distinguish a host among those sharing the same IP address. An example of such option is documented in [I-D.chen-intarea-v4-uid-header-option]. This IP option allows to convey an IPv4 address, an IPv6 prefix, a GRE key, IPv6 Flow Label, etc.
Another way for using IP option has been described in Section 4.6 of [RFC3022].
Unlike the solution presented in Section 4.3, this proposal can apply for any transport protocol. Nevertheless, it is widely known that routers (and other middleboxes) filter IP options. IP packets with IP options can be dropped by some IP nodes. Previous studies demonstrated that "IP Options are not an option" (Refer to [Not_An_Option], [Options]).
As a conclusion, using an IP option to convey a host-hint is not viable.
HOST_ID may be conveyed in a dedicated TCP Option. An example is specified in [I-D.wing-nat-reveal-option] which defines a new TCP Option called USER_HINT. This option encloses the TCP client's identifier (e.g., the lower 16 bits of their IPv4 address, their VLAN ID, VRF ID, subscriber ID). The address sharing device inserts this TCP Option into the TCP SYN packet.
Using a new TCP Option to convey the HOST_ID does not require any modification to the applications but it is applicable only for TCP-based applications. Applications relying on other transport protocols are therefore left unsolved.
[I-D.wing-nat-reveal-option] discusses the interference with other TCP Options.
The risk related to handling a new TCP Option is low as measured in [Options]. [I-D.abdo-hostid-tcpopt-implementation] provides a detailed implementation and experimentation report of HOST_ID TCP Option. [I-D.abdo-hostid-tcpopt-implementation] investigated in depth the impact of activation HOST_ID in host, address sharing function and the enforcement of policies at the server side. [I-D.abdo-hostid-tcpopt-implementation] reports a failure ratio of 0,103% among top 100000 websites.
Some downsides have been raised against defining a TCP Option to reveal a host identity:
More discussion about issues raised when extending TCP can be found at
Another option is to not require any change at the transport nor the IP levels but to convey at the application payload the required information which will be used to disambiguate hosts. This format and the related semantics depend on its application (e.g., HTTP, SIP, SMTP, etc.).
For HTTP, the X-Forwarded-For (XFF) or Forwarded-For ([I-D.ietf-appsawg-http-forwarded]) headers can be used to display the original IP address when an address sharing device is involved. Service Providers operating address sharing devices can enable the feature of injecting the XFF/Forwarded-For header which will enclose the original IPv4 address or the IPv6 prefix part (see the example shown in Figure 1). The address sharing device has to strip all included XFF/Forwarded-For headers before injecting their own. Servers may rely on the contents of this field to enforce some policies such as blacklisting misbehaving users. Note that XFF can also be logged by some servers (this is for instance supported by Apache).
Forwarded: for=192.0.2.1,for=[2001:db8::1] Forwarded: proto=https;by=192.0.2.15
Figure 1: Example of Forwarded-For
Not all applications impacted by the address sharing can support the ability to disclose the original IP address. Only a subset of protocols (e.g., HTTP) can rely on this solution.
For the HTTP case, to prevent users injecting invalid HOST_IDs, an initiative has been launched to maintain a list of trusted ISPs using XFF: See for example the list available at: [Trusted_ISPs] of trusted ISPs as maintained by Wikipedia. If an address sharing device is on the trusted XFF ISPs list, users editing Wikipedia located behind the address sharing device will appear to be editing from their "original" IP address and not from the NATed IP address. If an offending activity is detected, individual hosts can be blacklisted instead of all hosts sharing the same IP address.
XFF header injection is a common practice of load balancers. When a load balancer is in the path, the original content of any included XFF header should not be stripped. Otherwise the information about the "origin" IP address will be lost.
When several address sharing devices are crossed, XFF/Forwarded-For header can convey the list of IP addresses (e.g., Figure 1). The origin HOST_ID can be exposed to the target server.
XFF also introduces some implementation complexity if the HTTP packet is at or close to the MTU size.
It has been reported that some "poor" implementation may encounter some parsing issues when injecting XFF header.
For encrypted HTTP traffic, injecting XFF header may be broken.
The solution, referred to as Proxy Protocol [Proxy], does not require any application-specific knowledge. The rationale behind this solution is to prepend each connection with a line reporting the characteristics of the other side's connection as shown in the example depicted in Figure 2:
PROXY TCP4 192.0.2.1 192.0.2.15 56324 443\r\n
Figure 2: Example of PROXY conection report
Upon receipt of a message conveying this line, the server removes the line. The line is parsed to retrieve the transported protocol. The content of this line is recorded in logs and used to enforce policies.
This solution can be deployed in a controlled environment but it can not be deployed to all access services available in the Internet. If the remote server does not support the Proxy Protocol, the session will fail. Other complications will raise due to the presence of firewalls for instance.
As a consequence, this solution is broken and can not be recommended.
This solution does not require any action from the address sharing function to disclose a host identifier. Instead of assuming all transport ports are associated with one single host, each host under the same external IP address is assigned a restricted port set. These port sets are then advertised to remote servers using off-line means. This announcement is not required for the delivery of internal services (i.e., offered by the service provider deploying the address sharing function) relying on implicit identification.
Port sets assigned to hosts may be static or dynamic.
Port set announcements to remote servers do not require to reveal the identity of individual hosts but only to advertise the enforced policy to generate non-overlapping port sets (e.g., the transport space associated with an IP address is fragmented to contiguous blocks of 2048 port numbers).
The solution does not require defining new fields nor options; it is policy-based.
The solution may contradict the port randomization as identified in [RFC6269]. A mitigation would be to avoid assigning static port sets to individual hosts.
The method is convenient for the delivery of services offered by the service provider offering also the IP connectivity service.
[RFC5201] specifies an architecture which introduces a new namespace to convey an identity information.
This solution requires both the client and the server to support HIP [RFC5201]. Additional architectural considerations are to be taken into account such as the key exchanges, etc.
If the address sharing function is required to act as a UDP/TCP-HIP relay, this is not a viable option.
Another alternative is to convey the HOST_ID using a separate notification channel than the packets issued to invoke the service.
An implementation example is defined in [I-D.yourtchenko-nat-reveal-ping]. This solution relies on a mechanism where the address sharing function encapsulates the necessary differentiating information into an ICMP Echo Request packet that it sends in parallel with the initial session creation (e.g., SYN). The information included in the ICMP Request Data portion describes the five-tuples as seen on both of the sides of the address sharing function.
Another alternative is to retrieve the HOST_ID using a dedicated query channel.
An implementation example may rely on the Identification Protocol (IDENT, [RFC1413]). This solution assumes address sharing function implements the server part of IDENT while remote servers implement the client part of the protocol. IDENT needs to be updated (see [IDENT_NAT]) to be able to return a host identifier instead of the user-id as defined in [RFC1413]. The IDENT response syntax uses the same USERID field described in [RFC1413] but rather than returning a username, a host identifier (e.g., a 16 bit value) is returned [IDENT_NAT]. For any new incoming connection, the server contacts the IDENT server to retrieve the associated identifier. During that phase, the connection may be delayed.
The following Table 1 summarizes the approaches analyzed in this document.
+-----+------+------+------+-----+-----+-----+-----+-----+
|IP-ID| IP | TCP |HTTP |PROXY|Port | HIP |ICMP |IDENT|
| |Option|Option|Header| | Set | | | |
| | | |(XFF) | | | | | |
----------+-----+------+------+------+-----+-----+-----+-----+-----+
UDP | Yes | Yes | No | No | No | Yes | | Yes | No |
----------+-----+------+------+------+-----+-----+-----+-----+-----+
TCP | Yes | Yes | Yes | No | Yes | Yes | | Yes | Yes |
----------+-----+------+------+------+-----+-----+-----+-----+-----+
HTTP | Yes | Yes | Yes | Yes | Yes | Yes | | Yes | Yes |
----------+-----+------+------+------+-----+-----+-----+-----+-----+
Encrypted | Yes | Yes | Yes | No | Yes | Yes | | Yes | Yes |
Traffic | | | | | | | | | |
----------+-----+------+------+------+-----+-----+-----+-----+-----+
Success | 100%| 30% | 99% | 100% | Low | 100%|Low |~100%|~100%|
Ratio | | | | | | | | (6) | (6) |
----------+-----+------+------+------+-----+-----+-----+-----+-----+
Possible | Low | High | Low | Med | High| No | N/A | High|High |
Perf | to | | to | to | | | | | |
Impact | Med | | Med | High | | | | | |
----------+-----+------+------+------+-----+-----+-----+-----+-----+
OS TCP/IP | Yes | Yes | Yes | No | No | No | | Yes | Yes |
Modif | | | | | | | | | |
----------+-----+------+------+------+-----+-----+-----+-----+-----+
Deployable| Yes | Yes | Yes | Yes | No | Yes | No | Yes | Yes |
Today | | | | | | | | | |
----------+-----+------+------+------+-----+-----+-----+-----+-----+
Notes | (1) | | | (2) | | (1) | (4) | (7) | (1) |
| | | | | | (3) | (5) | | (7) |
----------+-----+------+------+------+-----+-----+-----+-----+-----+
Notes:
(1) Requires mechanism to advertise NAT is participating in this
scheme (e.g., DNS PTR record).
(2) This solution is widely deployed.
(3) When the port set is not advertised, the solution is less
efficient for third-party services.
(4) Requires the client and the server to be HIP-compliant and HIP
infrastructure to be deployed.
(5) If the client and the server are HIP-enabled, the address
sharing function does not need to insert an identifier. If the
client is not HIP-enabled, designing the device that performs
address sharing to act as a UDP/TCP-HIP relay is not viable.
(6) Implementation-specific.
(7) The solution is inefficient in various scenarios as discussed
in Section 5.
Figure 3: Table 1: Summary of analyzed solutions.
This document does not require any action from IANA.
The same security concerns apply for the injection of an IP option, TCP Option and application-related content (e.g., XFF) by the address sharing device. If the server trusts the content of the HOST_ID field, a third party user can be impacted by a misbehaving user to reveal a "faked" HOST_ID (e.g., original IP address).
HOST_ID may be used to leak information about the internal structure of a network behind an address sharing function. If this behavior is undesired for the network administrator, the address sharing function can be configured to strip any existing HOST_ID in received packets from internal hosts.
HOST_ID specification documents SHOULD elaborate further on threats inherent to each individual solution to convey the HOST_ID (e.g., use of the IP-ID field to count hosts behind a NAT [Count]).
Many thanks to D. Wing, C. Jacquenet and J. Halpern for their review, comments and inputs.
Thanks also to P. McCann, T. Tsou, Z. Dong, B. Briscoe, T. Taylor, M. Blanchet, D. Wing and A. Yourtchenko for the discussions in Prague.
Some of the issues related to defining a new TCP Option have been raised by L. Eggert.
Privacy text is provided by A. Cooper.
| [RFC2119] | Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. |
| [RFC0791] | Postel, J., "Internet Protocol", STD 5, RFC 791, September 1981. |
| [RFC6056] | Larsen, M. and F. Gont, "Recommendations for Transport-Protocol Port Randomization", BCP 156, RFC 6056, January 2011. |
| [RFC3022] | Srisuresh, P. and K. Egevang, "Traditional IP Network Address Translator (Traditional NAT)", RFC 3022, January 2001. |