draft-ietf-v6ops-scanning-implications-02.txt   draft-ietf-v6ops-scanning-implications-03.txt 
IPv6 Operations T. Chown IPv6 Operations T. Chown
Internet-Draft University of Southampton Internet-Draft University of Southampton
Intended status: Informational March 5, 2007 Intended status: Informational March 27, 2007
Expires: September 6, 2007 Expires: September 28, 2007
IPv6 Implications for Network Scanning IPv6 Implications for Network Scanning
draft-ietf-v6ops-scanning-implications-02 draft-ietf-v6ops-scanning-implications-03
Status of this Memo Status of this Memo
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This Internet-Draft will expire on September 6, 2007. This Internet-Draft will expire on September 28, 2007.
Copyright Notice Copyright Notice
Copyright (C) The IETF Trust (2007). Copyright (C) The IETF Trust (2007).
Abstract Abstract
The 128 bits of IPv6 address space is considerably bigger than the 32 The 128 bits of IPv6 address space is considerably bigger than the 32
bits of address space of IPv4. In particular, the IPv6 subnets to bits of address space of IPv4. In particular, the IPv6 subnets to
which hosts attach will by default have 64 bits of host address which hosts attach will by default have 64 bits of host address
space. As a result, traditional methods of remote TCP or UDP network space. As a result, traditional methods of remote TCP or UDP network
scanning to discover open or running services on a host will scanning to discover open or running services on a host will
potentially become less feasible, due to the larger search space in potentially become less feasible, due to the larger search space in
the subnet. In addition automated attacks, such as those performed the subnet. In addition automated attacks, such as those performed
by network worms, may be hampered. This document discusses this by network worms, that pick random host addresses to propagate to,
property of IPv6, and describes related issues for site may be hampered. This document discusses this property of IPv6 and
administrators of IPv6 networks to consider, which may be of describes related issues for IPv6 site network administrators to
importance when planning site address allocation and management consider, which may be of importance when planning site address
strategies. While traditional network scanning probes (whether by allocation and management strategies. While traditional network
individuals or automated via network worms) may become less common, scanning probes (whether by individuals or automated via network
administrators should be aware of other methods attackers may use to worms) may become less common, administrators should be aware of
discover IPv6 addresses on a target network, and be aware of other methods attackers may use to discover IPv6 addresses on a
appropriate measures to mitigate them. target network, and also be aware of appropriate measures to mitigate
them.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Target Address Space for Network Scanning . . . . . . . . . . 4 2. Target Address Space for Network Scanning . . . . . . . . . . 4
2.1. IPv4 . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 2.1. IPv4 . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.2. IPv6 . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 2.2. IPv6 . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.3. Reducing the IPv6 Search Space . . . . . . . . . . . . . . 4 2.3. Reducing the IPv6 Search Space . . . . . . . . . . . . . . 4
2.4. Dual-stack Networks . . . . . . . . . . . . . . . . . . . 5 2.4. Dual-stack Networks . . . . . . . . . . . . . . . . . . . 5
2.5. Defensive Scanning . . . . . . . . . . . . . . . . . . . . 5 2.5. Defensive Scanning . . . . . . . . . . . . . . . . . . . . 5
3. Alternatives for Attackers: Off-link . . . . . . . . . . . . . 5 3. Alternatives for Attackers: Off-link . . . . . . . . . . . . . 5
3.1. Gleaning IPv6 prefix information . . . . . . . . . . . . . 6 3.1. Gleaning IPv6 prefix information . . . . . . . . . . . . . 6
3.2. DNS Advertised Hosts . . . . . . . . . . . . . . . . . . . 6 3.2. DNS Advertised Hosts . . . . . . . . . . . . . . . . . . . 6
3.3. DNS Zone Transfers . . . . . . . . . . . . . . . . . . . . 6 3.3. DNS Zone Transfers . . . . . . . . . . . . . . . . . . . . 6
3.4. Log File Analysis . . . . . . . . . . . . . . . . . . . . 6 3.4. Log File Analysis . . . . . . . . . . . . . . . . . . . . 6
3.5. Application Participation . . . . . . . . . . . . . . . . 6 3.5. Application Participation . . . . . . . . . . . . . . . . 6
3.6. Transition Methods . . . . . . . . . . . . . . . . . . . . 7 3.6. Multicast Group Addresses . . . . . . . . . . . . . . . . 7
3.7. Transition Methods . . . . . . . . . . . . . . . . . . . . 7
4. Alternatives for Attackers: On-link . . . . . . . . . . . . . 7 4. Alternatives for Attackers: On-link . . . . . . . . . . . . . 7
4.1. General on-link methods . . . . . . . . . . . . . . . . . 7 4.1. General on-link methods . . . . . . . . . . . . . . . . . 7
4.2. Multicast or Other Service Discovery . . . . . . . . . . . 8 4.2. Intra-site Multicast or Other Service Discovery . . . . . 8
5. Site Administrator Tools . . . . . . . . . . . . . . . . . . . 8 5. Site Administrator Tools . . . . . . . . . . . . . . . . . . . 8
5.1. IPv6 Privacy Addresses . . . . . . . . . . . . . . . . . . 8 5.1. IPv6 Privacy Addresses . . . . . . . . . . . . . . . . . . 8
5.2. Cryptographically Generated Addresses (CGAs) . . . . . . . 9 5.2. Cryptographically Generated Addresses (CGAs) . . . . . . . 9
5.3. Non-use of MAC addresses in EUI-64 format . . . . . . . . 9 5.3. Non-use of MAC addresses in EUI-64 format . . . . . . . . 9
5.4. DHCP Service Configuration Options . . . . . . . . . . . . 9 5.4. DHCP Service Configuration Options . . . . . . . . . . . . 9
5.5. Rolling Server Addresses . . . . . . . . . . . . . . . . . 10 5.5. Rolling Server Addresses . . . . . . . . . . . . . . . . . 10
5.6. Application-Specific Addresses . . . . . . . . . . . . . . 10 5.6. Application-Specific Addresses . . . . . . . . . . . . . . 10
6. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . 10 6. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . 10
7. Security Considerations . . . . . . . . . . . . . . . . . . . 11 7. Security Considerations . . . . . . . . . . . . . . . . . . . 11
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
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then be subject of a subsequent attack. Today many IPv4 sites are then be subject of a subsequent attack. Today many IPv4 sites are
subjected to such probing on a recurring basis. subjected to such probing on a recurring basis.
The 128 bits of IPv6 [1] address space is considerably bigger than The 128 bits of IPv6 [1] address space is considerably bigger than
the 32 bits of address space in IPv4. In particular, the IPv6 the 32 bits of address space in IPv4. In particular, the IPv6
subnets to which hosts attach will by default have 64 bits of host subnets to which hosts attach will by default have 64 bits of host
address space [2]. As a result, traditional methods of remote TCP or address space [2]. As a result, traditional methods of remote TCP or
UDP network scanning to discover open or running services on a host UDP network scanning to discover open or running services on a host
will potentially become less feasible, due to the larger search space will potentially become less feasible, due to the larger search space
in the subnet. This document discusses this property of IPv6, and in the subnet. This document discusses this property of IPv6, and
describes related issues for site administrators of IPv6 networks to describes related issues for IPv6 site network administrators to
consider, which may be of importance when planning site address consider, which may be of importance when planning site address
allocation and management strategies. allocation and management strategies.
This document complements the transition-centric discussion of the This document complements the transition-centric discussion of the
issues that can be found in Appendix A of the IPv6 Transition/ issues that can be found in Appendix A of the IPv6 Transition/
Co-existence Security Considerations text [12], which takes a broad Co-existence Security Considerations text [12], which takes a broad
view of security issues for transitioning networks. view of security issues for transitioning networks.
The reader is also referred to a recent paper by Bellovin on worm The reader is also referred to a recent paper by Bellovin on worm
propagation strategies in IPv6 networks [13]. This paper discusses propagation strategies in IPv6 networks [13]. This paper discusses
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case, the probes are from an attacker outside a site boundary who is case, the probes are from an attacker outside a site boundary who is
trying to find weaknesses on any system in that network which they trying to find weaknesses on any system in that network which they
may then subsequently be able to compromise. The other case is may then subsequently be able to compromise. The other case is
scanning by worms that spread through (site) networks, looking for scanning by worms that spread through (site) networks, looking for
further hosts to compromise. Many worms, like Slammer, rely on such further hosts to compromise. Many worms, like Slammer, rely on such
address scanning methods to propagate, whether they pick subnets address scanning methods to propagate, whether they pick subnets
numerically (and thus probably topologically) close to the current numerically (and thus probably topologically) close to the current
victim, or subnets in random remote networks. victim, or subnets in random remote networks.
It must be remembered that the defence of a network must not rely It must be remembered that the defence of a network must not rely
solely on the obscurity of the hosts on that network. Such a feature solely on the unpredictable sparseness of the host addresses on that
or property is only one measure in a set of measures that may be network. Such a feature or property is only one measure in a set of
applied. However, with a growth in usage of IPv6 devices in open measures that may be applied. However, with a growth in usage of
networks likely, and security becoming more likely an issue for the IPv6 devices in open networks likely, and security becoming more
end devices, such obfuscation can be useful where its use is of likely an issue for the end devices, such obfuscation can be useful
little or no cost to the administrator to implement it. However, a where its use is of little or no cost to the administrator to
law of diminishing returns does apply. An administrator who implement it. However, a law of diminishing returns does apply. An
undertakes an address hiding policy should be aware that while IPv6 administrator who undertakes an address hiding policy through
host addresses may be picked that are likely to take significant time unpredictable sparseness should be aware that while IPv6 host
to discover by traditional scanning methods, there are other means by addresses may be assigned to hosts that are likely to take
which such addresses may be discovered. Implementing all of them may significant time to discover by traditional scanning methods, there
are other means by which such addresses may be discovered.
Implementing all of the mitigating methods described in this text may
be deemed unwarranted effort. But it is up to the site administrator be deemed unwarranted effort. But it is up to the site administrator
to be aware of the context and the options available, and in to be aware of the context and the options available, and in
particular what new methods may attackers use to glean IPv6 address particular what new methods may attackers use to glean IPv6 address
information, and how these can potentially be mitigated against. information, and how these can potentially be mitigated against.
This document is intended to be informational; there is not yet Finally, note that this document is currently intended to be
sufficient deployment experience for it to be considered BCP. informational; there is not yet sufficient deployment experience for
it to be considered BCP.
2. Target Address Space for Network Scanning 2. Target Address Space for Network Scanning
There are significantly different considerations for the feasibility There are significantly different considerations for the feasibility
of plain, brute force IPv4 and IPv6 address scanning. of plain, brute force IPv4 and IPv6 address scanning.
2.1. IPv4 2.1. IPv4
A typical IPv4 subnet may have 8 bits reserved for host addressing. A typical IPv4 subnet may have 8 bits reserved for host addressing.
In such a case, a remote attacker need only probe at most 256 In such a case, a remote attacker need only probe at most 256
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2.3. Reducing the IPv6 Search Space 2.3. Reducing the IPv6 Search Space
The IPv6 host address space through which an attacker may search can The IPv6 host address space through which an attacker may search can
be reduced in at least two ways. be reduced in at least two ways.
First, the attacker may rely on the administrator conveniently First, the attacker may rely on the administrator conveniently
numbering their hosts from [prefix]::1 upward. This makes scanning numbering their hosts from [prefix]::1 upward. This makes scanning
trivial, and thus should be avoided unless the host's address is trivial, and thus should be avoided unless the host's address is
readily obtainable from other sources (for example it is the site's readily obtainable from other sources (for example it is the site's
primary DNS or email MX server). Alternatively if hosts are numbered published primary DNS or email MX server). Alternatively if hosts
sequentially, or using any regular scheme, knowledge of one address are numbered sequentially, or using any regular scheme, knowledge of
may expose other available addresses to scan. one address may expose other available addresses to scan.
Second, in the case of statelessly autoconfiguring [1] hosts, the Second, in the case of statelessly autoconfiguring [1] hosts, the
host part of the address will usually take a well-known format that host part of the address will usually take a well-known format that
includes the Ethernet vendor prefix and the "fffe" stuffing. For includes the Ethernet vendor prefix and the "fffe" stuffing. For
such hosts, the search space can be reduced to 48 bits. Further, if such hosts, the search space can be reduced to 48 bits. Further, if
the Ethernet vendor is also known, the search space may be reduced to the Ethernet vendor is also known, the search space may be reduced to
24 bits, with a one probe per second scan then taking a less daunting 24 bits, with a one probe per second scan then taking a less daunting
194 days. Even where the exact vendor is not known, using a set of 194 days. Even where the exact vendor is not known, using a set of
common vendor prefixes can reduce the search. In addition, many common vendor prefixes can reduce the search. In addition, many
nodes in a site network may be procured in batches, and thus have nodes in a site network may be procured in batches, and thus have
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3.5. Application Participation 3.5. Application Participation
More recent peer-to-peer applications often include some centralised More recent peer-to-peer applications often include some centralised
server which coordinates the transfer of data between peers. The server which coordinates the transfer of data between peers. The
BitTorrent application builds swarms of nodes that exchange chunks of BitTorrent application builds swarms of nodes that exchange chunks of
files, with a tracker passing information about peers with available files, with a tracker passing information about peers with available
chunks of data between the peers. Such applications may offer an chunks of data between the peers. Such applications may offer an
attacker a source of peer IP addresses to probe. attacker a source of peer IP addresses to probe.
3.6. Transition Methods 3.6. Multicast Group Addresses
Where an Embedded RP [7] multicast group address is known, the
unicast address of the rendezvous point is implied by the group
address. Where unicast prefix based multicast group addresses [5]
are used, specific /64 link prefixes may also be disclosed in traffic
that goes off-site. An administrator may thus choose to put aside
/64 bit prefixes for multicast group addresses that are not in use
for normal unicast routing and addressing. Alternatively a site may
simply use their /48 site prefix allocation to generate RFC3306
multicast group addresses.
3.7. Transition Methods
Specific knowledge of the target network may be gleaned if that Specific knowledge of the target network may be gleaned if that
attacker knows it is using 6to4 [4], ISATAP [10], Teredo [11] or attacker knows it is using 6to4 [4], ISATAP [10], Teredo [11] or
other techniques that derive low-order bits from IPv4 addresses other techniques that derive low-order bits from IPv4 addresses
(though in this case, unless they are using IPv4 NAT, the IPv4 (though in this case, unless they are using IPv4 NAT, the IPv4
addresses may be probed anyway). addresses may be probed anyway).
For example, the current Microsoft 6to4 implementation uses the For example, the current Microsoft 6to4 implementation uses the
address 2002:V4ADDR::V4ADDR while older Linux and FreeBSD address 2002:V4ADDR::V4ADDR while older Linux and FreeBSD
implementations default to 2002:V4ADDR::1. This leads to specific implementations default to 2002:V4ADDR::1. This leads to specific
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the "all hosts" link local multicast address. Likewise any routers the "all hosts" link local multicast address. Likewise any routers
on link may be found via the "all routers" link local multicast on link may be found via the "all routers" link local multicast
address. An attacker may choose to probe in a slightly more address. An attacker may choose to probe in a slightly more
obfuscated way by probing the solicited node multicast address of a obfuscated way by probing the solicited node multicast address of a
potential target host. potential target host.
Where a host has already been compromised, its Neighbour Discovery Where a host has already been compromised, its Neighbour Discovery
cache is also likely to include information about active nodes on cache is also likely to include information about active nodes on
link, just as an ARP cache would do for IPv4. link, just as an ARP cache would do for IPv4.
4.2. Multicast or Other Service Discovery 4.2. Intra-site Multicast or Other Service Discovery
A site may also have site or organisational scope multicast A site may also have site or organisational scope multicast
configured, in which case application traffic, or service discovery, configured, in which case application traffic, or service discovery,
may be exposed site wide. An attacker may choose to use any other may be exposed site wide. An attacker may also choose to use any
service discovery methods supported by the site. other service discovery methods supported by the site.
There are also issues with disclosure from multicast itself. Where
an Embedded RP [7] multicast group address is known, the unicast
address of the rendezvous point is implied by the group address.
Where unicast prefix based multicast group addresses [5] are used,
specific /64 link prefixes may also be disclosed in traffic that goes
off-site. An administrator may thus choose to put aside /64 bit
prefixes for multicast group addresses that are not in use for normal
unicast routing and addressing.
5. Site Administrator Tools 5. Site Administrator Tools
There are some tools that site administrators can apply to make the There are some tools that site administrators can apply to make the
task for IPv6 network scanning attackers harder. These methods arise task for IPv6 network scanning attackers harder. These methods arise
from the considerations in the previous section. from the considerations in the previous section.
The author notes that at his current (university) site, there is no The author notes that at his current (university) site, there is no
evidence of general network scanning running across subnets. evidence of general network scanning running across subnets.
However, there is network scanning over IPv6 connections to systems However, there is network scanning over IPv6 connections to systems
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(temporary) privacy address. While an attacker may be able to port (temporary) privacy address. While an attacker may be able to port
scan that address if they do so quickly upon observing or otherwise scan that address if they do so quickly upon observing or otherwise
learning of the address, the threat or risk is reduced due to the learning of the address, the threat or risk is reduced due to the
time-constrained value of the address. One implementation of RFC3041 time-constrained value of the address. One implementation of RFC3041
already deployed has privacy addresses active for one day, with such already deployed has privacy addresses active for one day, with such
addresses reachable for seven days. addresses reachable for seven days.
Note that an RFC3041 host will usually also have a separate static Note that an RFC3041 host will usually also have a separate static
global IPv6 address by which it can also be reached, and that may be global IPv6 address by which it can also be reached, and that may be
DNS-advertised if an externally reachable service is running on it. DNS-advertised if an externally reachable service is running on it.
Both of these can be served by DHCPv6.
DHCPv6 can be used to serve normal global addresses and IPv6 Privacy
Addresses.
The implication is that while Privacy Addresses can mitigate the The implication is that while Privacy Addresses can mitigate the
long-term value of harvested addresses, an attacker creating an IPv6 long-term value of harvested addresses, an attacker creating an IPv6
application server to which clients connect will still be able to application server to which clients connect will still be able to
probe the clients by their Privacy Address as and when they visit probe the clients by their Privacy Address as and when they visit
that server. In the general context of hiding the addresses exposed that server. The duration for which Privacy Addresses are valid will
from a site, an administrator may choose to use IPv6 Privacy impact on the usefulness of such observed addresses to an external
Addresses. The duration for which these are valid will impact on the attacker. The frequency with which such address get recycled could
usefulness of such observed addresses to an external attacker. The be increased, though this may increase the complexity of local
frequency with which such address get recycled could be increased, network management for the administrator, since doing so will cause
though this will present the site administrator with more addresses more addresses to be used over time in the site.
to track the usage of.
It may be worth exploring whether firewalls can be adapted to allow It may be worth exploring whether firewalls can be adapted to allow
the option to block traffic initiated to a known IPv6 Privacy Address the option to block traffic initiated to a known IPv6 Privacy Address
from outside a network boundary. While some applications may from outside a network boundary. While some applications may
genuinely require such capability, it may be useful to be able to genuinely require such capability, it may be useful to be able to
differentiate in some circumstances. differentiate in some circumstances.
5.2. Cryptographically Generated Addresses (CGAs) 5.2. Cryptographically Generated Addresses (CGAs)
The use of Cryptographically Generated Addresses (CGAs) [9] may also The use of Cryptographically Generated Addresses (CGAs) [9] may also
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operating system currently defaults to generation of the 64 bit operating system currently defaults to generation of the 64 bit
interface identifier by use of random bits, and thus does not embed interface identifier by use of random bits, and thus does not embed
the MAC address. Where such a method exists as an option, an the MAC address. Where such a method exists as an option, an
administrator may wish consider use of that option. administrator may wish consider use of that option.
5.4. DHCP Service Configuration Options 5.4. DHCP Service Configuration Options
The administrator should configure DHCPv6 so that the first addresses The administrator should configure DHCPv6 so that the first addresses
allocated from the pool begins much higher in the address space than allocated from the pool begins much higher in the address space than
at [prefix]::1. Further, it is desirable that allocated addresses at [prefix]::1. Further, it is desirable that allocated addresses
are not sequential, nor have any predictable pattern to them. DHCPv6 are not sequential, nor have any predictable pattern to them.
implementors should support configuration options to allow such Unpredictable sparseness in the allocated addresses is a desirable
behaviour. property. DHCPv6 implementors should support configuration options
to allow such behaviour.
DHCPv6 also includes an option to use Privacy Extension [3] DHCPv6 also includes an option to use Privacy Extension [3]
addresses, i.e. temporary addresses, as described in Section 12 of addresses, i.e. temporary addresses, as described in Section 12 of
the DHCPv6 [6] specification. the DHCPv6 [6] specification.
5.5. Rolling Server Addresses 5.5. Rolling Server Addresses
Given the huge address space in an IPv6 subnet/link, and the support Given the huge address space in an IPv6 subnet/link, and the support
for IPv6 multiaddressing, whereby a node or interface may have for IPv6 multiaddressing, whereby a node or interface may have
multiple IPv6 valid addresses of which one is preferred for sending, multiple IPv6 valid addresses of which one is preferred for sending,
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have open IPv4 services running, they will remain exposed to have open IPv4 services running, they will remain exposed to
traditional probes over IPv4 transport. traditional probes over IPv4 transport.
This document has discussed the considerations a site administrator This document has discussed the considerations a site administrator
should bear in mind when considering IPv6 address planning issues and should bear in mind when considering IPv6 address planning issues and
configuring various service elements. It highlights relevant issues configuring various service elements. It highlights relevant issues
and offers some informational guidance for administrators. While and offers some informational guidance for administrators. While
some suggestions are currently more practical than others, it is up some suggestions are currently more practical than others, it is up
to individual administrators to determine how much effort they wish to individual administrators to determine how much effort they wish
to invest in 'address hiding' schemes, given that this is only one to invest in 'address hiding' schemes, given that this is only one
aspect of network security, and certainly not one to rely solely on. aspect of network security, and certainly not one to rely solely
But by implementing the basic principle of allocating 'random', non upon. But by implementing the basic principle of allocating
predictable addresses, some level of obfuscation can be cheaply addresses on the basis of unpredictable sparseness, some level of
deployed. obfuscation can be cheaply deployed.
7. Security Considerations 7. Security Considerations
There are no specific security considerations in this document There are no specific security considerations in this document
outside of the topic of discussion itself. outside of the topic of discussion itself.
8. IANA Considerations 8. IANA Considerations
There are no IANA considerations for this document. There are no IANA considerations for this document.
9. Acknowledgements 9. Acknowledgements
Thanks are due to people in the 6NET project (www.6net.org) for Thanks are due to people in the 6NET project (www.6net.org) for
discussion of this topic, including Pekka Savola, Christian Strauf discussion of this topic, including Pekka Savola, Christian Strauf
and Martin Dunmore, as well as other contributors from the IETF v6ops and Martin Dunmore, as well as other contributors from the IETF v6ops
and other mailing lists, including Tony Finch, David Malone, Bernie and other mailing lists, including Tony Finch, David Malone, Bernie
Volz, Fred Baker, Andrew Sullivan and Alex Petrescu. Volz, Fred Baker, Andrew Sullivan, Tony Hain, Dave Thaler and Alex
Petrescu.
10. Informative References 10. Informative References
[1] Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6) [1] Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6)
Specification", RFC 2460, December 1998. Specification", RFC 2460, December 1998.
[2] Thomson, S. and T. Narten, "IPv6 Stateless Address [2] Thomson, S. and T. Narten, "IPv6 Stateless Address
Autoconfiguration", RFC 2462, December 1998. Autoconfiguration", RFC 2462, December 1998.
[3] Narten, T. and R. Draves, "Privacy Extensions for Stateless [3] Narten, T. and R. Draves, "Privacy Extensions for Stateless
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