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Internet Draft Johan Ihr‰n
draft-ietf-dnsop-interim-signed-root-01.txt Autonomica
February 2003
Expires in six months
An Interim Scheme for Signing the Public DNS Root
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as
Internet-Drafts.
Internet-Drafts are draft documents valid for a maximum of six
months and may be updated, replaced, or obsoleted by other
documents at any time. It is inappropriate to use Internet-Drafts
as reference material or to cite them other than as "work in
progress."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
Abstract
This memo documents a proposed mechanism for a first stage of a
transition from an unsigned DNS root to a signed root, such that
the data in the root zone is accompanied by DNSSEC signatures to
allow validation.
The underlying reason for signing the root zone is to be able to
provide a more secure DNS hierarchy, where it is possible to
distinguish false answers from correct answers.
For the special case of the DNS root zone, an interim scheme is
proposed. This scheme is mostly aimed at securing the root zone
itself for technical and operational reasons, and to give
operational experience of DNSSEC.
1. Terminology
The key words "MUST", "SHALL", "REQUIRED", "SHOULD", "RECOMMENDED",
and "MAY" in this document are to be interpreted as described in
RFC 2119.
The term "zone" refers to the unit of administrative control in the
Domain Name System. "Name server" denotes a DNS name server that is
authoritative (i.e. knows all there is to know) for a DNS zone,
typically the root zone. A "resolver", finally, is a DNS "client",
i.e. an entity that sends DNS queries to authoritative nameservers
and interpret the results.
2. Motivation for signing the DNS root
In the special case of the root zone there are very strong reasons
to take a slow and conservative approach to any changes with
operational impact. Signing the root is such a change.
DNSSEC[RFC2535, RFC3090] has been in development for a number of
years now and still has not reached the point where the last flag
day is behind us.
However, during the years of DNSSEC development and refinement
[RFC2930, RFC3007, RFC3008, RFC3110, RFC3225, RFC3226, AD-secure,
Opt-in, Wild-card-optimize], the Internet has matured and more and
more businesses and other organizations have become dependent on
the stability and constant availability of the Internet.
It is therefore prudent to do everything in our power to ensure
that the DNS infrastructure works as well as possible and, when
appropriate and possible, adding enhancements and functionality.
The time is now right for yet another step of improvement by
signing the root zone. By doing that any Internet user that so
wishes will obtain the ability of verifying responses received from
the root nameservers.
Since this is new operational ground the objective is not maximum
security but rather an "Internet-wide" controlled experiment with a
signed root zone, where the trade off is that we utilize the fact
that there are operators in place that can help even though they
are not sufficiently staffed to do off-line signing in a 24x7
mode. For this reason it is fully possible to even do automatic
signing, since the purpose is to ensure that DNSSEC works
operationally with a signed root zone and gain experience from the
exercise.
It should be pointed out, however, that the experimental part is
only the added DNSSEC records. The traditional records in the root
zone remain unchanged and the new records will only be visible to
DNSSEC-aware resolvers that explicitly ask to see these new
records.
2.1. Motivation for signing the root zone now
The reason DNSSEC is not yet widely deployable is that the problem
of key management remains unsolved, especially where communication
between the zone administrators for a parent zone and child zone is
required.
However, during the past year a solution to this problem has been
found (in the form of a new record type, DS aka Delegation Signer)
[DS], discussed, implemented and tested. Therefore, it is finally
reasonable to consider DNSSEC deployment to be a real possibility
within the next 12 months.
In the case of the root zone the particular importance of managing
the transition with zero operational mistakes is a strong reason to
separate the signing of the zone itself, with the associated key
management issues, from the future management of signed delegations
(of top level domains).
Although support for Delegation Signer has been implemented and
tested it is not yet generally available except experimentally. For
this reason signed delegations for productions zones will have to
wait a bit longer. Furthermore, it will take some time to ensure
that the entire RSS aka Root Server System is capable of supporting
the protocol changes that accompany the new support for Delegation
Signer.
By signing now it will be possible to work through the operational
issues with signing the zone itself without at the same time having
to manage the additional complexity of signed delegations. Also, by
explicitly not supporting any signed delegations, it is also
possible to recover in a graceful way should new operational
problems turn up.
Because of these operational and technical issues there is a
"window of opportunity" to sign the root zone before the status of
implementation of "full DNSSEC", including Delegation Signer
support, change to "generally available", thereby increasing the
pressure for signed delegations from the root zone.
3. Trust in DNSSEC Keys
In the end the trust that a validating resolver will be able to put
in a key that it cannot validate within DNSSEC will have to be a
function of
* trust in the key issuer
* trust in the distribution method
* trust in extra, out-of-band verification
For any security apex, i.e. a node in the DNS hierarchy that issues
out-of-band "trusted keys", it is of critical importance that this
function produces a positive result (i.e. the resolver gains
sufficient confidence in the keys to decide to trust them). The
particular case of the root zone is no different, although possibly
it is more crucial than many other security apexes.
To ensure that the resulting trust is maximized it is necessary to
work with all the parameters that are available. In the case of the
key issuer there is the possibility of chosing a key issuer that
has a large "trust base" (i.e. is already trusted by a large
fraction of the resolver population). However, it is also possible
to expand the aggregated trust base by using multiple simultaneous
key issuers, as described in [Threshold-Validation].
Furthermore, with multiple trusted keys it will be possible for a
validating resolver to optimize for the "right compromise" between
* maximum security (as expressed by all available signatures by all
available keys verifying correctly
* maximum redundancy (as in the DNS lookup being validated if there
is any signature by any trusted key available)
Without multiple, independent trusted keys the rollover operation
will always be a dangerous operation where it is likely that some
fraction of the resolver population lose their ability to verify
lookups (and hence start to fail hard). I.e. the validating
resolver will be forced to adopt the "maximum security" policy,
since there is no alternative.
With multiple, independent trusted keys, however, it is possible to
design for improved redundancy by trusting lookups that are only
validated against a subset of the available keys. This will make
rollovers much less risky to the extent that it will be possible to
"survive" even emergency rollovers without any immediate
configuration chagnes in the validating resolver.
4. Interim signing of the root zone
At present all the authoritative servers for the root zone pull the
zone from a primary master. I.e. any changes at the primary master
will propagate via NOTIFY[RFC1996] and subsequent
AXFR/IXFR[RFC1995, AXFR-clarify] to the slave servers.
Between the primary master and the slaves transactions are signed
with TSIG[RFC2845] signatures. This mechanism is already in place,
and there is operational experience with periodic key rollover of
the TSIG keys.
4.1. Design philosophy
By introducing a signing step into the distribution of the zone
file complexity is increased. To avoid introducing new single
points of failure it is therefore important to ensure that any
added or changed steps are as redundant as possible.
The assumption is that the operators of the root servers are
trusted and that consistency of the zone among the root servers is
a crucial property that MUST be preserved in emergencies.
To ensure that consistency is maintained new single points of
failure SHOULD NOT be introduced by the signing step. Therefore a
structure where several parties have the ability to sign the zone
is proposed. Furthermore, such a design helps avoid any individual
party becoming a de facto single zone signing authority.
4.2. Proposed new management structure for the root zone
Taking into account the already existing infrastructure for
management of TSIG keys and rollover of these keys the following
structure is proposed:
* Day-to-day signing of the root zone is performed by a subgroup of
the root server operators referred to as "signing operators". For
this task individual, per-operator, Zone Signing Keys, ZSKs, are
used.
* The set of Zone Signing Keys are signed by several Key Signing
Keys, KSKs, at any particular time. The public part of KSKs in
use have to be statically configured as "trusted keys" in all
resolvers that want to be able to perform validation of
responses.
* Key rollover, the operation when an old KSK is exchanged for a
new KSK, is managed with minimal overlap and on a frequent basis
of no less than three times per year per KSK. The rollover
schedule is chosen to be frequent enough to not make long-term
trust possible while being low enough to not become operationally
infeasible.
4.2.1. Management and distribution of the zone file
The present, unsigned zone is published by the official slaves, the
"root nameservers", transferring the zone securely from a primary
master and subsequently using the data to respond to queries. This
mechanism is changed into:
* The unsigned root zone is transferred securely from the primary
master to a set of "signing primaries" managed by the operators
participating in signing the zone. This is done via the
traditional mechanisms NOTIFY + AXFR/IXFR + TSIG.
* The root nameservers change their configuration so that they
replace the present, single, primary master as the source of the
zone with a set of multiple possible "signing primaries" as
masters that provide the signed zone.
* When a new, unsigned zone, is published by the primary master it
will automatically be transferred to the pre-signing servers. The
responsible operator will then sign the new zone and publish it
from his signing primary. Since the serial number is higher than
what the official root nameservers presently have the official
root nameservers will all transfer the zone from this source
(because of the redundant configuration with multiple possible
masters for the signed zone).
* The operators that participate in signing rotate the signing
responsibility among themselves. Should the responsible operator
be unable to do this for any reason then any of the others are
able to do the signing instead. Traceability is maintained since
the zone signing keys are individual.
* To avoid having several "backup signing operators" possibly sign
at exactly the same time backups are allocated "time windows"
within which they are allowed to publish a signed zone.
With N signers, each signer is assigned a unique number R in
[1..N].
T = 2*k*((S-R) mod N)
T is time to wait in seconds, S current serial number. The length
of the window is k, thereby separating each signing window with
an interval where signing is not allowed.
The constant k is used to create sufficient separation of the
signers with respect to time used to sign and time needed to
distribute the zone. A reasonable value for k would be in the
order of 1800 seconds.
* Because the digital signatures in the signed root zone MUST NOT
expire it is necessary to ensure that the unsigned zone does
change sufficiently often to cause new signing to occur within
the validity period of the old signatures. This is most easily
accomplished by a dummy update that only increments the serial
number in the SOA record.
This new requirement will not cause any operational problems,
since in fact the root zone is maintained this way since several
years back.
4.2.2. Management of the Key Signing Keys
Key Signing Keys, KSKs, are periodically issued by a several "Key
Signing Key Holders", KSK holders, individually. These KSKs consist
of a private part that should always be kept secret and a public
part that should be published as widely as possible since it will
be used as a "trusted key" in resolver configurations.
The public part of each KSK should be included in the keyset for
"." as described in [Threshold-Validation]. I.e. the keyset will be
the union of the public parts of all KSKs and all ZSKs, Zone
Signing Keys.
* Each KSK holder should generate a sequence of KSKs where the
public parts will be used to include in the keyset for "." and
the private parts will be used for signing the keyset.
* Each KSK holder should, on request from the signing operators,
send in the public part of the KSK. The union of the public parts
of KSKs and the corresponding public parts of ZSKs, as collected
by the signing operators, constitute a "keyset".
* Each KSK holder should, on request from the signing operators,
sign the complete keyset with the private part of the associated
KSK and send in the resulting signature to the signing operators
for inclusion in the signed zone.
4.2.3. Management of the Zone Signing Keys and the keyset signatures
A subgroup of the root operators that choose to participate in
signing the zone each hold an individual "Zone Signing Key",
ZSK. The subgroup of operators may include all operators, but that
is not necessary as long as a sufficient number to achieve
redundancy in "signing ability" participates.
* The complete keyset (i.e. all the public parts of KSKs and ZSKs)
should be signed by each KSK holder individually, generating a
new signature for the keyset which is sent back to the signing
operators via an out-of-band mechanism.
* The signing operators should verify each received signature
against the corresponding key in the keyset and, unless invalid,
accept the signature into the set of signatures associated with
this keyset as described in [Threshold-Validation].
* The signing operators should include one of the keysets together
with all the KSK signatures in the zone file according to an
agreed upon rotation schedule.
4.2.4. Management of key rollover
The Key Signing Keys should, for this interim scheme, be relatively
short-lived and rolled over frequently. The direct intent is that
it should not be possible to put long term trust in the keys.
Therefore, by design, every resolver that chooses to configure
these as "trusted keys" (to be able to validate lookups) will have
to change the configuration periodically.
This is, after all, only an interim method of root zone signing.
* Key rollover is executed by changing from one signed keyset to
another. I.e. from a keyset signed by one set of KSKs to a keyset
signed by a partially different set of KSKs. By not rolling all
the KSKs at the same time redundancy is improved.
* Technically the signing operators are able to initiate key
rollover, although except for the case of a suspected key
compromise (with subsequent immediate key rollover) this ability
should only be used according to an established schedule.
* New Key Signing Keys will be published as widely as possible,
however exactly how and where to publish the keys is by itself an
area where it is necessary to acquire more experience. Especially
for the case of individual hosts and other devices this will be a
significant issue to deal with.
* Since the keys expire within a few months the aim is to make it
as difficult as possible to configure an interim key and then
forget about it long enough to still trust an interim key when a
long term design for signing the root zone emerges.
4.2.5. The role of the KSK holder
With multiple KSKs it is possible to have multiple individual KSK
holders. Each will perform the role of authenticating the identity
of the signing operators, through the act of signing the keyset
that includes the operator Zone Signing Keys.
The chain of authority that defines editorial control over the zone
contents is entirely separate from this and is in no way affected.
I.e. the KSK holder is only asserting identity of the holders of
ZSKs and does not in any way take part in issues regarding zone
contents. In this respect the role of a KSK holder is much like
that of a public notary or a Certificate Authority.
The primary function that the KSK holder is performing is adding
trust to the authenticity of the Zone Signing Keys and this trust
has to be propagated down to validating resolvers. Therefore an
obvious key characteristic of a KSK holder is that it should
already be trusted by as large a fraction of the "resolver
population" as possible. In this document that fraction is referred
to as the "trust base" of a KSK holder.
The key characteristics of a KSK holder will be entities that
* already are trusted by some part of the "resolver population",
i.e. have a "trust base"
* are multiple entities that complement each other (so that the
aggregated "trust base" grows)
* are willing to help work on methods for distributing their
trusted keys to the resolvers (hard problem)
The requirement on the individual KSK holders is that they must be
able to
* establish a secure out-of-band communication path in
collaboration with the signing operators which will be used for
authenticated exchange of the unsigned keyset and generated
signatures
* periodically generate strong keys using a good random number
generator
* manage their keys (i.e. use them for signing the operator keyset
and keeping the private key appropriately secret)
4.2.5.6. Suggestions for KSK holders
Regional Internet Registries, RIRs, are suggested to be one
suitable choice of KSK holders. However, since every KSK holder
will act on its own there is no requirement that all RIRs
participate, although more than one would be good. Other suitable
KSK holders may exist and as long as the requirements are met more
would be better within the packe size limitations that are
discussed in [Threshold-Validation].
The basis of the suggestion of RIRs is
* their neutrality
* their proven record of service to the Internet community
* that they don't have the domain name system as their primary
field of activity
* their geographical diversity
* the fact that they are, by definition, not a single entity, but
rather a collective of organizations.
5. Risk Analysis
A change in the management of the root zone is always a risk. But
that is all the more reason to do it carefully and after due
consideration, rather than as a result of an immediate and urgent
need. The gains of a signed root zone have to be judged against the
added complexity of the signing step.
However, added complexity, in one form or another, is basically
unavoidable. It is possible to argue for or against implementation
details, but in the end the benefits of a signed root will have to
be compared against some amount of added complexity. If the cost or
risk of this complexity is deemed to be too high, then it is not
possible to sign the DNS root zone. If that is the conclusion; then
it is obviously important to reach it as soon as possible.
The same is true for the need for operational experience with a
signed root zone. There is no method of acquiring this experience
except by signing the root zone, so that is what is being proposed.
Some identified scenarios:
5.1. What will the consequences of a signed root zone be for old
resolver software?
Nameservers that are authoritative for signed zones only give out
these signatures when explicitly asked for them. Therefore, the
presence of signatures in the root zone will not have any impact at
all on the majority of resolvers on the Internet that does not
validate lookups.
5.2. What happens if a signing operator fails to sign the zone on
time?
Literally nothing. I.e. the root zone that is published will be the
version prior to the last change. This is not believed to be a
major problem, since all changes to the data in the root zone are
characterized by long overlaps in time. Furthermore every change is
preceded by an administrative process that takes several days or
even weeks. Because of this, a failure to install a new version of
the root zone for even so long as a day will not noticeably change
the turn-around time for changes in the root zone.
5.3. What happens if several signing operators by mistake sign the
same version?
Literally nothing. One signing operator will be first, according to
the mechanism designed to separate the different backup signing
operators described in 3.3.1. The signed zone published by this
operator will be the version of the signed root zone that all the
root nameservers pick up.
When the second signing operator signs the zone the serial number
in the zone will be unchanged, and therefore the root nameservers
will not pick this zone up but instead stay with the first version.
5.4. What happens if the unsigned zone is compromised between the
primary master and the signing primaries?
This is basically the same threat as the zone being compromised
between the primary master and the root servers in the traditional
unsigned case. To guard against this possibility every zone
transfer is already protected by a TSIG signature.
Because of this the root zone file cannot get transferred to the
signing primaries unless the transaction signature matches, thereby
proving that the zone contents are uncompromised. The consequence
is that if the zone transfers are somehow compromised in transit,
they will not get signed and therefore the published root zone will
remain the signed version of the last uncompromised transfer.
5.5. What are the security implications for the new "signing
primaries"? Will they not have to be as secure as the primary
master is now?
Yes. However, the entire root server system is based upon trust,
i.e. the entire Internet is trusting the present root server system
because there is no alternative to doing that. This proposal is not
about changing the need for trust in the root server system. It is
about providing a method of determining authenticity of data,
something that is presently missing.
The root operators are already trusted to provide a root server
system based upon secure servers lacking validation mechanisms. It
is therefore a bit difficult to argue for not trusting them to
provide an improved system that adds exactly the lacking validation
mechanisms on a basis of not trusting them to secure the servers
involved. In both cases trust is involved, the difference is that
in the latter case validation is possible.
Furthermore, the proposed signing primaries will not need to have
publicly known addresses (just as the present primary master is not
publicly known), nor will they need to be able to communicate with
anyone outside the well defined set of servers that are part of the
root server system. Because of this it will be significantly easier
to secure the signing primaries than the already present task of
securing the DNS root nameservers.
5.6. What happens if a Zone Signing Key is compromised?
If this happens it is necessary to do an emergency rollover of the
keyset that includes the compromised key. I.e. the old keyset (and
its signatures) is replaced by a new keyset containing new ZSKs but
the same, uncompromised, KSKs and its signatures. Then the root
zone is re-signed using one of the new Zone Signing Keys.
This problem is not unique to a signed root zone, it is inherent in
any activity involving cryptographic keys.
Also note that there will be no need to change any configuration in
the resolver end. The rollover is an entirely atomic operation
inside the management structure of the root zone.
5.7. What happens if a Key Signing Key is compromised?
Depending on the trust model used by individual validating
resolvers one of two things will happen.
Resolvers that through local policy have chosen to trust this KSK
alone will need to change their configuration to instead trust
other KSKs, either available from other KSK holders or a new
trusted key made available by the same KSK holder that held the
compromised key.
Resolvers that have chosen to require multiple signatures by KSKs
for the root zone will not have to do any emergency key rollover at
all, since validation of lookups will still work, based upon the
integrity of the other trusted keys that have not been compromised.
In the management end it is necessary to do an emergency rollover
of the keyset including the compromised key and the suggested
method is by switching to a keyset that only changes the
compromised KSK and the ZSKs and keeps all other KSKs. Such keysets
should be prepared and signed in advance.
The new signed keyset is added to the root zone and then the zone
is re-signed using one of the new Zone Signing Keys. In this case,
since a Key Signing Key is changed, every resolver that choose to
trust that KSK holder will over time have to configure the new key
to be able to validate lookups.
This problem is not unique to a signed root zone, it is inherent in
any activity involving cryptographic keys.
5.8. Does the out-of-band communication between the signing operators
and the RIRs holding the key-signing keys add a new failure mode?
Yes. The traditional DNSSEC design assumes that (for an arbitrary
zone, not particularly for the root zone) the key-signing key is
managed by the same entity that manages the operator keys and hence
no communication issue exists.
In this case it is important that the signing operators do not hold
the responsibility for the key-signing keys. The root server
operators should only act as a "publishing service" for the root
zone contents, not as the entity that verifies correctness of the
published data.
However, apart from established secure methods of out-of-band
communication being available, there is also the very real
possibility of managing these exchanges with actual eye-to-eye
contact. Representatives for the RIRs and the root server operators
already meet on a regular basis during IETF meetings and the
different RIR meetings.
6. Security Considerations
Signing the DNS root zone is all about security. A signed root zone
may aid applications and organizations all over the Internet in
improving their security by enabling validation of DNS lookups.
Signing the root zone does add a new management step and therefore
it is important to ensure that possible failures in this management
step does not leave the published root zone in a state that is
actually worse than the original unsigned state.
The various failure modes that have been identified all show this
property of not being any worse than the unsigned case.
There is however one scenario that changes drastically with the
proposed distributed signing scheme and that is the consequences of
one signing operator intentionally changing the contents of the
root zone prior to the actual signing. In the unsigned case this
will cause the root zone to become inconsistent (i.e. the responses
vary depending upon which server it comes from), while in the
signed case the root zone will remain consistent but with erroneous
data in it.
It is my belief (this is impossible to prove) that the risk of
human error among the operators, however small, is still far
greater than the risk of willful misconduct. Therefore, the
proposed design that enforces consistency among the roots, is
actually also an improvement in stability and security.
Se further section 4 for a more complete risk analysis.
7. IANA Considerations
NONE.
8. References
8.1. Normative.
[RFC2535] Domain Name System Security Extensions. D. Eastlake.
March 1999.
[RFC3090] DNS Security Extension Clarification on Zone Status.
E. Lewis. March 2001.
[RFC1995] Incremental Zone Transfer in DNS. M. Ohta. August 1996.
[RFC1996] A Mechanism for Prompt Notification of Zone Changes
(DNS NOTIFY). P. Vixie. August 1996.
[RFC2845] Secret Key Transaction Authentication for DNS (TSIG).
P. Vixie, O. Gudmundsson, D. Eastlake, B. Wellington.
May 2000.
8.2. Informative.
[RFC2930] Secret Key Establishment for DNS (TKEY RR). D. Eastlake.
September 2000.
[RFC3007] Secure Domain Name System (DNS) Dynamic Update.
B. Wellington. November 2000.
[RFC3008] Domain Name System Security (DNSSEC) Signing Authority.
B. Wellington. November 2000.
[RFC3110] RSA/SHA-1 SIGs and RSA KEYs in the Domain Name System
(DNS). D. Eastlake 3rd. May 2001.
[RFC3225] Indicating Resolver Support of DNSSEC. D. Conrad.
December 2001.
[RFC3226] DNSSEC and IPv6 A6 aware server/resolver message size
requirements. O. Gudmundsson. December 2001.
[Delegation-Signer] Delegation Signer Resource Record.
O. Gudmundsson. October 2002. Work In Progress.
[AXFR-clarify] draft-ietf-dnsext-axfr-clarify-NN.txt DNS Zone
Transfer Protocol Clarifications. A. Gustafsson. March
2002. Work In Progress.
[AD-secure] draft-ietf-dnsext-ad-is-secure-NN.txt Redefinition of
DNS AD bit. B. Wellington, O Gudmundsson. June 2002.
Work In Progress.
[Opt-In] draft-ietf-dnsext-dnssec-opt-in-NN.txt DNSSEC Opt-In
R. Arends, M Kosters, D. Blacka. June 2002. Work In
Progress.
[Wild-card-optimize] draft-olaf-dnsext-dnssec-wildcard-
optimization-NN.txt DNSSEC Wildcard optimization.
O. Kolkman, J. Ihren, R. Arends. September 2002.
Work In Progress.
[Threshold-Validation]
draft-ihren-dnsop-threshold-validation-00.txt Threshold
validation: A Mechanism for Improved Trust and Redundancy
for DNSSEC Keys. J. Ihren. February 2003. Work In
Progress.
9. Acknowledgments.
To help me produce this document I have received lots and lots of
comments from many people around the Internet with suggestions for
lots and lots sorely needed improvements. Among the folks who
helped out are, in no particular order: Paul Vixie, Bill Manning,
Ôlafur Gumundsson, Rob Austein, Patrik F„ltstr÷m, Olaf Kolkman,
Harald Alvestrand, Suzanne Woolf, John M. Brown, Lars-Johan Liman
and M…ns Nilsson.
10. Authors' Address
Johan Ihr‰n
Autonomica AB
Bellmansgatan 30
SE-118 47 Stockholm, Sweden
johani@autonomica.se
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