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Network Working Group Henry Spencer
Internet Draft SP Systems
Expires: 26 Aug 2002 D. Hugh Redelmeier
Mimosa Systems
26 Feb 2002
IKE Implementation Issues
<draft-spencer-ipsec-ike-implementation-02.txt>
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
(If approved as an Informational RFC...) This memo provides
information for the Internet community. This memo does not specify
an Internet standard of any kind.
Distribution of this memo is unlimited.
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.
This Internet-Draft will expire on 26 Aug 2002.
Copyright Notice
Copyright (C) The Internet Society 2002. All Rights Reserved.
Spencer & Redelmeier [Page 1]
Internet Draft IKE Implementation Issues 26 Feb 2002
Table of Contents
1. Introduction ................................................... 3
2. Lower-level Background and Notes ............................... 4
2.1. Packet Handling .............................................. 4
2.2. Ciphers ...................................................... 5
2.3. Interfaces ................................................... 5
3. IKE Infrastructural Issues ..................................... 5
3.1. Continuous Channel ........................................... 5
3.2. Retransmission ............................................... 5
3.3. Replay Prevention ............................................ 6
4. Basic Keying and Rekeying ...................................... 7
4.1. When to Create SAs ........................................... 7
4.2. When to Rekey ................................................ 8
4.3. Choosing an SA ............................................... 9
4.4. Why to Rekey ................................................. 9
4.5. Rekeying ISAKMP SAs ......................................... 10
4.6. Bulk Negotiation ............................................ 10
5. Deletions, Teardowns, Crashes ................................. 11
5.1. Deletions ................................................... 11
5.2. Teardowns and Shutdowns ..................................... 12
5.3. Crashes ..................................................... 13
5.4. Network Partitions .......................................... 13
5.5. Unknown SAs ................................................. 14
6. Misc. IKE Issues .............................................. 16
6.1. Groups 1 and 5 .............................................. 16
6.2. To PFS Or Not To PFS ........................................ 16
6.3. Debugging Tools, Lack Thereof ............................... 16
6.4. Terminology, Vagueness Thereof .............................. 17
6.5. A Question of Identity ...................................... 17
6.6. Opportunistic Encryption .................................... 17
6.7. Authentication and RSA Keys ................................. 17
6.8. Misc. Snags ................................................. 18
7. Security Considerations ....................................... 19
8. References .................................................... 19
Authors' Addresses ............................................... 20
Full Copyright Statement ......................................... 21
Spencer & Redelmeier [Page 2]
Internet Draft IKE Implementation Issues 26 Feb 2002
Abstract
The current IPsec specifications for key exchange and connection
management, RFCs 2408 [ISAKMP] and 2409 [IKE], leave many aspects of
connection management unspecified, most prominently rekeying
practices. Pending clarifications in future revisions of the
specifications, this document sets down some successful experiences,
to minimize the extent to which new implementors have to rely on
unwritten folklore.
The Linux FreeS/WAN implementation of IPsec interoperates with almost
every other IPsec implementation. This document describes how the
FreeS/WAN project has resolved some of the gaps in the IPsec
specifications (and plans to resolve some others), and what
difficulties have been encountered, in hopes that this generally-
successful experience might be informative to new implementors.
This is offered as an Informational RFC.
This -02 revision mainly: discusses ISAKMP SA expiry during IPsec-SA
rekeying (4.5), revises the discussion of bidirectional Deletes
(5.1), suggests remembering the parameters of successful negotiations
for later use (4.2, 5.3), notes an unsuccessful negotiation from the
other end as a hint of a possibly broken connection (5.5), and adds
sections on network partitions (5.4), authentication methods and the
subtleties of RSA public keys (6.7), and miscellaneous
interoperability concerns (6.8).
1. Introduction
The current IPsec specifications for key exchange and connection
management, RFCs 2408 [ISAKMP] and 2409 [IKE], leave many aspects of
connection management unspecified, most prominently rekeying
practices. This is a cryptic puzzle which each group of implementors
has to struggle with, and differences in how the ambiguities and gaps
are resolved are potentially a fruitful source of interoperability
problems. We can hope that future revisions of the specifications
will clear this up. Meanwhile, it seems useful to set down some
successful experiences, to minimize the extent to which new
implementors have to rely on unwritten folklore.
The Linux FreeS/WAN implementation of IPsec interoperates with almost
every other IPsec implementation, and because of its free nature, it
also sees some use as a reference implementation by other
implementors. The high degree of interoperability is noteworthy
given its organizers' strong minimalist bias, which has caused them
to implement only a small subset of the full glory of IPsec. This
document describes how the FreeS/WAN project has resolved some of the
Spencer & Redelmeier [Page 3]
Internet Draft IKE Implementation Issues 26 Feb 2002
gaps in the IPsec specifications (and plans to resolve some others),
and what difficulties have been encountered, in hopes that this
generally-successful experience might be informative to new
implementors.
One small caution about applicability: this experience may not be
relevant to severely resource-constrained implementations.
FreeS/WAN's target environment is previous-generation PCs, now
available at trivial cost (often, within an organization, at no
cost), which have quite impressive CPU power and memory by the
standards of only a few years ago. Some of the approaches discussed
here may be inapplicable to implementations with severe external
constraints which prevent them from taking advantage of modern
hardware technology.
2. Lower-level Background and Notes
2.1. Packet Handling
FreeS/WAN implements ESP [ESP] and AH [AH] straightforwardly,
although AH sees little use among our users. Our ESP/AH
implementation cannot currently handle packets with IP options;
somewhat surprisingly, this has caused little difficulty. We insist
on encryption and do not support authentication-only connections, and
this has not caused significant difficulty either.
MTU and fragmentation issues, by contrast, have been a constant
headache. We will not describe the details of our current approach
to them, because it still needs work. One difficulty we have
encountered is that many combinations of packet source and packet
destination apparently cannot cope with an "interior minimum" in the
path MTU, e.g. where an IPsec tunnel intervenes and its headers
reduce the MTU for an intermediate link. This is particularly
prevalent when using common PC software to connect to large well-
known web sites; we think it is largely due to misconfigured
firewalls which do not pass ICMP Fragmentation Required messages.
The only solution we have yet found is to lie about the MTU of the
tunnel, accepting the (undesirable) fragmentation of the ESP packets
for the sake of preserving connectivity.
We currently zero out the TOS field of ESP packets, rather than
copying it from the inner header, on the grounds that it lends itself
too well to traffic analysis and covert channels. We provide an
option to restore RFC 2401 [IPSEC] copying behavior, but this appears
to see little use.
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2.2. Ciphers
We initially implemented both DES [DES] and 3DES [CIPHERS] for both
IKE and ESP, but after the Deep Crack effort [CRACK] demonstrated its
inherent insecurity, we dropped support for DES. Somewhat
surprisingly, our insistence on 3DES has caused almost no
interoperability problems, despite DES being officially mandatory. A
very few other systems either do not support 3DES or support it only
as an optional upgrade, which inconveniences a few would-be users.
There have also been one or two cases of systems which don't quite
seem to know the difference!
See also section 6.1 for a consequence of our insistence on 3DES.
2.3. Interfaces
We currently employ PF_KEY version 2 [PFKEY], plus various non-
standard extensions, as our interface between keying and ESP. This
has not proven entirely satisfactory. Our feeling now is that keying
issues and policy issues do not really lend themselves to the clean
separation that PF_KEY envisions.
3. IKE Infrastructural Issues
A number of problems in IPsec connection management become easier if
some attention is first paid to providing an infrastructure to
support solving them.
3.1. Continuous Channel
FreeS/WAN uses an approximation to the "continuous channel" model, in
which ISAKMP SAs are maintained between IKEs so long as any IPsec SAs
are open between the two systems. The resource consumption of this
is minor: the only substantial overhead is occasional rekeying.
IPsec SA management becomes significantly simpler if there is always
a channel for transmission of control messages. We suggest (although
we do not yet fully implement this) that inability to maintain (e.g.,
to rekey) this control path should be grounds for tearing down the
IPsec SAs as well.
As a corollary of this, there is one and only one ISAKMP SA
maintained between a pair of IKEs (although see sections 5.3 and 6.5
for minor complications).
3.2. Retransmission
The unreliable nature of UDP transmission is a nuisance. IKE
implementations should always be prepared to retransmit the most
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recent message they sent on an ISAKMP SA, since there is some
possibility that the other end did not get it. This means, in
particular, that the system sending the supposedly-last message of an
exchange cannot relax and assume that the exchange is complete, at
least not until a significant timeout has elapsed.
Systems must also retain information about the message most recently
received in an exchange, so that a duplicate of it can be detected
(and possibly interpreted as a NACK for the response).
The retransmission rules FreeS/WAN follows are: (1) if a reply is
expected, retransmit only if it does not appear before a timeout; and
(2) if a reply is not expected (last message of the exchange),
retransmit only on receiving a retransmission of the previous
message. Notably, in case (1) we do NOT retransmit on receiving a
retransmission, which avoids possible congestion problems arising
from packet duplication, at the price of slowing response to packet
loss. The timeout for case (1) is 10 seconds for the first retry, 20
seconds for the second, and 40 seconds for all subsequent retries
(normally only one, except when configuration settings call for
persistence and the message is the first message of Main Mode with a
new peer). These retransmission rules have been entirely successful.
(Michael Thomas of Cisco has pointed out that the retry timeouts
should include some random jitter, to de-synchronize hosts which are
initially synchronized by, e.g., a power outage. We already jitter
our rekeying times, as noted in section 4.2, but that does not help
with initial startup. We're implementing jittered retries, but
cannot yet report on experience with this.)
There is a deeper problem, of course, when an entire "exchange"
consists of a single message, e.g. the ISAKMP Informational Exchange.
Then there is no way to decide whether or when a retransmission is
warranted at all. This seems like poor design, to put it mildly (and
there is now talk of fixing it). We have no experience in dealing
with this problem at this time, although it is part of the reason why
we have delayed implementing Notification messages.
3.3. Replay Prevention
The unsequenced nature of UDP transmission is also troublesome,
because it means that higher levels must consider the possibility of
replay attacks. FreeS/WAN takes the position that systematically
eliminating this possibility at a low level is strongly preferable to
forcing careful consideration of possible impacts at every step of an
exchange. RFC 2408 [ISAKMP] section 3.1 states that the Message ID
of an ISAKMP message must be "unique". FreeS/WAN interprets this
literally, as forbidding duplication of Message IDs within the set of
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all messages sent via a single ISAKMP SA.
This requires remembering all Message IDs until the ISAKMP SA is
superseded by rekeying, but that is not costly (four bytes per sent
or received message), and it ELIMINATES replay attacks from
consideration; we believe this investment of resources is well
worthwhile. If the resource consumption becomes excessive--in our
experience it has not--the ISAKMP SA can be rekeyed early to collect
the garbage.
There is theoretically an interoperability problem when talking to
implementations which interpret "unique" more loosely and may re-use
Message IDs, but it has not been encountered in practice. This
approach appears to be completely interoperable.
The proposal by Andrew Krywaniuk [REPLAY], which advocates turning
the Message ID into an anti-replay counter, would achieve the same
goal without the minor per-message memory overhead. This may be
preferable, although it means an actual protocol change and more
study is needed.
4. Basic Keying and Rekeying
4.1. When to Create SAs
As Tim Jenkins [REKEY] pointed out, there is a potential race
condition in Quick Mode: a fast lightly-loaded Initiator might start
using IPsec SAs very shortly after sending QM3 (the third and last
message of Quick Mode), while a slow heavily-loaded Responder might
not be ready to receive them until after spending a significant
amount of time creating its inbound SAs. The problem is even worse
if QM3 gets delayed or lost.
FreeS/WAN's approach to this is what Jenkins called "Responder Pre-
Setup": the Responder creates its inbound IPsec SAs before it sends
QM2, so they are always ready and waiting when the Initiator sends
QM3 and begins sending traffic. This approach is simple and
reliable, and in our experience it interoperates with everybody.
(There is potentially still a problem if FreeS/WAN is the Initiator
and the Responder does not use Responder Pre-Setup, but no such
problems have been seen.) The only real weakness of Responder Pre-
Setup is the possibility of replay attacks, which we have eliminated
by other means (see section 3.3).
With this approach, the Commit Bit is useless, and we ignore it. In
fact, until quite recently we discarded any IKE message containing
it, and this caused surprisingly few interoperability problems;
apparently it is not widely used. We have recently been persuaded
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that simply ignoring it is preferable; preliminary experience with
this indicates that the result is successful interoperation with
implementations which set it.
4.2. When to Rekey
To preserve connectivity for user traffic, rekeying of a connection
(that is, creation of new IPsec SAs to supersede the current ones)
must begin before its current IPsec SAs expire. Preferably one end
should predictably start rekeying negotiations first, to avoid the
extra overhead of two simultaneous negotiations, although either end
should be prepared to rekey if the other does not. There is also a
problem with "convoys" of keying negotiations: for example, a "hub"
gateway with many IPsec connections can be inundated with rekeying
negotiations exactly one connection-expiry time after it reboots, and
the massive overload this induces tends to make this situation self-
perpetuating, so it recurs regularly. (Convoys can also evolve
gradually from initially-unsynchronized negotiations.)
FreeS/WAN has the concept of a "rekeying margin", measured in
seconds. If FreeS/WAN was the Initiator for the previous rekeying
(or the startup, if none) of the connection, it nominally starts
rekeying negotiations at expiry time minus one rekeying margin. Some
random jitter is added to break up convoys: rather than starting
rekeying exactly at minus one margin, it starts at a random time
between minus one margin and minus two margins. (The randomness here
need not be cryptographic in quality, so long as it varies over time
and between hosts. We use an ordinary PRNG seeded with a few bytes
from a cryptographic randomness source. The seeding mostly just
ensures that the PRNG sequence is different for different hosts, even
if they start up simultaneously.)
If FreeS/WAN was the Responder for the previous rekeying/startup, and
nothing has been heard from the previous Initiator at expiry time
minus one-half the rekeying margin, FreeS/WAN will initiate rekeying
negotiations. No jitter is applied; we now believe that it should be
jittered, say between minus one-half margin and minus one-quarter
margin.
Having the Initiator lead the way is an obvious way of deciding who
should speak first, since there is already an Initiator/Responder
asymmetry in the connection. Moreover, our experience has been that
Initiator lead gives a significantly higher probability of successful
negotiation! The negotiation process itself is asymmetric, because
the Initiator must make a few specific proposals which the Responder
can only accept or reject, so the Initiator must try to guess where
its "acceptable" region (in parameter space) might overlap with the
Responder's. We have seen situations where negotiations would
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succeed or fail depending on which end initiated them, because one
end was making better guesses. Given an existing connection, we KNOW
that the previous Initiator WAS able to initiate a successful
negotiation, so it should (if at all possible) take the lead again.
Also, the Responder should remember the Initiator's successful
proposal, and start from that rather than from his own default
proposals if he must take the lead; we don't currently implement this
completely but plan to.
FreeS/WAN defaults the rekeying margin to 9 minutes, although this
can be changed by configuration. There is also a configuration
option to alter the permissible range of jitter. The defaults were
chosen somewhat arbitrarily, but they work extremely well and the
configuration options are rarely used.
4.3. Choosing an SA
Once rekeying has occurred, both old and new IPsec SAs for the
connection exist, at least momentarily. FreeS/WAN accepts incoming
traffic on either old or new inbound SAs, but sends outgoing traffic
only on the new outbound ones. This approach appears to be
significantly more robust than using the old ones until they expire,
notably in cases where renegotiation has occurred because something
has gone wrong on the other end. It avoids having to pay meticulous
attention to the state of the other end, state which is difficult to
learn reliably given the limitations of IKE.
This approach has interoperated successfully with ALMOST all other
implementations. The only (well-characterized) problem cases have
been implementations which rely on receiving a Delete message for the
old SAs to tell them to switch over to the new ones. Since delivery
of Delete is unreliable, and support for Delete is optional, this
reliance seems like a serious mistake. This is all the more true
because Delete announces that the deletion has already occurred
[ISAKMP, section 3.15], not that it is about to occur, so packets
already in transit in the other direction could be lost. Delete
should be used for resource cleanup, not for switchover control.
(These matters are discussed further in section 5.)
4.4. Why to Rekey
FreeS/WAN currently implements only time-based expiry (life in
seconds), although we are working toward supporting volume-based
expiry (life in kilobytes) as well. The lack of volume-based expiry
has not been an interoperability problem so far.
Volume-based expiry does add some minor complications. In
particular, it makes explicit Delete of now-disused SAs more
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important, because once an SA stops being used, it might not expire
on its own. We believe this lacks robustness and is generally
unwise, especially given the lack of a reliable Delete, and expect to
use volume-based expiry only as a supplement to time-based expiry.
However, Delete support (see section 5) does seem advisable for use
with volume-based expiry.
We do not believe that volume-based expiry alters the desirability of
switching immediately to the new SAs after rekeying. Rekeying
margins are normally a small fraction of the total life of an SA, so
we feel there is no great need to "use it all up".
4.5. Rekeying ISAKMP SAs
The above discussion has focused on rekeying for IPsec SAs, but
FreeS/WAN applies the same approaches to rekeying for ISAKMP SAs,
with similar success.
One issue which we have noticed, but not explicitly dealt with, is
that difficulties may ensue if an IPsec-SA rekeying negotiation is in
progress at the time when the relevant ISAKMP SA gets rekeyed. The
IKE specification [IKE] hints, but does not actually say, that a
Quick Mode negotiation should remain on a single ISAKMP SA
throughout.
A reasonable rekeying margin will generally prevent the old ISAKMP SA
from actually expiring during a negotiation. Some attention may be
needed to prevent in-progress negotiations from being switched to the
new ISAKMP SA. Any attempt at pre-expiry deletion of the ISAKMP SA
must be postponed until after such dangling negotiations are
completed, and there should be enough delay between ISAKMP-SA
rekeying and a deletion attempt to (more or less) ensure that there
are no negotiation-starting packets still in transit from before the
rekeying.
At present, FreeS/WAN does none of this, and we don't KNOW of any
resulting trouble. With normal lifetimes, the problem should be
uncommon, and we speculate that an occasional disrupted negotiation
simply gets retried.
4.6. Bulk Negotiation
Quick Mode nominally provides for negotiating possibly-large numbers
of similar but unrelated IPsec SAs simultaneously [IKE, section 9].
Nobody appears to do this. FreeS/WAN does not support it, and its
absence has caused no problems.
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5. Deletions, Teardowns, Crashes
FreeS/WAN currently ignores all Notifications and Deletes, and never
generates them. This has caused little difficulty in
interoperability, which shouldn't be surprising (since Notification
and Delete support is officially entirely optional) but does seem to
surprise some people. Nevertheless, we do plan some changes to this
approach based on past experience.
5.1. Deletions
As hinted at above, we plan to implement Delete support, done as
follows. Shortly after rekeying of IPsec SAs, the Responder issues a
Delete for its old inbound SAs (but does not actually delete them
yet). The Responder initiates this because the Initiator started
using the new SAs on sending QM3, while the Responder started using
them only on (or somewhat after) receiving QM3, so there is less
chance of old-SA packets still being in transit from the Initiator.
The Initiator issues an unsolicited Delete only if it does not hear
one from the Responder after a longer delay.
Either party, on receiving a Delete for one or more of the old
outbound SAs of a connection, deletes ALL the connection's SAs, and
acknowledges with a Delete for the old inbound SAs. A Delete for
nonexistent SAs (e.g., SAs which have already been expired or
deleted) is ignored. There is no retransmission of unacknowledged
Deletes.
In the normal case, with prompt reliable transmission (except
possibly for loss of the Responder's initial Delete) and conforming
implementations on both ends, this results in three Deletes being
transmitted, resembling the classic three-way handshake. Loss of a
Delete after the first, or multiple losses, will cause the SAs not to
be deleted on at least one end. It appears difficult to do much
better without at least a distinction between request and
acknowledgement.
RFC 2409 section 9 "strongly suggests" that there be no response to
informational messages such as Deletes, but the only rationale
offered is prevention of infinite loops endlessly exchanging "I don't
understand you" informationals. Since Deletes cannot lead to such a
loop (and in any case, the nonexistent-SA rule prevents more than one
acknowledgement for the same connection), we believe this
recommendation is inapplicable here.
As noted in section 4.3, these Deletes are intended for resource
cleanup, not to control switching between SAs. But we expect that
they will improve interoperability with some broken implementations.
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We believe strongly that connections need to be considered as a
whole, rather than treating each SA as an independent entity. We
will issue Deletes only for the full set of inbound SAs of a
connection, and will treat a Delete for any outbound SA as equivalent
to deletion of all the outbound SAs for the associated connection.
The above is phrased in terms of IPsec SAs, but essentially the same
approach can be applied to ISAKMP SAs (the Deletes for the old ISAKMP
SA should be sent via the new one).
5.2. Teardowns and Shutdowns
When a connection is not intended to be up permanently, there is a
need to coordinate teardown, so that both ends are aware that the
connection is down. This is both for recovery of resources, and to
avoid routing packets through dangling SAs which can no longer
deliver them.
Connection teardown will use the same bidirectional exchange of
Deletes as discussed in section 5.1: a Delete received for current
IPsec SAs (not yet obsoleted by rekeying) indicates that the other
host wishes to tear down the associated connection.
A Delete received for a current ISAKMP SA indicates that the other
host wishes to tear down not only the ISAKMP SA but also all IPsec
SAs currently under the supervision of that ISAKMP SA. The 5.1
bidirectional exchange might seem impossible in this case, since
reception of an ISAKMP-SA Delete indicates that the other end will
ignore further traffic on that ISAKMP SA. We suggest using the same
tactic discussed in 5.1 for IPsec SAs: the first Delete is sent
without actually doing the deletion, and the response to receiving a
Delete is to do the deletion and reply with another Delete. If there
is no response to the first Delete, retry a small number of times and
then give up and do the deletion; apart from being robust against
packet loss, this also maximizes the probability that an
implementation which does not do the bidirectional Delete will
receive at least one of the Deletes.
When a host with current connections knows that it is about to shut
down, it will issue Deletes for all SAs involved (both IPsec and
ISAKMP), advising its peers (as per the meaning of Delete [ISAKMP,
section 3.15]) that the SAs have become useless. It will ignore
attempts at rekeying or connection startup thereafter, until it shuts
down.
It would be better to have a Final-Contact notification, analogous to
Initial-Contact but indicating that no new negotiations should be
attempted until further notice. Initial-Contact actually could be
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used for shutdown notification (!), but in networks where connections
are intended to exist permanently, it seems likely to provoke
unwanted attempts to renegotiate the lost connections.
5.3. Crashes
Systems sometimes crash. Coping with the resulting loss of
information is easily the most difficult problem we have found in
implementing robust IPsec systems.
When connections are intended to be permanent, it is simple to
specify renegotiation on reboot. With our approach to SA selection
(see section 4.3), this handles such cases robustly and well. We do
have to tell users that BOTH hosts should be set this way. In cases
where crashes are synchronized (e.g. by power interruptions), this
may result in simultaneous negotiations at reboot. We currently
allow both negotiations to proceed to completion, but our use-newest
selection method effectively ignores one connection or the other, and
when one of them rekeys, we notice that the new SAs replace those of
both old connections, and we then refrain from rekeying the other.
(This duplicate detection is desirable in any event, for robustness,
to ensure that the system converges on a reasonable state eventually
after it is perturbed by difficulties or bugs.)
When connections are not permanent, the situation is less happy. One
particular situation in which we see problems is when a number of
"Road Warrior" hosts occasionally call in to a central server. The
server is normally configured not to initiate such connections, since
it does not know when the Road Warrior is available (or what IP
address it is using). Unfortunately, if the server crashes and
reboots, any Road Warriors then connected have a problem: they don't
know that the server has crashed, so they can't renegotiate, and the
server has forgotten both the connections and their (transient) IP
addresses, so it cannot renegotiate.
We believe that the simplest answer to this problem is what John
Denker has dubbed "address inertia": the server makes a best-effort
attempt to remember (in nonvolatile storage) which connections were
active and what the far-end addresses were (and what the successful
proposal's parameters were), so that it can attempt renegotiation on
reboot. We have not implemented this yet, but intend to; Denker has
implemented it himself, although in a somewhat messy way, and reports
excellent results.
5.4. Network Partitions
A network partition, making the two ends unable to reach each other,
has many of the same characteristics as having the other end crash...
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until the network reconnects. It is desirable that recovery from
this be automatic.
If the network reconnects before any rekeying attempts or other IKE
activities occurred, recovery is fully transparent, because the IKEs
have no idea that there was any problem. (Complaints such as ICMP
Host Unreachable messages are unauthenticated and hence cannot be
given much weight.) This fits the general mold of TCP/IP: if nobody
wanted to send any traffic, a network outage doesn't matter.
If IKE activity did occur, the IKE implementation will discover that
the other end doesn't seem to be responding. The preferred response
to this depends on the nature of the connection. If it was intended
to be ephemeral (e.g. opportunistic encryption [OE]), closing it down
after a few retries is reasonable. If the other end is expected to
sometimes drop the connection without warning, it may not be
desirable to retry at all. (We support both these forms of
configurability, and indeed we also have a configuration option to
suppress rekeying entirely on one end.)
If the connection was intended to be permanent, however, then
persistent attempts to re-establish it are appropriate. Some degree
of backoff is appropriate here, so that retries get less frequent as
the outage gets prolonged. Backoff should be limited, so that re-
established connectivity is not followed by a long delay before a
retry. Finally, after many retries (say 24 hours' worth), it may be
preferable to just declare the connection down and rely on manual
intervention to re-establish it, should this be desirable. We do not
yet fully support all this.
5.5. Unknown SAs
A more complete solution to crashes would be for an IPsec host to
note the arrival of ESP packets on an unknown IPsec SA, and report it
somehow to the other host, which can then decide to renegotiate.
This arguably might be preferable in any case--if the non-rebooted
host has no traffic to send, it does not care whether the connection
is intact--but delays and packet loss will be reduced if the
connection is renegotiated BEFORE there is traffic for it. So
unknown-SA detection is best reserved as a fallback method, with
address inertia used to deal with most such cases.
A difficulty with unknown-SA detection is, just HOW should the other
host be notified? IKE provides no good way to do the notification:
Notification payloads (e.g., Initial-Contact) are unauthenticated
unless they are sent under protection of an ISAKMP SA. A "Security
Failures - Bad SPI" ICMP message [SECFAIL] is an interesting
alternative, but has the disadvantage of likewise being
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unauthenticated. It's fundamentally unlikely that there is a simple
solution to this, given that almost any way of arranging or checking
authentication for such a notification is costly.
We think the best answer to this is a two-step approach. An
unauthenticated Initial-Contact or Security Failures - Bad SPI cannot
be taken as a reliable report of a problem, but can be taken as a
hint that a problem MIGHT exist. Then there needs to be some
reliable way of checking such hints, subject to rate limiting since
the checks are likely to be costly (and checking the same connection
repeatedly at short intervals is unlikely to be worthwhile anyway).
So the rebooted host sends the notification, and the non-rebooted
host--which still thinks it has a connection--checks whether the
connection still works, and renegotiates if not.
Also, if an IPsec host which believes it has a connection to another
host sees an unsuccessful attempt by that host to negotiate a new
one, that is also a hint of possible problems, justifying a check and
possible renegotiation. ("Unsuccessful" here means a negotiation
failure due to lack of a satisfactory proposal. A failure due to
authentication failure suggests a denial-of-service attack by a third
party, rather than a genuine problem on the legitimate other end.)
As noted in section 4.2, it is possible for negotiations to succeed
or fail based on which end initiates them, and some robustness
against that is desirable.
We have not yet decided what form the notification should take. IKE
Initial-Contact is an obvious possibility, but has some
disadvantages. It does not specify which connection has had
difficulties. Also, the specification [IKE section 4.6.3.3] refers
to "remote system" and "sending system" without clearly specifying
just what "system" means; in the case of a multi-homed host using
multiple forms of identification, the question is not trivial.
Initial-Contact does have the fairly-decisive advantage that it is
likely to convey the right general meaning even to an implementation
which does not do things exactly the way ours does.
A more fundamental difficulty is what form the reliable check takes.
What is wanted is an "IKE ping", verifying that the ISAKMP SA is
still intact (it being unlikely that IPsec SAs have been lost while
the ISAKMP SA has not). The lack of such a facility is a serious
failing of IKE. An acknowledged Notification of some sort would be
ideal, but there is none at present. Some existing implementations
are known to use the private Notification values 30000 as ping and
30002 as ping reply, and that seems the most attractive choice at
present. If it is not recognized, there will probably be no reply,
and the result will be an unnecessary renegotiation, so this needs
strict rate limiting. (Also, when a new connection is set up, it's
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probably worth determining by experiment whether the other end
supports IKE ping, and remembering that.)
While we think this facility is desirable, and is about the best that
can be done with the poor tools available, we have not gotten very
far in implementation and cannot comment intelligently about how well
it works or interoperates.
6. Misc. IKE Issues
6.1. Groups 1 and 5
We have dropped support for the first Oakley Group (group 1), despite
it being officially mandatory, on the grounds that it is grossly too
weak to provide enough randomness for 3DES. There have been some
interoperability problems, mostly quite minor: ALMOST everyone
supports group 2 as well, although sometimes it has to be explicitly
configured.
We also support the quasi-standard group 5 [GROUPS]. This has not
been seriously exercised yet, because historically we offered group 2
first and almost everyone accepted it. We have recently changed to
offering group 5 first, and no difficulties have been reported.
6.2. To PFS Or Not To PFS
A persistent small interoperability problem is that the presence or
absence of PFS (for keys [IKE, section 5.5]) is neither negotiated
nor announced. We have it enabled by default, and successful
interoperation often requires having the other end turn it on in
their implementation, or having the FreeS/WAN end disable it. Almost
everyone supports it, but it's usually not the default, and
interoperability is often impossible unless the two ends somehow
reach prior agreement on it.
We do not explicitly support the other flavor of PFS, for identities
[IKE, section 8], and this has caused no interoperability problems.
6.3. Debugging Tools, Lack Thereof
We find IKE lacking in basic debugging tools. Section 5.4, above,
notes that an IKE ping would be useful for connectivity verification.
It would also be extremely helpful for determining that UDP/500
packets get back and forth successfully between the two ends, which
is often an important first step in debugging.
It's also quite common to have IKE negotiate a connection
successfully, but to have some firewall along the way blocking ESP.
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Users find this mysterious and difficult to diagnose. We have no
immediate suggestions on what could be done about it.
6.4. Terminology, Vagueness Thereof
The terminology of IPsec needs work. We feel that both the
specifications and user-oriented documentation would be greatly
clarified by concise, intelligible names for certain concepts.
We semi-consistently use "group" for the set of IPsec SAs which are
established in one direction by a single Quick Mode negotiation and
are used together to process a packet (e.g., an ESP SA plus an AH
SA), "connection" for the logical packet path provided by a
succession of pairs of groups (each rekeying providing a new pair,
one group in each direction), and "keying channel" for the
corresponding supervisory path provided by a sequence of ISAKMP SAs.
We think it's a botch that "PFS" is used to refer to two very
different things, but we have no specific new terms to suggest, since
we only implement one kind of PFS and thus can just ignore the other.
6.5. A Question of Identity
One specification problem deserves note: exactly when can an existing
phase 1 negotiation be re-used for a new phase 2 negotiation, as IKE
[IKE, section 4] specifies? Presumably, when it connects the same
two "parties"... but exactly what is a "party"?
As noted in section 5.4, in cases involving multi-homing and multiple
identities, it's not clear exactly what criteria are used for
deciding whether the intended far end for a new negotiation is the
same one as for a previous negotiation. Is it by Identification
Payload? By IP address? Or what?
We currently use a somewhat-vague notion of "identity", basically
what gets sent in Identification Payloads, for this, and this seems
to be successful, but we think this needs better specification.
6.6. Opportunistic Encryption
Further IKE challenges appear in the context of Opportunistic
Encryption [OE], but operational experience with it is too limited as
yet for us to comment usefully right now.
6.7. Authentication and RSA Keys
We provide two IKE authentication methods: shared secrets ("pre-
shared keys") and RSA digital signatures. (A user-provided add-on
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package generalizes the latter to limited support for certificates;
we have not worked extensively with it ourselves yet and cannot
comment on it yet.)
Shared secrets, despite their administrative difficulties, see
considerable use, and are also the method of last resort for
interoperability problems.
For digital signatures, we have taken the somewhat unorthodox
approach of using "bare" RSA public keys, either supplied in
configuration files or fetched from DNS, rather than getting involved
in the complexity of certificates. We encode our RSA public keys
using the DNS KEY encoding [DNSRSA] (aka "RFC 2537", although that
RFC is now outdated), which has given us no difficulties and which we
highly recommend. We have seen two difficulties in connection with
RSA keys, however.
First, while a number of IPsec implementations are able to take
"bare" RSA public keys, each one seems to have its own idea of what
format should be used for transporting them. We've had little
success with interoperability here, mostly because of key-format
issues; the implementations generally WILL interoperate successfully
if you can somehow get an RSA key into them at all, but that's hard.
X.509 certificates seem to be the lowest (!) common denominator for
key transfer.
Second, although the content of RSA public keys has been stable,
there has been a small but subtle change over time in the content of
RSA private keys. The "internal modulus", used to compute the
private exponent "d" from the public exponent "e" (or vice-versa) was
originally [RSA] [PKCS1v1] [SCHNEIER] specified to be (p-1)*(q-1),
where p and q are the two primes. However, more recent definitions
[PKCS1v2] call it "lambda(n)" and define it to be lcm(p-1, q-1); this
appears to be a minor optimization. The result is that private keys
generated with the new definition often fail consistency checks in
implementations using the old definition. Fortunately, it is seldom
necessary to move private keys around. Our software now consistently
uses the new definition (and thus will accept keys generated with
either definition), but our key generator also has an option to
generate old-definition keys, for the benefit of users who upgrade
their networks incrementally.
6.8. Misc. Snags
Nonce size is another characteristic that is neither negotiated nor
announced but that the two ends must somehow be able to agree on.
Our software accepts anything between 8 and 256, and defaults to 16.
These numbers were chosen rather arbitrarily, but we have seen no
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interoperability failures here.
Nothing in the ISAKMP [ISAKMP] or IKE [IKE] specifications says
explicitly that a normal Message ID must be non-zero, but a zero
Message ID in fact causes failures.
Similarly, there is nothing in the specs which says that ISAKMP
cookies must be non-zero, but zero cookies will in fact cause
trouble.
7. Security Considerations
Since this document discusses aspects of building robust and
interoperable IPsec implementations, security considerations permeate
it.
8. References
[AH] Kent, S., and Atkinson, R., "IP Authentication Header",
RFC 2402, Nov 1998.
[CIPHERS] Pereira, R., and Adams, R., "The ESP CBC-Mode Cipher
Algorithms", RFC 2451, Nov 1998.
[CRACK] Electronic Frontier Foundation, "Cracking DES: Secrets of
Encryption Research, Wiretap Politics and Chip Design",
O'Reilly 1998, ISBN 1-56592-520-3.
[DES] Madson, C., and Doraswamy, N., "The ESP DES-CBC Cipher
Algorithm", RFC 2405, Nov 1998.
[DNSRSA] D. Eastlake 3rd, "RSA/SHA-1 SIGs and RSA KEYs in the
Domain Name System (DNS)", RFC 3110, May 2001.
[ESP] Kent, S., and Atkinson, R., "IP Encapsulating Security
Payload (ESP)", RFC 2406, Nov 1998.
[GROUPS] Kivinen, T., and Kojo, M., "More MODP Diffie-Hellman
groups for IKE", <draft-ietf-ipsec-ike-modp-
groups-04.txt>, 13 Dec 2001 (work in progress).
[IKE] Harkins, D., and Carrel, D., "The Internet Key Exchange
(IKE)", RFC 2409, Nov 1998.
[IPSEC] Kent, S., and Atkinson, R., "Security Architecture for the
Internet Protocol", RFC 2401, Nov 1998.
Spencer & Redelmeier [Page 19]
Internet Draft IKE Implementation Issues 26 Feb 2002
[ISAKMP] Maughan, D., Schertler, M., Schneider, M., and Turner, J.,
"Internet Security Association and Key Management Protocol
(ISAKMP)", RFC 2408, Nov 1998.
[OE] Richardson, M., Redelmeier, D. H., and Spencer, H., "A
method for doing opportunistic encryption with IKE",
<draft-richardson-ipsec-opportunistic-06.txt>, 21 Feb 2002
(work in progress).
[PKCS1v1] Kaliski, B., "PKCS #1: RSA Encryption, Version 1.5", RFC
2313, March 1998.
[PKCS1v2] Kaliski, B., and Staddon, J., "PKCS #1: RSA Cryptography
Specifications, Version 2.0", RFC 2437, Oct 1998.
[PFKEY] McDonald, D., Metz, C., and Phan, B., "PF_KEY Key
Management API, Version 2", RFC 2367, July 1998.
[REKEY] Tim Jenkins, "IPsec Re-keying Issues", <draft-jenkins-
ipsec-rekeying-06.txt>, 2 May 2000 (draft expired, work no
longer in progress).
[REPLAY] Krywaniuk, A., "Using Isakmp Message Ids for Replay
Protection", <draft-krywaniuk-ipsec-antireplay-00.txt>, 9
July 2001 (work in progress).
[RSA] Rivest, R.L., Shamir, A., and Adleman, L., "A Method for
Obtaining Digital Signatures and Public-Key
Cryptosystems", Communications of the ACM v21n2, Feb 1978,
p. 120.
[SCHNEIER] Bruce Schneier, "Applied Cryptography", 2nd ed., Wiley
1996, ISBN 0-471-11709-9.
[SECFAIL] Karn, P., and Simpson, W., "ICMP Security Failures
Messages", RFC 2521, March 1999.
Authors' Addresses
Henry Spencer
SP Systems
Box 280 Stn. A
Toronto, Ont. M5W1B2
Canada
[email protected]
416-690-6561
Spencer & Redelmeier [Page 20]
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D. Hugh Redelmeier
Mimosa Systems Inc.
29 Donino Ave.
Toronto, Ont. M4N2W6
Canada
[email protected]
416-482-8253
Spencer & Redelmeier [Page 21]
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