Network Working Group E. Lear
Internet-Draft Cisco Systems GmbH
Expires: March 20, 2006 September 16, 2005
Simple Firewall Traversal Mechanisms and Their Pitfalls
draft-lear-callhome-description-00.txt
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Copyright (C) The Internet Society (2005).
Abstract
Many devices make use of so-called "Call Home" functionality in order
to be managed or updated, or to otherwise establish outbound
communication in the face of NATs, firewalls, and mobility. This
memo defines call home functionality, discusses the requirement for
firewall traversal, some mechanisms used, and security considerations
of those mechanisms. Several existing examples will be shown. This
memo also contains a proposal for making SNMP and ISMs a call-home
protocol.
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1. Introduction
In the early days of the networking it was recognized that some
devices would be intermittantly reachable. Mechanisms such as UUCP
were based on this notion, and support for systems requesting that
the server act as the client showed up in the Internet no later than
1982 inSMTP [2] and were formalized in Blocks Extensible eXchange
Protocol(BEEP) [3] in 2001.
However, in the early days of the Internet it also largely didn't
matter from a network security or transparency standpoint which
device initiated communication, because there was little if any
network security and everyone used public address space. With the
introduction of private address space [4]and firewalls the world
changed. Today a firewall with NAT functionality is a consumer
device, not to mention an interdepartmental device.
In addition, the complexity of IT relationships and the number of
vendors that support enterprises has changed the underlying
assumption that the enterprise actually manages its own network and
support devices, such as power distribution units. Often for small
businesses, today, the situation is reversed and it is the small
business that has limited access to even the network layer of their
data center service provider.
All of this leads us to the conclusion that a flexible means for
management applications to traverse firewalls is a useful approach in
the face of devices that intercept unacknowledged SYNS or keep
translation tables based on connection state.
2. What is Call Home?
"Call Home" (CH) refers simply to the notion of reversing the party
that traditionally initiates a communication. An early example of CH
is the SMTP "TURN" command where the SMTP server becomes the client
and the client becomes the server.
3. What is Call Home good for?
Call Home is useful for devices that do not retain a stable
accessible point within a network. For instance, a lap top or a
wireless phone may move from one location to another, and yet it
still is be desirable for that device to be managed when it is
online. Imagine what would be necessary in order to manage such a
device by having the manager contact it:
1. Either the DNS would have to be updated with the mobile devices
new address or the device would have to make use of MOBILE-IP
[ref];
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2. The device would have to remain in either the global address
space or within the same address space as the manager;
3. Because firewalls often only allow communications one way without
prior arrangement (if they have the capability at all), they
would have to be informed of the device's new location and
that the device is authorized to receive requests.
4. How is Call Home achieved?
Because TCP state is easily detected in the header via the ACK bit,
the most basic form of CH involves the device wishing to be managed
contacting the manager via TCP. This simple approach traverses
nearly all consumer and commercial firewalls by default.
Because connection state is not as easily discerned for protocols
based on UDP, firewalls may be more retiscent to pass UDP traffic and
simple NAT mapping timeouts may require contrived or dummy
transactions to retain the mapping, but the same principle would
apply.
5. How does CH change the nature of the communication?
There are several difference between the traditional connection
approach and Call Home. In the traditional case of a manager and an
agent, the manager would make a request of the agent at any point
when the manager wishes. In the case of Call Home, the manager must
wait at least until the agent has established a transport connection.
This also means that control of connection frequency passes from the
manager to the agent. If frequency is important either the behavior
must be codified somehow or the manager must pass these parameters to
the agent and the agent must use them.
Change of whose listening for new connections in the cases of TCP or
SCTP further means that a potential DDOS target passes from the agent
to the manager.
In the traditional case, a manager may use any local TCP or UDP port
to initiate a connection but must connect to the agent on a well
known (or at least prearranged) port. In the call home case, again
the roles are reversed, and it is the manager that must service
requests on a well known port.
In the traditional case, each agent has a stable well known address,
just as it has a well known port. In the case of Call Home, the
manager must maintain a stable well known address.
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6. Security Considerations
The nature of security of the communication is likely to change.
While there are many aspects of this problem, the common traditional
case requires that the agent somehow authenticate its host address
(either via X.509 certificate or SSH host key) and the manager
authenticates via public key or username and password. Once again,
with Call Home these roles are reversed: the manager authenticates
its host address and the agent authenticates via public key or
username and password.
Some applications might require some additional configuration,
therefore, in order to accomodate Call Home. For instance, SNMP
requires that the command generator be associated with a
SecurityName. If the agent initiates the connection, either it must
derive the security name from something like the host key or subject
in the certificate of a manager, or it must be preconfigured with a
username to associate the connection.
As we discuss elsewhere in this document Call Home reverses use of
well known ports and services. It is important for CH protocols to
make use of well known ports in order to respect the legitimate
wishes of firewall administrators. Such use makes (more) reasonable
the assumption that a port is blocked for a reason.
7. IANA Considerations
While much of this is protocol specific it is within the realm of
possibilities that with client/server protocols either a new port or
an SSH service name or a BEEP URN will be needed to indicate the
intent of the initiator of communication to "turn" it.
8. Summary
Call Home is a useful firewall and NAT traversal approach
applications can use to augment their existing approach in order to
establish communications with devices that sit behind NATs or
firewalls, or otherwise have intermittant connectivity.
9. Informational References
[1] Pleasant, M. and E. Lear, "Transcending Administrative domains
by Automating System Management Tasks in a Large Heterogeneous
Environment", Usenix Software Security Workshop , April 1989.
[2] Postel, J., "Simple Mail Transfer Protocol", STD 10, RFC 821,
August 1982.
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[3] Rose, M., "The Blocks Extensible Exchange Protocol Core",
RFC 3080, March 2001.
[4] Rekhter, Y., Moskowitz, R., Karrenberg, D., and G. de Groot,
"Address Allocation for Private Internets", RFC 1597,
March 1994.
[5] Lear, E. and K. Crozier, "Using the NETCONF Protocol over Blocks
Extensible Exchange Protocol (BEEP)", draft-ietf-netconf-beep-05
(work in progress), April 2005.
[6] Herzog, S., Boyle, J., Cohen, R., Durham, D., Rajan, R., and A.
Sastry, "COPS usage for RSVP", RFC 2749, January 2000.
[7] Rosenberg, J., Weinberger, J., Huitema, C., and R. Mahy, "STUN -
Simple Traversal of User Datagram Protocol (UDP) Through Network
Address Translators (NATs)", RFC 3489, March 2003.
Author's Address
Eliot Lear
Cisco Systems GmbH
Glatt-com
Glattzentrum, ZH CH-8301
Switzerland
Phone: +41 1 878 7525
Email: lear@cisco.com
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