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Network Working Group                                            E. Lear
Internet-Draft                                        Cisco Systems GmbH
Expires: April 20, 2006                                 October 17, 2005

        Simple Firewall Traversal Mechanisms and Their Pitfalls

Status of this Memo

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Copyright Notice

   Copyright (C) The Internet Society (2005).


   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 examples of how one would make SNMP over SSH,
   NETCONF over SSH, and interactive terminal access call-home

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1.  Introduction

   In the early days of the networking it was recognized that some
   devices would be intermittently reachable.  Mechanisms such as UUCP
   [1] 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 in SMTP [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 network address translator (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 and what is it good for?

   "Call Home" refers simply to the notion of reversing the party that
   traditionally initiates a communication.  An early example of Call
   Home includes the SMTP "TURN" command where the SMTP server becomes
   the client and the client becomes the server Various system
   management protocols such as Track [5][6] have offered similar
   functionality for quite some time.  Most PCs have some means to
   update their operating systems and virus definitions via a similar

   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

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   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
   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.

   Call Home also allows for more complex management relationships
   without the need of complex VPNs and access lists.  If an enterprise
   wished to make use of a contract service for printer maintenance,
   that service could monitor printers via the MIB defined in [7].  The
   same scenario could be envisioned for management of uninterrupted
   power supplies (UPS) via [9].  In either case the vendor has little
   need of general remote network access, and the enterprise has a
   desire to limit such access.

3.  How is Call Home achieved?

   Call Home already exists in those session-based unicast protocols
   where the allowed operations and responses do not differ based on who
   initiated the connection.  An example in the routing world would be
   BGP.  Once the connection is established each side authenticates to
   the other and the same protocol operations may be executed by either
   end.  In the application world, so-called "peer to peer" protocols
   that are used for (often illicit) file transfer also fit this

   Often, however, protocols are designed with client and server roles.
   Examples include SMTP, and NNTP.  In these cases, some additional
   support within the application is necessary.  In SMTP's case the TURN
   and ETRN capabilities provide a means for ends to switch roles of
   client and server.  In NNTP a separate mechanism to retrieve articles
   - NEWNEWS - allows transfer agents to retrieve articles in a similar
   (albeit not identical) way the IHAVE operation and a queue of

   The applicability of Call Home in circumstances other than those
   above is extremely limited.  For instance, protocols that are based
   on atomic transactions, such as DNS queries, have no need to reverse
   client and server roles.  Indeed one would wonder of the intent of a
   name server that attempted to require a client to make a query of it.
   Similarly, the notion of Call Home in a multicast environment is
   likely limited as well as it is not clear who would reverse roles.

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   Because TCP state is easily detected in the header via the ACK bit,
   call home is also most easily implemented in TCP.  Because connection
   state is not as easily discerned for protocols based on UDP as
   specifics of each protocol would need to be known and the
   communication unencrypted, firewalls may be more reticent 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.  Hence the usefulness of Call Home in a UDP environment
   may be limited.

4.  How does Call Home change the nature of the communication?

   There are several differences 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.

   A change of who is 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.

5.  Naming Issues

   One reason to make use of Call Home is that traditional names, such
   as domain names may not be useful to contact a device, particularly
   if its IP address changes, either because the device has moved or
   because it leases addresses from a pool.  While it is possible to
   make use of DNS in such circumstances through mechanisms such as
   dynamic update [10], such use requires that tight coupling between
   the subsystem invoked via Call Home and the DNS, and is not
   particularly meaningful when the connecting device resides behind a

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   NAT or a firewall.

   When implementing Call Home there are several possibilities for
   choice of naming system.  In some cases, no naming system may be
   needed.  In others, as may be the case with a consumer DSL or cable
   deployment, the customer username may be sufficient.  In other cases,
   domain names may be suitable.  In all cases where names are used the
   security of the binding between the name and the device or
   application must be considered.

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 or
   domain name (either via X.509 [11] 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 or domain name and the agent
   authenticates via public key or username and password.

   Some applications might require some additional configuration,
   therefore, in order to accommodate 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 pre-configured with a
   username to associate the connection.

6.1.  Threat / Trust Model Changes

   In a more traditional client server relationship, the client connects
   to request some service of the server (thus the terminology).  In a
   way, that does not change with Call Home, because in this case the
   client is requesting to be managed or is requesting that roles be
   reversed.  The server must still authorize this request.

   However, there are changes from the traditional model.  For instance,
   if the client is asking to be managed, the nature of attacks change
   to that of mechanisms such as dictionary attacks on a request port to
   approaches that trick the client into connecting to a bogus
   management server where bogus requests could be generated.  This can
   actually have some benefits to security by limiting dictionary and
   buffer overflow attacks, to centralized well protected points,
   provided that the communication initiated by the client is well
   protected with such mechanisms as SSL, TLS, and the like; and the
   client itself does not listen for requests.

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   Finally, the underlying application that makes use of Call Home will
   have to consider the sort of information that is being made available
   as a service.  Each application will have different sorts of threats
   and mitigations.  For instance, the author knows of no SMTP agent
   that implements TURN because while most SMTP users are comfortable
   with the risks of Man in the Middle attacks associated with
   masquerading as SMTP servers, the risk of someone masquerading as a
   client are considered unacceptable.

   In all cases, strong authentication of either end of the
   communication is recommended for exchange of sensitive information,
   regardless of who started it.

6.2.  Firewall Administration

   As we discuss elsewhere in this document Call Home reverses use of
   well known ports and services.  It is important for Call Home
   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.  A
   firewall administrator may wish to allow certain communications in a
   single direction.  Use of additional well known ports may be advised
   in certain circumstances.  However, the ability of devices and
   protocols to call home exists today through SSL connections, to give
   but one example.  Excessive barriers to inclusion of call home
   functionality in protocols risks inappropriate use of existing

7.  Example 1: NETCONF using SSH

   NETCONF [12] is a fairly simple client/server protocol.  NETCONF is
   mapped to several protocols, including SSH.[13] In order for NETCONF
   agents to call home some protocol operation must be passed to the
   manager for this purpose, and this operation can occur in the
   protocol mapping layer.  Thus, the simplest approach would be to have
   a new SSH subsystem called "netconf-turn".  When the SSH client
   invokes this subsystem, the SSH server either will initiate the the
   subsystem and proceed with NETCONF capabilities exchange from the
   point of view of a manager or refuse to initiate the subsystem.

   The nature of the NETCONF communication changes in that the manager
   must wait for the agent to connect, as mentioned above.  There are no
   events explicitly defined in NETCONF at this time and so there are no
   explicit functions that require deferral from a protocol standpoint.
   However, the manager cannot configure the agent until it connects and
   so completion of a configuration request may be deferred when a
   manager is not in communication with an agent.  The manager must

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   retain configuration requests and higher level application must be
   able to deal with such deferrals.

   From an authentication standpoint, the SSH server must determine
   whether based on the credentials given the client has appropriate
   access to be managed.  Each NETCONF management operation on the SSH
   server must be governed by those credentials.

   On the client, it would be a configuration error for it to invoke the
   netconf-turn subsystem on the manager and then not allow ANY
   operations, but each operation must be authorized based on the server
   identity passed up by the SSH subsystem.

8.  Example 2: SNMP over SSH

   Let us again first discuss the nature of the communication.  In the
   case of SNMP there are ostensibly two basic protocol operations -
   request and response.  While in theory either entity may make such
   requests in practice only one end issues GET, SET, or GET-BULK
   operations while the other end issues notifications.

   SNMP does not specify when GET, SET, and GET-BULK are to be executed,
   as these choices are left to the application or the user.  Therefore,
   the analysis given for NETCONF regarding deferral is just as
   applicable to SNMP.  However, in the case of notifications, SNMP does
   specify when these occur based on the MIB definitions.  Had the
   designers of SNMP version 3 not allowed for the SNMP-TARGET-MIB, a
   change to the protocol base would have been required.  But because
   such a MIB exists, all that remains is how it should be configured.
   There are two cases:
   o  It is desired that no events be deferred and the agent connect to
      the manager, just as would be the case in RFC 3430.  In this case,
      the SNMP-TARGET-MIB is configured externally to use (presumably)
      the SSHSM security model to contact the manager when a
      notification is to be sent.  The SSHSM will define initial
      connection semantics.
   o  It is desired that notifications be deferred until the manager
      contacts the agent.  Here once the SSHSM subsystem is invoked by
      the manager, a policy is triggered to configure the SNMP-TARGET-
      MIB to receive events appropriate to the manager.

   The following is speculative as work on [14] is not complete.  That
   document specifies a means to extend the SNMP protocol to use SSH.
   SSH establishes a session and will to SNMP via SSHSM a securityName
   that may be used for purposes of authorization.  Once established the
   connection may be used for any purpose, no matter the original
   purpose in a vein similar to that specified by RFC 3430 [15] provided

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   each end is properly authorized.  Once again, it would be a
   configuration error for a device to connect for the purposes of being
   monitored or configured by a manager to not accept any operations.
   It would similarly be a configuration error for a device to connect
   for purposes of sending notifications but then not have any possibly

9.  Example 3: Remote terminal access via Call Home SSH

   Consider the case of a device that is managed by administration that
   resides on the other (public, if you will) side of a firewall.  When
   the device starts it initiates an SSH connection, perhaps with the
   intent of starting a netconf or SSHSM session.  When a problem
   arises, however, the administrator may want interactive access to the
   device to debug the problem.  The administrator makes use of a tool
   on the network management station that causes the NMS to request a
   "session"connection, thus allowing the administrator interactive
   command line access even through the device initiated the connection.

   Section 6.1 of [16] rightly encourages client implementations to
   reject such requests.  However, if they are prepared to trust the
   device they are connecting to for maintenance and debugging purposes,
   the benefit may outweigh the risks.  In all cases, the session should
   be properly authorized, meaning that the agent should be configured
   to allow appropriate access to those who have appropriate access to
   the NMS, and the NMS should properly authenticate and authorize that

   In this example, a naming method must be employed at the very least
   by the NMS in order to properly identify the correct device to
   connect to.

10.  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.

11.  Summary

   Call Home is a useful - and in some circumstances necessary -
   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

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   intermittent connectivity.

12.  Informational References

   [1]   Nowitz, D., Lesk, M., and G. Chesson, "A Dial-8p Network of
         UNIX Systems", UNIX System 7 , August 1978.

   [2]   Postel, J., "Simple Mail Transfer Protocol", STD 10, RFC 821,
         August 1982.

   [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]   Nachbar, D., "When Network File Systems Aren't Enough:
         Automatic Software Distribution Revisited", Proceedings of
         Usenix Summer 1986 , June 1986.

   [6]   Pleasant, M. and E. Lear, "Transcending Administrative domains
         by Automating System Management Tasks in a Large Heterogeneous
         Environment", Usenix Software Security Workshop , April 1989.

   [7]   Bergman, R., Lewis, H., and I. McDonald, "Printer MIB v2",
         RFC 3805, June 2004.

   [8]   Johnson, D., Perkins, C., and J. Arkko, "Mobility Support in
         IPv6", RFC 3775, June 2004.

   [9]   Case, J., "UPS Management Information Base", RFC 1628,
         May 1994.

   [10]  Wellington, B., "Secure Domain Name System (DNS) Dynamic
         Update", RFC 3007, November 2000.

   [11]  International International Telephone and Telegraph
         Consultative Committee, "Information Technology - Open Systems
         Interconnection - The Directory: Authentication Framework",
         CCITT Recommendation X.509, November 1988.

   [12]  Enns, R., "NETCONF Configuration Protocol",
         draft-ietf-netconf-prot-08 (work in progress), September 2005.

   [13]  Wasserman, M. and T. Goddard, "Using the NETCONF Configuration
         Protocol over Secure Shell (SSH)", draft-ietf-netconf-ssh-04
         (work in progress), April 2005.

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   [14]  Harrington, D., "Secure Shell Security Model for SNMP",
         draft-harrington-isms-secshell-01 (work in progress),
         September 2005.

   [15]  Schoenwaelder, J., "Simple Network Management Protocol Over
         Transmission Control Protocol Transport Mapping", RFC 3430,
         December 2002.

   [16]  Lonvick, C. and T. Ylonen, "SSH Connection Protocol",
         draft-ietf-secsh-connect-25 (work in progress), March 2005.

Appendix A.  Changes
   o  From -02 to -03: Added naming and interactive terminal sections.
   o  From -01 to -02: reworded limitations of UDP and call home.
      Expanded security considerations.  Spell-checked.
   o  From -00 to -01: provided more detail on Call Home applicability
      in the cases of unicast session based versus other.  Discussed the
      difference between p2p protocols versus client server.  Provided
      more examples.

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Author's Address

   Eliot Lear
   Cisco Systems GmbH
   Glattzentrum, ZH  CH-8301

   Phone: +41 1 878 7525
   Email: lear@cisco.com

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