rfc2196.txt

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   There may be regulatory requirements that affect some aspects of your
   security policy (e.g., line monitoring).  The creators of the
   security policy should consider seeking legal assistance in the
   creation of the policy.  At a minimum, the policy should be reviewed
   by legal counsel.

   Once your security policy has been established it should be clearly
   communicated to users, staff, and management.  Having all personnel
   sign a statement indicating that they have read, understood, and
   agreed to abide by the policy is an important part of the process.
   Finally, your policy should be reviewed on a regular basis to see if
   it is successfully supporting your security needs.





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RFC 2196              Site Security Handbook              September 1997


2.3  Keeping the Policy Flexible

   In order for a security policy to be viable for the long term, it
   requires a lot of flexibility based upon an architectural security
   concept. A security policy should be (largely) independent from
   specific hardware and software situations (as specific systems tend
   to be replaced or moved overnight).  The mechanisms for updating the
   policy should be clearly spelled out.  This includes the process, the
   people involved, and the people who must sign-off on the changes.

   It is also important to recognize that there are exceptions to every
   rule.  Whenever possible, the policy should spell out what exceptions
   to the general policy exist.  For example, under what conditions is a
   system administrator allowed to go through a user's files.  Also,
   there may be some cases when multiple users will have access to the
   same userid.  For example, on systems with a "root" user, multiple
   system administrators may know the password and use the root account.

   Another consideration is called the "Garbage Truck Syndrome."  This
   refers to what would happen to a site if a key person was suddenly
   unavailable for his/her job function (e.g., was suddenly ill or left
   the company unexpectedly).  While the greatest security resides in
   the minimum dissemination of information, the risk of losing critical
   information increases when that information is not shared.  It is
   important to determine what the proper balance is for your site.

3.  Architecture

3.1  Objectives

3.1.1  Completely Defined Security Plans

   All sites should define a comprehensive security plan.  This plan
   should be at a higher level than the specific policies discussed in
   chapter 2, and it should be crafted as a framework of broad
   guidelines into which specific policies will fit.

   It is important to have this framework in place so that individual
   policies can be consistent with the overall site security
   architecture.  For example, having a strong policy with regard to
   Internet access and having weak restrictions on modem usage is
   inconsistent with an overall philosophy of strong security
   restrictions on external access.

   A security plan should define: the list of network services that will
   be provided; which areas of the organization will provide the
   services; who will have access to those services; how access will be
   provided; who will administer those services; etc.



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RFC 2196              Site Security Handbook              September 1997


   The plan should also address how incident will be handled.  Chapter 5
   provides an in-depth discussion of this topic, but it is important
   for each site to define classes of incidents and corresponding
   responses.  For example, sites with firewalls should set a threshold
   on the number of attempts made to foil the firewall before triggering
   a response?  Escallation levels should be defined for both attacks
   and responses.  Sites without firewalls will have to determine if a
   single attempt to connect to a host constitutes an incident? What
   about a systematic scan of systems?

   For sites connected to the Internet, the rampant media magnification
   of Internet related security incidents can overshadow a (potentially)
   more serious internal security problem.  Likewise, companies who have
   never been connected to the Internet may have strong, well defined,
   internal policies but fail to adequately address an external
   connection policy.

3.1.2  Separation of Services

   There are many services which a site may wish to provide for its
   users, some of which may be external.  There are a variety of
   security reasons to attempt to isolate services onto dedicated host
   computers.  There are also performance reasons in most cases, but a
   detailed discussion is beyond to scope of this document.

   The services which a site may provide will, in most cases, have
   different levels of access needs and models of trust.  Services which
   are essential to the security or smooth operation of a site would be
   better off being placed on a dedicated machine with very limited
   access (see Section 3.1.3 "deny all" model), rather than on a machine
   that provides a service (or services) which has traditionally been
   less secure, or requires greater accessability by users who may
   accidentally suborn security.

   It is also important to distinguish between hosts which operate
   within different models of trust (e.g., all the hosts inside of a
   firewall and any host on an exposed network).

   Some of the services which should be examined for potential
   separation are outlined in section 3.2.3. It is important to remember
   that security is only as strong as the weakest link in the chain.
   Several of the most publicized penetrations in recent years have been
   through the exploitation of vulnerabilities in electronic mail
   systems.  The intruders were not trying to steal electronic mail, but
   they used the vulnerability in that service to gain access to other
   systems.





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RFC 2196              Site Security Handbook              September 1997


   If possible, each service should be running on a different machine
   whose only duty is to provide a specific service.  This helps to
   isolate intruders and limit potential harm.

3.1.3  Deny all/ Allow all

   There are two diametrically opposed underlying philosophies which can
   be adopted when defining a security plan.  Both alternatives are
   legitimate models to adopt, and the choice between them will depend
   on the site and its needs for security.

   The first option is to turn off all services and then selectively
   enable services on a case by case basis as they are needed. This can
   be done at the host or network level as appropriate.  This model,
   which will here after be referred to as the "deny all" model, is
   generally more secure than the other model described in the next
   paragraph.  More work is required to successfully implement a "deny
   all" configuration as well as a better understanding of services.
   Allowing only known services provides for a better analysis of a
   particular service/protocol and the design of a security mechanism
   suited to the security level of the site.

   The other model, which will here after be referred to as the "allow
   all" model, is much easier to implement, but is generally less secure
   than the "deny all" model.  Simply turn on all services, usually the
   default at the host level, and allow all protocols to travel across
   network boundaries, usually the default at the router level.  As
   security holes become apparent, they are restricted or patched at
   either the host or network level.

   Each of these models can be applied to different portions of the
   site, depending on functionality requirements, administrative
   control, site policy, etc.  For example, the policy may be to use the
   "allow all" model when setting up workstations for general use, but
   adopt a "deny all" model when setting up information servers, like an
   email hub.  Likewise, an "allow all" policy may be adopted for
   traffic between LAN's internal to the site, but a "deny all" policy
   can be adopted between the site and the Internet.

   Be careful when mixing philosophies as in the examples above.  Many
   sites adopt the theory of a hard "crunchy" shell and a soft "squishy"
   middle.  They are willing to pay the cost of security for their
   external traffic and require strong security measures, but are
   unwilling or unable to provide similar protections internally.  This
   works fine as long as the outer defenses are never breached and the
   internal users can be trusted.  Once the outer shell (firewall) is
   breached, subverting the internal network is trivial.




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RFC 2196              Site Security Handbook              September 1997


3.1.4  Identify Real Needs for Services

   There is a large variety of services which may be provided, both
   internally and on the Internet at large.  Managing security is, in
   many ways, managing access to services internal to the site and
   managing how internal users access information at remote sites.

   Services tend to rush like waves over the Internet.  Over the years
   many sites have established anonymous FTP servers, gopher servers,
   wais servers, WWW servers, etc. as they became popular, but not
   particularly needed, at all sites.  Evaluate all new services that
   are established with a skeptical attitude to determine if they are
   actually needed or just the current fad sweeping the Internet.

   Bear in mind that security complexity can grow exponentially with the
   number of services provided.  Filtering routers need to be modified
   to support the new protocols.  Some protocols are inherently
   difficult to filter safely (e.g., RPC and UDP services), thus
   providing more openings to the internal network.  Services provided
   on the same machine can interact in catastrophic ways.  For example,
   allowing anonymous FTP on the same machine as the WWW server may
   allow an intruder to place a file in the anonymous FTP area and cause
   the HTTP server to execute it.

3.2  Network and Service Configuration

3.2.1  Protecting the Infrastructure

   Many network administrators go to great lengths to protect the hosts
   on their networks.  Few administrators make any effort to protect the
   networks themselves.  There is some rationale to this.  For example,
   it is far easier to protect a host than a network.  Also, intruders
   are likely to be after data on the hosts; damaging the network would
   not serve their purposes.  That said, there are still reasons to
   protect the networks.  For example, an intruder might divert network
   traffic through an outside host in order to examine the data (i.e.,
   to search for passwords).  Also, infrastructure includes more than
   the networks and the routers which interconnect them.  Infrastructure
   also includes network management (e.g., SNMP), services (e.g., DNS,
   NFS, NTP, WWW), and security (i.e., user authentication and access
   restrictions).

   The infrastructure also needs protection against human error.  When
   an administrator misconfigures a host, that host may offer degraded
   service.  This only affects users who require that host and, unless






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RFC 2196              Site Security Handbook              September 1997


   that host is a primary server, the number of affected users will
   therefore be limited.  However, if a router is misconfigured, all
   users who require the network will be affected.  Obviously, this is a
   far larger number of users than those depending on any one host.

3.2.2  Protecting the Network

   There are several problems to which networks are vulnerable.  The
   classic problem is a "denial of service" attack.  In this case, the
   network is brought to a state in which it can no longer carry
   legitimate users' data.  There are two common ways this can be done:
   by attacking the routers and by flooding the network with extraneous
   traffic.  Please note that the term "router" in this section is used
   as an example of a larger class of active network interconnection
   components that also includes components like firewalls, proxy-
   servers, etc.

   An attack on the router is designed to cause it to stop forwarding
   packets, or to forward them improperly.  The former case may be due
   to a misconfiguration, the injection of a spurious routing update, or
   a "flood attack" (i.e., the router is bombarded with unroutable
   packets, causing its performance to degrade).  A flood attack on a
   network is similar to a flood attack on a router, except that the
   flood packets are usually broadcast.  An ideal flood attack would be
   the injection of a single packet which exploits some known flaw in
   the network nodes and causes them to retransmit the packet, or