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Network Working Group M. Kaeo Request for Comments: 4778 Double Shot Security, Inc. Category: Informational January 2007

             Current Operational Security Practices in
               Internet Service Provider Environments

Status of This Memo

 This memo provides information for the Internet community.  It does
 not specify an Internet standard of any kind.  Distribution of this
 memo is unlimited.

Copyright Notice

 Copyright (C) The IETF Trust (2007).


 This document is a survey of the current practices used in today's
 large ISP operational networks to secure layer 2 and layer 3
 infrastructure devices.  The information listed here is the result of
 information gathered from people directly responsible for defining
 and implementing secure infrastructures in Internet Service Provider

Kaeo Informational [Page 1] RFC 4778 OPSEC Practices January 2007

Table of Contents

 1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  2
   1.1.  Scope  . . . . . . . . . . . . . . . . . . . . . . . . . .  2
   1.2.  Threat Model . . . . . . . . . . . . . . . . . . . . . . .  3
   1.3.  Attack Sources . . . . . . . . . . . . . . . . . . . . . .  4
   1.4.  Operational Security Impact from Threats . . . . . . . . .  5
   1.5.  Document Layout  . . . . . . . . . . . . . . . . . . . . .  7
 2.  Protected Operational Functions  . . . . . . . . . . . . . . .  8
   2.1.  Device Physical Access . . . . . . . . . . . . . . . . . .  8
   2.2.  Device Management - In-Band and Out-of-Band (OOB)  . . . . 10
   2.3.  Data Path  . . . . . . . . . . . . . . . . . . . . . . . . 16
   2.4.  Routing Control Plane  . . . . . . . . . . . . . . . . . . 18
   2.5.  Software Upgrades and Configuration
         Integrity/Validation . . . . . . . . . . . . . . . . . . . 22
   2.6.  Logging Considerations . . . . . . . . . . . . . . . . . . 26
   2.7.  Filtering Considerations . . . . . . . . . . . . . . . . . 29
   2.8.  Denial-of-Service Tracking/Tracing . . . . . . . . . . . . 30
 3.  Security Considerations  . . . . . . . . . . . . . . . . . . . 32
 4.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 32
 5.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 33
   5.1.  Normative References . . . . . . . . . . . . . . . . . . . 33
   5.2.  Informational References . . . . . . . . . . . . . . . . . 33
 Appendix A.  Protocol Specific Attacks . . . . . . . . . . . . . . 34
   A.1.  Layer 2 Attacks  . . . . . . . . . . . . . . . . . . . . . 34
   A.2.  IPv4 Protocol-Based Attacks  . . . . . . . . . . . . . . . 34
   A.3.  IPv6 Attacks . . . . . . . . . . . . . . . . . . . . . . . 36

1. Introduction

 Security practices are well understood by the network operators who
 have, for many years, gone through the growing pains of securing
 their network infrastructures.  However, there does not exist a
 written document that enumerates these security practices.  Network
 attacks are continually increasing and although it is not necessarily
 the role of an ISP to act as the Internet police, each ISP has to
 ensure that certain security practices are followed to ensure that
 their network is operationally available for their customers.  This
 document is the result of a survey conducted to find out what current
 security practices are being deployed to secure network

1.1. Scope

 The scope for this survey is restricted to security practices that
 mitigate exposure to risks with the potential to adversely impact
 network availability and reliability.  Securing the actual data
 traffic is outside the scope of the conducted survey.  This document

Kaeo Informational [Page 2] RFC 4778 OPSEC Practices January 2007

 focuses solely on documenting currently deployed security mechanisms
 for layer 2 and layer 3 network infrastructure devices.  Although
 primarily focused on IPv4, many of the same practices can (and
 should) apply to IPv6 networks.  Both IPv4 and IPv6 network
 infrastructures are taken into account in this survey.

1.2. Threat Model

 A threat is a potential for a security violation, which exists when
 there is a circumstance, capability, action, or event that could
 breach security and cause harm [RFC2828].  Every operational network
 is subject to a multitude of threat actions, or attacks, i.e., an
 assault on system security that derives from an intelligent act that
 is a deliberate attempt to evade security services, and violate the
 security policy of a system [RFC2828].  Many of the threats to a
 network infrastructure occur from an instantiation (or combination)
 of the following:
 Reconnaissance: An attack whereby information is gathered to
 ascertain the network topology or specific device information, which
 can be further used to exploit known vulnerabilities
 Man-In-The-Middle: An attack where a malicious user impersonates
 either the sender or recipient of a communication stream while
 inserting, modifying, or dropping certain traffic.  This type of
 attack also covers phishing and session hijacks.
 Protocol Vulnerability Exploitation: An attack that takes advantage
 of known protocol vulnerabilities due to design or implementation
 flaws to cause inappropriate behavior.
 Message Insertion: This can be a valid message (it could be a reply
 attack, which is a scenario where a message is captured and resent at
 a later time).  A message can also be inserted with any of the fields
 in the message being spoofed, such as IP addresses, port numbers,
 header fields, or even packet content.  Flooding is also part of this
 threat instantiation.
 Message Diversion/Deletion: An attack where legitimate messages are
 removed before they can reach the desired recipient, or are
 re-directed to a network segment that is normally not part of the
 data path.
 Message Modification: This is a subset of a message insertion attack
 where a previous message has been captured and modified before being
 retransmitted.  The message can be captured using a man-in-the-middle
 attack or message diversion.

Kaeo Informational [Page 3] RFC 4778 OPSEC Practices January 2007

 Note that sometimes denial-of-service attacks are listed as separate
 categories.  A denial-of-service is a consequence of an attack and
 can be the result of too much traffic (i.e., flooding), exploiting
 protocol exploitation, or inserting/deleting/diverting/modifying

1.3. Attack Sources

 These attacks can be sourced in a variety of ways:
 Active vs Passive Attacks
    An active attack involves writing data to the network.  It is
    common practice in active attacks to disguise one's address and
    conceal the identity of the traffic sender.  A passive attack
    involves only reading information off the network.  This is
    possible if the attacker has control of a host in the
    communications path between two victim machines, or has
    compromised the routing infrastructure to specifically arrange
    that traffic pass through a compromised machine.  There are also
    situations where mirrored traffic (often used for debugging,
    performance monitoring, or accounting purposes) is diverted to a
    compromised machine, which would not necessarily subvert any
    existing topology, and could be harder to detect.  In general, the
    goal of a passive attack is to obtain information that the sender
    and receiver would prefer to remain private [RFC3552].
 On-path vs Off-path Attacks
    In order for a datagram to be transmitted from one host to
    another, it generally must traverse some set of intermediate links
    and routers.  Such routers are naturally able to read, modify, or
    remove any datagram transmitted along that path.  This makes it
    much easier to mount a wide variety of attacks if you are on-path.
    Off-path hosts can transmit arbitrary datagrams that appear to
    come from any host but cannot necessarily receive datagrams
    intended for other hosts.  Thus, if an attack depends on being
    able to receive data, off-path hosts must first subvert the
    topology in order to place themselves on-path.  This is by no
    means impossible, but is not necessarily trivial [RFC3552].  A
    more subtle attack is one where the traffic-mirroring capability
    of a device is hijacked and the traffic is diverted to a
    compromised host since the network topology may not need to be

Kaeo Informational [Page 4] RFC 4778 OPSEC Practices January 2007

 Insider vs Outsider Attacks
    An "insider attack" is initiated from inside a given security
    perimeter by an entity that is authorized to access system
    resources, but uses them in a way not approved by those who
    granted the authorization.  An "outside attack" is initiated from
    outside the perimeter by an unauthorized or illegitimate user of
    the system.
 Deliberate Attacks vs Unintentional Events
    A deliberate attack is where a miscreant intentionally performs an
    assault on system security.  However, there are also instances
    where unintentional events cause the same harm, yet are performed
    without malicious intent.  Configuration errors and software bugs
    can be as devastating to network availability as any deliberate
    attack on the network infrastructure.
 The attack source can be a combination of any of the above, all of
 which need to be considered when trying to ascertain the impact any
 attack can have on the availability and reliability of the network.
 It is nearly impossible to stop insider attacks or unintentional
 events.  However, if appropriate monitoring mechanisms are in place,
 these attacks can also be detected and mitigated as with any other
 attack source.  The amount of effort it takes to identify and trace
 an attack is, of course, dependent on the resourcefulness of the
 attacker.  Any of the specific attacks discussed further in this
 document will elaborate on malicious behavior, which are sourced by
 an "outsider" and are deliberate attacks.  Some further elaboration
 will be given to the feasibility of passive vs active and on-path vs
 off-path attacks to show the motivation behind deploying certain
 security features.

1.4. Operational Security Impact from Threats

 The main concern for any of the potential attack scenarios is the
 impact and harm it can cause to the network infrastructure.  The
 threat consequences are the security violations that results from a
 threat action, i.e., an attack.  These are typically classified as
 (Unauthorized) Disclosure
    A circumstance or event whereby an entity gains access to data for
    which the entity is not authorized.

Kaeo Informational [Page 5] RFC 4778 OPSEC Practices January 2007

    A circumstance or event that may result in an authorized entity
    receiving false data and believing it to be true.
    A circumstance or event that interrupts or prevents the correct
    operation of system services and functions.  A broad variety of
    attacks, collectively called denial of service attacks, threaten
    the availability of systems and bandwidth to legitimate users.
    Many such attacks are designed to consume machine resources,
    making it difficult or impossible to serve legitimate users.
    Other attacks cause the target machine to crash, completely
    denying service to users.
    A circumstance or event that results in control of system services
    or functions by an unauthorized entity.  Most network
    infrastructure systems are only intended to be completely
    accessible to certain authorized individuals.  Should an
    unauthorized person gain access to critical layer 2/layer 3
    infrastructure devices or services, they could cause great harm to
    the reliability and availability of the network.
 A complete description of threat actions that can cause these threat
 consequences can be found in [RFC2828].  Typically, a number of
 different network attacks are used in combination to cause one or
 more of the above-mentioned threat consequences.  An example would be
 a malicious user who has the capability to eavesdrop on traffic.
 First, he may listen in on traffic for a while, doing reconnaissance
 work and ascertaining which IP addresses belong to specific devices,
 such as routers.  Were this miscreant to obtain information, such as
 a router password sent in cleartext, he can then proceed to
 compromise the actual router.  From there, the miscreant can launch
 various active attacks, such as sending bogus routing updates to
 redirect traffic or capture additional traffic to compromise other
 network devices.  While this document enumerates which
 countermeasures ISPs are deploying today, a useful generic analysis
 of actual backbone infrastructure attacks and the appropriate
 countermeasures can be found in [RTGWG].

Kaeo Informational [Page 6] RFC 4778 OPSEC Practices January 2007

1.5. Document Layout

 This document is a survey of current operational practices that
 mitigate the risk of being susceptible to any threat actions.  As
 such, the main focus is on the currently deployed security practices
 used to detect and/or mitigate attacks.  The top-level categories in
 this document are based on operational functions for ISPs and
 generally relate to what is to be protected.  This is followed by a
 description of which attacks are possible and the security practices
 currently deployed.  This will provide the necessary security
 services to help mitigate these attacks.  These security services are
 classified as follows:
 o  User Authentication
 o  User Authorization
 o  Data Origin Authentication
 o  Access Control
 o  Data Integrity
 o  Data Confidentiality
 o  Auditing/Logging
 o  DoS Mitigation
 In many instances, a specific protocol currently deployed will offer
 a combination of these services.  For example, Authentication,
 Authorization, and Accounting (AAA) can offer user authentication,
 user authorization, and audit/logging services, while the Secure
 SHell (SSH) Protocol can provide data origin authentication, data
 integrity, and data confidentiality.  The services offered are more
 important than the actual protocol used.  Note that access control
 will refer basically to logical access control, i.e., filtering.
 Each section ends with an additional considerations section that
 explains why specific protocols may or may not be used, and also
 gives some information regarding capabilities, which are not possible
 today due to bugs or lack of usability.

Kaeo Informational [Page 7] RFC 4778 OPSEC Practices January 2007

2. Protected Operational Functions

2.1. Device Physical Access

 Device physical access pertains to protecting the physical location
 and access of the layer 2 or layer 3 network infrastructure device.
 Physical security is a large field of study/practice in and of
 itself, arguably the largest, oldest, and most well-understood area
 of security.  Although it is important to have contingency plans for
 natural disasters, such as earthquakes and floods, which can cause
 damage to networking devices, this is out of the scope of this
 document.  Here, we concern ourselves with protecting access to the
 physical location and how a device can be further protected from
 unauthorized access if the physical location has been compromised,
 i.e., protecting the console access.  This is aimed largely at
 stopping an intruder with physical access from gaining operational
 control of the device(s).  Note that nothing will stop an attacker
 with physical access from effecting a denial-of-service attack, which
 can be easily accomplished by powering off the device or just
 unplugging some cables.

2.1.1. Threats/Attacks

 If any intruder gets physical access to a layer 2 or layer 3 device,
 the entire network infrastructure can be under the control of the
 intruder.  At a minimum, the intruder can take the compromised device
 out of service, causing network disruption, the extent of which
 depends on the network topology.  A worse scenario is where the
 intruder uses this device to crack the console password, gaining
 complete control of the device (perhaps without anyone detecting such
 a compromise, or to attach another network device onto a port and
 siphon off data with which the intruder can ascertain the network
 topology) and the entire network.
 The threat of gaining physical access can be realized in a variety of
 ways, even if critical devices are under high security.  Cases still
 occur where attackers have impersonated maintenance workers to gain
 physical access to critical devices that have caused major outages
 and privacy compromises.  Insider attacks from authorized personnel
 also pose a real threat and must be adequately recognized and

2.1.2. Security Practices

 For physical device security, equipment is kept in highly restrictive
 environments.  Only authorized users with card-key badges have access
 to any of the physical locations that contain critical network
 infrastructure devices.  These card-key systems keep track of who

Kaeo Informational [Page 8] RFC 4778 OPSEC Practices January 2007

 accessed which location and at what time.  Most cardkey systems have
 a fail-back "master key" in case the card system is down.  This
 "master key" usually has limited access and its use is also carefully
 logged (which should only happen if the card-key system is NOT
 All console access is always password protected and the login time is
 set to time out after a specified amount of inactivity - typically
 between 3-10 minutes.  The type of privileges that you obtain from a
 console login varies between separate vendor devices.  In some cases
 you get initial basic access and need to perform a second
 authentication step to get more privileged access (i.e., enable or
 root).  In other vendors, you get the more privileged access when you
 log into the console as root, without requiring a second
 authentication step.
 How ISPs manage these logins vary greatly, although many of the
 larger ISPs employ some sort of AAA mechanism to help automate
 privilege-level authorization and utilize the automation to bypass
 the need for a second authentication step.  Also, many ISPs define
 separate classes of users to have different privileges while logged
 onto the console.  Typically, all console access is provided via an
 out-of-band (OOB) management infrastructure, which is discussed in
 Section 2.2 of this document.

2.1.3. Security Services

 The following security services are offered through the use of the
 practices described in the previous section:
 o  User Authentication - All individuals who have access to the
    physical facility are authenticated.  Console access is
 o  User Authorization - An authenticated individual has implicit
    authorization to perform commands on the device.  In some cases,
    multiple authentication is required to differentiate between basic
    and more privileged access.
 o  Data Origin Authentication - Not applicable.
 o  Access Control - Not applicable.
 o  Data Integrity - Not applicable.
 o  Data Confidentiality - Not applicable.

Kaeo Informational [Page 9] RFC 4778 OPSEC Practices January 2007

 o  Auditing/Logging - All access to the physical locations of the
    infrastructure equipment is logged via electronic card-key
    systems.  All console access is logged (refer to Section 2.2 of
    this document for more details).
 o  DoS Mitigation - Not applicable.

2.1.4. Additional Considerations

 Physical security is relevant to operational security practices as
 described in this document, mostly from a console-access perspective.
 Most ISPs provide console access via an OOB management
 infrastructure, which is discussed in Section 2.2 of this document.
 The physical and logical authentication and logging systems should be
 run independently of each other and should reside in different
 physical locations.  These systems need to be secured to ensure that
 they themselves will not be compromised, which could give the
 intruder valuable authentication and logging information.
 Social engineering plays a big role in many physical access
 compromises.  Most ISPs have set up training classes and awareness
 programs to educate company personnel to deny physical access to
 people who are not properly authenticated or authorized to have
 physical access to critical infrastructure devices.

2.2. Device Management - In-Band and Out-of-Band (OOB)

 In-band management is generally considered to be device access, where
 the control traffic takes the same data path as the data that
 traverses the network.  Out-of-band management is generally
 considered to be device access, where the control traffic takes a
 separate path as the data that traverses the network.  In many
 environments, device management for layer 2 and layer 3
 infrastructure devices is deployed as part of an out-of-band
 management infrastructure, although there are some instances where it
 is deployed in-band as well.  Note that while many of the security
 concerns and practices are the same for OOB management and in-band
 management, most ISPs prefer an OOB management system, since access
 to the devices that make up this management network are more
 vigilantly protected and considered to be less susceptible to
 malicious activity.
 Console access is always architected via an OOB network.  Presently,
 the mechanisms used for either in-band management or OOB are via
 virtual terminal access (i.e., Telnet or SSH), Simple Network
 Management Protocol (SNMP), or HTTP.  In all large ISPs that were
 interviewed, HTTP management was never used and was explicitly

Kaeo Informational [Page 10] RFC 4778 OPSEC Practices January 2007

 disabled.  Note that file transfer protocols (TFTP, FTP, and SCP)
 will be covered in Section 2.5 of this document.

2.2.1. Threats/Attacks

 For device management, passive attacks are possible if someone has
 the capability to intercept data between the management device and
 the managed device.  The threat is possible if a single
 infrastructure device is somehow compromised and can act as a network
 sniffer, or if it is possible to insert a new device that acts as a
 network sniffer.
 Active attacks are possible for both on-path and off-path scenarios.
 For on-path active attacks, the situation is the same as for a
 passive attack, where either a device has to already be compromised
 or a device can be inserted into the path.  For off-path active
 attacks, where a topology subversion is required to reroute traffic
 and essentially bring the attacker on-path, the attack is generally
 limited to message insertion or modification. Confidentiality Violations

 Confidentiality violations can occur when a miscreant intercepts any
 management data that has been sent in cleartext or with weak
 encryption.  This includes interception of usernames and passwords
 with which an intruder can obtain unauthorized access to network
 devices.  It can also include other information, such as logging or
 configuration information, if an administrator is remotely viewing
 local logfiles or configuration information. Offline Cryptographic Attacks

 If username/password information was encrypted but the cryptographic
 mechanism used made it easy to capture data and break the encryption
 key, the device management traffic could be compromised.  The traffic
 would need to be captured either by eavesdropping on the network or
 by being able to divert traffic to a malicious user. Replay Attacks

 For a replay attack to be successful, the management traffic would
 need to first be captured either on-path or diverted to an attacker
 to later be replayed to the intended recipient.

Kaeo Informational [Page 11] RFC 4778 OPSEC Practices January 2007 Message Insertion/Deletion/Modification

 Data can be manipulated by someone in control of intermediary hosts.
 Forging data is also possible with IP spoofing, where a remote host
 sends out packets that appear to come from another, trusted host. Man-In-The-Middle

 A man-in-the-middle attack attacks the identity of a communicating
 peer rather than the data stream itself.  The attacker intercepts
 traffic that is sent from a management system to the networking
 infrastructure device and traffic that is sent from the network
 infrastructure device to the management system.

2.2.2. Security Practices

 OOB management is done via a terminal server at each location.  SSH
 access is used to get to the terminal server from where sessions to
 the devices are initiated.  Dial-in access is deployed as a backup if
 the network is not available.  However, it is common to use dial-
 back, encrypting modems, and/or one-time-password (OTP) modems to
 avoid the security weaknesses of plain dial-in access.
 All in-band management and OOB management access to layer 2 and layer
 3 devices is authenticated.  The user authentication and
 authorization is typically controlled by an AAA server (i.e., Remote
 Authentication Dial-in User Service (RADIUS) and/or Terminal Access
 Controller Access-Control System (TACACS+)).  Credentials used to
 determine the identity of the user vary from static username/password
 to one-time username/password schemes such as Secure-ID.  Static
 username/passwords are expired after a specified period of time,
 usually 30 days.  Every authenticated entity via AAA is an individual
 user for greater granularity of control.  Note that often the AAA
 server used for OOB management authentication is a separate physical
 device from the AAA server used for in-band management user
 authentication.  In some deployments, the AAA servers used for device
 management authentication/authorization/accounting are on separate
 networks to provide a demarcation for any other authentication
 For backup purposes, there is often a single local database entry for
 authentication that is known to a very limited set of key personnel.
 It is usually the highest privilege-level username/password
 combination, which in most cases is the same across all devices.
 This local device password is routinely regenerated once every 2-3
 months, and is also regenerated immediately after an employee who had
 access to that password leaves the company or is no longer authorized
 to have knowledge of that password.

Kaeo Informational [Page 12] RFC 4778 OPSEC Practices January 2007

 Each individual user in the AAA database is configured with specific
 authorization capability.  Specific commands are either individually
 denied or permitted, depending on the capability of the device to be
 accessed.  Multiple privilege levels are deployed.  Most individuals
 are authorized with basic authorization to perform a minimal set of
 commands, while a subset of individuals are authorized to perform
 more privileged commands.  Securing the AAA server is imperative and
 access to the AAA server itself is strictly controlled.  When an
 individual leaves the company, his/her AAA account is immediately
 deleted and the TACACS/RADIUS shared secret is reset for all devices.
 Some management functions are performed using command line interface
 (CLI) scripting.  In these scenarios, a dedicated user is used for
 the identity in scripts that perform CLI scripting.  Once
 authenticated, these scripts control which commands are legitimate,
 depending on authorization rights of the authenticated individual.
 SSH is always used for virtual terminal access to provide for an
 encrypted communication channel.  There are exceptions due to
 equipment limitations which are described in the additional
 considerations section.
 If SNMP is used for management, it is for read queries only and
 restricted to specific hosts.  If possible, the view is also
 restricted to only send the information that the management station
 needs, rather than expose the entire configuration file with the
 read-only SNMP community.  The community strings are carefully chosen
 to be difficult to crack and there are procedures in place to change
 these community strings between 30-90 days.  If systems support two
 SNMP community strings, the old string is replaced by first
 configuring a second, newer community string and then migrating over
 from the currently used string to the newer one.  Most large ISPs
 have multiple SNMP systems accessing their routers so it takes more
 then one maintenance period to get all the strings fixed in all the
 right systems.  SNMP RW is not used and is disabled by configuration.
 Access control is strictly enforced for infrastructure devices by
 using stringent filtering rules.  A limited set of IP addresses are
 allowed to initiate connections to the infrastructure devices and are
 specific to the services to which they are to limited (i.e., SSH and
 All device management access is audited and any violations trigger
 alarms that initiate automated email, pager, and/or telephone
 notifications.  AAA servers keep track of the authenticated entity as
 well as all the commands that were carried out on a specific device.
 Additionally, the device itself logs any access control violations
 (i.e., if an SSH request comes in from an IP address that is not

Kaeo Informational [Page 13] RFC 4778 OPSEC Practices January 2007

 explicitly permitted, that event is logged so that the offending IP
 address can be tracked down and investigations made as to why it was
 trying to access a particular infrastructure device)

2.2.3. Security Services

 The security services offered for device OOB management are nearly
 identical to those of device in-band management.  Due to the critical
 nature of controlling and limiting device access, many ISPs feel that
 physically separating the management traffic from the normal customer
 data traffic will provide an added level of risk mitigation and limit
 the potential attack vectors.  The following security services are
 offered through the use of the practices described in the previous
 o  User Authentication - All individuals are authenticated via AAA
 o  User Authorization - All individuals are authorized via AAA
    services to perform specific operations once successfully
 o  Data Origin Authentication - Management traffic is strictly
    filtered to allow only specific IP addresses to have access to the
    infrastructure devices.  This does not alleviate risk the from
    spoofed traffic, although when combined with edge filtering using
    BCP38 [RFC2827] and BCP84 [RFC3704] guidelines (discussed in
    Section 2.5), then the risk of spoofing is mitigated, barring a
    compromised internal system.  Also, using SSH for device access
    ensures that no one can spoof the traffic during the SSH session.
 o  Access Control - Management traffic is filtered to allow only
    specific IP addresses to have access to the infrastructure
 o  Data Integrity - Using SSH provides data integrity and ensures
    that no one has altered the management data in transit.
 o  Data Confidentiality - Using SSH provides data confidentiality.
 o  Auditing/Logging - Using AAA provides an audit trail for who
    accessed which device and which operations were performed.
 o  DoS Mitigation - Using packet filters to allow only specific IP
    addresses to have access to the infrastructure devices.  This
    limits but does not prevent spoofed DoS attacks directed at an
    infrastructure device.  However, the risk is lowered by using a
    separate physical network for management purposes.

Kaeo Informational [Page 14] RFC 4778 OPSEC Practices January 2007

2.2.4. Additional Considerations

 Password selection for any device management protocol used is
 critical to ensure that the passwords are hard to guess or break
 using a brute-force attack.
 IP security (IPsec) is considered too difficult to deploy, and the
 common protocol to provide for confidential management access is SSH.
 There are exceptions for using SSH due to equipment limitations since
 SSH may not be supported on legacy equipment.  In some cases,
 changing the host name of a device requires an SSH rekey event since
 the key is based on some combination of host name, Message
 Authentication Code (MAC) address, and time.  Also, in the case where
 the SSH key is stored on a route processor card, a re-keying of SSH
 would be required whenever the route processor card needs to be
 swapped.  Some providers feel that this operational impact exceeds
 the security necessary and instead use Telnet from trusted inside
 hosts (called 'jumphosts' or 'bastion hosts') to manage those
 devices.  An individual would first SSH to the jumphost and then
 Telnet from the jumphost to the actual infrastructure device, fully
 understanding that any passwords will be sent in the clear between
 the jumphost and the device to which it is connecting.  All
 authentication and authorization is still carried out using AAA
 In instances where Telnet access is used, the logs on the AAA servers
 are more verbose and more attention is paid to them to detect any
 abnormal behavior.  The jumphosts themselves are carefully controlled
 machines and usually have limited access.  Note that Telnet is NEVER
 allowed to an infrastructure device except from specific jumphosts;
 i.e., packet filters are used at the console server and/or
 infrastructure device to ensure that Telnet is only allowed from
 specific IP addresses.
 With thousands of devices to manage, some ISPs have created automated
 mechanisms to authenticate to devices.  As an example, Kerberos has
 been used to automate the authentication process for devices that
 have support for Kerberos.  An individual would first log in to a
 Kerberized UNIX server using SSH and generate a Kerberos 'ticket'.
 This 'ticket' is generally set to have a lifespan of 10 hours and is
 used to automatically authenticate the individual to the
 infrastructure devices.
 In instances where SNMP is used, some legacy devices only support
 SNMPv1, which then requires the provider to mandate its use across
 all infrastructure devices for operational simplicity.  SNMPv2 is
 primarily deployed since it is easier to set up than v3.

Kaeo Informational [Page 15] RFC 4778 OPSEC Practices January 2007

2.3. Data Path

 This section refers to how traffic is handled that traverses the
 network infrastructure device.  The primary goal of ISPs is to
 forward customer traffic.  However, due to the large amount of
 malicious traffic that can cause DoS attacks and render the network
 unavailable, specific measures are sometimes deployed to ensure the
 availability to forward legitimate customer traffic.

2.3.1. Threats/Attacks

 Any data traffic can potentially be attack traffic and the challenge
 is to detect and potentially stop forwarding any of the malicious
 traffic.  The deliberately sourced attack traffic can consist of
 packets with spoofed source and/or destination addresses or any other
 malformed packet that mangle any portion of a header field to cause
 protocol-related security issues (such as resetting connections,
 causing unwelcome ICMP redirects, creating unwelcome IP options, or
 packet fragmentations).

2.3.2. Security Practices

 Filtering and rate limiting are the primary mechanism to provide risk
 mitigation of malicious traffic rendering the ISP services
 unavailable.  However, filtering and rate limiting of data path
 traffic is deployed in a variety of ways, depending on how automated
 the process is and what the capabilities and performance limitations
 of the existing deployed hardware are.
 The ISPs that do not have performance issues with their equipment
 follow BCP38 [RFC2827] and BCP84 [RFC3704] guidelines for ingress
 filtering.  BCP38 recommends filtering ingress packets with obviously
 spoofed and/or 'reserved' source addresses to limit the effects of
 denial-of-service attacks, while BCP84 extends the recommendation for
 multi-homed environments.  Filters are also used to help alleviate
 issues between service providers.  Without any filtering, an
 inter-exchange peer could steal transit just by using static routes,
 and essentially redirect data traffic.  Therefore, some ISPs have
 implemented ingress/egress filters that block unexpected source and
 destination addresses not defined in the above-mentioned documents.
 Null routes and black-hole triggered routing [RFC3882] are used to
 deter any detected malicious traffic streams.  These two techniques
 are described in more detail in Section 2.8 below.
 Most ISPs consider layer 4 filtering useful, but it is only
 implemented if performance limitations allow for it.  Since it poses
 a large administrative overhead and ISPs are very much opposed to
 acting as the Internet firewall, Layer 4 filtering is typically

Kaeo Informational [Page 16] RFC 4778 OPSEC Practices January 2007

 implemented as a last option.  Netflow is used for tracking traffic
 flows, but there is some concern whether sampling is good enough to
 detect malicious behavior.
 Unicast Reverse Path Forwarding (RPF) is not consistently
 implemented.  Some ISPs are in the process of doing so, while other
 ISPs think that the perceived benefit of knowing that spoofed traffic
 comes from legitimate addresses are not worth the operational
 complexity.  Some providers have a policy of implementing uRPF at
 link speeds of Digital Signal 3 (DS3) and below, which was due to the
 fact that all hardware in the network supported uRPF for DS3 speeds
 and below.  At higher-speed links, the uRPF support was inconsistent
 and it was easier for operational people to implement a consistent

2.3.3. Security Services

 o  User Authentication - Not applicable.
 o  User Authorization - Not applicable.
 o  Data Origin Authentication - When IP address filtering per BCP38,
    BCP84, and uRPF are deployed at network edges it can ensure that
    any spoofed traffic comes from at least a legitimate IP address
    and can be tracked.
 o  Access Control - IP address filtering and layer 4 filtering is
    used to deny forbidden protocols and limit traffic destined for
    infrastructure device itself.  Filters are also used to block
    unexpected source/destination addresses.
 o  Data Integrity - Not applicable.
 o  Data Confidentiality - Not applicable.
 o  Auditing/Logging - Filtering exceptions are logged for potential
    attack traffic.
 o  DoS Mitigation - Black-hole triggered filtering and rate-limiting
    are used to limit the risk of DoS attacks.

2.3.4. Additional Considerations

 For layer 2 devices, MAC address filtering and authentication is not
 used in large-scale deployments.  This is due to the problems it can
 cause when troubleshooting networking issues.  Port security becomes
 unmanageable at a large scale where thousands of switches are

Kaeo Informational [Page 17] RFC 4778 OPSEC Practices January 2007

 Rate limiting is used by some ISPs, although other ISPs believe it is
 not really useful, since attackers are not well-behaved and it
 doesn't provide any operational benefit over the complexity.  Some
 ISPs feel that rate limiting can also make an attacker's job easier
 by requiring the attacker to send less traffic to starve legitimate
 traffic that is part of a rate limiting scheme.  Rate limiting may be
 improved by developing flow-based rate-limiting capabilities with
 filtering hooks.  This would improve the performance as well as the
 granularity over current capabilities.
 Lack of consistency regarding the ability to filter, especially with
 respect to performance issues, cause some ISPs not to implement BCP38
 and BCP84 guidelines for ingress filtering.  One such example is at
 edge boxes, where up to 1000 T1s connecting into a router with an
 OC-12 (Optical Carrier) uplink.  Some deployed devices experience a
 large performance impact with filtering, which is unacceptable for
 passing customer traffic through, though ingress filtering (uRPF)
 might be applicable at the devices that are connecting these
 aggregation routers.  Where performance is not an issue, the ISPs
 make a tradeoff between management versus risk.

2.4. Routing Control Plane

 The routing control plane deals with all the traffic that is part of
 establishing and maintaining routing protocol information.

2.4.1. Threats/Attacks

 Attacks on the routing control plane can be from both passive or
 active sources.  Passive attacks are possible if someone has the
 capability to intercept data between the communicating routing peers.
 This can be accomplished if a single routing peer is somehow
 compromised and can act as a network sniffer, or if it is possible to
 insert a new device that acts as a network sniffer.
 Active attacks are possible for both on-path and off-path scenarios.
 For on-path active attacks, the situation is the same as for a
 passive attack, where either a device has to already be compromised
 or a device can be inserted into the path.  This may lead to an
 attacker impersonating a legitimate routing peer and exchanging
 routing information.  Unintentional active attacks are more common
 due to configuration errors, which cause legitimate routing peers to
 feed invalid routing information to other neighboring peers.
 For off-path active attacks, the attacks are generally limited to
 message insertion or modification, which can divert traffic to
 illegitimate destinations, causing traffic to never reach its
 intended destination.

Kaeo Informational [Page 18] RFC 4778 OPSEC Practices January 2007 Confidentiality Violations

 Confidentiality violations can occur when a miscreant intercepts any
 of the routing update traffic.  This is becoming more of a concern
 because many ISPs are classifying addressing schemes and network
 topologies as private and proprietary information.  It is also a
 concern because the routing protocol packets contain information that
 may show ways in which routing sessions could be spoofed or hijacked.
 This in turn could lead into a man-in-the-middle attack, where the
 miscreants can insert themselves into the traffic path or divert the
 traffic path and violate the confidentiality of user data. Offline Cryptographic Attacks

 If any cryptographic mechanism was used to provide for data integrity
 and confidentiality, an offline cryptographic attack could
 potentially compromise the data.  The traffic would need to be
 captured either by eavesdropping on the network or by being able to
 divert traffic to a malicious user.  Note that by using
 cryptographically protected routing information, the latter would
 require the cryptographic key to already be compromised anyway, so
 this attack is only feasible if a device was able to eavesdrop and
 capture the cryptographically protected routing information. Replay Attacks

 For a replay attack to be successful, the routing control plane
 traffic would need to first be captured either on-path or diverted to
 an attacker to later be replayed to the intended recipient.
 Additionally, since many of these protocols include replay protection
 mechanisms, these would also need to be subverted, if applicable. Message Insertion/Deletion/Modification

 Routing control plane traffic can be manipulated by someone in
 control of intermediate hosts.  In addition, traffic can be injected
 by forging IP addresses, where a remote router sends out packets that
 appear to come from another, trusted router.  If enough traffic is
 injected to be processed by limited memory routers, it can cause a
 DoS attack. Man-In-The-Middle

 A man-in-the-middle attack attacks the identity of a communicating
 peer rather than the data stream itself.  The attacker intercepts
 traffic that is sent from one routing peer to the other and
 communicates on behalf of one of the peers.  This can lead to a
 diversion of the user traffic to either an unauthorized receiving

Kaeo Informational [Page 19] RFC 4778 OPSEC Practices January 2007

 party or cause legitimate traffic to never reach its intended

2.4.2. Security Practices

 Securing the routing control plane takes many features, which are
 generally deployed as a system.  Message Digest 5 (MD5)
 authentication is used by some ISPs to validate the sending peer and
 to ensure that the data in transit has not been altered.  Some ISPs
 only deploy MD5 authentication at the customers' request.  Additional
 sanity checks to ensure with reasonable certainty that the received
 routing update was originated by a valid routing peer include route
 filters and the Generalized TTL Security Mechanism (GTSM) feature
 [RFC3682] (sometimes also referred to as the TTL-Hack).  The GTSM
 feature is used for protocols such as the Border Gateway Protocol
 (BGP), and makes use of a packet's Time To Live (TTL) field (IPv4) or
 Hop Limit (IPv6) to protect communicating peers.  If GTSM is used, it
 is typically deployed only in limited scenarios between internal BGP
 peers due to lack of consistent support between vendor products and
 operating system versions.
 Packet filters are used to limit which systems can appear as a valid
 peer, while route filters are used to limit which routes are believed
 to be from a valid peer.  In the case of BGP routing, a variety of
 policies are deployed to limit the propagation of invalid routing
 information.  These include: incoming and outgoing prefix filters for
 BGP customers, incoming and outgoing prefix filters for peers and
 upstream neighbors, incoming AS-PATH filter for BGP customers,
 outgoing AS-PATH filter towards peers and upstream neighbors, route
 dampening and rejecting selected attributes and communities.
 Consistency between these policies varies greatly and there is a
 definite distinction whether the other end is an end-site vs an
 internal peer vs another big ISP or customer.  Mostly ISPs do
 prefix-filter their end-site customers, but due to the operational
 constraints of maintaining large prefix filter lists, many ISPs are
 starting to depend on BGP AS-PATH filters to/from their peers and
 upstream neighbors.
 In cases where prefix lists are not used, operators often define a
 maximum prefix limit per peer to prevent misconfiguration (e.g.,
 unintentional de-aggregation or neighbor routing policy
 mis-configuration) or overload attacks.  ISPs need to coordinate with
 each other what the expected prefix exchange is, and increase this
 number by some sane amount.  It is important for ISPs to pad the
 max-prefix number enough to allow for valid swings in routing
 announcements, preventing an unintentional shut down of the BGP
 session.  Individual implementation amongst ISPs are unique, and
 depending on equipment supplier(s), different implementation options

Kaeo Informational [Page 20] RFC 4778 OPSEC Practices January 2007

 are available.  Most equipment vendors offer implementation options
 ranging from just logging excessive prefixes being received, to
 automatically shutting down the session.  If the option of
 reestablishing a session after some pre-configured idle timeout has
 been reached is available, it should be understood that automatically
 reestablishing the session may potentially introduce instability
 continuously into the overall routing table if a policy
 mis-configuration on the adjacent neighbor is causing the condition.
 If a serious mis-configuration on a peering neighbor has occurred,
 then automatically shutting down the session and leaving it shut down
 until being manually cleared, is sometimes best and allows for
 operator intervention to correct as needed.
 Some large ISPs require that routes be registered in an Internet
 Routing Registry (IRR), which can then be part of the Routing Assets
 Database (RADb) - a public registry of routing information for
 networks in the Internet that can be used to generate filter lists.
 Some ISPs, especially in Europe, require registered routes before
 agreeing to become an eBGP peer with someone.
 Many ISPs also do not propagate interface IP addresses to further
 reduce attack vectors on routers and connected customers.

2.4.3. Security Services

 o  User Authentication - Not applicable.
 o  User Authorization - Not applicable.
 o  Data Origin Authentication - By using MD5 authentication and/or
    the TTL-hack, a routing peer can be reasonably certain that
    traffic originated from a valid peer.
 o  Access Control - Route filters, AS-PATH filters, and prefix limits
    are used to control access to specific parts of the network.
 o  Data Integrity - By using MD5 authentication, a peer can be
    reasonably certain that the data has not been modified in transit,
    but there is no mechanism to prove the validity of the routing
    information itself.
 o  Data Confidentiality - Not implemented.
 o  Auditing / Logging - Filter exceptions are logged.

Kaeo Informational [Page 21] RFC 4778 OPSEC Practices January 2007

 o  DoS Mitigation - Many DoS attacks are mitigated using a
    combination of techniques including: MD5 authentication, the GTSM
    feature, filtering routing advertisements to bogons, and filtering
    routing advertisements to one's own network.

2.4.4. Additional Considerations

 So far the primary concern to secure the routing control plane has
 been to validate the sending peer and to ensure that the data in
 transit has not been altered.  Although MD5 routing protocol
 extensions have been implemented, which can provide both services,
 they are not consistently deployed amongst ISPs.  Two major
 deployment concerns have been implementation issues, where both
 software bugs and the lack of graceful re-keying options have caused
 significant network down times.  Also, some ISPs express concern that
 deploying MD5 authentication will itself be a worse DoS attack victim
 and prefer to use a combination of other risk mitigation mechanisms
 such as GTSM (for BGP) and route filters.  An issue with GTSM is that
 it is not supported on all devices across different vendors'
 IPsec is not deployed since the operational management aspects of
 ensuring interoperability and reliable configurations is too complex
 and time consuming to be operationally viable.  There is also limited
 concern to the confidentiality of the routing information.  The
 integrity and validity of the updates are of much greater concern.
 There is concern for manual or automated actions, which introduce new
 routes and can affect the entire routing domain.

2.5. Software Upgrades and Configuration Integrity/Validation

 Software upgrades and configuration changes are usually performed as
 part of either in-band or OOB management functions.  However, there
 are additional considerations to be taken into account, which are
 enumerated in this section.

2.5.1. Threats/Attacks

 Attacks performed on system software and configurations can be both
 from passive or active sources.  Passive attacks are possible if
 someone has the capability to intercept data between the network
 infrastructure device and the system which is downloading or
 uploading the software or configuration information.  This can be
 accomplished if a single infrastructure device is somehow compromised
 and can act as a network sniffer, or if it is possible to insert a
 new device that acts as a network sniffer.

Kaeo Informational [Page 22] RFC 4778 OPSEC Practices January 2007

 Active attacks are possible for both on-path and off-path scenarios.
 For on-path active attacks, the situation is the same as for a
 passive attack, where either a device has to already be compromised
 or a device can be inserted into the path.  For off-path active
 attacks, the attacks are generally limited to message insertion or
 modification where the attacker may wish to load illegal software or
 configuration files to an infrastructure device.
 Note that similar issues are relevant when software updates are
 downloaded from a vendor site to an ISPs network management system
 that is responsible for software updates and/or configuration
 information. Confidentiality Violations

 Confidentiality violations can occur when a miscreant intercepts any
 of the software image or configuration information.  The software
 image may give an indication of exploits which the device is
 vulnerable to while the configuration information can inadvertently
 lead attackers to identify critical infrastructure IP addresses and
 passwords. Offline Cryptographic Attacks

 If any cryptographic mechanism was used to provide for data integrity
 and confidentiality, an offline cryptographic attack could
 potentially compromise the data.  The traffic would need to be
 captured either by eavesdropping on the communication path or by
 being able to divert traffic to a malicious user. Replay Attacks

 For a replay attack to be successful, the software image or
 configuration file would need to first be captured either on-path or
 diverted to an attacker to later be replayed to the intended
 recipient.  Additionally, since many protocols do have replay
 protection capabilities, these would have to be subverted as well in
 applicable situations. Message Insertion/Deletion/Modification

 Software images and configuration files can be manipulated by someone
 in control of intermediate hosts.  By forging an IP address and
 impersonating a valid host which can download software images or
 configuration files, invalid files can be downloaded to an
 infrastructure device.  This can also be the case from trusted
 vendors who may unbeknownst to them have compromised trusted hosts.
 An invalid software image or configuration file can cause a device to

Kaeo Informational [Page 23] RFC 4778 OPSEC Practices January 2007

 hang and become inoperable.  Spoofed configuration files can be hard
 to detect, especially when the only added command is to allow a
 miscreant access to that device by entering a filter allowing a
 specific host access and configuring a local username/password
 database entry for authentication to that device. Man-In-The-Middle

 A man-in-the-middle attack attacks the identity of a communicating
 peer rather than the data stream itself.  The attacker intercepts
 traffic that is sent between the infrastructure device and the host
 used to upload/download the system image or configuration file.
 He/she can then act on behalf of one or both of these systems.
 If an attacker obtained a copy of the software image being deployed,
 he could potentially exploit a known vulnerability and gain access to
 the system.  From a captured configuration file, he could obtain
 confidential network topology information, or even more damaging
 information, if any of the passwords in the configuration file were
 not encrypted.

2.5.2. Security Practices

 Images and configurations are stored on specific hosts that have
 limited access.  All access and activity relating to these hosts are
 authenticated and logged via AAA services.  When uploaded/downloading
 any system software or configuration files, either TFTP, FTP, or SCP
 can be used.  Where possible, SCP is used to secure the data transfer
 and FTP is generally never used.  All SCP access is username/password
 authenticated but since this requires an interactive shell, most ISPs
 will use shared key authentication to avoid the interactive shell.
 While TFTP access does not have any security measures, it is still
 widely used, especially in OOB management scenarios.  Some ISPs
 implement IP-based restriction on the TFTP server, while some custom
 written TFTP servers will support MAC-based authentication.  The
 MAC-based authentication is more common when using TFTP to bootstrap
 routers remotely.
 In most environments, scripts are used for maintaining the images and
 configurations of a large number of routers.  To ensure the integrity
 of the configurations, every hour the configuration files are polled
 and compared to the previously polled version to find discrepancies.
 In at least one environment these, tools are Kerberized to take
 advantage of automated authentication (not confidentiality).
 'Rancid' is one popular publicly available tool for detecting
 configuration and system changes.

Kaeo Informational [Page 24] RFC 4778 OPSEC Practices January 2007

 Filters are used to limit access to uploading/downloading
 configuration files and system images to specific IP addresses and
 The software images perform Cyclic Redundancy Checks (CRC) and the
 system binaries use the MD5 algorithm to validate integrity.  Many
 ISPs expressed interest in having software image integrity validation
 based on the MD5 algorithm for enhanced security.
 In all configuration files, most passwords are stored in an encrypted
 format.  Note that the encryption techniques used in varying products
 can vary and that some weaker encryption schemes may be subject to
 off-line dictionary attacks.  This includes passwords for user
 authentication, MD5-authentication shared secrets, AAA server shared
 secrets, NTP shared secrets, etc.  For older software that may not
 support this functionality, configuration files may contain some
 passwords in readable format.  Most ISPs mitigate any risk of
 password compromise by either storing these configuration files
 without the password lines or by requiring authenticated and
 authorized access to the configuration files that are stored on
 protected OOB management devices.
 Automated security validation is performed on infrastructure devices
 using Network Mapping (Nmap) and Nessus to ensure valid configuration
 against many of the well-known attacks.

2.5.3. Security Services

 o  User Authentication - All users are authenticated before being
    able to download/upload any system images or configuration files.
 o  User Authorization - All authenticated users are granted specific
    privileges to download or upload system images and/or
    configuration files.
 o  Data Origin Authentication - Filters are used to limit access to
    uploading/downloading configuration files and system images to
    specific IP addresses.
 o  Access Control - Filters are used to limit access to uploading/
    downloading configuration files and system images to specific IP
    addresses and protocols.
 o  Data Integrity - All systems use either a CRC-check or MD5
    authentication to ensure data integrity.  Also, tools such as
    rancid are used to automatically detect configuration changes.

Kaeo Informational [Page 25] RFC 4778 OPSEC Practices January 2007

 o  Data Confidentiality - If the SCP protocol is used then there is
    confidentiality of the downloaded/uploaded configuration files and
    system images.
 o  Auditing/Logging - All access and activity relating to
    downloading/uploading system images and configuration files are
    logged via AAA services and filter exception rules.
 o  DoS Mitigation - A combination of filtering and CRC-check/
    MD5-based integrity checks are used to mitigate the risks of DoS
    attacks.  If the software updates and configuration changes are
    performed via an OOB management system, this is also added

2.5.4. Additional Considerations

 Where the MD5 algorithm is not used to perform data-integrity
 checking of software images and configuration files, ISPs have
 expressed an interest in having this functionality.  IPsec is
 considered too cumbersome and operationally difficult to use for data
 integrity and confidentiality.

2.6. Logging Considerations

 Although logging is part of all the previous sections, it is
 important enough to be covered as a separate item.  The main issues
 revolve around what gets logged, how long are logs kept, and what
 mechanisms are used to secure the logged information while it is in
 transit and while it is stored.

2.6.1. Threats/Attacks

 Attacks on the logged data can be both from passive or active
 sources.  Passive attacks are possible if someone has the capability
 to intercept data between the recipient logging server and the device
 from which the logged data originated.  This can be accomplished if a
 single infrastructure device is somehow compromised and can act as a
 network sniffer, or if it is possible to insert a new device that
 acts as a network sniffer.
 Active attacks are possible for both on-path and off-path scenarios.
 For on-path active attacks, the situation is the same as for a
 passive attack, where either a device has to already be compromised,
 or a device can be inserted into the path.  For off-path active
 attacks, the attacks are generally limited to message insertion or
 modification that can alter the logged data to keep any compromise
 from being detected, or to destroy any evidence that could be used
 for criminal prosecution.

Kaeo Informational [Page 26] RFC 4778 OPSEC Practices January 2007 Confidentiality Violations

 Confidentiality violations can occur when a miscreant intercepts any
 of the logging data that is in transit on the network.  This could
 lead to privacy violations if some of the logged data has not been
 sanitized to disallow any data that could be a violation of privacy
 to be included in the logged data. Offline Cryptographic Attacks

 If any cryptographic mechanism was used to provide for data integrity
 and confidentiality, an offline cryptographic attack could
 potentially compromise the data.  The traffic would need to be
 captured either by eavesdropping on the network or by being able to
 divert traffic to a malicious user. Replay Attacks

 For a replay attack to be successful, the logging data would need to
 first be captured either on-path or diverted to an attacker and later
 replayed to the recipient. Message Insertion/Deletion/Modification

 Logging data could be injected, deleted, or modified by someone in
 control of intermediate hosts.  Logging data can also be injected by
 forging packets from either legitimate or illegitimate IP addresses. Man-In-The-Middle

 A man-in-the-middle attack attacks the identity of a communicating
 peer rather than the data stream itself.  The attacker intercepts
 traffic that is sent between the infrastructure device and the
 logging server or traffic sent between the logging server and the
 database that is used to archive the logged data.  Any unauthorized
 access to logging information could lead to the knowledge of private
 and proprietary network topology information, which could be used to
 compromise portions of the network.  An additional concern is having
 access to logging information, which could be deleted or modified so
 as to cover any traces of a security breach.

2.6.2. Security Practices

 When it comes to filtering, logging is mostly performed on an
 exception auditing basis (i.e., traffic that is NOT allowed is
 logged).  This is to assure that the logging servers are not
 overwhelmed with data, which would render most logs unusable.
 Typically the data logged will contain the source and destination IP

Kaeo Informational [Page 27] RFC 4778 OPSEC Practices January 2007

 addresses and layer 4 port numbers as well as a timestamp.  The
 syslog protocol is used to transfer the logged data between the
 infrastructure device to the syslog server.  Many ISPs use the OOB
 management network to transfer syslog data since there is virtually
 no security performed between the syslog server and the device.  All
 ISPs have multiple syslog servers - some ISPs choose to use separate
 syslog servers for varying infrastructure devices (i.e., one syslog
 server for backbone routers, one syslog server for customer edge
 routers, etc.)
 The timestamp is derived from NTP, which is generally configured as a
 flat hierarchy at stratum1 and stratum2 to have less configuration
 and less maintenance.  Consistency of configuration and redundancy is
 the primary goal.  Each router is configured with several stratum1
 server sources, which are chosen to ensure that proper NTP time is
 available, even in the event of varying network outages.
 In addition to logging filtering exceptions, the following is
 typically logged: routing protocol state changes, all device access
 (regardless of authentication success or failure), all commands
 issued to a device, all configuration changes, and all router events
 The main function of any of these log messages is to see what the
 device is doing as well as to try and ascertain what certain
 malicious attackers are trying to do.  Since syslog is an unreliable
 protocol, when routers boot or lose adjacencies, not all messages
 will get delivered to the remote syslog server.  Some vendors may
 implement syslog buffering (e.g., buffer the messages until you have
 a route to the syslog destination), but this is not standard.
 Therefore, operators often have to look at local syslog information
 on a device (which typically has very little memory allocated to it)
 to make up for the fact that the server-based syslog files can be
 incomplete.  Some ISPs also put in passive devices to see routing
 updates and withdrawals and do not rely solely on the device for log
 files.  This provides a backup mechanism to see what is going on in
 the network in the event that a device may 'forget' to do syslog if
 the CPU is busy.
 The logs from the various syslog server devices are generally
 transferred into databases at a set interval that can be anywhere
 from every 10 minutes to every hour.  One ISP uses Rsync to push the
 data into a database, and then the information is sorted manually by
 someone SSH'ing to that database.

Kaeo Informational [Page 28] RFC 4778 OPSEC Practices January 2007

2.6.3. Security Services

 o  User Authentication - Not applicable.
 o  User Authorization - Not applicable.
 o  Data Origin Authentication - Not implemented.
 o  Access Control - Filtering on logging host and server IP address
    to ensure that syslog information only goes to specific syslog
 o  Data Integrity - Not implemented.
 o  Data Confidentiality - Not implemented.
 o  Auditing/Logging - This entire section deals with logging.
 o  DoS Mitigation - An OOB management system is used and sometimes
    different syslog servers are used for logging information from
    varying equipment.  Exception logging tries to keep information to
    a minimum.

2.6.4. Additional Considerations

 There is no security with syslog and ISPs are fully cognizant of
 this.  IPsec is considered too operationally expensive and cumbersome
 to deploy.  Syslog-ng and stunnel are being looked at for providing
 better authenticated and integrity-protected solutions.  Mechanisms
 to prevent unauthorized personnel from tampering with logs is
 constrained to auditing who has access to the logging servers and
 ISPs expressed requirements for more than just UDP syslog.
 Additionally, they would like more granular and flexible facilities
 and priorities, i.e., specific logs to specific servers.  Also, a
 common format for reporting standard events so that modifying parsers
 after each upgrade of a vendor device or software is not necessary.

2.7. Filtering Considerations

 Although filtering has been covered under many of the previous
 sections, this section will provide some more insights to the
 filtering considerations that are currently being taken into account.
 Filtering is now being categorized into three specific areas: data
 plane, management plane, and routing control plane.

Kaeo Informational [Page 29] RFC 4778 OPSEC Practices January 2007

2.7.1. Data Plane Filtering

 Data plane filters control the traffic that traverses through a
 device and affects transit traffic.  Most ISPs deploy these kinds of
 filters at customer facing edge devices to mitigate spoofing attacks
 using BCP38 and BCP84 guidelines.

2.7.2. Management Plane Filtering

 Management filters control the traffic to and from a device.  All of
 the protocols that are used for device management fall under this
 category and include: SSH, Telnet, SNMP, NTP, HTTP, DNS, TFTP, FTP,
 SCP, and Syslog.  This type of traffic is often filtered per
 interface and is based on any combination of protocol, source and
 destination IP address, and source and destination port number.  Some
 devices support functionality to apply management filters to the
 device rather than to the specific interfaces (e.g., receive ACL or
 loopback interface ACL), which is gaining wider acceptance.  Note
 that logging the filtering rules can today place a burden on many
 systems and more granularity is often required to more specifically
 log the required exceptions.
 Any services that are not specifically used are turned off.
 IPv6 networks require the use of specific ICMP messages for proper
 protocol operation.  Therefore, ICMP cannot be completely filtered to
 and from a device.  Instead, granular ICMPv6 filtering is always
 deployed to allow for specific ICMPv6 types to be sourced or destined
 to a network device.  A good guideline for IPv6 filtering is in the
 Recommendations for Filtering ICMPv6 Messages in Firewalls [ICMPv6].

2.7.3. Routing Control Plane Filtering

 Routing filters are used to control the flow of routing information.
 In IPv6 networks, some providers are liberal in accepting /48s due to
 the still unresolved multihoming issues, while others filter at
 allocation boundaries, which are typically at /32.  Any announcement
 received that is longer than a /48 for IPv6 routing and a /24 for
 IPv4 routing is filtered out of eBGP.  Note that this is for
 non-customer traffic.  Most ISPs will accept any agreed upon prefix
 length from its customer(s).

2.8. Denial-of-Service Tracking/Tracing

 Denial-of-Service attacks are an ever-increasing problem and require
 vast amounts of resources to combat effectively.  Some large ISPs do
 not concern themselves with attack streams that are less than 1G in
 bandwidth - this is on the larger pipes where 1G is essentially less

Kaeo Informational [Page 30] RFC 4778 OPSEC Practices January 2007

 than 5% of an offered load.  This is largely due to the large amounts
 of DoS traffic, which continually requires investigation and
 mitigation.  At last count, the number of hosts making up large
 distributed DoS botnets exceeded 1 million hosts.
 New techniques are continually evolving to automate the process of
 detecting DoS sources and mitigating any adverse effects as quickly
 as possible.  At this time, ISPs are using a variety of mitigation
 techniques including: sinkhole routing, black hole triggered routing,
 uRPF, rate limiting, and specific control plane traffic enhancements.
 Each of these techniques will be detailed below.

2.8.1. Sinkhole Routing

 Sinkhole routing refers to injecting a more specific route for any
 known attack traffic, which will ensure that the malicious traffic is
 redirected to a valid device or specific system where it can be

2.8.2. Black Hole Triggered Routing

 Black hole triggered routing (also referred to as Remote Triggered
 Black Hole Filtering) is a technique where the BGP routing protocol
 is used to propagate routes which in turn redirects attack traffic to
 the null interface where it is effectively dropped.  This technique
 is often used in large routing infrastructures since BGP can
 propagate the information in a fast, effective manner, as opposed to
 using any packet-based filtering techniques on hundreds or thousands
 of routers (refer to the following NANOG presentation for a more
 complete description
 Note that this black-holing technique may actually fulfill the goal
 of the attacker if the goal was to instigate black-holing traffic
 that appeared to come from a certain site.  On the other hand, this
 black hole technique can decrease the collateral damage caused by an
 overly large attack aimed at something other than critical services.

2.8.3. Unicast Reverse Path Forwarding

 Unicast Reverse Path Forwarding (uRPF) is a mechanism for validating
 whether or not an incoming packet has a legitimate source address.
 It has two modes: strict mode and loose mode.  In strict mode, uRPF
 checks whether the incoming packet has a source address that matches
 a prefix in the routing table, and whether the interface expects to
 receive a packet with this source address prefix.  If the incoming
 packet fails the unicast RPF check, the packet is not accepted on the

Kaeo Informational [Page 31] RFC 4778 OPSEC Practices January 2007

 incoming interface.  Loose mode uRPF is not as specific and the
 incoming packet is accepted if there is any route in the routing
 table for the source address.
 While BCP84 [RFC3704] and a study on uRPF experiences [BCP84-URPF]
 detail how asymmetry, i.e., multiple routes to the source of a
 packet, does not preclude applying feasible paths strict uRPF, it is
 generally not used on interfaces that are likely to have routing
 asymmetry.  Usually for the larger ISPs, uRPF is placed at the
 customer edge of a network.

2.8.4. Rate Limiting

 Rate limiting refers to allocating a specific amount of bandwidth or
 packets per second to specific traffic types.  This technique is
 widely used to mitigate well-known protocol attacks such as the
 TCP-SYN attack, where a large number of resources get allocated for
 spoofed TCP traffic.  Although this technique does not stop an
 attack, it can sometimes lessen the damage and impact on a specific
 service.  However, it can also make the impact of a DoS attack much
 worse if the rate limiting is impacting (i.e., discarding) more
 legitimate traffic.

2.8.5. Specific Control Plane Traffic Enhancements

 Some ISPs are starting to use capabilities that are available from
 some vendors to simplify the filtering and rate limiting of control
 traffic.  Control traffic here refers to the routing control plane
 and management plane traffic that requires CPU cycles.  A DoS attack
 against any control plane traffic can therefore be much more damaging
 to a critical device than other types of traffic.  No consistent
 deployment of this capability was found at the time of this writing.

3. Security Considerations

 This entire document deals with current security practices in large
 ISP environments.  It lists specific practices used in today's
 environments and as such, does not in itself pose any security risk.

4. Acknowledgments

 The editor gratefully acknowledges the contributions of: George
 Jones, who has been instrumental in providing guidance and direction
 for this document, and the insightful comments from Ross Callon, Ron
 Bonica, Ryan Mcdowell, Gaurab Upadhaya, Warren Kumari, Pekka Savola,
 Fernando Gont, Chris Morrow, Ted Seely, Donald Smith, and the
 numerous ISP operators who supplied the information that is depicted
 in this document.

Kaeo Informational [Page 32] RFC 4778 OPSEC Practices January 2007

5. References

5.1. Normative References

 [RFC2827]     Ferguson, P. and D. Senie, "Network Ingress Filtering:
               Defeating Denial of Service Attacks which employ IP
               Source Address Spoofing", BCP 38, RFC 2827, May 2000.
 [RFC2828]     Shirey, R., "Internet Security Glossary", RFC 2828,
               May 2000.
 [RFC3552]     Rescorla, E. and B. Korver, "Guidelines for Writing RFC
               Text on Security Considerations", BCP 72, RFC 3552,
               July 2003.
 [RFC3682]     Gill, V., Heasley, J., and D. Meyer, "The Generalized
               TTL Security Mechanism (GTSM)", RFC 3682,
               February 2004.
 [RFC3704]     Baker, F. and P. Savola, "Ingress Filtering for
               Multihomed Networks", BCP 84, RFC 3704, March 2004.
 [RFC3882]     Turk, D., "Configuring BGP to Block Denial-of-Service
               Attacks", RFC 3882, September 2004.

5.2. Informational References

 [BCP84-URPF]  Savola, P., "Experiences from Using Unicast RPF", Work
               in Progress, November 2006.
 [ICMPv6]      Davies, E. and J. Mohacsi, "Recommendations for
               Filtering ICMPv6 Messages in Firewalls", Work
               in Progress, July 2006.
 [RTGWG]       Savola, P., "Backbone Infrastructure Attacks and
               Protections", Work in Progress, July 2006.

Kaeo Informational [Page 33] RFC 4778 OPSEC Practices January 2007

Appendix A. Protocol Specific Attacks

 This section will list many of the traditional protocol-based attacks
 that have been observed over the years to cause malformed packets
 and/or exploit protocol deficiencies.  Note that they all exploit
 vulnerabilities in the actual protocol itself and often, additional
 authentication and auditing mechanisms are now used to detect and
 mitigate the impact of these attacks.  The list is not exhaustive,
 but is a fraction of the representation of what types of attacks are
 possible for varying protocols.

A.1. Layer 2 Attacks

 o  ARP Flooding

A.2. IPv4 Protocol-Based Attacks

 o  IP Addresses, either source or destination, can be spoofed which
    in turn can circumvent established filtering rules.
 o  IP Source Route Option can allows attackers to establish stealth
    TCP connections.
 o  IP Record Route Option can disclose information about the topology
    of the network.
 o  IP header that is too long or too short can cause DoS attacks to
 o  IP Timestamp Option can leak information that can be used to
    discern network behavior.
 o  Fragmentation attacks which can vary widely - more detailed
    information can be found at
 o  IP ToS field (or the Differentiated Services (DSCP) field) can be
    used to reroute or reclassify traffic based on specified
 o  IP checksum field has been used for scanning purposes, for example
    when some firewalls did not check the checksum and allowed an
    attacker to differentiate when the response came from an end-
    system, and when from a firewall.
 o  IP TTL field can be used to bypass certain network-based intrusion
    detection systems and to map network behavior.

Kaeo Informational [Page 34] RFC 4778 OPSEC Practices January 2007

A.2.1. Higher Layer Protocol Attacks

 The following lists additional attacks, but does not explicitly
 numerate them in detail.  It is for informational purposes only.
 o  IGMP oversized packet
 o  ICMP Source Quench
 o  ICMP Mask Request
 o  ICMP Large Packet (> 1472)
 o  ICMP Oversized packet (>65536)
 o  ICMP Flood
 o  ICMP Broadcast w/ Spoofed Source (Smurf Attack)
 o  ICMP Error Packet Flood
 o  ICMP Spoofed Unreachable
 o  TCP Packet without Flag
 o  TCP Oversized Packet
 o  TCP FIN bit with no ACK bit
 o  TCP Packet with URG/OOB flag (Nuke Attack)
 o  SYN Fragments
 o  SYN Flood
 o  SYN with IP Spoofing (Land Attack)
 o  SYN and FIN bits set
 o  TCP port scan attack
 o  UDP spoofed broadcast echo (Fraggle Attack)
 o  UDP attack on diagnostic ports (Pepsi Attack)

Kaeo Informational [Page 35] RFC 4778 OPSEC Practices January 2007

A.3. IPv6 Attacks

 Any of the above-mentioned IPv4 attacks could be used in IPv6
 networks with the exception of any fragmentation and broadcast
 traffic, which operate differently in IPv6.  Note that all of these
 attacks are based on either spoofing or misusing any part of the
 protocol field(s).
 Today, IPv6-enabled hosts are starting to be used to create IPv6
 tunnels, which can effectively hide botnet and other malicious
 traffic if firewalls and network flow collection tools are not
 capable of detecting this traffic.  The security measures used for
 protecting IPv6 infrastructures should be the same as in IPv4
 networks, but with additional considerations for IPv6 network
 operations, which may be different from IPv4.

Author's Address

 Merike Kaeo
 Double Shot Security, Inc.
 3518 Fremont Avenue North #363
 Seattle, WA  98103
 Phone: +1 310 866 0165

Kaeo Informational [Page 36] RFC 4778 OPSEC Practices January 2007

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 contained in BCP 78, and except as set forth therein, the authors
 retain all their rights.
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Kaeo Informational [Page 37]

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