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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).
Abstract
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
environments.
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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
infrastructures.
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
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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.
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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
messages.
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
subverted.
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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
follows:
(Unauthorized) Disclosure
A circumstance or event whereby an entity gains access to data for
which the entity is not authorized.
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Deception
A circumstance or event that may result in an authorized entity
receiving false data and believing it to be true.
Disruption
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.
Usurpation
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].
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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.
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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
addressed.
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
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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
online/functional).
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
authenticated.
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.
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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
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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.
2.2.1.1. 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.
2.2.1.2. 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.
2.2.1.3. 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.
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2.2.1.4. 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.
2.2.1.5. 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
functions.
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.
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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
SNMP).
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
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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
section:
o User Authentication - All individuals are authenticated via AAA
services.
o User Authorization - All individuals are authorized via AAA
services to perform specific operations once successfully
authenticated.
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
devices.
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.
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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
servers.
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.
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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
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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
solution.
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
deployed.
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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.
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2.4.1.1. 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.
2.4.1.2. 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.
2.4.1.3. 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.
2.4.1.4. 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.
2.4.1.5. 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
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party or cause legitimate traffic to never reach its intended
destination.
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
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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.
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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'
products.
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.
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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.
2.5.1.1. 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.
2.5.1.2. 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.
2.5.1.3. 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.
2.5.1.4. 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
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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.
2.5.1.5. 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.
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Filters are used to limit access to uploading/downloading
configuration files and system images to specific IP addresses and
protocols.
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.
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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
protection.
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.
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2.6.1.1. 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.
2.6.1.2. 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.
2.6.1.3. 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.
2.6.1.4. 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.
2.6.1.5. 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
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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
(boot-up/flaps).
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.
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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
hosts.
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
files.
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.
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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
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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
analyzed.
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 http://www.nanog.org/mtg-0402/pdf/morrow.pdf).
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
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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.
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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.
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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
devices.
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 http://www-src.lip6.fr/homepages/
Fabrice.Legond-Aubry/www.ouah.org/fragma.html.
o IP ToS field (or the Differentiated Services (DSCP) field) can be
used to reroute or reclassify traffic based on specified
precedence.
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.
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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)
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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
U.S.A.
Phone: +1 310 866 0165
EMail: merike@doubleshotsecurity.com
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