INTERNET-DRAFT Verio Northwest
Valid for six months March 1998
Security Expectations for Internet Service Providers
<draft-ietf-grip-isp-04.txt>
Status of this Memo
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Copyright Notice
Copyright (C) The Internet Society (1998). All Rights Reserved.
Abstract
The purpose of this document is to express the general Internet
community's expectations of Internet Service Providers (ISPs) with
respect to security.
It is not the intent of this document to define a set of requirements
that would be appropriate for all ISPs, but rather to raise awareness
among ISPs of the community's expectations, and to provide the
community with a framework for discussion of security expectations
with current and prospective service providers.
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Table of Contents
1 Introduction
1.1 Conventions Used in this Document
2 Incident Response
2.1 ISPs and Security Incident Response Teams (SIRTs)
2.2 Assistance with Inbound Security Incidents
2.3 Assistance with Outbound or Transit Security Incidents
2.4 Notification of Vulnerabilities and Reporting Incidents
2.5 Contact Information
2.6 Communication and Authentication
3 Appropriate Use Policy
3.1 Announcement of Policy
3.2 Sanctions
4 Protection of the Community
4.1 Cooperation
4.2 Data Protection
4.3 Training
4.4 Registry Data Maintenance
5 Network Infrastructure
5.1 Routers
5.2 Switches, Terminal Servers, Modems and other Network Devices
5.3 Anonymous telnet and other unlogged connections
5.4 The Network Operation Centre (NOC) and Network Management
5.5 Physical Security
5.6 Routing Infrastructure
5.7 Ingress Filtering on Source Address
5.8 Egress Filtering on Source Address
5.9 Route Filtering
5.10 Directed Broadcast
6 Systems Infrastructure
6.1 Policy
6.2 System Management
6.3 Backup
6.4 Software Distribution
7 Domain Name Service (DNS)
7.1 DNS Server Management
7.2 Authoritative Domain Name Service
7.3 Resolution Service
8 Email and Mail Services
8.1 Mail Server Administration
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8.2 Secure Mail
8.3 Open Mail Relay
8.4 Message Submission
8.5 POP and IMAP Services
9 News Service (NNTP)
9.1 News Server Administration
9.2 Article Submission
9.3 Control Messages
9.4 Newsfeed Filters
10 Web-related Services
10.1 Webhosting Server Administration
10.2 Server Side Programs
10.3 Data and Databases
10.4 Logs and Statistics Reporting
10.5 Push and Streaming Services
10.6 Commerce
10.7 Content Loading and Distributed Authoring
10.8 Search Engines and other tools
11 References
12 Acknowledgements
13 Security Considerations
14 Author's Address
15 Full Copyright Statement
1 Introduction
The purpose of this document is to express the general Internet
community's expectations of Internet Service Providers (ISPs) with
respect to security.
A goal is that customers could have a framework to facilitate the
discussion of security with prospective service providers;
regrettably, such a discussion rarely takes place today.
Additionally, in informing ISPs of what the community hopes and
expects of them, a further goal is to encourage ISPs to become
proactive in making security not only a priority, but something to
which they point with pride when selling their services.
This document is addressed to Internet service purchasing decision-
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makers (customers) and ISPs.
It has been argued that vendors begin to care about security only
when prompted by customers. I hope that this document will encourage
both parties to more readily express how much they care about
security, and that discussion between the community and its ISPs will
be increased.
1.1 Conventions Used in this Document
The key words "REQUIRED", "MUST", "MUST NOT", "SHOULD", "SHOULD NOT",
and "MAY" in this document are to be interpreted as described in "Key
words for use in RFCs to Indicate Requirement Levels" [RFC2119].
2 Incident Response
A Security Incident Response Team (SIRT) is a team that performs,
coordinates, and supports the response to security incidents that
involve sites within a defined constituency. The Internet
community's expectations of SIRTs are described in a work in progress
called "Expectations for Security Incident Response".
2.1 ISPs and Security Incident Response Teams (SIRTs)
Some ISPs have SIRTs. However it should not be assumed that either
the ISP's connectivity customers or a site being attacked by a
customer of that ISP can automatically avail of the services of the
ISP's SIRT. ISP SIRTs are frequently provided as an added-cost
service, with the team defining as their constituency only those who
specifically subscribe to (and perhaps pay for) Incident Response
services.
Thus it's important to determine what incident response and security
resources are available to you, and define your incident response
escalation chain BEFORE an incident occurs.
Customers should find out if their ISP has a SIRT, and if so what the
charter, policies and services of that team are. This information is
best expressed using the SIRT template as shown in Appendix D of the
work in progress called "Expectations for Security Incident
Response". If the ISP doesn't have a SIRT they SHOULD describe what
role if any they will take in incident response, and SHOULD indicate
if there is any SIRT whose constituency would include the customer
and to whom incidents could be reported.
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2.2 Assistance with Inbound Security Incidents
When a security incident targeting one of their connectivity
customers occurs ISPs SHOULD inform the customer of the attack, and
provide assistance to
- trace the 'apparent' origin of the attack and attempt to
determine the veracity of each step in the path (keeping in
mind that the source address may be spoofed). In cases where
the source address is spoofed the ISP could trace the point
at which the bogusly addressed traffic entered the ISP's
network.
- obtain contact information for the source of the attack using
whois [RFC1834 and RFC1835], the DNS [RFC1034 and RFC1035] or
relevant common mailbox names [RFC2142].
- collect and protect evidence of the incident and guard against
its destruction or unintentional announcement.
If the event continues then, at the customer's request, ISPs may also
assist by logging in order to further diagnose the problem, or by
filtering certain types of traffic.
2.3 Assistance with Outbound or Transit Security Incidents
In the case where one of their connectivity customers appears to be
the source of a security incident an ISP will frequently be
contacted. Once the ISP is satisfied as to the authenticity of the
report, they should provide contact information so that the
administrators at the source site can be reached by the target site.
An ISP may also be contacted to assist with incidents that traverse
their network but use bogus source addresses, such as SYN flooding
attacks [CA-96.21.tcp_syn_flooding]. Assistance in this case would
consist of using network traces on a hop by hop basis to identify the
point at which the bogusly addressed traffic entered the ISP's
network. In tracing such incidents it's frequently necessary to
coordinating with adjacent ISPs to form a complete chain of response
teams along the path of the attack.
2.4 Notification of Vulnerabilities and Reporting of Incidents
ISPs should be proactive in notifying customers of security
vulnerabilities in the services they provide. In addition, as new
vulnerabilities in systems and software are discovered they should
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indicate whether their services are threatened by these risks.
When security incidents occur that affect components of an ISP's
infrastructure the ISP SHOULD promptly report to their customers
- who is coordinating response to the incident
- the vulnerability
- how service was affected
- what is being done to respond to the incident
- whether customer data may have been compromised
- what is being done to eliminate the vulnerability
- the expected schedule for response, assuming it can be
predicted
2.5 Contact Information
[RFC2142] states that sites (including ISPs) must maintain a mailbox
called SECURITY for network security issues, ABUSE for issues
relating to inappropriate public behaviour and NOC for issues
relating to network infrastructure. It also lists additional
mailboxes that are defined for receiving queries and reports relating
to specific services.
ISPs should consider using common URLs for expanded details on the
above (e.g., http://www.ISP-name-here.net/security/).
In addition, ISPs have a duty to make sure that their contact
information, in Whois, in the routing registry [RFC1786] or in any
other repository, is complete, accurate and reachable.
2.6 Communication and Authentication
ISPs SHOULD have clear policies and procedures on the sharing of
information about a security incident with their customers, with
other ISPs or SIRTs, with law enforcement or with the press and
general public.
ISPs SHOULD also be able to conduct such communication over a secure
channel. I recognise that in some jurisdictions secure channels
might not be permitted.
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3 Appropriate Use Policy
An Appropriate Use Policy (AUP) SHOULD clearly identify what
customers shall and shall not do on the various components of a
system or network, including the type of traffic allowed on the
networks. The AUP should be as explicit as possible to avoid
ambiguity or misunderstanding.
Whenever an ISP contracts with a customer to provide connectivity to
the Internet that contract SHOULD be governed by an AUP. The AUP
should be reviewed each time the contract is up for renewal, and in
addition the ISP should proactively notify customers as policies are
updated.
3.1 Announcement of Policy
In addition to communicating their AUP to their customers ISPs should
publish their policy in a public place such as their web site so that
the community can be aware of what the ISP considers appropriate and
can know what action to expect in the event of inappropriate
behaviour.
3.2 Sanctions
An AUP should be clear in stating what sanctions will be enforced in
the event of inappropriate behaviour, and ISPs should be forthcoming
in announcing to the community when such sanctions are imposed.
4 Protection of the Community
ISPs play a crucial role in helping to improve the security of the
Internet. This and following sections describe a number of issues
which, should they be addressed by ISPs in a coordinated and timely
way, would have a very positive effect on the security of the
network, and would make it much more difficult for the perpetrators
to cover their tracks.
Later sections cover in some detail issues related to specific
services such as ingress filtering and open mail relays. Such issues,
if addressed by all the ISPs in a concerted way, could have a very
positive effect.
4.1 Cooperation
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This section is about protecting the community. This requires that
we as a community work together to that end. It's worth observing
that many of the most significant successes in securing the Internet
could not have been achieved by anyone acting alone.
Cooperation should be put on legal ground. For example prior to
entering into peering agreements ISPs should specify the steps they
will take to cooperatively track security incidents that involve both
peers.
4.2 Data Protection
Many jurisdictions have the good fortune to have Data Protection
Legislation. Where such legislation exists ISPs should consider the
personal data they hold and, if necessary, register themselves as
Data Controllers and be prepared to make data available according to
the terms of the legislation. Given the global nature of the
Internet ISPs that are located where no such legislation exists
should at least familiarise themselves with their responsibilities by
reading a Data Protection Act (e.g., [DPR1984]).
4.3 Training
It's important that all ISP staff be trained to be security-conscious
at all times and to be able to make appropriate use of tools that
enhance security. Some issues that they should be particularly aware
of include the use of secure channels for confidential information,
the risk of attacks that use social engineering, management of data
used for authentication, and so on.
4.4 Registry Data Maintenance
ISPs are commonly responsible for maintaining the data that is stored
in global repositories such as the Internet Routing Registry (IRR)
and the APNIC, InterNIC and RIPE databases. Updates to this data
should only be possible using strong authentication.
5 Network Infrastructure
ISPs are responsible for managing the network infrastructure of the
Internet in such a way that it is
- reasonably resistant to known security vulnerabilities
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- not easily hijacked by attackers for use in subsequent attacks
5.1 Routers
Routers are an excellent platform from which to launch a security
attack, as well as being attractive targets of themselves.
Many routers allow an attacker to do dangerous things such as:
- sniff transit traffic
- manipulate routing tables to redirect traffic
- manipulate interface states to disrupt service
- create routing flaps which could potentially cause Denial of
Service for large parts of the Internet
- create packets with spoofed addresses, and with any desired flags
set
- initiate ICMP packet storms and other Denial of Service attacks
- 'black hole' traffic (e.g., by holding a local route to a null or
invalid interface, by holding a local route to an invalid
next-hop (one which does not itself have a corresponding route,
and does not have a default), or worst yet, by using a dynamic
routing protocol to advertise availability of a low-cost route
and thus actively drawing traffic toward the black hole)
- launder connections to third-party destinations, facilitated by
the router's lack of logging
Such threats are amplified because of the central part in the
networking infrastructure that routers occupy, and the large
bandwidth frequently available to them.
So access to routers SHOULD be based on one-time passwords or better
(e.g., Kerberos) and should be as restricted as possible.
Sessions SHOULD be encrypted to prevent sessions or data from being
stolen and to avoid replay attacks.
Routers SHOULD NOT run the 'small services', which are often enabled
by default. These may include bootp, chargen, daytime, discard, echo
and finger.
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5.2 Switches, Terminal Servers, Modems and other Network Devices
ISPs should be similarly vigilant in their configuration of other
network devices. Unfortunately many such devices deployed in the
field support only minimal authentication, do authorisation on an
all-or-nothing basis and do little or no logging. In the past ISPs
have been left with no trail to follow after their switches were
reconfigured, their terminal servers were used to launch attacks on
third parties or their Uninterruptable Power Supplies were shut down.
Where possible access to such devices should be restricted only to
legitimate network administrators.
Network infrastructure devices frequently don't support extensive
internal logging because they have no long-term storage, like hosts'
hard drives. Many support syslog or SNMP traps however, or at least
a short internal event log or debugging mode which can be captured
through the console or a remote login session.
Router or switch configurations should always be maintained on a tftp
server, so that they can be restored to previous configuration easily
and quickly. These backup configurations should obviously be
protected so that they cannot be tftp'd by unauthorised parties, or
overwritten with new bogus configurations.
5.3 Anonymous telnet and other unlogged connections
There are many network devices ranging from low-end routers to
printers that accept telnet connections without prompting for a
password. Obviously such devices, many of which don't maintain audit
trails, are very popular among attackers who wish to cover their
tracks.
If ISPs have such devices on their own network they should restrict
access to them. In addition, they should work with their customers
to block access to such devices from outside of the customer's
network.
The use of telnet to manage network elements is strongly discouraged.
5.4 The Network Operation Centre (NOC) and Network Management
A NOC is a crucial part of an ISP's infrastructure, and should be
operated with appropriate regard to security.
A NOC frequently has management control over the configuration
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information of network devices, and should be vigilant in restricting
access to that information. Loading of configuration information
into network devices is still frequently done using the TFTP protocol
[RFC1350], which not only lacks authentication and uses an insecure
channel, but calls for great care in configuration at the server end
[CA-91:18.Active.Internet.tftp.Attacks].
A NOC will generally perform a network monitoring function by polling
(e.g., with ICMP Echo) a set of network devices periodically. In
selecting the set of devices to be polled the crucial role of the
devices in 5.2 shouldn't be overlooked.
Beyond simple polling a NOC will also use a network management
protocol such as SNMP to communicate with network devices. Usually
this will be used to 'get' the value of various variables, such as
the number of packets received on a particular interface. However
the protocol can be used to 'set' variables, perhaps with serious
results (e.g., the device can be reconfigured). In any case, SNMPv1
uses only trivial authentication. Where possible SNMP should be used
as a read-only tool to 'get' information from remote devices, and the
information gotten should be treated as confidential.
A further use of SNMP is in trap reporting, whereby a management
station is notified when certain exceptions occur. This information
should also be considered confidential, and the NOC should take care
that such trap reporting cannot of itself become a Denial of Service
attack.
5.5 Physical Security
The physical security of every installation should be given
appropriate consideration. This is particularly so for co-located
facilities to which people from different organisations and with
different security policies have access.
Three types of co-location arrangements are of particular interest:
- customers co-locating equipment at an ISP's facility
- ISPs co-locating equipment at an external facility with
authorised 'remote hands'
- ISPs co-locating equipment at an external facility with no
authorised physical access
The first case is most likely to directly concern the customer. If
an ISP has a co-location facility for the hosting of customer-owned
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equipment many issues arise surrounding customer access to their co-
located equipment.
Ideally every customer SHOULD have a fully enclosed locking 'cage',
akin to a small room with walls and ceiling of heavy wire mesh
fencing, containing the racks in which their equipment is mounted.
Customers are allowed access to their own cage under escort by one of
the ISP's employees, or with keys that grant access specifically to
their cage.
This assignment of separate cages is expensive in terms of space
however, so many ISPs compromise by putting all co-located equipment
together in a single machine room, and managing the actions of
escorted customers very closely. However this may be insufficient to
prevent mishaps such as the accidental disconnection of another
customer's equipment. If a single machine room is used then the ISP
SHOULD provide separate locking cabinets for each co-location
customer in preferance to an
A customer SHOULD always be supervised while in the physical presence
of any equipment that is not their own, and SHOULD NOT be allowed to
touch, photograph, or examine equipment belonging to another
customer.
Also of concern is layer 2 security of co-located equipment. Customer
equipment SHOULD NOT be allowed to share a physical network segment
with hosts belonging to anyone else, whether another customer or the
ISP themselves. It's common for crackers to exploit weak security or
unencrypted remote logins on co-located customer-owned equipment to
take control of that equipment and put it into promiscuous listening
mode on the local network segment, thereby potentially compromising
the privacy and security of any other devices on that segment.
When ISPs co-locate network infrastructure components outside of
their own premises, such as at peering points or remote POPs,
security considerations are extremely important. These locations
often play a pivotal role in the network topology, and may be
particular targets for attack or vulnerable to accidents. Equipment
SHOULD ideally be fully enclosed in locking cabinets or cages to
limit physical access. If on-site spares are kept, they should
likewise be protected from tampering. Whenever possible, security
systems and logging card-swipe locks should be employed.
Installations should be inspected periodically for the addition of
unauthorised equipment which might be used to 'tap' a connection. As
with any other facility, hosts SHOULD NOT be attached to transit
segments, and hosts should never have unused physical interfaces
attached to network segments.
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5.6 Routing Infrastructure
An ISP's ability to route traffic to the correct destination depends
on routing policy as configured in the routing registries [RFC1786].
ISPs SHOULD ensure that the registry information that they maintain
can only be updated using strong authentication, and that the
authority to make updates is appropriately restricted.
Due care should also be taken in determining in whose routing
announcements you place greater trust when a choice of routes are
available to a destination. In the past bogus announcements have
resulted in traffic being 'black holed', or worse, hijacked. BGP
authentication SHOULD be used with routing peers.
The internal routing protocol that an ISP uses should be chosen with
security in mind. It should not be configured with the assumption
that route recalculations are rare and expensive as this would leave
the way open for a Denial of Service attack. Routing updates should
use the highest level of authentication supported by the internal
routing protocol.
If more specific routes to parts of the ISP's allocated address space
are heard from external peers then the ISP needs to be judicious in
deciding whether to accept the announcement. Only ISPs who have
allowed their CIDR address allocations to become fragmented (by
allowing customers to take addressess with them when they change
providers) have to face this decision.
5.7 Ingress Filtering on Source Address
The direction of such filtering is from the edge site (customer) to
the Internet.
Attackers frequently cover their tracks by using forged source
addresses. To divert attention from their own site the source
address they choose will generally be from an innocent remote site or
indeed from those addresses that are allocated for private Internets
[RFC1918]. In addition, forged source addresses are frequently used
in spoof-based attacks in order to exploit a trust relationship
between hosts.
To reduce the incidence of attacks that rely on forged source
addresses ISPs should do the following. At the boundary router with
each of their customers they SHOULD proactively filter all traffic
coming from the customer that has a source address of something other
than the addresses that have been assigned to that customer. For a
more detailed discussion of this topic see [RFC2267].
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There are (rare) circumstances where ingress filtering is not
currently possible, for example on large aggregation routers that
cannot take the additional load of applying packet filters. In
addition, such filtering can cause difficulty for mobile users.
Hence, while the use of this technique to prevent spoofing is
strongly encouraged, I realise that it is not always feasible.
In these rare cases where ingress filtering at the interface between
the customer and the ISP is not possible, the customer should be
encouraged to implement ingress filtering within their networks. In
general filtering should be done as close to the actual hosts as
possible.
5.8 Egress Filtering on Source Address
The direction of such filtering is from the Internet to the edge site
(customer).
There are many applications in widespread use on the Internet today
that grant trust to other hosts based only on ip address (e.g., the
Berkeley 'r' commands). These are susceptible to IP spoofing, as
described in [CA-95.01.IP.spoofing]. In addition, there are
vulnerabilities that depend on the misuse of supposedly locally
addresses, such as 'land' as described in [CA-97.28.Teardrop_Land].
To reduce the exposure of their customers to attacks that rely on
forged source addresses ISPs should do the following. At the
boundary router with each of their customers they SHOULD proactively
filter all traffic going to the customer that has a source address of
any of the addresses that have been assigned to that customer.
The circumstances described in 5.7 in which ingress filtering isn't
feasible similarly apply to egress filtering.
5.9 Route Filtering
Excessive routing updates can be leveraged by an attacker as a base
load on which to build a Denial of Service attack. At the very least
they will result in performance degradation.
ISPs SHOULD filter the routing announcements they hear, for example
to ignore routes to addresses allocated for private Internets, to
avoid bogus routes and to implement route dampening and aggregation
policy.
ISPs SHOULD implement techniques that reduce the risk of putting
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excessive load on routing in other parts of the network. These
include 'nailed up' routes, aggressive aggregation and route
dampening, all of which lower the impact on others when your internal
routing changes in a way that isn't relevant to them.
5.10 Directed Broadcast
The IP protocol allows for directed broadcast, the sending of a
packet across the network to be broadcast on to a specific subnet.
Very few practical uses for this feature exist, but several different
security attacks (primarily Denial of Service attacks making use of
the packet multiplication effect of the broadcast) use it.
Therefore, routers connected to a broadcast medium SHOULD NOT be
configured to allow directed broadcasts onto that medium.
If it is a packet to which the router would respond if received as a
unicast, it MAY send a (single) response. If it is not responding
(either because it's not appropriate, or has been configured not to)
it MAY send an ICMP error. It is also appropriate to silently
discard such packets. In any case such packets SHOULD be counted to
detect possible attempts to abuse this feature.
6 Systems Infrastructure
The way an ISP manages their systems is crucial to the security and
reliability of their network. A breach of their systems may
minimally lead to degraded performance or functionality, but could
lead to loss of data or the risk of traffic being eavesdropped (thus
leading to 'man-in-the-middle' attacks).
In general a 'horses for courses' approach to the provision of
systems services should be adopted (i.e., separate systems should be
used to deliver each distinct service). Apart from the benefits that
accrue in terms of easing systems administration it's widely
acknowledged that it's much easier to build secure systems if
different services (such as mail, news and web-hosting) are kept on
separate systems.
6.1 Policy
An ISP's policy with regard to privacy, authentication,
accountability, application of security patches, availability and
violations reporting should all be of interest to their customers,
and should be published in a public place such as the ISP's web site.
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A more detailed discussion of Security Policy can be found in the
Site Security Handbook [RFC2196].
6.2 System Management
All systems that perform critical ISP functions such as mail, news
and web-hosting, should be restricted such that access to them is
only available to the administrators of those services. That access
should be granted only following strong authentication, and should
take place over an encrypted link. Only the ports on which those
services listen should be reachable from outside of the ISP's systems
networks.
If the ISP provides login accounts to customers then the systems that
support this service should be isolated from the rest of the ISP's
systems networks.
If applications such as rdist are used for software distribution and
synchronisation then they should be used over a secure channel and
with strong authentication, for example over Secure Shell (ssh)
[SSH1997].
A system SHOULD NOT be attached to transit segments.
If reusable passwords are permitted then users should be educated
about how to choose and care for a password, and proactive password
checks, password aging and password guessers should be employed.
6.3 Backup
The importance of backups need not be stressed here. However backups
can become the weakest link in a system's security if appropriate
care isn't taken of backup media.
If backups are done across the network then a secure channel should
be used. If volumes are dumped to staging disks during the backup
process then access to the images on those staging disks should be as
restricted as possible.
Backups take on additional significance as audit data following a
security incident.
Ageing backup media should be destroyed rather than discarded.
6.4 Software Distribution
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ISPs frequently engage in application software distribution. The
integrity of the software should be assured by distributing with it a
checksum that has been produced with a strong digest function such as
SHA-1 [SHA].
7 Domain Name Service (DNS)
The DNS is critical to the daily activities of millions of Internet
users. Regrettably applications have frequently placed blind trust
in the information contained in the DNS, and in the availability of
the DNS. However prior to DNSSEC [RFC2065] the DNS protocol lacked
security, while widely used implementations of the DNS protocol
contain further severe vulnerabilities [VIX1995].
While this section indicates some methods in which the DNS can be
made more trustworthy and reliable it cannot be stressed too strongly
that name based authentication is inherently insecure.
7.1 DNS Server Administration
In addition to issues raised in section 6 ISPs will need to address
the following issues in administering their DNS servers:
- Service Monitoring.
The service availability (ability to answer queries) should be
monitored.
- Clock synchronisation.
Servers should synchronise their clocks using the NTP protocol
[RFC1305] with authentication. At least two NTP servers should
be used.
7.2 Authoritative Domain Name Service
An Authoritative Server is one that knows the content of a DNS zone
from local knowledge, and thus can answer queries about that zone
without needing to query other servers. Customers should consider
[RFC2182] when choosing secondary DNS servers.
ISPs commonly operate as secondary (or slave) servers for their
customers, and these servers may provide service for thousands of
zones. Regardless of the number of zones, administrators of these
servers should be familiar with the Operational Criteria for Root
Name Servers [RFC2010], and in particular should follow these
guidelines:
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- Recursion should be disabled for queries.
- Zone transfer should be restricted.
Apart from the significant load presented by zone transfer
with resultant exposure to Denial of Service attacks, ISPs
should recognise that some of their customers will consider the
contents of their zone files to be private.
- Performance Monitoring.
Key variables such as queries per second and average latency
should be monitored.
7.3 Resolution Service
ISPs commonly operate DNS resolution service for their customers. In
this scenario customers configure their DNS resolver (client) to
resolve queries from the ISP's DNS resolution servers. For
resolution servers ISPs should follow these guidelines:
- Recursion must be enabled for queries.
An implication is that ISPs should not use the same servers for
resolution service and authoritative DNS service.
- Zone transfer should be disallowed.
Even though there may be no zones to transfer, allowing zone
transfers would expose the servers to Denial of Service attacks.
- Performance Monitoring.
Key variables such as queries per second and average latency
should be monitored. In addition, the hosts generating the
highest number of requests should be periodically reported.
- Name server software.
A name server package should be run that is not vulnerable to
server cache poisoning where malicious or misleading data
received from a remote name server is cached and is then made
available to resolvers that request the cached data.
8 Email and Mail Services
Email has been the target of some of the most widely reported
security attacks, as well as thousands of juvenile hoaxes and pranks.
ISPs have a major role in protecting the community from abuse and in
educating their customers in appropriate technologies and in
appropriate uses of the technology.
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8.1 Mail Server Administration
In configuring mail servers ISPs should follow these guidelines:
- Mail software.
If possible software that uses a separate receiving/sending agent
and a processing agent should be used. A goal is that the
receiving/sending agent, which interfaces with remote mail
servers, can be run with reduced privilege.
- Restrict remote message queue starting.
On-demand queue runs (to facilitate customers who receive mail at
their own domain and don't have permanent connections) should be
restricted, preferably using a strong authentication mechanism.
Remote message queue starting is implemented using a variety of
mechanisms, one of which is the ETRN SMTP service extension as
described in [RFC1985].
- Disable VRFY and EXPN.
No more should be revealed about local users or delivery
mechanisms than is necessary.
- Clock synchronisation.
Servers should synchronise their clocks using the NTP protocol
[RFC1305] with authentication. At least two NTP servers should
be used.
- Exception Reporting.
Exceptional conditions such as repeated authentication failures,
mail loops and abnormal queue length should be trapped and
reported.
- Restrict Access to mail logs.
Mail logs should only be readable by system administrators.
8.2 Secure Mail
As indicated in 2.6, It's critical that ISPs, and in particular their
Security Incident Response personnel, have access to tools that allow
them to exchange email securely.
8.3 Open Mail Relay
An SMTP mail server is said to be running as an 'open' mail relay if
it is willing to accept and relay to non-local destinations mail
messages that do not originate locally (i.e., neither the originator
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nor the recipient address is local). Such open relays are frequently
used by 'spammers' to inject their Unsolicited Bulk E-mail (UBE)
while hiding their identity. There are very limited cases in which
an administrator can make a justifiable case for leaving a mail relay
on the Internet completely open.
The processes for restricting relaying are well documented. It's
regrettable that some major software vendors ship their Message
Transfer Agents (MTAs) with relaying open by default.
While this is an issue for the whole community, ISPs SHOULD be
particularly vigilant in disabling open relaying on mail servers that
they manage because their high-bandwidth connectivity makes them the
preferred injection point for UBE.
ISPs SHOULD also strongly encourage their customers to disable open
relaying on their mail servers. Sanctions for running an open mail
relay should be covered in an ISP's AUP.
8.4 Message Submission
To facilitate the enforcement of security policy message submission
should be done through the MAIL SUBMIT port (587) as proposed in the
work in progress called "Message Submission and Relay", rather than
through the SMTP port (25). In addition, message submissions should
be authenticated using the AUTH SMTP service extension as described
in the work in progess called "SMTP Service Extension for
Authentication". In this way the SMTP port (25) can be restricted to
local delivery only.
These two measures not only protect the ISP from serving as a UBE
injection point, but also help in tracking accountability for message
submission in the case where a customer sends UBE. Furthermore,
using the Submit port with SMTP AUTH has additional advantages over
IP address-based submission restrictions in that it gives the ISP's
customers the flexibility of being able to submit mail even when not
connected through the ISP's network (for example, while at work), is
more resistant to spoofing, and can be upgraded to newer
authentication mechanisms as they become available.
The (undocumented) XTND XMIT POP3 extension which allows clients to
send mail through the POP3 session rather than using SMTP may also be
considered. It also provides a way to support mobile users at sites
where open relaying is disabled, and has the benefit of an
authenticated connection and a better audit trail.
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8.5 POP and IMAP Services
ISPs who provide POP or IMAP access to mailboxes to their customers
SHOULD, at a minimum, support the CRAM-MD5 [RFC2195] or APOP
[RFC1939] authentication mechanisms. Support for stronger mechanisms
should be considered, as should disabling plaintext (user/password)
authentication.
9 News Service (NNTP)
As in the case of SMTP, the NNTP protocol [RFC977] used by News
suffers from a lack of authentication, and so it's trivial to forge
news postings. Forgeries can bypass the moderation process, cancel
legitimate articles and create havoc for sites that maintain an
active file.
The lack of encryption in the protocol and the manner in which many
news systems are maintained lead to privacy issues in that it's easy
for others to detect what newsgroups and articles you are reading.
9.1 News Server Administration
In configuring news servers ISPs should follow these guidelines:
- News software.
A news software package should be run that is not vulnerable to
maliciously formed news control messages or buffer overflows.
- Disable other services.
Given news' propensity to consume all available disk space and
CPU cycles it's particularly important that news systems do not
perform other services.
- Do not interpret batches.
If incoming batches of articles are supported they should not
be fed to a command interpreter.
- Restrict Access to news logs.
News logs should only be readable by system administrators.
- Authenticate approved headers.
If possible support for cryptographic authentication of approved
messages should be supported, particularly in the case of group
control messages.
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9.2 Article Submission
As many of the issues relating to open mail relays (8.3) apply to
news, ISPs should restrict article submission only to approved
customers. Further, the networks from which posting is allowed and
the newsgroups to which posting is allowed should be as restricted as
possible.
9.3 Control Messages
Control messages attempt to cause the news server to take action
beyond filing and passing on the article. Certain control messages,
because of the ease with which they can be forged, should be handled
with care. While it is up to the ISP to decide whether to take
action they must at least propagate control messages even if they do
not understand them.
- 'whogets', 'sendsys', 'version' should be ignored by ISPs.
- While 'cancel' messages must be acted on and propagated their
sheer volume can sometimes swamp service, and the fact that much
of that volume is computer-generated is worrying.
- Systems that require the maintenance of an active file should
exercise extreme caution in choosing which if any group control
messages (checkgroups, newgroup, rmgroup) should result in
action being taken.
9.4 Newsfeed Filters
The most obvious form of security problem with news is 'leakage' of
articles which are intended to have only restricted circulation. The
flooding algorithm is extremely good at finding any path by which
articles can leave a subnet with supposedly restrictive boundaries.
Substantial administrative effort is required to ensure that local
newsgroups remain local [SPE1994].
ISPs who provide customers with the ability to remotely manipulate
their inbound filters should use strong authentication for this
service.
ISPs should not propagate articles that are excessively crossposted.
10 or more cross-postings is widely agreed to be excessive.
ISPs should impose an upper limit on the article size that they will
propagate.
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10 Web-hosting Services
Sites frequently choose to out-source the operation and
administration of their site to an ISP, and security is often a
prominent motivator for doing so. The hosting of such sites and
provision of related services is the subject of this section.
Further information on the topic can be found in [GAR1997] and
[HUG1995].
10.1 Webhosting Server Administration
In addition to issues raised in section 6 ISPs will need to address
the following issues in administering their web-hosting servers:
- Service Monitoring.
The service availability (ability to answer HTTP requests) should
be monitored.
- Clock synchronisation.
Servers should synchronise their clocks using the NTP protocol
[RFC1305] with authentication. At least two NTP servers should
be used.
- DNS.
DNS lookups should not be performed on web clients when they
connect because they expose the web servers to DNS-based Denial
of Service attacks, and they adversely affect performance.
- DocumentRoot.
Everything below this directory should be subject to the
strictest scrutiny.
- UserDir.
Users other than administrators should not be permitted on the
server. If users have accounts then the 'UserDir' directive, if
permitted, SHOULD NOT access their private accounts. In
particular, scripts SHOULD NOT be permitted to be run from their
accounts.
- Process User and Group.
The web daemon SHOULD be run as a user and group that is set up
specifically for that purpose, and that user/group should have
minimal privilege. This user should be different from the
maintainers of the web content.
- Partitioning of Virtual Sites.
A single server that hosts multiple sites (virtual domains)
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SHOULD be set up such that all data, programs and logs for the
different sites are partitioned from each other such that no
access to the configuration or data of each other's sites is
possible. In addition, it should not be possible to access the
data or programs of one customer's site using a URL that has
the name of another customer's site in it's host part.
- Access Control.
Restricted access to certain parts of a site should be
facilitated using a strong authentication mechanism such as a
certificate or a one-time password device. An alternative is
to use well-chosen passwords in conjunction with SSL which at
least avoids passwords being passed across the network in
plaintext.
- Security Patches.
The stakes in running a web server are particularly high, so
administrators should be particularly vigilant in applying
security patches as they are released.
10.2 Server Side Programs
Server side programs such as those that use the Common Gateway
Interface (CGI) are crucial to the flexibility of the web as a
communications medium. However that flexibility introduces security
risks and a weak program threatens all of the virtual hosts on the
server that runs it. An ISP's policy with regard to what programs it
will allow is a good indicator of security policy in general.
ISPs should follow these guidelines on server side programs and CGIs:
- Security Policy.
ISPs should give their customers clear guidelines about how to
write secure programs for their hosting environment, and give
specific indications about what programming practices will result
in a program being rejected.
- Program Installation.
Customers should never be allowed to install their own programs.
All programs and scripts should be submitted to the ISP first to
be checked for conformance with security policy. The programs
SHOULD be installed such that only the server administrators have
permission to modify them.
- Process User and Group.
Programs SHOULD be run as a user and group that is set up
specifically for that purpose, and that user/group should have
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minimal privilege (many sites use 'nobody').
- Display by Browsers.
Programs SHOULD never be allowed to be viewed by browsers. One
implication of that is that they SHOULD NOT be put under the
DocumentRoot.
- Partitioning of Virtual Sites.
Programs SHOULD NOT be accessible through the site of another
customer on the same server, or to the webmaster of that other
customer.
- User Input.
Expressions SHOULD NOT be evaluated based on user input except
when used with the equivalent of Perl's tainting features.
- Processing Limit.
All programs SHOULD have a limit set on real and CPU time, and on
the amount of disk space that they can consume.
- Paths.
All paths SHOULD be full or starting at DocumentRoot, and the
PATH variable should be set by the server administrator.
10.3 Data and Databases
Data that is written by server-side programs such as CGI scripts
should be considered confidential and to prevent them being read by
browsers their permission should be such that they're not readable by
the web daemon process.
If access to a back-end database is provided then programs that
facilitate such access should have the least privilege that is
absolutely necessary.
Data that relates to state management (cookies) that is stored on the
server should be considered confidential and should not be accessible
from browsers.
10.4 Logs and Statistics Reporting
The logs generated by the web daemon process can be useful from the
security viewpoint in providing an audit trail of site activity,
however their more common use is for billing and for market and site
analysis.
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These logs should be considered highly confidential.
- The only manipulation of them done by the ISP should be that
which is necessary to generate billing information and
periodically rotate them.
- They should be stored outside of DocumentRoot to prevent access
by a browser to them.
- Access to them, whether in raw or summarised format, should be
provided to the customer over a secure channel.
10.5 Push and Streaming Services
ISPs frequently provide their customers with the ability to deliver
content using protocols other than HTTP. Where such add-on services
are provided, both the customer and the ISP should be aware of the
security implications of providing such services.
10.6 Commerce
Many ISPs set up the means whereby their customers can sell goods and
services through their web-hosted sites. Though a server that can
exchange information with a browser over SSL is sometimes described
as a 'secure server' this term can be misleading, and ISPs that host
commerce applications should consider the following:
- Encrypted Transactions.
Transactions should never be stored on the server in unencrypted
form. Public key cryptography should be used such that only the
customer can decrypt the transactions. Even when transactions
are passed directly to a financial institution and to the
customer some part of the transaction will have to be stored by
the ISP for audit trail purposes.
- Transaction Transfer.
If transactions are not processed immediately but instead are
transferred to the customer in batches then that transfer should
occur over a secure channel such as SSL and only after strong
authentication has taken place. Transaction files should be
carefully rotated so that every transaction occurs exactly once.
- Backups.
If transactions are written to backup media then the physical
security of the backup media should be assured.
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10.7 Content Loading and Distributed Authoring
The loading of content onto the ISP's server should happen over a
secure channel.
If server support for Distributed Authoring tools is enabled, then
this should be administered with great care to ensure that strong
authentication takes place and that access is given only to the
customer's virtual site, and only to that customer's content
maintainer.
10.8 Search Engines and other tools
ISPs frequently install tools such as search engines, link checkers
and so on for use by their customers. Many such tools create a very
great processing overhead when run and so running them on-demand
should not be allowed to avoid Denial of Service attacks.
Search engines should be configured so that their searches are
restricted to those parts of a site that are available to all.
The output of link checkers should be considered confidential, and
should only be available to the content maintainer of the customer's
site.
11 References
[CA-91:18.Active.Internet.tftp.Attacks] "Active Internet tftp
Attacks", ftp://info.cert.org/pub/cert_advisories/
[CA-95.01.IP.spoofing] "IP Spoofing Attacks and Hijacked Terminal
Connections", ftp://info.cert.org/pub/cert_advisories/
[CA-96.21.tcp_syn_flooding] "TCP SYN Flooding and IP Spoofing
Attacks", ftp://info.cert.org/pub/cert_advisories/
[CA-97.28.Teardrop_Land] "IP Denial-of-Service Attacks",
ftp://info.cert.org/pub/cert_advisories/
[DPR1984] The UK "Data Protection Act 1984 (c. 35)",
http://www.hmso.gov.uk/acts/acts1984/1984035.htm
[GAR1997] Garfinkel, S., "Web Security and Commerce",
O'Reilly and Associates, Sebastopol, CA, 1997.
[HUG1995] Hughes Jr., L., "Actually Useful Internet Security
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Internet Draft Security Expectations for ISPs 12 March 1998
Techniques", New Riders Publishing, Indianapolis, IN, 1995.
[RFC977] Kantor, B and P. Lapsley, "Network News Transfer Protocol",
RFC 977, February 1986.
[RFC1350] Sollins, K. R., "The TFTP Protocol (revision 2)", STD 33,
RFC 1350, July 1992.
[RFC1034] Mockapetris, P. V., "Domain names - concepts and
facilities", STD 13, RFC 1034, November 1987.
[RFC1035] Mockapetris, P. V., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987.
[RFC1305] Mills, D., "Network Time Protocol (Version 3)
Specification, Implementation", RFC 1305, March 1992.
[RFC1786] Bates, T., Gerich, E., Joncheray, L., Jouanigot, J-M.,
Karrenberg, D., Terpstra, M., and J. Yu, "Representation of IP
Routing Policies in a Routing Registry (ripe-81++)", RFC 1786,
March 1995.
[RFC1834] Gargano, J., and K. Weiss, "Whois and Network Information
Lookup Service", RFC 1834, August 1995.
[RFC1835] Deutsch, P., Schoultz, R., Faltstrom, P., and C. Weider,
"Architecture of the WHOIS++ service", RFC 1835, August 1995.
[RFC1918] Rekhter, Y., Moskowitz, B., Karrenberg, D., de Groot, G.
J., and E. Lear, "Address Allocation for Private Internets", BCP 5,
RFC 1918, February 1996.
[RFC1939] Myers, J., and M. Rose, "Post Office Protocol - Version
3", RFC 1939, May 1996.
[RFC1985] De Winter, J. "SMTP Service Extension for Remote Message
Queue Starting", RFC 1985, August 1996.
[RFC2010] Manning, B., and P. Vixie, "Operational Criteria for Root
Name Servers", RFC 2010, October 1996.
[RFC2065] Eastlake 3rd, D., and C. Kaufman, "Domain Name System
Security Extensions", RFC 2065, January 1997.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", RFC 2119, March 1997.
[RFC2142] Crocker, D., "Mailbox Names for Common Services, Roles and
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Internet Draft Security Expectations for ISPs 12 March 1998
Functions", RFC 2142, May 1997.
[RFC2195] Klensin, J., Catoe, R., and P. Krumviede, "IMAP/POP
AUTHorize Extension for Simple Challenge/Response", RFC 2195,
September 1997.
[RFC2196] Fraser, B., "Site Security Handbook", RFC 2196, September
1997.
[RFC2267] Ferguson, P., and D. Senie, "Network Ingress Filtering:
Defeating Denial of Service Attacks which employ IP Source
Address Spoofing", RFC 2267, January 1998.
[SHA] NIST, FIPS PUB 180-1: Secure Hash Standard, April 1995.
[SPE1994] Spencer, H., "News Article Format and Transmission",
ftp://ftp.zoo.toronto.edu/pub/news.txt.Z
[SSH1997] SSH (secure Shell) Remote Login Program,
http://www.cs.hut.fi/ssh/
[VIX1995] Vixie, P., "DNS and BIND Security Issues",
ftp://ftp.vix.com/pri/vixie/bindsec.psf, 1995.
12 Acknowledgements
I gratefully acknowledge the constructive comments received from
Nevil Brownlee, Randy Bush, Bill Cheswick, Barbara Y. Fraser, Randall
Gellens, Erik Guttman, Larry J. Hughes Jr., Klaus-Peter Kossakowski,
Michael A. Patton, Don Stikvoort and Bill Woodcock.
13 Security Considerations
This entire document discusses security issues.
14 Author's Address
Tom Killalea
Verio Northwest
15400 SE 30th Pl., Ste. 202
Bellevue, WA 98007-6546
USA
Phone: +1 425 649-7417
E-Mail: tomk@nw.verio.net
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15 Full Copyright Statement
Copyright (C) The Internet Society (1998). All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implmentation may be prepared, copied, published and
distributed, in whole or in part, without restriction of any kind,
provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
Internet organisations, except as needed for the purpose of
developing Internet standards in which case the procedures for
copyrights defined in the Internet Standards process must be
followed, or as required to translate it into languages other than
English.
The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns.
This document and the information contained herein is provided on an
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE."
This document expires Sep 15, 1998.
Killalea [Page 30]
