| Cisco Content Service Switch 11000 Series DNS Negative
Cache of Information DoS
Author : Cisco Systems Product Security Incident Response Team <email@example.com>
The Cisco Content Service Switch (CSS) 11000 and 11500 series switches
to certain Domain Name Service (DNS) name server record requests with
code and no Start of Authority (SOA) records, which can be negatively
some DNS name servers resulting in a potential denial-of-service attack
particular domain name hosted by a CSS. To be affected by this vulnerability,
CSS devices must be configured for Global Server Load Balancing. The CERT/CC
issued a vulnerability note on this issue (VU#714121). Cisco is providing
repaired software, and customers are urged to upgrade to repaired code.
This vulnerability in CSS is documented as Cisco Bug IDs CSCdz62499 and
This advisory will be available at
The CSS 11000 and 11500 series switches (formerly known as Arrowpoint)
of the CSS 11050, CSS 11150, CSS 11800 11501, 11503, and 11506 hardware
platforms. They run the Cisco WebNS software.
CSS 11000 and 11500 series switches running any WebNS software revision
affected by this vulnerability only if they are configured for Global
Load Balancing (also known as DNS Load Balancing).
To determine if your CSS equipment is configured for Global Server Load
Balancing, please check the configuration for the dns-server command.
command is not present, the configuration is not vulnerable to this issue.
No other Cisco product is currently known to be affected by this vulnerability.
Commonly, the name service in use by the Internet, DNS, uses various
types for queries between DNS servers and clients. The common record types
Address records (A-records), Name Server records (NS records), Mail Exchange
(MX records), Start of Authority records (SOA records), and Canonical
records (CNAME records). Each record or query type has various rules and
formats associated with it, including details about what may be cached,
may be trusted by other clients, etc.
Clients usually send queries to a local server, and that local server
further queries to other servers on behalf of that client in order to
a response for the client. When the local server receives the responses,
will cache the information for future use and will respond to the client.
The CSS 11000 and 11500 series switches have the ability to act as an
authoritative DNS name server and will only respond to DNS A-record requests.
If a CSS configured for DNS via the Global Server Load Balancing feature
receives a DNS request or query for an unsupported record type, the CSS
respond with rcode 4 "not implemented" or rcode 3 "NXDOMAIN,"
depending on the
version of WebNS. When an NXDOMAIN response code is received, the querying
server will typically stop attempting to resolve any other record type
name. For example, an NXDOMAIN response to the AAAA query may stop the
from sending an A query, though there may indeed be an A-record in existence.
Some resolvers that receive an NXDOMAIN response and support negative
will not query for A-records for the same name until the negatively cached
error response has expired, which can take an extended period of time.
When the DNS query received is for a legitimate host name but an unsupported
record type, these negative responses may be cached by various proxies
caching nameservers and will lead to apparent temporary service outages
other clients query the caching nameserver or proxy for the legitimate
name. Though network services are physically unaffected, end users are
dependent upon name resolution, and the lack of correct DNS information
result in effective service outages.
Cisco Bug ID CSCdz62499 was the first fix, which changed the response
rcode 3 to rcode 4. This result code is also negatively cached, so the
fix has been correctly addressed with Cisco Bug ID CSCea36989.
The CSS will now return an RFC 2308-compliant NODATA type 3 response,
an authoritative answer with rcode=NOERROR, answer=0, and no SOA. This
should cause the specific client to query for another A-record instead
continuing to query for the unsupported record type or using the negatively
cached error message or NXDOMAIN answer.
Exploitation of this vulnerability would result in a sporadic or partial
of service, affecting only the users of the DNS services that cache the
negative response information in response to an unsupported query type
that same userbase. The administrators of the affected CSS and associated
resources may not be aware of any exploitation, since there are no locally
apparent symptoms. Only certain user groups would be affected, which may
significant difficulty in troubleshooting customer reports of problems.
Software Versions and Fixes
The following table summarizes the CSS software releases affected by
described in this notice and provides scheduled dates on which the earliest
corresponding fixed releases will be available. Dates are tentative and
When selecting a release, keep in mind the following definitions.
* A maintenance release is the most heavily tested and highly recommended
* An interim release has much less testing than a maintenance release
should be selected only if no other suitable release fixes the defect.
In all cases, customers should exercise caution to be certain the devices
upgraded contain sufficient memory and that current hardware and software
configurations will continue to be supported properly by the new release.
| Affected | | Estimated |
| Version | Fixed Version | Date |
| | | Available |
| | 5.00.1.08S - | |
| | Interim Build | 2003-Apr-29 |
| 5.00.1.05 | | |
| | 5.00.2.01 - | 2003-May-30 |
| | Maintenance | |
| | Release | |
| | Upgrade to 6.10 - | |
| 5.03 | Maintenance | 2003-May-15 |
| | Release | |
| | 7.20.0.03 - | |
| | Maintenance | Available |
| | Release | Now |
| 7.10.1.02 | | |
| | 7.10.2.06 - | Available |
| | Maintenance | Now |
| | Release | |
Obtaining Fixed Software
Cisco is offering free software upgrades to remedy this vulnerability
affected customers. Customers may only install and expect support for
feature sets they have purchased.
Customers with contracts should obtain upgraded software through their
update channels. For most customers, this means that upgrades should be
obtained through the Software Center on Cisco's worldwide website at
Customers whose Cisco products are provided or maintained through prior
existing agreement with third-party support organizations such as Cisco
Partners, authorized resellers, or service providers should contact that
support organization for assistance with the upgrade, which should be
Customers who purchase direct from Cisco but who do not hold a Cisco
contract and customers who purchase through third-party vendors but are
unsuccessful at obtaining fixed software through their point of sale should
their upgrades by contacting the Cisco Technical Assistance Center (TAC).
those cases, customers may only upgrade to a later version of the same
as indicated by the applicable row in the Software Versions and Fixes
TAC contacts are as follows:
* +1 800 553 2447 (toll free from within North America)
* +1 408 526 7209 (toll call from anywhere in the world)
* e-mail: firstname.lastname@example.org
See http://www.cisco.com/warp/public/687/Directory/DirTAC.shtml for additional
TAC contact information, including special localized telephone numbers
instructions and e-mail addresses for use in various languages.
Please have your product serial number available and give the URL of
notice as evidence of your entitlement to a free upgrade. Free upgrades
non-contract customers must be requested through the TAC.
Please do not contact either "email@example.com" or "firstname.lastname@example.org"
for software upgrades.
The workaround for this issue is to disable Global Server Load Balancing
configure DNS records for the affected servers and domains on a separate
compliant DNS server until an upgrade to repaired versions can be installed.
Exploitation and Public Announcements
This vulnerability has been published by the CERT at
http://www.kb.cert.org/vuls/id/714121. CERT notes that this issue is not
new. The Cisco PSIRT is not aware of any malicious use of the vulnerabilities
described in this advisory, but because of the nature of this issue, it
be unlikely that exploitation would be noticed or reported.
Status of This Notice: FINAL
This is a final notice. Although Cisco cannot guarantee the accuracy
statements in this notice, all the facts have been checked to the best
ability. Cisco does not anticipate issuing updated versions of this notice
unless there is some material change in the facts. Should there be a
significant change in the facts, Cisco may update this notice.
This notice will be posted on Cisco's worldwide website at
In addition to worldwide web posting, a text version of this notice is
clear-signed with the Cisco PSIRT PGP key and is posted to the following
and Usenet news recipients.
* email@example.com (includes CERT/CC)
* Various internal Cisco mailing lists
Future updates of this advisory, if any, will be placed on Cisco's worldwide
website, but may or may not be actively announced on mailing lists or
newsgroups. Users concerned about this problem are encouraged to check
above URL for any updates.
| Revision | 2003-April-30 | Initial public |
| 1.0 | | release. |
Cisco Security Procedures
Complete information on reporting security vulnerabilities in Cisco products,
obtaining assistance with security incidents, and registering to receive
security information from Cisco, is available on Cisco's worldwide website
instructions for press inquiries regarding Cisco security notices. All
security advisories are available at http://www.cisco.com/go/psirt/.
This notice is Copyright 2003 by Cisco Systems, Inc. This notice may
redistributed freely after the release date given at the top of the text,
provided that redistributed copies are complete and unmodified, and include
date and version information.