Advisory ID: cisco-sa-20030430-dns
For Public Release 2003 April 30 08:00 UTC (GMT)
The Cisco Content Service Switch (CSS) 11000 and 11500 series switches
respond to certain Domain Name Service (DNS) name server record requests with
an error code and no Start of Authority (SOA) records, which can be negatively
cached by some DNS name servers resulting in a potential denial-of-service
attack for a 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
This vulnerability in CSS is documented as Cisco Bug IDs CSCdz62499 and
This advisory will be available at http://tools.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-20030430-dns.
The CSS 11000 and 11500 series switches (formerly known as Arrowpoint)
consist 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 are affected by this vulnerability only if they are configured for
Global Server 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. If this command is
not present, the configuration is not vulnerable to this
No other Cisco products are currently known to be affected by this vulnerability.
Commonly, the name service in use by the Internet, DNS, uses various
record types for queries between DNS servers and clients. The common record
types are Address records (A-records), Name Server records (NS records), Mail
Exchange (MX records), Start of Authority records (SOA records), and Canonical
Name records (CNAME records). Each record or query type has various rules and
formats associated with it, including details about what may be cached, what
may be trusted by other clients, etc.
Clients usually send queries to a local server, and that local server
may send further queries to other servers on behalf of that client in order to
formulate a response for the client. When the local server receives the
responses, it will cache the information for future use and will respond to the
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 will
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 for that
name. For example, an NXDOMAIN response to the AAAA query may stop the server
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 caching
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 or caching nameservers and will lead to apparent temporary service
outages when other clients query the caching nameserver or proxy for the
legitimate host name. Though network services are physically unaffected, end
users are dependent upon name resolution, and the lack of correct DNS
information can result in effective service outages.
Cisco Bug ID CSCdz62499 was the first fix, which changed the response
from rcode 3 to rcode 4. This result code is also negatively cached, so the
complete fix has been correctly addressed with Cisco Bug ID CSCea36989.
The CSS will now return an RFC 2308-compliant NODATA type 3 response,
which is an authoritative answer with rcode=NOERROR, answer=0, and no SOA. This
response should cause the specific client to query for another A-record instead
of continuing to query for the unsupported record type or using the negatively
cached error message or NXDOMAIN answer.
Cisco has not provided scores for the vulnerabilities in this advisory based on the Common Vulnerability Scoring System (CVSS). At the time of original publication, CVSS scoring in Cisco security publications was not yet adopted.
Exploitation of this vulnerability would result in a sporadic or
partial denial of service, affecting only the users of the DNS services that
cache the negative response information in response to an unsupported query
type from 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 cause significant difficulty in troubleshooting customer reports of
The following table summarizes the CSS software releases affected by
the defect described in this notice and provides scheduled dates on which the
earliest corresponding fixed releases will be available. Dates are tentative
and subject to change.
When selecting a release, keep in mind the following
A maintenance release is the most heavily tested
and highly recommended release.
An interim release has much less testing than a
maintenance release and should be selected only if no other suitable release
fixes the defect.
In all cases, customers should exercise caution to be certain the
devices to be upgraded contain sufficient memory and that current hardware and
software configurations will continue to be supported properly by the new
Estimated Date Available
5.00 up to and including 5.00.1.05
5.00 build 105
5.00.1.08S - Interim Build
5.00 build 108s
5.00.2.01 - Maintenance Release
5.00 build 201
5.01, 5.02, and 5.03
Upgrade to 6.10 - Maintenance Release
7.10 up to and including 7.10.1.02
7.10 build 102
7.20.0.03 - Maintenance Release
7.20 build 003
7.10.2.06 - Maintenance Release
7.10 build 206
Note: Bullet items in the table above provide information on build
identifier and file name (previous naming convention for the same
The workaround for this issue is to disable Global Server Load
Balancing and to configure DNS records for the affected servers and domains on
a separate compliant DNS server until an upgrade to repaired versions can be
Cisco has released software updates that address this vulnerability
for all affected customers. Customers may only install and expect support for
the feature sets they have purchased.
Customers with contracts should obtain software through their regular update channels. For most customers, software patches and bug fixes should be obtained through the Software Center on Cisco's worldwide website at
Customers whose Cisco products are provided or maintained through prior
or 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 free of
Customers who purchase direct from Cisco but who do not hold a Cisco
service contract and customers who purchase through third-party vendors but are unsuccessful in obtaining fixed software through their point of sale should obtain software patches and bug fixes by contacting the Cisco Technical Assistance Center (TAC). In
those cases, customers may only upgrade to a later version of the same release
as indicated by the applicable row in the Software
Versions and Fixes table. 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)
for additional TAC contact information, including special localized telephone
numbers and instructions and e-mail addresses for use in various
Please have your product serial number available and give the
of this notice as evidence of entitlement to a software patch or bug fix. Customers without service contracts should request a software patch or bug fix through the TAC.
Please do not contact either "email@example.com" or
"firstname.lastname@example.org" for software upgrades.
Customers with contracts should obtain software through their regular update channels. For most customers, software patches and bug fixes should be obtained through the Software Center on Cisco's worldwide website at http://www.cisco.com
Customers whose Cisco products are provided or maintained through prior or existing agreements with third-party support organizations, such as Cisco Partners, authorized resellers, or service providers should contact that support organization for guidance and assistance with the appropriate course of action in regards to this advisory.
The effectiveness of any workaround or fix is dependent on specific customer situations, such as product mix, network topology, traffic behavior, and organizational mission. Due to the variety of affected products and releases, customers should consult with their service provider or support organization to ensure any applied workaround or fix is the most appropriate for use in the intended network before it is deployed.
Customers who purchase direct from Cisco but do not hold a Cisco
service contract, and customers who purchase through third-party vendors but are unsuccessful in obtaining fixed software through their point of sale should obtain software patches and bug fixes by contacting the Cisco Technical Assistance Center (TAC). 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)
Customers should have the product serial number available and be prepared to provide the URL of this notice as evidence of entitlement to a software patch or bug fix. Customers without service contracts should request a software patch or bug fix through the TAC.
for additional TAC contact information, including localized telephone numbers,
and instructions and e-mail addresses for use in various languages.
This vulnerability has been published by the CERT at
. 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 may be unlikely that exploitation would be noticed or
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This advisory is 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 e-mail and Usenet news recipients.
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 the
above URL for any updates.
Initial public release.
Alternate build identifiers added for clarification in Software
Versions and Fixes.
Edited Affected Version column contents in the table to clarify