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AUSCERT External Security Bulletin Redistribution
ESB-2020.4493
BIG-IP system may respond with the NXDOMAIN status when it receives a DNS
query of a certain type on a CNAME wide IP
18 December 2020
===========================================================================
AusCERT Security Bulletin Summary
---------------------------------
Product: BIG-IP System
Publisher: F5 Networks
Operating System: Network Appliance
Impact/Access: Reduced Security -- Remote/Unauthenticated
Resolution: Patch/Upgrade
Original Bulletin:
https://support.f5.com/csp/article/K23022557
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K23022557: The BIG-IP system may respond with the NXDOMAIN status when it
receives a DNS query of a certain type on a CNAME wide IP
Original Publication Date: 07 Mar, 2017
Latest Publication Date: 17 Dec, 2020
Security Advisory Description
The BIG-IP system may respond with the NXDOMAIN status when it receives a DNS
query on a CNAME wide IP. This issue occurs when all of the following
conditions are met:
o The BIG-IP system is configured with a CNAME wide IP.
For example:
test.example.com
o The BIG-IP system is also configured with a DNS Express (DNSX) zone, but it
does not have a record for the CNAME wide IP.
o The BIG-IP system receives a DNS query of type MX or ANY for the CNAME wide
IP.
Impact
DNS cache resolvers that receive the response with the NXDOMAIN status use that
response for the entire domain name. As a result, DNS clients that use these
DNS cache resolvers for DNS queries of type A/AAAA may receive responses with
the NXDOMAIN status until the cache expires.
Symptoms
As a result of this issue, you may encounter one or more of the following
symptoms:
o BIG-IP iHealth lists Heuristic H645784 on the Diagnostics > Identified >
Medium screen.
o A DNS cache resolver receives a DNS response with the NXDOMAIN status when
querying the BIG-IP system for an MX record or ANY records of a CNAME wide
IP.
o A DNS client receives DNS responses with NXDOMAIN when querying an affected
DNS cache resolver for A/AAAA records.
Security Advisory Status
Status
F5 Product Development has assigned ID 644192 to this issue. F5 has confirmed
that this issue exists in the products listed in the Applies to (see versions)
box, located in the upper-right corner of this article. For information about
releases or hotfixes that resolve this issue, refer to the following table:
+------------------+-----------------+----------------------------------------+
|Type of Fix |Fixes Introduced |Related Articles |
| |In | |
+------------------+-----------------+----------------------------------------+
|Release |15.1.2 |K2200: Most recent versions of F5 |
| | |software |
+------------------+-----------------+----------------------------------------+
|Point release/ |14.1.3.1 |K9502: BIG-IP hotfix and point release |
|hotfix | |matrix |
+------------------+-----------------+----------------------------------------+
Security Advisory Recommended Actions
Workaround
To work around this issue, you can either create a TXT resource record for the
CNAME wide IP in ZoneRunner, or create an iRule that replaces the NXDOMAIN
status with a NOERROR status before sending out the response. To do so, perform
either of the following procedures:
o Creating a TXT resource record in ZoneRunner
o Creating an iRule to replace the DNS response status
Creating a TXT resource record in ZoneRunner
To create a TXT resource record for the CNAME wide IP in ZoneRunner, perform
the following procedure:
Impact of workaround: Performing the following procedure should not have a
negative impact on your system.
1. Log in to the Configuration utility.
2. Navigate to one of the following locations:
BIG-IP 11.5.0 and later
DNS > Zones > ZoneRunner > Zone List
BIG-IP 11.4.0 through 11.4.1
Global Traffic > ZoneRunner > Zone List
3. Click on the zone name of the affected CNAME wide IP. For example, if the
affected CNAME wide IP is test.example.com, the zone name should be
example.com.
Note: If the zone of the affected CNAME wide IP does not exist, you may
have to create one before proceeding to the next step. For more information
about creating a zone, refer to the Using ZoneRunner to Configure DNS Zones
chapter of the BIG-IP DNS Services: Implementations manual. For information
about how to locate F5 product manuals, refer to K12453464: Finding product
documentation on AskF5.
4. Click the Resource Records tab.
5. Click Create.
6. In the Name setting, type the affected CNAME wide IP.
For example:
test.example.com
7. For the TTL setting, enter your desired value in seconds.
8. For the Type setting, select TXT.
9. In the Text setting, type your desired text.
10. To save the changes, click Finished.
Creating an iRule to replace the DNS response status
To create an iRule to replace the DNS response status, perform the following
procedure:
Impact of workaround: The following iRule may not apply to all environments. F5
recommends authoring an iRule that works best for your application environment.
Additionally, adding an iRule increases the resources used by the associated
virtual server. Depending on the type and volume of the connections, the iRule
may introduce noticeable latency. F5 recommends testing any such changes in an
appropriate environment.
1. Log in to the Configuration utility.
2. Navigate to one of the following locations:
BIG-IP 11.5.0 and later
DNS > Delivery > iRules > iRule List
BIG-IP 11.4.0 through 11.4.1
Local Traffic > iRules > iRule List
3. Click Create.
4. In the Name setting, type a name for the iRule.
For example:
K23022557workaround
5. In the Definition setting, type an iRule similar to the following example,
replacing test.example.com with the affected CNAME wide IP:
when DNS_RESPONSE {
if { [DNS::question name] eq "test.example.com" } {
if { [DNS::header rcode] eq "NXDOMAIN" } {
DNS::header rcode NOERROR
DNS::authority clear
return
}
}
}
6. To save the iRule. click Finished.
7. Navigate to one of the following locations:
BIG-IP 11.5.0 and later
DNS > Delivery > Listeners > Listener List
BIG-IP 11.4.0 through 11.4.1
Local Traffic > Virtual Servers > Virtual Server List
8. Click the GTM/DNS listener. Typically the name of this listener ends in
_gtm.
9. Click the Resources tab.
10. In the iRules section. click Manage.
11. From the Available box, select the iRule you created in Steps 3 through 6.
12. To move the iRule to the Enabled box, click <<.
13. To save the changes, click Finished.
Acknowledgements
F5 would like to acknowledge Ray Krebs, Sam Cohoon, and Erik Espinoza of eBay
Inc. for bringing this issue to our attention, and for following the highest
standards of responsible disclosure.
Supplemental Information
o K41942608: Overview of AskF5 security advisory articles
o K9970: Subscribing to email notifications regarding F5 products
o K9957: Creating a custom RSS feed to view new and updated documents
o K4602: Overview of the F5 security vulnerability response policy
o K4918: Overview of the F5 critical issue hotfix policy
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