MDNS的漏洞报告——mdns的最大问题是允许广域网的mdns单播查询，这会暴露设备信息，或者被利用用于dns放大攻击

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Vulnerability Note VU#550620
Multicast DNS (mDNS) implementations may respond to unicast queries originating outside the local link
Original Release date: 31 三月 2015 | Last revised: 15 五月 2015
转自：http://www.kb.cert.org/vuls/id/550620
文中说得很明白，mdns的最大问题是允许广域网的mdns单播查询，这会暴露设备信息，或者被利用用于dns放大攻击。
解决方法：（1）考虑在WAN处屏蔽MDNS UDP端口5353的流量进入或离开，就是说不允许5353的mdns流量流入广域网。（2）禁用mDNS服务
　　



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Overview
　　Multicast DNS implementations may respond to unicast queries that originate from sources outside of the local link network. Such responses may disclose information about network devices or be used in denial-of-service (DoS) amplification attacks.
Description

Multicast DNS (mDNS) is a way for devices on a local link network to automatically discover other services and devices. In some implementations of mDNS, the mDNS server replies to unicast queries from outside the link local network (e.g., the WAN). This mDNS response may result in information disclosure of devices on the network. Furthermore, the information returned in the response is greater in size than the query and may be used for denial-of-service (DoS) amplification. 　　RFC 6762 Section 5.5 states the following: "In specialized applications there may be rare situations where it   makes sense for a Multicast DNS querier to send its query via unicast   to a specific machine.  When a Multicast DNS responder receives a   query via direct unicast, it SHOULD respond as it would for "QU"   questions, as described above in Section 5.4.  Since it is possible   for a unicast query to be received from a machine outside the local   link, responders SHOULD check that the source address in the query   packet matches the local subnet for that link (or, in the case of   IPv6, the source address has an on-link prefix) and silently ignore   the packet if not.    There may be specialized situations, outside the scope of this   document, where it is intended and desirable to create a responder   that does answer queries originating outside the local link." While unicast queries originating from outside the local link are not specifically disallowed, RFC 6762 recommends to ignore any such packets. Some implementations of mDNS do however respond to unicast queries originating outside the local link, possibly for specialized use cases beyond the scope of RFC 6762. In these circumstances, the mDNS response to a query from outside the local link allows for information disclosure about devices on the network, such as model number and operating system. Additionally, the mDNS response to a query from outside the local link may be used for denial of service amplification attacks, due to the larger response size compared to the query size. More information can be found in security researcher's blog.

Impact

An mDNS response to a unicast query originating outside of the local link network may result in information disclosure, such as disclosing the device type/model that responds to the request or the operating system running such software. The mDNS response may also be used to amplify denial of service attacks against other networks.

Solution

Block inbound and outbound mDNS on the WAN If such mDNS behavior is not a requirement for your organization, consider blocking the mDNS UDP port 5353 from entering or leaving your local link network.

Disable mDNS services Some software and devices may allow disabling of the mDNS services. Please consult with the vendor of your product.

Vendor Information (Learn More)

Despite attempts to analyze scan results, it is not entirely clear exactly which software responds to mDNS queries. Vendors have been alerted, but currently only a small number of devices have been confirmed to respond to unicast queries from the WAN. In Linux, the Avahi software is also known to allow unicast queries. Listed below are vendors that are affected, in the sense that their software or devices by default can respond to unicast queries from outside the link local network. While this technically follows established RFCs and is not a vulnerability in the normal sense, for reasons outlined above this may be unwanted behavior. If you are aware of a software or device that responds to mDNS unicast queries from outside the local link, please contact us.

VendorStatusDate NotifiedDate UpdatedAvahi mDNSAffected-31 Mar 2015CanonAffected10 Feb 201508 Apr 2015Hewlett-Packard CompanyAffected10 Feb 201520 Mar 2015IBM CorporationAffected10 Feb 201531 Mar 2015SynologyAffected10 Feb 201531 Mar 2015Cisco Systems, Inc.Not Affected10 Feb 201531 Mar 2015CitrixNot Affected10 Feb 201525 Mar 2015D-Link Systems, Inc.Not Affected10 Feb 201520 Mar 2015F5 Networks, Inc.Not Affected10 Feb 201531 Mar 2015Microsoft CorporationNot Affected10 Feb 201509 Mar 2015Ricoh Company Ltd.Not Affected10 Feb 201515 May 2015AppleUnknown10 Feb 201510 Feb 2015CentOSUnknown10 Feb 201510 Feb 2015Debian GNU/LinuxUnknown10 Feb 201510 Feb 2015Dell Computer Corporation, Inc.Unknown10 Feb 201510 Feb 2015If you are a vendor and your product is affected, let us know.View More »

CVSS Metrics (Learn More)
GroupScoreVectorBase6.4AV:N/AC:L/Au:N/C:P/I:N/A:PTemporal5.2E:POC/RL:W/RC:UREnvironmental3.9CDP:ND/TD:M/CR:ND/IR:ND/AR:NDReferences

• http://www.ietf.org/rfc/rfc6762.txt
  
• https://github.com/chadillac/mdns_recon