Channel 221, the SIE NX Domains channel is a source of DNS intelligence for DNS queries where the responding name server returned a “Non-Existent Domain” or “NXDomain” Response Code (RCODE). The name server returned this RCODE because the domain or hostname “does not exist” and could not be resolved. This channel is derived from Channel 220 DNS Errors, but only includes NXDomain RCODEs. This enables customers that are only interested in this specific response code to monitor it with a reduced bandwidth requirement.

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About Security Information Exchange (SIE)

The Security Information Exchange (SIE), from Farsight Security® Inc., is a scalable and adaptable real-time data streaming and information sharing platform. SIE collects and provides access to more than 200,000 observations per-second of raw data from its global sensor network. Farsight also applies unique and proprietary methods for improving usability of the data, directly sharing the refined intelligence with SIE customers and DNSDB®, one of the world’s largest passive DNS (pDNS) databases.

The diverse set of data available from SIE includes the following and is relevant and useful for practitioners in various technology roles:

Each unique set of data in SIE is known as a channel and the data acquired from a specific channel can be customized to meet the needs of each customer, enabling you to subscribe to and access only the channels needed to solve your problem. A channel in SIE may be the result from analyzing the data or a subset of data from other channels.

Why Passive DNS (pDNS)?

DNS is a critical component of Internet communication and almost all Internet transactions begin with a DNS query and response.

DNS serves as early warning and detection solution for phishing, spam, malicious and suspicious behaviors, and other attacks. DNS intelligence is considered the only source of “ground truth” information for the Internet.

Passive DNS (pDNS) begins with raw DNS traffic that is observed and collected by passive DNS sensors and contributed to Farsight’s Security Information Exchange (SIE) by pDNS sensor operators.

Farsight Security’s mission is to make the Internet a safer place. We provide security solutions that empower customers with meaningful and relevant intelligence. This information provides customers with insights about the network configuration of a threat and the surrounding network on the Internet for improving the value and impact of threat intelligence and research.

The Security Information Exchange (SIE), from Farsight Security Inc., is designed with privacy in mind. The passive DNS (pDNS) sensors do not collect Personally Identifiable Information (PII) from client resolvers (also known as stub) by deliberately collecting between recursive resolvers and authoritative servers.

The data from SIE enables security professionals to accurately identify, map, and protect their networks from cybercriminal activity by providing global visibility. It provides immediate access to a real-time global sensor network without the need to develop or deploy your own data collection infrastructure.

About SIE NX Domains

Channel 221 provides insights for DNS queries that return an “NXDomain” Response Code (RCODE). Billions of DNS queries occur every day and most return usable answers. However, in some cases DNS returns an RCODE because the domain you are interested in “does not exist”. A lot of interesting information can be discovered and learned about DNS by investigating “NXDomain” DNS messages.

Use Cases for SIE NX Domains

Customers can discover and learn a lot of interesting information about DNS by investigating these errors. By analyzing this DNS intelligence, customers can solve operational, security, risk, and brand protection related issues.

Detection of Domain Generation Algorithms (DGAs)

One significant use-case for Channel 221 NX Domains is identifying users infected with malware. Infected computers will attempt to contact command and control (C&C or C2) servers to receive instructions or commands on what it should do.

Criminals that create malware may use a Domain Generation Algorithm (DGA) to generate a large number of fallback domain names. This effort is an attempt to ensure the criminal does not lose control of the botnet when the primary C&C server is taken down by authorities. If the domain name of primary C&C server can not be resolved, the malware may attempt to resolve a fallback domain name created by the DGA and then connect to a secondary C&C server.

This behavior can generate an excessive amount of DNS resolution failures, which can be used to identify infected systems attempting to contact C&C servers.

DGAs created by malware authors are designed to prevent reverse-engineering. This makes is difficult for analysts and security practitioners to discover and identify C&C servers from DNS responses returning a non-zero Response Code (RCODE).

Additionally, malware has started to generate and test numerous domains to make it even more challenging to reverse engineer the DGA and track C&C servers. While that makes it more difficult to identify the C&C servers, the excessive amount of DNS erros makes it easier to identify the infected computers so they can be remediated.

See this for more on Domain Generation Algorithms.

Channel Information for SIE Channel 221

Channel Name NX Domains
Channel Number 221
Description Passive DNS observations where the responding server returned the “NXDomain” error.
Schema SIE:dnsnx
Bit Rate 35Mb/sec
Bit Rate (Max) 45Mb/sec
Payloads 45K/sec
Payloads (Max) 60K/sec

Note: Quoted bitrates and payloads are representative of SIE traffic as of June, 2021.

Access Methods for SIE Channel 221

SIE Direct Connect Yes
SIE Direct Connect Data Format NMSG
SIE Remote Access (SRA) Yes
SRA Data Format NMSG
SIE Batch Yes
SIE Batch Data Format NMSG
Advanced Exchange Access Middleware Daemon (AXAMD) No
AXAMD Data Format N/A

Data Format for SIE Channel 221

The NX Domains channel data uses the SIE NMSG dnsnx DNS Query and Response resource record schema that observes and collects “NXDomain” data returned from a query. Note, Channel 221 only returns a subset of the fields to reduce bandwidth requirements. The data available from this channel contains NMSG SIE:dnsnx type messages that include the following fields:

The NMSG header includes the following fields:

time Time when a “NXDomain” RCODE was observed in Channel 220.
vname Vendor Name, SIE.
mname Message type, dnsnx.
source Identifies pDNS operator contributing DNS data, not the sensor.
message Embedded JSON record describing the observed DNS Query and Response RR.

The embedded NMSG message payload is JSON formatted and includes the following fields:

qname Name of the queried resource record.
qclass Class of the query, most commonly IN.
qtype Type of the query.
response_ip IP address of the server responding to the query.
soa_rrname Authoritative domain name returned in the start of authority (SOA) record, if any.
response_time_sec Time in Unix epoch the response was received from server responding to the query.
response_time_nsec Time in nanoseconds for the response to be received.

Example Message from SIE Channel 221

Data acquired from Channel 221 NX Domains is returned in NMSG format. NMSG is an adaptable container format that allows for consistent or variable message types.

The nmsgtool program is a tool for acquiring a variety of different inputs, like data streams from the network, capturing data from network interfaces, reading data from files, or even standard input and making NMSG payloads available to one or more outputs. The nmsgtool program can acquire data from SIE Channel 221 and convert it to a ND-JSON (newline-delimited JSON) text format for display or additional processing and analysis. nmsgtool is a program written by Farsight and released as open source.

See the following pages for instructions on how to install software packages for a specific distribution.

After data for Channel 221 has been acquired, written, and saved to a file, you need to decode it to ND-JSON using nmsgtool. The [-r ch221_nxdomain.nmsg] option tells nmsgtool to read binary NMSG data from a file, [-c 1] limits the output to single NMSG payload, and [-J -] displays the record in ND-JSON format to stdout, which is typically the screen.

$ nmsgtool -r ch221_nxdomain.nmsg -c 1 -J -
{"time":"2020-01-13 17:46:47.995494902","vname":"SIE","mname":"dnsnx","source":"a1ba02cf",

If you want to display a pretty-printed output of ND-JSON formatted records, we recommend using jq, a lightweight and flexible command-line JSON processor.

The open source software package is available on Debian and can be installed using $ sudo apt-get install jq. The output from nmsgtool in JSON format [-J -] can be piped to jq using the following:

$ nmsgtool -r ch221_nxdomain.nmsg -c 1 -J - | jq '.'
  "time": "2020-01-13 17:46:47.995494902",
  "vname": "SIE",
  "mname": "dnsnx",
  "source": "a1ba02cf",
  "message": {
    "qname": "",
    "qclass": "IN",
    "qtype": "PTR",
    "response_ip": "",
    "soa_rrname": "",
    "response_time_sec": 1578937543,
    "response_time_nsec": 441845000

SIE Access Methods

Data from SIE can be accessed and acquired using the following methods:

For additional information about SIE access methods, please see the SIE Technical Overview document.

Direct Connect

SIE Direct Connect allows a customer to physically connect a server to the Farsight SIE network for maximum data throughput. This can be done in one of two ways:

If a blade server is leased from Farsight, it will be pre-installed with the essential software components needed to acquire, process, compress, buffer, and transfer data from SIE channels to the customer’s data center for additional analysis, enrichment, and storage.

If a customer uses their own server, an order can be submitted for a cross-connect to the SIE switches hosted at select Equinix data centers (Ashburn DC3 and Palo Alto SV8). An FSI account manager can help guide cross-connect provisioning details, hosting, or colocation options.

For additional information about SIE connection methods, please see the SIE Technical Overview document. A Farsight’s sales representatives is happy to share a copy of this document with you. This will help inform and guide you in understanding which connection method will work best for you.

SIE Remote Access (SRA)

SIE Remote Access (SRA) enables a customer to remotely connect to the Security Information Exchange (SIE) from anywhere on the Internet. SRA provides access to SIE channel data on customer’s local servers, allowing their analysis and processing systems to be located in their own data centers rather than physically co-located at a Farsight’s data center.

Due to the technical limitations of transporting high bitrate SIE channels across the Internet, the SRA access method is not available for all SIE channels. Please reference the SIE Channel Guide for channels that can be accessed using SRA:

SRA uses the Advanced Exchange Access (AXA) transport protocol which enables SRA sessions to perform the following:

The streaming search and filtering capabilities of AXA enables SRA to access and acquire meaningful and relevant data from SIE while avoiding the costs of transporting enormous volumes of data across the Internet.

Note: For high volume channels accessed using SRA, it is expected that customer’s will specify a search or filter for IP addresses and DNS domain names or hostnames of interest. The SRA service will only collect and send data matching the specified criteria across the Internet to the customer.

SIE Batch

SIE Batch provides on-demand access for downloading data from SIE channels using a RESTful API or web-based interface. You select the channel and duration of time you are interested in, and then download the data for analysis. The duration of available data is dependent on the channel, but is typically the most recent 12-18 hours. SIE Batch allows you to acquire data from SIE channel using two (2) methods:

Additional Information

About Farsight Security

Farsight Security, Inc. is the world’s leading provider of historical and real-time DNS intelligence solutions. We enable security teams to qualify, enrich, and correlate all sources of threat data and ultimately save time when it is most critical - during an attack or investigation. Our solutions provide enterprise, government, and security industry personnel and platforms with unmatched global visibility, context, and response. Farsight Security is headquartered in San Mateo, California, USA. To learn more about how we can empower your security, threat, and intelligence platforms and security organization with Farsight Security passive DNS (pDNS) and threat intelligence solutions, please visit us at or follow us on Twitter at @FarsightSecInc.