| Internet-Draft | Data for Filtered DNS | April 2022 |
| Wing, et al. | Expires 29 October 2022 | [Page] |
DNS filtering is widely deployed for network security, but filtered DNS responses lack information for the end user to understand the reason for the filtering. Existing mechanisms to provide detail to end users cause harm especially if the blocked DNS response is to an HTTPS website.¶
This document updates RFC8914's EXTRA-TEXT field to provide information on DNS filtering. This information can be parsed by the client and displayed, logged, or used for other purposes.¶
This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.¶
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This Internet-Draft will expire on 29 October 2022.¶
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DNS filters are deployed for a variety of reasons including endpoint security, parental filtering, and filtering required by law enforcement. Network-based security solutions such as firewalls and Intrusion Prevention Systems (IPS) rely upon network traffic inspection to implement perimeter-based security policies and operate by filtering DNS responses. In a home, DNS filtering is used for the same reasons as above and additionally for parental control. Internet Service Providers typically block access to some DNS domains due to a requirement imposed by an external entity (e.g., law enforcement agency) also performed using DNS-based content filtering.¶
Users of DNS services which perform filtering may wish to receive more information about such filtering to resolve problems with the filter -- for example to contact the administrator to allowlist a domain that was erroneously filtered or to understand the reason a particular domain was filtered. With that information, the user can choose another network, open a trouble ticket with the DNS administrator to resolve erroneous filtering, log the information, or other uses.¶
DNS responses can be filtered by sending a bogus (also called, "forged") A or AAAA response, NXDOMAIN error or empty answer, or an extended DNS error (EDE) code defined in [RFC8914]. Each of these methods have advantages and disadvantages that are discussed below:¶
The DNS response is forged to provide a list of IP addresses that points to an HTTP(S) server alerting the end user about the reason for blocking access to the requested domain (e.g., malware). When an HTTP(S) enabled domain name is blocked, the network security device (e.g., CPE, firewall) presents a block page instead of the HTTP response from the content provider hosting that domain. If an HTTP enabled domain name is blocked, the network security device intercepts the HTTP request and returns a block page over HTTP. If an HTTPS enabled domain is blocked, the block page is also served over HTTPS. In order to return a block page over HTTPS, man in the middle (MITM) is enabled on endpoints by generating a local root certificate and an accompanying (local) public/private key pair. The local root certificate is installed on the endpoint while the network security device(s) stores a copy of the private key. During the TLS handshake, the network security device modifies the certificate provided by the server and (re)signs it using the private key from the local root certificate.¶
However, configuring the local root certificate on endpoints is not a viable option in several deployments like home networks, schools, Small Office/Home Office (SOHO), and Small/ Medium Enterprise (SME). In these cases, the typical behavior is that the filtered DNS response points to a server that will display the block page. If the client is using HTTPS (via web browser or another application) this results in a certificate validation error which gives no information to the end-user about the reason for the DNS filtering. Browsers will display errors such as "The security certificate presented by this website was not issued by a trusted certificate authority" (Internet Explorer/Edge"), "The site's security certificate is not trusted" (Chrome), "This Connection is Untrusted" (Firefox), "Safari can't verify the identity of the website..." (Safari on MacOS). Applications might display even more cryptic error messages.¶
For both DNS filtering mechanisms described above, the DNS server can return extended error codes Blocked, Censored, Filtered, or Forged Answer defined in Section 4 of [RFC8914]. However, these codes only explain that filtering occurred but lack detail for the user to diagnose erroneous filtering.¶
No matter which type of response is generated (forged IP address(es), NXDOMAIN or empty answer, even with an extended error code), the user who triggered the DNS query has little chance to understand which entity filtered the query, how to report a mistake in the filter, or why the entity filtered it at all. This document describes a mechanism to provide such detail.¶
One of the other benefits of this approach is to eliminate the need to "spoof" block pages for HTTPS resources. This is achieved since clients implementing this approach would be able to display a meaningful error message, and would not need to connect to such a block page. This approach thus avoids the need to install a local root certificate authority on those IT-managed devices.¶
This document describes a format for computer-parsable data in the EXTRA-TEXT field of Extended DNS Errors [RFC8914].¶
This document does not recommend DNS filtering but provides a mechanism for better transparency to explain to the users why some DNS queries are filtered.¶
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119][RFC8174] when, and only when, they appear in all capitals, as shown here.¶
This document uses terms defined in DNS Terminology [RFC8499].¶
"Requestor" refers to the side that sends a request. "Responder" refers to an authoritative, recursive resolver or other DNS component that responds to questions. Other terminology is used here as defined in the RFCs cited by this document.¶
"Encrypted DNS" refers to any encrypted scheme to convey DNS messages, for example, DNS-over-HTTPS [RFC8484], DNS-over-TLS [RFC7858], or DNS-over-QUIC [I-D.ietf-dprive-dnsoquic].¶
Servers compliant with this specification send I-JSON data in the EXTRA-TEXT field [RFC8914] using the Internet JSON (I-JSON) message format [RFC7493].¶
This document defines the following JSON names:¶
New JSON names MUST be defined in the IANA registry (Section 7), consist only of lower-case ASCII characters, digits, and hyphens (that is, Unicode characters U+0061 through 007A, U+0030 through U+0039, and U+002D). These names MUST be 63 characters or shorter and it is RECOMMENDED they be as short as possible.¶
To reduce packet overhead the generated JSON SHOULD be as short as possible: short domain names, concise text in the values for the "j" and "o" names, and minified JSON (that is, without spaces or line breaks between JSON elements).¶
The JSON data can be parsed to display to the user, logged, or otherwise used to assist the end-user or IT staff with troubleshooting and diagnosing the cause of the DNS filtering.¶
When generating a DNS query, the client MUST include the OPT pseudo-RR [RFC6891] to elicit the Extended DNS Error option [RFC8914] in the DNS response.¶
When the DNS server filters its DNS response to an A or AAAA record query, the DNS response MAY contain an empty answer, NXDOMAIN, or a forged A or AAAA response, as desired by the DNS server. In addition, if the query contained the OPT pseudo-RR the DNS server MAY return more detail in the EXTRA-TEXT field as described in Section 4.3.¶
Servers may decide to return small TTL values in filtered DNS responses (e.g., 2 seconds) to handle domain category and reputation updates.¶
On receipt of a DNS response with an Extended DNS Error option, the following actions are performed if the EXTRA-TEXT field contains valid JSON:¶
An example showing the nameserver at 'ns.example.net' that filtered a DNS "A" record query for 'example.org' is shown in Figure 1.¶
{
"c": ["tel:+358-555-1234567", "sips:bob@bobphone.example.com",
"https://ticket.example.com?d=example.org&t=1650560748"],
"j": "malware present for 23 days",
"o": "example.net Filtering Service"
}
In Figure 2 the same content is shown with minified JSON (no whitespace, no blank lines) with '\' line wrapping per [RFC8792].¶
============== NOTE: '\' line wrapping per RFC 8792 ===============
{"c":["tel:+358-555-1234567","sips:bob@bobphone.example.com", \
"https://ticket.example.com?d=example.org&t=1650560748"], \
"j":"malware present for 23 days","o":"example.net Filtering \
Service"}
Security considerations in Section 6 of [RFC8914] apply to this document.¶
To minimize impact of active on-path attacks on the DNS channel, the client validates the response as described in Section 4.3.¶
A client might choose to display the information in the EXTRA-TEXT field if and only if the encrypted resolver has sufficient reputation, according to some local policy (e.g. user configuration, administrative configuration, or a built-in list of respectable resolvers). This limits the ability of a malicious encrypted resolver to cause harm. If the client decides not to display the all of the information in the EXTRA-TEXT field, it can be logged for diagnostics purpose and the client can only display the resolver hostname that blocked the domain and error description for the EDE code to the end-user.¶
When displaying the free-form text of "c" and "j", the browser SHOULD NOT make any of those elements into actionable (clickable) links.¶
An attacker might inject (or modify) the EDE EXTRA-TEXT field with an DNS proxy or DNS forwarder that is unaware of EDE. Such a DNS proxy or DNS forwarder will forward that attacker-controlled EDE option. To prevent such an attack, clients supporting this document MUST discard the EDE option if their DNS server does not signal EDE support via RESINFO [I-D.reddy-add-resolver-info]. As recommended in [I-D.reddy-add-resolver-info], RESINFO should be retrieved over an encrypted DNS channel or integrity protected with DNSSEC.¶
This document requests IANA to register the "application/json+structured-dns-error" media type in the "Media Types" registry [IANA-MediaTypes]. This registration follows the procedures specified in [RFC6838]:¶
Type name: application
Subtype name: json+structured-dns-error
Required parameters: N/A
Optional parameters: N/A
Encoding considerations: as defined in Section NN of [RFCXXXX].
Security considerations: See Section NNN of [RFCXXXX].
Interoperability considerations: N/A
Published specification: [RFCXXXX]
Applications that use this media type: Section NNNN of [RFCXXXX].
Fragment identifier considerations: N/A
Additional information: N/A
Person & email address to contact for further information: IETF,
iesg@ietf.org
Intended usage: COMMON
Restrictions on usage: none
Author: See Authors' Addresses section.
Change controller: IESG
Provisional registration? No¶
This section is to be removed before publishing as an RFC.¶