Hi Mukund, Many thanks for the detailed review.
We updated the spec to address many of your suggestions. We added in particular text to explain the rationale for having c/j mandatory for IT teams and also added text to explain why suberr is defined rather than consuming ede codes. However, no changes were made for your comments about, e.g., translation/TTL, as that part of the text was added to address previous comments we received from the WG. FWIW, a diff can be seen at: https://author-tools.ietf.org/iddiff?url1=draft-ietf-dnsop-structured-dns-error-03&url2=draft-ietf-dnsop-structured-dns-error-05&difftype=--html Thanks again. Cheers, Med > -----Message d'origine----- > De : DNSOP <dnsop-boun...@ietf.org> De la part de Mukund Sivaraman > Envoyé : mardi 27 juin 2023 19:29 > À : dnsop@ietf.org > Objet : [DNSOP] Review of draft-ietf-dnsop-structured-dns-error-03 > > We're implementing RFC 8914 for error reporting from NIOS's DNS > features, and so I read this draft today: > > > DNS Operations Working Group > D. Wing > > Internet-Draft > Citrix > > Updates: 8914 (if approved) > T. Reddy > > Intended status: Standards Track > Nokia > > Expires: 28 November 2023 > N. Cook > > > Open-Xchange > > M. > Boucadair > > > Orange > > 27 > May > > 2023 > > > > Structured Error Data for Filtered DNS > > draft-ietf-dnsop-structured-dns-error-03 > > > Abstract > > > DNS filtering is widely deployed for various reasons, > including > > network security. However, filtered DNS responses lack > information > > for end users to understand the reason for the filtering. > > What is claimed in this introduction does not appear entirely > accurate after RFC 8914. > > * RFC 8914 allows filtered DNS responses to contain the EDE EDNS > option > that can contain a populated EXTRA-TEXT field specifically for > end-users. > > * Whether the EXTRA-TEXT field is populated, and what is entered > there > is optional. Several of our customers would not even want to > indicate > that responses have been filtered (we've had trouble with even > returning an SOA record in RPZ-rewritten answers), so the > optional > nature of this field is fine. > > It appears the authors want to structure it so it can be used to > display a generated error-page within a web-browser, to avoid > loading an error-page via HTTPS, which is a fine goal. > > > Existing mechanisms to provide explanatory details to end users > cause > > harm especially if the blocked DNS response is to an HTTPS > server. > > I suggest rewording this sentence as it reads as though the > blocked DNS response is sent to a HTTPS server. Instead, I assume > what the authors are attempting to state is that blocked DNS > responses may cause harm if they contain address records that > cause web traffic for a web-domain to be directed to a different > local HTTPS server. > > > > 1. Introduction > > > DNS filters are deployed for a variety of reasons, e.g., > endpoint > > security, parental filtering, and filtering required by law > > enforcement. > > I suggest keeping the above part of the paragraph ... > > > 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 network, DNS filtering > is used > > for the same reasons as above and additionally for parental > control. > > Internet Service Providers (ISPs) 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. > > ... and removing the part above as it's unnecessary extra detail, > which may not age well. > > > Users of DNS services that perform filtering may wish to > receive more > > explanatory information about such a filtering to resolve > problems > > s/such a filtering/such filtering/ > > > with the filter -- for example to contact the administrator > to > > allowlist a DNS domain that was erroneously filtered or to > > understand > > s/allowlist/allow/ > > > the reason a particular domain was filtered. With that > information, > > a user can choose another network, open a trouble ticket with > the > > DNS > > s/a user can choose another network/a user can choose to use > another network/ > > > administrator to resolve erroneous filtering, log the > information, or > > other uses. > > s/or other uses/or do something else with it/ > > > For the DNS filtering mechanisms described in Section 3 the > DNS > > server can return extended error codes Blocked, 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 filterings. > > > 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. > > This has to be reworded, because the user can understand what > happened from the EXTRA-TEXT field of RFC 8914 if the information > is provided. > This draft's purpose appears to be about structuring such > information for display in a UI, so the authors should describe it > so. > > > One of the other benefits of the approach described in this > document > > 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 is a fine goal and I like it, but: > > (1) Is the need to spoof a website gone? What about clients that > do not support this draft? They may still receive e.g. address > records from a resolver that performs filtering, and attempt to > use those addresses. > > (2) Are web-browser developers on board with implementing such a > draft? > > > This document describes a format for computer-parsable data > in the > > EXTRA-TEXT field of [RFC8914]. It updates Section 2 of > [RFC8914] > > which says the information in EXTRA-TEXT field is intended > for human > > consumption (not automated parsing). > > This is the abstract of the draft that describes its purpose > neatly; I suggest placing this text in the abstract. > > > 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. > > > 2. Conventions and Definitions > > > 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. > > > "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 [RFC9250]. > > > The document refers to an Extended DNS Error (EDE) using its > purpose, > > not its INFO-CODE as per Table 3 of [RFC8914]. "Forged > Answer", > > "Blocked", and "Filtered" are thus used to refer to "Forged > Answer > > (4)", "Blocked (15)", and "Filtered (17)". > > > 3. DNS Filtering Techniques and Their Limitations > > > 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 code defined in [RFC8914]. > > Does this enumerate all the cases? RPZ and some other lying > features in DNS implementations allow returning forged answers for > other types too. I suggest removing the "A or AAAA" part, or > starting the paragraph as "DNS responses to address queries can be > filtered...". > > I don't understand the last part, i.e., "DNS responses can be > filtered by sending ... an Extended DNS Error code defined in > [RFC8914]". How are responses filtered by sending an EDE EDNS > option? > > > Each of these methods have advantages and disadvantages that are > > discussed below: > > > > > > > Wing, et al. Expires 28 November 2023 > [Page 4] > > > > > > Internet-Draft Structured DNS Error > May 2023 > > > > 1. 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., Customer Premises > Equipment (CPE) > > or 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 stores a copy > of the > > private key. During the TLS handshake, the on-path > 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), or > 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 a > > 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. > > > * Enterprise networks do not assume that all the > connected > > devices are managed by the IT team or Mobile Device > Management > > (MDM) devices, especially in the quite common Bring > Your Own > > Device (BYOD) scenario. In addition, the local root > > certificate cannot be installed on IoT devices without > a > > device management tool. > > > * An end user does not know why the connection was > prevented > > and, consequently, may repeatedly try to reach the > domain but > > with no success. Frustrated, the end user may switch > to an > > alternate network that offers no DNS filtering against > malware > > and phishing, potentially compromising both security > and > > privacy. Furthermore, certificate errors train users > to click > > through certificate errors, which is a bad security > practice. > > To eliminate the need for an end user to click through > > certificate errors, an end user may manually install a > local > > root certificate on a host device. Doing so, however, > is also > > a bad security practice as it creates a security > > vulnerability > > > that may be exploited by a MITM attack. When a > manually > > installed local root certificate expires, the user has > to > > (again) manually install the new local root > certificate. > > The above is a good explanation, but it appears excessive, > deviating from the gist of the draft. Instead, the authors could > concisely write that a client may see certificate errors when > attempting to browse error pages due to the redirection of HTTPS > traffic to an unauthentic webserver. > > > > 2. The DNS response is forged to provide a NXDOMAIN response > to > > cause the DNS lookup to terminate in failure. In this > case, an > > end user does not know why the domain cannot be reached > and may > > repeatedly try to reach the domain but with no success. > > Frustrated, the end user may use insecure connections to > reach > > the domain, potentially compromising both security and > privacy. > > This appears to be a good example where having a structured error > is an advantage over the current practice. > > > 3. The extended error codes Blocked and Filtered defined in > > Section 4 of [RFC8914] can be returned by a DNS server to > provide > > additional information about the cause of a DNS error. > > > 4. These extended error codes do not suffer from the > limitations > > discussed in bullets (1) and (2), but the user still does > not > > know the exact reason nor he/she is aware of the exact > entity > > I suggest removing "he/she"; it reads well without it. > > > blocking the access to the domain. For example, a DNS > server may > > block access to a domain based on the content category > such as > > "Malware" to protect the endpoint from malicious > software, > > "Phishing" to prevent the user from revealing sensitive > > information to the attacker, etc. A user needs to know > the > > contact details of the IT/InfoSec team to raise a > complaint. > > A user does not "need to know". "A user may want to know" is > perhaps better text here. Depending on the feature implementing > filtering (e.g., RPZ vs. parental control) I doubt if some of our > customers would even indicate if rewriting/filtering occurred > going by past activity. So all this is optional. > > > 5. When a DNS resolver or forwarder forwards the received > EDE > > option, the EXTRA-TEXT field only conveys the source of > the error > > (Section 3 of [RFC8914]) and does not provide additional > textual > > information about the cause of the error. > > This claim does not appear accurate. RFC 8914 section 3 mentions > "the source of the error SHOULD be attributed in the EXTRA-TEXT > field" in the context of forwarding only, and just that the source > is also included alongside any other text that may also be > present. I don't see that there are any limitations on what may be > included in the EXTRA-TEXT field, and there could even be multiple > EDE options. > > > 4. I-JSON in EXTRA-TEXT Field > > > DNS servers that are compliant with this specification send > I-JSON > > data in the EXTRA-TEXT field [RFC8914] using the Internet > JSON > > (I-JSON) message format [RFC7493]. > > Reword as "DNS servers that are compliant with this specification > send data in the EXTRA-TEXT field [RFC8914] encoded using the > Internet JSON > (I-JSON) message format [RFC7493]." > > However, using some kind of binary encoding may reduce the size of > the EDE option. DNS client software can decode the binary encoding > for human-readable presentation. > > > Note that [RFC7493] was based on [RFC7159], but [RFC7159] > was > > replaced by [RFC8259] > . > > This document defines the following JSON names: > > > c: (contact) The contact details of the IT/InfoSec team to > report > > mis-classified DNS filtering. This field is structured as > an > > array of contact URIs (e.g., tel, sips, https). At least > one > > contact URI MUST be included. This field is mandatory. > > I suggest not making it mandatory. The operator may wish to > indicate that filtering occurred without attracting flak from some > users. > > Also, just how many IT teams respond or take action when something > is reported to them? Not all do. > > > j: (justification) 'UTF-8'-encoded [RFC5198] textual > justification > > > for this particular DNS filtering. The field should be > treated > > only as diagnostic information for IT staff. This field > is > > mandatory. > > I suggest not making it mandatory. If it is mandatory, some > operators may put whitespace or "N/A" in the field's value. > > > s: (suberror) the suberror code for this particular DNS > filtering. > > This field is optional. > > There are 16 bits in RFC 8914's INFO-CODEs, allowing a total of > 65536 values. I suggest adding more values there than starting > another registry. Try to keep text and constructs as simple as it > can be. > There's already an RCODE, and an EDE INFO-CODE, and the draft's > proposing a 3rd level of suberror code. This is excessive. > > > o: (organization) 'UTF-8'-encoded human-friendly name of the > > organization that filtered this particular DNS query. > This field > > is optional. > > > New JSON names can be defined in the IANA registry introduced > in > > Section 11.2. Such names MUST 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). Also, > these > > names MUST be 63 characters or shorter and it is RECOMMENDED > they > > be as short as possible. > > > The text in the "j" and "o" names can include international > > characters. If the text is displayed in a language not known > to the > > end user, browser extensions to translate to user's native > language > > can be used. > > I suggest deleting this paragraph. It has already been mentioned > elsewhere that the fields are UTF-8 encoded. Describing browser > extentions to translate strings deviates from the topic of this > draft. > > > 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). > > s/packet overhead/DNS message size/ > > It isn't exactly "overhead" if it's the contents. > > > 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. > > > 5. Protocol Operation > > > 5.1. Client Generating Request > > > When generating a DNS query the client includes the EDE > option > > (Section 2 of [RFC8914]) in the OPT pseudo-RR [RFC6891] to > elicit the > > EDE option in the DNS response. > > Is this what this draft requires, or is the sentence above > claiming that RFC 8914 requires it as well? > > > It SHOULD use an OPTION-LENGTH of 2, > > the INFO-CODE field set to "0" (Other Error), and an empty > EXTRA-TEXT > > field. This signal indicates that the client desires that > the server > > responds in accordance with the present specification. > > Why? > > This draft appears to describe a simple extension of EDE, limited > to structuring the value in the EXTRA-TEXT field. > > > 5.2. Server Generating 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 > > (less ideally) forged A or AAAA response, as desired by the > DNS > > server. > > Again, limiting to A/AAAA is not correct. Other RR types are also > filtered by filtering features in implementation today. > > > 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 5.3. > > > Servers may decide to return small TTL values in filtered DNS > > responses (e.g., 2 seconds) to handle domain category and > reputation > > updates. > > Suggesting TTLs here appears off-topic. The draft is just about > structuring the EDE.EXTRA-TEXT field. > > > Because the DNS client signals its EDE support (Section 5.1) > and > > because EDE support is signaled via a non-cached OPT resource > record > > (Section 6.2.1 of [RFC6891]) the EDE-aware DNS server can > tailor its > > filtered response to be most appropriate to that client's EDE > > support. If EDE support is signaled in the query the server > MUST NOT > > return the "Forged Answer" extended error code because the > client can > > take advantage of EDE's more sophisticated error reporting > (e.g., > > "Filtered", "Blocked"). Continuing to send "Forged Answer" > even to > > an EDE-supporting client will cause the persistence of the > drawbacks > > described in Section 3. > > Why? What if forged-answer info-code is returned? A web user-agent > can ignore the answer section and use the structured EXTRA-TEXT > field anyway. The paragraph above is not very clear. > > > 5.3. Client Processing Response > > > On receipt of a DNS response with an EDE option from a DNS > responder, > > the following actions are performed on the EXTRA-TEXT field: > > > * Verify the field contains valid JSON. If not, the > requestor MUST > > discard data in the EXTRA-TEXT field. > > > * The response MUST be received over an encrypted DNS > channel. If > > not, the requestor MUST discard data in the EXTRA-TEXT > field. > > Why is this additional requirement present over RFC 8914? RFC 8914 > does not prevent unencrypted DNS transports from carrying a human > readable EXTRA-TEXT field (which may be displayed by a client > program such as 'dig'). > > > * Servers which don't support this specification might use > plain > > text in the EXTRA-TEXT field so that requestors SHOULD > properly > > s/in the EXTRA-TEXT field so that/field, and so/ > > > > 6. Interoperation with RPZ Servers > > > This section discusses operation with an RPZ server [RPZ] > that > > indicates filtering with a NXDOMAIN response with the > Recursion > > Available bit cleared (RA=0). > > > When a DNS client supports this specification it includes the > EDE > > option in its DNS query. > > > If the server does not support this specification and is > performing > > RPZ filtering, the server ignores the EDE option in the DNS > query and > > replies with NXDOMAIN and RA=0. The DNS client can continue > to > > accept such responses. > > > If the server does support this specification and is > performing RPZ > > filtering, the server can use the EDE option in the query to > identify > > an EDE-aware client and respond appropriately (that is, by > generating > > a response described in {#server-response}) as NXDOMAIN and > RA=0 are > > not necessary when generating a response to such a client. > > If NXDOMAIN is not necessary, what RCODE will be returned in RPZ- > rewritten responses where NXDOMAIN is returned today? > > > Thank you for attempting this. Some of what is described appears > as an improvement over what is practised currently. > > The review above may seem overly critical because it mostly > contains suggestions to correct/change text, rather than praise > for what is good. > > However, overall, after reading the draft till the end and > considering what it proposes, I think that based on the INFO-CODE, > having a web browser display the EXTRA-TEXT field of RFC 8914 as a > text string would serve the purpose of displaying information > (whatever the operator desires to show) about what filtering > occurred, and any other free-text such as contact information. > Perhaps additional EDE INFO-CODEs may be added to the RFC 8914 > registry as required, and it may be prescribed that browsers avoid > loading web-pages with address records from forged answers and > instead display the EXTRA-TEXT field. > > This JSON encoding of the same and adding a registry for fields > within it appears excessive for the purpose it aims to serve. > > Mukund ____________________________________________________________________________________________________________ Ce message et ses pieces jointes peuvent contenir des informations confidentielles ou privilegiees et ne doivent donc pas etre diffuses, exploites ou copies sans autorisation. 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