DNS Query and Response Defaults
Table of contents
- Overview
- Application of this specification
- Default setting in DNS Query
- Default setting in EDNS Query
- Default setting in DNSSEC Query
- Default handling of a DNS Response
- Default handling of an EDNS Response
- Default handling of a DNSSEC response
- Appendix A: UDP Message size setting in EDNS
Overview
Almost all test cases emit DNS queries and react to the responses. This document defines the default "setting" of a DNS query and the default handling of "setting" in the DNS response. The meaning of "setting" here will be clear from the specification below.
Application of this specification
This specification applies to all test case specifications that has an explicit reference to this document.
Default setting in DNS Query
A DNS Query has the following default setting. A test case specification can refer to a DNS Query with one or several changes to the Parameters overriding the default values. If a Parameter is specified as "fixed" (with an "X" in that column) then the default value cannot be overidden.
| Parameter | Default value | Fixed | Comment |
|---|---|---|---|
| Protocol | UDP | ||
| OpCode | "query" | X | |
| QR flag | unset | X | |
| AA flag | unset | X | |
| TC flag | unset | X | |
| RD flag | unset | ||
| RA flag | unset | X | |
| AD flag | unset | ||
| CD flag | unset | ||
| RCODE | "NoError" | X | |
| Query Name | - | Must be defined in test case | |
| Query Type | - | Must be defined in test case | |
| Query Class | "IN" | ||
| EDNS | no | No OPT record is included |
Default setting in EDNS Query
An EDNS Query inherit the default setting from a DNS Query except for the parameters specified below. If a Parameter is specified as "fixed" (with an "X" in that column) then the default value cannot be overidden.
A test case specification can refer to an EDNS Query with one or several changes to the Parameters.
| Parameter | Default value | Fixed | Comment |
|---|---|---|---|
| EDNS | OPT record included | X | |
| EDNS UDP Message size | 512 | See Appendix A | |
| EDNS Extended RCODE | no | ||
| EDNS Version | 0 | ||
| EDNS DO flag | unset | ||
| EDNS Z flag | unset | ||
| EDNS0 Option | none |
Default setting in DNSSEC Query
A DNSSEC Query inherits the default setting from an EDNS Query except for the parameter specified below. If a Parameter is specified as "fixed" (with an "X" in that column) then the default value cannot be overidden.
A test case specification can refer to a DNSSEC Query with one or several changes to the Parameters.
| Parameter | Default value | Fixed | Comment |
|---|---|---|---|
| EDNS DO flag | set | X | |
| EDNS UDP Message size | 1232 | See Appendix A | |
Default handling of a DNS Response
A DNS Response is a response to a DNS Query. Unless specified in the test case specification, the items in the response are handled as listed in the table below.
Validation of DNS message
If a Response Item is specified as "fixed" (with an "X" in that column) then the requirement, as specified under "Default handling", must be met for the response to be considered to be a DNS response.
| Response Item | Default handling | Fixed | Comment |
|---|---|---|---|
| OpCode | Require value to be "response" | X | |
| QR flag | Require flag to be set | X | |
| AA flag | - | Defined in test case | |
| TC flag | Re-query over TCP if set | ||
| RD flag | ignore | ||
| RA flag | ignore | ||
| AD flag | ignore | ||
| CD flag | ignore | ||
| RCODE | - | Defined in test case | |
| Query Name | ignore | ||
| Query Type | ignore | ||
| Query Class | Require value to be same as in the query | Normally "IN" | |
| EDNS | ignore |
Extraction of DNS records
-
Owner name and record type of a DNS record are compared, by default, against Query Name and Query Type in the question section in the query, not in the response.
-
When fetching records from the answer section, these are the default criteria:
- Only records matching Query Name and Query Type are fetched. Any RRSIG records meeting the next criterion are also fetched.
- RRSIG records matching Query Name and covering Query Type are fetched.
- CNAME records are ignored unless Query Type is CNAME.
-
When the test case specification states that a CNAME chain is to be followed, the default handling is to only follow a CNAME chain, and fetch the records, if the CNAME chain is valid.
- The chain is, by default, considered to be valid if the following criteria
are met:
- It must be possible to arrange all CNAME records from the answer section into a contiguous logical chain with a possible addition of a non-CNAME record whose owner name matches the RDATA of the last CNAME record.
- The owner name of the first CNAME record in the chain must match Query Name.
- The last record in the chain must either be a CNAME or a record matching Query Type.
- For each owner name of the CNAME records in the chain there must not be any additional records in the answer section with the same owner name besides RRSIG and NSEC records (that may exist).
- CNAME records not part of a valid CNAME chain are by default ignored.
- The chain is, by default, considered to be valid if the following criteria
are met:
-
Authority and additional sections are by default ignored.
The test case specification may prescribe that CNAME records are handled in a different way than the default above.
Default handling of an EDNS Response
An EDNS response is a response to an EDNS Query. An EDNS Response inherits the default handling from a DNS Response except for the response items specified below. Unless specified in the test case specification, the items in the response are handled using the default handling.
| Response Item | Default handling | Comment |
|---|---|---|
| EDNS | Take note if OPT is missing | Further actions to be specified in the test case specification |
Default handling of a DNSSEC response
A DNSSEC response is a response to a DNSSEC Query. A DNSSEC Response inherits the default handling from a EDNS Response except for the response items specified below. Unless specified in the test case specification, the items in the response are handled using the default handling.
| Response Item | Default handling | Comment |
|---|---|---|
| EDNS DO flag | Take note if DO flag is missing | Further actions to be specified in the test case specification |
Appendix A: UDP Message size setting in EDNS
In non-DNSSEC messages the Zonemaster choice is to set the EDNS UDP Message size to 512 to prevent any firewalls from blocking a response. This guarantees that the response is never larger than the non-EDNS limit, assuming that the remote server respects the setting.
In DNSSEC messages the Zonemaster choice is to set the EDNS UDP Message size to a value that makes fragmentation unlikely, even though fragmentation of UDP messages is supported. 1280 is the smallest MTU supported by IPv6. When 48 bytes for IPv6 and UDP headers are removed the value is 1232 bytes, which is the Zonemaster choice and also the recommendation in DNS flag day 2020.
At the time of writing there is an active IETF draft, not yet an RFC, that recommends 1400 bytes as the EDNS UDP Message size. See IP Fragmentation Avoidance in DNS over UDP.
Choosing a small value is permitted. It might result in more truncated responses and requerying over TCP.