Support for Authoritative Domains, NXDOMAIN Responses, NS and SOA Records
When an Avi DNS virtual service has a pass-through pool (of back-end servers) configured, and the FQDNs are not found in the DNS table, the Avi DNS virtual service proxies these requests to the pool of servers. An exception is when Avi is configured with an authoritative domain, and the queried FQDN is within the authoritative domain, in which case an NXDOMAIN is returned.
Starting with release 17.2.4, an Avi Vantage DNS virtual service includes a Start of Authority (SOA) record with its NXDOMAIN (and other) replies.
NOTE: Responses to SOA queries are not supported prior to Avi Vantage release 18.2.5. Refer to the Support for SOA rdata Queries section of this article.
Related Reading
Features
An NXDOMAIN (“the domain does Not eXist”) response is accompanied by an SOA record if the incoming query’s domain is a subdomain of one of the configured authoritative domains in the DNS application profile.
Negative caching, i.e., the caching of the fact of non-existence of a record, is determined by name servers authoritative for a zone which must include the Start of Authority (SOA) record when reporting no data of the requested type exists. The value of the minimum field of the SOA record and the TTL of the SOA itself is used to establish the TTL for the negative answer.
If the query’s FQDN matches an entry in the DNS table but the query type is not supported, by default the Avi SE generates a NOERROR response, optionally with an SOA record if the domain matches a configured authoritative domain.
Configuration Using the Avi UI
Refer to figure 1.
Queries for FQDNs that are subdomains of the authoritative domain names and do not have any DNS record in Avi are dropped or the NXDOMAIN response is sent. The Avi System-DNS profile comes preconfigured to respond to unhandled DNS requests. However, when creating a DNS profile afresh, the default value of the (Options for) Invalid DNS Query processing field is set to drop unhandled requests. Change it to respond to ensure NXDOMAIN responses get sent when appropriate.
When an NXDOMAIN reply is appropriate for an FQDN that ends with one of the authoritative domains, the value appearing in the Negative TTL field will be incorporated into the attached SOA record. Note the 30-second default; the allowed range is 1 to 86400 seconds.
An Avi DNS VS need not have a back-end DNS server pool. If it does have a back-end pool, the Avi DNS’ Service Engines will only load balance to it if the FQDN is not a subdomain of one of those configured in the Authoritative Domain Names field. All are configured with Ends-With semantics.
Values in the Valid subdomains field are for validity checking and thus optional. If not configured, all subdomains of acme.com will be processed and looked up in the DNS table.
Figure 1. Setting DNS options with the Avi Vantage application profile editor
Configuration Using the Avi CLI
In the below example, we see the before and after configurations of the System-DNS application profile. Various applicationprofile:dns_service_profile subcommands are used to:
- Define the authoritative domain names. In this example, they are
acme.comandcoyote.com. - Enable NXDOMAIN responses. To do this, the value of
error_responseis changed fromDNS_ERROR_RESPONSE_NONE(the default) toDNS_ERROR_RESPONSE_ERROR. Thenegative_caching_ttlis left unchanged from its 30-second default. - Specify subdomains of
acme.comfor which the DNS can provide an IP address. The subdomains aresales.acme.com,docs.acme.com, andsupport.acme.com. These subdomains are for validity checking and thus optional. If not configured, all subdomains ofacme.comandcoyote.comwill be processed and looked up in the DNS table.
[admin:10-10-25-20]: > configure applicationprofile System-DNS
Updating an existing object. Currently, the object is:
+-------------------------+---------------------------------------------------------+
| Field | Value |
+-------------------------+---------------------------------------------------------+
| uuid | applicationprofile-fdb6a5d6-bbf8-4f15-b851-f436b599992c |
| name | System-DNS |
| type | APPLICATION_PROFILE_TYPE_DNS |
| dns_service_profile | |
| num_dns_ip | 1 |
| ttl | 30 sec |
| error_response | DNS_ERROR_RESPONSE_NONE |
| edns | False |
| dns_over_tcp_enabled | True |
| aaaa_empty_response | True |
| negative_caching_ttl | 30 sec |
| ecs_stripping_enabled | True |
| preserve_client_ip | False |
| tenant_ref | admin |
+-------------------------+---------------------------------------------------------+
[admin:10-10-25-20]: applicationprofile> dns_service_profile
[admin:10-10-25-20]: applicationprofile:dns_service_profile> authoritative_domain_names acme.com
[admin:10-10-25-20]: applicationprofile:dns_service_profile> authoritative_domain_names coyote.com
[admin:10-10-25-20]: applicationprofile:dns_service_profile> error_response dns_error_response_error
Overwriting the previously entered value for error_response
[admin:10-10-25-20]: applicationprofile:dns_service_profile> domain_names sales.acme.com
[admin:10-10-25-20]: applicationprofile:dns_service_profile> domain_names docs.acme.com
[admin:10-10-25-20]: applicationprofile:dns_service_profile> domain_names support.acme.com
[admin:10-10-25-20]: applicationprofile:dns_service_profile> save
[admin:10-10-25-20]: applicationprofile> save
+---------------------------------+---------------------------------------------------------+
| Field | Value |
+---------------------------------+---------------------------------------------------------+
| uuid | applicationprofile-fdb6a5d6-bbf8-4f15-b851-f436b599992c |
| name | System-DNS |
| type | APPLICATION_PROFILE_TYPE_DNS |
| dns_service_profile | |
| num_dns_ip | 1 |
| ttl | 30 sec |
| error_response | DNS_ERROR_RESPONSE_ERROR |
| domain_names[1] | sales.acme.com |
| domain_names[2] | docs.acme.com |
| domain_names[3] | support.acme.com |
| edns | False |
| dns_over_tcp_enabled | True |
| aaaa_empty_response | True |
| authoritative_domain_names[1] | acme.com |
| authoritative_domain_names[2] | coyote.com |
| negative_caching_ttl | 30 sec |
| ecs_stripping_enabled | True |
| preserve_client_ip | False |
| tenant_ref | admin |
+---------------------------------+---------------------------------------------------------+
[admin:10-10-25-20]: >Support for SOA rdata Queries
Starting with release 18.2.5, Avi Vantage supports SOA queries for configured authoritative domains, and the customization of SOA fields MNAME and RNAME (refer to RFC 1035), which are configured using the Avi CLI configuration sub-command applicationprofile>dns_service_profile to supply two corresponding parameters:
- name_server — The <domain-name> of the name server that was the original or primary source of data for this zone. This field is used in SOA records pertaining to all domain names specified as authoritative domain names. If not configured, domain name is used as name server in SOA response.
- admin_email — Email address of the administrator responsible for this zone . This field is used in SOA records pertaining to all domain names specified as authoritative domain names. If not configured, the default value hostmaster is used in SOA responses.
CLI Example
[admin:10-10-25-20]: applicationprofile> dns_service_profile admin_email john_doe@acme.com
[admin:10-10-25-20]: applicationprofile:dns_service_profile> name_server roadrunner.com
[admin:10-10-25-20]: applicationprofile:dns_service_profile> save
[admin:10-10-25-20]: applicationprofile> save
+---------------------------------+---------------------------------------------------------+
| Field | Value |
+---------------------------------+---------------------------------------------------------+
| uuid | applicationprofile-fdb6a5d6-bbf8-4f15-b851-f436b599992c |
| name | System-DNS |
| type | APPLICATION_PROFILE_TYPE_DNS |
| dns_service_profile | |
| num_dns_ip | 1 |
| ttl | 30 sec |
| error_response | DNS_ERROR_RESPONSE_ERROR |
| domain_names[1] | sales.acme.com |
| domain_names[2] | docs.acme.com |
| domain_names[3] | support.acme.com |
| edns | False |
| dns_over_tcp_enabled | True |
| aaaa_empty_response | True |
| authoritative_domain_names[1] | acme.com |
| authoritative_domain_names[2] | coyote.com |
| negative_caching_ttl | 30 sec |
| name_server | roadrunner.com |
| admin_email | john_doe@acme.com |
| ecs_stripping_enabled | True |
| preserve_client_ip | False |
| tenant_ref | admin |
+---------------------------------+---------------------------------------------------------+
[admin:10-10-25-20]: >
When a SOA request is made, the SOA response is sent in the answer section. For non-existent records of domains for which Avi Vantage is the authority, the response is sent in the authority section.