Identifying The Effective DNS Firewall Rule

Domain expertise for working out which single Route 53 Resolver DNS Firewall rule decides the outcome for a domain when a VPC has several rule groups associated and more than one rule could match. Covers the two-layer evaluation order (rule group association priority, then rule priority within a group), the first-match-wins behavior that makes a high-priority Allow or Alert shadow a lower-priority Block, and how to read the live configuration to name the winning rule.

This is a read and diagnostic workflow. It inspects configuration and explains the result; it does not create or change rules. For authoring blocking rules see the blocking-malicious-domains reference, and for fanning rule groups across accounts see the centralizing-dns-firewall reference.

Execute commands using the AWS MCP server when connected (sandboxed execution, audit logging, observability). Fall back to the AWS CLI otherwise.

Table of Contents

• Overview
• Workflow
• How DNS Firewall picks the winning rule
• Decision: is a rule actually reachable?
• Troubleshooting
• Procedure
• Additional Resources

Workflow

To identify the effective rule for a domain, follow the procedure exactly. See the Procedure section below.

The procedure covers:

• Listing the rule groups associated with the VPC and sorting them by association priority
• Listing the rules in each candidate group and sorting them by rule priority
• Finding the first rule whose domain list matches the domain, in evaluation order
• Reporting the winning rule, its action, and any lower-priority rules it shadows
• Surfacing the console link so the customer can see the rule group and its rules

How DNS Firewall picks the winning rule

DNS Firewall evaluates in a fixed order and stops at the first match:

1. Across rule groups: the VPC’s rule group associations are processed by association priority, lowest number first. Each association on a VPC has a unique priority (valid range 100 to 9900; the console help panel uses 101 to 9899).
2. Within a rule group: rules are processed by rule priority, lowest number first.
3. First match wins and halts inspection. A rule matches when the query satisfies all of:
   • The query name matches the rule’s domain list (or the rule is an Advanced rule that detects the query by behavior)
   • The query type (Qtype) matches the rule’s Qtype filter. If the rule has no Qtype set, it matches all query types. If a Qtype is set (e.g., A, AAAA, MX, TXT), only queries of that type match that rule. When a match occurs the resolver applies the rule’s action and stops. All three actions are terminating:
   • Allow — permit the query, stop inspecting.
   • Alert — permit the query, log an alert, stop inspecting.
   • Block — block the query, respond per the block mode (NODATA, NXDOMAIN, or OVERRIDE), log the block, stop inspecting.

Because every action halts inspection, a broad Allow or Alert in a higher-priority group prevents a Block in a lower-priority group from ever running for that domain. That shadowing is the usual reason a domain the customer expected to be blocked is not.

Constraints:

• You MUST combine both layers in order (association priority, then rule priority within the group) rather than reasoning about one rule group in isolation
• You MUST treat the first matching rule as the effective one and report that an Allow or Alert halts inspection, so any matching rule in a lower-priority position never applies
• You MUST evaluate domain matching from most specific to least specific: a rule matches when its domain list contains the exact query name or a parent the list covers via a *. wildcard. Wildcard semantics: the * can only appear as the leftmost label; *.example.com matches any subdomain (e.g., sub.example.com, deep.sub.example.com) but does NOT match the apex example.com itself. Check the query name and each parent label, not just an exact string
• You MUST account for query type (Qtype) filtering: a rule with a Qtype set (e.g., A, AAAA, MX) only matches queries of that type. A rule with no Qtype set matches all query types. When debugging “why didn’t my rule match,” verify the rule’s Qtype matches the query type
• You MUST account for AWS-managed domain lists, whose contents cannot be listed or downloaded. The domain may be in a managed list the customer did not author; determine a managed-list match from Resolver query logs or AWS-provided test domains, not by enumerating the list
• You MUST account for DNS Firewall Advanced rule types (DGA, DNS tunneling, threat categories, content categories), which have no customer domain list and match by detection or categorization rather than list membership; such a rule can be the effective or shadowing rule, and the Allow action is not available for Advanced rules
• You MUST account for domain redirection behavior: a rule’s FirewallDomainRedirectionAction setting determines whether CNAME/DNAME chain targets are also inspected against firewall rules (INSPECT_REDIRECTION_DOMAIN) or trusted without further inspection (TRUST_REDIRECTION_DOMAIN). When a query follows a CNAME chain, this setting affects which domains in the chain are evaluated against rules

Decision: is a rule actually reachable?

Troubleshooting

A domain the customer expected to be blocked is allowed

A higher-priority Allow or Alert rule matches the domain first and halts inspection. Find the first matching rule in evaluation order; the Block sits below it and never runs.

Behavior changed after adding a rule group

A newly associated group took a lower (higher-priority) association number, or a Profile-managed group was added, changing which group is evaluated first. Re-sort the associations by priority.

Two associations appear to have the same priority

Each association on a VPC must have a unique priority. If a tool reports a clash, an association was just changed; re-read the associations before concluding.

The matching rule is not one the customer wrote

The domain is in an AWS-managed domain list referenced by a rule. Check managed-list rules, not just custom lists.

A rule with the right domain does not match

The rule has a Qtype filter (e.g., A only) that does not match the query type (e.g., the query is AAAA or MX). Check the rule’s Qtype setting; if it is set, only queries of that type trigger it.

Procedure

Overview

This procedure determines which DNS Firewall rule decides the outcome for a domain on a VPC that has one or more rule groups associated. It reads the rule group associations and their priorities, reads the rules and their priorities within each candidate group, finds the first match in evaluation order, and reports the effective rule and what it shadows. It changes nothing.

Parameters

• vpc_id (required): The VPC whose effective rule you want to determine (e.g., vpc-0abc123).
• region (required): The AWS Region the VPC is in (e.g., us-east-1).
• domain (required): The domain name to evaluate (e.g., example.com).

Constraints for parameter acquisition:

• You MUST ask for all required parameters upfront in a single prompt
• You SHOULD prefer read-only or least-privilege credentials; this workflow only reads

Steps

1. Verify dependencies

Constraints:

• You MUST confirm credentials with aws sts get-caller-identity
• You MUST REQUIRE provisioning credentials through ephemeral mechanisms (IAM roles via instance profiles, SSO/IAM Identity Center session credentials, or aws sts assume-role) rather than long-lived IAM user access keys.
• You SHOULD use read-only credentials; no step here writes

2. List the rule group associations on the VPC and sort by association priority

Constraints:

• You MUST list the associations for the VPC and order them by Priority, lowest first:

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  aws route53resolver list-firewall-rule-group-associations \
    --vpc-id {vpc_id} --region {region}

• The lowest Priority association is evaluated first. Note each association’s FirewallRuleGroupId in that order

• A VPC can have one Route 53 Profile, and rule groups applied through that Profile may not appear in list-firewall-rule-group-associations. You MUST also check for a Profile on the VPC and enumerate its rule group resource associations, folding them into the same priority-ordered walk as the directly associated rule groups:

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  aws route53profiles list-profile-associations --resource-id {vpc_id} --region {region}
  aws route53profiles list-profile-resource-associations \
    --profile-id {profile_id} --region {region}

3. List the rules in each candidate group and sort by rule priority

Constraints:

• For each associated rule group, in association-priority order, you MUST list its rules ordered by rule Priority, lowest first:

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  aws route53resolver list-firewall-rules \
    --firewall-rule-group-id {rule_group_id} --region {region}

• For a rule that references a FirewallDomainListId, resolve whether {domain} (or a parent the list matches via a *. wildcard) is in that list. Custom lists can be enumerated:

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  aws route53resolver list-firewall-domains \
    --firewall-domain-list-id {domain_list_id} --region {region}

• AWS-managed domain lists CANNOT be enumerated or downloaded; list-firewall-domains does not return their contents. For a rule backed by a managed list, determine a match from Resolver query logs (see Step 3a) or by testing an AWS-provided test domain, not by listing the list

• Advanced rule types (DGA, DNS tunneling) have no FirewallDomainListId; they match by detection. Treat such a rule as a potential match by its rule type and confirm from query logs

3a. Confirm the match from Resolver query logs (authoritative)

When Resolver query logging is enabled on the VPC, the logs name the rule that acted on a query and are the authoritative way to confirm the effective rule, especially for managed-list and Advanced rules whose contents you cannot enumerate.

Constraints:

• You SHOULD use query logs when available. The log record reports firewall_rule_group_id, firewall_rule_action, and the matched firewall_domain_list_id for the query
• You MUST account for one caveat: these firewall fields populate for ALERT and BLOCK actions but NOT for ALLOW. A query permitted by a shadowing Allow rule will not name that rule in the logs, so a domain that resolves with no firewall fields may still have matched an Allow. Fall back to the configuration walk (Step 4) to find a shadowing Allow

4. Find the first match in evaluation order

Constraints:

• You MUST walk the rules in combined order (association priority, then rule priority within the group) and stop at the first rule that matches {domain} AND the query type — by exact name, by a parent wildcard in its domain list, or by detection for an Advanced rule. A rule with a Qtype filter that does not match the query’s type is skipped even if the domain matches
• You MUST report that rule as the effective one, with its action (ALLOW, ALERT, or BLOCK plus the block response mode), and explain that the match halts inspection
• You MUST list any lower-priority rules that also match {domain} and note they are shadowed and never apply, so the customer sees why a Block below an Allow or Alert has no effect

5. Surface the console link

Constraints:

• You MUST present the rule group detail view for the winning rule’s group, filling {region} with the VPC’s Region and {ruleGroupId} with the matched rule group’s ID. The fragment is RulegroupId (lowercase g):

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  https://{region}.console.aws.amazon.com/vpcconsole/home?region={region}#DNSFirewallRuleGroupDetails:RulegroupId={ruleGroupId}

• If no rule matches {domain} in any associated group, you MUST say so plainly: DNS Firewall takes no action and the query resolves normally

• You SHOULD note that this “resolves normally” outcome assumes the firewall is healthy. If the firewall cannot be reached, the VPC’s DNS Firewall fail-open setting decides the result; the default is fail-closed (the query is blocked), unless fail-open is enabled

Example

Example input

Example output

Troubleshooting

A domain expected to be blocked is allowed

A higher-priority Allow or Alert matched first and halted inspection (Step 4). The Block sits in a lower-priority position and never runs.

Behavior changed after adding a rule group

A new association took a higher-priority (lower-numbered) slot, or a Profile-managed group was added. Re-sort the associations (Step 2).

The matching rule is a managed-list rule

The domain is in an AWS-managed domain list, not a custom one. Resolve managed lists too (Step 3).

Security Considerations

• You SHOULD use least-privilege IAM credentials provisioned through ephemeral mechanisms (IAM roles, SSO/IAM Identity Center session credentials, or aws sts assume-role) rather than long-lived IAM user access keys. This workflow only reads, so read-only credentials suffice.
• Where you read Resolver query logs to confirm the effective rule, you SHOULD ensure the log destination is encrypted at rest (KMS on CloudWatch Logs, SSE-S3/SSE-KMS on S3, or server-side encryption on a Data Firehose stream), because query logs reveal the domains a VPC resolves.
• You SHOULD report the effective rule and any shadowed rules accurately rather than assuming a Block applies, since a shadowing Allow or Alert silently changes the security outcome for a domain the customer believes is blocked.

Additional Resources

• Rule actions in DNS Firewall (Route 53 Developer Guide) (opens in a new tab)
• Define priority (Route 53 Console help panel) (opens in a new tab)
• Filter DNS traffic using Route 53 Resolver DNS Firewall (VPC User Guide) (opens in a new tab)