Cloud DNS forwarding zones let you configure target name servers for specific private zones. Using a forwarding zone is one way to implement outbound DNS forwarding from your VPC network.
A Cloud DNS forwarding zone is a special type of Cloud DNS private zone. Instead of creating records within the zone, you specify a set of forwarding targets. Each forwarding target is an IP address of a DNS server, located in your VPC network, or in an on-premises network connected to your VPC network by Cloud VPN or Cloud Interconnect.
Forwarding targets and routing methods
Cloud DNS supports three types of targets and offers standard or private methods for routing traffic to them.
|Forwarding target||Description||Standard routing supports||Private routing supports||Source of requests|
|Type 1||An internal IP address of a Google Cloud VM or an internal TCP/UDP load balancer in the same VPC network that is authorized to use the forwarding zone.||Only RFC 1918 IP addresses—traffic always routed through an authorized VPC network.||Any internal IP address, including non-RFC 1918 private IP addresses and privately re-used public IP addresses—traffic always routed through an authorized VPC network.||
|Type 2||An IP address of an on-premises system, connected to the VPC network authorized to query the forwarding zone, using Cloud VPN or Cloud Interconnect.||Only RFC 1918 IP addresses—traffic always routed through an authorized VPC network.||Any internal IP address, including non-RFC 1918 private IP addresses and privately re-used public IP addresses—traffic always routed through an authorized VPC network.||
|Type 3||An external IP address of a DNS name server accessible to the internet or the external IP address of a Google Cloud resource; for example, the external IP address of a VM in another VPC network.||Only internet-routable external IP addresses—traffic always routed to the internet or to the external IP address of a Google Cloud resource.||Private routing isn't supported—make sure that private routing is not selected.||Google Public DNS source ranges|
You can choose one of the two following routing methods when you add the forwarding target to the forwarding zone:
Standard routing: Routes traffic through an authorized VPC network or over the internet based on whether the forwarding target is an RFC 1918 IP address. If the forwarding target is an RFC 1918 IP address, Cloud DNS classifies the target as either a Type 1 or Type 2 target, and routes requests through an authorized VPC network. If the target is not an RFC 1918 IP address, Cloud DNS classifies the target as Type 3, and expects the target to be internet accessible.
Private routing: Always routes traffic through an authorized VPC network, regardless of the target's IP address (RFC 1918 or not). Consequently, only Type 1 and Type 2 targets are supported.
To access a Type 1 or a Type 2 forwarding target, Cloud DNS uses routes in the authorized VPC network where the DNS client is located. These routes define a secure path to the forwarding target:
To send traffic to Type 2 targets, Cloud DNS can use either custom dynamic routes or custom static routes, except for custom static routes with network tags. To reply, Type 2 forwarding targets use routes in your on-premises network.
For additional guidance about network requirements for Type 1 and Type 2 targets, see forwarding target network requirements.
Forwarding target selection order
Cloud DNS lets you configure a list of alternative name servers for an outbound server policy and a list of forwarding targets for a forwarding zone.
In case of multiple forwarding targets, Cloud DNS uses an internal algorithm to select a forwarding target. This algorithm ranks each forwarding target.
To process a request, Cloud DNS first tries a DNS query by contacting the forwarding target with the highest ranking. If that server does not respond, Cloud DNS repeats the request to the next highest ranked forwarding target. If no forwarding targets reply, Cloud DNS synthesizes a SERVFAIL response.
The ranking algorithm is automatic, and the following factors increase the ranking of a forwarding target:
- The higher the number of successful DNS responses processed by the forwarding target. Successful DNS responses include NXDOMAIN responses.
- The lower the latency (round-trip time) for communicating with the forwarding target.
Use forwarding zones
VMs in a VPC network can use a Cloud DNS forwarding zone in the following cases:
- The VPC network has been authorized to use the Cloud DNS forwarding zone. You can authorize multiple VPC networks in the same project to use the forwarding zone.
- The guest operating system of each VM in the VPC network uses
the VM's metadata server
169.254.169.254as its name server.
Overlapping forwarding zones
Because Cloud DNS forwarding zones are a type of Cloud DNS managed private zone, you can create multiple zones that overlap. VMs configured as described earlier resolve a record according to the name resolution order, using the zone with the longest suffix. For more information, see Overlapping zones.
Caching and forwarding zones
Cloud DNS caches responses for queries sent to Cloud DNS forwarding zones. Cloud DNS maintains a cache of responses from reachable forwarding targets for the shorter of the following time spans:
- 60 seconds
- The duration of the record's time to live (TTL)
When all of the forwarding targets for a forwarding zone become unreachable, Cloud DNS maintains its cache of the previously requested records in that zone for the duration of each record's TTL.
When to use peering instead
Cloud DNS cannot use transitive routing to connect to a forwarding target. To illustrate an invalid configuration, consider the following scenario:
You've used Cloud VPN or Cloud Interconnect to connect an on-premises network to a VPC network named
You've used VPC Network Peering to connect VPC network
vpc-net-b. You've configured
vpc-net-ato export custom routes, and
vpc-net-bto import them.
You've created a forwarding zone whose forwarding targets are located in the on-premises network to which
vpc-net-ais connected. You've authorized
vpc-net-bto use that forwarding zone.
Resolving a record in a zone served by the forwarding targets fails in this
scenario, even though there is connectivity from
vpc-net-b to your on-premises
network. To demonstrate this failure, perform the following tests from a
Query the VM's metadata server
169.254.169.254for a record defined in the forwarding zone. This query fails (expectedly) because Cloud DNS does not support transitive routing to forwarding targets. To use a forwarding zone, a VM must be configured to use its metadata server.
Query the forwarding target directly for that same record. Although Cloud DNS does not use this path, this query demonstrates that transitive connectivity succeeds.
You can use a Cloud DNS peering zone to fix this invalid scenario:
- Create a Cloud DNS peering zone authorized for
- Create a forwarding zone authorized for
vpc-net-awhose forwarding targets are on-premises name servers.
You can perform these steps in any order. After completing these steps,
Compute Engine instances in both
vpc-net-b can query
the on-premises forwarding targets.
For instructions on how to create forwarding zones, see Create a forwarding zone.
DNS peering lets you send requests for records that come from one zone's namespace to another VPC network. For example, a SaaS provider can give a SaaS customer access to DNS records it manages.
To provide DNS peering, you must create a Cloud DNS peering zone and configure it to perform DNS lookups in a VPC network where the records for that zone's namespace are available. The VPC network where the DNS peering zone performs lookups is called the DNS producer network.
The peering zone is only visible to VPC networks that are selected during zone configuration. The VPC network that is authorized to use the peering zone is called the DNS consumer network.
After Google Cloud resources in the DNS consumer network are authorized, they can perform lookups for records in the peering zone's namespace as if they were in the DNS producer network. Lookups for records in the peering zone's namespace follow the DNS producer network's name resolution order.
Therefore, Google Cloud resources in the DNS consumer network can look up records in the zone's namespace from the following sources available in the DNS producer network:
- Cloud DNS managed private zones authorized for use by the DNS producer network.
- Cloud DNS managed forwarding zones authorized for use by the DNS producer network.
- Compute Engine internal DNS names in the DNS producer network.
- An alternative name server, if an outbound server policy has been configured in the DNS producer network.
DNS peering limitations and key points
Keep the following in mind when configuring DNS peering:
- DNS peering is a one-way relationship. It allows Google Cloud resources in the DNS consumer network to look up records in the peering zone's namespace as if the Google Cloud resources were in the DNS producer network.
- The DNS producer and consumer networks must be VPC networks.
- While DNS producer and consumer networks are typically part of the same organization, cross-organizational DNS peering is also supported.
- DNS peering and VPC Network Peering are different services. DNS peering can be used with VPC Network Peering, but VPC Network Peering is not required for DNS peering.
- Transitive DNS peering is supported, but only through a single transitive hop.
In other words, no more than three VPC networks (with the
network in the middle being the transitive hop) can be involved. For example,
you can create a peering zone in
vpc-net-b, and then create a peering zone in
- If you are using DNS peering to target a forwarding zone, the target VPC network with the forwarding zone must contain a VM, a VLAN attachment, or a Cloud VPN tunnel located in the same region as the source VM that uses the DNS peering zone. For details about this limitation, see Forwarding queries from VMs in a consumer VPC network to a producer VPC network not working.
To create a peering zone, you must have the DNS Peer IAM role for the project that contains the DNS producer network.
For instructions on how to create a peering zone, see Create a peering zone.
Managed reverse lookup zones
A managed reverse lookup zone is a private zone with a special attribute that instructs Cloud DNS to perform a PTR lookup against Compute Engine DNS data.
PTR records for RFC 1918 addresses in private zones
To perform reverse lookups with custom PTR records for RFC 1918 addresses, you must create a Cloud DNS private zone that is at least as specific as one of the following example zones. This is a consequence of the longest suffix matching described in Name resolution order.
Example zones include the following:
17.172.in-addr.arpa., ... through
If you create a Cloud DNS private zone for RFC 1918 addresses, custom PTR records that you create for VMs in that zone are overridden by the PTR records that internal DNS creates automatically. This is because internal DNS PTR records are created in the previous list of more specific zones.
For example, suppose you create a managed private zone for
the following PTR record for a VM whose IP address is
188.8.131.52.in-addr.arpa. -> test.example.domain
PTR queries for
184.108.40.206.in-addr.arpa. are answered by the more specific
10.in-addr.arpa. internal DNS zone. The PTR record in your Cloud DNS
private zone for
test.example.domain is ignored.
To override the automatically created internal DNS PTR
records for VMs, make sure that you create your custom PTR records in a private
zone that is at least as specific as one of the zones presented in this section.
For example, if you create the following PTR record in a private zone for
10.in-addr.arpa., your record overrides the automatically generated
220.127.116.11.in-addr.arpa. -> test.example.domain.
If you only need to override a portion of a network block, you can create
more specific reverse Cloud DNS private zones. For example, a private
5.10.in-addr.arpa. can be used to hold PTR records that override any
internal DNS PTR records that are automatically created
for VMs with IP addresses in the
10.5.0.0/16 address range.
For instructions on how to create a managed reverse lookup zone, see Create a managed reverse lookup zone.
Two zones overlap with each other when the origin domain name of one zone is either identical to or is a subdomain of the origin of the other zone. For example:
- A zone for
gcp.example.comand another zone for
gcp.example.comoverlap because the domain names are identical.
- A zone for
dev.gcp.example.comand a zone for
dev.gcp.example.comis a subdomain of
Rules for overlapping zones
Cloud DNS enforces the following rules for overlapping zones:
Overlapping public zones are not allowed on the same Cloud DNS name servers. When you create overlapping zones, Cloud DNS attempts to put them on different name servers. If that is not possible, Cloud DNS fails to create the overlapping zone.
A private zone can overlap with any public zone.
Private zones scoped for different VPC networks can overlap with each other. For example, two VPC networks can each have a database VM named
database.gcp.example.comin a zone
gcp.example.com. Queries for
database.gcp.example.comreceive different answers according to the zone records defined in the zone authorized for each VPC network.
Two private zones that have been authorized to be accessible from the same VPC network cannot have identical origins unless one zone is a subdomain of the other. The metadata server uses longest suffix matching to determine which origin to query for records in a given zone.
Query resolution examples
Google Cloud resolves Cloud DNS zones as described in Name resolution order. When determining the zone to query for a given record, Cloud DNS tries to find a zone that matches as much of the requested record as possible (longest suffix matching).
Unless you have specified an alternative name server in an outbound server policy, Google Cloud first attempts to find a record in a private zone (or forwarding zone or peering zone) authorized for your VPC network before it looks for the record in a public zone.
The following examples illustrate the order that the metadata server uses when
querying DNS records. For each of these examples, suppose that you have created
two private zones,
dev.gcp.example.com, and authorized
access to them from the same VPC network. Google Cloud
handles the DNS queries from VMs in a VPC network in the
The metadata server uses public name servers to resolve a record for
myapp.example.com.(note the trailing dot) because there is no private zone for
example.comthat has been authorized for the VPC network. Use
digfrom a Compute Engine VM to query the VM's default name server:
For details, see Query for the DNS name using the metadata server.
The metadata server resolves the record
myapp.gcp.example.comusing the authorized private zone
gcp.example.comis the longest common suffix between the requested record name and available authorized private zones.
NXDOMAINis returned if there's no record for
myapp.gcp.example.comdefined in the
gcp.example.comprivate zone, even if there is a record for
myapp.gcp.example.comdefined in a public zone.
Similarly, queries for
myapp.dev.gcp.example.comare resolved according to records in the authorized private zone
NXDOMAINis returned if there is no record for
dev.gcp.example.comzone, even if there is a record for
myapp.dev.gcp.example.comin another private or public zone.
myapp.prod.gcp.example.comare resolved according to records in the private zone
gcp.example.comis the longest common suffix between the requested DNS record and the available private zones.
Split horizon DNS example
You can use a combination of public and private zones in a split horizon DNS configuration. Private zones enable you to define different responses to a query for the same record when the query originates from a VM within an authorized VPC network. Split horizon DNS is useful whenever you need to provide different records for the same DNS queries depending on the originating VPC network.
Consider the following split horizon example:
- You've created a public zone,
gcp.example.com, and you've configured its registrar to use Cloud DNS name servers.
- You've created a private zone,
gcp.example.com, and you've authorized your VPC network to access this zone.
In the private zone, you've created a single record as shown in the following table.
In the public zone, you've created two records.
The following queries are resolved as described:
- A query for
myrecord1.gcp.example.comfrom a VM in your VPC network returns
- A query for
myrecord1.gcp.example.comfrom the internet returns
- A query for
myrecord2.gcp.example.comfrom a VM in your VPC network returns an
NXDOMAINerror because there's no record for
myrecord2.gcp.example.comin the private zone
- A query for
myrecord2.gcp.example.comfrom the internet returns
Cross-project binding zones
Cross-project binding zones let you keep the ownership of the DNS namespace of the service project independent of the ownership of the DNS namespace of the entire VPC network.
A typical Shared VPC setup has service projects that take ownership of a virtual machine (VM) application or services, while the host project takes ownership of the VPC network and network infrastructure. Often, a DNS namespace is created from the VPC network's namespace to match the service project's resources. For such a setup, it can be easier to delegate the administration of each service project's DNS namespace to the administrators of each service project (which are often different departments or businesses). Cross-project binding lets you separate the ownership of the DNS namespace of the service project from the ownership of the DNS namespace of the entire VPC network.
The following figure shows a typical Shared VPC setup with DNS peering.
The following figure shows a setup using cross-project binding. Cloud DNS lets each service project create and own its DNS zones, but still have it bound to the shared network that the host project owns. This allows for better autonomy and a more precise permission boundary for DNS zone administration.
Cross-project binding provides the following:
- Service project administrators and users can create and manage their own DNS zones.
- You don't need to create a placeholder VPC network.
- Host project administrators don't have to manage the service project.
- IAM roles still apply at the project level.
- All the DNS zones are directly associated with the Shared VPC network.
- Any-to-any DNS resolution is readily available. Any VM in the Shared VPC network can resolve associated zones.
- There is no transitive hop limit. You can manage it in a hub and spoke design.
For instructions on how to create a cross-project binding zone, see Create a cross-project binding zone.
- To create, update, list, and delete managed zones, see Manage zones.
- To find solutions for common issues that you might encounter when using Cloud DNS, see Troubleshoot.
- To get an overview of Cloud DNS, see Cloud DNS overview.