Mastering GCP Multi-Org Hybrid Cloud DNS: Solving Outbound DNS and Asymmetric Routing Concerns…

Mastering GCP Multi-Org Hybrid Cloud DNS: Solving Outbound DNS and Asymmetric Routing Concerns (Part 2)

Hybrid cloud integration requires bidirectional configuration. While getting on-premises queries resolved in the cloud is critical (as we discussed in Part 1), cloud workloads must also resolve hostnames for legacy databases, mainframes, and applications that remain in your local data centers (and now multiple external Cloud Provider networks).

However, reaching back from Google Cloud to on-premises introduces a network routing challenge that can disrupt hybrid connectivity: Asymmetric Routing. In this post, we will explain how outbound Cloud DNS routing works, explore why asymmetric routing occurs in multi-VPC or multi-organization setups, outline the challenges of using NAT to address it, and detail the recommended design for centralized outbound resolution.

1. Mechanics of the Outbound Engine

When you configure a Cloud DNS Forwarding Zone, Google Cloud does not forward queries using the individual private IP addresses of your VM instances. Instead, the platform leverages a specific, system-generated IP block: the Cloud DNS Egress source range (35.199.192.0/19).

Whenever a cloud workload needs to resolve an on-premises resource, the outbound flow follows these precise steps:

1. The Query Trigger: A virtual machine (VM) in a workload VPC queries an on-premises hostname (e.g., db.onprem.example.com).
2. Zone Match: Cloud DNS detects a matching Cloud DNS Forwarding Zone configured for that domain.
3. Egress Allocation: Cloud DNS forwards the query to your specified on-premises DNS server, rewriting the source IP of the packet to an address within the Cloud DNS Egress source range (35.199.192.0/19).
4. Link Traversal: The packet traverses your hybrid connectivity link — such as VLAN attachments on a Dedicated Interconnect — to reach your local data center.

2. The Core Problem: Asymmetric Routing

While using a standardized, shared range like the Cloud DNS Egress source range (35.199.192.0/19) simplifies outbound forwarding at a platform level, it introduces significant architectural challenges in multi-VPC and multi-organization environments.

Consider a scenario where Customer X has two distinct environments — Customer X (Org A) and Customer X (Org B) — each with its own independent VPC and hybrid connectivity link to the same on-premises data center. If both VPCs independently use Cloud DNS outbound forwarding to query the same local DNS server, the on-premises router faces a path selection conflict:

1. Both VPCs advertise the exact same Cloud DNS Egress source range (35.199.192.0/19) route back to on-premises via Border Gateway Protocol (BGP).
2. The on-premises router now sees two identical, valid BGP paths to reach 35.199.192.0/19 (one via Org A's link, one via Org B's link).
3. When the local DNS server replies to a query, the on-premises router must choose a path. Due to BGP path selection or ECMP, it may route the response back via the “wrong” hybrid link (e.g., sending Org A’s query response down Org B’s Interconnect).
4. Because stateful firewalls or the Cloud DNS service on that incorrect path lack any connection state for the original query, the packet is subsequently dropped.

3. Evaluating On-Premises NAT/SNAT Workarounds

When security and network architects first run into asymmetric routing drops, their common initial approach is often to configure Source NAT (SNAT) on-premises or write custom routing policies to map the Cloud DNS Egress source range to unique, local IP addresses.

While we do not recommend this as a primary architecture, many customers do NAT the 35.199.192.0/19 outbound forwarding range in practice to get around asymmetric routing when strict isolation prevents a shared hub. However, this pattern carries important operational and supportability trade-offs due to the following reasons:

• The 5-Tuple Requirement: Cloud DNS is highly stateful. It strictly expects the response to match a precise 5-tuple (source/destination IPs, ports, and protocol) and, crucially, to arrive on the exact same virtual interface that sent the original query. Introducing intermediate NAT devices requires careful state-table alignment so that return traffic consistently arrives back on the originating hybrid interface.
• Post-NAT “Diamond Routing”: Even if you configure NAT, the underlying routing decisions on your local routers can still fall victim to “diamond routing” topologies, which can direct return packets into the wrong VPC context if symmetric policy-based routing is not strictly maintained.
• Firewall Complexity: Attempting to NAT this range increases administrative complexity. On-premises security teams must manually coordinate, track, and align firewall rules for both the translated NAT IP ranges and the original 35.199.192.0/19 range.
• Support Limitations: Google Cloud Support does not assist with or troubleshoot non-standard, NAT-based DNS forwarding architectures, which may experience instability under high load or network reconvergence.

4. The Recommended Approach: Centralized Outbound Hub

To avoid asymmetric routing entirely without resorting to manual NAT configurations, we recommend implementing a Centralized Outbound Hub architecture.

Under this pattern, we restrict outbound path advertisement to a single, designated gateway:

• The Producer VPC: Designate a single “Producer” VPC — for instance, within Customer X (Org A) — to handle all outbound forwarding to on-premises. Only the Cloud Routers associated with this specific VPC advertise the Cloud DNS Egress source range (35.199.192.0/19) via BGP to your on-premises routers. Because the local router sees only one path to the egress range, asymmetric routing is effectively prevented by design.
• Cross-Organizational DNS Peering: Workload VPCs (Consumers) residing in any other environment, such as Customer X (Org B), do not advertise the egress range. Instead, they leverage Google Cloud’s intra- or inter-organizational DNS Peering to direct their outbound queries to the central Producer VPC. The Producer VPC then forwards the queries over its dedicated link. When designing this topology, always peer every spoke VPC directly back to the central Producer Hub VPC (1 hop) rather than chaining peering zones across multiple VPCs. Cloud DNS enforces a maximum of two transitive peering zone hops; if a customer uses two peering zone hops, any Google-managed services connected to the terminal workload VPC (such as Cloud SQL, GKE, or Vertex AI via Private Services Access) will be unable to use that DNS resolution.
• The Engineering Trade-off: While this architecture guarantees symmetric return paths, it circumvents strict network isolation between your environments. This is because VPC Network Peering is required to facilitate the underlying DNS peering control plane relationship. For organizations with absolute, zero-trust isolation mandates, this trade-off must be carefully weighed during the design phase.

5. Series Summary & Best Practices

To ensure a reliable outbound hybrid DNS setup, follow these recommended practices:

• Centralize Outbound Advertisements: Designate a single hub VPC or organization to advertise the Cloud DNS Egress source range (35.199.192.0/19) to your local network.
• Consolidate with DNS Peering: Connect your workloads across different VPCs and organizations to your central hub using direct hub-and-spoke DNS Peering zones (respecting the two-hop transitive peering limit).
• Prefer Centralized Peering Over NAT Where Possible: Although on-premises NAT/SNAT for the 35.199.192.0/19 outbound forwarding range is used by some organizations when environments cannot be peered, a centralized DNS hub avoids stateful NAT maintenance and keeps the deployment within standard supported architectures.
• Align On-Premises Firewalls: Work closely with your security team to ensure that local firewalls explicitly allow inbound UDP and TCP port 53 traffic coming from the 35.199.192.0/19 range.

Series Summary

By pairing a centralized Inbound Transit VPC (Part 1) with a single Outbound DNS Producer Hub (Part 2) connected to spoke VPCs via single-hop DNS Peering, enterprises can achieve deterministic, symmetric hybrid DNS resolution at scale — without the operational fragility of manual A-record databases or stateful NAT workarounds.

Additional hybrid DNS architectures may be considered and implemented depending on specific workload segmentation and isolation requirements. However, these alternatives must be evaluated holistically against your overall network design and organizational security policies to avoid unexpected routing or compliance issues.

Mastering GCP Multi-Org Hybrid Cloud DNS: Solving Outbound DNS and Asymmetric Routing Concerns… was originally published in Google Cloud – Community on Medium, where people are continuing the conversation by highlighting and responding to this story.

Source Credit: https://medium.com/google-cloud/mastering-hybrid-cloud-dns-solving-outbound-dns-and-asymmetric-routing-concerns-part-2-8edb3de65b76?source=rss—-e52cf94d98af—4

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