How should security teams design relay paths for workloads in hard NAT or firewalled environments?

Security teams should prefer direct connections first, then use relays as a controlled fallback when network conditions block peer-to-peer paths. The relay should be customer-managed where possible, placed close to the workloads it serves, and limited to traffic inside the trusted network boundary. Keep the relay surface area small with a single exposed port and maintain end-to-end encryption.

Why This Matters for Security Teams

Relay paths are not just a networking convenience. In hard NAT or heavily firewalled environments, they become part of the trust boundary for machine traffic, so design mistakes can turn a fallback path into a permanent dependency. Security teams often underestimate how quickly relays accumulate privilege, especially when they are introduced to preserve connectivity for certificates, secrets, or workload-to-workload control traffic.

The operational risk is that relays can mask deeper identity and reachability problems. If a team cannot observe which workload is connecting, why it is connecting, and whether that path is still required, the relay becomes an opaque middlebox rather than a controlled safeguard. That is why NHI governance guidance increasingly pairs network design with workload identity, inventory, and lifecycle controls, as discussed in NHIMG’s Ultimate Guide to NHIs — What are Non-Human Identities. In practice, many security teams encounter relay abuse only after a blocked path, an expired certificate, or a lateral movement attempt has already exposed the gap.

How It Works in Practice

The safest pattern is to treat relay paths as a controlled exception, not the default architecture. Start with direct connectivity where possible. If NAT traversal or upstream firewall policy blocks peer-to-peer traffic, introduce a relay that is customer-managed, deployed close to the workloads it serves, and constrained to the smallest possible blast radius. That usually means one exposed port, narrow routing, and explicit allowlisting for source and destination workload sets.

For identity, the relay should not authenticate traffic by network location alone. Current guidance suggests binding the session to workload identity, such as SPIFFE IDs or short-lived OIDC tokens, so the relay can verify what the workload is rather than merely where it came from. The SPIFFE workload identity specification is useful here because it defines a cryptographic identity primitive that works even when IPs are unstable or private. That aligns with NHIMG’s Guide to SPIFFE and SPIRE, which is relevant when teams need identity continuity across dynamic infrastructure.

• Prefer direct connections first, then use a relay only when network controls block peer paths.
• Keep relay traffic inside the trusted network boundary whenever possible.
• Use end-to-end encryption so the relay cannot inspect or modify payloads.
• Attach authentication and authorization to workload identity, not static source IPs.
• Log relay admissions, denied sessions, and certificate or token failures for auditability.

Operationally, this should be paired with security controls from NIST SP 800-53 Rev 5 Security and Privacy Controls, especially around access enforcement, auditing, and system communications protection. These controls tend to break down when the relay is placed in a shared DMZ with no workload-level policy, because the path then becomes a generic transit service instead of a bounded trust mechanism.

Common Variations and Edge Cases

Tighter relay controls often increase deployment overhead, requiring organisations to balance connectivity reliability against operational complexity. That tradeoff becomes sharper in hybrid estates, multi-cloud topologies, and segmented enterprise networks where firewall rules are owned by separate teams and ports change slowly. In those environments, the relay may need to support both inbound brokered sessions and outbound-only initiation, but best practice is evolving and there is no universal standard for this yet.

One common exception is when the relay must cross administrative domains. In that case, keep it from becoming a general-purpose tunnel. Limit protocol support, isolate relay instances per application or tenant, and rotate any service credentials frequently so the fallback path does not outlive the incident that created it. Another edge case is certificate-heavy workloads, where expiry and renewal failures can make relay traffic appear healthy while the underlying identity has already failed. NHIMG’s Critical Gaps in Machine Identity Management report notes that 45% of organisations identify certificate expiry as the leading cause of outages, which is exactly why relay design should be paired with certificate and secret lifecycle monitoring.

For teams managing many machine identities, the same report also shows 57% lack a complete inventory of their machine identities, making relay attribution harder when troubleshooting or investigating abuse. The practical answer is to keep the relay simple, observable, and identity-bound, so it remains a fallback path rather than a shadow network. That is especially important when the firewall or NAT rule set is so restrictive that operators start punching broad exceptions just to restore service.

Related resources from NHI Mgmt Group

• How should security teams design role models to reduce role sprawl in complex identity environments?
• How should security teams design a workspace that reduces tool sprawl without weakening access controls?
• How do security teams evaluate whether transparent credential injection is actually safe for cloud workloads?
• How do secure-by-design base images change the balance between platform teams and security teams?