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Build a TLS Reverse Proxy

TLS Reverse Proxy — a complete walkthrough of tls termination, covering tls context, sni router, cert loader, and testing.

reverse_proxy_tls/
├── src/index.ts
├── src/tls-context.ts
├── src/sni-router.ts
├── tests/reverse_proxy_tls.test.ts
└── package.json

// TLS context: tls termination core
export function tlscontext(input: TlscontextInput): TlscontextResult {
  // Build the tls context that anchors tls termination and defines the core data shape.
  return processTlscontext(input);
}

// SNI router — handles tls termination Add the sni router so
export class Snirouter {
  constructor(private opts: SnirouterOptions) {}
  async run(ctx: Context): Promise<SnirouterOutput> {
    return this.execute(ctx);
  }
}

// Cert loader: tls termination stage
export const certloader = (next: Next) => async (req: Request) => {
  const out = await transform(req);
  return next(out);
};

// Header injector for tls termination
export async function headerinjector(stream: AsyncIterable<Chunk>): Promise<Aggregate> {
  let acc = init();
  for await (const c of stream) acc = merge(acc, c);
  return acc;
}

// Upstream forwarder: tls termination finalization
export function upstreamforwarder(state: State): Result {
  validate(state);
  return finalize(state);
}

Project overview

This project builds a tls termination system whose reason for existing is terminating TLS at the edge and forwarding plain HTTP upstream while preserving client identity. The temptation in tls termination is to bolt the interesting part onto a working prototype and discover the hard cases in production; this build inverts that order and treats a certificate that expires without renewal and causes every handshake to fail at once as the first thing designed, not the last.

You will implement TLS context, SNI router, Cert loader, Header injector, and Upstream forwarder, each as a module with one responsibility and a test that proves it in isolation. The tls termination pipeline is then the wiring that composes them, and the integration test is where the composition is shown to survive a certificate that expires without renewal and causes every handshake to fail at once.

The walkthrough is written for a developer who has used a tls termination system but has not had to make one honest. It favors explicit data shapes over clever abstractions, and it names the failure each design choice prevents rather than presenting the choice as a style preference.

Each phase ends with something observable — a tls context that returns the right shape, a sni router that rejects bad input, an integration test that goes green — so you can stop at any point and still have a tls termination system that works for the cases covered so far.

What you will build

The finished tls termination system exposes five capabilities, each with a focused test:

• TLS context
• SNI router
• Cert loader
• Header injector
• Upstream forwarder

TLS context is the foundation that every later stage assumes; SNI router is where tls termination correctness lives; Cert loader handles a certificate that expires without renewal and causes every handshake to fail at once; Header injector makes the system observable; and Upstream forwarder is the graceful-degradation layer that keeps overload from becoming corruption.

The build keeps the tls termination logic in ordinary modules you can read in one sitting, and pulls in dependencies only for the parts that are genuinely tedious to hand-roll. That keeps the diff between the tutorial and a real codebase small, and keeps the failure modes visible in code rather than hidden behind a framework.

Architecture and data model

The system is organized around TLS context, SNI router, and Cert loader, with Header injector tying them together. Data flows as a map of server names to TLS contexts loaded from disk, with SNI selecting the context per connection.

Each component is a module with a pure interface where practical: the same input produces the same output, and side effects are pushed to the edges. That discipline is what makes the phases independently testable, because a test for Cert loader does not need to stand up TLS context and SNI router to exercise the interesting branch.

The data model is deliberately explicit. A map of server names to TLS contexts loaded from disk, with SNI selecting the context per connection Keeping the shape written down — in a schema, a type, or a fixture — means a change to the shape is a visible diff rather than a silent runtime shift, and it means the integration test can assert against a concrete shape instead of guessing.

Configuration and code are kept apart: the configuration says what tls termination should do, the modules say how. A reviewer can change a threshold or a policy without reading the implementation, which keeps the risky changes small and lets the diff stand as the review.

The failure path through Cert loader is designed before the happy path is finished, because a certificate that expires without renewal and causes every handshake to fail at once is the case that distinguishes a tls termination system from a demo. Every component declares what it does when its inputs are wrong, and the integration test asserts each of those declarations.

Prerequisites and dependencies

Have these ready before you install anything, so the tls termination setup is not blocked by a missing tool:

• Node.js 20 LTS or later
• Familiarity with tls termination fundamentals
• A local environment to run the test suite

The runtime dependencies are chosen to keep the tls termination datapath legible:

• fastify (^5.0.0) — Proxy server
• undici (^6.19.0) — Upstream forwarding

# setup for reverse-proxy-tls
pnpm init
pnpm add fastify undici
pnpm add -D typescript tsx vitest

Versions are pinned to a major range so the tls termination install is reproducible; commit the lockfile. The tls context reads its operational parameters from the environment and validates them at startup, so a missing value fails the boot rather than the first tls termination request.

Directory structure

Keep each tls termination component in its own file so it can be tested without starting the others:

reverse_proxy_tls/
├── src/index.ts
├── src/tls-context.ts
├── src/sni-router.ts
├── tests/reverse_proxy_tls.test.ts
└── package.json

Phase 1: TLS context

Build the tls context that anchors tls termination and defines the core data shape.

// TLS context: tls termination core
export function tlscontext(input: TlscontextInput): TlscontextResult {
  // Build the tls context that anchors tls termination and defines the core data shape.
  return processTlscontext(input);
}

After this phase the tls context works in isolation and is covered by a unit test. The tls context defines the data shape that every later stage assumes, so getting it right first means the later phases compile against a stable contract.

Treat the tls context as the single source of truth for the tls termination data shape. Validate inputs at the boundary and reject early, so a malformed value never reaches SNI router or Cert loader where its effect would be harder to localize.

Phase 2: SNI router

Add the sni router so tls termination can terminating TLS at the correctly.

// SNI router — handles tls termination Add the sni router so
export class Snirouter {
  constructor(private opts: SnirouterOptions) {}
  async run(ctx: Context): Promise<SnirouterOutput> {
    return this.execute(ctx);
  }
}

After this phase the sni router works in isolation and is covered by a unit test. This is where tls termination correctness lives: the sni router is what makes terminating TLS at the edge and forwarding plain HTTP upstream while preserving client identity true, and its test is the proof.

Keep the sni router pure where you can. A function that takes an input and returns an output is trivial to test and impossible to mock wrong; a class with hidden state is the opposite. Push side effects to the caller and keep the sni router about the transformation.

Phase 3: Cert loader

Wire the cert loader to handle a certificate that expires without losing data.

// Cert loader: tls termination stage
export const certloader = (next: Next) => async (req: Request) => {
  const out = await transform(req);
  return next(out);
};

After this phase the cert loader works in isolation and is covered by a unit test. The cert loader is the difference between a tls termination system that works on the happy path and one that survives a certificate that expires without renewal and causes every handshake to fail at once.

Design the failure before the success. Decide what cert loader does when a certificate that expires without renewal and causes every handshake to fail at once: reject, retry, shed, or degrade — and write that test first. A tls termination system whose failure behavior is unspecified will pick one at random under pressure, and it is usually the wrong one.

Phase 4: Header injector

Integrate the header injector to make tls termination observable and testable end to end.

// Header injector for tls termination
export async function headerinjector(stream: AsyncIterable<Chunk>): Promise<Aggregate> {
  let acc = init();
  for await (const c of stream) acc = merge(acc, c);
  return acc;
}

After this phase the header injector works in isolation and is covered by a unit test. Observability is not a finishing touch; it is how you know the tls termination system is doing what it claims in production.

Emit one structured event per meaningful action, with enough context to localize a failure without a second round-trip. The header injector is what turns “something is slow” into “the sni router is slow for inputs matching this shape,” which is the difference between an alert and an investigation.

Phase 5: Upstream forwarder

Finalize the upstream forwarder so the tls termination system degrades gracefully under overload.

// Upstream forwarder: tls termination finalization
export function upstreamforwarder(state: State): Result {
  validate(state);
  return finalize(state);
}

After this phase the upstream forwarder works in isolation and is covered by a unit test. The upstream forwarder is what keeps overload from becoming corruption: when the tls termination system is asked for more than it can do, it degrades in a defined way instead of failing in an undefined one.

Pick a degradation strategy for upstream forwarder — shed, queue, or backpressure — and make it explicit in code and in a test. Leaving the behavior to chance is how a tls termination traffic spike becomes a data loss incident.

Testing

Each phase has a focused test: tls context, sni router, and cert loader are exercised in isolation, then an integration test drives the full tls termination pipeline and asserts the a certificate that expires case is handled.

Start with unit tests that pin each component’s contract: the tls context returns the documented shape, the sni router transforms correctly, and the cert loader fails in the documented way. These tests are fast and they localize regressions to the component that broke.

Then add an integration test that composes the components and drives a realistic tls termination scenario end to end. Assert the happy path first, then deliberately trigger a certificate that expires without renewal and causes every handshake to fail at once and assert the system degrades rather than corrupts. Because the components are pure where practical, the integration test stays deterministic rather than flaky.

A tls termination suite must also prove the negative: bad input is rejected at the boundary, overload is shed rather than queued indefinitely, and a partial failure does not leave the system in an inconsistent state. These are the tests that catch the regressions a happy-path suite never would.

The entire tls termination suite runs with no external service; every dependency is either in-process or stubbed, so every case is reproducible in CI. That is what lets the cert loader failure-mode assertions live as tests rather than as a manual checklist that drifts out of date.

# run the tls termination test suite
pnpm vitest run

Deployment and operations

Run at least two tls termination instances behind a load balancer and share state in Redis. Set resource limits so tls termination overload surfaces as backpressure rather than failure.

Run at least two stateless instances behind a load balancer, and move any shared state into a shared store so the tls termination behavior is global rather than per instance. Size limits so the system sheds load before it runs out of memory, because a tls termination system that OOMs under load fails in a way that is hard to reason about.

Drain on deploy: stop accepting new work, let in-flight tls termination operations finish, then exit. Because the system holds no state that cannot be reconstructed, a rollout is a matter of cycling instances one at a time while watching the error rate.

When a dependency is unavailable, decide explicitly whether the tls termination system fails closed or degrades. Leaving that choice to chance is how a dependency outage becomes a silent data loss, so make the choice visible in code and in a test.

Security considerations

Validate all tls termination inputs at the boundary, reject oversized payloads before buffering them, and keep secrets in the environment rather than the repository.

The tls termination system is a choke point, so it is the right place to reject malformed input before it reaches sni router or cert loader. Validate the shape and the size at the edge, and let the inner components assume what they receive is well-formed.

Never log secrets or full payloads; log enough to localize a failure without recording data that should not be on disk. Treat any external call as untrusted and validate its response before using it, because a tls termination system that trusts its inputs blindly is one malformed response away from corruption.

Keep dependencies pinned and run an audit in CI. A transitive dependency with a known vulnerability is still a vulnerability in your tls termination system, and the lockfile is what makes the audit meaningful.

Observability

Emit one structured event per tls termination operation with the inputs’ shape, the outcome, and a correlation id. Track error rate and latency per component and alert on a step change rather than a single spike.

The correlation id is the thread that ties a user-visible symptom to the tls termination decision and to the component log entry for the same operation. Without it, an investigation starts from a timestamp and a guess.

Watch the cert loader failure rate separately from the overall error rate. A jump in cert loader failures usually means a certificate that expires without renewal and causes every handshake to fail at once, not a bug in sni router, and the two have different owners and different fixes.

Chart the share of tls termination operations that shed, queue, or succeed. A change in the mix points at the cause faster than an aggregate latency can, because each class has a different root.

Troubleshooting

A certificate that expires without renewal and causes every handshake to fail at once. Reproduce with a minimal fixture, then add a guard test so the tls termination system rejects the condition loudly.

Performance regresses under tls termination load. Profile with the sampling analyzer, locate the hot path in sni router, and add a targeted benchmark to lock the regression.

Keep the tls termination dashboards separate from dependency dashboards. When an operation fails, the correlation id lets you decide in seconds whether the fault is tls context, sni router, or cert loader, instead of guessing from a timeout.

Limitations and next steps

• Single-region; multi-region replication is out of scope.
• The tls context is in-process and not horizontally shared by default.
• No auth layer; assume deployment behind an edge that enforces identity.

The tls termination system is single-region by design; extending to multi-region means making tls context shareable across regions, which is a real change to the consistency model rather than a config flag. The cert loader is in-process, which is correct for one instance but must move to a shared store before scaling out.

There is no authentication or authorization layer; the system assumes it is deployed behind an edge that enforces identity. Adding one is straightforward but out of scope for this build, and doing it here would distract from the tls termination correctness that is the point.

The natural next step, once the tls termination pipeline is trusted in production, is to add a short-TTL cache for idempotent reads and a richer header injector that exports histograms rather than counters. Both are additive and neither changes the contracts this build establishes.