fd0f4a0365
6d224861 came from a --no-checkout worktree whose empty index made the
commit drop every file except two. This restores 05b50d2b's full tree and
correctly adds stacks/stem95su/gdrive-sync.tf + the service-catalog stem95su
entry. Forward-only (parent=6d224861, no force-push); [ci skip] since the
live infra was never applied from the broken commit.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
181 lines
8.8 KiB
Markdown
181 lines
8.8 KiB
Markdown
# Mailserver PROXY protocol — research & decision
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Last updated: 2026-04-18 (original research). **Outcome implemented 2026-04-19 — see [UPDATE](#update-2026-04-19) below.**
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> ## UPDATE (2026-04-19)
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>
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> This doc describes the research that led to the Phase-6 rollout. **Option C
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> (pfSense HAProxy + PROXY v2)** was chosen and is now live. Operational
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> state, cutover history, bootstrap, and rollback procedures live in
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> [`mailserver-pfsense-haproxy.md`](mailserver-pfsense-haproxy.md).
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>
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> This file is retained as a decision record — it explains *why* Option A
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> (pod-pinning via nodeSelector) was rejected mid-session in favour of
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> Option C, and documents the MetalLB upstream limitation (PROXY injection
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> is explicitly won't-implement). Future debates of "why don't we just pin
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> the pod?" should land here first.
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## TL;DR
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**MetalLB does not and will not inject PROXY protocol headers.** The original plan
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(`/home/wizard/.claude/plans/let-s-work-on-linking-temporal-valiant.md`, task
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`code-rtb`) assumed MetalLB could be configured to emit PROXY v1/v2 on behalf of
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the `mailserver` LoadBalancer Service. That assumption is wrong at the product
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level. MetalLB is a control-plane-only announcer (ARP/NDP for L2 mode, BGP for
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L3 mode); it never touches the L4 payload.
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As a result, there is no single Terraform change that can flip
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`externalTrafficPolicy: Local` → `Cluster` on the `mailserver` Service while
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preserving the real client IP for Postfix/postscreen and Dovecot. Three
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alternative paths exist (see below); none is trivial.
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## Environment (verified 2026-04-18)
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- **MetalLB version**: `quay.io/metallb/controller:v0.15.3` /
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`quay.io/metallb/speaker:v0.15.3` (5 speakers).
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- **Advertisement type**: L2Advertisement `default` bound to IPAddressPool
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`default` (10.0.20.200–10.0.20.220). No BGPAdvertisements.
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- **Service**: `mailserver/mailserver` — type `LoadBalancer`, `loadBalancerIPs:
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10.0.20.202`, `externalTrafficPolicy: Local`,
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`healthCheckNodePort: 30234`, 5 ports (25, 465, 587, 993, 9166/dovecot-metrics).
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- **Pod**: single replica today, RWO PVCs prevent horizontal scale without
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further work (`mailserver-data-encrypted`, `mailserver-letsencrypt-encrypted`).
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## Why the original plan fails
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### MetalLB never touches packets
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> *"MetalLB is controlplane only, making it part of the dataplane means we
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> would be responsible for the performance of the system, so more bugs to
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> fight, I personally don't see that happening."*
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> — MetalLB maintainer `champtar`, 2021-01-06
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> (issue [#797 — Feature Request: Supporting Proxy Protocol v2](https://github.com/metallb/metallb/issues/797))
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Issue #797 is closed as "won't implement". Repeat asks in 2022–2023 got the
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same answer. The v0.15.3 API surface confirms this: no
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`proxyProtocol`/`haproxy`/`protocol: proxy` field exists on `IPAddressPool`,
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`L2Advertisement`, `BGPAdvertisement`, or as a Service annotation.
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Only managed-cloud LBs (AWS NLB, Azure LB, OCI, DO, OVH, Scaleway, etc.) offer
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PROXY protocol as a tick-box. MetalLB's equivalents are:
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| MetalLB feature | Does it preserve client IP? | Comment |
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|---|---|---|
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| `externalTrafficPolicy: Local` (current) | Yes, via iptables DNAT on the speaker node | Forces pod↔speaker colocation on L2 mode. This is the pain we wanted to avoid. |
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| `externalTrafficPolicy: Cluster` | No — kube-proxy SNATs to the node IP | The problem we would re-introduce if we flipped without PROXY injection. |
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| PROXY protocol injection | N/A — not implemented | Dead end. |
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### The `Local` trap is real, but narrower than it seems
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Today's `Local` policy means the ARP announcer node must also host the mailserver
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pod. MetalLB always picks a single speaker to advertise the VIP (leader
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election per IP), so in practice exactly one node matters at any moment. A pod
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rescheduled to a different node silently drops inbound SMTP/IMAP until a GARP
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flip or node cordon.
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The only pods on our cluster that see this same class of risk are Traefik
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(3 replicas + PDB `minAvailable=2`, so 2 of 3 nodes always have a pod) and
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mailserver (1 replica). Traefik survives because the pods outnumber the nodes
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that could be the speaker at once; the mailserver cannot.
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## Alternative paths (ranked by effort)
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### Option A — Pin the mailserver pod to a specific node (SIMPLEST)
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Add `nodeSelector` on the mailserver Deployment pointing at a label that's also
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stamped on the MetalLB speaker we want to advertise the VIP from, and use
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MetalLB's [node selector](https://metallb.io/configuration/_advanced_l2_configuration/#specify-network-interfaces-that-lb-ip-can-be-announced-from)
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on `L2Advertisement.spec.nodeSelectors` to pin the announcer to the same node.
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Trade-offs:
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- Zero changes to Postfix/Dovecot configs.
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- Keeps `externalTrafficPolicy: Local` — real client IP keeps arriving.
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- Loses HA (the whole point of the MetalLB layer) but reflects reality — one
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replica, one PVC, no HA today anyway.
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- Drain of that node requires a planned cutover, but that's no worse than
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today's silent failure mode.
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Implementation (~10 lines of Terraform):
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```hcl
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# In stacks/mailserver/modules/mailserver/main.tf, on the Deployment:
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node_selector = { "viktorbarzin.me/mailserver-anchor" = "true" }
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# In stacks/platform (or wherever the MetalLB CRs live):
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resource "kubernetes_manifest" "mailserver_l2ad" {
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manifest = {
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apiVersion = "metallb.io/v1beta1"
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kind = "L2Advertisement"
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metadata = { name = "mailserver", namespace = "metallb-system" }
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spec = {
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ipAddressPools = ["default"]
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nodeSelectors = [{ matchLabels = { "viktorbarzin.me/mailserver-anchor" = "true" } }]
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}
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}
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}
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```
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Plus a node label via `kubectl label node k8s-node3 viktorbarzin.me/mailserver-anchor=true`.
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**Recommendation: this is the shortest path to eliminating the silent-drop
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failure mode** without taking on a new proxy tier.
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### Option B — Put a HAProxy sidecar in front of Postfix/Dovecot
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Stand up an in-cluster HAProxy with PROXY v2 enabled on the frontend and
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`send-proxy-v2` on the backend to `mailserver:25/465/587/993`. Expose HAProxy
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via a new MetalLB Service with `externalTrafficPolicy: Cluster` + kube-proxy
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DSR workaround (still loses client IP at that layer), or run HAProxy on the
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host-network of the same node (back to Option A's colocation).
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Trade-offs:
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- Introduces one more network hop and TLS-termination decision for every
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SMTP connect.
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- HAProxy needs its own cert rotation (or `tls-passthrough`) — adds moving
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parts to an already crowded mailserver module.
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- Doesn't actually solve the colocation problem on its own — HAProxy itself
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needs to receive the client IP, so we are back to externalTrafficPolicy
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constraints for HAProxy.
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**Recommendation: avoid unless we also get HA for mailserver itself, which
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needs RWX storage + DB split-brain work — out of scope.**
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### Option C — Replace MetalLB with a different LB for this Service
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Candidates: [kube-vip](https://kube-vip.io/) (supports eBPF-based DSR but not
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PROXY injection either), [Cilium LB](https://docs.cilium.io/en/stable/network/lb-ipam/)
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(preserves client IP via DSR in hybrid mode), or a dedicated HAProxy running on
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pfSense and NAT-forwarding 25/465/587/993 with PROXY headers to a
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ClusterIP-exposed mailserver. Cilium requires a CNI migration (we run Calico
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today); pfSense HAProxy is genuinely feasible but belongs in a different bd
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task.
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**Recommendation: track as P3 follow-up under a new bd task if Option A proves
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insufficient.**
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## Decision
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Do nothing in this session beyond this runbook + the bd note. The `code-rtb`
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task as written is not executable — MetalLB cannot inject PROXY headers, and
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the Postfix/Dovecot config changes the plan proposed would not receive the
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header they expect, they would hang waiting for it and then timeout (5s per
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connection).
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Follow-up work filed as bd child tasks (if user wants to pursue):
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- **Option A — pin mailserver + L2Advertisement nodeSelectors** (new bd task)
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- **Option C — HAProxy on pfSense with PROXY v2 to a ClusterIP** (new bd task)
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## References
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- [MetalLB issue #797 — Feature Request: Supporting Proxy Protocol v2](https://github.com/metallb/metallb/issues/797) (closed, won't implement)
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- [MetalLB PR #796 — Source IP Preservation discussion](https://github.com/metallb/metallb/issues/796)
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- Postfix [postscreen_upstream_proxy_protocol](https://www.postfix.org/postconf.5.html#postscreen_upstream_proxy_protocol) — expects the PROXY header *on every incoming connection*; if absent, postscreen drops after `postscreen_upstream_proxy_timeout`.
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- Dovecot [haproxy_trusted_networks](https://doc.dovecot.org/settings/core/#core_setting-haproxy_trusted_networks) — treats the header as mandatory for listed source networks.
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- Cluster state verified against: `kubectl -n metallb-system get pods`,
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`kubectl get ipaddresspools.metallb.io -A`,
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`kubectl get l2advertisements.metallb.io -A`,
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`kubectl get bgpadvertisements.metallb.io -A`,
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`kubectl -n mailserver get svc mailserver -o yaml`.
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