infra/docs/runbooks/mailserver-proxy-protocol.md

Mailserver PROXY protocol — research & decision

Last updated: 2026-04-18

TL;DR

MetalLB does not and will not inject PROXY protocol headers. The original plan (/home/wizard/.claude/plans/let-s-work-on-linking-temporal-valiant.md, task code-rtb) assumed MetalLB could be configured to emit PROXY v1/v2 on behalf of the mailserver LoadBalancer Service. That assumption is wrong at the product level. MetalLB is a control-plane-only announcer (ARP/NDP for L2 mode, BGP for L3 mode); it never touches the L4 payload.

As a result, there is no single Terraform change that can flip externalTrafficPolicy: Local → Cluster on the mailserver Service while preserving the real client IP for Postfix/postscreen and Dovecot. Three alternative paths exist (see below); none is trivial.

Environment (verified 2026-04-18)

• MetalLB version: quay.io/metallb/controller:v0.15.3 / quay.io/metallb/speaker:v0.15.3 (5 speakers).
• Advertisement type: L2Advertisement default bound to IPAddressPool default (10.0.20.200–10.0.20.220). No BGPAdvertisements.
• Service: mailserver/mailserver — type LoadBalancer, loadBalancerIPs: 10.0.20.202, externalTrafficPolicy: Local, healthCheckNodePort: 30234, 5 ports (25, 465, 587, 993, 9166/dovecot-metrics).
• Pod: single replica today, RWO PVCs prevent horizontal scale without further work (mailserver-data-encrypted, mailserver-letsencrypt-encrypted).

Why the original plan fails

MetalLB never touches packets

"MetalLB is controlplane only, making it part of the dataplane means we would be responsible for the performance of the system, so more bugs to fight, I personally don't see that happening." — MetalLB maintainer champtar, 2021-01-06 (issue #797 — Feature Request: Supporting Proxy Protocol v2)

Issue #797 is closed as "won't implement". Repeat asks in 2022–2023 got the same answer. The v0.15.3 API surface confirms this: no proxyProtocol/haproxy/protocol: proxy field exists on IPAddressPool, L2Advertisement, BGPAdvertisement, or as a Service annotation.

Only managed-cloud LBs (AWS NLB, Azure LB, OCI, DO, OVH, Scaleway, etc.) offer PROXY protocol as a tick-box. MetalLB's equivalents are:

MetalLB feature	Does it preserve client IP?	Comment
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.
externalTrafficPolicy: Cluster	No — kube-proxy SNATs to the node IP	The problem we would re-introduce if we flipped without PROXY injection.
PROXY protocol injection	N/A — not implemented	Dead end.

The Local trap is real, but narrower than it seems

Today's Local policy means the ARP announcer node must also host the mailserver pod. MetalLB always picks a single speaker to advertise the VIP (leader election per IP), so in practice exactly one node matters at any moment. A pod rescheduled to a different node silently drops inbound SMTP/IMAP until a GARP flip or node cordon.

The only pods on our cluster that see this same class of risk are Traefik (3 replicas + PDB minAvailable=2, so 2 of 3 nodes always have a pod) and mailserver (1 replica). Traefik survives because the pods outnumber the nodes that could be the speaker at once; the mailserver cannot.

Alternative paths (ranked by effort)

Option A — Pin the mailserver pod to a specific node (SIMPLEST)

Add nodeSelector on the mailserver Deployment pointing at a label that's also stamped on the MetalLB speaker we want to advertise the VIP from, and use MetalLB's node selector on L2Advertisement.spec.nodeSelectors to pin the announcer to the same node.

Trade-offs:

• Zero changes to Postfix/Dovecot configs.
• Keeps externalTrafficPolicy: Local — real client IP keeps arriving.
• Loses HA (the whole point of the MetalLB layer) but reflects reality — one replica, one PVC, no HA today anyway.
• Drain of that node requires a planned cutover, but that's no worse than today's silent failure mode.

Implementation (~10 lines of Terraform):

# In stacks/mailserver/modules/mailserver/main.tf, on the Deployment:
node_selector = { "viktorbarzin.me/mailserver-anchor" = "true" }

# In stacks/platform (or wherever the MetalLB CRs live):
resource "kubernetes_manifest" "mailserver_l2ad" {
  manifest = {
    apiVersion = "metallb.io/v1beta1"
    kind       = "L2Advertisement"
    metadata   = { name = "mailserver", namespace = "metallb-system" }
    spec = {
      ipAddressPools = ["default"]
      nodeSelectors  = [{ matchLabels = { "viktorbarzin.me/mailserver-anchor" = "true" } }]
    }
  }
}

Plus a node label via kubectl label node k8s-node3 viktorbarzin.me/mailserver-anchor=true.

Recommendation: this is the shortest path to eliminating the silent-drop failure mode without taking on a new proxy tier.

Option B — Put a HAProxy sidecar in front of Postfix/Dovecot

Stand up an in-cluster HAProxy with PROXY v2 enabled on the frontend and send-proxy-v2 on the backend to mailserver:25/465/587/993. Expose HAProxy via a new MetalLB Service with externalTrafficPolicy: Cluster + kube-proxy DSR workaround (still loses client IP at that layer), or run HAProxy on the host-network of the same node (back to Option A's colocation).

Trade-offs:

• Introduces one more network hop and TLS-termination decision for every SMTP connect.
• HAProxy needs its own cert rotation (or tls-passthrough) — adds moving parts to an already crowded mailserver module.
• Doesn't actually solve the colocation problem on its own — HAProxy itself needs to receive the client IP, so we are back to externalTrafficPolicy constraints for HAProxy.

Recommendation: avoid unless we also get HA for mailserver itself, which needs RWX storage + DB split-brain work — out of scope.

Option C — Replace MetalLB with a different LB for this Service

Candidates: kube-vip (supports eBPF-based DSR but not PROXY injection either), Cilium LB (preserves client IP via DSR in hybrid mode), or a dedicated HAProxy running on pfSense and NAT-forwarding 25/465/587/993 with PROXY headers to a ClusterIP-exposed mailserver. Cilium requires a CNI migration (we run Calico today); pfSense HAProxy is genuinely feasible but belongs in a different bd task.

Recommendation: track as P3 follow-up under a new bd task if Option A proves insufficient.

Decision

Do nothing in this session beyond this runbook + the bd note. The code-rtb task as written is not executable — MetalLB cannot inject PROXY headers, and the Postfix/Dovecot config changes the plan proposed would not receive the header they expect, they would hang waiting for it and then timeout (5s per connection).

Follow-up work filed as bd child tasks (if user wants to pursue):

• Option A — pin mailserver + L2Advertisement nodeSelectors (new bd task)
• Option C — HAProxy on pfSense with PROXY v2 to a ClusterIP (new bd task)

References

• MetalLB issue #797 — Feature Request: Supporting Proxy Protocol v2 (closed, won't implement)
• MetalLB PR #796 — Source IP Preservation discussion
• Postfix postscreen_upstream_proxy_protocol — expects the PROXY header on every incoming connection; if absent, postscreen drops after postscreen_upstream_proxy_timeout.
• Dovecot haproxy_trusted_networks — treats the header as mandatory for listed source networks.
• Cluster state verified against: kubectl -n metallb-system get pods, kubectl get ipaddresspools.metallb.io -A, kubectl get l2advertisements.metallb.io -A, kubectl get bgpadvertisements.metallb.io -A, kubectl -n mailserver get svc mailserver -o yaml.