[PR #6289] [client] Fix fwmark bit 0x20 conflict with OVH OKS kube-proxy MASQUERADE rules #25281

New Issue

saavagebueno · 2026-08-05T07:05:48-04:00

saavagebueno commented

2026-08-05 07:05:48 -04:00

📋 Pull Request Information

Original PR: https://github.com/netbirdio/netbird/pull/6289
Author: @elprofessor-de
Created: 5/29/2026
Status: 🔄 Open

Base: main ← Head: fix/fwmark-ovh-oks-conflict

📝 Commits (1)

6daa99a [client] Fix fwmark conflict with OVH OKS kube-proxy rules

📊 Changes

1 file changed (+6 additions, -4 deletions)

View changed files

📝 client/net/net.go (+6 -4)

📄 Description

Describe your changes

On OVH Managed Kubernetes (OKS) nodes, a pre-installed iptables rule in
mangle POSTROUTING matches packets where bit 0x20 is set in the fwmark,
and strips bit 0x4000 — which is kube-proxy's MASQUERADE bit set by
KUBE-MARK-MASQ. As a result, KUBE-POSTROUTING bails out, MASQUERADE
never happens, and NodePort replies never reach the client.

Since v0.40, the three PreroutingFwmark* constants use values 0x1BD20,
0x1BD21, 0x1BD22 — all of which have bit 0x20 set in their lower byte.
This was introduced in #3623 as a sequential grouping choice (after
0x1BD10/0x1BD11), with no intention of using that specific bit.

Root cause confirmed with nft monitor trace: 0x4000 is added by
KUBE-MARK-MASQ then immediately stripped by the OVH hairpin rule. That
rule's counter shows ~4M packet matches vs ~2.4K for legitimate hairpin
traffic — almost entirely Netbird-marked packets. Manually deleting the rule
fixes NodePort access immediately.

Fix: shift the three prerouting marks from 0x1BD2x to 0x1BD4x
(0x40 has bit 0x20 clear). No other code changes are needed as all
consumers reference these constants.

Constant	Before	After
`PreroutingFwmarkRedirected`	`0x1BD20`	`0x1BD40`
`PreroutingFwmarkMasquerade`	`0x1BD21`	`0x1BD41`
`PreroutingFwmarkMasqueradeReturn`	`0x1BD22`	`0x1BD42`

These marks are managed entirely by the Netbird daemon at start/stop —
no user-visible config or persistent state is affected.

Issue ticket number and link

No existing issue — bug found in production on OVH OKS.

Stack

Checklist

Is it a bug fix
Is a typo/documentation fix
Is a feature enhancement
It is a refactor
Created tests that fail without the change (if possible)
This change does not modify the public API, gRPC protocols, functionality behavior, CLI / service flags, or introduce a new feature — OR I have discussed it with the NetBird team beforehand.

Documentation

Documentation is not needed for this change (internal constants, no user-facing behavior change)

Summary by CodeRabbit

Chores
- Updated internal firewall mark constants to prevent conflicts with network routing behavior.

_{🔄 This issue represents a GitHub Pull Request. It cannot be merged through Gitea due to API limitations.}

## 📋 Pull Request Information **Original PR:** https://github.com/netbirdio/netbird/pull/6289 **Author:** [@elprofessor-de](https://github.com/elprofessor-de) **Created:** 5/29/2026 **Status:** 🔄 Open **Base:** `main` ← **Head:** `fix/fwmark-ovh-oks-conflict` --- ### 📝 Commits (1) - [`6daa99a`](https://github.com/netbirdio/netbird/commit/6daa99a505374ce65e6a4104622311f6b7866aa5) [client] Fix fwmark conflict with OVH OKS kube-proxy rules ### 📊 Changes **1 file changed** (+6 additions, -4 deletions) <details> <summary>View changed files</summary> 📝 `client/net/net.go` (+6 -4) </details> ### 📄 Description ## Describe your changes On OVH Managed Kubernetes (OKS) nodes, a pre-installed iptables rule in `mangle POSTROUTING` matches packets where bit `0x20` is set in the fwmark, and strips bit `0x4000` — which is kube-proxy's MASQUERADE bit set by `KUBE-MARK-MASQ`. As a result, `KUBE-POSTROUTING` bails out, MASQUERADE never happens, and NodePort replies never reach the client. Since v0.40, the three `PreroutingFwmark*` constants use values `0x1BD20`, `0x1BD21`, `0x1BD22` — all of which have bit `0x20` set in their lower byte. This was introduced in #3623 as a sequential grouping choice (after `0x1BD10`/`0x1BD11`), with no intention of using that specific bit. **Root cause confirmed with `nft monitor trace`**: `0x4000` is added by `KUBE-MARK-MASQ` then immediately stripped by the OVH hairpin rule. That rule's counter shows ~4M packet matches vs ~2.4K for legitimate hairpin traffic — almost entirely Netbird-marked packets. Manually deleting the rule fixes NodePort access immediately. **Fix**: shift the three prerouting marks from `0x1BD2x` to `0x1BD4x` (`0x40` has bit `0x20` clear). No other code changes are needed as all consumers reference these constants. | Constant | Before | After | |---|---|---| | `PreroutingFwmarkRedirected` | `0x1BD20` | `0x1BD40` | | `PreroutingFwmarkMasquerade` | `0x1BD21` | `0x1BD41` | | `PreroutingFwmarkMasqueradeReturn` | `0x1BD22` | `0x1BD42` | These marks are managed entirely by the Netbird daemon at start/stop — no user-visible config or persistent state is affected. ## Issue ticket number and link No existing issue — bug found in production on OVH OKS. ## Stack  ### Checklist - [x] Is it a bug fix - [ ] Is a typo/documentation fix - [ ] Is a feature enhancement - [ ] It is a refactor - [ ] Created tests that fail without the change (if possible) - [x] This change does **not** modify the public API, gRPC protocols, functionality behavior, CLI / service flags, or introduce a new feature — **OR** I have discussed it with the NetBird team beforehand. ## Documentation - [x] Documentation is **not needed** for this change (internal constants, no user-facing behavior change)  ## Summary by CodeRabbit * **Chores** * Updated internal firewall mark constants to prevent conflicts with network routing behavior.  [![Review Change Stack](https://storage.googleapis.com/coderabbit_public_assets/review-stack-in-coderabbit-ui.svg)](https://app.coderabbit.ai/change-stack/netbirdio/netbird/pull/6289?utm_source=github_walkthrough&utm_medium=github&utm_campaign=change_stack)   --- <sub>🔄 This issue represents a GitHub Pull Request. It cannot be merged through Gitea due to API limitations.</sub>

saavagebueno added the pull-request label 2026-08-05 07:05:48 -04:00

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Reference: DYNR/netbird#25281