[PR #6781] [client] Evaluate IP fragments against firewall ACLs #28970

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saavagebueno · 2026-08-05T08:07:13-04:00

saavagebueno commented

2026-08-05 08:07:13 -04:00

Original Pull Request: https://github.com/netbirdio/netbird/pull/6781

State: closed
Merged: Yes

Describe your changes

The userspace firewall passed every IP fragment through without ACL evaluation, because gopacket does not decode the transport header of a fragment (not even the first one). This makes fragments follow the same ACL path as whole packets.

Decode and ACL-evaluate the first fragment of a datagram (IPv4 and IPv6) and record its verdict, keyed by the reassembly tuple (source, destination, protocol, identification).
Trailing fragments inherit the recorded verdict; a fragment with no allowed first fragment on record is dropped (fail closed). Downstream reassembly by the kernel/netstack is unchanged.
Drop first fragments too small to hold the full transport header, and drop fragments that overlap the already-inspected header (RFC 1858 / RFC 3128; RFC 5722 for IPv6).
Bound the verdict table with a TTL and a cap (NB_FRAGMENT_MAX_ENTRIES, default 16384), failing closed at capacity.
Emit netflow drop events for denied first fragments, matching non-fragmented packets.

Added unit tests and benchmarks for the fragment paths (v4/v6, tiny-fragment, overlap, conntrack, routed, TTL, capacity); the attacker-amplifiable paths stay zero-allocation and at or below the normal-packet baseline.

Issue ticket number and link

Stack

0.74.7-branch - ⚠️ No PR associated with branch
- #6781 👈

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 (link the issue / Slack thread in the description). See CONTRIBUTING.md.

By submitting this pull request, you confirm that you have read and agree to the terms of the Contributor License Agreement.

Documentation

Select exactly one:

I added/updated documentation for this change
Documentation is not needed for this change (explain why)

Internal firewall packet-handling behavior; no user-facing configuration beyond the optional NB_FRAGMENT_MAX_ENTRIES tuning env var.

Summary by CodeRabbit

New Features
- Added secure handling for fragmented IPv4 and IPv6 traffic.
- Fragmented packets are now evaluated consistently against inbound and routed firewall rules.
- Allowed first fragments permit matching trailing fragments, while unknown, expired, overlapping, or unsupported fragments are blocked.
- Added fail-closed safeguards when fragment tracking capacity is reached.
Bug Fixes
- Improved firewall drop logging and flow reporting for denied fragmented traffic.
Tests
- Added comprehensive coverage and performance benchmarks for fragmented packet scenarios.

**Original Pull Request:** https://github.com/netbirdio/netbird/pull/6781 **State:** closed **Merged:** Yes --- ## Describe your changes The userspace firewall passed every IP fragment through without ACL evaluation, because gopacket does not decode the transport header of a fragment (not even the first one). This makes fragments follow the same ACL path as whole packets. - Decode and ACL-evaluate the first fragment of a datagram (IPv4 and IPv6) and record its verdict, keyed by the reassembly tuple (source, destination, protocol, identification). - Trailing fragments inherit the recorded verdict; a fragment with no allowed first fragment on record is dropped (fail closed). Downstream reassembly by the kernel/netstack is unchanged. - Drop first fragments too small to hold the full transport header, and drop fragments that overlap the already-inspected header (RFC 1858 / RFC 3128; RFC 5722 for IPv6). - Bound the verdict table with a TTL and a cap (`NB_FRAGMENT_MAX_ENTRIES`, default 16384), failing closed at capacity. - Emit netflow drop events for denied first fragments, matching non-fragmented packets. Added unit tests and benchmarks for the fragment paths (v4/v6, tiny-fragment, overlap, conntrack, routed, TTL, capacity); the attacker-amplifiable paths stay zero-allocation and at or below the normal-packet baseline. ## Issue ticket number and link ## Stack - `0.74.7-branch` - :warning: No PR associated with branch  - \#6781 :point\_left: ### Checklist - [x] Is it a bug fix - [ ] Is a typo/documentation fix - [ ] Is a feature enhancement - [ ] It is a refactor - [x] 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 (link the issue / Slack thread in the description). See [CONTRIBUTING.md](https://github.com/netbirdio/netbird/blob/main/CONTRIBUTING.md#discuss-changes-with-the-netbird-team-first). > By submitting this pull request, you confirm that you have read and agree to the terms of the [Contributor License Agreement](https://github.com/netbirdio/netbird/blob/main/CONTRIBUTOR_LICENSE_AGREEMENT.md). ## Documentation Select exactly one: - [ ] I added/updated documentation for this change - [x] Documentation is **not needed** for this change (explain why) Internal firewall packet-handling behavior; no user-facing configuration beyond the optional `NB_FRAGMENT_MAX_ENTRIES` tuning env var.  ## Summary by CodeRabbit - **New Features** - Added secure handling for fragmented IPv4 and IPv6 traffic. - Fragmented packets are now evaluated consistently against inbound and routed firewall rules. - Allowed first fragments permit matching trailing fragments, while unknown, expired, overlapping, or unsupported fragments are blocked. - Added fail-closed safeguards when fragment tracking capacity is reached. - **Bug Fixes** - Improved firewall drop logging and flow reporting for denied fragmented traffic. - **Tests** - Added comprehensive coverage and performance benchmarks for fragmented packet scenarios.

saavagebueno added the pull-request label 2026-08-05 08:07:13 -04:00

saavagebueno closed this issue

2026-08-05 08:07:13 -04:00

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Reference: DYNR/netbird#28970