[neddp]

What is this change about?

Follow-up to #484 (DNSDefaultRoute=no). Even after that fix, BOSH's network configuration was silently ignored on Azure VMs - the agent wrote the file but it was never applied.

The root cause: systemd-networkd selects .network files by pure alphabetical basename order across all config directories. BOSH wrote 10_eth0.network (underscore), but Azure cloud-init/netplan generates /run/systemd/network/10-netplan-eth0.network (hyphen). Since _ (0x5F) > - (0x2D), the netplan file sorted last and won - the opposite of what bosh-agent intended.

Fix: rename the generated file to 09-eth0.network. The 09- prefix sorts before all 10-* files, so BOSH's configuration takes effect on every systemctl restart systemd-networkd regardless of what netplan wrote at first boot.

Please provide contextual information

Only affects Azure, where cloud-init/netplan generates /run/systemd/network/10-netplan-eth0.network. Other IaaS providers do not ship netplan.io in the stemcell.

What do the two .network files look like?

On Azure, two files compete for eth0. systemd-networkd picks the alphabetically-first basename across all config dirs:

	netplan (cloud-init)	BOSH (this PR)
Path	/run/systemd/network/10-netplan-eth0.network	/etc/systemd/network/09-eth0.network
Sorts	10-netplan-…	09-eth0… ← comes first (09 < 10)
[Match]	Driver=hv_netvsc + PermanentMACAddress=… + Name=eth0	Name=eth0
DHCP=	ipv4	yes
DNSDefaultRoute=	(absent, defaults to yes)	no ← routes DNS through BOSH-DNS
DNS=	(absent)	<from cloud config>
LinkLocalAddressing=	ipv6	(absent)

Before this fix, BOSH wrote 10_eth0.network (underscore). Since _ (0x5F) > - (0x2D), the netplan file won and DNSDefaultRoute=no was never applied.

What tests have you run against this PR?

• go test ./platform/net/... - all pass
• Manual validation with a custom stemcell built from this patched agent, deployed to an Azure environment via BOSH:
  • Before (stock stemcell): networkctl status eth0 → Network File: /run/systemd/network/10-netplan-eth0.network
  • After (patched stemcell): networkctl status eth0 → Network File: /etc/systemd/network/09-eth0.network
  • Confirmed via journalctl -u systemd-networkd: at first boot netplan wins, then bosh-agent restarts networkd and the BOSH file takes over
  • Fix survives VM restart (/etc/ persists, /run/ is cleared on reboot)

How should this change be described in bosh-agent release notes?

Fix BOSH network config being ignored on Azure: the generated .network file is now named so it takes precedence over the cloud-init/netplan file.

Does this PR introduce a breaking change?

No. Only the filename changes (10_<iface>.network → 09-<iface>.network).

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• platform/net/ubuntu_net_manager.go
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Walkthrough

The network manager now generates systemd-networkd interface filenames as 09-<interface>.network instead of 10_<interface>.network. A non-Windows test helper constructs interface file paths. Ubuntu networking tests use the helper for expected paths, file lookups, and fixture creation. A test checks that the eth0 filename sorts before the netplan filename.

Suggested reviewers: rkoster

Priority: Normal

Merge Risk: Minimal · up to 111cb

The filename change and related tests are aligned, with no actionable merge risk identified.

Pre-merge checks: 4

Check name	Status	Explanation
Title check		The title names the main change: BOSH network config on Azure was ignored when netplan was present. It follows the conventional prefix style and is specific enough for a reader scanning history. The t…
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Out of Scope Changes check		Check skipped because no linked issues were found for this pull request.
Description check		The pull request description covers the change, root cause, affected environment, implementation details, tests, Azure validation, release notes, and breaking-change assessment. It does not include th…

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The title names the main change: BOSH network config on Azure was ignored when netplan was present. It follows the conventional prefix style and is specific enough for a reader scanning history. The title is consistent with the objectives (renaming the generated file from 10_ to 10- so it sorts before the netplan file) and with the file summaries. It is 74 characters, which is within a normal length for a title.

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The pull request description covers the change, root cause, affected environment, implementation details, tests, Azure validation, release notes, and breaking-change assessment. It does not include the template sections for tagging reviewers or documenting AI review feedback, but the required technical information is otherwise complete.

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Inline comments:
Review comments at @platform/net/ubuntu_net_manager_helpers_test.go:
- Line 8: Update the networking test around SetupNetworking to create the
netplan fixture after setup, then sort the directory listing and assert that the
literal 10-eth0.network path precedes 10-netplan-eth0.network. This verifies the
expected filename ordering and catches a regression to an underscore in the
interface path.

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Files selected for processing: 4

• platform/net/ubuntu_net_manager.go
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Review comments at @platform/net/ubuntu_net_manager.go:
- Line 353: Update the netFile test helper and the ordering test to expect the
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[a-hassanin]

LGTM

[neddp]

During the community meeting there was a question about why we implement a workaround for the config file instead of simply cleaning up the netplan file cloud-init generates.

Cloud-init is used on Azure only - it was introduced in #235. Microsoft requires it for provisioning Ubuntu VMs on Azure. Removing it would break VM deployment entirely - cloud-init is what gets the VM connected during the early boot sequence so Azure can complete provisioning.

The current flow on deploy and restart: cloud-init runs early in the boot sequence and writes a netplan networkd unit to get the VM connected for Azure provisioning. After that, bosh-agent runs and writes its own networkd unit with the DNS and network config from the manifest.

Without this fix, both files exist and cloud-init's 10-netplan-eth0.network could win depending on ordering. This PR names the BOSH unit 09-eth0.network so it always sorts before cloud-init's file - systemd-networkd picks the first match, so BOSH's config wins.

[s4heid]

The rename makes sense for this collision. However, more generally speaking I think it is not great that we have two configuration producers that are trying to co-manage the network config. As a follow-up, I’d suggest to make the ownership contract more explicit:

• cloud-init/netplan provide Azure provisioning and bootstrap connectivity
• BOSH owns the complete configuration of its managed interfaces afterward
• Azure’s hardware exclusions retain precedenc (network config that is provided by azure-vm-utils)

Some ideas how to make the ownership more explicit:

1. Cloud-init should finish setting up the initial network and writing BOSH’s settings before the agent applies its network configuration. Make that dependency explicit in the Azure stemcell’s service startup order. Once BOSH takes over, prevent later cloud-init/netplan operations from independently renaming or reconfiguring those interfaces. The 09-* filename only controls which networkd profile wins.
2. After applying configuration, check that networkd selected the expected BOSH profile (the Network File exposed by networkctl status) and report unexpected selection explicitly. Successfully writing the file didn’t protect us from this original failure...
3. writeNetworkInterfaces should recognize its own profiles, including legacy 10_<iface>.network files, rather than treating unrelated entries under /etc/systemd/network as stale. Today, writeNetworkInterfaces treats almost everything under /etc/systemd/network as stale unless it was just generated or has the special excluded suffix.

[a-hassanin]

@s4heid Thanks for your review and recommendations. Let us follow up on the issues created by @neddp above. For now, we should bring this in to reduce the current DNS load on the Azure infrastructure.