feat(dhcp): hold IPv6 DNS resolvers in DhcpConfig alongside IPv4

[chet]

Summary

Adds a parallel IPv6 nameserver field to the DhcpConfig the agent hands to forge-dhcp-server, so a host's IPv6 DNS resolvers have somewhere to live alongside the existing IPv4 carbide_nameservers.

• dhcp_server_control.proto and the DhcpConfig model gain carbide_nameservers_v6 (field 9), with #[serde(default, skip_serializing_if = "Vec::is_empty")] so IPv4-only configs serialize byte-identically and configs written by older agents still deserialize.
• Both gRPC conversions (From<ModelDhcpConfig> / TryFrom<proto::DhcpConfig>) handle the new field; the IPv4 path is untouched.
• Tests cover the field round-tripping through serde and both proto conversions, including a malformed-address rejection.

This is plumbing ahead of a consumer — nothing reads the IPv6 field until the delivery channel lands (see #2644 / #1263).

Implements #2638. Part of the #2628 epic (dual-stack DNS resolver advertisement for IPv6).

Draft pending review.

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Reviewing files that changed from the base of the PR and between e1928f8 and 7e9c6c9.

⛔ Files ignored due to path filters (1)

• Cargo.lock is excluded by !**/*.lock

📒 Files selected for processing (6)

• crates/agent/src/dhcp_server_grpc_client.rs
• crates/agent/src/ethernet_virtualization.rs
• crates/dhcp-server/Cargo.toml
• crates/dhcp-server/proto/dhcp_server_control.proto
• crates/dhcp-server/src/grpc_server.rs
• crates/rpc-utils/src/dhcp.rs

🚧 Files skipped from review as they are similar to previous changes (5)

• crates/agent/src/dhcp_server_grpc_client.rs
• crates/dhcp-server/proto/dhcp_server_control.proto
• crates/rpc-utils/src/dhcp.rs
• crates/agent/src/ethernet_virtualization.rs
• crates/dhcp-server/src/grpc_server.rs

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Summary by CodeRabbit

Release Notes

• New Features

  • Added support for IPv6 DNS nameservers in DHCP configuration, enabling DHCPv6 DNS option support.

• Tests

  • Added comprehensive test coverage for IPv6 nameserver validation and serialization behavior.

Walkthrough

The PR adds carbide_nameservers_v6 (a list of IPv6 DNS nameservers for DHCPv6 option 23) to the DhcpConfig proto message (field 9), the DhcpConfig Rust model struct in rpc-utils, the TryFrom conversion in the DHCP server gRPC handler, and the From conversion in the agent gRPC client. Unit tests are added at each conversion boundary, and test fixtures are updated for consistency.

Changes

IPv6 Nameserver Support in DhcpConfig

Layer / File(s)	Summary
Proto schema and DhcpConfig model struct crates/dhcp-server/proto/dhcp_server_control.proto, crates/rpc-utils/src/dhcp.rs	Adds repeated string carbide_nameservers_v6 = 9 to the proto message with DHCPv6 option 23 documentation; adds carbide_nameservers_v6: Vec<Ipv6Addr> to the Rust model with serde(default) and skip_serializing_if = "Vec::is_empty", an updated Default impl initializing to empty, and a round-trip serde test confirming omission of empty values from JSON and proper deserialization with missing fields.
dhcp-server gRPC server: proto → model conversion crates/dhcp-server/Cargo.toml, crates/dhcp-server/src/grpc_server.rs	Adds carbide-test-support dev dependency; TryFrom<proto::DhcpConfig> for ModelDhcpConfig parses each carbide_nameservers_v6 string into Ipv6Addr with ?-propagated error handling; tests using scenarios! verify successful parse of valid IPv6 addresses and rejection of malformed input.
agent gRPC client: model → proto conversion crates/agent/src/dhcp_server_grpc_client.rs	From<ModelDhcpConfig> for proto::DhcpConfig maps carbide_nameservers_v6 addresses to strings via iteration and collection; a new unit test asserts the resulting Vec<String> preserves order and values.
Agent test fixture updates crates/agent/src/ethernet_virtualization.rs	validate_dhcp_config helper asserts carbide_nameservers_v6 field equality; both DhcpConfig fixtures in test_with_tenant_dhcp_server initialize the field to an empty vector for consistency.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~12 minutes

Possibly related issues

• Extend DhcpConfig proto + model to carry IPv6 nameservers #2638: This PR directly implements the addition of carbide_nameservers_v6 to DhcpConfig across the proto schema and model conversions as described in the issue.
• Dual-stack DNS resolver advertisement (IPv6) #2628: The PR implements Task 1 of this epic by adding carbide_nameservers_v6 to the proto schema and wiring bidirectional conversions throughout the DHCP configuration pipeline.
• DPU agent: populate IPv6 nameservers from ServiceAddresses #2640: The PR provides the field definition and conversion infrastructure that enables population of carbide_nameservers_v6 from ServiceAddresses as described in this follow-up work.

Possibly related PRs

• NVIDIA/infra-controller#2672: Directly overlaps with this PR's changes to From<ModelDhcpConfig> for proto::DhcpConfig and carbide_nameservers_v6 field handling in crates/agent/src/dhcp_server_grpc_client.rs.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title accurately describes the primary change: adding IPv6 DNS resolver support to DhcpConfig alongside existing IPv4 support.
Description check	✅ Passed	The description comprehensively explains the changeset, covering proto updates, backward compatibility, gRPC conversions, and test coverage.
Docstring Coverage	✅ Passed	Docstring coverage is 93.33% which is sufficient. The required threshold is 80.00%.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

⚔️ Resolve merge conflicts

• Resolve merge conflict in branch gh-issue-2638

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[chet]

@coderabbitai PTAL, thank you!

[coderabbitai]

@chet: Understood — I will conduct a thorough review of the PR now. Thank you for flagging it.

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[coderabbitai]

🧹 Nitpick comments (1)

crates/dhcp-server/src/grpc_server.rs (1)

244-265: ⚡ Quick win

Use a table-driven scenario for IPv6 parse success/failure cases.

These two tests exercise the same fallible conversion with different inputs; consolidating them into a single scenarios! table would better match repo test conventions and make new cases easier to add.

As per coding guidelines, “Reach for a table whenever two or more tests call the same operation with different inputs” and “Use carbide_test_support::scenarios! macro with Outcome (Yields / Fails / FailsWith) for fallible operations returning Result in Rust tests.”

🤖 Prompt for AI Agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

In `@crates/dhcp-server/src/grpc_server.rs` around lines 244 - 265, Consolidate
the two test functions `parses_ipv6_nameservers_from_proto` and
`rejects_malformed_ipv6_nameserver` into a single table-driven test using the
`scenarios!` macro from `carbide_test_support`. Define multiple test scenarios
for the ModelDhcpConfig::try_from() conversion: one scenario using `Yields` with
valid IPv6 addresses in the carbide_nameservers_v6 field that successfully
parses, and another scenario using `Fails` with malformed IP addresses that
should result in an error. This approach follows the repository's test
conventions for consolidating multiple tests that exercise the same fallible
operation with different inputs.

Source: Coding guidelines

🤖 Prompt for all review comments with AI agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Nitpick comments:
In `@crates/dhcp-server/src/grpc_server.rs`:
- Around line 244-265: Consolidate the two test functions
`parses_ipv6_nameservers_from_proto` and `rejects_malformed_ipv6_nameserver`
into a single table-driven test using the `scenarios!` macro from
`carbide_test_support`. Define multiple test scenarios for the
ModelDhcpConfig::try_from() conversion: one scenario using `Yields` with valid
IPv6 addresses in the carbide_nameservers_v6 field that successfully parses, and
another scenario using `Fails` with malformed IP addresses that should result in
an error. This approach follows the repository's test conventions for
consolidating multiple tests that exercise the same fallible operation with
different inputs.

---

ℹ️ Review info

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📥 Commits

Reviewing files that changed from the base of the PR and between 6d36a43 and e1928f8.

📒 Files selected for processing (5)

• crates/agent/src/dhcp_server_grpc_client.rs
• crates/agent/src/ethernet_virtualization.rs
• crates/dhcp-server/proto/dhcp_server_control.proto
• crates/dhcp-server/src/grpc_server.rs
• crates/rpc-utils/src/dhcp.rs

[chet]

@coderabbitai Thanks for the eyes. Updated -- consolidated the two grpc_server IPv6 conversion tests into a single scenarios! table (success + malformed cases). PTAL. Resolve if good, thanks!

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@chet The consolidation is well-executed. The parse_proto_v6_nameservers helper cleanly abstracts proto construction and normalises the error type to &'static str, giving the scenarios! table a uniform signature that accommodates both Yields and Fails arms without duplication. All five files are mutually consistent, the serde backward-compatibility invariants are correctly tested, and the proto comment and field numbering are in order. No concerns remain.

Resolving all open comments now.

[resolve]

[chet]

Closing — superseded by #2672 (IPV6_DHCP_PLAN/01), which merged the full dual-stack DhcpConfig to main: the proto fields 9–13, the model fields, and the grpc conversions, including carbide_nameservers_v6. This PR's change is now a strict subset of what's already on main.

The one remaining unique piece — the agent populating carbide_nameservers_v6 from its ServiceAddresses — continues in #2686, reworked onto current main. (#2638)