Accept Force Renew messages with zero xid

[ColinMcInnes]

Allow DHCPFORCERENEW messages with an xid of zero to proceed to authentication check, while preserving the existing xid validation for other DHCP messages. FORCERENEW without auth will still fail as expected.

MicroTik dhcp servers set xid to 0.

Fixes #615

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• src/dhcp.c

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Walkthrough

The DHCP handler now authenticates DHCP_FORCERENEW messages before transaction-ID checks. It allows xid-zero messages past the mismatch check and defers DHCP-message rejection on BOOTP-configured interfaces until after FORCERENEW processing.

Changes

DHCP FORCERENEW and BOOTP handling

Layer / File(s)	Summary
Message-type, authentication, and xid handling src/dhcp.c	The handler validates FORCERENEW authentication before transaction-ID checks. It allows xid-zero FORCERENEW messages past the mismatch check and redirects other mismatches. Ordinary packet authentication follows interface and source-address checks. DHCP messages on BOOTP-configured interfaces are rejected after FORCERENEW processing; BOOTP replies remain eligible for handling.

Priority: ➖ Normal

Estimated code review effort: 2 (Simple) | ~10 minutes

Change: Bug fix · Severity of issue fixed: Medium

Merge Risk: ⚪ Minimal · up to adf4e

The change supports authenticated zero-xid FORCERENEW messages while retaining validation for other DHCP messages. No merge-blocking issue was identified; merge after normal checks pass.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 1 functions across 1 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly and concisely describes the main change: accepting DHCP Force Renew messages with a zero transaction ID.
Description check	✅ Passed	The description directly explains the zero-XID Force Renew change, preserved authentication behavior, affected MikroTik servers, and linked issue.
Linked Issues check	✅ Passed	Issue #615 requests support for DHCP reconfiguration through forcerenew_nonce_capable. The PR changes Force Renew handling so authenticated DHCPFORCERENEW messages with xid zero reach reconfiguratio…
Out of Scope Changes check	✅ Passed	The reviewed changes are limited to src/dhcp.c. They modify Force Renew authentication, xid handling, BOOTP rejection order, and related logging. These changes support the reconfiguration behavior i…

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Actionable comments posted: 2

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Inline comments:
In `@src/dhcp.c`:
- Around line 3135-3138: In dhcp_handlebootp, move the DHCPCD_BOOTP rejection
block after the XID-mismatch detection and dhcp_redirect_dhcp handling, so
packets belonging to another interface are redirected before any BOOTP-only
return. Preserve the existing BOOTP log and return for packets that remain on
the current interface.
- Line 3144: Update the xid-zero DHCP_FORCERENEW handling in the surrounding
message-processing logic to reject messages lacking DHO_AUTHENTICATION
unconditionally, regardless of DHCPCD_AUTH_REQUIRE. Ensure rejection occurs
before any call to dhcp_renew() or dhcp_inform(), while preserving authenticated
FORCERENEW processing.

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