Fix UDP Socket Binding Issue on Linux for IPv4 and IPv6

[jkralik]

On Linux, when using UDP sockets, both IPv6 and IPv4 addresses are bound to the same socket by default. This can cause issues when sending and receiving packets across different address families. Specifically, if we receive a packet from an IPv4 address, attempting to send a response using an IPv6 address fails.

Fixes #581

Summary by CodeRabbit

• New Features

  • Enhanced UDP connection handling for mixed IPv4 and IPv6 addresses to ensure correct packet transmission.
  • Improved test flexibility by dynamically retrieving network interfaces and addresses for UDP connection tests.

• Bug Fixes

  • Adjusted error handling in tests to ensure failures are correctly reported when suitable interfaces are not found.

• Tests

  • Expanded test cases for better coverage, including new scenarios for Linux operating systems.

[coderabbitai]

Walkthrough

The pull request introduces enhancements to the writeTo method of the UDPConn struct in the net package, improving the handling of UDP connections for mixed IPv4 and IPv6 addresses. Additionally, it updates the test cases in net/connUDP_internal_test.go to dynamically manage network interfaces and improve test flexibility. A new helper function is added for better interface management, and the overall robustness of the tests is enhanced.

Changes

File	Change Summary
net/connUDP.go	Enhanced writeTo method to handle mixed IPv4 and IPv6 addresses by introducing packetConnIPv4.
net/connUDP_internal_test.go	Improved TestUDPConnWriteWithContext with dynamic interface retrieval and added test scenarios for Linux. Introduced getInterfaceIndex helper function.

Assessment against linked issues

Objective	Addressed	Explanation
No error on server side with go-coap versions > 3.1.6 (#581)	✅

Suggested reviewers

• coderabbitai

Poem

In the meadow where rabbits play,
A fix was made to save the day.
UDP hops with joy anew,
IPv4 and six, a perfect view!
With tests that dance and interfaces bright,
Code flows smoothly, oh what a sight! 🐇✨

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Recent review details

Configuration used: CodeRabbit UI
Review profile: CHILL

Commits

Files that changed from the base of the PR and between 99a5d13 and 924009a.

Files selected for processing (2)

• net/connUDP.go (1 hunks)
• net/connUDP_internal_test.go (4 hunks)

Additional comments not posted (7)

net/connUDP_internal_test.go (6)

338-349: LGTM!

The new getInterfaceIndex function improves code reuse and clarity by encapsulating the logic for finding a suitable multicast interface. The error handling ensures that a failure to find a suitable interface results in a test failure, rather than returning an empty interface.

---

Line range hint 23-123: LGTM!

The enhancements to the TestUDPConnWriteWithContext function improve the test's flexibility and relevance to the actual network environment. The test now dynamically retrieves the appropriate network interface and its associated IPv4 address for the peer connection, rather than using a hardcoded address. The expanded test cases cover a wider range of scenarios, including Linux-specific configurations, and the updated WriteWithOptions method enhances the functionality being tested.

---

Line range hint 125-261: Skipped review.

This function has not been modified in the current changes.

---

Line range hint 263-308: Skipped review.

This function has not been modified in the current changes.

---

Line range hint 351-449: LGTM!

The modifications to the TestUDPConnWriteToAddr function improve the test's flexibility and relevance to the actual network environment. The test now uses the getInterfaceIndex function to retrieve a suitable network interface for testing, and the test cases have been updated to use the retrieved interface and its associated IP addresses. The test covers a wide range of scenarios, including IPv4 and IPv6 configurations, closed connections, and various options such as interface, source IP, and multicast hop limit.

---

Line range hint 451-547: Skipped review.

This function has not been modified in the current changes.

net/connUDP.go (1)

519-525: LGTM!

The changes introduced in the writeTo method enhance the handling of UDP connections, particularly in scenarios involving mixed IPv4 and IPv6 addresses. The new logic checks if the destination address is IPv4 and if the current connection is using an IPv6 packet connection. If both conditions are met, it creates a new packetConnIPv4 instance that wraps the existing connection, allowing the method to send packets correctly from an IPv4 address. This adjustment ensures compatibility when sending packets in environments where both IPv4 and IPv6 addresses are bound to the same socket, addressing a potential issue in packet transmission.

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[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 76.03%. Comparing base (99a5d13) to head (924009a).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #582      +/-   ##
==========================================
+ Coverage   75.82%   76.03%   +0.21%     
==========================================
  Files          73       73              
  Lines        5915     5918       +3     
==========================================
+ Hits         4485     4500      +15     
+ Misses       1045     1036       -9     
+ Partials      385      382       -3     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[sonarcloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
100.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarCloud

[jkralik]

@coderabbitai review

[coderabbitai]

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