[client] Fix IPv6 address formatting in DNS address construction

[mango766]

Summary

Fixes #5601
Related to #4074

Several places in the DNS client code construct server addresses using fmt.Sprintf("%s:%d", ip, port). This produces malformed addresses when the IP is IPv6, since IPv6 addresses contain colons and need brackets when combined with a port (e.g., [2606:4700:4700::1111]:53 instead of 2606:4700:4700::1111:53).

This causes DNS resolution to fail with dial udp: address 2606:4700:4700::1111:53: too many colons in address when IPv6 nameservers are configured.

Changes

Replaced fmt.Sprintf("%s:%d", ip, port) with net.JoinHostPort() in:

• client/internal/routemanager/dnsinterceptor/handler.go — upstream DNS address for the DNS interceptor (most critical, directly causes query failures)
• client/internal/dns/service_listener.go — DNS listener address and port binding probes
• client/internal/routemanager/client/client.go — DNS address for dynamic route handlers

net.JoinHostPort() is the standard Go idiom for this — it automatically wraps IPv6 addresses in brackets. The rest of the codebase already uses this function in many places (e.g., peer/worker_ice.go, ssh/detection/detection.go).

Test plan

• Verify existing DNS tests still pass (go test ./client/internal/dns/...)
• Verify route manager tests pass (go test ./client/internal/routemanager/...)
• Configure an IPv6 nameserver (e.g., Cloudflare 2606:4700:4700::1111) and verify DNS queries succeed
• Verify IPv4 nameserver behavior is unchanged

Summary by CodeRabbit

• Refactor
  • Enhanced internal DNS address handling with standardized formatting utilities, improving code consistency and maintainability across core networking components.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: 41b5c730-99d1-4fda-8908-792ff324f962

📥 Commits

Reviewing files that changed from the base of the PR and between 3e6baea and e9474f9.

📒 Files selected for processing (3)

• client/internal/dns/service_listener.go
• client/internal/routemanager/client/client.go
• client/internal/routemanager/dnsinterceptor/handler.go

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📝 Walkthrough

Walkthrough

This PR fixes IPv6 address formatting in DNS address construction by replacing fmt.Sprintf with net.JoinHostPort across three DNS-related files. The fix ensures IPv6 addresses are properly bracketed when combined with ports.

Changes

Cohort / File(s)	Summary
DNS Service Listener client/internal/dns/service_listener.go	Replaced fmt.Sprintf("%s:%d", ip, port) with net.JoinHostPort(ip.String(), strconv.Itoa(port)) in DNS listener address construction to support IPv6 addresses.
Route Manager DNS Handling client/internal/routemanager/client/client.go, client/internal/routemanager/dnsinterceptor/handler.go	Replaced DNS address string formatting with net.JoinHostPort() for dynamic route handlers and upstream DNS interceptor to properly format IPv6 addresses with bracketed notation.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~10 minutes

Suggested reviewers

• pappz

Poem

🐰 IPv6 brackets now in place,
No more colons in the space,
DNS queries flow with grace,
Addresses formatted just right! 🌐

🚥 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%.	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: fixing IPv6 address formatting in DNS address construction across the client code.
Description check	✅ Passed	The description provides a comprehensive summary of the problem, lists all changed files, explains the rationale, and includes a test plan. However, the PR description template sections are not formally filled out per the repository template.
Linked Issues check	✅ Passed	All four locations identified in issue #5601 have been addressed: dnsinterceptor/handler.go, service_listener.go (two locations), and client/client.go. The changes replace fmt.Sprintf with net.JoinHostPort, correctly fixing IPv6 address formatting.
Out of Scope Changes check	✅ Passed	All changes are directly scoped to fixing IPv6 address formatting in DNS address construction across the three files. No unrelated modifications are present.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

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• Post copyable unit tests in a comment

📝 Coding Plan

• Generate coding plan for human review comments

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[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

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

See analysis details on SonarQube Cloud

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[lixmal]

Can you please stick to the PR template?
Thanks for the contribution!