add troubleshooting section for Docker MTU issues

[vschnei]

Details

This pull request updates the README.txt documentation to include a new section on Docker MTU Troubleshooting (see issue #191 )
.
This change is to help users diagnose and resolve connectivity issues caused by MTU mismatches between Docker container networks and host/underlay networks (cloud).

The added section provides:

• diagnostic steps
• guidance on capturing PMTU-related issues
• permanent configuration examples

Testing
Since this is a documentation-only change, no unit or integration tests apply.

Documentation

The new MTU section has been added directly to README.txt.

Style

The new content follows the existing README formatting.

Closing issues

#191

Summary by Sourcery

Documentation:

• Document Docker MTU mismatch symptoms, diagnostic steps, and configuration options for temporary and permanent fixes in the README.

[sourcery-ai]

Reviewer's Guide

Adds a new Docker MTU troubleshooting section to the README, documenting how to diagnose and permanently fix MTU-related connectivity issues between containers and external networks.

Flow diagram for Docker MTU troubleshooting steps

flowchart TD
  A[Container cannot reach external hosts] --> B[Run ip link show on host to check MTU of physical and bridge interfaces]
  B --> C[From container, run curl tests and inspect etc_resolv_conf]
  C --> D[Capture ICMP traffic on host with tcpdump while reproducing issue]
  D --> E{Suspect PMTU mismatch?}
  E -- No --> F[Investigate other network causes]
  E -- Yes --> G[Temporarily lower container eth0 MTU using nsenter and ip link set]
  G --> H[Test connectivity again from container]
  H --> I{Connectivity restored?}
  I -- No --> F
  I -- Yes --> J[Choose permanent fix]
  J --> K[Set global Docker daemon MTU in etc_docker_daemon_json and restart Docker]
  J --> L[Or set compose network MTU via networks_default_driver_opts in docker_compose_yaml]
  K --> M[Recreate networks_or_compose_stack]
  L --> M
  M --> N[Verify MTU with ip link and confirm with test curl from container]

Loading

File-Level Changes

Change	Details	Files
Document Docker MTU troubleshooting steps, temporary verification, and permanent configuration fixes in the README.	Explain MTU mismatch symptoms between container networks and host/underlay networks and their impact on TCP connectivity. Provide quick diagnostic checks from both host and container, including MTU inspection, connectivity tests, and ICMP capture with tcpdump. Describe a temporary MTU change procedure inside a running container’s network namespace to confirm PMTU-related issues. Outline permanent MTU configuration options via Docker daemon global setting and docker-compose network driver options, including example snippets and post-change verification steps.	README.md

Possibly linked issues

• Add docker MTU section to README #191: The PR adds the Docker MTU troubleshooting README section with diagnostics and configuration, exactly matching the issue request.

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[sourcery-ai]

Hey there - I've reviewed your changes - here's some feedback:

• Consider formatting the subsections "Quick checks", "Temporary verification", and "Permanent fixes" as markdown subheadings (e.g., ### Quick checks) for better readability and consistent structure within the README.
• The examples hard-code an MTU of 1400; it may help users if you add a brief note on how to determine an appropriate MTU value for their environment rather than implying 1400 is always correct.

Prompt for AI Agents

Please address the comments from this code review:

## Overall Comments
- Consider formatting the subsections "Quick checks", "Temporary verification", and "Permanent fixes" as markdown subheadings (e.g., `### Quick checks`) for better readability and consistent structure within the README.
- The examples hard-code an MTU of 1400; it may help users if you add a brief note on how to determine an appropriate MTU value for their environment rather than implying 1400 is always correct.

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[uniqueg]

Fantastic - thanks a lot, @vschnei ! Maybe mention somewhere what MTU means, but otherwise gtg

[vschnei]

@uniqueg Thanks for the quick review.
Changes are done.