fix(qr-server): pass host/port to FastMCP for Docker compatibility#372
Open
malbari wants to merge 1 commit intomodelcontextprotocol:mainfrom
Open
fix(qr-server): pass host/port to FastMCP for Docker compatibility#372malbari wants to merge 1 commit intomodelcontextprotocol:mainfrom
malbari wants to merge 1 commit intomodelcontextprotocol:mainfrom
Conversation
When FastMCP is created with the default host=127.0.0.1, MCP SDK 1.26+ automatically enables DNS rebinding protection that only allows requests from localhost. This breaks Docker where requests come from host.docker.internal, causing 421 Invalid Host header errors. By passing host=HOST (0.0.0.0 for Docker), DNS rebinding protection is not auto-enabled, allowing requests from any host.
@modelcontextprotocol/ext-apps
@modelcontextprotocol/server-basic-react
@modelcontextprotocol/server-basic-vanillajs
@modelcontextprotocol/server-budget-allocator
@modelcontextprotocol/server-cohort-heatmap
@modelcontextprotocol/server-customer-segmentation
@modelcontextprotocol/server-map
@modelcontextprotocol/server-pdf
@modelcontextprotocol/server-scenario-modeler
@modelcontextprotocol/server-shadertoy
@modelcontextprotocol/server-sheet-music
@modelcontextprotocol/server-system-monitor
@modelcontextprotocol/server-threejs
@modelcontextprotocol/server-transcript
@modelcontextprotocol/server-video-resource
@modelcontextprotocol/server-wiki-explorer
commit: |
This file contains hidden or bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters.
Learn more about bidirectional Unicode characters
Sign up for free
to join this conversation on GitHub.
Already have an account?
Sign in to comment
1 participant
Add this suggestion to a batch that can be applied as a single commit.This suggestion is invalid because no changes were made to the code.Suggestions cannot be applied while the pull request is closed.Suggestions cannot be applied while viewing a subset of changes.Only one suggestion per line can be applied in a batch.Add this suggestion to a batch that can be applied as a single commit.Applying suggestions on deleted lines is not supported.You must change the existing code in this line in order to create a valid suggestion.Outdated suggestions cannot be applied.This suggestion has been applied or marked resolved.Suggestions cannot be applied from pending reviews.Suggestions cannot be applied on multi-line comments.Suggestions cannot be applied while the pull request is queued to merge.Suggestion cannot be applied right now. Please check back later.
Problem
When using the qr-server in a Docker environment, requests fail with 421 Invalid Host header errors.
Root Cause
MCP SDK 1.26+ introduced automatic DNS rebinding protection. When FastMCP is created with the default host=127.0.0.1, it automatically enables this protection which only allows requests from localhost, 127.0.0.1, and ::1.
In Docker deployments, the backend container sends requests with Host: host.docker.internal, which gets rejected.
Solution
Pass host=HOST and port=PORT to the FastMCP constructor. Since HOST defaults to 0.0.0.0 (for Docker compatibility), this prevents the auto-enabling of DNS rebinding protection, allowing requests from any host.
Testing