Picking a default port based on the server hash

Author: geelen

src/client.ts

@@ -151,7 +151,7 @@ async function runClient(
 }
 
 // Parse command-line arguments and run the client
-parseCommandLineArgs(process.argv.slice(2), 3333, 'Usage: npx tsx client.ts <https://server-url> [callback-port]')
+parseCommandLineArgs(process.argv.slice(2), 'Usage: npx tsx client.ts <https://server-url> [callback-port]')
   .then(({ serverUrl, callbackPort, headers, transportStrategy }) => {
     return runClient(serverUrl, callbackPort, headers, transportStrategy)
   })

src/lib/utils.ts

@@ -4,19 +4,19 @@ import { SSEClientTransport } from '@modelcontextprotocol/sdk/client/sse.js'
 import { StreamableHTTPClientTransport } from '@modelcontextprotocol/sdk/client/streamableHttp.js'
 import { Transport } from '@modelcontextprotocol/sdk/shared/transport.js'
 import { OAuthClientInformationFull, OAuthClientInformationFullSchema } from '@modelcontextprotocol/sdk/shared/auth.js'
+import { OAuthCallbackServerOptions } from './types'
+import { getConfigFilePath, readJsonFile } from './mcp-auth-config'
+import express from 'express'
+import net from 'net'
+import crypto from 'crypto'
+import fs from 'fs/promises'
 
 // Connection constants
 export const REASON_AUTH_NEEDED = 'authentication-needed'
 export const REASON_TRANSPORT_FALLBACK = 'falling-back-to-alternate-transport'
 
 // Transport strategy types
 export type TransportStrategy = 'sse-only' | 'http-only' | 'sse-first' | 'http-first'
-import { OAuthCallbackServerOptions } from './types'
-import { getConfigFilePath, readJsonFile } from './mcp-auth-config'
-import express from 'express'
-import net from 'net'
-import crypto from 'crypto'
-import fs from 'fs/promises'
 
 // Package version from package.json
 export const MCP_REMOTE_VERSION = require('../../package.json').version
@@ -355,8 +355,7 @@ export function setupOAuthCallbackServer(options: OAuthCallbackServerOptions) {
   return { server, authCode, waitForAuthCode }
 }
 
-async function findExistingClientPort(serverUrl: string): Promise<number | undefined> {
-  const serverUrlHash = getServerUrlHash(serverUrl)
+async function findExistingClientPort(serverUrlHash: string): Promise<number | undefined> {
   const clientInfo = await readJsonFile<OAuthClientInformationFull>(serverUrlHash, 'client_info.json', OAuthClientInformationFullSchema)
   if (!clientInfo) {
     return undefined
@@ -370,6 +369,13 @@ async function findExistingClientPort(serverUrl: string): Promise<number | undef
   return parseInt(localhostRedirectUri.port)
 }
 
+function calculateDefaultPort(serverUrlHash: string): number {
+  // Convert the first 4 bytes of the serverUrlHash into a port offset
+  const offset = parseInt(serverUrlHash.substring(0, 4), 16)
+  // Pick a consistent but random-seeming port from 3335 to 49151
+  return 3335 + (offset % 45816)
+}
+
 /**
  * Finds an available port on the local machine
  * @param preferredPort Optional preferred port to try first
@@ -403,11 +409,10 @@ export async function findAvailablePort(preferredPort?: number): Promise<number>
 /**
  * Parses command line arguments for MCP clients and proxies
  * @param args Command line arguments
- * @param defaultPort Default port for the callback server if specified port is unavailable
  * @param usage Usage message to show on error
  * @returns A promise that resolves to an object with parsed serverUrl, callbackPort and headers
  */
-export async function parseCommandLineArgs(args: string[], defaultPort: number, usage: string) {
+export async function parseCommandLineArgs(args: string[], usage: string) {
   // Process headers
   const headers: Record<string, string> = {}
   let i = 0
@@ -457,17 +462,19 @@ export async function parseCommandLineArgs(args: string[], defaultPort: number,
     log(usage)
     process.exit(1)
   }
+  const serverUrlHash = getServerUrlHash(serverUrl)
+  const defaultPort = calculateDefaultPort(serverUrlHash)
 
   // Use the specified port, or the existing client port or fallback to find an available one
-  const [existingClientPort, availablePort] = await Promise.all([findExistingClientPort(serverUrl), findAvailablePort(defaultPort)])
+  const [existingClientPort, availablePort] = await Promise.all([findExistingClientPort(serverUrlHash), findAvailablePort(defaultPort)])
   let callbackPort: number
 
   if (specifiedPort) {
     if (existingClientPort && specifiedPort !== existingClientPort) {
       log(
         `Warning! Specified callback port of ${specifiedPort}, which conflicts with existing client registration port ${existingClientPort}. Deleting existing client data to force reregistration.`,
       )
-      await fs.rm(getConfigFilePath(getServerUrlHash(serverUrl), 'client_info.json'))
+      await fs.rm(getConfigFilePath(serverUrlHash, 'client_info.json'))
     }
     log(`Using specified callback port: ${specifiedPort}`)
     callbackPort = specifiedPort

src/proxy.ts

@@ -135,7 +135,7 @@ to the CA certificate file. If using claude_desktop_config.json, this might look
 }
 
 // Parse command-line arguments and run the proxy
-parseCommandLineArgs(process.argv.slice(2), 3334, 'Usage: npx tsx proxy.ts <https://server-url> [callback-port]')
+parseCommandLineArgs(process.argv.slice(2), 'Usage: npx tsx proxy.ts <https://server-url> [callback-port]')
   .then(({ serverUrl, callbackPort, headers, transportStrategy }) => {
     return runProxy(serverUrl, callbackPort, headers, transportStrategy)
   })