## Subject

Private networking (RAILWAY_PRIVATE_DOMAIN) not resolving - causing egress charges

## Issue Description

I'm experiencing issues with Railway's private networking between services in the same project and region. The RAILWAY_PRIVATE_DOMAIN variable is not resolving correctly, forcing me to use the TCP proxy which incurs egress fees.

### Environment

- **Project:** i3atlas

- **Services affected:**

- Web application service (Next.js app)

- MongoDB database service

- **Region:** Both services deployed in the same region

- **Issue:** Private networking hostname does not resolve

### What I'm Trying to Achieve

Connect my web application to MongoDB using Railway's private networking to avoid egress fees, as recommended in Railway's documentation.

### Current Setup

**MongoDB Service Variables:**

```

MONGO_INITDB_ROOT_USERNAME=mongo

MONGO_INITDB_ROOT_PASSWORD=[password]

RAILWAY_PRIVATE_DOMAIN=[should resolve to private hostname]

MONGO_URL=mongodb://${{MONGO_INITDB_ROOT_USERNAME}}:${{MONGO_INITDB_ROOT_PASSWORD}}@${{RAILWAY_PRIVATE_DOMAIN}}:27017/i3atlas?authSource=admin

```

**App Service Variable:**

```

MONGO_URL=${{MongoDB.MONGO_URL}}

```

When MONGO_URL resolves, it becomes:

```

mongodb://mongo:[password]@mongodb.railway.internal:27017/i3atlas?authSource=admin

```

### The Problem

**Error in application logs:**

```

MongoServerSelectionError: getaddrinfo ENOTFOUND mongodb.railway.internal

at async s (.next/server/app/api/cron/update-trajectory/route.js:1:861)

code: undefined,

[cause]: MongoNetworkError: getaddrinfo ENOTFOUND mongodb.railway.internal

errorLabelSet: Set(1) { 'ResetPool' },

beforeHandshake: false,

[cause]: Error: getaddrinfo ENOTFOUND mongodb.railway.internal

errno: -3007,

code: 'ENOTFOUND',

syscall: 'getaddrinfo',

hostname: 'mongodb.railway.internal'

```

The hostname mongodb.railway.internal (resolved from RAILWAY_PRIVATE_DOMAIN) cannot be found via DNS lookup from my application service.

### Troubleshooting Already Done

I have verified the following:

1. **Both services are in the same Railway project**

2. **Both services are deployed in the same region**

3. **MongoDB service is running and healthy**

4. **Connection string format is correct** (works with TCP proxy)

5. **Tried both mongodb.railway.internal and MongoDB.railway.internal** (service name with capital M)

6. **App service has been redeployed multiple times** to pick up new variables

7. **No duplicate environment variables**

### Current Workaround

I'm currently using the TCP proxy as a workaround:

**MongoDB Service:**

```

MONGO_URL=mongodb://${{MONGO_INITDB_ROOT_USERNAME}}:${{MONGO_INITDB_ROOT_PASSWORD}}@${{RAILWAY_TCP_PROXY_DOMAIN}}:${{RAILWAY_TCP_PROXY_PORT}}/i3atlas?authSource=admin

```

This resolves to:

```

mongodb://mongo:[password]@maglev.proxy.rlwy.net:28191/i3atlas?authSource=admin

```

**This works**, but Railway warns:

```

This variable references a public endpoint through this variable:

MONGO_URL -> MongoDB.MONGO_URL -> MongoDB.RAILWAY_TCP_PROXY_DOMAIN

Connecting to a public endpoint will incur egress fees.

```

### What I Need Help With

1. **Why is RAILWAY_PRIVATE_DOMAIN / mongodb.railway.internal not resolving** between my services?

2. **How can I enable proper private networking** to avoid egress fees?

3. **Is there a configuration or migration step required** for my project to use Railway's private networking?

4. **Is this related to Railway's V2 networking migration?** Does my project need to be migrated?

### Expected Behavior

According to Railway's documentation, services in the same project should be able to communicate via private networking using RAILWAY_PRIVATE_DOMAIN or <service-name>.railway.internal, which should not incur egress fees.

### Actual Behavior

Private networking hostnames do not resolve via DNS, resulting in ENOTFOUND errors. I'm forced to use the TCP proxy which incurs egress charges.

### Additional Context

- I previously had a working setup using the TCP proxy with MONGODB_URI

- I recently migrated to MONGO_URL to clean up variable naming

- The connection works perfectly with the TCP proxy endpoint

- Only the private networking hostname resolution is failing

### Request

Please investigate why private networking is not working for my project and provide guidance on how to enable it properly to avoid egress fees.

Thank you for your help!