# Railway Support: Redis IPv6 Connectivity Issue

**Project**: mixpost (`0fdaabcf-5cc8-473f-b8c5-fd27ce7bef9d`)
**Environment**: production
**Severity**: Critical - App Down 5+ hours
**URL**: https://social.brokerkit.app

## Problem

Mixpost container cannot connect to KeyDB (Redis) over private network. MySQL works fine, only Redis fails.

## Symptoms

```bash
# Application timeouts
curl -I https://social.brokerkit.app/mixpost/login
# Result: timeout after 10 seconds

# Redis connection error in logs
PhpRedisConnector.php line 181: Connection timed out

# DNS resolution fails
nc -zv keydb.railway.internal 6379
# Result: getaddrinfo: nodename nor servname provided, or not known

# Horizon crashes every 60 seconds
INFO exited: horizon_00 (exit status 1; not expected)
```

## What Works ✅

- MySQL: `mysql.railway.internal:3306` connects instantly
- KeyDB via TCP proxy: `caboose.proxy.rlwy.net:20086` works
- Container startup: All processes running (nginx, PHP-FPM)

## What Fails ❌

- Redis private network: `keydb.railway.internal:6379` timeouts
- DNS resolution: Only for KeyDB, not MySQL
- All HTTP requests: Hang waiting for Redis

## Configuration (Verified Correct)

**KeyDB Service**:
```
RAILWAY_PRIVATE_DOMAIN=keydb.railway.internal
KEYDB_URL=redis://default:<pwd>@keydb.railway.internal:6379
```

**Mixpost Service**:
```
REDIS_HOST=keydb.railway.internal
REDIS_PORT=6379
DB_HOST=mysql.railway.internal  # This works
```

## Key Question

Why does `mysql.railway.internal` resolve/connect successfully but `keydb.railway.internal` fails DNS resolution? Both services are in same project/environment.

## IPv6 Consideration

Railway docs say private networking is IPv6-only. PHP's Redis extension (phpredis) may not support IPv6 properly, but MySQL client does. Is this a known issue?

## Diagnostics Run

- Verified all environment variables correct
- Confirmed KeyDB service is running and healthy
- Tested MySQL (works) vs Redis (fails) from same container
- KeyDB accessible via public TCP proxy (proves it's working)
- Issue isolated to private network IPv6 connectivity

## Timeline

- Oct 19 19:16 UTC: Last successful deployment
- Oct 20 ~20:00 UTC: Issue began (during AWS outage)
- Oct 21 00:54 UTC: Root cause identified

## Temporary Workaround

Using TCP proxy (not ideal):
```bash
REDIS_HOST=caboose.proxy.rlwy.net
REDIS_PORT=20086
```

This works but uses public internet and incurs egress costs.

## Questions

1. Why does KeyDB DNS fail when MySQL DNS works?
2. Is there an IPv6 routing issue specific to Redis/KeyDB?
3. Should we use `${{KeyDB.RAILWAY_PRIVATE_DOMAIN}}` instead?
4. Any recent private networking changes?

## Supporting Info

**Services**:
- Mixpost: `4ebcc492-214c-40fd-a122-6ddbea06d74e`
- KeyDB: `9892ea7a-413c-409c-81a0-e2d99d2dd09b`
- MySQL: `1ea08289-be8f-4a7b-9784-6c95d67858a3`

Need guidance on fixing private network DNS resolution for Redis. Production app completely down.