dotnet-devcert-trust

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• SKILL.md

SKILL.md (first lines)

# .NET Dev Certificate Trust on Linux

## When to Use This Skill

Use this skill when:
- Redis TLS connections fail with `UntrustedRoot` or `RemoteCertificateNameMismatch` in Aspire
- `dotnet dev-certs https --check --trust` returns exit code 7
- HTTPS localhost connections fail with certificate validation errors
- After running `dotnet dev-certs https --clean` and needing to restore trust
- Setting up a new Linux dev machine for .NET HTTPS development
- Aspire dashboard or inter-service gRPC calls fail with TLS errors
- Upgrading from Aspire < 13.1.0 (which didn't use TLS on Redis by default)

## The Problem

On Windows and macOS, `dotnet dev-certs https --trust` handles everything automatically — it generates the certificate, installs it in the user store, and adds it to the system trust store. On Linux, **it does almost nothing useful**. The command generates the cert and places it in the user store, but:

1. It does **not** export the certificate to the system CA directory
2. It does **not** run `update-ca-certificates` to rebuild the CA bundle
3. It does **not** add the cert to browser trust stores (NSS/NSSDB)
4. The `--trust` flag silently succeeds but the cert remains untrusted

This means .NET applications, OpenSSL, curl, and browsers all reject the dev certificate — even though `dotnet dev-certs https --check` reports it exists.

### Why This Surfaces with Aspire 13.1.0+

Prior to Aspire 13.1.0, Redis connections used plaintext. Starting with 13.1.0, Aspire enables TLS on Redis by default. If your dev cert isn't trusted at the system level, Redis connections fail immediately with:

```
System.Security.Authentication.AuthenticationException:
  The remote certificate is invalid because of errors in the certificate chain: UntrustedRoot
```

## How Linux Certificate Trust Works

Understanding the architecture prevents cargo-cult debugging:

```
┌─────────────────────────────────────────────────────┐
│ Application (.NET, curl, OpenSSL)                   │
│   reads: /etc/ssl/certs/ca-certificates.crt         │
│          (consolidated CA bundle)                    │
└──────────────────────┬──────────────────────────────┘
                       │ built by
┌──────────────────────▼──────────────────────────────┐
│ update-ca-certificates                              │
│   reads from:                                        │
│     /usr/share/ca-certificates/      (distro CAs)   │
│     /usr/local/share/ca-certificates/ (local CAs)   │
│   writes to:                                         │
│     /etc/ssl/certs/ca-certificates.crt (bundle)     │
│     /etc/ssl/certs/*.pem (individual symlinks)      │
└─────────────────────────────────────────────────────┘
```

**Key insight:** Placing a `.crt` file in `/usr/local/share/ca-certificates/` is necessary but **not sufficient**. The consolidated bundle at `/etc/ssl/certs/ca-certificates.crt` must be rebuilt by running `update-ca-certificates`. Applications read the bundle, not the individual files.

## 5-Point Diagnostic Procedure

Run these checks in order. Stop at the first FAIL and apply its fix before continuing.

### Check 1: Dev Cert Existence

```bash
dotnet dev-certs https --check
echo "Exit code: $?"
```

| Exit Code | Meaning | Action |
|-----------|---------|--------|
| 0 | Cert exists in user store | PASS — continue |
| Non-zero | No valid dev cert | Run `dotnet dev-certs https` |

### Check 2: System Trust Store — Single Cert, Correct Permissions

```bash
ls -la /usr/local/share/ca-certificates/ | grep -iE 'dotnet|aspnet'
```

| Result | Meaning |
|--------|---------|
| Only `dotnet-dev-cert.crt` with `-rw-r--r--` (644) | PASS |
| Multiple cert files, wrong permissions, or stale `aspnet*` files | FAIL |

**Common stale files from previous sessions:**

| File | Problem |
|------|---------|
| `aspnetcore-dev.crt` | Often created with `0600` permissions (unreadable by `update-ca-certificates`) |
| `aspnet/https.crt` | Old convention, may have a different fingerprint than current dev cert |
| `dotnet-dev-cert.crt` with `0600` | Correct name but wrong permissions |

**Fix:**
```bash
# Remove ALL stale cert files
sudo rm -f /usr/local/share/ca-certificates/aspnetcore-dev.crt
sudo rm -rf /usr/local/share/ca-certificates/aspnet/

# Ensure correct permissions on the dev cert (if it exists)
sudo chmod 644 /usr/local/share/ca-certificates/dotnet-dev-cert.crt
```

### Check 3: CA Bundle Inclusion

This is the most commonly failed check. The cert file exists but was never added to the bundle.

```bash
openssl verify -CAfile /etc/ssl/certs/ca-certificates.crt \
  /usr/local/share/ca-certificates/dotnet-dev-cert.crt
```

| Result | Meaning |
|--------|---------|
| `dotnet-dev-cert.crt: OK` | PASS — cert is in the consolidated bundle |
| `error 20 at 0 depth lookup: unable to get local issuer certificate` | FAIL — bundle was never rebuilt |
| `error 2 at 0 depth lookup: unable to get issuer certificate` | FAIL — same issue, different OpenSSL version |

**Fix:**
```bash

From ComeOnOliver/skillshub, MIT.

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