# Grpc.Core.RpcException: Status(StatusCode="Unavailable", Detail="failed to connect to all addresses; last error: UNKNOWN: No match found for server name")

- **ID:** `dotnet/grpc-client-ssl-error`
- **Domain:** dotnet
- **Category:** network_error
- **Verification:** ai_generated
- **Fix Rate:** 80%

## Root Cause

gRPC client SSL/TLS handshake fails because the server certificate's Subject Alternative Name (SAN) does not match the hostname used in the client channel address.

## Version Compatibility

| Version | Status | Introduced | Deprecated |
|---------|--------|------------|------------|
| Grpc.Net.Client 2.49.x | active | — | — |
| Grpc.Net.Client 2.50.x | active | — | — |
| Grpc.Net.Client 2.51.x | active | — | — |
| Grpc.Net.Client 2.52.x | active | — | — |

## Workarounds

1. **Ensure the server certificate's SAN includes the exact hostname used in the client channel address (e.g., myserver.local).** (90% success)
   ```
   Ensure the server certificate's SAN includes the exact hostname used in the client channel address (e.g., myserver.local).
   ```
2. **Use the correct hostname in the gRPC channel, matching the certificate SAN, especially when behind a load balancer.** (85% success)
   ```
   Use the correct hostname in the gRPC channel, matching the certificate SAN, especially when behind a load balancer.
   ```
3. **If using self-signed certificates for development, add the certificate to trusted root store and use matching hostname.** (70% success)
   ```
   If using self-signed certificates for development, add the certificate to trusted root store and use matching hostname.
   ```

## Dead Ends

- **** — Disabling SSL validation entirely (using HttpClientHandler.ServerCertificateCustomValidationCallback) creates security vulnerability and doesn't fix SAN mismatch. (95% fail)
- **** — Using IP address instead of hostname in channel address still fails if certificate doesn't have IP SAN. (90% fail)
- **** — Setting GRPC_DNS_RESOLVER environment variable doesn't affect SSL certificate validation. (85% fail)