A proxy or load balancer is accepting TCP connections but not forwarding them to Snowflake. An SSL inspection appliance is intercepting the connection but failing to complete the TLS handshake. The Snowflake endpoint is experiencing service degradation — connections accepted but not processed. A connection blackhole: TCP SYN accepted but no data returned (firewall ACL asymmetry). The HTTP connection timeout is too short for the network latency to the Snowflake endpoint

How do I fix JDBC 12?

Test with curl: curl -v https:// .snowflakecomputing.com/ — check if the HTTP response is received.. Bypass the proxy if possible and test direct connectivity to isolate the issue.. Check the Snowflake status page for degraded service in your region.. Increase the JDBC connection timeout in the connection URL: ?loginTimeout=60.. Capture a packet trace (tcpdump/Wireshark) to see if the TCP connection is established but no data flows.

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Power BI Refresh Error:
JDBC 12

What does this error mean?

The JDBC driver established a TCP connection to Snowflake but received no HTTP response. The server accepted the TCP connection but did not return an HTTP response within the timeout, suggesting a proxy interception or a network appliance absorbing the connection.

Common causes

1A proxy or load balancer is accepting TCP connections but not forwarding them to Snowflake
2An SSL inspection appliance is intercepting the connection but failing to complete the TLS handshake
3The Snowflake endpoint is experiencing service degradation — connections accepted but not processed
4A connection blackhole: TCP SYN accepted but no data returned (firewall ACL asymmetry)
5The HTTP connection timeout is too short for the network latency to the Snowflake endpoint

How to fix it

1Test with curl: curl -v https://<account>.snowflakecomputing.com/ — check if the HTTP response is received.
2Bypass the proxy if possible and test direct connectivity to isolate the issue.
3Check the Snowflake status page for degraded service in your region.
4Increase the JDBC connection timeout in the connection URL: ?loginTimeout=60.
5Capture a packet trace (tcpdump/Wireshark) to see if the TCP connection is established but no data flows.

Frequently asked questions

How is JDBC 12 different from JDBC 1 (Connection Refused)?

JDBC 1 means the TCP SYN was rejected — nothing is listening. JDBC 12 means the TCP handshake succeeded but the HTTP layer returned nothing — something is listening but not responding correctly.

Can this be caused by IP-level firewall rules?

Yes — asymmetric ACLs can allow the inbound SYN but drop subsequent packets, causing a TCP session that appears established on the client but never delivers data. A packet trace confirms this pattern.

Official documentation: https://docs.snowflake.com/en/user-guide/client-connectivity-troubleshooting/error-messages