Connecting to Azure PostgreSQL with libpq 12 in a Kerberos environment

If you are using Azure PostgreSQL and have upgraded your client side libpq to version 12 (which can happen automatically for example if you use the PostgreSQL apt repositories), you may see connection attempts fail with symptoms like:

$ psql -dpostgres -UXXXXX_dba@ZZZ-db01
psql: server closed the connection unexpectedly
This probably means the server terminated abnormally
before or while processing the request.

With no log information whatsoever available. This can happen if your client is in a Kerberos environment and has valid Kerberos credentials (which can be verified with the klist command). In this case, PostgreSQL 12 will attempt to negotiate GSSAPI encryption with the server, and it appears the connection handler in Azure PostgreSQL is unable to handle this and just kills the connection.

When running the same thing against a local PostgreSQL server prior to version 12, a message like the following will show up in the log:

2020-02-20 10:48:08 CET [35666]: [2-1] client= FATAL:  unsupported frontend protocol 1234.5680: server supports 2.0 to 3.0

This is a clear indicator of what's going on, but unfortunately the information isn't always available when connecting to a managed cloud service, such as Azure PostgreSQL. The hard error from Azure also prevents libpq from retrying without GSSAPI encryption, which is what would happen when connecting to a regular PostgreSQL backend or for example through pgbouncer.

The fix/workaround? Disable GSSAPI encryption in the client:

$ export PGGSSENCMODE=disable
$ psql -dpostgres -UXXXXX_dba@ZZZ-db01
Password for user XXXXX_dba@ZZZ-db01:
psql (11.6 (Ubuntu 11.6-1.pgdg16.04+1), server 9.5.20)
SSL connection (protocol: TLSv1.2, cipher: ECDHE-RSA-AES256-GCM-SHA384, bits: 256, compression: off)
Type "help" for help.


If you have this type of issue, it's probably worth putting this environment variable in your startup scripts. It can also be set using the gssencmode parameter as part of the connection string, in environments where this is more convenient.


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