JDBC connection over IPv6

Question

Hi
I have an MS SQL Server 2005 Database Server and a Client computer on two different subnets.
There is also a DNS Server, which contains both an IPv4 (A) and IPv6 (AAAA) DNS record for the Database Server.
I've disabled IPv4 routing between the two subnets so that the Database Server is only accessible by IPv6 from the Client computer.
I am then unable to connect to the Database Server from the Client using JDBC.
I have tried increasing the loginTimeout in the JDBC connection string, but this doesn't appear to make any difference.
I get a java.net.ConnectException in about 20 seconds.
If I remove the IPv4 (A) record of the Database Server from the DNS Server, the JDBC connection then succeeds.
Does JDBC iterate over all the possible addresses of the Database Server?
Is the loginTimeout the correct parameter to try to increase, to allow it to iterate over the possible addresses?
There is a similar problem with MS SQL Client - see http://msdn.microsoft.com/en-us/library/ms378428(SQL.90).aspx
But increasing the connection timeout allows a successful connection.
Thanks
Alain

Answer 1

jschell wrote:
ahkal01 wrote:
Alain: A machine may have multiple IPv6 addresses, as well as an IPv4 address.
You may not be able to get to the machine via all the addresses, depending on the network route.
If a driver gives up on the first DNS lookup (which probably will be the IPv4 address), it'll never try the IPv6 addresses. However that is NOT what you are testing.
As described in your first post there is a route. Routing works if the final location is found. After that a refused connection is a refused connection.
Alain: In my test, IPv4 routing is disabled between the two subnets. So there is no IPv4 routing between the two machines, only IPv6.
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But even so, to me it is still two different IP addresses. Just as if you try to create your own replication by having two database servers on different IPv4 addresses. If the client is supposed to be using one address then the DNS should be set up that way.
Or use the address. Because the point is not the IP address, the point is the DNS look up.
Alain: With IPv6, it is common for computers to be assigned more than one address for routing under different hierarchies.
If you're saying that the DNS lookup should be set up to return only one IP address to client computers trying to connect to the DB Server, DNS will need to be set up differently depending on where the client computer is.
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Again looking at it from the point of someone developing a driver the only option I would allow is that one might be able to specify a configuration value that says to try a IPv6 address first. That would be in the driver, not JDBC. So you can look to your driver for that.
There is a similar problem with MS SQL Client - see http://msdn.microsoft.com/en-us/library/ms378428(SQL.90).aspx
But increasing the connection timeout allows a successful connection.
I do not see anywhere in that link that it claims that the setup you are using would work Alain: apologies. It was the wrong url link.
I meant, http://blogs.msdn.com/sql_protocols/archive/2005/10/12/480192.aspx.The scenario there is SQL Server is configured to listen only on IPv6 addresses and disable all IPv4 addresses so that only IPv6 connection can be accepted. The workaround is to specify the IPv6 address of the target machine explicitly to force SNI to use IPv6 connection directly or to use longer timeout value. I don't see that in that blog.
It says that if you have addresses that some different behavior might result. It specifically refers to using an IPv4 address (not DNS) and failing on that and then attempting to use a IPv6 address (again not a DNS) and even in that case it might fail.
Alain: The blog is talking about a hostname resolving into multiple addresses. I quote from the blog, with bold for emphasis,
"In most cases, the connection string does not need to be modified if the <servername> is specified using server hostname or FQDN (Full Qualified Domain Name). If the server machine has dual-stack, *its hostname or FQDN will be resolved into multiple IP addresses*, including at lease one IPv4 address and multiple IPv6 addresses. *And SNI will attempt to establish connections using these IP addresses in order and use the first connection that succeeds*. IPv4 addresses are attempted first if both IPv4 and IPv6 addresses are present. This logic is transparent to the users of ODBC, OLEDB or ADO.NET.
And it doesn't say anything about that some clients already do this but merely that they can.
Alain: SNI does, from the quote above. A test also shows that it does try all the addresses that DNS returns, with the caveat that the connection timeout has to be increased.
Unfortunately it looks like the MS JDBC driver doesn't do likewise.
Myself I don't really consider that a valid option for most use cases for drivers. If I have a data center then I am not normally going to be using both types of addresses. And if I am both will work. And if there are variation in subnets then I would expect that if DNS is in use then it would be set up to correctly represent that.Thanks for your input on this thread. Much appreciated.