Posts Tagged ‘ORA-12545’

Resolving Oracle networking problems – ORA-12545.

The original version of the below article was created by Ed Stevens and could be find here.

Continuing our discussion of resolving Oracle networking problems, I want to focus on “ORA-12545: Connect failed because target host or object does not exist”.

To recap what we’ve covered so far, when an oracle client requests a connection to a database, it has to provide a “connect identifier”, which sqlnet then translates to a “connect descriptor”. The connect descriptor specifies the transport protocol (usually tcp), the IP address of the database server, the port being used by the listener, and the service name the database has registered with the listener. This name resolution is usually done by looking up the connect identifier in the client’s tnsnames.ora file, though there are other methods available as well. Failure to find an entry from which to derive the connect descriptor will result in an “ORA-12154: TNS:could not resolve the connect identifier specified”, which I delved into here.

If sqlnet is able to determine the connect descriptor, the next step of the process is to pass that information to the OS’s network layer to be routed to the specified IP address. It is problems at this point that we will focus on in this post.

For this demonstration, I am using an Oracle 32bit client on Windows 7 Pro, connecting to an Oracle database on Windows 2008R2. Name resolution is through tnsames.ora, which looks like this:

# tnsnames.ora Network Configuration File: c:\oracle\11.2.0\CLIENT\network\admin\tnsnames.ora
# Generated by Oracle configuration tools.
myorcl =
(ADDRESS = (PROTOCOL = TCP)(HOST = orclsvr)(PORT = 1521))

The key information we are focusing on is the “HOST = orclsvr” on line 7. First, let’s make a good connection to prove that everything is working correctly, then we will break it:

Case #1
We know that in order for a message to be routed across a network, we need an IP address. This is like placing a telephone call. If I want to call Moe, I can’t just pick up my phone and dial “Moe”. The telephone company switchboards don’t know anything about Moe. There has to be some mechanism to translate “Moe” to 1-555-123-4567. Likewise the network routers don’t know anything about “orclsvr”. For your telephone you would have some sort of directory to tell you what Moe’s number is. In a corporate environment, you probably have a DNS server to tell the network stack that “orclsvr” is The other mechanism, and the one that trumps a DNS lookup, is a file on the client, named simply “hosts”. On Unix, this will be at /etc/hosts. On my Windows 7 machine it is at c:\Windows\system32\drivers\etc\hosts. Given Microsoft’s propensity for reshuffling the deck with each new release of Windows, I can’t promise that is where the file should be located on your machine.

My hosts file looks like this: localhost orclsvr

For those not familiar with this file, the format is

ipaddress alias1 alias2 .... aliasN

Note: it is a good habit to enter an empty line at the end of this file.

All hosts files should have the same first line, equating IP address to the alias “localhost”.  All other entries typically have two aliases, one with the unqualified server name, the other with the fully qualified servername.domain. That is by convention and for everyone’s convenience, but the fact is these are just aliases for the IP address, and like any alias can actually be anything you want. I will demonstrate that after taking care of the business at hand.

At this point, we know that we told the network to route our request to ‘orclsvr’, and by using the local hosts file, it was able to translate ‘orclsvr’ to ‘’. Let’s fix it so that it can’t make that translation, and see what results. We will do that by removing the entry for ‘orclsvr from the hosts file: localhost fubar

Then test:

And we have our error. Please notice that we did NOT touch our tnsnames.ora file, which we’ve already proven to be good. This error is simply Oracle reporting what the OS network returned. Of course, since it resulted from a mismatch between the HOST parameter in tnsnames and the entries in the hosts file, the proper fix is could be in either file. It is often suggested to avoid this problem by hard-coding the IP address in the tnsnames (HOST=, but I consider that to be a hack taken by those who do not understand how net name resolution works. It is certainly poor practice to hard-code an IP address any place an alias can be used. Just think of the problems caused by hard-coded IP addresses when the network administrator restructures the net.
This particular error is exactly analogous to “ORA-12154: TNS:could not resolve the connect identifier specified”. With that error, sqlnet couldn’t find a tnsnames entry for “myorcl” to translate to a connect descriptor; with this error, the OS network layer couldn’t find a hosts entry for ‘orclsvr’ to translate to an IP address.

Case #2
For our next trial, let’s restore the proper alias to the hosts file, but equate it to a bogus IP address.

C:\>type C:\WINDOWS\system32\drivers\etc\hosts localhost orclsvr

Then make some tests:

A slightly different error, sqlnet simply reporting what was returned by the OS network layer – compare to the result of the ‘ping’ command.

Case #3
One more test. We will restore the hosts to its pristine condition, then shutdown the db server and test.

C:\>type C:\WINDOWS\system32\drivers\etc\hosts localhost orclsvr

Then tests:

Of course, having a correct IP address in hosts, but the target server down, is really no different that an incorrect IP address to start with.

Case #4
I would like to do one more test to serve as the lead-in to the next few posts, dealing with problems on the db server rather than the client. For this test, the db server is up, but the listener has been shut down.

and repeating the tests:

Along with the previous posts in this series, this covers the TNS and network configurations that can be controlled completely on the client side. We have touched on a couple of server-side issues. Future posts will dig into that side of the equation.


Resolving Oracle networking problems – general introduction.

The original version of the below article was created by Ed Stevens and could be find here.

Some of the most frequently problems I have to deal in my work are resolving Oracle networking problems. Tracing the problem isn’t rocket science, but I often see people not paying attention to (or not trusting) specific error messages and riding off in all directions at once. A computer will always do exactly what it is told. The problem comes in that we often don’t really know everything we’re telling it. So let me try to explain a little about how Oracle handles a request to “connect me to my database” and actually locates a database running on a machine on the other side of the planet (or even on the very machine from which the request originated).

Before digging in, let’s talk about a very simple concept that an amazing number of people struggle with. For purposes of the current discussion there are two “entities”, or processes, involved. First there is the server process. Depending on one’s semantic precision and the context in which the term is used, the “server” could refer to the database, the database instance, the database server process, or the physical computer on which any of these execute. In terms of network routing it all comes back to a specific box with a specific IP address. The second process is the client process. That is the process that is requesting the connection to (and services from) the database. Again, depending on one’s semantic precision and context, the term “client” could refer to a process or a physical computer, but for our purposes it means the process. And this process could be running on any computer, including (understand this) the same computer that is acting as the server. In this case, it is still a client and the fact that it is running on the server computer is totally coincidental and irrelevant.

So let’s say you are using sqlplus. You issue this statement to start it and connect to your database:

C:\> sqlplus scott/tiger@myorcl

Of course, the first thing that will happen really has nothing to do with Oracle. First, the OS must locate an executable called ‘sqlplus’, load it, and pass it the rest of the command line (scott/tiger@myorcl) do with as it sees fit. And what sqlplus sees fit is to ask TNS to make a connection to “myorcl”, using the userid “scott” and the password “tiger” as its authentication credentials. So TNS has to figure out what is meant by “myorcl”. By default it will do this by looking in a file called tnsnames.ora. Since we are still at the client making the request, this file must be found on the client machine. By default it will be found in $ORACLE_HOME/network/admin (for example: c:\oracle\11.2.0\CLIENT\network\admin).

Let’s make it easy and suppose our tnsnames file has this entry:

myorcl =
			(ADDRESS = (PROTOCOL = TCP)(HOST = orclsvr)(PORT = 1521))
			(SERVICE_NAME = orcl)

TNS will look in your tnsnames.ora for an entry called ‘myorcl’. Finding it, a request is sent through the normal OS network stack to (PORT = 1521) on (HOST = orclsvr) using (PROTOCOL = TCP), asking for a connection to (SERVICE_NAME = orcl). Notice where it got this information from the entry in the tnsnames file (this entry is known as the “connect identifier”). Also notice that what is going on here is the resolution of an alias “myorcl” to an actual destination “orcl”. In this respect the tnsnames.ora file serves the same purpose for sqlnet as the OS’s “hosts” file serves for the standard network stack.

Where is (HOST = orclsvr) on the network? When the request gets passed from TNS to the standard network stack, the name ‘orclsvr’ will get resolved to an IP address, either via a local “hosts” file, via DNS, or possibly other less used mechanisms. You can also hard-code the IP address (HOST = in the tnsnames.ora.

Once the Ip address is determined, the standard networking process delivers the message to the designated port (PORT = 1521) on the designated host/IP address. Hopefully, there is an Oracle database listener on “orclsvr” configured to listen on the specified port, and that listener knows about SERVICE_NAME = orcl. If so, the listener will spawn a server process to act as the intermediary between your client and the database instance. Communication to that server process will be on a different port, selected by the listener. At that point the listener is out of the process and continues to await other connection requests coming in on its configured port.

Before running sqlplus (or the other application which connects to the database), you could test communication between the client and the listener. We will use tnsping to complete this step. It’s a common misconception that tnsping tests connectivity to the instance or database. In actual fact, it only tests connectivity to the listener. Here, we will use it to prove that:
a) the tnsnames.ora has the correct hostname and port
b) that there is a listener listening on the specified host and port.

You can issue tnsping like this:

C:\> tnsping myorcl

If it is successful you will see something like this:

If not, here are some common errors, and some suggestions for fixing them:
First, there may not be an entry for ‘myorcl’ in your tnsnames. In that case you get “ORA-12154: TNS:could not resolve the connect identifier specified“. I’ll expand on the various reasons ‘myorcl’ may not have been found at a later date, but make no mistake, if you receive a ORA-12154, it is an absolute certainty your request never got past this point. You are wasting your time trying to solve this by looking at your listener. If you can’t place a telephone call because you don’t know the number (can’t find your telephone directory – aka “tnsnames.ora” – or can’t find the party you are looking for listed in it – no entry for orcl) you don’t look for problems at the telephone switchboard.

Maybe the entry for ‘myorcl’ was found, but ‘orclsvr’ couldn’t be resolved to an IP address (say there was no entry for ‘orclsvr’ in the local hosts file). This will result in “ORA-12545: Connect failed because target host or object does not exist“.

Maybe there was an entry for “orclsvr” in the local hosts file, but it specified a bad IP address. This will result in “ORA-12545: Connect failed because target host or object does not exist“.

Maybe the IP was good, but there is no listener running: “ORA-12541: TNS:no listener“.

Maybe the IP was good, there is a listener at orclsvr, but it is listening on a different port => “ORA-12560: TNS:protocol adapter error“.

Maybe the IP was good, there is a listener at orclsvr, it is listening on the specified port, but doesn’t know about SERVICE_NAME = orcl => “ORA-12514: TNS:listener does not currently know of service requested in connect descriptor“.

Maybe the IP was good, there is a listener at orclsvr, it is listening on the specified port, knows about SERVICE_NAME = orcl, but you have other application running on the same port as the listener. In that case you don’t get any errors – tnsping just hangs for a long time…

Ok, that is how we get *from* the client connection request *to* the listener. What about the listener’s part of all this?

The listener is very simple. It runs on the server (not the client) and it’s job is to listen for connection requests and make the connection (server process) between the client and the database instance. Once that connection is made, the listener is out of the picture. If you were to kill the listener, all existing connections would continue.

The listener is configured with the listener.ora file, but if that file doesn’t exist, the listener is quite capable of starting up with all default values. One common mistake with the listener configuration is to specify “HOST=localhost” or “HOST=127.0.01”. This is a NONROUTABLE ip address. LOCALHOST and IP address always mean “this machine on which I am sitting”. So, *all* computers are known as “localhost” or “”. If you specify this address in your listener configuration, the listener will only be capable of receiving requests from the machine on which it is running. If you specified that address in your tnsnames file, the request would be routed to the machine on which the requesting client resides. Probably not what you want.

From here I have a few ideas for future posts, each focusing on potential complications at each step of the process.