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Repair Windows errors before they cause bigger problemsFix Now →Fix the driver behind crashes, sound loss and screen glitchesFind Drivers →The title alone does not reveal why an Oracle migration tool cannot connect. Common causes include an incorrect host, port, connect identifier or service name; a stopped or unreachable listener; blocked network traffic; invalid or insufficiently privileged credentials; and tool-specific setup requirements. Start with the complete error and the stage where it occurs, then check the connection details and network path before applying instructions for a specific migration product.
What information identifies the cause?
Before changing settings, record the details that distinguish a connection failure from a migration setup problem:
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- The migration tool and its version.
- The source and target database products and versions, and where each runs.
- Which endpoint the tool is trying to reach, and whether it connects directly or through a cloud service.
- The connection method, including a connect descriptor or alias, service name, wallet, or other authentication method in use.
- The complete error text and code, plus the step where it appears: test connection, source discovery, repository setup, or migration.
These details matter because SQL Developer Migrations, the Database Migration Assistant for Unicode (DMU), and OCI Database Migration have different connection workflows. Do not apply one product’s repository or cloud settings to another.
Why won’t my Oracle migration tool connect?
Check the connection inputs and account first. Oracle’s Database Tools troubleshooting guidance recommends verifying the connection string, host, TCP port, username, password, and wallet if one is used. Also confirm that the account is valid, enabled, unlocked, has an unexpired password, and has the grants required for the operation.
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Where possible, test the same connection string and authentication details from another suitable Oracle client, such as SQLcl, or from an approved shell or bastion host. If that client connects while the migration tool fails, the comparison points toward the tool’s configuration, driver, or workflow—but does not by itself prove which is at fault. If both fail, investigate shared inputs, account state, listener, and network access.
For DMU, Oracle lists the target connection details as an administrative username and password, host name, TNS listener port, and database service name. Its getting-started guide, updated June 2024, also directs users to check supported database and operating-system configurations. Consult documentation for the DMU release actually in use rather than treating that guide as a current compatibility matrix.
What does the Oracle error code point to?
Use the exact code to choose the next check; similar-looking connection failures can have different causes.
ORA-12541: no listener
Oracle describes ORA-12541 as: “Cannot connect. No listener at host_port.” Check that the connection descriptor points to the intended host and port, then confirm that a listener is running and listening there. Oracle’s ORA-12541 guidance specifically calls for checking the connection string and listener status. If the listener runs on a different host or port than the client is using, correct the descriptor or the listener configuration as appropriate.
ORA-12154: connect identifier not resolved
This error points to failure to resolve the supplied connect identifier. Check that the alias is spelled correctly and that the client is using the intended naming configuration, including the correct tnsnames.ora location when connecting by alias. Oracle’s Oracle Net Services troubleshooting guide for Database 19c covers naming-related errors and diagnostic steps; use documentation appropriate to your installed Oracle Net version.
ORA-12170: connection timeout
A timeout means the connection attempt did not complete in time; it does not identify one universal root cause. Check whether the client can reach the destination host and TCP port, then investigate routing, security rules, listener responsiveness, and any intervening firewall. If the host is cloud-hosted, verify that the configured path matches its actual public or private access model.
ORA-12514: listener does not know the requested service
Compare the service name in the connection request with the services registered with the listener. Confirm that the request targets the intended database or pluggable database (PDB), and that the service is available. A reachable listener can still reject a request for a service it does not recognize.
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First establish where the client and database are, and which route is intended to carry the connection. Then ask the responsible network or database administrator to verify that the database is running, accepts TCP connections, and is reachable through the approved route and security controls.
For Oracle Cloud Infrastructure Database Tools, Oracle’s troubleshooting guidance identifies the VCN, subnet, applicable gateways, route tables, security lists, and network security groups as relevant connection settings. For a private database, check private endpoint setup. For a shared Autonomous Database accessed through a public IP with an access control list (ACL), confirm that the client’s address or CIDR block is allowed. For a customer-managed database, check the host firewall’s rules for the database TCP port.
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Do not open a database port to the public internet as a generic troubleshooting step. Verify the permitted route for the actual deployment and change only the relevant controls. If a connection previously worked, check for changes to database or service state, VCN configuration, IAM permissions, group membership, or database privileges.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Which migration-tool setup should you check?
Only investigate product-specific prerequisites after identifying the tool and the stage that fails. A repository permission error, for example, is not the same diagnosis as a timeout to the database host.
Oracle SQL Developer Migrations
SQL Developer Migrations uses a migration repository and requires setup for third-party source connections. Oracle’s SQL Developer Migrations getting-started guidance lists RESOURCE, CREATE SESSION, and CREATE VIEW among the repository schema privileges. It also describes JDBC driver configuration for certain third-party source connections. Check these requirements if the failure occurs during repository setup or when connecting to a non-Oracle source; they do not replace checking the Oracle endpoint’s host, service, account, and network path.
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Database Migration Assistant for Unicode (DMU)
For DMU, check that the target connection uses the intended host, TNS listener port, service name, and administrative account. Also verify that the database and operating-system combination is supported by the DMU release in use. The June 2024 DMU getting-started guide describes these connection fields, but does not establish a current compatibility matrix for every later release.
OCI Database Migration
OCI Database Migration uses database connection resources for source and target endpoints. In the migration workflow, an Oracle PDB source must be represented by its PDB connection; CDB details may also be needed. Oracle notes that a CDB connection is not required for an offline migration. Review the current connection management guidance and migration creation guidance for the selected workflow.
Transfer methods have different constraints. A database link uses a direct SQL*Net connection. Oracle recommends Data Pump through a database link only for small databases because parallelism cannot be enabled with that method. These are OCI Database Migration details, not general instructions for SQL Developer Migrations or DMU.
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What should you give your database or network administrator?
If the checks do not resolve the failure, provide a concise, sanitized diagnostic bundle rather than only saying that the tool cannot connect:
- Migration product, version, and the exact workflow step that fails.
- Source and target database products and versions, plus whether each endpoint is local, on-premises, or cloud-hosted.
- Full error text and code, including any nested Oracle Net message.
- The intended host, port, service name or connect identifier, and whether a wallet or private endpoint is involved. Do not send passwords, private keys, or wallet secrets in an unsecured message.
- Whether the same endpoint and credentials work from another approved client, and from which network location.
- Recent changes to account status, database services, routes, firewall rules, cloud access controls, or tool configuration.
That information helps separate a naming or descriptor mistake from a listener, network, account, or migration-product prerequisite problem without assuming a root cause the error has not established.
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