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Bad file descriptor is the operating system’s EBADF error attached to an iperf3 failure. It means a local iperf3 process tried to use a descriptor that was not valid for that operation; by itself, it does not show that the remote host, firewall, cable, or network is at fault. Start with the complete output, identify the operation named before the colon, and compare what the client and server logged.
What “Bad file descriptor” means in iperf3
The text after the colon is EBADF, an operating-system error. The words before it identify the operation that failed, such as sending a control message, reading a stream socket, or calling select(). Those stages involve different descriptors, so the suffix alone cannot identify the cause.
A prior error may also matter: cleanup after an earlier failure can close a descriptor, and a later code path may then try to use it. Read the log from the beginning rather than treating the final line as a complete diagnosis.
What to check first
- Capture both endpoints’ complete output. Include the lines before the final error, and note whether each message came from the client or server. One endpoint’s failure may trigger a different message at the other.
- Identify the failed operation. Read the text before
Bad file descriptor. A control-message write, stream-socket read, cookie reception, andselect()failure are not interchangeable. - Verify the server and port. Confirm that iperf3 is running on the server and listening on the port the client targets. Debian bug #972109 describes a connection attempt to a non-running server with Debian’s iperf3 package version 3.9-1; the client reported “unable to send control message: Bad file descriptor.” The report also noted that an earlier version gave clearer connection-refused wording. This is a version-specific historical example, not proof that every such error means the server is down. Read Debian bug #972109.
- Record the test context. Note both iperf3 versions, the selected transport, how long the test ran, and whether it used unusual duration or traffic conditions. These details help distinguish unlike failures.
How to interpret common messages
unable to send control message: Bad file descriptor
The failed operation was a control-message write using a local descriptor. Check earlier output, server availability, and the state of the control connection at both endpoints. The message does not by itself implicate the network.
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unable to read from stream socket: Bad file descriptor
A stream-socket read failed. Use the protocol and endpoint logs to establish which stream and stage were involved. In ESnet issue #1852, an SCTP test using iperf 3.1.3 stopped after several intervals; the server logged a stream-read EBADF while the client reported that the control socket had closed unexpectedly. The report does not establish a root cause. Read ESnet iperf issue #1852.
unable to receive cookie at server: Bad file descriptor
This wording identifies cookie reception on an accepted data-stream socket. ESnet issue #1882 records cookie and parameter EBADF messages in a server log for version 3.18. The issue text says the cookie message was also present in 3.15, so it does not confirm a 3.18 regression. Read ESnet iperf issue #1882.
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select failed: Bad file descriptor
A descriptor included in the select() set was rejected; the message does not establish that the descriptor was the control socket. Historical reports show this wording in distinct situations: an iperf 3.2/3.2rc1 long-running test using -t 0 ended after about 15 seconds, with unexpected control-socket closure on the client and a server-side select failure (ESnet issue #645); another report described delayed UDP packets on a constrained, queued link before the server printed the error (ESnet issue #753). These individual cases do not establish a universal cause.
control socket has closed unexpectedly
This message has no errno suffix. Its meaning depends on the endpoint and surrounding state: a client-side empty read indicates end-of-file from the peer, while the server’s read path may also return after a ten-second wait. Correlate it with the other endpoint’s log instead of treating the wording alone as a definitive diagnosis.
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Use reports as examples, not universal explanations
iperf3 error reports cover different versions, transports, test lengths, and endpoint behavior. Debian’s 3.9-1 report concerns a non-running server; the 2017 and 2018 ESnet reports describe different long-running and UDP conditions; the 2025 reports cover an SCTP test with iperf 3.1.3 and server messages in 3.18. They are useful for recognizing possible contexts, but individual reports do not establish prevalence or prove that the same cause applies to a new test.
When sharing a failure for diagnosis, include the full client and server logs, both versions, transport, test duration, and the port used. Those details preserve the distinctions needed to investigate the failed operation.
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