Quick wins for a faster PC:
Repair Windows errors before they cause bigger problemsFix Now →Scan for outdated or missing drivers - takes under a minuteDriver Scan →Special offer. See more information about Outbyte and uninstall instructions. Please review EULA and Privacy policy.
Bad file descriptor is the operating system’s EBADF error: iperf3 tried to use a local descriptor that was invalid for the operation. It does not, by itself, prove a firewall, cable, remote peer, or other single cause. Read the words before the colon, then compare the complete client and server logs to find which stage failed.
What “Bad file descriptor” means in iperf3
iperf3 reports an operation-specific error followed by the operating system error. The operation identifies what the process was trying to do; EBADF says the descriptor used for that operation was invalid at that point. The suffix alone does not explain why it became invalid. An earlier failure may have triggered cleanup, for example, and a later code path may then try to use a descriptor that has already been closed.
As an Amazon Associate I earn from qualifying purchases.
That distinction matters: a control-message write, a stream-socket read, cookie reception, and select() are different stages. select failed means a descriptor in the set passed to select() was rejected; the wording does not establish that the control socket was the descriptor involved. The iPerf3 Blog explains the distinction between iperf3’s operation message and the appended operating-system error.
The Tool Desk
Outbyte PC Repair FREEClear out junk files and repair common Windows errorsFree Scan →Outbyte Driver Updater FREEFix the driver behind crashes, sound loss and screen glitchesFind Drivers →Special offer. See more information about Outbyte and uninstall instructions. Please review EULA and Privacy policy.
What to check first
- Save the full output from both endpoints. Include lines preceding the final error. The first failure may explain why a later operation encountered an invalid descriptor.
- Identify the endpoint and operation. Note whether the client or server printed the message and copy the exact text before the colon. Do not assume the two endpoints will report the same failure.
- Verify the server and port. Confirm the server process is running and listening on the port the client is using. Debian bug #972109 documents a historical iperf3 package version 3.9-1 case where connecting to a non-running server produced “unable to send control message: Bad file descriptor.” That is a version-specific example, not a general explanation for every EBADF. Debian bug #972109.
- Record both iperf3 versions and the test conditions. Write down the client and server versions, selected transport (such as TCP, UDP, or SCTP), test duration, and any unusual link conditions. A version pair or transport can help distinguish unlike cases.
- Compare the logs before changing the network. Check for an earlier connection, read, or parameter error. The suffix does not establish that network hardware or a firewall is at fault.
How to read common iperf3 error messages
| Message | What it identifies | What to inspect |
|---|---|---|
unable to send control message: Bad file descriptor |
A local descriptor used for a control-message write was invalid. | Earlier errors, the control connection, and whether the server is running at the expected port. |
unable to read from stream socket: Bad file descriptor |
A stream-socket read failed with EBADF. | Which endpoint printed it, the selected protocol, and both endpoint logs to identify the stream and preceding events. |
unable to receive cookie at server: Bad file descriptor |
The server’s cookie-reception operation on an accepted data-stream socket failed. | The preceding server output and version. A 2025 report records this wording on a 3.18 server and notes that it was also present in 3.15; it does not establish a confirmed regression. ESnet iperf issue #1882. |
select failed: Bad file descriptor |
A descriptor in the set supplied to select() was rejected. |
Full logs, endpoint, transport, duration, and preceding errors. The message does not name the descriptor as the control socket. |
control socket has closed unexpectedly |
The text reports unexpected control-socket closure and has no EBADF suffix. | Which endpoint printed it and what the other endpoint logged. A client-side empty read can indicate end-of-file from the peer; on the server, the read path may also return after a ten-second wait. The wording alone is not a definitive root-cause diagnosis. |
Why the client and server messages may differ
The same run can fail at different points in each process, so compare the endpoint logs rather than treating one line as the whole diagnosis. In an SCTP issue report using iperf 3.1.3, the server reported a stream-read EBADF while the client reported an unexpected control-socket closure. The report records that particular case; it does not establish a general SCTP cause. ESnet iperf issue #1852.
#1 Best Overall
Sale
UGREEN Cat 8 Ethernet Cable 6FT, High Speed Braided 40Gbps 2000Mhz Network Cord Cat8 RJ45 Shielded Indoor Heavy Duty LAN Cables Compatible with Gaming PC PS5 PS4 PS3 Xbox Modem Router 6FT
- 40 Gbps 2000 Mhz High Speed: The Cat 8 ethernet cable support max. 40 Gbps data transfer and 2000 MHz Brandwith, ideal for gaming and streaming, greatly improving upload and download speed, sound, image and resolution quality
- Excellent Anti-interference: The ethernet cable comes with 4 shielded foiled twisted pairs (F/FTP), pure copper core and gold-plated RJ45 connector, reducing interference, noise and crosstalk, making network speed faster and more stable
- Marvelous Durability: Internet cable wrapped with quality cotton braided cord, which makes the LAN cable stronger and more durable. The test proves that this internet cable can be bent at least 10000 times without broken, very suitable for long-term use
- PoE Supported: All lengths of ethernet cord can support the PoE power supply function except 65ft. You don't need additional power supply when installing a PoE camera, which is very convenient and safe
- Wide Compatibility: With the RJ45 Connector, network cable can be perfectly compatible with computers, laptops, modems, routers, PS5, X-Box and other networking devices. It can also be fully backward compatible with Cat7, Cat6e, Cat6, Cat5e, Cat5
Other reports likewise describe individual historical conditions, not prevalence or a universal current bug. An iperf 3.2/3.2rc1 report involving -t 0 described a run ending after about 15 seconds, with unexpected control-socket closure on the client and select failed: Bad file descriptor on the server. ESnet iperf issue #645. A separate UDP report attributed delayed packets to a constrained, queued link and later recorded the server’s select failed message; that is the reporter’s account of one case, not a broadly measured finding. ESnet iperf issue #753.
Quick Recap
SaleBestseller No. 1
Bestseller No. 3
Bestseller No. 4
Best Value
MORELECS Cat 7 Flat Ethernet Cable 6.6FT,10Gbps,Braided,Shielded(3FT-150FT)
- [Flat Design, Zero Cable Clutter] - Lies perfectly flat against walls, under rugs, along baseboards, and through tight spaces without kinks, tangles, or messy coils. Customers praise it for effortless installation and clean cable management that blends into any room.
- [REINFORCED BRAIDED CONSTRUCTION FOR LONG‑LASTING PERFORMANCE] - Premium cotton braided jacket paired with reinforced RJ45 connectors delivers outstanding durability, rigorously tested for over 15,000 bend cycles. Many customers describe this ethernet cable as rock‑solid and well‑crafted, ideal for long‑term daily use with no worries about premature wear‑and‑tear or connection failure
- [10GBPS SPEED & 600MHZ BANDWIDTH — GAMING, STREAMING & FIBER READY] - Delivers 10Gbps data transfer rate with 600MHz bandwidth for PS5, Xbox, 4K streaming, and fiber internet. Customers report stable performance and fast speeds. Backward compatible with Cat 6 and Cat 5e devices
- [STP SHIELDING & GOLD-PLATED RJ45 — MINIMIZES EMI/RFI INTERFERENCE] - 100% bare copper STP shielding helps protect signal integrity when routed near power cords. Gold-plated RJ45 connectors resist corrosion. Compatible with 2.5GB network card
- [Works with Everything — Router, Modem, PS5, Xbox, PC, Smart TV, Printer More ] - Full backward compatibility with Cat7, Cat6, Cat6a, and Cat5e devices means this one cable works with all your home or office equipment today, and future upgrades tomorrow. Works with 10/100/1000/10G/40G BASE-T speeds. Includes 36-month warranty with free replacement support
Rank #4
Smolink Cat 8 Ethernet Cable, 50ft 40Gbps 2000MHz RJ45 LAN Cable
- Cat 8 Speed, Cat 5/5e Value Enjoy Cat 8 Ethernet cable performance at a Cat 5/5e-level value. With up to 40Gbps speed and 2000MHz bandwidth, this high speed internet cable delivers more bandwidth than standard Cat 5 and Cat 5e cables, helping support smooth gaming, streaming, video calls, large file transfers and everyday wired network use.
- 40Gbps Speed, Wide Compatibility This Cat 8 Ethernet cable supports up to 40Gbps data transfer and 2000MHz bandwidth for fast, reliable internet performance. Standard RJ45 connectors are backward compatible with Cat7, Cat6, Cat6a and Cat5e devices, including routers, modems, switches, gaming PCs, PS5, PS4, Xbox, smart TVs, laptops and printers.
- Stable U/FTP Shielding Each of the 4 twisted pairs is individually wrapped with aluminum foil to help reduce crosstalk, noise, and signal interference. Combined with RJ45 connectors on both ends, the U/FTP design helps maintain cleaner signal transmission for a stable and reliable wired network connection.
- Nylon Braided Durability The nylon braided jacket adds everyday durability while keeping the cable flexible and easy to route. Reinforced construction helps the cord handle bending, pulling and frequent plugging, making it a reliable choice for desks, gaming rooms, home offices and long-term network setups.
- 50ft Reach for More Setups The 50 ft length makes it easier to connect devices across rooms, along walls, under desks or around corners. Great for router-to-PC connections, modem-to-TV setups, gaming consoles, workstations, printers and other home network equipment that needs a longer Ethernet cable.
Rank #3
DbillionDa Cat 8 Ethernet Cable, 6FT 40Gbps 2000MHz RJ45 LAN Cable
- Designed for Outdoor & Direct Burial Installations – Heavy-duty double-shielded Cat8 Ethernet cable minimizes EMI/RFI interference and delivers stable long-distance performance. Waterproof, anti-corrosion PVC jacket allows safe direct burial and reliable use in outdoor or indoor environments.
- 26AWG for Stable High-Load Networks – Thicker 26AWG conductors provide faster, more stable data transmission than standard 32AWG cables. Ideal for high-performance home networks, gaming setups, smart homes, and data-intensive applications.
- F/FTP Shielding & Hyper-Speed Performance: Cat8 Ethernet cable constructed with 4 shielded foiled twisted pairs and 26AWG OFC conductors; supports bandwidth up to 2000 MHz and data transmission speeds up to 40 Gbps, effectively reducing signal interference and ensuring stable connections. Ideal for low-latency gaming, 4K/8K streaming, and high-speed internet connections.
- RJ45 Connectors & Wide Compatibility: Cat8 Ethernet cable with two shielded RJ45 connectors; compatible with networking switches, IP cameras, routers, Nintendo Switch, modems, PS3, PS4, Xbox, patch panels, servers, smart TVs, and more; works with Cat7, Cat6, Cat5e, and Cat5 devices
- Weatherproof & UV Resistant: Outdoor-rated Cat8 Ethernet cable with UV-resistant PVC jacket; withstands direct sunlight, extreme cold, humidity, and hot weather; anti-aging and durable; Includes 18-month support.
Rank #2
Jadaol Cat6/Cat6A Ethernet Cable 50FT Flat with Clips 10Gbps Network, White
- Cat 6 performance at a Cat5e price but with higher bandwidth
- High Performance Cat6, 30 AWG, RJ45 Ethernet Patch Cable provides universal connectivity for LAN network components such as PCs,computer servers,printers,routers,switch boxes,network media players,NAS,VoIP phones
- Jadaol cat6 standard cable support Cat8 and Cat7 network and provides performance of up to 250 MHz 10Gbps and is suitable for 10BASE-T, 100BASE-TX (Fast Ethernet), 1000BASE-T/1000BASE-TX (Gigabit Ethernet) and 10GBASE-T (10-Gigabit Ethernet)
- UTP(Unshielded Twisted Pair) patch cable with RJ45 gold-plated Connectors and are made of 100% bare copper wire, ensure minimal noise and interference
- The unique flat cable shape allows for a cleaner and safer installation. You can easily and seamlessly make the cable run along walls, follow edges & corners or even make it completely invisible by sliding it under a carpet.
What the message does—and does not—tell you
- It tells you: a local iperf3 operation encountered an invalid descriptor, and the operation named in the message narrows down the stage involved.
- It does not tell you: the underlying cause, which descriptor was involved when the message does not specify it, or whether the remote peer, firewall, cable, or network is responsible.
- Use issue reports as case examples: Debian bug #972109 was filed in 2020; issues #645 and #753 date to 2017 and 2018; issues #1852 and #1882 date to 2025. Their versions, transports, and conditions differ, so none alone proves that your run has the same cause.
Product prices and availability are accurate as of the date/time indicated and are subject to change. Any price and availability information displayed on Amazon at the time of purchase will apply.
