Solution for Speedtest CLI error “Failed to find a working test server”

Once, when running the Speedtest CLI on one Linux server, I noticed the following error:

speedtest --selection-details
   Speedtest by Ookla
Selecting server:
       2933: Failed; Thüga SmartService - Hof
       4578: Failed; SWW Wunsiedel GmbH - Wunsiedel
      25733: Failed; TaNET West s.r.o. - Tachov
       6518: Failed; Thueringer Netkom GmbH - Weimar
      11519: Failed; TELE AG - Leipzig
       3313: Failed; suec//dacor GmbH - Coburg
       3823: Failed; Newone - Ilmenau
       6416: Failed; ForWiFi s.r.o. - Plzen
      10708: Failed; PilsFree, z.s - Plzen
       2495: Failed; IBH IT-Service GmbH - Dresden
[error] Server Selection - Failed to find a working test server. (NoServers)

I tried to specify the speedtest server manually:

speedtest --server-id=25733
   Speedtest by Ookla
     Server: TaNET West s.r.o. - Tachov (id = 25733)
        ISP: Hetzner Online GmbH
[error] Failed to resolve host name. Cancelling test suite.
[error] Host resolve failed: Connection refused

If you run the Speedtest CLI in verbose mode, you can see that it is trying to establish connections with the servers:

speedtest -vvv
...
completed with response code 200
...
[info] Server count: 10
[info] app.version: 1.0.0.2 (5ae238b)
[info] Resolving host in thread.
[info] Resolving host in thread.
[info] Resolving host in thread.
[info] Resolving host in thread.
[info] Resolved address 1: 91.137.110.250
[info] Resolving host in thread.
[info] Opening socket to '91.137.110.250'
[info] Resolving host in thread.
[info] Resolving host in thread.
[info] Resolving host in thread.
[info] Resolved address 1: 109.73.31.177
[info] Opening socket to '109.73.31.177'
[info] Resolving host in thread.
[info] Resolved address 1: 5.61.162.244
[info] Opening socket to '5.61.162.244'
...

At the same time, the Internet on the server worked and the working DNS servers were specified in the /etc/resolv.conf file.

I found the cause of errors in the firewall, which blocked outgoing connections (OUTPUT) and after I allowed the server all outgoing connections, the Speedtest CLI started working.

iptables -P OUTPUT ACCEPT

See also my articles:
How to configure IPTables

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