erl.exe -noshell -eval erlang:get_cookie() -s init stop -name undefined failed with Can't set long node name!

[inikulshin]

Describe the bug
erl.exe -noshell -eval erlang:get_cookie() -s init stop -name undefined failed with

  Can't set long node name!
  Please check your configuration
  
  2025-04-14 15:58:55.230000 crash_report        
      initial_call: {net_kernel,init,['Argument__1']}
      pid: <0.64.0>
      registered_name: []
      error_info: {exit,{error,badarg},[{gen_server,init_it,6,[{file,"gen_server.erl"},{line,961}]},{proc_lib,init_p_do_apply,3,[{file,"proc_lib.erl"},{line,241}]}]}
      ancestors: [net_sup,kernel_sup,<0.47.0>]
      message_queue_len: 0
      messages: []
      links: [<0.61.0>]
      dictionary: [{longnames,true}]
      trap_exit: true
      status: running
      heap_size: 6772
      stack_size: 28
      reductions: 2516
  2025-04-14 15:58:55.230000 supervisor_report   
      supervisor: {local,net_sup}
      errorContext: start_error
      reason: {'EXIT',nodistribution}
      offender: [{pid,undefined},{id,net_kernel},{mfargs,{net_kernel,start_link,[#{name=>undefined,supervisor=>net_sup,name_domain=>longnames,clean_halt=>true}]}},{restart_type,permanent},{significant,false},{shutdown,2000},{child_type,worker}]
  2025-04-14 15:58:55.230000 supervisor_report   
      supervisor: {local,kernel_sup}
      errorContext: start_error
      reason: {shutdown,{failed_to_start_child,net_kernel,{'EXIT',nodistribution}}}
      offender: [{pid,undefined},{id,net_sup},{mfargs,{erl_distribution,start_link,[]}},{restart_type,permanent},{significant,false},{shutdown,infinity},{child_type,supervisor}]
  2025-04-14 15:58:55.230000 crash_report        
      initial_call: {application_master,init,['Argument__1','Argument__2','Argument__3','Argument__4']}
      pid: <0.46.0>
      registered_name: []
      error_info: {exit,{{shutdown,{failed_to_start_child,net_sup,{shutdown,{failed_to_start_child,net_kernel,{'EXIT',nodistribution}}}}},{kernel,start,[normal,[]]}},[{application_master,init,4,[{file,"application_master.erl"},{line,142}]},{proc_lib,init_p_do_apply,3,[{file,"proc_lib.erl"},{line,241}]}]}
      ancestors: [<0.45.0>]
      message_queue_len: 1
      messages: [{'EXIT',<0.47.0>,normal}]
      links: [<0.45.0>,<0.44.0>]
      dictionary: []
      trap_exit: true
      status: running
      heap_size: 376
      stack_size: 28
      reductions: 169
  2025-04-14 15:58:55.230000 std_info            
      application: kernel
      exited: {{shutdown,{failed_to_start_child,net_sup,{shutdown,{failed_to_start_child,net_kernel,{'EXIT',nodistribution}}}}},{kernel,start,[normal,[]]}}
      type: permanent
  Kernel pid terminated (application_controller) ("{application_start_failure,kernel,{{shutdown,{failed_to_start_child,net_sup,{shutdown,{failed_to_start_child,net_kernel,{'EXIT',nodistribution}}}}},{kernel,start,[normal,[]]}}}")

To Reproduce
I can't reproduce it, same command worked thousands times before. But maybe as a code authors you can think of reproduction scenario/environment.

Expected behavior
Must succeed, according to https://www.erlang.org/docs/26/man/erl#flags

If Name is set to undefined the node will be started in a special mode optimized to be the temporary client of another node.

Affected versions
26.2.5 on Windows

[garazdawi]

The system cannot figure out your longname automatically, so you need to set it.

erl.exe -noshell -eval erlang:get_cookie() -s init stop -name undefined@example.com

alternatively you need to configure your system so that the long domain can be detected.

[inikulshin]

@garazdawi

Afaiu, I can just use -sname undefined, at least for my purpose (to create .erlang.cookie file).

But firstly, -name with special value undefined worked for months if not for years, and is still working for most of the times. So, at least sometimes erl.exe knows to work with special value -name undefined without hostname.

And secondly, imo documentation should describe it better.

[garazdawi]

Afaiu, I can just use -sname undefined, at least for my purpose (to create .erlang.cookie file).

yes, short name normally always works as it does not need a working DNS server.

But firstly, -name with special value undefined worked for months if not for years, and is still working for most of the times. So, at least sometimes erl.exe knows to work with special value -name undefined without hostname.

Something changed with the configuration of your network/machine or with the way that Erlang detects this. Do you have the same issue if you use older Erlang releases?

And secondly, imo documentation should describe it better.

Agreed, the documentation can be improved here. I don't know much about how you configure long vs short names on Windows, so maybe you can help out?

[inikulshin]

Something changed with the configuration of your network/machine or with the way that Erlang detects this.

For sure something in our environment was changed, but I don't know what. We use Erlang only for RabbitMQ and we never specially configured it.

Do you have the same issue if you use older Erlang releases?

We use -name undefined for few months and with 26.2.5 only, but we saw this crash for the first time only few days ago.

Agreed, the documentation can be improved here.

Imo, the code should be fixed, and not the documentation.

For -sname option, undefined is now well-known, reserved value and it should be the same for -name option. I don't know the implementation, but if for Dynamic Node Name hostname part is ignored after all, using undefined@localhost or undefined@example.com is very confusing even if documented.

Checking for undefined (for Dynamic Node Name) should be done before any other [s]name parsing and matching.

short name normally always works as it does not need a working DNS server.

-name undefined should also normally always work and should not need a working DNS server.

[josevalim]

For what is worth, we heard similar reports in Livebook, which also used -name. For example, if you join another network or another computer joined your network with the same name as yours, then your operating system may change the name accordingly, and we also saw computer names being set to invalid host names automatically, such as empty strings or number sequences, which we could not reproduce by calling anything via the command line. Next time it happens, try grabbing the machine’s name and see if it is valid, but it all pointed out to be an operating system or network issue.

[IngelaAndin]

We will try to get to the bottom of this, maybe it is an oversight of some kind when handling dynamic nodes. This might require some cross team effort, so alas I think this will happen first after summer vacation period.