ssh.exe local port forwarding limited to 32 connections

[scottmckenzie]

I have an ssh server which I am using to tunnel connections to remote servers. In my .ssh\config file I have lots (>32) of LocalForward directives for this server like so:
LocalForward 127.0.0.1:13389 remoteserver.localnet:3389

When my config file contains more than 32 LocalForward directives I can no longer connect to my jumphost. I receive the error select: Overlapped I/O operation is in progress.. If I limit the LocalForward directives to 32 then I can successfully connect.

This seems to have been introduced in release 3_19_2016 which I believe included new I/O code. Release 2_25_2016 allows me to connect to my jumphost with more than 32 local port forwardings.

[manojampalam]

This is due to a internal enforced limit. This can be upped a bit more but making it boundless requires a multi threaded solution that I would like to avoid.
A work around would be to make a copy of ssh.exe and ssh_config in a separate directory, split LocalForward directives between ssh_configs and run separate instances of ssh.exe

[jhudsoncedaron]

@manojampalam Ah yes you ran into an inherent design flaw in Windows. It's not the first you will hit.

Knowing the flaws involved when I attempted to plan a port, I'm surprised that MS didn't just decide to fix the Windows problems first. Problem #1: Can't wait for thousands of handles per thread. Problem #2: Can't create anything like a PTY so a console app that wants to change TTY settings does not signal the server end and so it doesn't pass down the network pipe. Problem #3: Trying UTF-8 on the console explodes, but essentially all ssh links are either UTF-8 Text or binary and ssh can't tell the difference.

[nkrepo]

I am using "OpenSSH for Windows" version 0.0.22.0 at
Microsoft Windows 10 Pro 64-bit 10.0.16299 (Fall Creators Update)

@manojampalam
The number of local ports that can be forwarded is limited to 16 instead of 32.
When I try to forward more than 16 local ports I get the following error:
"select: Not enough space"

[manojampalam]

@nkrepo there were no changes in the limit. Can you share your repro steps?

[nkrepo]

@manojampalam
I used a simple PowerShell script that called ssh.exe and specified more than 16 local port forwards like the following.

ssh -L 3301:192.168.0.1:1123 `  
       ... `
      -L 3317:192.168.0.17:1123 `
      [email protected]

[manojampalam]

Try with -4 parameter. My guess is you are ending up listening on IPv6 address too for each forwarded port. That will double the number of sockets you are listening on.

[nkrepo]

@manojampalam you are correct it was binding to both IPv4 and IPv6 address now it works up to 32 local port forwards. I had disabled IPv6 for the physical network adapter but not system-wide, therefore the Loopback Pseudo-Interface still has an IPv6 address.

Get-NetIPAddress -ifIndex 1

IPAddress         : ::1
InterfaceIndex    : 1
InterfaceAlias    : Loopback Pseudo-Interface 1
AddressFamily     : IPv6
...
PolicyStore       : ActiveStore

IPAddress         : 127.0.0.1
InterfaceIndex    : 1
InterfaceAlias    : Loopback Pseudo-Interface 1
AddressFamily     : IPv4
...
PolicyStore       : ActiveStore