[Buildroot] [PATCH] system: add option for standalone telnetd on target

Alexey Brodkin Alexey.Brodkin at synopsys.com
Thu Mar 12 08:04:33 UTC 2015

Hi Peter,

On Wed, 2015-03-11 at 17:53 +0100, Peter Korsgaard wrote:
> >>>>> "Alexey" == Alexey Brodkin <Alexey.Brodkin at synopsys.com> writes:
> Hi,
> >> Any specific reason why you don't just configure a root password and
>  >> enable dropbear instead?
>  > Well I though of telnet as an essential replacement of serial console
>  > especially for development boards.
> These days I would say ssh is much more common.
>  > So my main intention was to get the most convenient tool for wide range
>  > of developers. 
>  > For example in Windows if I'm not mistaken Telnet client is available
>  > right from MS, while SSH client is always 3rd-party program like Putty. 
> If I'm not mistaken Windows no longer comes with telnet out of the box.
> E.G. first Google hit:
> http://social.technet.microsoft.com/wiki/contents/articles/910.windows-7-enabling-telnet-client.aspx

Well probably it was in days of WinXP when Telnet was pre-installed.
Still as you may see from the article - there's a way to install
"native" Telnet client from Windows update/software sources.

>  > Also ability to not set password is convenient here - because people
>  > will ask "what's the password" otherwise. Still in case of devboards we
>  > have limited access to the network for foreigners so we may not care
>  > much about paranoid safety.
> Don't they ask the same about the serial login password?

That's exactly the point for serial port as well as for Telnet we may
not use password for root - which is a default case in Buildroot.

>  > Indeed your proposal may work if my motivation is not convincing enough.
> I can still be convinced, but my initial thought is that it isn't
> really a common enough use case / we should promote ssh instead.

I tried your proposal with Dropbear but frankly with not much luck.
What I did:
 [1] Enabled "dropbear": BR2_PACKAGE_DROPBEAR=y
 [2] Set root password: BR2_TARGET_GENERIC_ROOT_PASSWD="xxx"

What's nice Dropbear auto-starts on boot. But...

Now on attempt to ssh to the target I see:
 $ ssh root at
 root at's password: 
 PTY allocation request failed on channel 0
 shell request failed on channel 0

Even though I see devpts is correctly mounted to /dev/pts and /dev/ptmx

So probably I'm missing something.

Another inconvenience I discovered with SSH - every time I boot my
target it gets new fingerprint and then on attempt to ssh to the target
I see:
 $ ssh root at
 Someone could be eavesdropping on you right now (man-in-the-middle
 It is also possible that a host key has just been changed.
 The fingerprint for the ECDSA key sent by the remote host is
 Please contact your system administrator.
 Add correct host key in /home/abrodkin/.ssh/known_hosts to get rid of
this message.
 Offending ECDSA key in /home/abrodkin/.ssh/known_hosts:49
 ECDSA host key for has changed and you have requested
strict checking.
 Host key verification failed.

I may assume this is because I have filesystem built-in kernel (vmlinux)
so between boots filesystem doesn't preserve any information - but in
case of simulators we usually don't have any other options.


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