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

Mike Williams mike at mikebwilliams.com
Thu Mar 12 12:59:03 UTC 2015


On Thu, Mar 12, 2015 at 4:04 AM, Alexey Brodkin
<Alexey.Brodkin at synopsys.com> wrote:
> 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:
>  --->8---
>  $ ssh root at
>  root at's password:
>  PTY allocation request failed on channel 0
>  shell request failed on channel 0
>  --->8---
> Even though I see devpts is correctly mounted to /dev/pts and /dev/ptmx
> exists.
> 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:
>  --->8---
>  $ ssh root at
>  @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
>  @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
>  Someone could be eavesdropping on you right now (man-in-the-middle
> attack)!
>  It is also possible that a host key has just been changed.
>  The fingerprint for the ECDSA key sent by the remote host is
>  82:b8:c2:cf:88:d6:19:77:60:23:ff:9b:cc:3e:3d:2c.
>  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 solved this by copying the SSH keys in /etc to the filesystem
overlay. SSH won't regenerate them every boot if they already exist,
so it will speed up your boot time and get rid of this warning. I'm
not sure you'd want to do that for your production builds though.

>  --->8---
> 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.
> -Alexey
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