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Multiple forced commands being executed

 

 

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oliver at obeattie

Jan 20, 2011, 3:49 PM

Post #1 of 5 (3520 views)
Permalink
Multiple forced commands being executed

Hi there,

I am having a very strange problem with SSH. Essentially, I'm using
forced commands to restrict access based on public key (there are
around 2000 public keys). It appears to work okay, but when I look at
the ssh -v output I see that the client/server is actually executing
all the forced commands for RSA keys (I am connecting with an RSA key)
until it "hits" my key.

Anyone have any idea why this is happening? I have no clue where to
even look for hints as to what would cause this…

Here's an example of the output I am seeing (condensed, the real
output is ~3000 lines):

OpenSSH_5.2p1, OpenSSL 0.9.8l 5 Nov 2009
debug1: Authentication succeeded (publickey).
debug2: fd 5 setting O_NONBLOCK
debug2: fd 6 setting O_NONBLOCK
debug1: channel 0: new [client-session]
debug3: ssh_session2_open: channel_new: 0
debug2: channel 0: send open
debug1: Requesting no-more-sessions [at] openssh
debug1: Entering interactive session.
debug1: Remote: Forced command: gitosis-serve osjokine
debug1: Remote: Port forwarding disabled.
debug1: Remote: X11 forwarding disabled.
debug1: Remote: Agent forwarding disabled.
debug1: Remote: Pty allocation disabled.
[... hundreds more like this ...]
debug1: Remote: Forced command: gitosis-serve obeattie
debug1: Remote: Port forwarding disabled.
debug1: Remote: X11 forwarding disabled.
debug1: Remote: Agent forwarding disabled.
debug1: Remote: Pty allocation disabled.
debug1: Remote: Forced command: gitosis-serve osjokine
debug1: Remote: Port forwarding disabled.
debug1: Remote: X11 forwarding disabled.
debug1: Remote: Agent forwarding disabled.
debug1: Remote: Pty allocation disabled.
[... hundreds more again ...]
debug1: Remote: Forced command: gitosis-serve obeattie
debug1: Remote: Port forwarding disabled.
debug1: Remote: X11 forwarding disabled.
debug1: Remote: Agent forwarding disabled.
debug1: Remote: Pty allocation disabled.
debug2: callback start

—Oliver


dev at naturalnik

Jan 22, 2011, 2:24 AM

Post #2 of 5 (3405 views)
Permalink
Re: Multiple forced commands being executed [In reply to]

-----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA1

Hi Oliver,

this is essentially the point of the forced commands. SSH will execute
them, no matter what the client actually provides as a command.

If you instead want to jsut verify if the command is allowed, you will
need a wrapper script as forced command that checks the
$SSH_ORIGINAL_COMMAND environment variable and then decides what to do.

Again, the forced-commands-only is for forcing a command, not for
verifying it.

- -nik
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oliver at obeattie

Jan 22, 2011, 2:27 AM

Post #3 of 5 (3457 views)
Permalink
Re: Multiple forced commands being executed [In reply to]

Hi Dominik,

Thanks for your reply, but I'm not sure I've properly explained what I
mean. In essence, from what I can see, it isn't just executing the
forced command for the key that is being used, it executes the
commands for *every* RSA key in the authorized_keys file, meaning I
get hundreds of commands being run for each login. The program is
itself checking the $SSH_ORIGINAL_KEY.

Hope this explains it better.

—Oliver



On 22 January 2011 09:43, Dominik George <nik [at] naturalnet> wrote:
> Hi Oliver,
>
> this is essentially the point of the forced commands. SSH will execute
> them, no matter what the client actually provides as a command.
>
> If you instead want to jsut verify if the command is allowed, you will
> need a wrapper script as forced command that checks the
> $SSH_ORIGINAL_COMMAND environment variable and then decides what to do.
>
> Again, the forced-commands-only is for forcing a command, not for
> verifying it.
>
> -nik
>
>> Hi there,
>>
>> I am having a very strange problem with SSH. Essentially, I'm using
>> forced commands to restrict access based on public key (there are
>> around 2000 public keys). It appears to work okay, but when I look at
>> the ssh -v output I see that the client/server is actually executing
>> all the forced commands for RSA keys (I am connecting with an RSA key)
>> until it "hits" my key.
>>
>> Anyone have any idea why this is happening? I have no clue where to
>> even look for hints as to what would cause this…
>>
>> Here's an example of the output I am seeing (condensed, the real
>> output is ~3000 lines):
>>
>> OpenSSH_5.2p1, OpenSSL 0.9.8l 5 Nov 2009
>> debug1: Authentication succeeded (publickey).
>> debug2: fd 5 setting O_NONBLOCK
>> debug2: fd 6 setting O_NONBLOCK
>> debug1: channel 0: new [client-session]
>> debug3: ssh_session2_open: channel_new: 0
>> debug2: channel 0: send open
>> debug1: Requesting no-more-sessions [at] openssh
>> debug1: Entering interactive session.
>> debug1: Remote: Forced command: gitosis-serve osjokine
>> debug1: Remote: Port forwarding disabled.
>> debug1: Remote: X11 forwarding disabled.
>> debug1: Remote: Agent forwarding disabled.
>> debug1: Remote: Pty allocation disabled.
>> [... hundreds more like this ...]
>> debug1: Remote: Forced command: gitosis-serve obeattie
>> debug1: Remote: Port forwarding disabled.
>> debug1: Remote: X11 forwarding disabled.
>> debug1: Remote: Agent forwarding disabled.
>> debug1: Remote: Pty allocation disabled.
>> debug1: Remote: Forced command: gitosis-serve osjokine
>> debug1: Remote: Port forwarding disabled.
>> debug1: Remote: X11 forwarding disabled.
>> debug1: Remote: Agent forwarding disabled.
>> debug1: Remote: Pty allocation disabled.
>> [... hundreds more again ...]
>> debug1: Remote: Forced command: gitosis-serve obeattie
>> debug1: Remote: Port forwarding disabled.
>> debug1: Remote: X11 forwarding disabled.
>> debug1: Remote: Agent forwarding disabled.
>> debug1: Remote: Pty allocation disabled.
>> debug2: callback start
>>
>> —Oliver
>>
>
>
>


dev at naturalnik

Jan 22, 2011, 2:50 AM

Post #4 of 5 (3411 views)
Permalink
Re: Multiple forced commands being executed [In reply to]

-----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA1

Hi Oliver,

oh, sure, I didn't exactly get that. I wil ltry to reproduce that ...

- -nik

Am 22.01.2011 11:27, schrieb Oliver Beattie:
> Hi Dominik,
>
> Thanks for your reply, but I'm not sure I've properly explained
> what I mean. In essence, from what I can see, it isn't just
> executing the forced command for the key that is being used, it
> executes the commands for *every* RSA key in the authorized_keys
> file, meaning I get hundreds of commands being run for each login.
> The program is itself checking the $SSH_ORIGINAL_KEY.
>
> Hope this explains it better.
>
> —Oliver
>
>
>
> On 22 January 2011 09:43, Dominik George <nik [at] naturalnet>
> wrote:
>> Hi Oliver,
>>
>> this is essentially the point of the forced commands. SSH will
>> execute them, no matter what the client actually provides as a
>> command.
>>
>> If you instead want to jsut verify if the command is allowed, you
>> will need a wrapper script as forced command that checks the
>> $SSH_ORIGINAL_COMMAND environment variable and then decides what
>> to do.
>>
>> Again, the forced-commands-only is for forcing a command, not
>> for verifying it.
>>
>> -nik
>>
>>> Hi there,
>>>
>>> I am having a very strange problem with SSH. Essentially, I'm
>>> using forced commands to restrict access based on public key
>>> (there are around 2000 public keys). It appears to work okay,
>>> but when I look at the ssh -v output I see that the
>>> client/server is actually executing all the forced commands for
>>> RSA keys (I am connecting with an RSA key) until it "hits" my
>>> key.
>>>
>>> Anyone have any idea why this is happening? I have no clue
>>> where to even look for hints as to what would cause this…
>>>
>>> Here's an example of the output I am seeing (condensed, the
>>> real output is ~3000 lines):
>>>
>>> OpenSSH_5.2p1, OpenSSL 0.9.8l 5 Nov 2009 debug1: Authentication
>>> succeeded (publickey). debug2: fd 5 setting O_NONBLOCK debug2:
>>> fd 6 setting O_NONBLOCK debug1: channel 0: new
>>> [client-session] debug3: ssh_session2_open: channel_new: 0
>>> debug2: channel 0: send open debug1: Requesting
>>> no-more-sessions [at] openssh debug1: Entering interactive
>>> session. debug1: Remote: Forced command: gitosis-serve
>>> osjokine debug1: Remote: Port forwarding disabled. debug1:
>>> Remote: X11 forwarding disabled. debug1: Remote: Agent
>>> forwarding disabled. debug1: Remote: Pty allocation disabled.
>>> [... hundreds more like this ...] debug1: Remote: Forced
>>> command: gitosis-serve obeattie debug1: Remote: Port forwarding
>>> disabled. debug1: Remote: X11 forwarding disabled. debug1:
>>> Remote: Agent forwarding disabled. debug1: Remote: Pty
>>> allocation disabled. debug1: Remote: Forced command:
>>> gitosis-serve osjokine debug1: Remote: Port forwarding
>>> disabled. debug1: Remote: X11 forwarding disabled. debug1:
>>> Remote: Agent forwarding disabled. debug1: Remote: Pty
>>> allocation disabled. [... hundreds more again ...] debug1:
>>> Remote: Forced command: gitosis-serve obeattie debug1: Remote:
>>> Port forwarding disabled. debug1: Remote: X11 forwarding
>>> disabled. debug1: Remote: Agent forwarding disabled. debug1:
>>> Remote: Pty allocation disabled. debug2: callback start
>>>
>>> —Oliver
>>>
>>
>>
>>

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mark-secure at aufflick

Feb 2, 2011, 7:35 PM

Post #5 of 5 (3368 views)
Permalink
Re: Multiple forced commands being executed [In reply to]

FYI - ssh version is:

OpenSSH_5.4p1 FreeBSD-20100308, OpenSSL 0.9.8n 24 Mar 2010

And now I am using my own smtp not Gmail's blacklisted ones hopefully this message will actually make it to the list...


On 03/02/2011, at 1:44 PM, Mark Aufflick wrote:

> FYI I am seeing exactly the same issue. The -vv output on the client
> shows running a huge number of commands (not totally sure if it
> correlates to the number of entries in authorized_keys or just the
> number preceding the matched key - I think it's the latter) and
> judging by the pid of the command that finally ends up being run those
> log entries are actually related to commands being run and abandoned.
>
> It's like the forced command is executed prior to the key being
> checked? Besides wasting resources it's potentially doing things you
> don't expect (what if the command had a side effect) and slowing down
> the connection.
>
> I assume it's not intended behaviour? Neither the command="command"
> section of the ssh man page nor the ForceCommand section of
> sshd_config make any reference to it so I presume not.
>
> Let me know if I can provide any more useful info - I'm going to have
> a quick hunt in the sshd code to see if I can find anything obvious,
> but I'm not familiar with the code base so don't hold your breath.
>
> Cheers,
>
> Mark.
> --
> Mark Aufflick
> http://mark.aufflick.com/about/contact
> http://pumptheory.com/about

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