ssh tunnel problems

[Kevin Martin]

Rick Bilonick wrote:
> On Sun, 2008-06-22 at 17:36 +0930, Tim wrote:
>   
>> On Sat, 2008-06-21 at 19:34 -0400, Rick Bilonick wrote:
>>     
>>> What on the work server could be preventing the reverse tunnel from
>>> working? On the server I do use hosts.allow to only allow ssh from my
>>> home computer. Could this possibly prevent the reverse tunnel from
>>> working? Or is the problem on my home computer?
>>>       
>> Firewalling?  But we can only guess without detailed information about
>> the network configuration, on both sides.
>>
>> I would still try completely avoiding referring to localhost, anywhere.
>> It's awfully confusing trying to follow your steps, and I can forsee you
>> getting tangled in a knot.
>>
>> -- 
>> [tim at localhost ~]$ uname -r
>> 2.6.25.6-55.fc9.i686
>>
>> Don't send private replies to my address, the mailbox is ignored.  I
>> read messages from the public lists.
>>
>>
>>
>>     
>
> How do you explain that this works fine when going from my home computer
> to an account on my ISP's computer? I followed an example posted on the
> web (which DID have one mistake in using "localhost" which I corrected -
> but the other use of "localhost" is AFAIK correct). In order to do a
> reverse tunnel, don't you have to point to localhost in order to use the
> forwarded port? 
>
> I don't see this as confusing:
>
> (on my.work.server which is behind a firewall that blocks incoming ssh
> but not outgoing ssh)
>
>   
>> ssh -R 2022:my.work.server:22 me at home.computer
>>     
>
> where "my.work.server" is the IP address for my.work.server and
> "home.computer" is the IP address for my home.computer. This sets up the
> port forwarding for a reverse tunnel (that's the -R option). If on
> home.computer I do:
>
>   
>> netstat -an | grep 2022
>>     
>
> it shows that home.computer is listening to port 2022.
>
> Then, to use the reverse tunnel (again on home.computer):
>
>   
>> ssh -p 2022 accnt at localhost
>>     
>
> where "accnt" is the user account on my.work.server and I use the
> password for accnt on my.work.server. This should allow me then to go
> through the ssh tunnel in the reverse direction (getting through the
> firewall that is blocking the use of incoming ssh from the home computer
> to the my.work.server).
>
> Even after removing everything in hosts.allow on my.work.server, I still
> can't connect.
>
> This SAME set up works fine if I set up the tunnel from my home computer
> to my account on my ISP's server. And yes I'm using "localhost" similar
> to what I show above. And I've tried it from my.work.server to my
> account on my ISP but have the same problem so the problem is something
> on my.work.server. 
>
> Is it possible for the firewall to block a reverse tunnel (without
> blocking outgoing ssh)?
>
> Rick B.
>
>   
Rick,

 What do you see if you add -v to the "ssh -p 2022 accnt at localhost"?  
Also, what if you, instead of using localhost, use the ip address of 
your eth0 interface in the previous command?  If you do that and, in 
another terminal, do a "tcpdump -i eth0 -vv -l port 2022" do you see the 
connection attempt being made to the port?  Also try adding -v to  the 
connection where you are creating the tunnel and then watch the output 
of that connection as you try to make a connection back over the tunnel.

FWIW, I just did your exact setup with two machines that I have and it 
worked perfectly (prompted me for passwords and then the logins 
worked).   I added -v to both the tunnel creation and then the reverse 
use of the tunnel and saw some fun stuff.

Kevin


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