Re: can't ssh from host to remote

>>>>>> Now that we know what an IP address is, lets move on to the "Physical
>>>>>> Link".
>>>>>> Well, a cable will do...
>>>>>> In the wireless world, the "Association" is the link.
>>>>>> And how do you validate that?
>>>>>> iwconfig will tell you what (if anything) you are associated to.  No
>>>>>> association, no link, no connection, no SSH.
>>>>>> ifconfig will tell you what (if anything) you are wired to.  No wire,
>>>>>> no link, no connection, no SSH.
>>>>>> Ain't that simple?   ;-)
>>>>>> So we have a link...
>>>>>> And we have IP addresses in the same subnet.
>>>>>> So we are connected!!!   8-)
>>>>>> Not so fast Armando!!!
>>>>>> The fact that your addresses match is not necessarily a validation,
>>>>>> because each computer may be connected to a different router providing the
>>>>>> same NAT(ed) address!
>>>>>> NAT?
>>>>>> Yes NAT (Network Address Translation protocol), but that's yet
>>>>>> another book, so lets water it down:
>>>>>> NAT is the protocol that allows you to have an 'outside visible
>>>>>> address' and an 'inside invisible network' in a router.
>>>>>> NAT (as Netmask) was implemented mainly to alleviate the IPv4
>>>>>> shortage address because of the 'class A,B,C or D' mistake, but as a
>>>>>> byproduct, you can 'hide' behind it, which provides some level of security.
>>>>>>  How you hide is yet another bookshelf and essentially means that you
>>>>>> cannot access devices 'behind the router' unless the device initiates the
>>>>>> connection first, and that's how you raise a WEB site from 'behind the
>>>>>> router' and why you can SSH from 'inside to outside the router' but not the
>>>>>> other way around, so lets move on...
>>>>>> So, how do we know that we are connected to the same router?
>>>>>> Ah, glad you asked:
>>>>>> ARP!
>>>>>> Or Address Resolution Protocol.
>>>>>> *ALL* data transmission is done at OSI layer 2.
>>>>>> Quick implementation manual:
>>>>>> OSI layer 1: Cable or association.
>>>>>> OSI layer 2: MAC address.
>>>>>> OSI layer 3: IP address.
>>>>>> Your network doesn't know (and doesn't care) about IP addresses.  The
>>>>>> IP address is there to resolve the MAC address.
>>>>>> When you say:
>>>>>> ping 192.168.0.1
>>>>>> that generates a 'who has' request from the ARP protocol.
>>>>>> That request is broadcasted to anyone on the physical link (OSI layer
>>>>>> 1)
>>>>>> The device with the IP address interrogated by 'who has' answers with
>>>>>> its MAC address.
>>>>>> This IP/MAC address pair is then saved to the ARP table.
>>>>>> >From there on (and even though the IP address goes along in the
>>>>>> TCP/IP header) all transmissions are sent to the MAC address.
>>>>>> But then again, how do you know that your 2 boxes are talking to the
>>>>>> same router?
>>>>>> arp -n|grep 192.168.1.1
>>>>>> Same MAC?
>>>>>> Same box.
>>>>>> Different MAC?
>>>>>> Same Michael...   ;-)
>>>>>> What do we know so far?
>>>>>> Well, we know something about line 3 of the very first paragraph.
>>>>>> What about line 2?
>>>>>> Type
>>>>>> which ssh
>>>>>> You have it or not, and you know what to do, so lets move to line 1.
>>>>>> We now need to troubleshoot the SSH server.
>>>>>> Well, that boils down to 2 things, it is working or not...
>>>>>> You *KNOW* that the SSH server is 'listening' (although not
>>>>>> necessarily working) when you can connect to the 'port'
>>>>>> Port?
>>>>>> Yeah, port...
>>>>>> Lets move on up in the OSI model to the application layer.
>>>>>> In order to establish a TCP connection you need an IP connection and
>>>>>> a port (or a socket and a port)
>>>>>> The port is to the application what the IP address is to the MAC.
>>>>>> So if the port is listening, the application is awake.
>>>>>> And how do we know?
>>>>>> There are only 975143684 possible ways to validate a 'port is open'
>>>>>> (or listening) but I am a simple boring guy, so I do:
>>>>>> telnet localhost 22
>>>>>> I either get an answer or not.
>>>>>> If I get an answer, then we are most likely all good, but if I don't
>>>>>> get an answer then the ramifications are staggering and I'm not even going
>>>>>> to think about it.
>>>>>> In order to check that the other port listens then you:
>>>>>> telnet ${REMOTE} 22
>>>>>> Again, we either get an answer or not.  And the 'not' means another
>>>>>> Sunday drive to the library...
>>>>>> Finally, why 22?
>>>>>> Because that's the SSH port and it is defined in the configuration
>>>>>> file, which you can change to further complicate your (or someone else's)
>>>>>> life.
>>>>>> But who and where defined 22 as the SSH port?
>>>>>> grep -i ssh /etc/services
>>>>>> And who wrote /etc/services?
>>>>>> http://www.iana.org/
>>>>>> And how do I know all this crap?
>>>>>> Because I finished LFS!!!!    ;-)
>>>>>> I hope you see everything now as clear as mud.
>>>>>> Keep this message handy, you'll need to read it several times...
>>>>>> Keep in mind that what I have written here is a GROSS
>>>>>> oversimplification of several bookshelves contained in several buildings
>>>>>> and written along several decades all over the World, it's free advice, you
>>>>>> can't sue me...   :)
>>>>>> And always remember:
>>>>>> For every question there exists a simple, direct and wrong answer.
>>>>>> if you have any question,
>>>>>> you will get any answer...
>>>>>> ET
>>>>>> PS: Research project:
>>>>>> Why doesn't 'ping' use a port?
>>>>>> Why is 'ping' 'setuid(ed)'
>>>>>> What are 'routable' networks?
>>>>>> What are 'non-routable' networks?
>>>>>> What does it mean if you get and IP address like 169.254.0.0/16
>>>>>> Why do you always have a 127.0.0.1 address in your boxes?
>>>>>> Who defines (and where are the documents that define) all these
>>>>>> protocols? (RFC anyone?)