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?)