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?)