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?)