> Going the other way, you have no rules to pass
> the communication through.

Why were rules written for the second router but not the first? Is it
because it was connected first? Could we write the rules we need?


:-)~MIKE~(-:


On Fri, Jul 18, 2014 at 3:34 PM, Gilbert T. Gutierrez, Jr. <
mailing-lists@phoenixinternet.net> wrote:

>  NAT is the reason. The ping is being translated from one network to
> another as well as telnet. Going the other way, you have no rules to pass
> the communication through.
>
> Gilbert
>
>
> On 7/18/2014 2:44 PM, Michael Havens wrote:
>
> so according to your tutorial 192.168.0.x is not on the same subnet as
> 192.168.1.x. If that is correct why can I ssh (and ping and telnet....)
> from client to host but not host to client?
>
> :-)~MIKE~(-:
>
>
> On Fri, Jul 18, 2014 at 12:30 PM, Michael Havens <bmike1@gmail.com> wrote:
>
>>  telnet localhost 22 from the server received no answer from the client
>> telnet 192.168.1.101 22 from the client received no answer from the server
>>
>>  I'll get back to you about the research project
>>   (and as a private message)
>>
>> :-)~MIKE~(-:
>>
>>
>> On Fri, Jul 18, 2014 at 6:41 AM, <kitepilot@kitepilot.com> wrote:
>>
>>> Hello Michael:
>>> the 'Net' is a hodgepodge of protocols, all abiding to the 'OSI Layer
>>> Model' to work properly (http://en.wikipedia.org/wiki/OSI_model).
>>> Troubleshooting your SSH connection should be a fairly simple
>>> proposition, because there are only so many moving parts (Three!).
>>> As anything under the OSI model, nothing on an upper layer will work
>>> unless the necessary components of the lower layer are working.
>>> AND you *HAVE* to troubleshoot each layer separately.
>>> So how does this go?
>>> Well, lets take a look at your SSH problem...
>>> 1.- In order for the SSH connection to work you need 3 things:
>>> 1.1.- a SSH server,
>>> 1.2.- a SSH client and,
>>> 1.3.- a TCP/IP connection.
>>> *EACH* one of the lines above is a separate project and *HAS* to be
>>> addressed as such.
>>> Lets cover the basics first, the TCP/IP connection:
>>> You *HAVE* to *KNOW* The Mantra:
>>> "In order for any 2 devices to establish a TCP connection they have to
>>> share a physical link and they need addresses in the same subnet".
>>> The statement above is a pretty dense one, and has several implications,
>>> number one being: What does "subnet" mean?
>>> Another is: what about IPs in different subnets?
>>> We'll get there...
>>> As there are already several books written (and to be written) about the
>>> few lines above, I'll water it down to the bare minimum:
>>> The subnet is defined via the netmask, and implies that "ON" parts of
>>> the netmask are always equal in all the addresses on a network segment, so:
>>> Network:
>>> 192.168.0.0/24 or
>>> 192.168.0.0 with netmask 255.255.255.0 means that
>>> *ALL* the addresses in *THIS* network are going to look like
>>> 192.168.0.${SOMETHING_ELSE}
>>> '192.168.0' is the "Network", and "${SOMETHING_ELSE}" is the "Host".
>>> You can not use "Host 0" (because that defines the network) and you can
>>> not use the highest number (255) because that's the 'broadcast address'.
>>> Which means that any '/24" (slash 24) network can address 254 'hosts'.
>>> Network:
>>> 192.168.0.0/16 or
>>> 192.168.0.0 with netmask 255.255.0.0 means that
>>> *ALL* the addresses in *THIS* network are going to look like
>>> 192.168.${SOMETHING_ELSE}.${SOMETHING_ELSE}
>>> '192.168' is the "Network", and "${SOMETHING_ELSE}.${SOMETHING_ELSE}" is
>>> the "Host".
>>> You can not use "Host 0.0" (because that defines the network) and you
>>> can not use the highest number (255.255) because that's the 'broadcast
>>> address'.
>>> Which means that any '/16" (slash 16) network can address 65534 'hosts'.
>>> The reason why '255' is the highest number is because IPv4 addresses
>>> (and netmasks) are represented in memory in 4 bytes, each number one byte.
>>> Bytes are 8 bits, but that's a different book that you need to read too,
>>> lets move on with the network.
>>> Things get pretty interesting (and math pretty convoluted) when you
>>> define networks like 192.168.127.0/25
>>> If yo want to see all variations, you can be lazy (like me) and run:
>>> ipcalc 192.168.0.127/25
>>> Finally, "Netmasks" are a patch to the first defined (and shortsighted)
>>> 'Address Type' as class A,B,C or D, but I'll let you research that
>>> yourself.
>>>
>>> Well, that's all good, but how do you talk to other addresses?, I talk
>>> to google.com...
>>> That's a valid question, but
>>> 1.- it is not part of *THIS* SSH problem and
>>> 2.- you don't 'talk to google'.
>>> We'll talk more about how devices find each other in a network down
>>> below, but in order to talk to devices outside your network you need the
>>> 'Routing Protocol' (implemented at [SURPRISE!] 'the router') which is
>>> nothing else than a table of rules stating 'this IP goes that way'.  In
>>> your case, all addresses go the same place (the router) so the router
>>> becomes the 'Default Gateway'.  As to resolve google, you need the DNS, but
>>> you knew that...   :)
>>>
>>> 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?)
>>>
>>>
>>> Michael Havens writes:
>>>
>>>> okay, so I bought a used computer to do Linux from scratch on. Well, I'm
>>>> going to ssh from my primary computer to the new computer but got a
>>>> 'Connection timed out' error. After googling for a bit I discovered ufw
>>>> was
>>>> to blame.
>>>> after I disabled the firewall I could ssh from 192.168.1.101 <parasite>
>>>> to
>>>> 192.168.0.4 <host>
>>>> the error I got going the other way was the connection timed out error:
>>>> ssh mike@192.168.1.101
>>>> ssh: connect to host 192.168.1.101 port 22: Connection timed out
>>>> After googling some more I thought perhaps openssh-server wasn't
>>>> installed... but it is. So please.... what is the problem? I verifed
>>>> openssh-client is installed but I don't know what it could be. Could you
>>>> help me out?
>>>> :-)~MIKE~(-:
>>>>
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>>
>>
>
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