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zmodem - what type of protocol is it..? layer 3 or layer 4 ? more...

Posted on 2001-06-06
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Last Modified: 2013-12-23
guys,

is zmodem (zmtx/zmrx) a layer 3 (like tcp/udp) or layer 4 protocol (like ftp?).. I am running a server with no telnet and only SSH2 access, and we use zmodem to transfer files to and fro our local PCs. I would like to know what type of protocol is it and how it relates to firewall rules / port numbers...

does zmodem tunnel through SSH in this case?
Please advice.
Thanks
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Question by:thiamwah
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by:jlevie
ID: 6159432
zmodem is a serial file transfer protocl and as such is completely separate from networking protocols. It was originally delevloped as a replacement for xmodem, which in turn was developed as an alternative to kermit. All of these are a means of copying files over a simple serial connection like you'd have in the case of a direct modem dial-up directly to a remote server. They were very popular and widely used before the days of dial-up networking and are quite efficient over low speed dial-up connections (<9600baud).

The most appropriate way to transfer files between a remote client and a remote server that only offers ssh access is to use scp. The putty client for windows (http://www.chiark.greenend.org.uk/~sgtatham/putty/) includes a command line scp program and there are Gui clients for windows like WinSCP (http://winscp.vse.cz/eng/).
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by:thiamwah
ID: 6162181
so how does zmodem relate to firewall rules...?
As since I am using SSH2 to telnet, when I use zmodem to transfer or put files in the server, is it tunnelled through SSH (TCP/port 22)?

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by:newmang
ID: 6162238
The short answer is that it doesn't. The firewall controls the ability to connect various addresses and ports through it, generally it doesn't look at the contents of the packets passed through it. zmodem is an application to application protocol not a comms protocol.
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jlevie earned 200 total points
ID: 6163977
newmang is quite correct, but another way of stating the relationship of zmodem to firewall rules is that since zmodem is an embedded application protocol that works "under" some other commuication protocol, it doesn't require special adjustments on a firewall. The communications protocol (ssh in this case) provides the transport conduit and zmodem simply uses that connection.
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