Openssh Configuration Files - Red Hat ENTERPRISE LINUX 3 Reference Manual

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19.3.3. Channels
After a successful authentication over the SSH transport layer, multiple channels are opened via a
technique called multiplexing
sessions and for forwarded X11 sessions.
Both clients and servers can create a new channel. Each channel is then assigned a different number
on each end of the connection. When the client attempts to open a new channel, the clients sends
the channel number along with the request. This information is stored by the server and is used to
direct communication to that channel. This is done so that different types of sessions will not affect
one another and so that when a given session ends, its channel can be closed without disrupting the
primary SSH connection.
Channels also support flow-control, which allows them to send and receive data in an orderly fashion.
In this way, data is not sent over the channel until the client receives a message that the channel is
open.
The client and server negotiate the characteristics of each channel automatically, depending on the
type of service the client requests and the way the user is connected to the network. This allows
great flexibility in handling different types of remote connections without having to change the basic
infrastructure of the protocol.
19.4. OpenSSH Configuration Files
OpenSSH has two different sets of configuration files: one for client programs (
and one for the server daemon (
System-wide SSH configuration information is stored in the
— Contains Diffie-Hellman groups used for the Diffie-Hellman key exchange which is
• moduli
critical for constructing a secure transport layer. When keys are exchanged at the beginning of an
SSH session, a shared, secret value is created which cannot be determined by either party alone.
This value is then used to provide host authentication.
— The system-wide default SSH client configuration file. It is overridden if one is
• ssh_config
also present in the user's home directory (
— The configuration file for the
• sshd_config
•
ssh_host_dsa_key
• ssh_host_dsa_key.pub
— The RSA private key used by the
• ssh_host_key
tocol.
• ssh_host_key.pub
protocol.
• ssh_host_rsa_key
protocol.
• ssh_host_rsa_key.pub
protocol.
User-specific SSH configuration information is stored in the user's home directory within the
directory:
~/.ssh/
4. A multiplexed connection consists of several signals being sent over a shared, common medium. With SSH,
different channels are sent over a common secure connection.
4
. Each of these channels handles communication for different terminal
).
sshd
~/.ssh/config
— The DSA private key used by the
— The DSA public key used by the
— The RSA public key used by the
— The RSA private key used by the
— The RSA public key used by the
/etc/ssh/
).
daemon.
sshd
daemon.
sshd
sshd
daemon for version 1 of the SSH pro-
sshd
daemon for version 1 of the SSH
sshd
daemon for version 2 of the SSH
sshd
sshd
Chapter 19. SSH Protocol
, , and
ssh scp
directory:
daemon.
for version 2 of the SSH
)
sftp