CHAPTER 3 105Secondary ServerFigure 99. Dynamic DNS Update - Secondary ServerA secondary DNS server can be used e.g. if no connection to theprimary DNS server can be accomplished. An alternative DNS ser-vice with identical or different host name can be used for the sec-ondary DNS server.Note: In principle, a secondary DNS server is not necessary for adynamic DNS update.SSH TunnelThe SSH tunneling enables other hosts on the internet to connectto the A850 by using a SSH server as a proxy, even if the A850 hasno official IP address or cannot be directly reached from the inter-net, as long as the following preconditions are met:• The A850 must be able to initiate SSH connections to a SSHserver which can be reached from the internet. If there is afirewall between the A850 and the internet, connections tothe SSHserver's IP address and port must be allowed by thefirewall.• The SSH server must support SSH version 2 and allow remoteport forwarding without a parallel shell connection.• The user must create an account on the SSH server.The A850 supports two SSH tunnel connections: a primary which istried first and a secondary which is only used if the primary con-nection can't be established. Both SSH servers must supportremote port forwarding (for more than one port if necessary).Note: You can also perform the Console Commands of the A850from a remote PC through a SSH tunnel if you define a“Remote Telnet port” at the SSH server.
