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Building Embedded Linux Systems by Karim Yaghmour

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Secure Communication with SSH

Though you can easily communicate with your target using Telnet, it is a very insecure protocol and its vulnerabilities are widely documented. The user password, for instance, is transmitted in clear text from the client to the server. It would therefore be rather unprudent, and in most cases downright dangerous, to include a Telnet daemon in your product in the hopes of being able to remotely fix problems once the product is at the client's site. Instead, it would be much preferable to use a protocol that relies on strong encryption and other mechanisms to ensure the communication's confidentiality. The best way to do this currently is to use the SSH protocol and related tool suite. SSH uses public-key cryptography to guarantee end-to-end communication encryption while being fairly easy to use and deploy.

Because SSH is an IETF standard, there are a few competing implementations, some of which are proprietary commercial products. The main open source implementation is OpenSSH. Although there are other open source implementations, they are either very difficult to cross-compile or have dependencies that make them impractical for use in embedded Linux systems. I will therefore devote most of this section to discussing OpenSSH. We will briefly review the other open source implementations, because they may eventually become usable in embedded Linux systems.

An embedded system that can be accessed through SSH runs the same SSH server software usually ...

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