Telnet 相对于 SSH 的优势？

Question

SSH is the recommended protocol for remote configurations. Still, telnet is being used in many devices. Are there any advantages of telnet over SSH?

Answer 1

Hmm, let's see:

• Telnet is simpler (as others have noted already);

• Telnet is obviously faster, as the protocol is much more trivial and there is no key exchange and no encryption involved;

• Telnet is less vulnerable

Wait, stop, WHAT !?

Well, yes, telnet protocol is plain-text, so you can just sniff the connection and now you know the password and everything else. And that is a well known fact indeed.

How hard would it be to actually sniff the particular telnet session, depends on the network setup and a bunch of other things, and might range from being completely trivial to extremely hard to do.

But aside the (obvious) lack of encryption, when it comes to protocol and the service implementation(s) itself, which one is less vulnerable in overall ?

Let's take a look in the CVE database:

Telnet: https://cve.mitre.org/cgi-bin/cvekey.cgi?keyword=telnet

There were 5 vulnerabilities, registered in 2016, 3 of them are just "hardcoded credentials", which is more of a vendor error than a real service implementation or protocol flaw.

Now, SSH: https://cve.mitre.org/cgi-bin/cvekey.cgi?keyword=ssh

25 (!) vulnerabilities (year 2016), ranging from the "hard-coded" credentials to allowing the selection of low-security encryption algorithms, issues which allow for denial-of-service attacks or reading the private keys from the remote process memory and so on.

So there were obviously many more SSH related vulnerabilities than Telnet vulnerabilities detected in 2016, and Telnet is a winner here.

That is actually pretty logical, taking that SSH is a much more complex protocol, and a typical SSH implementation will have many more features, like X11 forwarding, file transfer, tunnels etc,requiring a more complex code, and making a much wider "attack surface"

Please take the above with a grain of salt, Telnet is still plain-text and it is widely regarded as an outdated protocol, so you definitely have to use a decent SSH implementation instead.

Just make sure that it is configured properly (eg switch off features you are not going to use), and keep it up to date at all times.

At the same time, you have to remember that sometimes "obvious things" are not always that "obvious", when you look at them at a bit different angle, and that is the point of this post.

Answer 2

telnet 可能更简单，但也可能是历史原因。