Telnet servers potentially lead to nonterminating acknowledgment loops

[jerrytesting]

Hello,

In the implementation of telnet servers until version 3.0 and even the latest commit 32b5b17, potential nonterminating acknowledgment loops have been found in telnet servers.

In order to prevent nonterminating acknowledgment loops, one rule made by RFC854 is that, a request must not be acknowledged if a party receives what appears to be the request to enter some mode it is already in.

However, when the negotiated options are already disabled, servers still respond to DONT and WONT requests with WONT or DONT commands. Hence, potential infinite acknowledgment loops exist in the telnet server during execution, which may lead to denial of service and excessive CPU consumption.

Could you have a check? Thanks a lot.