Telnet rule doesn't work

[Paul Li <paul () scybersecurity com>]

I'm using Snort 2.9.9 on Ubuntu 16.04. Trying to build a telnet login
detection rule as the following:

alert tcp $TELNET_SERVERS 23 -> $EXTERNAL_NET any (msg:"TELNET login
incorrect"; content:"Login incorrect"; nocase;classtype:bad-unknown;
sid:429; rev:2; priority:1;)


This rule looks good to me but it doesn't fire when failed TELNET occurs.
Any thing missing in this rule?

NOTE: At the same time, I created a SSH rule as the following that works
well:

alert tcp $EXTERNAL_NET any -> $HOME_NET any (msg:"SSH login attempt";
flow:to_server,established; content:"SSH-"; sid:10000002; rev:3;
classtype:attempted-user;)

Thanks,
Paul

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