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Linux - Security This forum is for all security related questions.
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Old 05-30-2001, 09:34 PM   #1
Registered: Feb 2001
Location: MI, USA
Distribution: Debian Linux 100% GNU
Posts: 210

Rep: Reputation: 31

I accidently stumbeled on something here....
I wanted to access my server via telnet from my other computer connected vi different internet interface, but I hated to turn on telnet port for security reasons of course.
What I did is actually turned it on for about 30 seconds just long enough to login to it then I went back after I loged in and executed the command #/sbin/service xinetd restart after I ran ntvsys to turn it off again....The funny thing is I am still connected via telnet but no new sessions can be open again....
Any thoughs on that...How secure is my connection afterward..Is it still unsecure as if the tellnet port was still active or else .....
Old 05-31-2001, 06:08 AM   #2
Registered: Apr 2001
Location: London
Posts: 408

Rep: Reputation: 31

When a TCP based application is loaded the app has to register with TCP.
Servers do this when they issue a TCP passive open while your client does this with an active open.

Once registered the application "telnet" is linked with a virtual circuit to the TCP stack with a LISTEN state.
Your client sends a connection request, then the server issues a SYN-RECEIVED, once the handshake has been made the circuit is ESTABLISHED, so by downing Telnet this circuit is still active until your client issues a FIN flag and tares down the connection.

Answering your question:
It's secure as your connection is the only one allowed since it established the handshake with telnet.
Someone will need to try and spoof your IP + guess and correctly match your next random sequencer number in-order to hijack your connection, which on Linux is close to imposable. "windows now that's a different matter"

Old 05-31-2001, 08:08 PM   #3
Registered: Feb 2001
Location: MI, USA
Distribution: Debian Linux 100% GNU
Posts: 210

Original Poster
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Hey thanks for the very technical explanation...good stuff!!!
Anyway it was just a thought. Of course it cannot be practical..I still need to think of a good firewall security solution for my box.
Thanks again.
Old 06-01-2001, 07:07 AM   #4
Senior Member
Registered: May 2001
Location: Left Coast - Canada
Distribution: s l a c k w a r e
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get ssh (openssh). install it and comment out the telnet/ftp/rlogin stuff outta your inetd.conf.

use ssh instead of telnet
use scp instead of ftp

more words of advice for young people. (Bill Burroughs woulda told ya the same thing)
Old 06-01-2001, 05:18 PM   #5
Registered: Feb 2001
Location: MI, USA
Distribution: Debian Linux 100% GNU
Posts: 210

Original Poster
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Yea, thanks. I am gonna look into that..Although my luck the version I am gonna use will have some big security hole...What else is new!



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