Telnet service_limit error
Previously I posted Inactive TELNET session timeout, How To Check Number of SSH and Telnet session and today I would like to share something related to TELNET again. In fact, I know telnet is NOT secure.
Referring to /var/log/xinetd.log(telnet parked under xinetd services), if you found something look like FAIL: telnet service_limit mean telnet instances are over limit.
09/8/26@10:43:23: FAIL: telnet service_limit from=192.168.10.180
09/8/26@10:43:23: FAIL: telnet service_limit from=192.168.10.180
09/8/26@10:43:24: FAIL: telnet service_limit from=192.168.10.180
09/8/26@10:43:24: FAIL: telnet service_limit from=192.168.10.180
09/8/26@10:43:25: FAIL: telnet service_limit from=192.168.10.180
09/8/26@10:43:25: FAIL: telnet service_limit from=192.168.10.180
09/8/26@10:43:26: FAIL: telnet service_limit from=192.168.10.180
09/8/26@10:43:26: FAIL: telnet service_limit from=192.168.10.180
09/8/26@10:43:27: FAIL: telnet service_limit from=192.168.10.180
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How To Check Which Port Is Listern or Open on Linux
For some security reason you may configure SSH or any other protocol using different kind of port number on Linux server. Sometimes it’s important to know which ports are listern or open to the system network, it may open for network instruction or hacking.
Basically there are few methods to see which ports are open on Linux.
“How to check and open ports in Linux”
Option 1:
Check /etc/services file
planetmy:/ # cat /etc/services | grep xxx (xxx = port number)
If the command return no output mean no port configure to listen on the particular port number. For port SSH/22, you should be able to see:
ssh 22/tcp # SSH Remote Login Protocol
ssh 22/udp # SSH Remote Login Protocol
Option 2:
Use netstat command – Print network connections, routing tables, interface statistics, masquerade connections, and multicast memberships.
planetmy:/ # netstat -nan | grep 22
tcp 0 0 0.0.0.0:22 0.0.0.0:* LISTEN 7110/sshd
If the command output return ‘LISTEN’, mean the particular port is open or listen on network.
Option 3:
use lsof command – list open files
planetmy:/ # lsof -i -n -P|grep 631
cupsd 17934 lp 0u IPv4 56540196 TCP *:631 (LISTEN)
cupsd 17934 lp 2u IPv4 56540197 UDP *:631
Read more
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How To Check Number of SSH and Telnet session
How to check number of Telnet/SSH session:
Telnet
netstat | grep telnet -c
netstat -n | grep :23 -c
SSH
netstat |grep ssh -c
netstat -n | grep :22 -c
Read more
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Inactive TELNET session timeout
The previous post about “Why Telnet Not Working From Different Network” solved my stupid mistake and I have discover another interesting regarding telnet.
Normally if anyone ‘telnet’ to server box and inactive(mean without login) for 60 seconds, the system will keep disconnect automatically. Due to certain reasons, I have to extend the inactive telnet session timeout to one hour.
The solution is simple but I have a difficulty before and I would like to share with everyone.
Edit /etc/login.defs, look for LOGIN_TIMEOUT and change from 60 second to 3600 seconds which is 1 hour.
#vi /etc/login.defs
# Max time in seconds for login
LOGIN_TIMEOUT 3600
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Why Telnet Not Working From Different Network
Due to certain reason, I have to enable telnet services on one of my server for some testing today. In fact, I know telnet is NOT secure but who care?
First of all, edit “/etc/xinetd.d/telnet“, modify “disable =no” and restart XINETD with “/etc/init.d/xinetd restart” command. Run “netstat -nap |grep 23” to make sure port 23 is open. Yes telnet service is working for me now.
After a while, my colleague complain that he can’t telnet from his laptop. Weird! I found out that telnet is not accept any connection from other VLAN network.
I have check the /var/log/messages, /var/log/xinetd.log and /var/log/secure but I can’t found any clue. Too bad! Firewall is not running and I try getting help from network team and he’s confirmed that telnet protocol is not block in switch or router level within the LAN.
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