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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August 26, 2009 · Filed Under Open Source, Tips

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SSH Remote Access Authentication Tips

My friend was try to setup Linux SSH public key authentication without password and he’s keep getting password prompt. I will not going to go through how to setup SSH public key authentication here and I just would like to share few troubleshoot tips in order to fix the similar problem here:

1. Checking /var/log/messages file
Basically SSH hints are written to “/var/log/messages” file, you should be able to see any error messages here. For an example “Authentication refused: bad ownership or
modes for directory /root cannot set to 775“. Perhaps you’re require to change /root folder permission to make it work (may apply to other /home folder as well).
Type #chmod 755 /root or #chmod 700 /root can solve authentication problem.

2. Permissions
Please make sure you have correct file and folder permission as suggested below.
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October 10, 2008 · Filed Under Open Source, Tips

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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
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September 12, 2008 · Filed Under How's To, Open Source, Tips

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SSH problem

I can’t login to the server using SSH and the TELNET service is NOT enable by default. Thanks god, I have DELL DRAC enable, I managed to login using console.

I’m getting an error messages in /var/log/messages and SSH connection refused error on one of my SuSE Linux box as below:

Aug 2 16:20:05 planetmy sshd[22531]: fatal: /var/lib/empty must be owned by root and not group or world-writable.
Aug 20 16:20:06 planetmy sshd[22546]: fatal: /var/lib/empty must be owned by root and not group or world-writable.
Aug 20 16:20:19 planetmy sshd[22797]: fatal: /var/lib/empty must be owned by root and not group or world-writable.
Aug 20 16:20:47 planetmy sshd[22838]: fatal: /var/lib/empty must be owned by root and not group or world-writable.
Aug 20 16:21:37 planetmy sshd[23598]: fatal: /var/lib/empty must be owned by root and not group or world-writable.

I found one of the stupid guy issue chmod 755 command in /var/log/messages:

Aug 20 16:15:37 planetmy sudo: username1 : TTY=pts/63 ; PWD=/ ; USER=root ; COMMAND=/usr/bin/ find ./ -type d -exec chmod 775 {}

Basically he try to changed all folder with permission 775(rwxrwxr-x) on Linux server.

Looking at man 8 sshd:

/var/lib/empty
chroot(2) directory used by sshd during privilege separation in the pre-authentication phase. The directory should not contain any files and must be owned by root and not group or world- writable.

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August 21, 2008 · Filed Under Tips

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How to DENY SSH access for certain user on Linux

Due to some security reason, you may require to block certain user SSH access to Linux box.

Edit the sshd_config file, the location will sometimes be different depend on Linux distribution, but it’s usually in /etc/ssh/.

Open the file up while logged on as root:

vi /etc/ssh/sshd_config

Insert a line:

DenyUsers username1 username2 username3 username4

Referring to #man sshd_config:

DenyUsers
separated by spaces. Login is disallowed for user names that
match one of the patterns. â*â and â?â can be used as wildcards
in the patterns. Only user names are valid; a numerical user ID
is not recognized. By default, login is allowed for all users.
If the pattern takes the form USER@HOST then USER and HOST are
separately checked, restricting logins to particular users from
particular hosts.

Save it and restart SSH services. Basically username1, username2, username3 & username4 SSH login is disallowed.
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August 18, 2008 · Filed Under How's To, Open Source, Tips

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