Category: Linux Server Administration Tips

I was testing the WP Mail SMTP plugin to send emails in WordPress via SMTP servers but when it was trying to connect to the remote SMTP server, it kept getting error “Connection refused”.

So I tried:

telnet smtp.xxxx.com 587

To test the connection. Turned out my own server is preventing the outgoing request because of the permission denied error by CSF as I didn’t have enough privilege as an ordinary user of the server:

grep -i smtp_block /path/to/csf.conf

grep: /path/to/csf.conf: Permission denied

Needless to say, the telnet connection successfully went through to the remote SMTP server when I logged in as root.

So the solution is simple. Just add the user to /path/to/csf.conf for SMTP privilege:

SMTP_ALLOWGROUP = "mail,mailman,youruser"

That’s it. Now youruser is capable of connecting to remote SMTP servers.

For example, a command like this could run a few days if there are so many images:

tar zcf xxxx.images.tgz *

So it’s best to put it in background when started, by nohup:

nohup tar zcf xxxx.images.tgz * > nohup.log 2>&1 &

If you have multiple servers you’d probably need rsync to transfer files among servers via SSH. An error like this, however, will occur when CSF protects the servers against malicious SSH connection attempts:

ssh: connect to host example.com port 9999: Connection refused
rsync: connection unexpectedly closed (0 bytes received so far) [Receiver]
rsync error: unexplained error (code 255) at io.c(226) [Receiver=x.x.x]

The solution is very simple. Just whitelist each server IP on the other server and vice versa:

csf -a 1.1.1.1

Wherein 1.1.1.1 is the other server. And perform the same:

csf -a 2.2.2.2

On the other server.

That’s it. Now you can freely SSH from and to the 2 servers.

When I ssh into my Debian Squeeze server and start up tail -f to watch a log file or anything else which uses ctrl-c to exit/stop, ctrl-c does not work. It prints the ^C character on screen and just keeps right on going.

Is there a setting somewhere that can be tweaked or maybe a different key combo that needs pressed?

Search results suggest that it’s a pretty common problem to Debian and that tinkering with the getty settings in /etc/inittab can fix it, but I’m hesitant to mess around in there too much. I may just take a snapshot and then see what happens.

Definitely an inittab thing, in case anyone else runs into this.
This line used to be at the top of the getty stuff

Code:

8:2345:respawn:/sbin/getty 38400 hvc0

I just moved it to the bottom and now ctrl-c works when connected using ssh.

Code:

 1:2345:respawn:/sbin/getty 38400 tty1
 2:23:respawn:/sbin/getty 38400 tty2
 3:23:respawn:/sbin/getty 38400 tty3
 4:23:respawn:/sbin/getty 38400 tty4
 5:23:respawn:/sbin/getty 38400 tty5
 6:23:respawn:/sbin/getty 38400 tty6
 8:2345:respawn:/sbin/getty 38400 hvc0

To delete all content in any directory, including all sub-directories and files, I’ve been using this:

rm -rf somedir/*

If it is to delete all content of the current directory:

rm -rf *

However, it turns out ‘rm -rf’ doesn’t remove hidden files such as .htaccess (Files with a name starting with a dot are hidden in Linux). To delete all the hidden files as well, I have to run a 2nd command:

rm -rf .??*

The ‘find’ command in Linux systems searches through a directory and return files that satisfy certain criteria. For instance, to find a file that contains the string ‘needle text’ in the ‘mydocs’ directory:

find mydocs -type f -exec grep -l "needle text" {} \;

The problem of this approach is that it would search through ALL files in this directory including the binary ones such as images, executables and zip packages. Sensibly, we would only want to search through text files for a specific string. If there are far too many of binary files in present, it’d be a significant waste of CPU usage and time to get what you want, because it’s totally unnecessary to go through the binary files.

To achieve this, use this version of the above command:

find mydocs -type f -exec grep -l "needle text" {} \; -exec file {} \; | grep text | cut -d ':' -f1

I asked the question at stackoverflow.com and peoro came up with this solution. It works great.

Basically, the bold part checks each file’s mime type information and only searches the files that have ‘text’ in its mime type description. According to the Linux ‘file’ command manual, we can be fairly sure that files with ‘text’ in its mime type string are text files AND all text files have ‘text’ in its mime type description string.

Thus far the best way to do this

find -type f -exec grep -Il . {} \;

Or for a particular needle text:

find -type f -exec grep -Il "needle text" {} \;

The -I option to grep tells it to immediately ignore binary files and the . option along with the -l will make it immediately match text files so it goes very fast.