Quick way to add swap – Ubuntu

 

One of the quickest way to avoid out of memory errors is by adding some swap space. Swap is an area on hard drive where operating system can temporally store data that it can no longer hold in RAM.

Use following way to add additional swap space in Ubuntu:

First check about swap space with free command:

free -m

Lets say if we need 10GB of swap, for this first create a 10GB file with following command:

sudo fallocate -l 10G /swapfile

View details about created swapfile:

ls -hl /swapfile

Output:

-rw-r–r– 1 root root 10G May 19 06:54 /swapfile

Change permissions of swapfile:

sudo chmod 600 /swapfile

Set swap space with following command:

sudo mkswap /swapfile

Check for new swap space:

sudo swapon -s

Output:

Filename                Type        Size    Used    Priority
/swapfile                               file        10485756    2700    -2

Make swap file permanent by adding it in /etc/fstab

/swapfile none swap sw 0 0

Get Thread and Heap dump – Java

 

Thread and Heap dumps are useful for debugging any Java programme.

To get thread dump:

By using jstack command we can get thread dump of a Java process.

Example:

$ jstack PID

where PID is Java process id for which we want to get thread dump.

To get heap dump:

By using jmap command we can get heap dump of a Java process.

Exmaple:

$ jmap -heap PID

where PID is Java process id for which we want to get heap dump.

ping ip with port

 

Ports are a concept of UDP and TCP. Ping messages are technically referred to as ICMP Echo Request and ICMP Echo Reply which are part of ICMP. ICMP, TCP, and UDP are “siblings”; they are not based on each other, but are three separate protocols that run on top of IP.

Therefore you can not ping a port. What you can do, is use a port scanner like nmap and check if ip/host and port are accessible.

$ nmap -p 80 www.google.com

Output:

Starting Nmap 6.40 ( http://nmap.org ) at 2015-12-02 14:38 IST
Nmap scan report for http://www.google.com (74.125.200.106)
Host is up (0.085s latency).
Other addresses for http://www.google.com (not scanned): 74.125.200.147 74.125.200.104 74.125.200.103 74.125.200.105 74.125.200.99
rDNS record for 74.125.200.106: sa-in-f106.1e100.net
PORT STATE SERVICE
80/tcp open http

Nmap done: 1 IP address (1 host up) scanned in 0.64 seconds

You can also use telnet onofri.org 80

Ubuntu 14.04 – Restore defaults of top bar in gnome session fallback

Due to some reason my top bar in gnome-panel got messed up and it not appearing as intended.

To fix this I tried to restore top bar in gnome-panel by using following command.

$ dconf reset -f /org/gnome/gnome-panel/
$ killall gnome-panel

After running above commands it appeared as intended.

Lighttpd both http and https

 

We can configure both http & https on lighttpd webserver.

Use following configuration to serve both http & https requests:

server.document-root = "/var/www"
server.upload-dirs = ( "/var/cache/lighttpd/uploads" )
accesslog.filename = "/var/log/lighttpd/access.log"
server.errorlog = "/var/log/lighttpd/error.log"
server.pid-file = "/var/run/lighttpd.pid"
server.username = "www-data"
server.groupname = "www-data"
server.port = 80

#Configuration for https
$SERVER["socket"] == ":443" {
  ssl.engine = "enable"
  ssl.pemfile = "/etc/lighttpd/certs/www.example.com.pem"
}

In above configuration http://www.example.com.pem file should contain both the private key and the certificate.

After updating lighttpd.conf file restart lighttpd:

$ service lighttpd restart

After restarting both http & https should work for your host.

iptables start on boot – Ubuntu/Debian

 

By default Ubuntu/Debian OS wont load iptable on boot.

So to load iptables on boot use follow procedure:

Save iptable conf with iptables-save command:

iptables-save > /etc/iptables.conf

Next, open file with name /etc/network/if-up.d/iptables and add following content:

#!/bin/sh
iptables-restore < /etc/iptables.conf

Change permission file /etc/network/if-up.d/iptables as executable with following command:

chmod +x /etc/network/if-up.d/iptables

Now, iptables will restored on boot.

Whenever you change iptable rules don’t forgot to update iptables.conf file. Use following command to update iptables.conf file:

iptables-save > /etc/iptables.conf

Crontab Hack – run every 5th, 15th, 25th … mins

To set crontab to run on every 5th, 15th, 25th, 35th, 45th, and 55th use following command:

5-55/10 * * * * command

This will execute your command on every 10th minute between 5 mins to 55 mins.

Python – ImportError: No module named memcache

While trying to connect memcached using python I got following error:

ImportError: No module named memcache

Since I imported memcache & it’s not available in my server I got the error message saying that “No module named memcache”.

To resolve this issue we need to install python-memcache module.

Use following command to install python-memcache:

$ pip install python-memcache

If  your OS is Ubuntu/Debian following command also works for you:

$ apt-get install python-memcache

After installing python-memcache we can seamlessly connect to memcached.

Ubuntu desktop change nameservers permanently

To change nameservers permanently in Ubuntu Desktop Version we need to update /etc/resolvconf/resolv.conf.d/head file with desired list.

Following is the procedure to change nameservers permanently:

Open /etc/resolvconf/resolv.conf.d/head file with your favourite editor:

sudo vim /etc/resolvconf/resolv.conf.d/head

Add following lines to /etc/resolvconf/resolv.conf.d/head, save & exit from the file

nameserver 4.2.2.2
nameserver 208.67.222.222

Run following command to change nameservers permanently:

sudo resolvconf -u

After running above commands nameservers in your desktop version will get updated with new list as mentioned in /etc/resolvconf/resolv.conf.d/head file.
To conform this open /etc/resolve.conf and validate.

fatal error: Python.h: No such file or directory

While trying to install some python module I got following exception:

fatal error: Python.h: No such file or directory

Reason for exception:

If you haven’t properly installed the header files and static libraries for python dev this issue may occur.

To resolve this issue install python-dev module by using following command:

$ sudo apt-get install python-dev