Archive for the ‘ How tos ’ Category

How to copy to a clipboard in Linux

Problem statement – You have a file with hundreds or thousands of lines and you want to copy the contents of this file and paste it to an external application, for instance to a browser.

Solution – The first attempt is to try to cat the file and scroll down with your mouse to select each line. This is time consuming or in some cases might not work if there are too many lines as some of the lines will ‘scroll out of the terminal’. One way of getting around this is to use “xclip” – a command line interface to X selections (clipboard).

In my case I wanted to copy the contents of ‘/tmp/ipaddresses.txt’ file to a browser for blogging. The file had 10000 lines. I used the following commands, first to install xclip and then to copy the file contents to a clipboard –

apt-get -y install xclip
xclip -sel cli < /tmp/ipaddresses.txt

The xclip command basically does a selection (-sel) from the file into the clipboard(-cli), where you can copy paste to any other external application.


How to fake or spoof x-forwarded-for header

The x-forwarded-for header is a way of identifying the IP address of the original client when a web server is sitting behind a proxy or load-balancer. The load-balancer does get the actual client IP as it directly sets up the TCP session with the load-balancer. But the x-forwarded-for address might contain a list of comma separated IP addresses in addition to the immediate client IP. It is these extra IPs that we can spoof and the procedure is similar to modifying any HTTP header such as user agent.

import requests
r = requests.get('', headers=headers)
if r.ok:

How the log likes like on an nginx access log –, - - [19/Mar/2017:16:43:55 -0700] "GET /index.html HTTP/1.0" 200 1311 "-" "python-requests/2.2.1 CPython/2.7.6 Linux/3.13.0-121-generic", - - [19/Mar/2017:16:53:55 -0700] "GET /index.html HTTP/1.0" 200 1311 "-" "python-requests/2.2.1 CPython/2.7.6 Linux/3.13.0-121-generic", - - [19/Mar/2017:16:58:55 -0700] "GET /index.html HTTP/1.0" 200 1311 "-" "python-requests/2.2.1 CPython/2.7.6 Linux/3.13.0-121-generic"

The take away is not to trust any IPs in the x-forwarded-for list apart from the load balancer IP and the immediate client IP which made a direct call to the load balancer. If we trust our load balancer, we can also reliably identify the immediate client IP. The rest of the IPs in the x-forwarded-for list can be ignored.

References –

git – add local files to a git repository in local file system (bare git repo).

In this blow, I will show you how you can turn your local files into a github style repository. In my case I had files in `/etc/puppet` that I wanted to version control, but I wanted to push to a bare repository in the same machine or localhost. Here are the steps I followed –

Files to version control : /etc/puppet
Bare git repository that we will push changes in /etc/puppet : /var/lib/puppet/gitrepo/

1. Create a github style git repository in /var/lib/puppet/gitrepo

root@linubuvmb:/# mkdir -p /var/lib/puppet/gitrepo && cd /var/lib/puppet/gitrepo
root@linubuvmb:/var/lib/puppet/gitrepo# git --bare init
Initialized empty Git repository in /var/lib/puppet/gitrepo/

2. Initialize files as git repository

root@linubuvmb:/# cd /etc/puppet
root@linubuvmb:/etc/puppet# git init
Initialized empty Git repository in /etc/puppet/.git/
root@linubuvmb:/etc/puppet# git add .
root@linubuvmb:/etc/puppet# git commit -m 'First commit'
[master (root-commit) b71ef42] First commit
 50 files changed, 3913 insertions(+)
 create mode 100644 auth.conf
 create mode 100644 environments/example_env/README.environment
 create mode 100755 etckeeper-commit-post
 create mode 100755 etckeeper-commit-pre
 create mode 100644 fileserver.conf
 create mode 100644 manifests/base.pp
 create mode 100644 manifests/nodes.pp
 create mode 100644 manifests/site.pp
 create mode 100644 modules/apache/manifests/init.pp

3. Add bare repo as remote

root@linubuvmb:/etc/puppet# git remote add origin file:///var/lib/puppet/gitrepo/

4. Push to local git repository

root@linubuvmb:/etc/puppet# git push -u origin master
Counting objects: 84, done.
Delta compression using up to 2 threads.
Compressing objects: 100% (70/70), done.
Writing objects: 100% (84/84), 129.33 KiB | 0 bytes/s, done.
Total 84 (delta 6), reused 0 (delta 0)
To file:///var/lib/puppet/gitrepo/
 * [new branch]      master -> master
Branch master set up to track remote branch master from origin.
root@linubuvmb:/etc/puppet# git status
On branch master
Your branch is up-to-date with 'origin/master'.

Changes not staged for commit:
  (use "git add <file>..." to update what will be committed)
  (use "git checkout -- <file>..." to discard changes in working directory)

        modified:   puppet.conf

no changes added to commit (use "git add" and/or "git commit -a")
root@linubuvmb:/etc/puppet# git commit -a
[master f57997d] test
 1 file changed, 1 deletion(-)
root@linubuvmb:/etc/puppet# git status
On branch master
Your branch is ahead of 'origin/master' by 1 commit.
  (use "git push" to publish your local commits)

nothing to commit, working directory clean

Reference –

List shared or dynamic libraries required by a program

In Linux, the


is used to find out the shared libraries or dependencies required by a program if it is a dynamic executable. ldd requires the full path to the executable as input.

For instance, the Linux ps command depends on the following shared or dynamic libraries –

[root@kauai rtc0]# ldd $(which ps) =>  (0x00007ffeb6277000) => /lib64/ (0x0000003ef6200000) => /lib64/ (0x0000003ef4e00000) => /lib64/ (0x0000003ef4a00000) => /lib64/ (0x0000003ef5600000)
	/lib64/ (0x0000003ef4600000)

You can also use the ldd command to find out if an executable has an expected dependencies. In this case, we expect that the htpasswd, login and sshd commands depend on the crypt library as they prompt a user for a password for authentication purposes –

[root@kauai rtc0]# ldd $(which htpasswd) |grep crypt => /lib64/ (0x00007f010c8ab000)

[root@kauai rtc0]# ldd $(which login) | grep crypt => /lib64/ (0x0000003efd200000)

[root@kauai rtc0]# ldd $(which sshd) | grep crypt => /usr/lib64/ (0x00007ffb0b1f2000) => /lib64/ (0x00007ffb0a988000) => /lib64/ (0x00007ffb0a015000)

References –

Getting date from Real time clock (RTC) without using the date command or any other Linux time related commands.

In Linux, the “Real Time Clock” tracks wall clock time and is battery backed so that it works even with system power off. The RTC has no concept of time zone or daylight saving, it defaults to UTC. One of the user interfaces that the Linux Kernel exposes is


and we will use the files in that directory to directly read time related data from the RTC.

* Files –

[root@ns3 rtc0]# ls /sys/class/rtc/rtc0
date  dev  device  hctosys  max_user_freq  name  power  since_epoch  subsystem  time  uevent  wakealarm

* Date and time in UTC

[root@ns3 rtc0]# cat date
[root@ns3 rtc0]# cat time

* The maximum interrupt rate an unprivileged user may request from this RTC.

# cat max_user_freq

* The name of the RTC corresponding to this sysfs directory

[root@ns3 rtc0]# cat name

* The number of seconds since the epoch according to the RTC

[root@ns3 rtc0]# cat since_epoch

* Status information is reported through the pseudo-file /proc/driver/rtc

[root@ns3 rtc0]# cat /proc/driver/rtc
rtc_time        : 23:06:58
rtc_date        : 2015-01-19
alrm_time       : 01:00:02
alrm_date       : ****-**-**
alarm_IRQ       : no
alrm_pending    : no
24hr            : yes
periodic_IRQ    : no
update_IRQ      : no
HPET_emulated   : no
DST_enable      : no
periodic_freq   : 1024
batt_status     : okay

References –

Real Time Clock (RTC) Drivers for Linux

Nginx / Apache – log real client IP or x-forwarded-for address.

Web servers such as Nginx or Apache when configured as reverse proxy behind a load balancer, they log the IP address of the load balancer in the access logs as the source IP. For practical use cases, you will usually want to log the actual client IP addresses.

In this setup, Nginx is setup to mimic a load balancer (reverse proxy) with multiple Apache web servers as backend.

1. Nginx snippet configuration to set x_forwarded_for proxy header –

server {
  listen 80;
  listen 443 default ssl;
  proxy_set_header Host $host;
  proxy_set_header X-Real-IP $remote_addr;
  proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;

2. Apache snippet Configuration to capture x_forwarded_for header in the access logs –

<VirtualHost *:443>
    DocumentRoot /var/www/homenet
    LogFormat "%h %l %u %t \"%r\" %>s %b \"%{Referer}i\" \"%{User-Agent}i\"" combined
    LogFormat "%{X-Forwarded-For}i %l %u %t \"%r\" %>s %b \"%{Referer}i\" \"%{User-Agent}i\"" proxy
    SetEnvIf X-Forwarded-For "^.*\..*\..*\..*" forwarded
    CustomLog "logs/" combined env=!forwarded
    CustomLog "logs/" proxy env=forwarded

Before making the above custom changes , the logs showed the load balancer IP only – - - [19/Mar/2015:16:21:10 -0700] "GET /signup.php HTTP/1.0" 200 1237 - - [19/Mar/2015:16:21:11 -0700] "GET /login.php HTTP/1.0" 200 1715

After the change the client IP ( was logged – - - [19/Mar/2015:16:26:43 -0700] "GET / HTTP/1.0" 200 1311 "" "Mozilla/5.0 (X11; Ubuntu; Linux i686; rv:35.0) Gecko/20100101 Firefox/35.0" - - [19/Mar/2015:16:26:44 -0700] "GET /signup.php HTTP/1.0" 200 1237 "" "Mozilla/5.0 (X11; Ubuntu; Linux i686; rv:35.0) Gecko/20100101 Firefox/35.0"

References –