List shared or dynamic libraries required by a program

In Linux, the

ldd

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)
	linux-vdso.so.1 =>  (0x00007ffeb6277000)
	libselinux.so.1 => /lib64/libselinux.so.1 (0x0000003ef6200000)
	libproc-3.2.8.so => /lib64/libproc-3.2.8.so (0x0000003ef4e00000)
	libc.so.6 => /lib64/libc.so.6 (0x0000003ef4a00000)
	libdl.so.2 => /lib64/libdl.so.2 (0x0000003ef5600000)
	/lib64/ld-linux-x86-64.so.2 (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
	libcrypt.so.1 => /lib64/libcrypt.so.1 (0x00007f010c8ab000)

[root@kauai rtc0]# ldd $(which login) | grep crypt
	libcrypt.so.1 => /lib64/libcrypt.so.1 (0x0000003efd200000)

[root@kauai rtc0]# ldd $(which sshd) | grep crypt
	libcrypto.so.10 => /usr/lib64/libcrypto.so.10 (0x00007ffb0b1f2000)
	libcrypt.so.1 => /lib64/libcrypt.so.1 (0x00007ffb0a988000)
	libk5crypto.so.3 => /lib64/libk5crypto.so.3 (0x00007ffb0a015000)

References –

http://man7.org/linux/man-pages/man1/ldd.1.html

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

 /sys/class/rtc/rtc{N} 

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
2015-01-19
[root@ns3 rtc0]# cat time
23:05:05

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

# cat max_user_freq
64

* The name of the RTC corresponding to this sysfs directory

[root@ns3 rtc0]# cat name
rtc_cmos

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

[root@ns3 rtc0]# cat since_epoch
1421708627

* 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

Update – Eritrean Ethiopian Internet radio

The server hosting the Internet radio for Eritrean and Ethiopian mostly Tigrigna music has been migrated to a new infrastructure and thus the public Internet IP address of the streaming radio has changed. Please use this URL to get the latest streaming address or save the below updated streaming playlist file –

.
NumberOfEntries=1
File1=http://162.247.78.0:8000/test.mp3
Title1=Eritrean Ethiopian - Tigrigna
Length1=-1
Version=2

Download the stream playlist in Linux –
wget -O tigrigna-music.pls http://linuxfreelancer.com:8000/listen.pls

Tag – Eritrean music.
        

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>
    ServerAdmin webmaster@home.net
    DocumentRoot /var/www/homenet
    ServerName todo.home.net
...
    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/todo.home.net-ssl_access_log" combined env=!forwarded
    CustomLog "logs/todo.home.net-ssl_access_log" proxy env=forwarded
 </Virtual>

Before making the above custom changes , the logs showed the load balancer IP 192.168.10.162 only –

192.168.10.162 - - [19/Mar/2015:16:21:10 -0700] "GET /signup.php HTTP/1.0" 200 1237
192.168.10.162 - - [19/Mar/2015:16:21:11 -0700] "GET /login.php HTTP/1.0" 200 1715

After the change the client IP (192.168.10.105) was logged –

192.168.10.105 - - [19/Mar/2015:16:26:43 -0700] "GET / HTTP/1.0" 200 1311 "https://todo.home.net/login.php" "Mozilla/5.0 (X11; Ubuntu; Linux i686; rv:35.0) Gecko/20100101 Firefox/35.0"
192.168.10.105 - - [19/Mar/2015:16:26:44 -0700] "GET /signup.php HTTP/1.0" 200 1237 "https://todo.home.net/" "Mozilla/5.0 (X11; Ubuntu; Linux i686; rv:35.0) Gecko/20100101 Firefox/35.0"

References –

https://www.nginx.com/resources/wiki/start/topics/examples/likeapache/

Google Chrome browse is not using my host DNS settings.

Environment –
Operating System : CentOS release 6.8
Google Chrome: Version 55.0.2883.87 (64-bit)

I have an Intranet DNS server with internal domain name. The domain name is resolved internally by my DNS server to an internal private IP address. With Firefox I could always visit my internal site without issues, but recently I installed Chrome browser into my CentOS desktop and when I tried to visit my internal site, it was directing me to an Internet site which I don’t own. Apparently Google Chrome was ignoring my dns setting and using its own name servers.

My first attempt – flush DNS on Chrome (failed)
I went to the DNS configuration for chrome and cleared host cache. The dns settings clearly showed my ‘nameservers’ as the 192.168.1.1 (my internal name servers) and yet Chrome was not using it. Even after cleaning the host cache and flushing sockets, it didn’t help.

Chrome setting to view and manage DNS settings –

chrome://net-internals/#dns

My second attempt – block IPv6 (worked)
After running tcpdump on port 53, Chrome was calling an IPv6 address 2001:4860:4860::8888 to resolve domains.

Added below lines to

 /etc/sysctl.conf 

in order to disable IPv6 temporarily for a test –

net.ipv6.conf.all.disable_ipv6 = 1
net.ipv6.conf.default.disable_ipv6 = 1

And executed the command

 sysctl -p 

to apply the new kernel settings. After this change and flushing dns, I was able to visit my internal site.

I am guessing the DNS IPs used by Chrome as somehow internally coded, and haven’t been able to find those settings.

References –

https://www.reddit.com/r/chrome/comments/1xj69t/chrome_ignoring_system_dns_and_using_google_dns/

https://www.howtogeek.com/197804/how-to-clear-the-google-chrome-dns-cache-on-windows/