mjhjmtu

I tried installing the following package:

[root@localhost ~]# rpm -i libmicrohttpd-0.9.22-1.el7.centos.x86_64.rpm

But I got an error for failed dependencies:

[root@localhost ~]# rpm -i libmicrohttpd-0.9.22-1.el7.centos.x86_64.rpm
error: Failed dependencies:
libgnutls.so.28()(64bit) is needed by libmicrohttpd-0.9.22-1.el7.centos.x86_64
libgnutls.so.28(GNUTLS_1_4)(64bit) is needed by libmicrohttpd-0.9.22-1.el7.centos.x86_64

Naturally I tried installing these dependencies:

[root@localhost ~]# yum install libgnutls.so.28
Loaded plugins: fastestmirror
Loading mirror speeds from cached hostfile
 * base: mirrors.vooservers.com
 * epel: pkg.adfinis-sygroup.ch
 * extras: mirrors.melbourne.co.uk
 * updates: mirrors.melbourne.co.uk
Package gnutls-3.3.26-9.el7.i686 already installed and latest version
Nothing to do

But as you can see above I am told that the package is "already installed" and there is "nothing to do".

This makes no sense to me if the package is already installed then why is the installation of libmicrohttpd-0.9.22-1.el7.centos.x86_64.rpm failing because it can not find this dependency?

You have installed gnutls-3.3.26-9.el7.i686, but that is not libgnutls.so.28()(64bit), it is 32bits.

You need to install gnutls-3.3.26-9.el7.x64_86, which is the correct architecture to match what you're trying to install.

Try this,

You were installing i686 which is a 32-bit library.The error clearly states that libmicrohttpd needs a 64-bit library.

Refer this link to get the required RPM.

You can also configure yum via /etc/yum.conf to install the best suited package for the underlying architecture. The parameter to be modified in yum.conf is multilib_policy.

multilib_policy Can be set to 'all' or 'best'. 
multilib_policy=all

All means install all possible arches for any package you want to install. Therefore yum install foo will install foo.i386 and foo.x86_64 on x86_64, if it is available. Best means install the best arch for this platform, only. For more refer this page and this answer.

To know the underlying archietcture , run
uname -m

it is always a better idea to use yum directly in order to let yum manage the dependencies for you:

yum install libmicrohttpd-0.9.22-1.el7.centos.x86_64.rpm