Apache mod for creating entropy.

bb1da082 add install instructions · by Georg Hopp


Apache module to generate entropy.


Add the following lines to your apache config file.

LoadModule entropy_module     modules/mod_entropy.so
SetInputFilter ENTROPY


This is an apache module the creates random values from incoming requests. It will only work on systems that have the RNDADDENTROPY ioctl, i only know of linux.

If you use encryption on your pages either for ssl, by using mcrypt for stuff or ssh, gpg, etc. you will use the systems random source. This is filled up by the kernel via some irq and user interaction. So it fills quite slowly and it is limited. And it fills even slower on a server where no Keyboard Interrupts or such will happen most of the time.

This problem is addressed by the existence of /dev/urandom which does some magic, so that the request for random numbers will never block. But the quality of these random numbers is a matter of discussion right now.

So i came up with the idea that user interachtion on a webserver (request) related to time are also entropic in nature and decided to build an apache module to fill the kernel random pool by that way. This should increase the quality as well as the responsiveness of encryption.

Actually i must confess that i have no really clue what a random number makes it cryptographically "good" and i would be happy to get some feedback on this.

Build and install

You can build and install this module as follows:

cd mod_entropy
make install


Add cap_sys_admin to apache.

You need libcap being installed on your system to do this. Assuming your apache binary is /usr/sbin/apache2 do the following as root:

setcap cap_net_bind_service,cap_sys_admin=ep /usr/sbin/apache2

And then make sure apache is started by the user configured in http.conf. This should prevent right drop while changing the effective user id. If apache2 does not start it most likely is related to some access rights. Just have a look in the error log as mentioned in this.

ATTENTION: This module adds data to the kernel random number pool. To do this the apache process needs CAP_SYS_ADMIN. Without any role based access control this is true only for the root user.

An alternative is to assign CAP_SYS_ADMIN to the apache process. This still seems not to be the ideal solution as this would give the apache process access to several system internals like de-/activation of swap devices mount/unmount, etc. Anyway, this is the best i could figure out.

It would be a good to have a special capabilty just for random pool administration but actually i have no clue if and how this might be possible. Anyway this still might lead to problems with the security of your encryption as an attacker might be able to add own random values to the random pool which in turn might compromize your encryption.

Actually i have no good solution for this...maybe it is not a good idea at all to generate random numbers this way, i would be lucky to get feedback on this issue.


  • Apache 2


Just write me an email.


This program is free software: you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation, either version 3 of the License, or
(at your option) any later version.

This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
GNU General Public License for more details.

You should have received a copy of the GNU General Public License
along with this program.  If not, see <http://www.gnu.org/licenses/>.


Georg Hopp georg@steffers.org