Download Proxy-Server Based Firewalls Lecture Notes on

Computer and Network Security by Avi Kak
Lecture 19
• The call to socksify as shown above invokes a shell script of that
name (that resides in /usr/bin/ in a standard install of Dante).
Basically, all it does is to set the LD_PRELOAD environment variable to the libdsocks library that resides in the libdsocks.so
dynamically linkable file.
• By setting the LD_PRELOAD environment variable (assuming your
platform allows it), ‘socksify’ saves you from the trouble of having
to recompile your client application so as to redirect the system
networking calls to the proxy server. [As explained in the ‘README.usage’ document that comes with the Dante install, this only works with non-setuid applications. The LD PRELOAD
environment variable is usually ignored by setuid applications. When a previously written client application can be compiled and linked to dynamically, you can socksify it by linking it with the libdsocks
shared library by supplying the linking command with the ‘-ldsocks’ option assuming that the file
libdsocks.so is at the standard location (otherwise, you must provide the pathname to this location with the ‘-L pathname’ option). If such dynamic linkage is not possible, you can always resort to
static recompilation of your client application. See the file ‘README.usage’ mentioned above for further
information on how to do this.
]
• All of the presentation so far has been from a Linux perspective. There is an implementation of the socks protocol, called
SocksCAP, that enables Windows based TCP and UDP networking clients to traverse a socks firewall. Visit http://www.socks.
permeo.com/ for further information.
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