Thursday, June 17, 2010

the linux proxy problem

For those of you who use linux for anything more than web browsing (in university/office) must be aware of the problems a proxy can pose. In many places as in my institute, you need to necessarily use a specified proxy server to access outside world, needing authentication for your credentials.
In my college, a common login registered in a central ldap server provides for all authentication services (used for course registration/fees payments/emails/proxy/...). Hence it is very important to protect it. Here i will show one way to avoid anyone easily getting your password.

Network proxy loophole in GNOME:
If you are using GNOME (default Fedora/Ubuntu) and you set your proxy details in "system->preferences->network proxy" then you open a simple loophole in the settings.
After setting your username/password, open a new terminal and type
    echo $http_proxy
Now you can clearly see your password as
Now since many people come to your rooms in colleges you can see how simple it is to get your credentials.

Is there a way out:
There may be other ways, but here's the one which i follow. I create a local forwarding proxy server on my own computer and direct all applications to use that proxy. The settings for my proxy server are written in a file only readable by the root.
What follows is a step-by-step guide to set it up. Tested on Fedora

What do i use:
I use a small proxy server 3proxy, you could also use any other proxy server such as squid. In fact i used to use squid before i came to know of 3proxy (when it was packaged in fedora). Squid is a much more feature rich and heavy proxy. When i was using it had a bug whereby it would do at least 100 cpu wakeups per second, using precious power on my laptop. This may have been fixed by now.

 On Fedora systems you can do
    yum install 3proxy
A similar command for apt-get may work on Ubuntu (i've never tried)

The configuration you need to do is

  1. Open the file /etc/3proxy.cfg in editor of your choice as root
  2. Locate the line containing 'proxy -n'
  3. Above this line, upto the line 'dnspr', comment out all uncommented lines and instead add the following lines:

    auth iponly

    allow * *,<local_IPs> * * * *
    allow * * * * * * *
    parent 1000 http <> <port> <proxy_user> <proxy_pass>
    proxy -n

    The values in angle brackets need to be replaced by you configuration The values for my college are given in parenthesis
    <local IPs> = ips not connected through proxy []
    <> = proxy server []
    <port> = proxy port [80]
    <proxy_user> = proxy authentication username
    <proxy_pass> = proxy authentication password
  4. Comment out all lines with the content:

  5. Save the file
  6. as root run (this will make the file only readable by root user)
        chmod o-rwx /etc/3proxy.cfg
        chkconfig 3proxy on
  7. ??
  8. profit
The details of the 3proxy.cfg file are documented at

Now in whichever application you need to set the proxy server, set it as

without any authentication.
Thats it, now only root knows your ldap password, and no one else can snoop

If you automatically want to set the proxy environment variable of the whole system, then you can create a file /etc/profile.d/ with the following content

export http_proxy=
export https_proxy=$http_proxy
export ftp_proxy=$http_proxy

Many (not all) programs on linux use these environment variables to get proxy settings.

To set multiple proxies (different hosts go through different proxies) you can do something like below (see 3proxy.cfg manual for much more detail and many other options):
# direct connection allow *,<local_IPs> * * # through proxy1 allow * * <hosts_thru_proxy1> * * parent 1000 http <> <port> <proxy_user>  # through proxy2 allow * * <hosts_thru_proxy2> * * parent 1000 http <> <port> <proxy_user>  # through proxy3 allow * * <hosts_thru_proxy3> * * parent 1000 http <> <port> <proxy_user> allow * * * * * proxy -n