I move between three different network configurations with my powerbook in an average day. Two of these configurations have proxy servers and one does not. Mac OS X has really excellent network location support that lets me configure this stuff once so that switching locations is as simple as invoking QuickSilver, typing the first few letters of the network location and BAM.

Most applications automatically pick up the new proxy configuration but some do not, like Firefox (which is one of three big reasons I still use Safari). I do a lot of work from the command line with network based tools such as curl, wget, port, http_ping, links, svn, etc. None of these use the system proxy settings but most support specifying a proxy server via the http_proxy environment variable.

I’ve searched high and low for a mechanism that would handle setting the http_proxy variable based on my current network location but have come up with nothing.

First, you need to create a file /etc/http_proxy that specifies the proxy servers for each Network Location you have setup in your Network Preferences (If anyone can figure out how to get the proxy information directly please let me know. I can get the current network location but not information about it). The file might look something like this:

Work = http://proxy.example.com:80
Library = http://proxy.library.com:80

The keys are the names of your network locations and the values are in http://proxy-host:proxy-port form.

Next, you’ll need to put the following script somewhere along your $PATH named proxy-config and give it a chmod +x too.

 #!/bin/bash 
 
 # source this into your current to have the proxy 
 # environment variables needed by many command line 
 # apps setup correctly.
 
 # get the current network location name
 netloc=$(/usr/sbin/scselect 2>&1 | egrep '^ \* ' | \
          sed 's:.*(\(.*\)):\1:')
 
 # find the proxy in /etc/http_proxy based on the 
 # current location
 http_proxy=$(egrep "$netloc[ \t]*=" /etc/http_proxy | \
              sed 's/.*=[ \t]*\(.*\)/\1/')
 
 if [ -n "$http_proxy" ]; then
   export http_proxy
   export HTTP_PROXY="$http_proxy"
 else
   unset http_proxy
   unset HTTP_PROXY
 fi
 
 # the rest of this is used for symlink commands
 bn=$(basename $0)
 if [ "$bn" != "proxy-config" ]; then
     dn=$(cd $(dirname $0); pwd)
     for p in $(echo $PATH | sed 's/:/ /g'); do
       [ "$p" != "$dn" ] && [ -x "$p/$bn" ] && exec "$p/$bn" "[email protected]"
     done
 fi

This script has two usage scenarios. You can source this into your current shell to have the http_proxy set correctly based on your current network location:

$ . proxy-config
$ echo $http_proxy
http://proxy.example.com:80

Alternatively, you can create symlinks to the proxy-config script using the names of commands that require http_proxy and the script will automatically set the variable and exec the real command.

Got that? No? Okay, let’s move on.

Pretend you have /usr/bin/curl and you put the script from above at /usr/local/bin/proxy-config. You can get curl to use the approriate proxy settings by doing something like this:

# mkdir /usr/local/proxybin
# cd /usr/local/proxybin
# ln -s ../bin/proxy-config curl
# ls -l
total 4
lrwxr-xr-x  1 root wheel 19 May 11 13:30 curl -> ../bin/proxy-config

As long as /usr/local/proxybin is on your $PATH before /usr/bin, executing curl will actuall call proxy-config. proxy-config will then setup the proxy settings and exec /usr/bin/curl.

Now just create a symlink just like the one made for curl for anything else that requires proxy settings and enjoy network location support from the command line.