- Fotis' Blog - https://fotisl.com/blog -

Setting default options and bindings for sockets.

Recently one of the people I follow at twitter (yes, I have a twitter account, you can follow me [1]!) asked if anyone knew an option for lynx which would make it bind to a certain interface. I searched the manual page but there was no such option. Some programs, like netkit telnet, have an option to bind to a certain address, which can be very useful and especially when you’ve joined a VPN.

The first thing that came to my mind is write a library which would automatically bind new sockets to a certain address and then use the environment variable LD_PRELOAD so that the runtime linker would load it when a new program was run. The library should overwrite the socket(2) function and replace it with one which would run the original function to create the socket and then¬†immediately¬†bind it to an address. This address would be taken from an environment variable. And since I wrote the wrapper for socket(2) I could add some extra functionality such as setting various options with setsockopt(2). I wanted the program to be very efficient so it shouldn’t lookup the old socket(2) function each time the wrapper was called. In order to do this I should store the original address of the function at some variable at the beginning. This could be done in a check in the wrapper function and if the address was equal to NULL, then all initialization would take place. However, that would mean an extra check at every call. So I created a constructor which would be called when the library was initially loaded. I used a very low priority so that the constructor was run before any other constructors which could open a socket.

The program is named sockopts and you can find it at my main page [2], under the programs section [3]. For the moment I have tested it at my local box having one ethernet interface and an openvpn running with a tap interface. And it works great! If you have a feature request or you found that something is broken you can leave a comment here or just email me.