The file forceIP.c is for an LD_PRELOAD shim for Linux to intercept networking API calls and bind sockets to a specified source address.

It is meant to be used when a VPN is operating but is not the default gateway. A scheme for this is described on the wiki page "Running OpenVPN on Linux without VPN as Default Gateway" found here:

https://github.com/tool-maker/VPN_just_for_torrents/wiki/Running-OpenVPN-on-Linux-without-VPN-as-Default-Gateway

Since the address to which we force a bind will not be the IP address of the default gateway interface, you will need to set up source address routing. You need two commands similar to this:

sudo ip -4 route add default via 10.4.0.1 dev tun0 table 1234
sudo ip -4 rule add from 10.4.36.17 table 1234

In that tun0 was the name of the local interface for the VPN and 10.4.0.1 was the gateway IP address for the VPN. And 10.4.36.17 was the IP address of the local interface for the VPN, to which this shim can bind all sockets for a network program. The first line adds a non-default routing table numbered as 1234. The second line adds a routing policy rule that says that any packet coming from a socket bound to address 10.4.36.17 should use the routing table numbered 1234 rather than the normal default routing table.

To build:

# install gcc if not already installed
sudo apt-get update
sudo apt-get install gcc

# create target folder and upload source
mkdir ~/forceIP
pushd ~/forceIP
curl https://raw.githubusercontent.com/tool-maker/forceIP/main/forceIP.c > forceIP.c
ls -la

# compile it
gcc -nostartfiles -fpic -shared forceIP.c -o forceIP.so -ldl
ls -la

# leave target folder
popd

To have this shim preload before a network program starts, set the LD_PRELOAD environment variable before starting the program:

export LD_PRELOAD=~/forceIP/forceIP.so

Also before starting the network program, set other environment variables ("export fIP_?=...") as desired:

  fIP_TRACE        - trace all calls (set to anything)
  fIP_BINDADDR     - IPv4 address to force as source address for calls to "bind"
  fIP_BINDADDR6    - IPv6 address to force as source address for calls to "bind"
                     bind to localhost, 127.0.0.0, ::1 is left alone
  fIP_BINDLOCAL    - force bind source address for localhost too (set to anything)
  fIP_LISTENADDR   - IPv4 address to force as source address for calls to "listen"
  fIP_LISTENADDR6  - IPv6 address to force as source address for calls to "listen"
                     listen on localhost, 127.0.0.0, ::1 is left alone
  fIP_LISTENLOCAL  - force listen source address for localhost too (set to anything)
  fIP_CONNECTADDR  - IPv4 address to force as source address for calls to "connect", "sendto" and "sendmsg"
  fIP_CONNECTADDR6 - IPv6 address to force as source address for calls to "connect", "sendto" and "sendmsg"
                     connect to destination localhost, 127.0.0.0, ::1 is left alone
  fIP_DNSSKIP      - do not modify UDP packets with destination port 53 (DNS) (set to anything)

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For an explanation of the function of the LD_PRELOAD environment variable in Linux see the man page for ld.so. For example here:

https://www.man7.org/linux/man-pages/man8/ld.so.8.html

This shim provides alternative versions of several network API functions (getaddrinfo, bind,listen, connect, socket, sendto, sendmsg). These then make the calls to the "real" API functions, using function pointers to them obtained using the dlsym function call. For a description of the dlsym function see for example here:

https://man7.org/linux/man-pages/man3/dlsym.3.html

The calls to dlsym are done in a shared library "constructor" function. For an explanation of this see the description of the "constructor" attribute here:

https://gcc.gnu.org/onlinedocs/gcc-11.2.0/gcc/Common-Function-Attributes.html#Common-Function-Attributes

For a description of the functions that this shim intercepts see for example here:

https://man7.org/linux/man-pages/man3/getaddrinfo.3.html

https://man7.org/linux/man-pages/man2/bind.2.html

https://man7.org/linux/man-pages/man2/listen.2.html

https://man7.org/linux/man-pages/man2/connect.2.html

https://man7.org/linux/man-pages/man2/socket.2.html

https://man7.org/linux/man-pages/man2/sendto.2.html

https://man7.org/linux/man-pages/man2/sendmsg.2.html

For an overview of IP, UDP and TCP programming in Linux see for example here:

https://man7.org/linux/man-pages/man7/ip.7.html

https://man7.org/linux/man-pages/man7/ipv6.7.html

https://man7.org/linux/man-pages/man7/socket.7.html

https://man7.org/linux/man-pages/man7/udp.7.html

https://man7.org/linux/man-pages/man7/tcp.7.html

It is not immediately obvious which network function calls need to be intercepted, other than bind. But this information from the ip.7 man page above provides some insight:

An ephemeral port is allocated to a socket in the following circumstances:

• the port number in a socket address is specified as 0 when calling bind(2);

• listen(2) is called on a stream socket that was not previously bound;

• connect(2) was called on a socket that was not previously bound;

• sendto(2) is called on a datagram socket that was not previously bound.

This implies that functions other than bind, listen, connect and sendto do not need to be intercepted.