Socat

More on socat

socat is a "multipurpose relay" often used to serve binary exploitation challenges in CTFs. Essentially, it transfers stdin and stdout to the socket and also allows simple forking capabilities. The following is an example of how you could host a binary on port 5000:

socat tcp-l:5000,reuseaddr,fork EXEC:"./vuln",pty,stderr

Most of the command is fairly logical (and the rest you can look up). The important part is that in this scenario we don't have to redirect file descriptors, as socat does it all for us.

What is important, however, is pty mode. Because pty mode allows you to communicate with the process as if you were a user, it takes in input literally - including DELETE characters. If you send a \x7f - a DELETE - it will literally delete the previous character (as shown shortly in my Dream Diary: Chapter 1 writeup). This is incredibly relevant because in 64-bit the \x7f is almost always present in glibc addresses, so it's not quite so possible to avoid (although you could keep rerunning the exploit until the rare occasion you get an 0x7e... libc base).

To bypass this we use the socat pty escape character \x16 and prepend it to any \x7f we send across.