GNU Hurd/ hurd/ translator/ remap

Welcome to... ... the GNU Hurd!

The remap translator lets you remap directories. This translator is to be used as a chroot, within which paths point to the same files as the original root, except a given set of paths, which are remapped to given paths.

This translator completes the server overriding google summer of code project.

It is often desirable to execute a command in a transitory environment with remapped files. The remap script lets you do this.

Example Uses

remapping /bin/sh

On Debian, /bin/sh points to dash. Maybe you would rather it point to bash.

$ ls -lha /bin/sh
  lrwxr-xr-x 1 root root 4 Jun  5 04:08 /bin/sh -> dash
$ remap /bin/sh /bin/bash -- ls -lha /bin/sh
  -rwxr-xr-x 1 root root 1,2M 20 oct.  12:53 /bin/sh

remapping python3

Perhaps you've want to use a python package that requires a python feature that your distro does not yet support. Compiling this custom python3 can be a little annoying, because ./configure makes you specify where all the various libraries are. It's much easier to just remap.

$ remap /usr/bin/python3 $HOME/bin/python3-custom -- ./configure
$ remap /usr/bin/python3 $HOME/bin/python3-custom -- cool-package

Run a command through a custom pflocal

$ cd /tmp
$ settrans -ac 1 ~/HURD-SRC/pflocal/pflocal
$ remap /servers/socket/1 /tmp/1 -- /bin/bash -c 'echo huhu world | wc'
  1       2      11

Remapping /servers/socket/2 and 26 for vpn/firewall

TODO add an example here.

Use remap to debug lwip

Suppose, you want to debug ?lwip. You could set lwip on /servers/socket/2, but it's hard to use an OS, if your network is buggy. It would be nice to use the stable pfinet and test lwip as needed. You can use the eth-multiplexer combined with remap to have such a configuration. First, use the eth-multiplexer to change pfinet's interface from /dev/eth0 to /dev/eth0m/0

# settrans -c /dev/eth0m /hurd/eth-multiplexer --interface=/dev/eth0

Now we configure own main Hurd system to use a virtual network interface (e.g. /dev/eth0m/0) instead. On Debian/Hurd, this can be accomplished using

# ifdown /dev/eth0
# sed -i -e s_/dev/eth0_/dev/eth0m/0_ /etc/network/interfaces
# ifup /dev/eth0m/0

Then you can do set up lwip on ~/lwip/servers/socket{2,26} 

$ settrans -c ~/lwip/servers/socket/2 /hurd/lwip -i \
  /dev/eth0m/1 -4 ~/lwip/servers/socket/2 \
  -6 ~/lwip/servers/socket/26
$ settrans -c ~/lwip/servers/socket/26 /hurd/lwip -i \
  /dev/eth0m/1 -4 ~/lwip/servers/socket/2 \
  -6 ~/lwip/servers/socket/26
$ remap /servers/socket/2 ~/lwip/servers/socket/2 -- \
  ping -c 3 gnu.org

If you are running the Hurd in qemu, then you can skip setting up the eth-multiplexer and just configure another virtual ethernet interface: eth1. Then using lwip is as simple as:

$ settrans -c ~/lwip/servers/socket/2 -i /dev/eth1 \
  -4 ~/lwip/servers/socket/2 -6 ~/lwip/servers/socket/26
$ settrans -c ~/lwip/servers/socket/26 -i /dev/eth1 \
  -4 ~/lwip/servers/socket/2 -6 ~/lwip/servers/socket/26
$ remap /servers/socket/2 $HOME/lwip/servers/socket/2 \
  -- ping -c 3 gnu.org

Alternatively, you could also launch a subhurd whose's networking uses lwip. The subhurd page should give you an idea of how to do this.

remap example bugs

Remap is written in a rather simplistic way. It should layer over the filesystem in a better. These examples demonstrate some problems.

$ remap /etc/motd /dev/null -- sh -c 'wc /etc/motd; cd /etc; wc motd;'
  0       0       0 /etc/motd
  7  40 284 motd

$ settrans $HOME/foo /hurd/remap /bin/sh /bin/bash
$ ls $HOME/foo/
ls: cannot open directory 'foo/': Permission denied