How to setup SFTP (in order to make sshfs work)

[unclechu]

How could I setup SFTP in order to make sshfs work? I made a simple script to start an SSHD server:

{ pkgs, lib }:

let
  hostKeyPath = toString
    /data/data/com.termux.nix/files/home/.ssh/ssh-host-key;

  # Add authorized keys to: ~/.ssh/authorized_keys
  sshdConfigFile = pkgs.writeText "sshd-config" ''
    HostKey "${lib.escape ["\""] hostKeyPath}"
    Port 8044
    Subsystem sftp "${lib.escape ["\""] e.sftp-server}"
  '';

  executables = {
    bash = "${pkgs.bash}/bin/bash";
    sshd = "${pkgs.openssh}/bin/sshd";
    sftp-server = "${pkgs.openssh}/libexec/sftp-server";
  };

  e = executables;
  esc = lib.escapeShellArg;

  run-sshd = pkgs.writeTextFile rec {
    name = "run-sshd";
    executable = true;
    destination = "/bin/${name}";
    text = ''
      #! ${e.bash}
      ${esc e.sshd} -f ${esc sshdConfigFile} -e -D
    '';
    checkPhase = ''(
      set -o xtrace
      ${builtins.concatStringsSep "\n" (
        map (x: "[[ -x ${esc x} ]]") (builtins.attrValues executables)
      )}
    )'';
  };
in
  run-sshd // { inherit sshdConfigFile; }

The regular ssh works fine but if I try to mount via sshfs I get an error like “Connection closed by the user". Here is the end of the log of sshfs call:

...
debug1: Sending subsystem: sftp
debug1: pledge: fork
debug1: client_input_channel_req: channel 0 rtype exit-status reply 0
debug1: client_input_channel_req: channel 0 rtype [email protected] reply 0
debug1: channel 0: free: client-session, nchannels 1
Transferred: sent 4872, received 3844 bytes, in 0.3 seconds
Bytes per second: sent 18440.4, received 14549.4
debug1: Exit status 255
remote host has disconnected

[Gerschtli]

It might be that the path to the sftp-server executable is not found because it may be running outside of proot.. Not sure how to debug that though but the arch wiki says that this error message happens when the sftp-server executable is not valid: https://wiki.archlinux.org/title/SSHFS#Remote_host_has_disconnected

[unclechu]

@Gerschtli I’ve also tried internal-sftp, it makes no difference.

[unclechu]

Also I don’t think it should run outside proot since I run run-sshd from inside of the Nix-on-Droid shell?

[azuwis]

Looks like a proot problem proot-me/proot#243

[SonicStark]

Looks like a proot problem proot-me/proot#243

A patched sftp-server, as a workaround, can mitigate the problem.
See proot-me/proot#243 (comment)

[unclechu]

@SonicStark I tried to patch it like this:

{ pkgs ? import <nixpkgs> {}
}:

let
  openssh = pkgs.openssh.overrideAttrs (old: {
    patchPhase = (old.patchPhase or "") + ''
      sed -i 's/\(platform_disable_tracing(\)1\();\)/\10\2/' sftp-server.c
    '';
  });
in

openssh

But for some reason it failed much later like this (cannot create directory '/var': Permission denied):

...
make[1]: Entering directory '/build/openssh-9.7p1/openbsd-compat'
make[1]: Nothing to be done for 'all'.
make[1]: Leaving directory '/build/openssh-9.7p1/openbsd-compat'
/nix/store/k71apxkm38m3g34k01sb6zhysi0y7gph-coreutils-9.5/bin/mkdir -p /nix/store/k1zkv58z9qgk53skli5l2ahfrcd3ljhk-openssh-9.7p1/bin
/nix/store/k71apxkm38m3g34k01sb6zhysi0y7gph-coreutils-9.5/bin/mkdir -p /nix/store/k1zkv58z9qgk53skli5l2ahfrcd3ljhk-openssh-9.7p1/etc/ssh
/nix/store/k71apxkm38m3g34k01sb6zhysi0y7gph-coreutils-9.5/bin/mkdir -p /nix/store/k1zkv58z9qgk53skli5l2ahfrcd3ljhk-openssh-9.7p1/bin
/nix/store/k71apxkm38m3g34k01sb6zhysi0y7gph-coreutils-9.5/bin/mkdir -p /nix/store/k1zkv58z9qgk53skli5l2ahfrcd3ljhk-openssh-9.7p1/share/man/man1
/nix/store/k71apxkm38m3g34k01sb6zhysi0y7gph-coreutils-9.5/bin/mkdir -p /nix/store/k1zkv58z9qgk53skli5l2ahfrcd3ljhk-openssh-9.7p1/share/man/man5
/nix/store/k71apxkm38m3g34k01sb6zhysi0y7gph-coreutils-9.5/bin/mkdir -p /nix/store/k1zkv58z9qgk53skli5l2ahfrcd3ljhk-openssh-9.7p1/share/man/man8
/nix/store/k71apxkm38m3g34k01sb6zhysi0y7gph-coreutils-9.5/bin/mkdir -p /nix/store/k1zkv58z9qgk53skli5l2ahfrcd3ljhk-openssh-9.7p1/libexec
/nix/store/k71apxkm38m3g34k01sb6zhysi0y7gph-coreutils-9.5/bin/mkdir -p -m 0755 /var/empty
/nix/store/k71apxkm38m3g34k01sb6zhysi0y7gph-coreutils-9.5/bin/mkdir: cannot create directory '/var': Permission denied
make: *** [Makefile:394: install-files] Error 1
error: builder for '/nix/store/2flc2rag0fvzymw2n3b79dqa9173lr0h-openssh-9.7p1.drv' failed with exit code 2;
       last 10 log lines:
       > /nix/store/k71apxkm38m3g34k01sb6zhysi0y7gph-coreutils-9.5/bin/mkdir -p /nix/store/k1zkv58z9qgk53skli5l2ahfrcd3ljhk-openssh-9.7p1/bin
       > /nix/store/k71apxkm38m3g34k01sb6zhysi0y7gph-coreutils-9.5/bin/mkdir -p /nix/store/k1zkv58z9qgk53skli5l2ahfrcd3ljhk-openssh-9.7p1/etc/ssh
       > /nix/store/k71apxkm38m3g34k01sb6zhysi0y7gph-coreutils-9.5/bin/mkdir -p /nix/store/k1zkv58z9qgk53skli5l2ahfrcd3ljhk-openssh-9.7p1/bin
       > /nix/store/k71apxkm38m3g34k01sb6zhysi0y7gph-coreutils-9.5/bin/mkdir -p /nix/store/k1zkv58z9qgk53skli5l2ahfrcd3ljhk-openssh-9.7p1/share/man/man1
       > /nix/store/k71apxkm38m3g34k01sb6zhysi0y7gph-coreutils-9.5/bin/mkdir -p /nix/store/k1zkv58z9qgk53skli5l2ahfrcd3ljhk-openssh-9.7p1/share/man/man5
       > /nix/store/k71apxkm38m3g34k01sb6zhysi0y7gph-coreutils-9.5/bin/mkdir -p /nix/store/k1zkv58z9qgk53skli5l2ahfrcd3ljhk-openssh-9.7p1/share/man/man8
       > /nix/store/k71apxkm38m3g34k01sb6zhysi0y7gph-coreutils-9.5/bin/mkdir -p /nix/store/k1zkv58z9qgk53skli5l2ahfrcd3ljhk-openssh-9.7p1/libexec
       > /nix/store/k71apxkm38m3g34k01sb6zhysi0y7gph-coreutils-9.5/bin/mkdir -p -m 0755 /var/empty
       > /nix/store/k71apxkm38m3g34k01sb6zhysi0y7gph-coreutils-9.5/bin/mkdir: cannot create directory '/var': Permission denied
       > make: *** [Makefile:394: install-files] Error 1
       For full logs, run 'nix log /nix/store/2flc2rag0fvzymw2n3b79dqa9173lr0h-openssh-9.7p1.drv'.

[SonicStark]

@unclechu Does it fail without the patch? Looks like /var is missing in your fs...

[unclechu]

I tired to rebuild it inside Nix-on-Droid environment. Maybe it’ll actually fail without a patch.