Rsync daemon fails to start if PTY not allocated on Android

[schuelermine]

I’m running a static build of Rsync for ARM that I built using Nix (nixpkgs#pkgsCross.aarch64-android.pkgsStatic.rsync) on an Android phone (specifically a Google Pixel 10 Pro XL running GrapheneOS) via ADB.

Rsync fails to start if it’s launched without a PTY. ADB shell creates a PTY if it’s run without a specific command and thus launches an interactive shell, but doesn’t otherwise (unless you pass -tt). The way I go about driving an ADB shell is usually by launching the noninteractive shell and piping a script to it due to ADB’s strange handling of arguments.

I’m running Rsync with the following command line:

/data/local/tmp/rsync --daemon --no-detach --config=/data/local/tmp/rsyncd.conf --verbose --log-file=/data/local/tmp/rsyncd.log

The config file reads:

address = 127.0.0.1
port = 60873
[user]
path = /storage/emulated/0/
read only = false

The log file shows the following error message:

2026/08/30 17:31:02 [9693] malformed address : Name does not resolve
2026/08/30 17:31:02 [9693] connect from UNKNOWN ()
2026/08/30 17:31:02 [9693] rsync: [Receiver] failed to set socket option SO_KEEPALIVE: Permission denied (13)

Rsync itself prints to stderr:

@RSYNCD: 32.0 sha512 sha256 sha1 md5 md4
@ERROR: protocol startup error

I’m not sure if this an Rsync bug or an ADB or Android bug.

[steadytao]

The current evidence does not yet show that PTY allocation is the cause. The daemon starts, accepts a connection and then reports SO_KEEPALIVE: Permission denied. Could you provide the exact client-side command, rsync --version, build config and a run with socket options cleared or traced? That should distinguish Android socket restrictions from ADB transport behaviour.

[schuelermine]

There is no client side command. The Rsync daemon crashes immediately on launch.

It's obviously possible that Android socket restrictions cause this, but it's somehow triggered by the PTY allocation. It doesn't crash if I pass -tt.

[steadytao]

Okay, I might know what that would be now. Give me a little bit.