mjhjmtu

$ SSH_AUTH_SOCK= rsync x brick:
Enter passphrase for key '/home/alan/.ssh/id_rsa': 
alan@brick.carrier.duckdns.org: Permission denied (publickey).
rsync: connection unexpectedly closed (0 bytes received so far) [sender]
rsync error: unexplained error (code 255) at io.c(226) [sender=3.1.3]

rsync supports arbitrary remote shells with --rsh. How is the rsh, e.g. ssh, able to write messages, and not have them mis-interpreted as coming from the rsync process on the server?

SSH is prompting and reading the login secrets through a separate file descriptor; it makes sure to open the current TTY.

Most programs take care to read login secrets from a TTY, so that they can stop the password from being shown by temporarily disabling the "echo" on the TTY. I would expect them to make sure to write the prompt to the same TTY, for robustness sake.

$ SSH_AUTH_SOCK= strace -f rsync x brick:
...
[pid 26255] openat(AT_FDCWD, "/dev/tty", O_RDWR) = 4
[pid 26255] ioctl(4, TCGETS, B38400 opost isig icanon echo ...) = 0
[pid 26255] ioctl(4, SNDCTL_TMR_CONTINUE or TCSETSF, B38400 opost isig icanon -echo ...) = 0
...
[pid 26255] write(4, "Enter passphrase for key '/home/"..., 51Enter passphrase for key '/home/alan/.ssh/id_rsa': ) = 51
[pid 26255] read(4, "", 1) = 0
[pid 26255] write(4, "n", 1
) = 1
[pid 26255] ioctl(4, SNDCTL_TMR_CONTINUE or TCSETSF, B38400 opost isig icanon echo ...) = 0
...

SSH printed the error message to standard error (open file number 2), not standard output (open file number 1) which rsync would use for its protocol.

[pid 26255] write(2, "alan@brick.carrier.duckdns.org: "..., 64alan@brick.carrier.duckdns.org: Permission denied (publickey).
) = 64
[pid 26255] exit_group(255) = ?