[SOLVED] ssh command fails via su -c

VP_Chestnut · 08-05-2020, 06:58 AM

I run this command as root from machine1 (CentOS6) :

Code:

[root@machine1] : su - user1 -c "ssh -v user2@machine2"

An OpenSSH window opens, I type the password in it, and log in machine2 as user2.

Just before the window pops up the verbose mode of ssh says :

Code:

debug1: Next authentication method: password
debug1: read_passphrase: can't open /dev/tty: No such device or address
debug1: permanently_drop_suid: 31078

(31078 being user1's UID)

I tried the same on an machine equivalent to machine1 but in CentOS8 :

Code:

[root@machine1bis] : su - user1 -c "ssh -v user2@machine2"

No pop-up window appears, I can't type any password and can't log in machine2.

I get :

Code:

debug1: Next authentication method: password
debug1: read_passphrase: can't open /dev/tty: No such device or address
debug1: Authentications that can continue: publickey,gssapi-keyex,gssapi-with-mic,password
Permission denied, please try again.
debug1: read_passphrase: can't open /dev/tty: No such device or address
debug1: Authentications that can continue: publickey,gssapi-keyex,gssapi-with-mic,password
Permission denied, please try again.
debug1: read_passphrase: can't open /dev/tty: No such device or address
debug1: Authentications that can continue: publickey,gssapi-keyex,gssapi-with-mic,password
debug1: No more authentication methods to try.
Permission denied (publickey,gssapi-keyex,gssapi-with-mic,password).

I don't know how to get this works on machine1bis, as on machine1.

Keith Hedger · 08-05-2020, 06:50 PM

why switch to user1 then login as user2?

VP_Chestnut · 08-06-2020, 02:08 AM

user1 exists only on machine1, and user2 on machine2.
The command line is part of an install script run as root.

scasey · 08-06-2020, 02:12 AM

Not sure that answers the question…
Why not simply do

Code:

ssh -v user2@machine2

without the su to user1?

I think the errors are because user1 doesn’t exist on machine2... But that’s just a guess.

Turbocapitalist · 08-06-2020, 02:30 AM

Interactive SSH requires allocation of a TTY on the client side. Since su does not do that, you cannot use it with programs which require a controlling TTY. In short, you can't get there from here.

Instead, you can use sudo if it is properly configured.

Code:

sudo -u user1 "ssh -v user2@machine2"
sudo -u user1 ssh -v user2@machine2

sudo -u user1 sh -c "ssh -v user2@machine2"

In contrast to su it will give you a TTY and thus allow use of the SSH client interactively.

Othwerwise, rewrite your workflow so that an interactive session is not needed.

VP_Chestnut · 08-10-2020, 06:46 AM

The 2nd of the 2 commands works fine, thank you, I understand where the problem was now.

To answer to scasey the actual command in the script is 'ssh-copy-id', in order to have a connection from user1@machine1 to user2@machine2 without password.
I tested with 'ssh' to have the verbose output.

MadeInGermany · 08-10-2020, 05:28 PM

Try also

Code:

sudo -u user1 ssh -v user2@machine2

No quotes! (su and sh -c need quotes, but not sudo.)

VP_Chestnut · 08-11-2020, 06:15 AM

Yes it works indeed !