I think I have started to understand: pam_info messages (together with
pam_error messages) are packed together into SSH2_MSG_USERAUTH_BANNER
messages. I have not checked it yet but, probably the pam_prompt
triggers either the sent of this message to client, or the display of
these messages in the client.
Can somebody tell me if this is the case? What I am trying to do is: I
have a script that produces a URL that is supposed to be sent to the
client, and wait while polling a web service. When the user has
authenticated on this server, the script is notified and returns zero or
one. Therefore, I need to find a way to send a string to the ssh client,
and have it display this string.
Thank you for your time,
Felix
---
Felix Rubio
"Don't believe what you're told. Double check."
On 2017-03-31 10:44, Felix Rubio wrote:
Hi all,
I had posted a related question to pam developers list, but I have
continued investigating and seems to be a question for this mailing
list. I have a script to be used by pam_exec when logging into a
system through openssh client. This script writes a message in its
stdout, that is passed to ssh_thread_conv(auth-pam.c) by a call to
pam_info, in pam_exec.so. The message is passed to ssh_msg_send, which
issues a write into the corresponent file descriptor.
The problem is: I never get to see that message in the ssh client.
OTOH, if I modify pam_exec module to issue a pam_prompt instead of a
pam_info, the message appears on the ssh client. The relevant code for
both functionalities in auth-pam.c is:
static int
sshpam_thread_conv(int n, sshpam_const struct pam_message **msg,
struct pam_response **resp, void *data)
{
.....
case PAM_PROMPT_ECHO_OFF:
case PAM_PROMPT_ECHO_ON:
buffer_put_cstring(&buffer,
PAM_MSG_MEMBER(msg, i, msg));
if (ssh_msg_send(ctxt->pam_csock,
PAM_MSG_MEMBER(msg, i, msg_style), &buffer)
== -1)
goto fail;
if (ssh_msg_recv(ctxt->pam_csock, &buffer) ==
-1)
goto fail;
if (buffer_get_char(&buffer) != PAM_AUTHTOK)
goto fail;
reply[i].resp = buffer_get_string(&buffer,
NULL);
break;
case PAM_ERROR_MSG:
case PAM_TEXT_INFO:
buffer_put_cstring(&buffer,
PAM_MSG_MEMBER(msg, i, msg));
if (ssh_msg_send(ctxt->pam_csock,
PAM_MSG_MEMBER(msg, i, msg_style), &buffer)
== -1)
goto fail;
break;
.....
}
As can be seen, the only difference (I have also checked the contents
of ssh_msg_send) is in the fact that, when pam_prompt is used, a
ssh_msg_send is followed by a ssh_msg_recv, whereas for a pam_info,
only the ssh_msg_send is performed.
The OpenSSH version is 7.5p1, and the operating system is CentOS7.3.
Can somebody advice on whether this is a client or a server issue,
and... yeah.. what can I do?
Thank you!
Felix
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