pam_unix(sudo:auth): conversation failed, auth could not identify password for [username]

Question

I'm using ansible to provision my Centos 7 produciton cluster. Unfortunately, execution of below command results with ansible Tiemout and Linux Pluggable Authentication Modules (pam) error conversation failed.

The same ansible command works well, executed against virtual lab mad out of vagrant boxes.

Ansible Command

$ ansible master_server -m yum -a 'name=vim state=installed' -b -K -u lukas -vvvv
123.123.123.123 | FAILED! => {
    "msg": "Timeout (7s) waiting for privilege escalation prompt: \u001b[?1h\u001b=\r\r"
}

SSHd Log

# /var/log/secure
Aug 26 13:36:19 master_server sudo: pam_unix(sudo:auth): conversation failed
Aug 26 13:36:19 master_server sudo: pam_unix(sudo:auth): auth could not identify password for [lukas]

Answer 1

I've found the problem. It turned out to be PAM's auth module problem! Let me describe how I got to the solution.

Context:

I set up my machine for debugging - that is I had four terminal windows opened.

• 1st terminal (local machine): Here, I was executing ansible prduction_server -m yum -a 'name=vim state=installed' -b -K -u username
• 2nd terminal (production server): Here, I executed journalctl -f (system wide log).
• 3rd terminal (production server): Here, I executed tail -f /var/log/secure (log for sshd).
• 4th terminal (production server): Here, I was editing vi /etc/pam.d/sudo file.

Every time, I executed command from 1st terminal I got this errors:

# ansible error - on local machine
Timeout (7s) waiting for privilege escalation prompt error.

# sshd error - on remote machine
pam_unix(sudo:auth): conversation failed
pam_unix(sudo:auth):  [username]

I showed my entire setup to my colleague, and he told me that the error had to do something with "PAM". Frankly, It was the first time that I've heard about PAM. So, I had to read this PAM Tutorial. I figured out, that error relates to auth interface located in /etc/pam.d/sudo module. Diging over the internet, I stambled upon this pam_permit.so module with sufficient controll flag, that fixed my problem!

Solution

Basically, what I added was auth sufficient pam_permit.so line to /etc/pam.d/sudo file. Look at the example below.

$ cat /etc/pam.d/sudo
#%PAM-1.0
# Fixing ssh "auth could not identify password for [username]"
auth       sufficient   pam_permit.so

# Below is original config
auth       include      system-auth
account    include      system-auth
password   include      system-auth
session    optional     pam_keyinit.so revoke
session    required     pam_limits.so
session    include      system-auth

Conclusion:

I spent 4 days to arrive to this solution. I stumbled upon over a dozens solutions that did not worked for me, starting from "duplicated sudo password in ansible hosts/config file", "ldap specific configuration" to getting advice from always grumpy system admins!

Note:

Since, I'm not expert in PAM, I'm not aware if this fix affects other aspects of the system, so be cautious over blindly copy pasting this code! However, if you are expert on PAM please share with us alternative solutions or input. Thanks!

Answer 2

Assuming the lukas user is a local account, you should look at how the pam_unix.so module is declared in your system-auth pam file. But more information about the user account and pam configuration is necessary for a specific answer.

---

While adding auth sufficient pam_permit.so is enough to gain access. Using it in anything but the most insecure test environment would not be recommended. From the pam_permit man page:

   pam_permit is a PAM module that always permit access. It does nothing
   else.

So adding pam_permit.so as sufficient for authentication in this manner will completely bypass the security for all users.