sshansiblecentospamsshd

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


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]

Solution

  • 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.

    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!