In order to allow the 2FA in my domain, I setup a LinOTP server to manage the map between tokens and users (from LDAP) in my realm.
So, I configured the PAM stack to integrate this authentication method also for SSH sessions:
# /etc/pam.d/sshd
# =========================================================
#%PAM-1.0
auth required pam_sepermit.so
# OTP Check
auth [success=1 default=ignore] pam_python.so\
/lib/security/pam_linotp.py nosslhostnameverify nosslcertverify\
url=https://mylinotpsrv.local/validate/simplecheck realm=MYDOMAIN debug
auth requisite pam_deny.so
auth substack password-auth
auth include postlogin
# Used with polkit to reauthorize users in remote sessions
-auth optional pam_reauthorize.so prepare
account required pam_nologin.so
account include password-auth
password include password-auth
# pam_selinux.so close should be the first session rule
session required pam_selinux.so close
session required pam_loginuid.so
# pam_selinux.so open should only be followed by sessions to be executed in the user context
session required pam_selinux.so open env_params
session required pam_namespace.so
session optional pam_keyinit.so force revoke
session include password-auth
session include postlogin
# Used with polkit to reauthorize users in remote sessions
-session optional pam_reauthorize.so prepare
Then, I tried to open a connection: ssh vdn@MYDOMAIN.LOC@192.168.0.12, and:
- the OTP (provided, in my case, by Google Authenticator) is correctly validated;
- the username/pwd correspondence is correctly checked.
But, when the double-check above is done, I obtain an error: Write Error: Broken Pipe.
The logs are as follows:
/var/log/secure
====================================================================================================
Mar 9 15:25:09 mflinux01 sshd[8215]: Set /proc/self/oom_score_adj to 0
Mar 9 15:25:09 mflinux01 sshd[8215]: Connection from 192.168.0.13 port 33926 on 192.168.0.12 port 22
Mar 9 15:25:09 mflinux01 sshd[8215]: Postponed keyboard-interactive for vdn@MYDOMAIN.LOC from 192.168.0.13 port 33926 ssh2 [preauth]
Mar 9 15:25:17 mflinux01 sshd[8215]: Postponed keyboard-interactive/pam for vdn@MYDOMAIN.LOC from 192.168.0.13 port 33926 ssh2 [preauth]
Mar 9 15:25:20 mflinux01 sshd[8217]: pam_sss(sshd:auth): authentication success; logname= uid=0 euid=0 tty=ssh ruser= rhost=192.168.0.13 user=vdn@MYDOMAIN.LOC
Mar 9 15:25:20 mflinux01 sshd[8220]: pam_krb5[8220]: got error -1 (Unknown code ____ 255) while obtaining tokens for cern.ch
Mar 9 15:25:20 mflinux01 sshd[8215]: Postponed keyboard-interactive/pam for vdn@MYDOMAIN.LOC from 192.168.0.13 port 33926 ssh2 [preauth]
Mar 9 15:25:20 mflinux01 sshd[8215]: Accepted keyboard-interactive/pam for vdn@MYDOMAIN.LOC from 192.168.0.13 port 33926 ssh2
Mar 9 15:25:20 mflinux01 sshd[8215]: fatal: PAM: pam_setcred(): Failure setting user credentials
/var/log/message
====================================================================================================
Mar 9 15:25:01 mflinux01 systemd: Created slice user-988.slice.
Mar 9 15:25:01 mflinux01 systemd: Starting user-988.slice.
Mar 9 15:25:01 mflinux01 systemd: Started Session 12 of user pcp.
Mar 9 15:25:01 mflinux01 systemd: Starting Session 12 of user pcp.
Mar 9 15:25:03 mflinux01 systemd: Removed slice user-988.slice.
Mar 9 15:25:03 mflinux01 systemd: Stopping user-988.slice.
Mar 9 15:25:09 mflinux01 pam_linotp[8217]: start pam_linotp.py authentication: 1, ['/lib/security/pam_linotp.py', 'nosslhostnameverify', 'nosslcertverify', 'url=https://192.168.0.14/validate/simplecheck', 'realm=MYDOMAIN', 'debug']
Mar 9 15:25:09 mflinux01 pam_linotp[8217]: got no password in authtok - trying through conversation
Mar 9 15:25:16 mflinux01 pam_linotp[8217]: got password: 932410
Mar 9 15:25:16 mflinux01 pam_linotp[8217]: calling url https://192.168.0.14/validate/simplecheck {'realm': 'MYDOMAIN', 'user': 'vdn@MYDOMAIN.LOC', 'pass': '932410'}
Mar 9 15:25:17 mflinux01 pam_linotp[8217]: :-)
Mar 9 15:25:17 mflinux01 pam_linotp[8217]: user successfully authenticated
Mar 9 15:25:20 mflinux01 sshd: Please note: pam_linotp does not support setcred
By looking for a solution on the web, I also setup the /etc/ssh/ssh_config and /etc/ssh/sshd_config adding ClientAliveInterval 120 and ServerAliveInterval 120, but the error persists.
Take into account that, by removing auth requisite pam_deny.so from the PAM stack, the OTP is of course not properly validated (it is always right), but after the username/pwd check, the SSH authentication works.
Do you have any clue about the resolution of this problem?
Note: all the Linux machines in my environment are based on CentOS 7.
UPDATE:
As follows, you can find the current version of password-auth PAM conf:
#%PAM-1.0
# This file is auto-generated.
# User changes will be destroyed the next time authconfig is run.
auth required pam_env.so
auth [default=1 success=ok] pam_localuser.so
auth [success=done ignore=ignore default=die] pam_unix.so nullok try_first_pass
auth requisite pam_succeed_if.so uid >= 1000 quiet_success
auth sufficient pam_sss.so forward_pass
auth sufficient pam_krb5.so use_first_pass
auth required pam_deny.so
account required pam_unix.so broken_shadow
account sufficient pam_localuser.so
account sufficient pam_succeed_if.so uid < 1000 quiet
account [default=bad success=ok user_unknown=ignore] pam_sss.so
account [default=bad success=ok user_unknown=ignore] pam_krb5.so
account required pam_permit.so
password requisite pam_pwquality.so try_first_pass local_users_only retry=3 authtok_type=
password sufficient pam_unix.so sha512 shadow nullok try_first_pass use_authtok
password sufficient pam_sss.so use_authtok
password sufficient pam_krb5.so use_authtok
password required pam_deny.so
session optional pam_keyinit.so revoke
session required pam_limits.so
-session optional pam_systemd.so
session optional pam_oddjob_mkhomedir.so umask=0077
session [success=1 default=ignore] pam_succeed_if.so service in crond quiet use_uid
session required pam_unix.so
session optional pam_sss.so
session optional pam_krb5.so
