(public key and OTP) or (password and OTP)
我的目标是开发一个 ansible playbook,以在 Ubuntu Server 18.04 主机上部署此类多因素 ssh 登录。
我遵循了指南这里并且它基本上有效。我遇到的唯一问题是需要添加auth required pam_permit.so
到末尾,/etc/pam.d/sshd
以便绕过没有~/.google_authenticator
文件的用户的 OTP 检查。
本指南设置为使用AuthenticationMethods publickey,password publickey,keyboard-interactive
,其中/etc/ssh/sshd_config
要求所有登录都使用公钥,而我尝试实施的 MFA 方案允许使用 OTP 密码。但是,任何其他排列AuthenticationMethods
似乎都会完全破坏身份验证。
例如,只有连续两次输入正确的 OTP 才AuthenticationMethods password,keyboard-interface
允许有文件的用户登录,而没有文件的用户则可以使用任意字符串作为密码登录。~/.google_authenticator
~/.google_authenticator
我尝试了很多选项,但我开始认为这种配置是不可能的(参见类似的帖子这里)。为什么会这样?我越来越相信 PAM 是罪魁祸首,但我不明白为什么它AuthenticationMethods publickey,keyboard-interface password,keyboard-interface
不能开箱即用
这是我的/etc/pam.d/sshd
:
# PAM configuration for the Secure Shell service
# Standard Un*x authentication.
# Commented out by me @include common-auth
# Disallow non-root logins when /etc/nologin exists.
account required pam_nologin.so
# Uncomment and edit /etc/security/access.conf if you need to set complex
# access limits that are hard to express in sshd_config.
# account required pam_access.so
# Standard Un*x authorization.
@include common-account
# SELinux needs to be the first session rule. This ensures that any
# lingering context has been cleared. Without this it is possible that a
# module could execute code in the wrong domain.
session [success=ok ignore=ignore module_unknown=ignore default=bad] pam_s$
# Set the loginuid process attribute.
session required pam_loginuid.so
# Create a new session keyring.
session optional pam_keyinit.so force revoke
# Standard Un*x session setup and teardown.
@include common-session
# Print the message of the day upon successful login.
# This includes a dynamically generated part from /run/motd.dynamic
# and a static (admin-editable) part from /etc/motd.
session optional pam_motd.so motd=/run/motd.dynamic
session optional pam_motd.so noupdate
# Print the status of the user's mailbox upon successful login.
session optional pam_mail.so standard noenv # [1]
# Set up user limits from /etc/security/limits.conf.
session required pam_limits.so
# Read environment variables from /etc/environment and
# /etc/security/pam_env.conf.
session required pam_env.so # [1]
# In Debian 4.0 (etch), locale-related environment variables were moved to
# /etc/default/locale, so read that as well.
session required pam_env.so user_readenv=1 envfile=/etc/default/locale
# SELinux needs to intervene at login time to ensure that the process starts
# in the proper default security context. Only sessions which are intended
# to run in the user's context should be run after this.
session [success=ok ignore=ignore module_unknown=ignore default=bad] pam_selinux.so open
# Standard Un*x password updating.
@include common-password
### Added by me ###
auth required pam_google_authenticator.so nullok
auth required pam_permit.so
和我的/etc/ssh/sshd_config
:
# $OpenBSD: sshd_config,v 1.101 2017/03/14 07:19:07 djm Exp $
# This is the sshd server system-wide configuration file. See
# sshd_config(5) for more information.
# This sshd was compiled with PATH=/usr/bin:/bin:/usr/sbin:/sbin
# The strategy used for options in the default sshd_config shipped with
# OpenSSH is to specify options with their default value where
# possible, but leave them commented. Uncommented options override the
# default value.
#Port 22
#AddressFamily any
#ListenAddress 0.0.0.0
#ListenAddress ::
#HostKey /etc/ssh/ssh_host_rsa_key
#HostKey /etc/ssh/ssh_host_ecdsa_key
#HostKey /etc/ssh/ssh_host_ed25519_key
# Ciphers and keying
#RekeyLimit default none
# Logging
#SyslogFacility AUTH
#LogLevel INFO
# Authentication:
#LoginGraceTime 2m
#PermitRootLogin prohibit-password
#StrictModes yes
#MaxAuthTries 6
#MaxSessions 10
#PubkeyAuthentication yes
# Expect .ssh/authorized_keys2 to be disregarded by default in future.
#AuthorizedKeysFile .ssh/authorized_keys .ssh/authorized_keys2
#AuthorizedPrincipalsFile none
#AuthorizedKeysCommand none
#AuthorizedKeysCommandUser nobody
# For this to work you will also need host keys in /etc/ssh/ssh_known_hosts
#HostbasedAuthentication no
# Change to yes if you don't trust ~/.ssh/known_hosts for
# HostbasedAuthentication
#IgnoreUserKnownHosts no
# Don't read the user's ~/.rhosts and ~/.shosts files
#IgnoreRhosts yes
# To disable tunneled clear text passwords, change to no here!
#PasswordAuthentication yes
#PermitEmptyPasswords no
# Change to yes to enable challenge-response passwords (beware issues with
# some PAM modules and threads)
### changed by me ###----------------------------------------------------------------
ChallengeResponseAuthentication yes
# Kerberos options
#KerberosAuthentication no
#KerberosOrLocalPasswd yes
#KerberosTicketCleanup yes
#KerberosGetAFSToken no
# GSSAPI options
#GSSAPIAuthentication no
#GSSAPICleanupCredentials yes
#GSSAPIStrictAcceptorCheck yes
#GSSAPIKeyExchange no
# Set this to 'yes' to enable PAM authentication, account processing,
# and session processing. If this is enabled, PAM authentication will
# be allowed through the ChallengeResponseAuthentication and
# PasswordAuthentication. Depending on your PAM configuration,
# PAM authentication via ChallengeResponseAuthentication may bypass
# the setting of "PermitRootLogin without-password".
# If you just want the PAM account and session checks to run without
# PAM authentication, then enable this but set PasswordAuthentication
# and ChallengeResponseAuthentication to 'no'.
UsePAM yes
### Added by me ###------------------------------------------------------------------
AuthenticationMethods publickey,keyboard-interactive password,keyboard-interactive
#AllowAgentForwarding yes
#AllowTcpForwarding yes
#GatewayPorts no
X11Forwarding yes
#X11DisplayOffset 10
#X11UseLocalhost yes
#PermitTTY yes
PrintMotd no
#PrintLastLog yes
#TCPKeepAlive yes
#UseLogin no
#PermitUserEnvironment no
#Compression delayed
#ClientAliveInterval 0
#ClientAliveCountMax 3
#UseDNS no
#PidFile /var/run/sshd.pid
#MaxStartups 10:30:100
#PermitTunnel no
#ChrootDirectory none
#VersionAddendum none
# no default banner path
#Banner none
# Allow client to pass locale environment variables
AcceptEnv LANG LC_*
# override default of no subsystems
Subsystem sftp /usr/lib/openssh/sftp-server
# Example of overriding settings on a per-user basis
#Match User anoncvs
# X11Forwarding no
# AllowTcpForwarding no
# PermitTTY no
# ForceCommand cvs server
PasswordAuthentication yes
答案1
问题源于 PAM,它几乎使得这变得不可能。
核心问题是公钥部分由 ssh 守护进程处理,而键盘交互始终由 PAM 处理(并且密码也经常由 PAM 处理,具体取决于配置)。因此,PAM 无法知道是否已设置公钥,并且无法将 ssh 配置为根据是否使用公钥来随意选择不同的 AuthenticationMethods。
但是 ssh做当公钥已设置并验证为已批准时,设置环境变量 SSH_AUTH_INFO_0,PAM 可以看到此变量。因此,您要做的就是编写一个 pam_exec 脚本,该脚本根据 SSH_AUTH_INFO_0 是否设置为某项而以不同的方式退出。并且 pam_exec 可以使用 success=1 跳过一行(如果需要,可以跳过多行,在我修改它之前,RHEL8 上的代码已经是意大利面条式代码了,这确实使事情变得更加复杂)。
我使用了一个 perl 脚本:
#!/usr/bin/perl
if ($ENV{'SSH_AUTH_INFO_0'} =~ /^publickey /) {
exit(0);
} else {
exit(1);
}
并且它使用此行调用(success=2 是因为预先存在的代码,我不得不跳过 pam_sss.so 后面的 pam_deny(它是“足够的”并且没有遇到拒绝)。是否使用“ignore”或“ok”取决于 sshd 的其余 PAM 堆栈。(并且在某些情况下,如果您在输入密码之前提示输入 OTP,您可能能够使用“success=done”,尽管当我认为它应该工作时,我无法让它工作。)
auth [success=2 default=ignore] pam_exec.so quiet /usr/local/sbin/ssh-pubkey-check
您还需要在 sshd_config 中设置这行有点奇怪的内容:
AuthenticationMethods publickey,keyboard-interactive keyboard-interactive
它首先尝试公钥和键盘交互。如果成功,PAM 将从环境变量中得知它可以完全跳过密码提示。如果失败(因为没有提供公钥),它接下来只尝试键盘交互,但在这种情况下,PAM 将提示输入 OTP(留给学生练习)和密码提示。
还有更多内容,但这一切都取决于您现有的 PAM 设置是什么样的,以及您要根据什么进行身份验证(本地密码文件、LDAP、NIS 等等),所以我无法提供一刀切的解决方案。
还要注意,使用环境变量来控制身份验证步骤感觉不太好,理论上可能会导致只需要 OTP 而不需要其他令牌的漏洞。尽管在我能想到的每种情况下,如果有人可以远程或本地设置 PAM 从 sshd 调用时看到的环境变量,那么您的系统已经受到攻击,这不是额外的风险。
答案2
请注意,“pam_permit.so”是一个危险的模块。
我认为你的说法与你的两条规则相矛盾:
身份验证需要 pam_google_authenticator.so nullok
相对
身份验证需要 pam_permit.so
使用“nullok”关键字应该足以实现您想要做的事情。没有设置 google-authentication 的用户也可以登录,而其他用户则需要输入验证码。 身份验证方法公钥、键盘交互/etc/ssh/sshd_config 文件里面就可以了。
此外,您还有两个选项可以尝试:在文件中列出用户:/etc/authusers 稍后您可以参考sshd 配置文件
身份验证充分 pam_securid.so
身份验证需要 pam_deny.so
auth requisite pam_listfile.so item=用户感知=允许文件=/etc/authusers
...或使用用户组:
sshd 配置文件
身份验证 [成功 = 完成 默认 = 忽略] pam_succeed_if.so 用户组“groupname_here”