配置文件

Question

我有一个普通的 Linux 用户：

1. 只允许通过 ssh 登录来执行单个命令，不允许执行其他命令
2. 可能永远不会一次登录多次

第一个限制是使用command用户authorized_keys文件中的选项实现的，并且可以正常工作。

第二个限制是使用 PAM 来强制执行的/etc/security/limits.conf。当我将它与前面的限制结合使用时，这部分似乎根本不起作用，但否则可以工作。也就是说，如果我将 ssh 登录限制为单个命令，我可以根据需要多次同时登录。如果我不使用该command选项，则不允许我在给定时间登录多次。 （登录被拒绝并出现常见错误Too many logins for user 'X'.：）

command为什么在使用该选项时 PAM 无法限制登录authorized_keys？我该如何同时强制执行上述两个限制？

sshd 配置为使用 pam ( UsePAM yes)。

附录：经过进一步调查，似乎正在针对所有 ssh 连接（指定和不指定选项）maxlogins评估限制。command但是，由于登录/pty 会话是唯一进行计数的会话类型maxlogins，因此只要交互式会话的数量保持在以下水平，就可以打开任意数量的并行非交互式会话maxlogins。

我可以看到两种方法可以解决这个问题：

1. 让 sshd 本身强制执行每个用户的单个连接限制
2. 找到一种方法让 linux 计算非交互式 shellmaxlogins

我浏览了两者的手册页sshd (8)，sshd_config(8)但无法找到任何相关内容。我可以强制使用 tty 客户端（例如ssh -tt），但这会将限制执行留给客户端。

配置文件

我正在尝试在基于 Debian 的 docker 容器中进行设置。除了添加到 的四行之外，/etc/pam.d/sshd和 与上游相比都没有变化。/etc/ssh/sshd_configsshd_config

/etc/pam.d/sshd

# PAM configuration for the Secure Shell service

# Standard Un*x authentication.
@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_selinux.so close

# 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

/etc/ssh/sshd_config

#       $OpenBSD: sshd_config,v 1.103 2018/04/09 20:41:22 tj 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)
ChallengeResponseAuthentication no

# 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

#AllowAgentForwarding yes
#AllowTcpForwarding yes
#GatewayPorts no
X11Forwarding yes
#X11DisplayOffset 10
#X11UseLocalhost yes
#PermitTTY yes
PrintMotd no
#PrintLastLog yes
#TCPKeepAlive yes
#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
PubkeyAcceptedKeyTypes            ssh-ed25519
HostKeyAlgorithms                 ssh-ed25519
ChallengeResponseAuthentication   no
PasswordAuthentication            no

Answer 1

这不是这个问题的一般答案，但它确实解决了不允许并发登录而只允许运行单个命令的问题：

如果您愿意回避使用来限制用户，则可以将的选项maxlogins的值用 包裹起来。那是：authorized_keyscommandflock

command="flock /path/to/lock ORIGINAL_COMMAND ARGS...", ssh-ed25519 AAAAA...

根据您的用例，您可能想研究一些集群选项（noblock、超时等）。