I can see two ways I can tackle this:

I can see two ways I can tackle this problem:

Configuration Files

I'm attempting to set this up in a Debian-based docker container. Both /etc/pam.d/sshd and /etc/ssh/sshd_config are unchanged from upstream except for the four lines added to sshd_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

I have a normal Linux user that is:

1. only allowed to login via ssh to execute a single command and nothing else
2. may never login more the once at a single time

The first restriction is implemented using the command option in the user's authorized_keys file, and works without issue.

The second restriction is implemented using PAM to enforce /etc/security/limits.conf. This part doesn't seem to work at all when I use it in conjunction with the previous restriction, but works otherwise. That is, If I limit ssh logins to a single command, I can log in concurrently as many times as I want. If I do not use the command option, I am not allowed to login more than once at a given time. (login is rejected with the usual error: Too many logins for user 'X'.)

Why is PAM failing to restrict logins when the command option is used in authorized_keys and what can I do to enforce the two aforementioned restrictions as the same time?

sshd is configured to use pam (UsePAM yes).

Addendum: Upon further investigation, it appears that maxlogins is in fact being respected in all cases. However, because login/pty sessions are the only type of sessions being counted against the limit, an unlimited number of parallel non-interactive sessions can be started as long as the number of interactive sessions is bellow maxlogins.

I can see two ways I can tackle this:

1. have sshd itself enforce the single connection per user restriction
2. find a way to get linux to count non-interactive shells against maxlogins

I've skimed through the manual pages for both sshd (8) and sshd_config(8) but was unable to find anything relevant. I can force a tty client side (e.g. ssh -tt) but that would leave restriction enforcement in the hands of the clients.

I have a normal Linux user that is:

1. only allowed to login via ssh to execute a single command and nothing else
2. may never login more the once at a single time

The first restriction is implemented using the command option in the user's authorized_keys file, and works without issue.

The second restriction is implemented using PAM to enforce /etc/security/limits.conf. This part doesn't seem to work at all when I use it in conjunction with the previous restriction, but works otherwise. That is, If I limit ssh logins to a single command, I can log in concurrently as many times as I want. If I do not use the command option, I am not allowed to login more than once at a given time. (login is rejected with the usual error: Too many logins for user 'X'.)

Why is PAM failing to restrict logins when the command option is used in authorized_keys and what can I do to enforce the two aforementioned restrictions as the same time?

sshd is configured to use pam (UsePAM yes).

Addendum: Upon further investigation, it appears that the maxlogins restriction is being evaluated for all ssh connections (both with the command option specified and without). However, because login/pty sessions are the only type of sessions being counted against maxlogins, any number of parallel non-interactive sessions can be opened as long as the number of interactive sessions remains bellow maxlogins.

I can see two ways I can tackle this:

1. have sshd itself enforce the single connection per user restriction
2. find a way to get linux to count non-interactive shells against maxlogins

I've skimed through the manual pages for both sshd (8) and sshd_config(8) but was unable to find anything relevant. I can force a tty client side (e.g. ssh -tt) but that would leave restriction enforcement in the hands of the clients.