OpenSSH Differences

Most integration surprises on an ssh lane are an assumption carried over from sshd. This page is the list of them. An end-hop is a complete SSH server for the paths it implements, not a drop-in sshd. It authenticates certificates, resolves accounts, spawns processes and honours login shells the way sshd does. It does not read sshd_config, does not invoke PAM, and does not implement most of what an OpenSSH deployment accumulates.

`sshd_config` mapping

Start here, because it shows which parts of an ssh lane are a rename of something you already know and which parts are genuinely new. The left column is what capabilities_allowed accepts; see Configuration.

Capability	`sshd_config` equivalent	Its default there
`shell`	none. `ForceCommand` replaces what runs; nothing refuses a shell as such	a shell is available
`exec`	none. Same keyword, same lack of a distinction from `shell`	a command is available
`env`	`AcceptEnv`, `PermitUserEnvironment`	accepts none
`sftp`	`Subsystem sftp …`; `ForceCommand internal-sftp`; `ChrootDirectory`	declared in the shipped config
`subsystem`	`Subsystem <name> <command>`	nothing available unless declared
`local_forward`	`AllowTcpForwarding`, `PermitOpen`, `DisableForwarding`	permitted, to any destination
`remote_forward`	`AllowTcpForwarding`, `PermitListen`, `GatewayPorts`	permitted, to any bind address
`pty`	`PermitTTY`	permitted
`agent_forward`	`AllowAgentForwarding`	permitted
`x11`	`X11Forwarding`	denied

The capabilities this design inspects are the ones OpenSSH cannot name. There is no keyword separating shell from exec, and none that treats a file operation as a statement — the closest control is ForceCommand, which replaces what runs rather than judging it. That is the gap this lane fills. AcceptEnv is the one keyword that is already default-deny, and it narrows by variable name, which is exactly how an env_set guardrail matches. This design does not copy that default — an omitted capabilities_allowed admits env. A deployment that wants OpenSSH’s answer writes the list without env.

Assumptions to drop

You may assume	What actually happens
NSS — LDAP, SSSD, AD, `systemd-homed` accounts	only `/etc/passwd` and `/etc/group` are consulted. This is not a build flag: the login shell is read from `/etc/passwd` directly, so even a build that resolves the account through NSS refuses the session for having no passwd entry. Put the served accounts in the image, or run one listener per account
`AuthorizedPrincipalsFile` / `AuthorizedPrincipalsCommand`	not read, and there is no equivalent. This is the mapping layer that lets `sshd` turn a principal like `alice@corp.example` into the account `devuser`. Without it, principals must be literal login names; carry identity in `-I` or a namespaced extension
`authorized_keys`	not read. Certificates only — a bare public key is refused, and there is no per-user state to hold an exception in
passwords, keyboard-interactive	no such method is offered, so there is nothing to brute-force. This is why a listener on a public address is defensible
`force-command`, `verify-required`, any critical option but `source-address`	the certificate is refused, not admitted with the option ignored. Fail-closed and loud, but it is a real interop edge with an existing CA
a KRL, `RevokedKeys`	there is no revocation. The serial is recorded in the audit trail but never checked. A short `-V` is the whole expiry story
host certificates	`host_key` takes a plain private key. A client with an `@cert-authority` line in `known_hosts` will not match it — distribute or pin the host public key
`AllowUsers`, `DenyUsers`, `PermitRootLogin`, `Match` blocks	not read, no equivalent. Nothing refuses a session as root; what bounds a privileged session is the capability list and the guardrail chain, which run identically whatever the account
PAM	not invoked. No `pam_limits`, no `pam_access`, no session modules
`utmp` / `wtmp` / `lastlog`	not written. `who` and `last` will not show these sessions — the audit trail is where they are recorded
`motd`, login banners	not printed
`ChrootDirectory`	no equivalent
`SSH_TTY`	not set. `SSH_CONNECTION` and `SSH_CLIENT` are, in `sshd`’s own spelling

What a session gets

So .profile and audit rules behave:

USER, LOGNAME, HOME, SHELL, PATH, plus TERM when a pty was allocated, and any env variables that passed the guardrails.
SSH_CONNECTION and SSH_CLIENT.
The Sidecar’s own environment is never inherited — it holds the Sidecar’s configuration and possibly its credentials.
An interactive session runs <shell> -l, so profile files are sourced. A command runs <shell> -c, so they are not. Same split as sshd.
Supplementary groups are applied in the child, setgroups → setgid → setuid, before exec.

Not implemented

Capability	Status
`remote_forward`	the refusal works; the return path that would make it function was never built
`agent_forward`	no handler exists
`x11`	no handler exists
any `subsystem` other than `sftp`	no handler exists
unix-socket forwarding, tun-device forwarding	separate channel types with no capability name. Refused by construction — an unhandled channel type is rejected

Naming any of the first four in capabilities_allowed fails at load, because the config would be asking for something this version cannot do. Refusing the request at runtime instead would leave an operator believing a capability is on when it is not.

Where it is stricter

Not everything on this page is a subtraction.

	`sshd`	An `ssh` lane
A certificate with no principals	valid for every login name	refused
A certificate that names nobody	no such concept — the key id is free-form and may be empty	refused, whichever field `identity.subject` reads
An unknown critical option	refused (same)	refused (same)
Forwarding destinations	permitted to any destination by default	denied until `destinations_allowed` names one
Client environment variables	`AcceptEnv` names them; guardrails cannot judge them	admitted by capability, then judged by name with an `env_set` rule
A command	runs, or is replaced wholesale by `ForceCommand`	read as a statement, matched by rule, classified by the analyzer, audited in full
A file operation	the subsystem is on or off	one statement per operation per path, each with a verdict recorded before the filesystem is touched

The last two rows are the point of the lane. The rest of this page is the price.

Host Configuration

The accounts, groups and shells that have to be in place before any of this applies.

Configuration

Every attribute, the full capability surface, and what each one can carry.

Getting Started

Sidecar

Features

Control Plane Setup

`sshd_config` mapping

Assumptions to drop

What a session gets

Not implemented

Where it is stricter

Next

Host Configuration

Configuration

​sshd_config mapping

​Assumptions to drop

​What a session gets

​Not implemented

​Where it is stricter

​Next

Host Configuration

Configuration

`sshd_config` mapping

Assumptions to drop

What a session gets

Not implemented

Where it is stricter

Next