A connection that cannot point at the machine it is running on

"Nothing runs on the LLeMbas host" is the sentence the absent sandbox and the
absent local MCP rest on, and an SSH profile aimed at 127.0.0.1 walked straight
past it -- through a real login, with every gate in policy.py still applying,
onto the machine holding the database and the Fernet key. From the SSH layer
down it is indistinguishable from a container on the network, so nothing here
could have noticed.

One switch, three positions: never, one named port, anywhere. The middle one is
the one with a real use -- a container that published its SSH port on the
loopback interface is genuinely somewhere else -- and port 22 is refused even
there, because that one is this host's own sshd.

Enforced in five places, because a row can predate a setting: saving a profile,
`session.resolve` (the control every agent tool, the terminal and the canvas go
through), the composer's picker, browsing, and the draft the panels open against
before a chat exists. Check refuses before it opens its socket rather than after.

And the recognition never resolves a name on the request path. `refusal` runs
several times per page render; the first version of this looked names up inline
and the suite went from two minutes to not finishing. Literal forms are decided
from the string, a name is settled where a network call is already expected, and
the answer lives on the row. The gap that leaves is written down rather than
discovered.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

src/lembas/api/agents.py

@@ -25,6 +25,7 @@ from lembas.api.pages import sidebar_context
 from lembas.db.models import AUTH_METHODS, AUTH_PASSWORD, SshProfile
 from lembas.services import settings_store
 from lembas.services.agent import draft as draft_service
+from lembas.services.agent import hosts
 from lembas.services.agent import index as index_service
 from lembas.services.agent import jobs as jobs_service
 from lembas.services.agent import ssh as ssh_service
@@ -119,6 +120,9 @@ def _detail(
             else "",
             "has_key": bool(profile.private_key_encrypted),
             "problem": ssh_service.available(),
+            # Empty on the new-connection page, where there is no host yet to
+            # ask about -- the answer arrives when it is submitted.
+            "refused": hosts.refusal_for(db, profile) if profile.host else "",
         },
     )
 
@@ -130,7 +134,11 @@ async def agents_page(request: Request, db: Db, user: RequiredUser, saved: str =
         "agents/index.html",
         {
             **sidebar_context(db, user),
-            "profiles": _owned(db, user.id),
+            "profiles": (owned := _owned(db, user.id)),
+            # Keyed by id rather than resolved in the template, because the
+            # template has no session and this is a question about instance
+            # settings, not about the row.
+            "refusals": {p.id: hosts.refusal_for(db, p) for p in owned},
             "saved": saved,
             "problem": ssh_service.available(),
             "enabled": bool(settings_store.agents(db).get("enabled")),
@@ -179,6 +187,18 @@ def _problem(db: Db, profile: SshProfile, owner_id: str, *, existing_id: str = "
     if not profile.username:
         return "A connection needs a username to log in as."
 
+    # Saving is one of the two moments a DNS lookup is affordable, so this is
+    # where a *name* pointing at loopback is settled and written to the row for
+    # every later request to read for free. See services/agent/hosts.py.
+    #
+    # Not the last word -- `session.resolve` refuses one that was saved before an
+    # administrator moved the switch, and has to, because a row can predate a
+    # setting. This is here so the refusal arrives while somebody is looking at
+    # the form that caused it rather than at an agent chat with no tools.
+    resolved = hosts.restamp(profile)
+    if refused := hosts.refusal(db, profile.host, profile.port, resolved=resolved):
+        return refused
+
     clash = db.scalar(
         select(SshProfile).where(
             SshProfile.owner_id == owner_id, SshProfile.name == profile.name
@@ -224,7 +244,11 @@ async def browse_profile(
     entries: list = []
     error = ""
 
-    if hint := ssh_service.available():
+    if refused := hosts.refusal_for(db, profile):
+        # First, because this one opens a connection and the others only explain
+        # why one would fail.
+        error = refused
+    elif hint := ssh_service.available():
         error = hint
     elif not profile.host_key:
         # connect_kwargs would raise the same thing, but a picker that opens on
@@ -300,6 +324,12 @@ async def draft_target(db: Db, user: RequiredUser, profile_id: str, dir: str = "
     finds the shell already running there rather than opening a second one.
     """
     profile = _profile(db, user, profile_id)
+    # A draft is what the terminal and the canvas open against before a chat
+    # exists, so refusing here is refusing the whole new-chat path. `resolve`
+    # would refuse it anyway once a chat existed; this stops the panel opening
+    # on a target it will not be allowed to use.
+    if refused := hosts.refusal_for(db, profile):
+        raise HTTPException(status.HTTP_403_FORBIDDEN, refused)
     draft = draft_service.remember(user.id, profile.id, dir or profile.default_dir or "")
     return {"id": draft.id, "dir": draft.project_dir}
 
@@ -406,6 +436,18 @@ async def check_profile(request: Request, db: Db, user: RequiredUser, profile_id
     """
     profile = _profile(db, user, profile_id)
 
+    # Before anything is sent. Check is the one button here that opens a socket,
+    # so a refused connection must not get one -- and the reason belongs in the
+    # place somebody just pressed rather than in a log.
+    #
+    # The other moment a lookup is affordable, and the one that catches a name
+    # whose DNS moved after it was saved: this button is how somebody finds out
+    # a connection has stopped working, so it is the right place to find out why.
+    hosts.restamp(profile)
+    db.commit()
+    if refused := hosts.refusal_for(db, profile):
+        return render(request, "agents/_check.html", {"profile": profile, "error": refused})
+
     try:
         line, fingerprint = await ssh_service.capture_host_key(
             profile.host, profile.port, timeout=profile.connect_timeout