The guest still has no network interface, and now that is the design rather than
a gap. Its only route out is an HTTP CONNECT proxy: agent CLI -> 127.0.0.1:3128
in the guest -> vsock 9002 -> a per-VM Unix socket on the host -> TLS to an
allow-listed host.
Why not TAP + iptables, which is what the Firecracker write-ups do — measured,
not argued:
- `ip tuntap add` is DENIED to the daemon user (needs CAP_NET_ADMIN), so TAP
would need root to pre-provision devices, the same privilege detour the
loop-mounted rootfs already forced.
- tank's FORWARD policy is DROP with Docker and Tailscale chains, so rules
would have to be inserted at position 1; appended ones die silently.
- a leaked TAP is a new class of host litter to reap.
CONNECT needs no privilege at all and is better on the merits: the client hands
us the HOSTNAME, so resolution happens host-side and the guest needs no DNS or
resolv.conf; the allow-list is by name, not address; and nothing in the guest can
reach the network except through one function. The guest end parses nothing and
enforces nothing, so a compromised agent cannot argue with the policy.
Rests on one measured fact: `claude` honours HTTPS_PROXY. With the proxy at a
closed port, `claude -p` fails ConnectionRefused instead of answering.
THE RESULT: a real agent turn now completes inside a VM with no network card, on
subscription auth — `claude -p` replies VM-OK. The selftest asks for it whenever
CLAUDE_CODE_OAUTH_TOKEN is present and SKIPS loudly when it is not, since it
spends a little of the plan.
The audit log earns its keep immediately: during that turn the proxy logged
`egress DENIED http-intake.logs.us5.datadoghq.com` — the CLI's telemetry, which
the mission container permits today without anyone deciding to.
Three bugs found by the checks rather than by review:
- `env_pairs` returned early when a caller sent no env, so the proxy address
was never added and `curl` in a VM with a working tunnel reported "Could not
resolve host". Absent env means "the caller sent none", not "this command
needs no environment".
- the deny check PASSED for the wrong reason — DNS was failing, so nothing was
refused by the allow-list at all. It now requires a 403 from the proxy, so it
cannot go green on a broken tunnel.
- `host_allowed` accepted `evil.test/api.anthropic.com`, which ends with an
allowed suffix. Hostnames are now validated against a character class, which
also refuses IP literals so an address cannot sidestep a list of names.
- `BufReader::into_inner()` discards buffered bytes: wrapping the stream twice
would have dropped the start of the TLS handshake and stalled a tunnel that
looked established. One reader now spans the request, and anything buffered
past the headers is forwarded as payload.
`iproute2` is in agent-toolchain because it is load-bearing: the guest's `lo`
starts DOWN, and while it is down a listener on loopback BINDS and then refuses
every connection with ENETUNREACH. fcagent finds `ip` by absolute path — as pid 1
its PATH comes from the kernel, and execvp's fallback excludes /usr/sbin, where
Debian puts it.
Egress needs both ends up, so `create` reports `egress` and the guest's `ping`
reports its own half. A VM without it is legal but never silent.
Verified on tank: 16/16 with backend=claude (create 1428 ms), 12/12 on the
default rootfs, no leaked processes, VM dirs or proxy sockets. 457 tests pass,
clippy clean.
Co-Authored-By: Claude Opus 5 <[email protected]>
