Egress proxy

Default-deny HTTP(S) proxy with allow rules and secrets.

grain can run a host-side HTTP(S) forward proxy that guest VMs use via HTTPS_PROXY / HTTP_PROXY. Outbound traffic is default-deny: only explicit allow rules (host, optional method, optional path prefix) pass. Rules may also inject an Authorization header from the host secrets store so the guest never holds the real token.

guest ──HTTPS_PROXY──► grain-proxy (host) ──► internet
                         │
                         ├ allow rules (host, method, path prefix)
                         └ secrets (inject Bearer on match)

The proxy is a separate process from the control-plane daemon (grain up) so a proxy crash does not take down VM management. Both share ~/.grain/secrets.

Quick start

# 1) start proxy (listens 0.0.0.0:3128 by default)
grain proxy up

# 2) create a client token (optional but recommended)
grain proxy client create agent
# prints: token <hex>
# guest:  export HTTPS_PROXY=http://<token>@10.0.2.2:3128

# 3) allow only the APIs you need
grain secret set openai-key --value 'sk-…'
grain proxy allow --host api.openai.com --secret openai-key
# HTTPS from the guest uses CONNECT; host match is enough (path not visible)

# Plain HTTP can also match method/path and inject Authorization:
grain proxy allow --host api.example.com --method POST --path /v1/ --secret openai-key

# 4) launch a VM with proxy env baked in
grain new --proxy -n agent

# or set manually inside the guest:
#   export HTTPS_PROXY=http://TOKEN@10.0.2.2:3128
#   export HTTP_PROXY=$HTTPS_PROXY
#   export NO_PROXY=localhost,127.0.0.1,10.0.2.0/24

List and remove rules:

grain proxy ls
grain proxy deny rule-1
grain proxy down

Listen address (important)

Config (~/.grain/config.yaml):

proxy_listen: 0.0.0.0:3128

CLI override:

grain proxy up --listen 0.0.0.0:3128

Firewall: binding 0.0.0.0 accepts connections on all host interfaces. On multi-user or exposed machines, restrict with a host firewall (e.g. allow only loopback + the SLIRP path) or put the host behind a private network. Create proxy clients so a token is required; until the first client exists, the proxy accepts unauthenticated connections (convenient for local dev, not for shared hosts).

Allow rules

Rules are stored in ~/.grain/proxy/rules.json.

HTTPS (CONNECT): browsers and curl open a tunnel; the proxy only sees host:port. Path/method allowlists do not apply inside the TLS session. Use host allow rules for HTTPS. Secret injection on CONNECT is not possible (payload is opaque TLS); inject only works for plain HTTP proxied requests (or tools that speak HTTP to the proxy with absolute URLs).

Default deny: if no rule matches, the proxy returns 403.

Secret injection

grain secret set gh-token --value 'ghp_…'
grain proxy allow --host api.github.com --path / --secret gh-token

On a matching plain-HTTP request, the proxy sets:

Authorization: Bearer <secret bytes>

If the secret already starts with Bearer or Basic , it is used as the full header value.

Client authentication

grain proxy client create myvm

Clients live in ~/.grain/proxy/clients.json (mode 0600). When any client exists, the proxy requires a token via:

• URL userinfo: http://TOKEN@10.0.2.2:3128
• Or Proxy-Authorization: Basic … / Bearer …

Guest integration

grain new --proxy

Creates a client token if none exist, then merges cloud-init that writes /etc/profile.d/grain-proxy.sh and /etc/environment with HTTP_PROXY / HTTPS_PROXY / NO_PROXY pointing at http://TOKEN@10.0.2.2:<port>.

Manual / profile

#cloud-config
write_files:
  - path: /etc/profile.d/grain-proxy.sh
    permissions: '0644'
    content: |
      export HTTPS_PROXY=http://TOKEN@10.0.2.2:3128
      export HTTP_PROXY=$HTTPS_PROXY
      export NO_PROXY=localhost,127.0.0.1,10.0.2.0/24

Coding agents

See recipes/coding-agent.md for a sandbox with a mounted repo; add grain proxy up + allow rules for model/API hosts and grain new --proxy so the agent cannot reach the open internet.

Security model

1. Default deny — no implicit open egress through the proxy.
2. Allowlist — only listed hosts (and optional method/path for HTTP).
3. Host-held secrets — tokens stay on the host; guests get a proxy client token only (revocable by rotating clients).
4. Process isolation — proxy pid/log under ~/.grain/proxy/; separate from grain up.
5. Not a network policy engine — guests can still use non-proxy egress (raw TCP via SLIRP) unless you add further host controls. The proxy is an application-level control for tools that honor HTTP(S)_PROXY.

Files

Related

• Networking — SLIRP, hostfwd, ports
• Recipes: coding agent