fix(landjail): allow TCP bind under Landlock

[openmarmot]

fix(landjail): allow TCP bind under Landlock

Problem

With --jail-type landjail, processes that listen on a local TCP port fail immediately (e.g. opencode web → ServeError / bind permission denied).

Landlock ABI V4 covers both bind_tcp and connect_tcp. Landjail used landlock.V4.RestrictNet(...) with only ConnectTCP(proxyPort) rules. Because bind was handled but never allowed, all binds were denied (EACCES), while connect to the proxy still worked.

Change

In landjail/child.go, restrict connect only:

• Handled rights: AccessNetConnectTCP
• Allowed: connect to the Boundary proxy port
• Bind/listen: unrestricted (not handled by Landlock)

Egress remains proxy-only. Local servers inside the jail can bind normally.

Docs updated in docs/architecture.md.

How to test

make build

# 1) Bind was broken before; should succeed now
./boundary --jail-type landjail --proxy-port 18080 --allow "domain=*" -- \
  python3 -c 'import socket; s=socket.socket(); s.bind(("127.0.0.1", 4096)); print("BIND_OK")'

# 2) Direct egress still blocked
./boundary --jail-type landjail --proxy-port 18080 --allow "domain=*" -- \
  python3 -c 'import socket; s=socket.socket(); s.connect(("1.1.1.1", 443))'
# expect: PermissionError / errno 13

# 3) Real workload that needs a listen port
./boundary --jail-type landjail --proxy-port 18080 --allow "domain=*" -- \
  opencode web --port 4096
# expect: opencode web works

[SasSwart]

Hi there @openmarmot,

Apologies for the delay. I've been out ill for a bit.
This looks good. It fixes something that I've been running into myself.
I'd like to do some minor quick local testing when I'm back in the office and then we'll move ahead with this!

Thanks for the contribution.