greywall/docs/configuration.md

Configuration

Fence reads settings from ~/.fence.json by default (or pass --settings ./fence.json). Config files support JSONC.

Example config:

{
  "network": {
    "allowedDomains": ["github.com", "*.npmjs.org", "registry.yarnpkg.com"],
    "deniedDomains": ["evil.com"]
  },
  "filesystem": {
    "denyRead": ["/etc/passwd"],
    "allowWrite": [".", "/tmp"],
    "denyWrite": [".git/hooks"]
  },
  "command": {
    "deny": ["git push", "npm publish"]
  }
}

Config Inheritance

You can extend built-in templates or other config files using the extends field. This reduces boilerplate by inheriting settings from a base and only specifying your overrides.

Extending a template

{
  "extends": "code",
  "network": {
    "allowedDomains": ["private-registry.company.com"]
  }
}

This config:

• Inherits all settings from the code template (LLM providers, package registries, filesystem protections, command restrictions)
• Adds private-registry.company.com to the allowed domains list

Extending a file

You can also extend other config files using absolute or relative paths:

{
  "extends": "./base-config.json",
  "network": {
    "allowedDomains": ["extra-domain.com"]
  }
}

{
  "extends": "/etc/fence/company-base.json",
  "filesystem": {
    "denyRead": ["~/company-secrets/**"]
  }
}

Relative paths are resolved relative to the config file's directory. The extended file is validated before merging.

Detection

The extends value is treated as a file path if it contains / or \, or starts with .. Otherwise it's treated as a template name.

Merge behavior

• Slice fields (domains, paths, commands) are appended and deduplicated
• Boolean fields use OR logic (true if either base or override enables it)
• Integer fields (ports) use override-wins semantics (0 keeps base value)

Chaining

Extends chains are supported—a file can extend a template, and another file can extend that file. Circular extends are detected and rejected. Maximum chain depth is 10.

See templates.md for available templates.

Network Configuration

Field	Description
allowedDomains	List of allowed domains. Supports wildcards like *.example.com
deniedDomains	List of denied domains (checked before allowed)
allowUnixSockets	List of allowed Unix socket paths (macOS)
allowAllUnixSockets	Allow all Unix sockets
allowLocalBinding	Allow binding to local ports
allowLocalOutbound	Allow outbound connections to localhost, e.g., local DBs (defaults to allowLocalBinding if not set)
httpProxyPort	Fixed port for HTTP proxy (default: random available port)
socksProxyPort	Fixed port for SOCKS5 proxy (default: random available port)

Wildcard Domain Access

Setting allowedDomains: ["*"] enables relaxed network mode:

• Direct network connections are allowed (sandbox doesn't block outbound)
• Proxy still runs for apps that respect HTTP_PROXY
• deniedDomains is only enforced for apps using the proxy

Warning

Security tradeoff: Apps that ignore HTTP_PROXY will bypass deniedDomains filtering entirely.

Use this when you need to support apps that don't respect proxy environment variables.

Filesystem Configuration

Field	Description
denyRead	Paths to deny reading (deny-only pattern)
allowWrite	Paths to allow writing
denyWrite	Paths to deny writing (takes precedence)
allowGitConfig	Allow writes to .git/config files

Command Configuration

Block specific commands from being executed, even within command chains.

Field	Description
deny	List of command prefixes to block (e.g., ["git push", "rm -rf"])
allow	List of command prefixes to allow, overriding deny
useDefaults	Enable default deny list of dangerous system commands (default: true)

Example:

{
  "command": {
    "deny": ["git push", "npm publish"],
    "allow": ["git push origin docs"]
  }
}

Default Denied Commands

When useDefaults is true (the default), fence blocks these dangerous commands:

• System control: shutdown, reboot, halt, poweroff, init 0/6
• Kernel manipulation: insmod, rmmod, modprobe, kexec
• Disk operations: mkfs*, fdisk, parted, dd if=
• Container escape: docker run -v /:/, docker run --privileged
• Namespace escape: chroot, unshare, nsenter

To disable defaults: "useDefaults": false

Command Detection

Fence detects blocked commands in:

• Direct commands: git push origin main
• Command chains: ls && git push or ls; git push
• Pipelines: echo test | git push
• Shell invocations: bash -c "git push" or sh -lc "ls && git push"

Other Options

Field	Description
allowPty	Allow pseudo-terminal (PTY) allocation in the sandbox (for MacOS)

See Also

• Config templates: docs/templates/
• Workflow guides: docs/recipes/