ModelRouter/internal/lua/plugin_http.go

package lua

// Outbound HTTP for plugins.
//
// The billing plugin never needed this: everything it reports arrives on a hook
// payload that the gateway produced while forwarding a request. A quota plugin
// is the first thing that must REACH OUT on its own — Command Code exposes
// /alpha/billing/credits and friends, and there is no way to learn a plan's
// remaining balance without asking.
//
// That capability is also the sharpest edge in the plugin model. A plugin is a
// single .lua file the WebUI installs from a textarea: untrusted, browser-facing
// code that runs in the gateway process. Handing it a general-purpose HTTP
// client would make every install a standing SSRF primitive against whatever
// the gateway can reach — the cloud metadata endpoint, the cluster control
// plane, whatever else sits on 127.0.0.1 in this deployment. Reading the
// environment for credentials is a second door out.
//
// So the bridge is deliberately narrow:
//
//   - Destination must match a host that a CONFIGURED SOURCE already talks to.
//     Those are operator-declared, not plugin-declared: a plugin cannot widen
//     its own reach by inventing a URL, and an existing source's host is
//     exactly the set of upstreams this deployment already trusts with API keys.
//   - http:// on a non-loopback host is refused. Credentials belong in HTTPS.
//   - Loopback needs no explicit opt-in: it is the only way a plugin can read
//     the gateway's own API (the billing rules UI does exactly that), and it
//     cannot leave the machine.
//   - Every request is bounded by a timeout and the response is size-capped, so
//     a plugin cannot wedge a worker goroutine or exhaust memory.
//   - Credentials are NEVER read from the environment here. A plugin that needs
//     a key must be given it (state, or the caller's own config), which keeps
//     the grant visible to the operator instead of silently ambient.
//
// Redirects are NOT followed: the allowlist is checked per hop, and a redirect
// to an unlisted host would otherwise walk straight past it. A 3xx is returned
// as-is so the plugin can see it.

import (
	"fmt"
	"io"
	"net"
	"net/http"
	"net/url"
	"strings"
	"time"

	golua "github.com/aarzilli/golua/lua"
)

// httpDefaultTimeout bounds a whole request; httpMaxResponseBytes caps the body
// a plugin can pull into the gateway's heap.
const (
	httpDefaultTimeout = 15 * time.Second
	httpMaxTimeout     = 60 * time.Second
	httpMaxResponse    = 4 << 20 // 4 MiB
)

// httpClient is shared by every plugin request. It deliberately has no
// ProxyFromEnvironment: an operator who proxies upstreams configures the proxy
// per source, and a plugin request should not silently inherit one.
var httpClient = &http.Client{
	Timeout: httpDefaultTimeout,
	Transport: &http.Transport{
		Proxy:                 nil,
		DialContext:           (&net.Dialer{Timeout: 10 * time.Second, KeepAlive: 30 * time.Second}).DialContext,
		ForceAttemptHTTP2:     true,
		MaxIdleConns:          16,
		IdleConnTimeout:       60 * time.Second,
		TLSHandshakeTimeout:   10 * time.Second,
		ExpectContinueTimeout: time.Second,
	},
	// See above: a redirect is a hop the allowlist has not seen.
	CheckRedirect: func(*http.Request, []*http.Request) error {
		return http.ErrUseLastResponse
	},
}

// httpGlobal is the Lua name of the outbound bridge.
const httpGlobal = "__llmsproxy_http"

// httpAllowlist is the set of hosts plugins may contact: the host of every
// configured source's base_url.
type httpAllowlist struct {
	hosts map[string]bool
}

func newHTTPAllowlist(hosts []string) *httpAllowlist {
	m := make(map[string]bool, len(hosts))
	for _, h := range hosts {
		if h != "" {
			m[strings.ToLower(h)] = true
		}
	}
	return &httpAllowlist{hosts: m}
}

// allows reports whether host:port may be contacted. Ports 80/443 are implied
// by the bare host so a source configured as https://api.example.com/v1 does not
// have to spell the port.
func (a *httpAllowlist) allows(host string, port string) bool {
	host = strings.ToLower(host)
	if host == "" {
		return false
	}
	if a.hosts[host] {
		return true
	}
	switch port {
	case "", "80", "443":
		return a.hosts[host]
	}
	return a.hosts[net.JoinHostPort(host, port)]
}

// checkURL validates a plugin-supplied URL against the allowlist. It returns
// the parsed URL plus a reason when it is refused.
func (a *httpAllowlist) checkURL(raw string) (*url.URL, error) {
	u, err := url.Parse(strings.TrimSpace(raw))
	if err != nil {
		return nil, fmt.Errorf("bad url: %v", err)
	}
	switch strings.ToLower(u.Scheme) {
	case "https":
	case "http":
		// Plain HTTP is only tolerated on loopback, where it stays on this
		// machine and is how the gateway's own plain-HTTP dev port is reached.
		if !isLoopbackHost(u.Hostname()) {
			return nil, fmt.Errorf("refusing plain http to non-loopback host %q: use https", u.Hostname())
		}
	default:
		return nil, fmt.Errorf("unsupported scheme %q", u.Scheme)
	}
	if !a.allows(u.Hostname(), u.Port()) && !isLoopbackHost(u.Hostname()) {
		// Loopback needs no entry: it cannot leave the machine, and the billing
		// rules UI reads the gateway's own API over plain HTTP in dev setups.
		return nil, fmt.Errorf("host %q is not an allowed outbound destination "+
			"(allowed: the hosts of configured sources, plus loopback)", u.Hostname())
	}
	return u, nil
}

func isLoopbackHost(h string) bool {
	if h == "localhost" {
		return true
	}
	ip := net.ParseIP(strings.Trim(h, "[]"))
	return ip != nil && ip.IsLoopback()
}

// httpRequestSpec is the decoded argument table Lua passes to __llmsproxy_http.
type httpRequestSpec struct {
	URL     string
	Method  string
	Headers map[string]string
	Body    string
	Timeout time.Duration
}

// luaHTTPFn builds the golua closure. It takes one Lua table argument and
// returns (responseTable, nil) or (nil, errorMessage) — never raises, so a
// plugin mistake cannot take the gateway with it.
func luaHTTPFn(L *golua.State, allow *httpAllowlist) golua.LuaGoFunction {
	return func(L *golua.State) int {
		spec, err := readHTTPRequestSpec(L)
		if err != nil {
			L.PushNil()
			L.PushString(err.Error())
			return 2
		}
		u, err := allow.checkURL(spec.URL)
		if err != nil {
			L.PushNil()
			L.PushString(err.Error())
			return 2
		}
		resp, err := performHTTP(u, spec)
		if err != nil {
			L.PushNil()
			L.PushString(err.Error())
			return 2
		}
		pushHTTPResponse(L, resp)
		L.PushNil()
		return 2
	}
}

func performHTTP(u *url.URL, spec httpRequestSpec) (*httpResult, error) {
	method := strings.ToUpper(spec.Method)
	if method == "" {
		method = http.MethodGet
	}
	timeout := spec.Timeout
	if timeout <= 0 || timeout > httpMaxTimeout {
		timeout = httpDefaultTimeout
	}

	var body io.Reader
	if spec.Body != "" {
		body = strings.NewReader(spec.Body)
	}
	req, err := http.NewRequest(method, u.String(), body)
	if err != nil {
		return nil, fmt.Errorf("build request: %v", err)
	}
	for k, v := range spec.Headers {
		req.Header.Set(k, v)
	}
	if req.Header.Get("User-Agent") == "" {
		req.Header.Set("User-Agent", "ModelRouter-plugin")
	}

	client := *httpClient
	client.Timeout = timeout
	resp, err := client.Do(req)
	if err != nil {
		return nil, fmt.Errorf("request failed: %v", err)
	}
	defer resp.Body.Close()

	data, err := io.ReadAll(io.LimitReader(resp.Body, httpMaxResponse+1))
	if err != nil {
		return nil, fmt.Errorf("read body: %v", err)
	}
	if len(data) > httpMaxResponse {
		return nil, fmt.Errorf("response exceeds %d byte cap", httpMaxResponse)
	}
	return &httpResult{
		Status:  resp.StatusCode,
		Headers: flattenHeaders(resp.Header),
		Body:    string(data),
	}, nil
}

type httpResult struct {
	Status  int
	Headers map[string]string
	Body    string
}

func flattenHeaders(h http.Header) map[string]string {
	out := make(map[string]string, len(h))
	for k := range h {
		out[strings.ToLower(k)] = h.Get(k)
	}
	return out
}