agent-sandbox/router.go

package main

import (
	"bufio"
	"fmt"
	"os"
	"regexp"
	"sort"
	"strconv"
	"strings"
)

const (
	// Every .domain entry resolves here; exact hosts get 127.1.x.y of their own.
	wildcardIP = "127.0.0.2"
	portBase   = 10000
)

var hostRE = regexp.MustCompile(`^\.?[a-z0-9]([a-z0-9-]*[a-z0-9])?(\.[a-z0-9]([a-z0-9-]*[a-z0-9])?)*$`)

// entry is one allowlist line; port is empty for plain web (80/443) access.
type entry struct{ host, port string }

// parseAllowlist returns the entries of allowlist file path, sorted bytewise
// on "host port" and unique.
func parseAllowlist(path string) ([]entry, error) {
	f, err := os.Open(path)
	if err != nil {
		return nil, err
	}
	defer func() { _ = f.Close() }()

	seen := map[string]bool{}
	var keys []string
	sc := bufio.NewScanner(f)
	for n := 1; sc.Scan(); n++ {
		line, _, _ := strings.Cut(sc.Text(), "#")
		fields := strings.Fields(line)
		if len(fields) == 0 {
			continue
		}
		if len(fields) > 1 {
			return nil, fmt.Errorf("%s:%d: one entry per line", path, n)
		}
		line = strings.ToLower(fields[0])
		host, port := line, ""
		if i := strings.LastIndex(line, ":"); i >= 0 {
			host, port = line[:i], line[i+1:]
		}
		if !hostRE.MatchString(host) {
			return nil, fmt.Errorf("%s:%d: invalid host '%s'", path, n, host)
		}
		if port != "" {
			p, err := strconv.Atoi(port)
			if err != nil || strings.ContainsAny(port, "+-") || p < 1 || p > 65535 {
				return nil, fmt.Errorf("%s:%d: invalid port '%s'", path, n, port)
			}
			if strings.HasPrefix(host, ".") {
				return nil, fmt.Errorf("%s:%d: ports need an exact host, not '%s'", path, n, host)
			}
			port = strconv.Itoa(p)
			if p == 80 || p == 443 {
				port = ""
			}
		}
		if k := host + " " + port; !seen[k] {
			seen[k] = true
			keys = append(keys, k)
		}
	}
	if err := sc.Err(); err != nil {
		return nil, err
	}
	sort.Strings(keys)
	entries := make([]entry, len(keys))
	for i, k := range keys {
		entries[i].host, entries[i].port, _ = strings.Cut(k, " ")
	}
	return entries, nil
}

// routerConfig renders dnsmasq.conf and nginx.conf for the router listening on
// rip, and the sandbox's loopback forwards as "bind:port:router:router_port"
// words.
func routerConfig(entries []entry, rip string) (dnsmasq, nginx, forwards string) {
	var hosts []string
	isHost := map[string]bool{}
	for _, e := range entries {
		if !isHost[e.host] {
			isHost[e.host] = true
			hosts = append(hosts, e.host)
		}
	}
	ip := map[string]string{}
	i := 0
	for _, h := range hosts {
		if !strings.HasPrefix(h, ".") {
			ip[h] = fmt.Sprintf("127.1.%d.%d", i/250, i%250+1)
			i++
		}
	}

	var tcp strings.Builder
	fwd := []string{"0.0.0.0:443:" + rip + ":443", "0.0.0.0:80:" + rip + ":80"}
	j := 0
	for _, e := range entries {
		if e.port == "" {
			continue
		}
		fmt.Fprintf(&tcp, "  server { listen %s:%d; set $up %s:%s; proxy_pass $up; }\n", rip, portBase+j, e.host, e.port)
		fwd = append(fwd, fmt.Sprintf("%s:%s:%s:%d", ip[e.host], e.port, rip, portBase+j))
		j++
	}

	var d strings.Builder
	fmt.Fprintf(&d, "no-resolv\nno-hosts\nbind-interfaces\nlisten-address=%s\naddress=/#/\n", rip)
	for _, h := range hosts {
		if strings.HasPrefix(h, ".") {
			fmt.Fprintf(&d, "address=/%s/%s\n", h[1:], wildcardIP)
		} else {
			fmt.Fprintf(&d, "host-record=%s,%s\n", h, ip[h])
		}
	}

	// nginx "hostnames" rejects an exact key that a .domain key already covers.
	var m strings.Builder
	for _, h := range hosts {
		if !strings.HasPrefix(h, ".") && isHost["."+h] {
			continue
		}
		fmt.Fprintf(&m, "    %s 1;\n", h)
	}

	nginx = strings.NewReplacer("@RIP@", rip, "@MAP@", m.String(), "@TCP@", tcp.String()).Replace(nginxTemplate)
	return d.String(), nginx, strings.Join(fwd, " ")
}

// @RESOLVER@ is filled in by the router's start.sh.
const nginxTemplate = `load_module /usr/lib/nginx/modules/ngx_stream_module.so;
worker_processes 1;
pid /tmp/nginx.pid;
error_log stderr warn;
events { worker_connections 1024; }

stream {
  resolver @RESOLVER@ ipv6=off valid=30s;
  map $ssl_preread_server_name $tls_allowed {
    hostnames;
    default 0;
@MAP@
  }
  map $tls_allowed $tls_upstream {
    1 $ssl_preread_server_name:443;
    default 127.0.0.1:9;
  }
  server {
    listen @RIP@:443;
    ssl_preread on;
    proxy_pass $tls_upstream;
  }
@TCP@}

http {
  resolver @RESOLVER@ ipv6=off valid=30s;
  access_log off;
  map $host $http_allowed {
    hostnames;
    default 0;
@MAP@
  }
  server {
    listen @RIP@:80;
    if ($http_allowed = 0) { return 403; }
    location / {
      proxy_pass http://$host;
      proxy_set_header Host $host;
    }
  }
}
`