mitmproxy/libmproxy/flow.py

"""
    This module provides more sophisticated flow tracking. These match requests
    with their responses, and provide filtering and interception facilities.
"""
import subprocess, sys, json, hashlib, Cookie, cookielib, base64, copy, re
import time
import netstring, filt, script, utils, encoding, proxy
from email.utils import parsedate_tz, formatdate, mktime_tz
import controller, version

class RunException(Exception):
    def __init__(self, msg, returncode, errout):
        Exception.__init__(self, msg)
        self.returncode = returncode
        self.errout = errout


class Headers:
    def __init__(self, lst=None):
        if lst:
            self.lst = lst
        else:
            self.lst = []

    def _kconv(self, s):
        return s.lower()

    def __eq__(self, other):
        return self.lst == other.lst

    def __getitem__(self, k):
        ret = []
        k = self._kconv(k)
        for i in self.lst:
            if self._kconv(i[0]) == k:
                ret.append(i[1])
        return ret

    def _filter_lst(self, k, lst):
        new = []
        for i in lst:
            if self._kconv(i[0]) != k:
                new.append(i)
        return new

    def __setitem__(self, k, hdrs):
        k = self._kconv(k)
        new = self._filter_lst(k, self.lst)
        for i in hdrs:
            new.append((k, i))
        self.lst = new

    def __delitem__(self, k):
        self.lst = self._filter_lst(k, self.lst)

    def __contains__(self, k):
        for i in self.lst:
            if self._kconv(i[0]) == k:
                return True
        return False

    def add(self, key, value):
        self.lst.append([key, str(value)])

    def get_state(self):
        return [tuple(i) for i in self.lst]

    @classmethod
    def from_state(klass, state):
        return klass([list(i) for i in state])

    def copy(self):
        lst = copy.deepcopy(self.lst)
        return Headers(lst)

    def __repr__(self):
        """
            Returns a string containing a formatted header string.
        """
        headerElements = []
        for itm in self.lst:
            headerElements.append(itm[0] + ": " + itm[1])
        headerElements.append("")
        return "\r\n".join(headerElements)

    def match_re(self, expr):
        """
            Match the regular expression against each header (key, value) pair.
        """
        for k, v in self.lst:
            s = "%s: %s"%(k, v)
            if re.search(expr, s):
                return True
        return False

    def read(self, fp):
        """
            Read a set of headers from a file pointer. Stop once a blank line
            is reached.
        """
        ret = []
        name = ''
        while 1:
            line = fp.readline()
            if not line or line == '\r\n' or line == '\n':
                break
            if line[0] in ' \t':
                # continued header
                ret[-1][1] = ret[-1][1] + '\r\n ' + line.strip()
            else:
                i = line.find(':')
                # We're being liberal in what we accept, here.
                if i > 0:
                    name = line[:i]
                    value = line[i+1:].strip()
                    ret.append([name, value])
        self.lst = ret

    def replace(self, pattern, repl, *args, **kwargs):
        """
            Replaces a regular expression pattern with repl in both header keys
            and values. Returns the number of replacements made.
        """
        nlst, count = [], 0
        for i in self.lst:
            k, c = re.subn(pattern, repl, i[0], *args, **kwargs)
            count += c
            v, c = re.subn(pattern, repl, i[1], *args, **kwargs)
            count += c
            nlst.append([k, v])
        self.lst = nlst
        return count


class HTTPMsg(controller.Msg):
    def decode(self):
        """
            Alters Response object, decoding its content based on the current
            Content-Encoding header and changing Content-Encoding header to
            'identity'.
        """
        ce = self.headers["content-encoding"]
        if not ce or ce[0] not in encoding.ENCODINGS:
            return
        self.content = encoding.decode(
            ce[0],
            self.content
        )
        del self.headers["content-encoding"]

    def encode(self, e):
        """
            Alters Response object, encoding its content with the specified
            coding. This method should only be called on Responses with
            Content-Encoding headers of 'identity'.
        """
        self.content = encoding.encode(e, self.content)
        self.headers["content-encoding"] = [e]


class Request(HTTPMsg):
    FMT = '%s %s HTTP/1.1\r\n%s\r\n%s'
    FMT_PROXY = '%s %s://%s:%s%s HTTP/1.1\r\n%s\r\n%s'
    def __init__(self, client_conn, host, port, scheme, method, path, headers, content, timestamp=None):
        self.client_conn = client_conn
        self.host, self.port, self.scheme = host, port, scheme
        self.method, self.path, self.headers, self.content = method, path, headers, content
        self.timestamp = timestamp or utils.timestamp()
        self.close = False
        controller.Msg.__init__(self)

        # Have this request's cookies been modified by sticky cookies or auth?
        self.stickycookie = False
        self.stickyauth = False

    def anticache(self):
        """
            Modifies this request to remove headers that might produce a cached
            response. That is, we remove ETags and If-Modified-Since headers.
        """
        delheaders = [
            "if-modified-since",
            "if-none-match",
        ]
        for i in delheaders:
            del self.headers[i]

    def anticomp(self):
        """
            Modifies this request to remove headers that will compress the
            resource's data.
        """
        self.headers["accept-encoding"] = ["identity"]

    def constrain_encoding(self):
        """
            Limits the permissible Accept-Encoding values, based on what we can
            decode appropriately.
        """
        if self.headers["accept-encoding"]:
            self.headers["accept-encoding"] = [', '.join([
                e for e in encoding.ENCODINGS if e in self.headers["accept-encoding"][0]
            ])]

    def set_replay(self):
        self.client_conn = None

    def is_replay(self):
        if self.client_conn:
            return False
        else:
            return True

    def load_state(self, state):
        if state["client_conn"]:
            if self.client_conn:
                self.client_conn.load_state(state["client_conn"])
            else:
                self.client_conn = ClientConnect.from_state(state["client_conn"])
        else:
            self.client_conn = None
        self.host = state["host"]
        self.port = state["port"]
        self.scheme = state["scheme"]
        self.method = state["method"]
        self.path = state["path"]
        self.headers = Headers.from_state(state["headers"])
        self.content = base64.decodestring(state["content"])
        self.timestamp = state["timestamp"]

    def get_state(self):
        return dict(
            client_conn = self.client_conn.get_state() if self.client_conn else None,
            host = self.host,
            port = self.port,
            scheme = self.scheme,
            method = self.method,
            path = self.path,
            headers = self.headers.get_state(),
            content = base64.encodestring(self.content),
            timestamp = self.timestamp,
        )

    @classmethod
    def from_state(klass, state):
        return klass(
            ClientConnect.from_state(state["client_conn"]),
            str(state["host"]),
            state["port"],
            str(state["scheme"]),
            str(state["method"]),
            str(state["path"]),
            Headers.from_state(state["headers"]),
            base64.decodestring(state["content"]),
            state["timestamp"]
        )

    def __hash__(self):
        return id(self)

    def __eq__(self, other):
        return self.get_state() == other.get_state()

    def copy(self):
        c = copy.copy(self)
        c.headers = self.headers.copy()
        return c

    def hostport(self):
        if (self.port, self.scheme) in [(80, "http"), (443, "https")]:
            host = self.host
        else:
            host = "%s:%s"%(self.host, self.port)
        return host

    def url(self):
        return "%s://%s%s"%(self.scheme, self.hostport(), self.path)

    def set_url(self, url):
        parts = utils.parse_url(url)
        if not parts:
            return False
        self.scheme, self.host, self.port, self.path = parts
        return True

    def is_response(self):
        return False

    def assemble(self, _proxy = False):
        """
            Assembles the request for transmission to the server. We make some
            modifications to make sure interception works properly.
        """
        headers = self.headers.copy()
        utils.del_all(
            headers,
            [
                'proxy-connection',
                'keep-alive',
                'connection',
                'content-length',
                'transfer-encoding'
            ]
        )
        if not 'host' in headers:
            headers["host"] = [self.hostport()]
        content = self.content
        if content is not None:
            headers["content-length"] = [str(len(content))]
        else:
            content = ""
        if self.close:
            headers["connection"] = ["close"]
        if not _proxy:
            return self.FMT % (self.method, self.path, str(headers), content)
        else:
            return self.FMT_PROXY % (self.method, self.scheme, self.host, self.port, self.path, str(headers), content)

    def replace(self, pattern, repl, *args, **kwargs):
        """
            Replaces a regular expression pattern with repl in both the headers
            and the body of the request. Returns the number of replacements
            made.
        """
        self.content, c = re.subn(pattern, repl, self.content, *args, **kwargs)
        self.path, pc = re.subn(pattern, repl, self.path, *args, **kwargs)
        c += pc
        c += self.headers.replace(pattern, repl, *args, **kwargs)
        return c


class Response(HTTPMsg):
    FMT = '%s\r\n%s\r\n%s'
    def __init__(self, request, code, msg, headers, content, timestamp=None):
        self.request = request
        self.code, self.msg = code, msg
        self.headers, self.content = headers, content
        self.timestamp = timestamp or utils.timestamp()
        controller.Msg.__init__(self)
        self.replay = False

    def _refresh_cookie(self, c, delta):
        """
            Takes a cookie string c and a time delta in seconds, and returns
            a refreshed cookie string.
        """
        c = Cookie.SimpleCookie(str(c))
        for i in c.values():
            if "expires" in i:
                d = parsedate_tz(i["expires"])
                if d:
                    d = mktime_tz(d) + delta
                    i["expires"] = formatdate(d)
                else:
                    # This can happen when the expires tag is invalid.
                    # reddit.com sends a an expires tag like this: "Thu, 31 Dec
                    # 2037 23:59:59 GMT", which is valid RFC 1123, but not
                    # strictly correct according tot he cookie spec. Browsers
                    # appear to parse this tolerantly - maybe we should too.
                    # For now, we just ignore this.
                    del i["expires"]
        return c.output(header="").strip()

    def refresh(self, now=None):
        """
            This fairly complex and heuristic function refreshes a server
            response for replay.

                - It adjusts date, expires and last-modified headers.
                - It adjusts cookie expiration.
        """
        if not now:
            now = time.time()
        delta = now - self.timestamp
        refresh_headers = [
            "date",
            "expires",
            "last-modified",
        ]
        for i in refresh_headers:
            if i in self.headers:
                d = parsedate_tz(self.headers[i][0])
                if d:
                    new = mktime_tz(d) + delta
                    self.headers[i] = [formatdate(new)]
        c = []
        for i in self.headers["set-cookie"]:
            c.append(self._refresh_cookie(i, delta))
        if c:
            self.headers["set-cookie"] = c

    def set_replay(self):
        self.replay = True

    def is_replay(self):
        return self.replay

    def load_state(self, state):
        self.code = state["code"]
        self.msg = state["msg"]
        self.headers = Headers.from_state(state["headers"])
        self.content = base64.decodestring(state["content"])
        self.timestamp = state["timestamp"]

    def get_state(self):
        return dict(
            code = self.code,
            msg = self.msg,
            headers = self.headers.get_state(),
            timestamp = self.timestamp,
            content = base64.encodestring(self.content)
        )

    @classmethod
    def from_state(klass, request, state):
        return klass(
            request,
            state["code"],
            str(state["msg"]),
            Headers.from_state(state["headers"]),
            base64.decodestring(state["content"]),
            state["timestamp"],
        )

    def __eq__(self, other):
        return self.get_state() == other.get_state()

    def copy(self):
        c = copy.copy(self)
        c.headers = self.headers.copy()
        return c

    def is_response(self):
        return True

    def assemble(self):
        """
            Assembles the response for transmission to the client. We make some
            modifications to make sure interception works properly.
        """
        headers = self.headers.copy()
        utils.del_all(
            headers,
            ['proxy-connection', 'connection', 'keep-alive', 'transfer-encoding']
        )
        content = self.content
        if content is not None:
            headers["content-length"] = [str(len(content))]
        else:
            content = ""
        if self.request.client_conn.close:
            headers["connection"] = ["close"]
        proto = "HTTP/1.1 %s %s"%(self.code, str(self.msg))
        data = (proto, str(headers), content)
        return self.FMT%data

    def replace(self, pattern, repl, *args, **kwargs):
        """
            Replaces a regular expression pattern with repl in both the headers
            and the body of the response. Returns the number of replacements
            made.
        """
        self.content, c = re.subn(pattern, repl, self.content, *args, **kwargs)
        c += self.headers.replace(pattern, repl, *args, **kwargs)
        return c


class ClientDisconnect(controller.Msg):
    def __init__(self, client_conn):
        controller.Msg.__init__(self)
        self.client_conn = client_conn


class ClientConnect(controller.Msg):
    def __init__(self, address):
        """
            address is an (address, port) tuple, or None if this connection has
            been replayed from within mitmproxy.
        """
        self.address = address
        self.close = False
        self.requestcount = 0
        self.connection_error = None
        controller.Msg.__init__(self)

    def __eq__(self, other):
        return self.get_state() == other.get_state()

    def load_state(self, state):
        self.address = state

    def get_state(self):
        return list(self.address) if self.address else None

    @classmethod
    def from_state(klass, state):
        if state:
            return klass(state)
        else:
            return None

    def copy(self):
        return copy.copy(self)


class Error(controller.Msg):
    def __init__(self, request, msg, timestamp=None):
        self.request, self.msg = request, msg
        self.timestamp = timestamp or utils.timestamp()
        controller.Msg.__init__(self)

    def load_state(self, state):
        self.msg = state["msg"]
        self.timestamp = state["timestamp"]

    def copy(self):
        return copy.copy(self)

    def get_state(self):
        return dict(
            msg = self.msg,
            timestamp = self.timestamp,
        )

    @classmethod
    def from_state(klass, state):
        return klass(
            None,
            state["msg"],
            state["timestamp"],
        )

    def __eq__(self, other):
        return self.get_state() == other.get_state()

    def replace(self, pattern, repl, *args, **kwargs):
        """
            Replaces a regular expression pattern with repl in both the headers
            and the body of the request. Returns the number of replacements
            made.
        """
        self.msg, c = re.subn(pattern, repl, self.msg, *args, **kwargs)
        return c












class ClientPlaybackState:
    def __init__(self, flows, exit):
        self.flows, self.exit = flows, exit
        self.current = None

    def count(self):
        return len(self.flows)

    def done(self):
        if len(self.flows) == 0 and not self.current:
            return True
        return False

    def clear(self, flow):
        """
           A request has returned in some way - if this is the one we're
           servicing, go to the next flow.
        """
        if flow is self.current:
            self.current = None

    def tick(self, master, testing=False):
        """
            testing: Disables actual replay for testing.
        """
        if self.flows and not self.current:
            n = self.flows.pop(0)
            n.request.client_conn = None
            self.current = master.handle_request(n.request)
            if not testing and not self.current.response:
                #begin nocover
                master.replay_request(self.current)
                #end nocover
            elif self.current.response:
                master.handle_response(self.current.response)


class ServerPlaybackState:
    def __init__(self, headers, flows, exit):
        """
            headers: A case-insensitive list of request headers that should be
            included in request-response matching.
        """
        self.headers, self.exit = headers, exit
        self.fmap = {}
        for i in flows:
            if i.response:
                l = self.fmap.setdefault(self._hash(i), [])
                l.append(i)

    def count(self):
        return sum([len(i) for i in self.fmap.values()])

    def _hash(self, flow):
        """
            Calculates a loose hash of the flow request.
        """
        r = flow.request
        key = [
            str(r.host),
            str(r.port),
            str(r.scheme),
            str(r.method),
            str(r.path),
            str(r.content),
        ]
        if self.headers:
            hdrs = []
            for i in self.headers:
                v = r.headers[i]
                # Slightly subtle: we need to convert everything to strings
                # to prevent a mismatch between unicode/non-unicode.
                v = [str(x) for x in v]
                hdrs.append((i, v))
            key.append(repr(hdrs))
        return hashlib.sha256(repr(key)).digest()

    def next_flow(self, request):
        """
            Returns the next flow object, or None if no matching flow was
            found.
        """
        l = self.fmap.get(self._hash(request))
        if not l:
            return None
        return l.pop(0)


class StickyCookieState:
    def __init__(self, flt):
        """
            flt: A compiled filter.
        """
        self.jar = {}
        self.flt = flt

    def ckey(self, m, f):
        """
            Returns a (domain, port, path) tuple.
        """
        return (
            m["domain"] or f.request.host,
            f.request.port,
            m["path"] or "/"
        )

    def handle_response(self, f):
        for i in f.response.headers["set-cookie"]:
            # FIXME: We now know that Cookie.py screws up some cookies with
            # valid RFC 822/1123 datetime specifications for expiry. Sigh.
            c = Cookie.SimpleCookie(str(i))
            m = c.values()[0]
            k = self.ckey(m, f)
            if cookielib.domain_match(f.request.host, k[0]):
                self.jar[self.ckey(m, f)] = m

    def handle_request(self, f):
        if f.match(self.flt):
            for i in self.jar.keys():
                match = [
                    cookielib.domain_match(i[0], f.request.host),
                    f.request.port == i[1],
                    f.request.path.startswith(i[2])
                ]
                if all(match):
                    l = f.request.headers["cookie"]
                    f.request.stickycookie = True
                    l.append(self.jar[i].output(header="").strip())
                    f.request.headers["cookie"] = l


class StickyAuthState:
    def __init__(self, flt):
        """
            flt: A compiled filter.
        """
        self.flt = flt
        self.hosts = {}

    def handle_request(self, f):
        if "authorization" in f.request.headers:
            self.hosts[f.request.host] = f.request.headers["authorization"]
        elif f.match(self.flt):
            if f.request.host in self.hosts:
                f.request.headers["authorization"] = self.hosts[f.request.host]


class Flow:
    def __init__(self, request):
        self.request = request
        self.response, self.error = None, None
        self.intercepting = False
        self._backup = None

    @classmethod
    def from_state(klass, state):
        f = klass(None)
        f.load_state(state)
        return f

    @classmethod
    def script_deserialize(klass, data):
        try:
            data = json.loads(data)
        except Exception:
            return None
        return klass.from_state(data)

    def get_state(self, nobackup=False):
        d = dict(
            request = self.request.get_state() if self.request else None,
            response = self.response.get_state() if self.response else None,
            error = self.error.get_state() if self.error else None,
            version = version.IVERSION
        )
        if nobackup:
            d["backup"] = None
        else:
            d["backup"] = self._backup
        return d

    def load_state(self, state):
        self._backup = state["backup"]
        if self.request:
            self.request.load_state(state["request"])
        else:
            self.request = Request.from_state(state["request"])

        if state["response"]:
            if self.response:
                self.response.load_state(state["response"])
            else:
                self.response = Response.from_state(self.request, state["response"])
        else:
            self.response = None

        if state["error"]:
            if self.error:
                self.error.load_state(state["error"])
            else:
                self.error = Error.from_state(state["error"])
        else:
            self.error = None

    def modified(self):
        # FIXME: Save a serialization in backup, compare current with
        # backup to detect if flow has _really_ been modified.
        if self._backup:
            return True
        else:
            return False

    def backup(self):
        self._backup = self.get_state(nobackup=True)

    def revert(self):
        if self._backup:
            self.load_state(self._backup)
            self._backup = None

    def match(self, pattern):
        if pattern:
            if self.response:
                return pattern(self.response)
            elif self.request:
                return pattern(self.request)
        else:
            return True

    def kill(self, master):
        self.error = Error(self.request, "Connection killed")
        if self.request and not self.request.acked:
            self.request.ack(None)
        elif self.response and not self.response.acked:
            self.response.ack(None)
        master.handle_error(self.error)
        self.intercepting = False

    def intercept(self):
        self.intercepting = True

    def accept_intercept(self):
        if self.request:
            if not self.request.acked:
                self.request.ack()
            elif self.response and not self.response.acked:
                self.response.ack()
            self.intercepting = False

    def replace(self, pattern, repl, *args, **kwargs):
        """
            Replaces a regular expression pattern with repl in all parts of the
            flow . Returns the number of replacements made.
        """
        c = self.request.replace(pattern, repl, *args, **kwargs)
        if self.response:
            c += self.response.replace(pattern, repl, *args, **kwargs)
        if self.error:
            c += self.error.replace(pattern, repl, *args, **kwargs)
        return c


class State(object):
    def __init__(self):
        self._flow_map = {}
        self._flow_list = []
        self.view = []

        # These are compiled filt expressions:
        self._limit = None
        self.intercept = None
        self._limit_txt = None

    @property
    def limit_txt(self):
        return self._limit_txt

    def flow_count(self):
        return len(self._flow_map)

    def active_flow_count(self):
        c = 0
        for i in self._flow_list:
            if not i.response and not i.error:
                c += 1
        return c

    def add_request(self, req):
        """
            Add a request to the state. Returns the matching flow.
        """
        f = Flow(req)
        self._flow_list.append(f)
        self._flow_map[req] = f
        if f.match(self._limit):
            self.view.append(f)
        return f

    def add_response(self, resp):
        """
            Add a response to the state. Returns the matching flow.
        """
        f = self._flow_map.get(resp.request)
        if not f:
            return False
        f.response = resp
        if f.match(self._limit) and not f in self.view:
            self.view.append(f)
        return f

    def add_error(self, err):
        """
            Add an error response to the state. Returns the matching flow, or
            None if there isn't one.
        """
        if err.request:
            f = self._flow_map.get(err.request)
            if not f:
                return None
        else:
            return None
        f.error = err
        if f.match(self._limit) and not f in self.view:
            self.view.append(f)
        return f

    def load_flows(self, flows):
        self._flow_list.extend(flows)
        for i in flows:
            self._flow_map[i.request] = i
        self.recalculate_view()

    def set_limit(self, txt):
        if txt:
            f = filt.parse(txt)
            if not f:
                return "Invalid filter expression."
            self._limit = f
            self._limit_txt = txt
        else:
            self._limit = None
            self._limit_txt = None
        self.recalculate_view()

    def set_intercept(self, txt):
        if txt:
            f = filt.parse(txt)
            if not f:
                return "Invalid filter expression."
            self.intercept = f
            self.intercept_txt = txt
        else:
            self.intercept = None
            self.intercept_txt = None

    def recalculate_view(self):
        if self._limit:
            self.view = [i for i in self._flow_list if i.match(self._limit)]
        else:
            self.view = self._flow_list[:]

    def delete_flow(self, f):
        if f.request in self._flow_map:
            del self._flow_map[f.request]
        self._flow_list.remove(f)
        if f.match(self._limit):
            self.view.remove(f)
        return True

    def clear(self):
        for i in self._flow_list[:]:
            self.delete_flow(i)

    def accept_all(self):
        for i in self._flow_list[:]:
            i.accept_intercept()

    def revert(self, f):
        f.revert()

    def killall(self, master):
        for i in self._flow_list:
            i.kill(master)



class FlowMaster(controller.Master):
    def __init__(self, server, state):
        controller.Master.__init__(self, server)
        self.state = state
        self.server_playback = None
        self.client_playback = None
        self.kill_nonreplay = False
        self.script = None

        self.stickycookie_state = False
        self.stickycookie_txt = None

        self.stickyauth_state = False
        self.stickyauth_txt = None

        self.anticache = False
        self.anticomp = False
        self.autodecode = False
        self.refresh_server_playback = False

    def add_event(self, e, level="info"):
        """
            level: info, error
        """
        pass

    def get_script(self, path):
        """
            Returns an (error, script) tuple.
        """
        s = script.Script(path, self)
        try:
            s.load()
        except script.ScriptError, v:
            return (v.args[0], None)
        ret = s.run("start")
        if not ret[0] and ret[1]:
            return ("Error in script start:\n\n" + ret[1][1], None)
        return (None, s)

    def load_script(self, path):
        """
            Loads a script. Returns an error description if something went
            wrong.
        """
        r = self.get_script(path)
        if r[0]:
            return r[0]
        else:
            self.script = r[1]

    def set_stickycookie(self, txt):
        if txt:
            flt = filt.parse(txt)
            if not flt:
                return "Invalid filter expression."
            self.stickycookie_state = StickyCookieState(flt)
            self.stickycookie_txt = txt
        else:
            self.stickycookie_state = None
            self.stickycookie_txt = None

    def set_stickyauth(self, txt):
        if txt:
            flt = filt.parse(txt)
            if not flt:
                return "Invalid filter expression."
            self.stickyauth_state = StickyAuthState(flt)
            self.stickyauth_txt = txt
        else:
            self.stickyauth_state = None
            self.stickyauth_txt = None

    def start_client_playback(self, flows, exit):
        """
            flows: A list of flows.
        """
        self.client_playback = ClientPlaybackState(flows, exit)

    def stop_client_playback(self):
        self.client_playback = None

    def start_server_playback(self, flows, kill, headers, exit):
        """
            flows: A list of flows.
            kill: Boolean, should we kill requests not part of the replay?
        """
        self.server_playback = ServerPlaybackState(headers, flows, exit)
        self.kill_nonreplay = kill

    def stop_server_playback(self):
        self.server_playback = None

    def do_server_playback(self, flow):
        """
            This method should be called by child classes in the handle_request
            handler. Returns True if playback has taken place, None if not.
        """
        if self.server_playback:
            rflow = self.server_playback.next_flow(flow)
            if not rflow:
                return None
            response = Response.from_state(flow.request, rflow.response.get_state())
            response.set_replay()
            flow.response = response
            if self.refresh_server_playback:
                response.refresh()
            flow.request.ack(response)
            return True
        return None

    def tick(self, q):
        if self.client_playback:
            e = [
                self.client_playback.done(),
                self.client_playback.exit,
                self.state.active_flow_count() == 0
            ]
            if all(e):
                self.shutdown()
            self.client_playback.tick(self)

        if self.server_playback:
            if self.server_playback.exit and self.server_playback.count() == 0:
                self.shutdown()

        return controller.Master.tick(self, q)

    def load_flows(self, fr):
        """
            Load flows from a FlowReader object.
        """
        for i in fr.stream():
            if i.request:
                self.handle_request(i.request)
            if i.response:
                self.handle_response(i.response)
            if i.error:
                self.handle_error(i.error)

    def process_new_request(self, f):
        if self.stickycookie_state:
            self.stickycookie_state.handle_request(f)
        if self.stickyauth_state:
            self.stickyauth_state.handle_request(f)

        if self.anticache:
            f.request.anticache()
        if self.anticomp:
            f.request.anticomp()

        if self.server_playback:
            pb = self.do_server_playback(f)
            if not pb:
                if self.kill_nonreplay:
                    f.kill(self)
                else:
                    f.request.ack()

    def process_new_response(self, f):
        if self.stickycookie_state:
            self.stickycookie_state.handle_response(f)

    def replay_request(self, f):
        """
            Returns None if successful, or error message if not.
        """
        #begin nocover
        if f.intercepting:
            return "Can't replay while intercepting..."
        if f.request:
            f.request.set_replay()
            if f.request.content:
                f.request.headers["content-length"] = [str(len(f.request.content))]
            f.response = None
            f.error = None
            self.process_new_request(f)
            rt = proxy.RequestReplayThread(f, self.masterq)
            rt.start()
        #end nocover

    def run_script(self, name, *args, **kwargs):
        if self.script:
            ret = self.script.run(name, *args, **kwargs)
            if not ret[0] and ret[1]:
                e = "Script error:\n" + ret[1][1]
                self.add_event(e, "error")

    def handle_clientconnect(self, cc):
        self.run_script("clientconnect", cc)
        self.add_event("Connect from: %s:%s"%cc.address)
        cc.ack()

    def handle_clientdisconnect(self, r):
        self.run_script("clientdisconnect", r)
        s = "Disconnect from: %s:%s"%r.client_conn.address
        self.add_event(s)
        if r.client_conn.requestcount:
            s = "    -> handled %s requests"%r.client_conn.requestcount
            self.add_event(s)
        if r.client_conn.connection_error:
            self.add_event(
                "   -> error: %s"%r.client_conn.connection_error, "error"
            )
        r.ack()

    def handle_error(self, r):
        f = self.state.add_error(r)
        if f:
            self.run_script("error", f)
        if self.client_playback:
            self.client_playback.clear(f)
        r.ack()
        return f

    def handle_request(self, r):
        f = self.state.add_request(r)
        self.run_script("request", f)
        self.process_new_request(f)
        return f

    def handle_response(self, r):
        f = self.state.add_response(r)
        if f:
            self.run_script("response", f)
        if self.client_playback:
            self.client_playback.clear(f)
        if not f:
            r.ack()
        self.process_new_response(f)
        return f


class FlowWriter:
    def __init__(self, fo):
        self.fo = fo
        self.ns = netstring.FileEncoder(fo)

    def add(self, flow):
        d = flow.get_state()
        s = json.dumps(d)
        self.ns.write(s)


class FlowReadError(Exception):
    @property
    def strerror(self):
        return self.args[0]


class FlowReader:
    def __init__(self, fo):
        self.fo = fo
        self.ns = netstring.decode_file(fo)

    def stream(self):
        """
            Yields Flow objects from the dump.
        """
        try:
            for i in self.ns:
                data = json.loads(i)
                yield Flow.from_state(data)
        except netstring.DecoderError:
            raise FlowReadError("Invalid data format.")