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听说3月1日 GitHub 被DDoS攻击了，好像挺严重的。
来看看怎么使用mc攻击~
mc 首先通过cmd line指定UDP端口，然后初始化libevent实例，初始化线程，
int main (int argc, char **argv) {
...
settings_init();
...
while (-1 != (c = getopt(argc, argv,
...
"U:" /* UDP port number to listen on */
...
case 'U':
settings.udpport = atoi(optarg);
udp_specified = true;
break;
...
))) {
...
if (tcp_specified && !udp_specified) {
settings.udpport = settings.port;
} else if (udp_specified && !tcp_specified) {
settings.port = settings.udpport;
}
...
main_base = event_init();
...
thread_init(settings.num_threads, main_base);
...
/* create unix mode sockets after dropping privileges */
if (settings.socketpath != NULL) {
errno = 0;
if (server_socket_unix(settings.socketpath,settings.access)) {
vperror("failed to listen on UNIX socket: %s", settings.socketpath);
exit(EX_OSERR);
}
}
/* create the listening socket, bind it, and init */
if (settings.socketpath == NULL) {
...
// TCP
errno = 0;
if (settings.port && server_sockets(settings.port, tcp_transport,
portnumber_file)) {
vperror("failed to listen on TCP port %d", settings.port);
exit(EX_OSERR);
}
/*
* initialization order: first create the listening sockets
* (may need root on low ports), then drop root if needed,
* then daemonise if needed, then init libevent (in some cases
* descriptors created by libevent wouldn't survive forking).
*/
/* create the UDP listening socket and bind it */
errno = 0;
if (settings.udpport && server_sockets(settings.udpport, udp_transport,
portnumber_file)) {
vperror("failed to listen on UDP port %d", settings.udpport);
exit(EX_OSERR);
}
...
/* enter the event loop */
if (event_base_loop(main_base, 0) != 0) {
retval = EXIT_FAILURE;
}
...
}
在此之前初始化了一些设置，可以看到默认端口是11211，有4个worker线程。
static void settings_init(void) {
...
settings.port = 11211;
settings.udpport = 11211;
/* By default this string should be NULL for getaddrinfo() */
settings.inter = NULL;
settings.maxbytes = 64 * 1024 * 1024; /* default is 64MB */
...
settings.chunk_size = 48; /* space for a modest key and value */
settings.num_threads = 4; /* N workers */
...
}
可以检查一下：
$ echo "stats settings" | nc localhost 11211
STAT maxbytes 67108864
STAT maxconns 1024
STAT tcpport 11211
STAT udpport 11211
STAT inter NULL
...
STAT chunk_size 48
STAT num_threads 4
...
END
随后线程初始化，main_base 是分发任务的主线程，创建管道用于libevent通知。主要调用了setup_thread初始化线程信息数据结构，最后创建并初始化线程，代码段都是 worker_libevent。
void thread_init(int nthreads, struct event_base *main_base) {
...
threads = calloc(nthreads, sizeof(LIBEVENT_THREAD));
if (! threads) {
perror("Can't allocate thread descriptors");
exit(1);
}
dispatcher_thread.base = main_base;
dispatcher_thread.thread_id = pthread_self();
for (i = 0; i < nthreads; i++) {
int fds[2];
if (pipe(fds)) {
perror("Can't create notify pipe");
exit(1);
}
threads[i].notify_receive_fd = fds[0];
threads[i].notify_send_fd = fds[1];
setup_thread(&threads[i]);
/* Reserve three fds for the libevent base, and two for the pipe */
stats.reserved_fds += 5;
}
/* Create threads after we've done all the libevent setup. */
for (i = 0; i < nthreads; i++) {
create_worker(worker_libevent, &threads[i]);
}
...
}
这里看到了 thread_libevent_process 指针，在设置线程初始化数据时，设置为me->notify_receive_fd 管道的libevent读事件。
static void setup_thread(LIBEVENT_THREAD *me) {
me->base = event_init();
if (! me->base) {
fprintf(stderr, "Can't allocate event base\n");
exit(1);
}
/* Listen for notifications from other threads */
event_set(&me->notify_event, me->notify_receive_fd,
EV_READ | EV_PERSIST, thread_libevent_process, me);
event_base_set(me->base, &me->notify_event);
if (event_add(&me->notify_event, 0) == -1) {
fprintf(stderr, "Can't monitor libevent notify pipe\n");
exit(1);
}
me->new_conn_queue = malloc(sizeof(struct conn_queue));
if (me->new_conn_queue == NULL) {
perror("Failed to allocate memory for connection queue");
exit(EXIT_FAILURE);
}
cq_init(me->new_conn_queue);
...
}
当管道可读时回调此函数。从队列中取出一个任务，随后调conn_new。
static void thread_libevent_process(int fd, short which, void *arg) {
LIBEVENT_THREAD *me = arg;
CQ_ITEM *item;
char buf[1];
if (read(fd, buf, 1) != 1)
if (settings.verbose > 0)
fprintf(stderr, "Can't read from libevent pipe\n");
switch (buf[0]) {
case 'c':
item = cq_pop(me->new_conn_queue);
if (NULL != item) {
conn *c = conn_new(item->sfd, item->init_state, item->event_flags,
item->read_buffer_size, item->transport, me->base);
...
}
}
conn_new为新的请求建立一个连接结构体。这里只填充conn结构体。主要在 libevent 中注册函数指针event_handler。
conn *conn_new(const int sfd, enum conn_states init_state,
const int event_flags,
const int read_buffer_size, enum network_transport transport,
struct event_base *base) {
{
/* data */
};
conn *c = conn_from_freelist();
if (NULL == c) {
if (!(c = (conn *)calloc(1, sizeof(conn)))) {
fprintf(stderr, "calloc()\n");
return NULL;
}
MEMCACHED_CONN_CREATE(c);
c->rbuf = c->wbuf = 0;
c->rbuf = (char *)malloc((size_t)c->rsize);
c->wbuf = (char *)malloc((size_t)c->wsize);
...
c->msglist = (struct msghdr *)malloc(sizeof(struct msghdr) * c->msgsize);
...
}// if
....
c->sfd = sfd;
...
c->item = 0;
...
event_set(&c->event, sfd, event_flags, event_handler, (void *)c);
event_base_set(base, &c->event);
c->ev_flags = event_flags;
if (event_add(&c->event, 0) == -1) {
...
}
...
return c;
}
当有新的连接的时候将会回调此函数。
void event_handler(const int fd, const short which, void *arg) {
conn *c;
c = (conn *)arg;
assert(c != NULL);
c->which = which;
/* sanity */
...
drive_machine(c);
return;
}
client connect 后，memcached server主线程被唤醒，然后调用event_handler()->drive_machine()，进入这个状态机。从别处代码看，只有tcp或UNIX域套接字才会进行conn_listening，即accept过程。conn_waiting等待新的命令请求，conn_read 为读取数据，读完请求后转换 conn 的状态，然后就是解析执行命令咯。在conn_mwrite状态下回复数据；在transmit中最终调用sendmsg写给套接字。
static void drive_machine(conn *c) {
bool stop = false;
int sfd, flags = 1;
socklen_t addrlen;
struct sockaddr_storage addr;
int nreqs = settings.reqs_per_event;
int res;
const char *str;
assert(c != NULL);
while (!stop) {
switch(c->state) {
case conn_listening:
addrlen = sizeof(addr);
if ((sfd = accept(c->sfd, (struct sockaddr *)&addr, &addrlen)) == -1) {
...
}
...
case conn_waiting:
if (!update_event(c, EV_READ | EV_PERSIST)) {
if (settings.verbose > 0)
fprintf(stderr, "Couldn't update event\n");
conn_set_state(c, conn_closing);
break;
}
conn_set_state(c, conn_read);
stop = true;
break;
case conn_read:
res = IS_UDP(c->transport) ? try_read_udp(c) : try_read_network(c);
switch (res) {
case READ_NO_DATA_RECEIVED:
conn_set_state(c, conn_waiting);
break;
...
}
break;
case conn_parse_cmd :
if (try_read_command(c) == 0) {
/* we need more data! */
conn_set_state(c, conn_waiting);
}
break;
...
case conn_nread:
if (c->rlbytes == 0) {
complete_nread(c);
break;
}
/* first check if we have leftovers in the conn_read buffer */
if (c->rbytes > 0) {
int tocopy = c->rbytes > c->rlbytes ? c->rlbytes : c->rbytes;
if (c->ritem != c->rcurr) {
memmove(c->ritem, c->rcurr, tocopy);
}
...
}
/* now try reading from the socket */
res = read(c->sfd, c->ritem, c->rlbytes);
...
case conn_write:
...
/* fall through... */
case conn_mwrite:
if (IS_UDP(c->transport) && c->msgcurr == 0 && build_udp_headers(c) != 0) {
if (settings.verbose > 0)
fprintf(stderr, "Failed to build UDP headers\n");
conn_set_state(c, conn_closing);
break;
}
switch (transmit(c)) {
case TRANSMIT_COMPLETE:
if (c->state == conn_mwrite) {
...
/* XXX: I don't know why this wasn't the general case */
if(c->protocol == binary_prot) {
conn_set_state(c, c->write_and_go);
} else {
...
}
case TRANSMIT_INCOMPLETE:
case TRANSMIT_HARD_ERROR:
break; /* Continue in state machine. */
case TRANSMIT_SOFT_ERROR:
stop = true;
break;
}
break;
...
case conn_closing:
if (IS_UDP(c->transport))
conn_cleanup(c);
else
conn_close(c);
stop = true;
break;
...
}
return;
}
上文用到的读取UDP，直接调recvfrom，此处从客户端接受数据，将读取到的指令放到rbuf中。
static enum try_read_result try_read_udp(conn *c) {
int res;
assert(c != NULL);
c->request_addr_size = sizeof(c->request_addr);
res = recvfrom(c->sfd, c->rbuf, c->rsize,
0, &c->request_addr, &c->request_addr_size);
...
memmove(c->rbuf, c->rbuf + 8, res);
c->rbytes = res;
c->rcurr = c->rbuf;
return READ_DATA_RECEIVED;
}
return READ_NO_DATA_RECEIVED;
}
主函数中配置的模式，允许客户端以几种方式向mc server发请求 UDP只要绑定之后，直接读取 sfd 就OK，在这里看出它 conn 初始状态应为 conn_read，而 TCP 对应的 conn 初始状态应该为 conn_listening。
static int server_sockets(int port, enum network_transport transport,
FILE *portnumber_file) {
if (settings.inter == NULL) {
return server_socket(settings.inter, port, transport, portnumber_file);
} else {
// tokenize them and bind to each one of them..
char *b;
int ret = 0;
char *list = strdup(settings.inter);
if (list == NULL) {
fprintf(stderr, "Failed to allocate memory for parsing server interface string\n");
return 1;
}
for (char *p = strtok_r(list, ";,", &b);
...
ret |= server_socket(p, the_port, transport, portnumber_file);
}
free(list);
return ret;
}
}
针对每个interface绑定。
static int server_socket(const char *interface,
int port,
enum network_transport transport,
FILE *portnumber_file) {
...
hints.ai_socktype = IS_UDP(transport) ? SOCK_DGRAM : SOCK_STREAM;
if (port == -1) {
port = 0;
}
snprintf(port_buf, sizeof(port_buf), "%d", port);
error= getaddrinfo(interface, port_buf, &hints, &ai);
...
for (next= ai; next; next= next->ai_next) {
conn *listen_conn_add;
if ((sfd = new_socket(next)) == -1) {
...
continue;
}
#ifdef IPV6_V6ONLY
...
#endif
setsockopt(sfd, SOL_SOCKET, SO_REUSEADDR, (void *)&flags, sizeof(flags));
if (IS_UDP(transport)) {
maximize_sndbuf(sfd);
} else {
...
}
if (bind(sfd, next->ai_addr, next->ai_addrlen) == -1) {
...
} else {
success++;
if (!IS_UDP(transport) && listen(sfd, settings.backlog) == -1) {
...
}
if (IS_UDP(transport)) {
// UDP
int c;
for (c = 0; c < settings.num_threads_per_udp; c++) {
/* this is guaranteed to hit all threads because we round-robin */
dispatch_conn_new(sfd, conn_read, EV_READ | EV_PERSIST,
UDP_READ_BUFFER_SIZE, transport);
}
} else {
if (!(listen_conn_add = conn_new(sfd, conn_listening,
EV_READ | EV_PERSIST, 1,
transport, main_base))) {
...
}
listen_conn_add->next = listen_conn;
listen_conn = listen_conn_add;
}
}
freeaddrinfo(ai);
/* Return zero iff we detected no errors in starting up connections */
return success == 0;
}
设置了socket的发送缓冲大小为，取默认值，然后和设置的最大值二分查找，取最后的最大值。
/*
* Sets a socket's send buffer size to the maximum allowed by the system.
*/
// defined somewhere else
#define MAX_SENDBUF_SIZE (256 * 1024 * 1024)
static void maximize_sndbuf(const int sfd) {
...
if (getsockopt(sfd, SOL_SOCKET, SO_SNDBUF, &old_size, &intsize) != 0) {
...
}
min = old_size;
max = MAX_SENDBUF_SIZE;
while (min <= max) {
avg = ((unsigned int)(min + max)) / 2;
if (setsockopt(sfd, SOL_SOCKET, SO_SNDBUF, (void *)&avg, intsize) == 0) {
last_good = avg;
min = avg + 1;
} else {
max = avg - 1;
}
}
...
}
然后分发新的连接到线程池中的一个线程中，就是在一个线程的wq中加入一个任务，并通过管道给相应的线程发信，向一个休眠线程写字符，已注册事件会被触发，随后调thread_libevent_process（上文setup_thread 中线程pd被设置到 event 中）
void dispatch_conn_new(int sfd, enum conn_states init_state, int event_flags,
int read_buffer_size, enum network_transport transport) {
// CQ_ITEM connection queue item
CQ_ITEM *item = cqi_new();
char buf[1];
int tid = (last_thread + 1) % settings.num_threads;
LIBEVENT_THREAD *thread = threads + tid;
...
cq_push(thread->new_conn_queue, item);
MEMCACHED_CONN_DISPATCH(sfd, thread->thread_id);
buf[0] = 'c';
if (write(thread->notify_send_fd, buf, 1) != 1) {
...
}
}
这里看到mc可通过UDP模式将放大的数据返回给client，所以可以利用这个特性执行攻击，利用网络上的mc放大攻击效果。
看一下协议：RFC768
User Datagram Protocol
----------------------
...
protocol is transaction oriented, and delivery and duplicate protection
are not guaranteed. Applications requiring ordered reliable delivery of
streams of data should use the Transmission Control Protocol (TCP) [2].
Format
------
0 7 8 15 16 23 24 31
+--------+--------+--------+--------+
| Source | Destination |
| Port | Port |
+--------+--------+--------+--------+
| | |
| Length | Checksum |
+--------+--------+--------+--------+
|
| data octets ...
+---------------- ...
User Datagram Header Format
Fields
------
Length字段占2字节。所以UDP协议单次最大发送数据为2 ^ 16 = 65535 = 64KB。UDP协议不基于连接，可直接发送数据报到目标机器。因为UDP协议无连接，直接发数据到target，不需三次握手。target也不好验证客户源IP。
我们先批量set 多一点大value到远程开放 memcached server上，过期也设置长一点，然后利用UDP伪造源地址在memcached server get 存储的value，请求时间段尽量集中，这样就将数据通过mc server Reflect 到target，实现DRDoS过程。
2月底，dormando Release了 1.5.6，该版本默认关闭了UDP启动：
https://groups.google.com/forum/#!topic/memcached/pu6LAIbL_Ks
若想预防，可以升级新版，也可以网络层做限制。也可以启动 memcached 加入 -U 0启动参数，表达式短路后就不会server_sockets，禁止监听udp协议。
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