All you need to know about SYN floods

Date: 09 Apr 2012
Author: Erik Dubbelboer

SYN cookies

So one day I noticed /var/log/syslog on one of our servers was filled with the following message:

TCP: Possible SYN flooding on port 80. Sending cookies.

This message can come a from a SYN DDOS, but in our case it was because of the amount of new connections one of our application was receiving. The syslog message is emitted when the SYN backlog of a socket is full.

The kernel documentation has the following to say about SYN cookies:

Note, that syncookies is fallback facility.
It MUST NOT be used to help highly loaded servers to stand
against legal connection rate. If you see SYN flood warnings
in your logs, but investigation shows that they occur
because of overload with legal connections, you should tune
another parameters until this warning disappear.
See: tcp_max_syn_backlog, tcp_synack_retries, tcp_abort_on_overflow.

syncookies seriously violate TCP protocol, do not allow
to use TCP extensions, can result in serious degradation
of some services (f.e. SMTP relaying), visible not by you,
but your clients and relays, contacting you. While you see
SYN flood warnings in logs not being really flooded, your server
is seriously misconfigured.

To fix this problem I started by increasing the net.ipv4.tcp_max_syn_backlog kernel parameter. On our Ubuntu system the default was 2048 so I changed it to 4096 and restarted our application.

Nothing changed and the flooding messages still kept being emitted. I tried tuning some more parameters like tcp_synack_retries and netdev_max_backlog but nothing helped. Finally a friend pointed out to me that I could be looking at the actual kernel source to find out why the message was still being emitted.

Going back to the source

The obvious place to start this search was the function that actually emitted the message.

I found the function in net/ipv4/tcp_ipv4.c:

static void syn_flood_warning(const struct sk_buff *skb)
  const char *msg;

  if (sysctl_tcp_syncookies)
    msg = "Sending cookies";
    msg = "Dropping request";

  pr_info("TCP: Possible SYN flooding on port %d. %s.\n",
        ntohs(tcp_hdr(skb)->dest), msg);

This function was only being called from one point, another function in net/ipv4/tcp_ipv4.c:

int tcp_v4_conn_request(struct sock *sk, struct sk_buff *skb)
  // ...

  if (inet_csk_reqsk_queue_is_full(sk) && !isn) {
    if (net_ratelimit())

  // ...

This function is called every time a new connection is set up. The net_ratelimit() is only there to prevent the syslog from being flooded with messages. So I followed the inet_csk_reqsk_queue_is_full() function to see what caused it to return true.

The function is defined in include/net/inet_connection_sock.h:

static inline int inet_csk_reqsk_queue_is_full(const struct sock *sk)
  return reqsk_queue_is_full(&inet_csk(sk)->icsk_accept_queue);

It simply calles another function in include/net/request_sock.h:

static inline int reqsk_queue_is_full(const struct request_sock_queue *queue)
  return queue->listen_opt->qlen >> queue->listen_opt->max_qlen_log;

So this is the actual check to see if the backlog is full. At a first glance the function looks a bit strange. The queue length is bit shifted by a max length variable. qlen is an integer so shifting it to the left will decrease it’s value. max_qlen_log is a base 2 logarithm of the max queue length (which can only be a power of 2). So when qlen is smaller than the maximum queue length all 1 bits will be shifted out and the return value will be 0. When qlen is larger it will still have bits set to 1 and the function will return a positive number.

So where is max_qlen_log set?

Size of the backlog

Searching for an assignment of max_qlen_log I found only one place.


int reqsk_queue_alloc(struct request_sock_queue *queue,
                      unsigned int nr_table_entries)
  // ...

  nr_table_entries = min_t(u32, nr_table_entries, sysctl_max_syn_backlog);
  nr_table_entries = max_t(u32, nr_table_entries, 8);
  nr_table_entries = roundup_pow_of_two(nr_table_entries + 1);

  // ...

  for (lopt->max_qlen_log = 3;
       (1 << lopt->max_qlen_log) < nr_table_entries;

  // ...

The reqsk_queue_alloc() function is called each time a new socket starts listening for connections. As you can see from the code max_qlen_log will depend on the nr_table_entries argument.

First nr_table_entries is bound to the 8,sysctl_max_syn_backlog range. This is the first sign of a kernel parameter, namely net.ipv4.tcp_max_syn_backlog, that I tried to tune. Apparently it has some effect on the backlog size but it only specifies a maximum, not the actual value like many resources would have you believe.

nr_table_entries then it is incremented by 1 (which still seems strange to me, see below) and rounded to the nearest power of 2. The for loop then sets max_qlen_log to the base 2 logarithm of nr_table_entries.

So from this function I found the maximum size of the backlog is bound by net.ipv4.tcp_max_syn_backlog but the actual size is determined by the nr_table_entries argument. Time to find out where reqsk_queue_alloc() is called.

The only place it is called from is in net/ipv4/inet_connection_sock.c:

int inet_csk_listen_start(struct sock *sk, const int nr_table_entries)
  // ...

  reqsk_queue_alloc(&icsk->icsk_accept_queue, nr_table_entries);

  // ...

The inet_csk_listen_start() function does nothing with the nr_table_entries argument and is itself called in net/ipv4/af_inet.c:

int inet_listen(struct socket *sock, int backlog)
  // ...

  err = inet_csk_listen_start(sk, backlog);

  // ...

This function doesn’t change the backlog size either. The function is not called directly in the source but is assign to a function pointer inside the inet_stream_ops struct in net/ipv4/af_inet.c.

The pointer in the struct is called in net/socket.c:

SYSCALL_DEFINE2(listen, int, fd, int, backlog)
  // ...

  int somaxconn;

  // ...

    somaxconn = sock_net(sock->sk)->core.sysctl_somaxconn;

    if ((unsigned)backlog > somaxconn)
      backlog = somaxconn;

    // ...

    sock->ops->listen(sock, backlog);

  // ...

Now this is the actual listen() syscall which has a backlog argument. This function also seems to put an upper limit on the backlog size. sock_net(sock->sk)->core.sysctl_somaxconn is another kernel parameter controlled by net.core.somaxconn. It defaults to the SOMAXCONN macro which equals 128 on our system.

128 is quite low so I increased it to 4096 as well. I restarted our application but to my supprise the flooding message still kept being emitted.

Lucky the application we are using is open source. So I opened up the source and found the application calling listen() with a backlog of again SOMAXCONN. After changing this to 1000000 (why not set it very high and let the kernel parameters limit it?), recompiling and restarting the application the message finally stopped.

Note: kernel 3.3 does exactly the same as 2.6 (on which this post is based)

A reasonably backlog size

In the reqsk_queue_alloc() function you can see an array of request_sock * pointers the size of nr_table_entries is allocated. On a 64 bit system the size of the request_sock is 56 bytes. Plus the 8 bytes for the pointer makes it around 64 bytes per entry. So 4096 entries would only take up 0.25 MB. 4096 should be enough for most servers but you can see that increasing it even more wouldn’t be a problem.

Keep in mind that setting the backlog to 4096 will actually make it 8192 entries big. This because of the strange + 1 in nr_table_entries = roundup_pow_of_two(nr_table_entries + 1);. This is the reason that software like nginx, redis and apache all set the backlog to 511.


The main thing I learned from this all is that using open source software allows you to track and fix problems that closed source software would not.

Also fixing the SYN flooding problem requires you to modify net.ipv4.tcp_max_syn_backlog, net.core.somaxconn and the backlog size passed to the listen() syscall.

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