It MUST NOT be used to help highly loaded servers to stand against legal connection rate. For recommended alternatives see tcp_max_syn_backlog, tcp_synack_retries, and tcp_abort_on_overflow.įurther down, they state: Note, that syncookies is fallback facility. It is not recommended as a tuning mechanism for heavily loaded servers to help with overloaded or misconfigured conditions. It can cause problems for clients and relays. This is a violation of the TCP protocol, and conflicts with other areas of TCP such as TCP extensions. This should be used as a last resort, if at all. See the section of the kernel documentation for tcp_syncookies - BOOLEAN for some further information regarding this feature: The syncookies feature attempts to protect a socket from a SYN flood attack. In this case, this would not be indicative of a real SYN flooding attack, but to the TCP/IP stack it looks like it exhibits the same characteristics and the kernel responds by reporting a possible ( fake) attack. In the case of MarkLogic, this message can appear if the rate of incoming messages is perceived to the kernel as being unusually high. By ignoring half-open connections, SYN floods are no longer a problem.
Syn flood attack full#
Instead it relies on the sequence number in the following ACK datagram that the ACK follows a SYN and a SYN-ACK which establishes full communication between client and server. If SYN cookies are enabled, then the kernel doesn't track half-open connections. A connection which is in the process of being established is also known as embryonic connection. The term half-open refers to TCP connections whose state is out of synchronization between the two communicating hosts, possibly due to a crash of one side. Due to the fact that there is a limit to how many half open connections that the kernel can maintain at any given time, this is where the problem becomes characterized as an attack. without SYN cookies), TCP connections will be kept half-open after receiving the first SYN because of the handshake mechanism used to establish TCP connections. In the event of a real attack, a SYN flood will most likely originate from a fake IP address during an attack, the client performing the "flood" is not waiting for the SYN-ACK response back from the server it is attacking. The goal of this is to firmly establish communication on both the server and the client. The process (or pattern) described above is known as Three Way Handshaking. Under normal operation, your kernel should acknowledge these incoming SYNs with a SYN-ACK, are not followed by ACK messages from the client. You would expect to see evidence of a SYN flood when a "flood" of TCP SYN messages are sent to the host. If the value returned is 1 (as per the example above), then tcp_syncookies are enabled for this host Possible SYN flooding A SYN flood is a form of denial-of-service attack in which an attacker sends a succession of SYN requests to a target's system in an attempt to consume enough server resources to make the system unresponsive to legitimate traffic. You can check for this by viewing the contents of /proc/sys/net/ipv4/tcp_syncookies $ cat /proc/sys/net/ipv4/tcp_syncookies
The tcp_syncookies configuration is likely enabled on your system. This may also be seen as part of the output from a call to dmesg and could possibly follow a stack trace, for example: ? audit_syscall_entry+0x1d7/0x200 ? system_call_fastpath+0x16/0x1b Some customers have reported seeing kernel level messages like this in their /var/log/messages file: Jan 31 17:41:46 ml-c1-u3 kernel: TCP: Possible SYN flooding on port 7999.
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