syncookies man page on FreeBSD

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SYNCACHE(4)		 BSD Kernel Interfaces Manual		   SYNCACHE(4)

NAME
     syncache, syncookiessysctl(8) MIBs for controlling TCP SYN caching

SYNOPSIS
     sysctl net.inet.tcp.syncookies
     sysctl net.inet.tcp.syncookies_only

     sysctl net.inet.tcp.syncache.hashsize
     sysctl net.inet.tcp.syncache.bucketlimit
     sysctl net.inet.tcp.syncache.cachelimit
     sysctl net.inet.tcp.syncache.rexmtlimit
     sysctl net.inet.tcp.syncache.count

DESCRIPTION
     The syncache sysctl(8) MIB is used to control the TCP SYN caching in the
     system, which is intended to handle SYN flood Denial of Service attacks.

     When a TCP SYN segment is received on a port corresponding to a listen
     socket, an entry is made in the syncache, and a SYN,ACK segment is
     returned to the peer.  The syncache entry holds the TCP options from the
     initial SYN, enough state to perform a SYN,ACK retransmission, and takes
     up less space than a TCP control block endpoint.  An incoming segment
     which contains an ACK for the SYN,ACK and matches a syncache entry will
     cause the system to create a TCP control block with the options stored in
     the syncache entry, which is then released.

     The syncache protects the system from SYN flood DoS attacks by minimizing
     the amount of state kept on the server, and by limiting the overall size
     of the syncache.

     Syncookies provides a way to virtually expand the size of the syncache by
     keeping state regarding the initial SYN in the network.  Enabling
     syncookies sends a cryptographic value in the SYN,ACK reply to the client
     machine, which is then returned in the client's ACK.  If the correspond‐
     ing entry is not found in the syncache, but the value passes specific
     security checks, the connection will be accepted.	This is only used if
     the syncache is unable to handle the volume of incoming connections, and
     a prior entry has been evicted from the cache.

     Syncookies have a certain number of disadvantages that a paranoid admin‐
     istrator may wish to take note of.	 Since the TCP options from the ini‐
     tial SYN are not saved, they are not applied to the connection, preclud‐
     ing use of features like window scale, timestamps, or exact MSS sizing.
     As the returning ACK establishes the connection, it may be possible for
     an attacker to ACK flood a machine in an attempt to create a connection.
     While steps have been taken to mitigate this risk, this may provide a way
     to bypass firewalls which filter incoming segments with the SYN bit set.

     To disable the syncache and run only with syncookies, set
     net.inet.tcp.syncookies_only to 1.

     The syncache implements a number of variables in the
     net.inet.tcp.syncache branch of the sysctl(3) MIB.	 Several of these may
     be tuned by setting the corresponding variable in the loader(8).

     hashsize	  Size of the syncache hash table, must be a power of 2.
		  Read-only, tunable via loader(8).

     bucketlimit  Limit on the number of entries permitted in each bucket of
		  the hash table.  This should be left at a low value to mini‐
		  mize search time.  Read-only, tunable via loader(8).

     cachelimit	  Limit on the total number of entries in the syncache.
		  Defaults to (hashsize × bucketlimit), may be set lower to
		  minimize memory consumption.	Read-only, tunable via
		  loader(8).

     rexmtlimit	  Maximum number of times a SYN,ACK is retransmitted before
		  being discarded.  The default of 3 retransmits corresponds
		  to a 45 second timeout, this value may be increased depend‐
		  ing on the RTT to client machines.  Tunable via sysctl(3).

     count	  Number of entries present in the syncache (read-only).

     Statistics on the performance of the syncache may be obtained via
     netstat(1), which provides the following counts:

     syncache entries added
		       Entries successfully inserted in the syncache.

     retransmitted     SYN,ACK retransmissions due to a timeout expiring.

     dupsyn	       Incoming SYN segment matching an existing entry.

     dropped	       SYNs dropped because SYN,ACK could not be sent.

     completed	       Successfully completed connections.

     bucket overflow   Entries dropped for exceeding per-bucket size.

     cache overflow    Entries dropped for exceeding overall cache size.

     reset	       RST segment received.

     stale	       Entries dropped due to maximum retransmissions or lis‐
		       ten socket disappearance.

     aborted	       New socket allocation failures.

     badack	       Entries dropped due to bad ACK reply.

     unreach	       Entries dropped due to ICMP unreachable messages.

     zone failures     Failures to allocate new syncache entry.

     cookies received  Connections created from segment containing ACK.

SEE ALSO
     netstat(1), tcp(4), loader(8), sysctl(8)

HISTORY
     The existing syncache implementation first appeared in FreeBSD 4.5.  The
     original concept of a syncache originally appeared in BSD/OS, and was
     later modified by NetBSD, then further extended here.

AUTHORS
     The syncache code and manual page were written by Jonathan Lemon
     ⟨jlemon@FreeBSD.org⟩.

BSD			       January 22, 2008				   BSD
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