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


     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


     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
     corresponding 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
     administrator may wish to take note of.  Since the TCP options from the
     initial SYN are not saved, they are not applied to the connection,
     precluding 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
                  minimize 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

     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
                  depending on the RTT to client machines.  Tunable via

     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
                       listen 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.


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


     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.


     The syncache code and manual page were written by Jonathan Lemon