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SYNCACHE(4)            FreeBSD Kernel Interfaces Manual            SYNCACHE(4)

     syncache, syncookies - sysctl(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

     cachelimit       Limit on the total number of entries in the syncache.
                      Defaults to (hashsize x 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 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
                           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
     <[email protected]>.

FreeBSD 11.1-RELEASE-p4        January 22, 2008        FreeBSD 11.1-RELEASE-p4
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