NAME

     syncache, syncookies — sysctl(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 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 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 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.

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>.