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     ng_tcpmss - netgraph node to adjust TCP MSS option


     #include <netgraph.h>
     #include <netgraph/ng_tcpmss.h>


     The tcpmss node type is designed to alter the Maximum Segment Size option
     of TCP packets.  This node accepts an arbitrary number of hooks.
     Initially a new hook is considered unconfigured.  The NG_TCPMSS_CONFIG
     control message is used to configure a hook.


     This node type supports the generic control messages, plus the following.

     NGM_TCPMSS_CONFIG (config)
             This control message configures node to do given MSS adjusting on
             a particular hook.  It requires the struct ng_tcpmss_config to be
             supplied as an argument:

             struct ng_tcpmss_config {
                     char            inHook[NG_HOOKSIZ];
                     char            outHook[NG_HOOKSIZ];
                     uint16_t        maxMSS;

             This means: packets received on inHook would be checked for TCP
             MSS option and the latter would be reduced down to maxMSS if it
             exceeds maxMSS.  After that, packets would be sent to hook

     NGM_TCPMSS_GET_STATS (getstats)
             This control message obtains statistics for a given hook.  The
             statistics are returned in struct ng_tcpmss_hookstat:

             struct ng_tcpmss_hookstat {
                     uint64_t        Octets;         /* total bytes */
                     uint64_t        Packets;        /* total packets */
                     uint16_t        maxMSS;         /* maximum MSS */
                     uint64_t        SYNPkts;        /* TCP SYN packets */
                     uint64_t        FixedPkts;      /* changed packets */

     NGM_TCPMSS_CLR_STATS (clrstats)
             This control message clears statistics for a given hook.

     NGM_TCPMSS_GETCLR_STATS (getclrstats)
             This control message obtains and clears statistics for a given


     In the following example, packets are injected into the tcpmss node using
     the ng_ipfw(4) node.

           # Create tcpmss node and connect it to ng_ipfw node
           ngctl mkpeer ipfw: tcpmss 100 qqq

           # Adjust MSS to 1452
           ngctl msg ipfw:100 config ’{ inHook="qqq" outHook="qqq" maxMSS=1452 }’

           # Divert traffic into tcpmss node
           ipfw add 300 netgraph 100 tcp from any to any tcpflags syn out via fxp0

           # Let packets continue with ipfw after being hacked
           sysctl net.inet.ip.fw.one_pass=0


     This node shuts down upon receipt of an NGM_SHUTDOWN control message, or
     when all hooks have been disconnected.


     netgraph(4), ng_ipfw(4)


     The ng_tcpmss node type was implemented in FreeBSD 6.0.


     Alexey Popov 〈〉
     Gleb Smirnoff 〈


     When running on SMP, system statistics may be broken.