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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 outHook.

     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 hook.


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

           # 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.