NG_TCPMSS(4)						   BSD Kernel Interfaces Manual 					      NG_TCPMSS(4)

NAME

     ng_tcpmss -- netgraph node to adjust TCP MSS option

SYNOPSIS

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

DESCRIPTION

     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.

CONTROL MESSAGES

     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 sup-
	  plied 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.

EXAMPLES

     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

SHUTDOWN

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

SEE ALSO

     netgraph(4), ng_ipfw(4)

HISTORY

     The ng_tcpmss node type was implemented in FreeBSD 6.0.

AUTHORS

     Alexey Popov <lollypop@flexuser.ru>
     Gleb Smirnoff <glebius@FreeBSD.org>

BUGS

     When running on SMP, system statistics may be broken.

BSD
								   June 9, 2005 							       BSD