.\" Copyright (c) 2005 Gleb Smirnoff
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.\" $FreeBSD: head/share/man/man4/ng_tcpmss.4 267938 2014-06-26 21:46:14Z bapt $
.\"
.Dd June 9, 2005
.Dt NG_TCPMSS 4
.Os
.Sh NAME
.Nm ng_tcpmss
.Nd "netgraph node to adjust TCP MSS option"
.Sh SYNOPSIS
.In netgraph.h
.In netgraph/ng_tcpmss.h
.Sh DESCRIPTION
The
.Nm 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
.Dv NG_TCPMSS_CONFIG
control message is used to configure a hook.
.Sh CONTROL MESSAGES
This node type supports the generic control messages, plus the following.
.Bl -tag -width foo
.It Dv NGM_TCPMSS_CONFIG Pq Ic config
This control message configures node to do given MSS adjusting on
a particular hook.
It requires the
.Vt "struct ng_tcpmss_config"
to be supplied as an argument:
.Bd -literal
struct ng_tcpmss_config {
	char		inHook[NG_HOOKSIZ];
	char		outHook[NG_HOOKSIZ];
	uint16_t	maxMSS;
}
.Ed
.Pp
This means: packets received on
.Va inHook
would be checked for TCP MSS option and the latter would be
reduced down to
.Va maxMSS
if it exceeds
.Va maxMSS .
After that, packets would be sent to hook
.Va outHook .
.It Dv NGM_TCPMSS_GET_STATS Pq Ic getstats
This control message obtains statistics for a given hook.
The statistics are returned in
.Vt "struct ng_tcpmss_hookstat" :
.Bd -literal
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 */
};
.Ed
.It Dv NGM_TCPMSS_CLR_STATS Pq Ic clrstats
This control message clears statistics for a given hook.
.It Dv NGM_TCPMSS_GETCLR_STATS Pq Ic getclrstats
This control message obtains and clears statistics for a given hook.
.El
.Sh EXAMPLES
In the following example, packets are injected into the
.Nm tcpmss
node using the
.Xr ng_ipfw 4
node.
.Bd -literal -offset indent
# 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
.Ed
.Sh SHUTDOWN
This node shuts down upon receipt of an
.Dv NGM_SHUTDOWN
control message, or when all hooks have been disconnected.
.Sh SEE ALSO
.Xr netgraph 4 ,
.Xr ng_ipfw 4
.Sh HISTORY
The
.Nm
node type was implemented in
.Fx 6.0 .
.Sh AUTHORS
.An Alexey Popov Aq Mt lollypop@flexuser.ru
.An Gleb Smirnoff Aq Mt glebius@FreeBSD.org
.Sh BUGS
When running on SMP, system statistics may be broken.