xref: /freebsd/share/man/man4/ng_tcpmss.4 (revision c604c8765123f1c6f029ae00dfc12d1e0057a7ad)

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.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/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.
To configure a hook, user should send
.Dv NG_TCPMSS_CONFIG
control message to node.
.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
particular hook.
It requires the
.Vt "struct ng_tcpmss_config"
to be supplied as 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
.Qq inHook
would be checked for TCP MSS option and the latter would be
reduced down to
.Qq maxMSS ,
if it exceeds
.Qq maxMSS .
After that packets would be sent to hook
.Qq outHook .
.It Dv NGM_TCPMSS_GET_STATS Pq Ic getstats
This control message obtains statistics for the 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
.Pp
.It Dv NGM_TCPMSS_CLR_STATS Pq Ic clrstats
This control message clears statistics for the given hook.
.It Dv NGM_TCPMSS_GETCLR_STATS Pq Ic getclrstats
This control message obtains and clears statistics for the given hook.
.El
.Sh EXAMPLES
In the following example packets are injected into
.Nm
node with help of
.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 lollypop@flexuser.ru
and
.An Gleb Smirnoff Aq glebius@FreeBSD.org
.Sh BUGS
When running on SMP system statistics may be broken.