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Date:      Mon, 3 Apr 2017 22:12:11 +0800
From:      Julian Elischer <julian@freebsd.org>
To:        "Andrey V. Elsukov" <ae@FreeBSD.org>, src-committers@freebsd.org, svn-src-all@freebsd.org, svn-src-head@freebsd.org
Subject:   Re: svn commit: r316435 - in head: sbin/ipfw sys/conf sys/modules sys/modules/ipfw_pmod sys/netpfil/ipfw/pmod
Message-ID:  <2fb0e146-8486-09c3-0c44-75c71a74fc2f@freebsd.org>
In-Reply-To: <201704030307.v3337mfs039014@repo.freebsd.org>
References:  <201704030307.v3337mfs039014@repo.freebsd.org>

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On 3/4/17 11:07 am, Andrey V. Elsukov wrote:
> Author: ae
> Date: Mon Apr  3 03:07:48 2017
> New Revision: 316435
> URL: https://svnweb.freebsd.org/changeset/base/316435

it was always my intention to hook netgraph modules into ipfw in this way

>
> Log:
>    Add ipfw_pmod kernel module.
>    
>    The module is designed for modification of a packets of any protocols.
>    For now it implements only TCP MSS modification. It adds the external
>    action handler for "tcp-setmss" action.
>    
>    A rule with tcp-setmss action does additional check for protocol and
>    TCP flags. If SYN flag is present, it parses TCP options and modifies
>    MSS option if its value is greater than configured value in the rule.
>    Then it adjustes TCP checksum if needed. After handling the search
>    continues with the next rule.
>    
>    Obtained from:	Yandex LLC
>    MFC after:	2 weeks
>    Relnotes:	yes
>    Sponsored by:	Yandex LLC
>    No objection from: #network
>    Differential Revision:	https://reviews.freebsd.org/D10150
>
> Added:
>    head/sys/modules/ipfw_pmod/
>    head/sys/modules/ipfw_pmod/Makefile   (contents, props changed)
>    head/sys/netpfil/ipfw/pmod/
>    head/sys/netpfil/ipfw/pmod/ip_fw_pmod.c   (contents, props changed)
>    head/sys/netpfil/ipfw/pmod/pmod.h   (contents, props changed)
>    head/sys/netpfil/ipfw/pmod/tcpmod.c   (contents, props changed)
> Modified:
>    head/sbin/ipfw/ipfw.8
>    head/sbin/ipfw/ipfw2.c
>    head/sbin/ipfw/ipfw2.h
>    head/sys/conf/NOTES
>    head/sys/conf/files
>    head/sys/conf/options
>    head/sys/modules/Makefile
>
> Modified: head/sbin/ipfw/ipfw.8
> ==============================================================================
> --- head/sbin/ipfw/ipfw.8	Mon Apr  3 02:44:40 2017	(r316434)
> +++ head/sbin/ipfw/ipfw.8	Mon Apr  3 03:07:48 2017	(r316435)
> @@ -1,7 +1,7 @@
>   .\"
>   .\" $FreeBSD$
>   .\"
> -.Dd March 15, 2017
> +.Dd April 3, 2017
>   .Dt IPFW 8
>   .Os
>   .Sh NAME
> @@ -1118,6 +1118,20 @@ It is also possible to use the
>   keyword with setdscp.
>   If the tablearg value is not within the 0..64 range, lower 6 bits of supplied
>   value are used.
> +.It Cm tcp-setmss Ar mss
> +Set the Maximum Segment Size (MSS) in the TCP segment to value
> +.Ar mss .
> +The kernel module
> +.Cm ipfw_pmod
> +should be loaded or kernel should have
> +.Cm options IPFIREWALL_PMOD
> +to be able use this action.
> +This command does not change a packet if original MSS value is lower than
> +specified value.
> +Both TCP over IPv4 and over IPv6 are supported.
> +Regardless of matched a packet or not by the
> +.Cm tcp-setmss
> +rule, the search continues with the next rule.
>   .It Cm reass
>   Queue and reassemble IP fragments.
>   If the packet is not fragmented, counters are updated and
>
> Modified: head/sbin/ipfw/ipfw2.c
> ==============================================================================
> --- head/sbin/ipfw/ipfw2.c	Mon Apr  3 02:44:40 2017	(r316434)
> +++ head/sbin/ipfw/ipfw2.c	Mon Apr  3 03:07:48 2017	(r316435)
> @@ -36,6 +36,7 @@
>   #include <pwd.h>
>   #include <stdio.h>
>   #include <stdarg.h>
> +#include <stdint.h>
>   #include <stdlib.h>
>   #include <string.h>
>   #include <sysexits.h>
> @@ -238,6 +239,7 @@ static struct _s_x rule_eactions[] = {
>   	{ "nat64lsn",		TOK_NAT64LSN },
>   	{ "nat64stl",		TOK_NAT64STL },
>   	{ "nptv6",		TOK_NPTV6 },
> +	{ "tcp-setmss",		TOK_TCPSETMSS },
>   	{ NULL, 0 }	/* terminator */
>   };
>   
> @@ -272,6 +274,7 @@ static struct _s_x rule_actions[] = {
>   	{ "call",		TOK_CALL },
>   	{ "return",		TOK_RETURN },
>   	{ "eaction",		TOK_EACTION },
> +	{ "tcp-setmss",		TOK_TCPSETMSS },
>   	{ NULL, 0 }	/* terminator */
>   };
>   
> @@ -4007,6 +4010,26 @@ chkarg:
>   		fill_cmd(action, O_CALLRETURN, F_NOT, 0);
>   		break;
>   
> +	case TOK_TCPSETMSS: {
> +		u_long mss;
> +		uint16_t idx;
> +
> +		idx = pack_object(tstate, "tcp-setmss", IPFW_TLV_EACTION);
> +		if (idx == 0)
> +			errx(EX_DATAERR, "pack_object failed");
> +		fill_cmd(action, O_EXTERNAL_ACTION, 0, idx);
> +		NEED1("Missing MSS value");
> +		action = next_cmd(action, &ablen);
> +		action->len = 1;
> +		CHECK_ACTLEN;
> +		mss = strtoul(*av, NULL, 10);
> +		if (mss == 0 || mss > UINT16_MAX)
> +			errx(EX_USAGE, "invalid MSS value %s", *av);
> +		fill_cmd(action, O_EXTERNAL_DATA, 0, (uint16_t)mss);
> +		av++;
> +		break;
> +	}
> +
>   	default:
>   		av--;
>   		if (match_token(rule_eactions, *av) == -1)
>
> Modified: head/sbin/ipfw/ipfw2.h
> ==============================================================================
> --- head/sbin/ipfw/ipfw2.h	Mon Apr  3 02:44:40 2017	(r316434)
> +++ head/sbin/ipfw/ipfw2.h	Mon Apr  3 03:07:48 2017	(r316435)
> @@ -284,6 +284,8 @@ enum tokens {
>   	TOK_INTPREFIX,
>   	TOK_EXTPREFIX,
>   	TOK_PREFIXLEN,
> +
> +	TOK_TCPSETMSS,
>   };
>   
>   /*
>
> Modified: head/sys/conf/NOTES
> ==============================================================================
> --- head/sys/conf/NOTES	Mon Apr  3 02:44:40 2017	(r316434)
> +++ head/sys/conf/NOTES	Mon Apr  3 03:07:48 2017	(r316435)
> @@ -971,6 +971,9 @@ device		lagg
>   #
>   # IPFIREWALL_NPTV6 adds support for in kernel NPTv6 in ipfw.
>   #
> +# IPFIREWALL_PMOD adds support for protocols modification module. Currently
> +# it supports only TCP MSS modification.
> +#
>   # IPSTEALTH enables code to support stealth forwarding (i.e., forwarding
>   # packets without touching the TTL).  This can be useful to hide firewalls
>   # from traceroute and similar tools.
>
> Modified: head/sys/conf/files
> ==============================================================================
> --- head/sys/conf/files	Mon Apr  3 02:44:40 2017	(r316434)
> +++ head/sys/conf/files	Mon Apr  3 03:07:48 2017	(r316435)
> @@ -4230,6 +4230,8 @@ netpfil/ipfw/nptv6/ip_fw_nptv6.c	optiona
>   	ipfirewall_nptv6
>   netpfil/ipfw/nptv6/nptv6.c	optional inet inet6 ipfirewall \
>   	ipfirewall_nptv6
> +netpfil/ipfw/pmod/ip_fw_pmod.c	optional inet ipfirewall_pmod
> +netpfil/ipfw/pmod/tcpmod.c	optional inet ipfirewall_pmod
>   netpfil/pf/if_pflog.c		optional pflog pf inet
>   netpfil/pf/if_pfsync.c		optional pfsync pf inet
>   netpfil/pf/pf.c			optional pf inet
>
> Modified: head/sys/conf/options
> ==============================================================================
> --- head/sys/conf/options	Mon Apr  3 02:44:40 2017	(r316434)
> +++ head/sys/conf/options	Mon Apr  3 03:07:48 2017	(r316435)
> @@ -426,6 +426,7 @@ IPFIREWALL_NAT64_DIRECT_OUTPUT	opt_ipfw.
>   IPFIREWALL_NPTV6	opt_ipfw.h
>   IPFIREWALL_VERBOSE	opt_ipfw.h
>   IPFIREWALL_VERBOSE_LIMIT	opt_ipfw.h
> +IPFIREWALL_PMOD		opt_ipfw.h
>   IPSEC			opt_ipsec.h
>   IPSEC_DEBUG		opt_ipsec.h
>   IPSEC_SUPPORT		opt_ipsec.h
>
> Modified: head/sys/modules/Makefile
> ==============================================================================
> --- head/sys/modules/Makefile	Mon Apr  3 02:44:40 2017	(r316434)
> +++ head/sys/modules/Makefile	Mon Apr  3 03:07:48 2017	(r316435)
> @@ -173,6 +173,7 @@ SUBDIR=	\
>   	ipfw_nat \
>   	${_ipfw_nat64} \
>   	${_ipfw_nptv6} \
> +	${_ipfw_pmod} \
>   	${_ipmi} \
>   	ip6_mroute_mod \
>   	ip_mroute_mod \
> @@ -443,6 +444,7 @@ _toecore=	toecore
>   _if_enc=	if_enc
>   _if_gif=	if_gif
>   _if_gre=	if_gre
> +_ipfw_pmod=	ipfw_pmod
>   .if ${MK_IPSEC_SUPPORT} != "no"
>   _ipsec=		ipsec
>   .endif
>
> Added: head/sys/modules/ipfw_pmod/Makefile
> ==============================================================================
> --- /dev/null	00:00:00 1970	(empty, because file is newly added)
> +++ head/sys/modules/ipfw_pmod/Makefile	Mon Apr  3 03:07:48 2017	(r316435)
> @@ -0,0 +1,8 @@
> +# $FreeBSD$
> +
> +.PATH: ${SRCTOP}/sys/netpfil/ipfw/pmod
> +
> +KMOD=   ipfw_pmod
> +SRCS=   ip_fw_pmod.c tcpmod.c opt_inet.h opt_inet6.h
> +
> +.include <bsd.kmod.mk>
>
> Added: head/sys/netpfil/ipfw/pmod/ip_fw_pmod.c
> ==============================================================================
> --- /dev/null	00:00:00 1970	(empty, because file is newly added)
> +++ head/sys/netpfil/ipfw/pmod/ip_fw_pmod.c	Mon Apr  3 03:07:48 2017	(r316435)
> @@ -0,0 +1,101 @@
> +/*-
> + * Copyright (c) 2017 Yandex LLC
> + * Copyright (c) 2017 Andrey V. Elsukov <ae@FreeBSD.org>
> + * All rights reserved.
> + *
> + * Redistribution and use in source and binary forms, with or without
> + * modification, are permitted provided that the following conditions
> + * are met:
> + *
> + * 1. Redistributions of source code must retain the above copyright
> + *    notice, this list of conditions and the following disclaimer.
> + * 2. Redistributions in binary form must reproduce the above copyright
> + *    notice, this list of conditions and the following disclaimer in the
> + *    documentation and/or other materials provided with the distribution.
> + *
> + * THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
> + * IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
> + * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
> + * IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
> + * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
> + * NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
> + * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
> + * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
> + * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
> + * THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
> + */
> +
> +#include <sys/cdefs.h>
> +__FBSDID("$FreeBSD$");
> +
> +#include <sys/param.h>
> +#include <sys/systm.h>
> +#include <sys/kernel.h>
> +#include <sys/lock.h>
> +#include <sys/malloc.h>
> +#include <sys/module.h>
> +#include <sys/rwlock.h>
> +#include <sys/socket.h>
> +
> +#include <net/if.h>
> +#include <net/vnet.h>
> +
> +#include <netinet/in.h>
> +#include <netinet/ip_var.h>
> +#include <netinet/ip_fw.h>
> +
> +#include <netpfil/ipfw/ip_fw_private.h>
> +#include <netpfil/ipfw/pmod/pmod.h>
> +
> +static int
> +vnet_ipfw_pmod_init(const void *arg __unused)
> +{
> +	int error;
> +
> +	error = tcpmod_init(&V_layer3_chain, IS_DEFAULT_VNET(curvnet));
> +	return (error);
> +}
> +
> +static int
> +vnet_ipfw_pmod_uninit(const void *arg __unused)
> +{
> +
> +	tcpmod_uninit(&V_layer3_chain, IS_DEFAULT_VNET(curvnet));
> +	return (0);
> +}
> +
> +static int
> +ipfw_pmod_modevent(module_t mod, int type, void *unused)
> +{
> +
> +	switch (type) {
> +	case MOD_LOAD:
> +	case MOD_UNLOAD:
> +		break;
> +	default:
> +		return (EOPNOTSUPP);
> +	}
> +	return (0);
> +}
> +
> +static moduledata_t ipfw_pmod_mod = {
> +	"ipfw_pmod",
> +	ipfw_pmod_modevent,
> +	0
> +};
> +
> +/* Define startup order. */
> +#define	IPFW_PMOD_SI_SUB_FIREWALL	SI_SUB_PROTO_IFATTACHDOMAIN
> +#define	IPFW_PMOD_MODEVENT_ORDER	(SI_ORDER_ANY - 128) /* after ipfw */
> +#define	IPFW_PMOD_MODULE_ORDER		(IPFW_PMOD_MODEVENT_ORDER + 1)
> +#define	IPFW_PMOD_VNET_ORDER		(IPFW_PMOD_MODEVENT_ORDER + 2)
> +
> +DECLARE_MODULE(ipfw_pmod, ipfw_pmod_mod, IPFW_PMOD_SI_SUB_FIREWALL,
> +    IPFW_PMOD_MODULE_ORDER);
> +MODULE_DEPEND(ipfw_pmod, ipfw, 3, 3, 3);
> +MODULE_VERSION(ipfw_pmod, 1);
> +
> +VNET_SYSINIT(vnet_ipfw_pmod_init, IPFW_PMOD_SI_SUB_FIREWALL,
> +    IPFW_PMOD_VNET_ORDER, vnet_ipfw_pmod_init, NULL);
> +VNET_SYSUNINIT(vnet_ipfw_pmod_uninit, IPFW_PMOD_SI_SUB_FIREWALL,
> +    IPFW_PMOD_VNET_ORDER, vnet_ipfw_pmod_uninit, NULL);
>
> Added: head/sys/netpfil/ipfw/pmod/pmod.h
> ==============================================================================
> --- /dev/null	00:00:00 1970	(empty, because file is newly added)
> +++ head/sys/netpfil/ipfw/pmod/pmod.h	Mon Apr  3 03:07:48 2017	(r316435)
> @@ -0,0 +1,36 @@
> +/*-
> + * Copyright (c) 2017 Yandex LLC
> + * Copyright (c) 2017 Andrey V. Elsukov <ae@FreeBSD.org>
> + * All rights reserved.
> + *
> + * Redistribution and use in source and binary forms, with or without
> + * modification, are permitted provided that the following conditions
> + * are met:
> + *
> + * 1. Redistributions of source code must retain the above copyright
> + *    notice, this list of conditions and the following disclaimer.
> + * 2. Redistributions in binary form must reproduce the above copyright
> + *    notice, this list of conditions and the following disclaimer in the
> + *    documentation and/or other materials provided with the distribution.
> + *
> + * THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
> + * IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
> + * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
> + * IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
> + * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
> + * NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
> + * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
> + * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
> + * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
> + * THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
> + *
> + * $FreeBSD$
> + */
> +
> +#ifndef	_IP_FW_PMOD_H_
> +#define	_IP_FW_PMOD_H_
> +
> +int tcpmod_init(struct ip_fw_chain *ch, int first);
> +void tcpmod_uninit(struct ip_fw_chain *ch, int last);
> +#endif /* _IP_FW_PMOD_H_ */
> +
>
> Added: head/sys/netpfil/ipfw/pmod/tcpmod.c
> ==============================================================================
> --- /dev/null	00:00:00 1970	(empty, because file is newly added)
> +++ head/sys/netpfil/ipfw/pmod/tcpmod.c	Mon Apr  3 03:07:48 2017	(r316435)
> @@ -0,0 +1,246 @@
> +/*-
> + * Copyright (c) 2017 Yandex LLC
> + * Copyright (c) 2017 Andrey V. Elsukov <ae@FreeBSD.org>
> + * All rights reserved.
> + *
> + * Redistribution and use in source and binary forms, with or without
> + * modification, are permitted provided that the following conditions
> + * are met:
> + *
> + * 1. Redistributions of source code must retain the above copyright
> + *    notice, this list of conditions and the following disclaimer.
> + * 2. Redistributions in binary form must reproduce the above copyright
> + *    notice, this list of conditions and the following disclaimer in the
> + *    documentation and/or other materials provided with the distribution.
> + *
> + * THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
> + * IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
> + * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
> + * IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
> + * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
> + * NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
> + * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
> + * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
> + * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
> + * THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
> + */
> +
> +#include "opt_inet.h"
> +#include "opt_inet6.h"
> +
> +#include <sys/cdefs.h>
> +__FBSDID("$FreeBSD$");
> +
> +#include <sys/param.h>
> +#include <sys/systm.h>
> +#include <sys/errno.h>
> +#include <sys/kernel.h>
> +#include <sys/mbuf.h>
> +#include <sys/module.h>
> +#include <sys/socket.h>
> +
> +#include <net/if.h>
> +#include <net/if_var.h>
> +#include <net/pfil.h>
> +#include <net/vnet.h>
> +
> +#include <netinet/in.h>
> +#include <netinet/ip.h>
> +#include <netinet/ip_var.h>
> +#include <netinet/tcp.h>
> +#include <netinet/ip_fw.h>
> +#include <netinet/ip6.h>
> +
> +#include <netpfil/ipfw/ip_fw_private.h>
> +#include <netpfil/ipfw/pmod/pmod.h>
> +
> +#include <machine/in_cksum.h>
> +
> +static VNET_DEFINE(uint16_t, tcpmod_setmss_eid) = 0;
> +#define	V_tcpmod_setmss_eid	VNET(tcpmod_setmss_eid)
> +
> +static int
> +tcpmod_setmss(struct mbuf **mp, struct tcphdr *tcp, int tlen, uint16_t mss)
> +{
> +	struct mbuf *m;
> +	u_char *cp;
> +	int optlen, ret;
> +	uint16_t oldmss, csum;
> +
> +	m = *mp;
> +	ret = IP_FW_DENY;
> +	if (m->m_len < m->m_pkthdr.len) {
> +		/*
> +		 * We shouldn't have any data, IP packet contains only
> +		 * TCP header with options.
> +		 */
> +		*mp = m = m_pullup(m, m->m_pkthdr.len);
> +		if (m == NULL)
> +			return (ret);
> +	}
> +	/* Parse TCP options. */
> +	for (tlen -= sizeof(struct tcphdr), cp = (u_char *)(tcp + 1);
> +	    tlen > 0; tlen -= optlen, cp += optlen) {
> +		if (cp[0] == TCPOPT_EOL)
> +			break;
> +		if (cp[0] == TCPOPT_NOP) {
> +			optlen = 1;
> +			continue;
> +		}
> +		if (tlen < 2)
> +			break;
> +		optlen = cp[1];
> +		if (optlen < 2 || optlen > tlen)
> +			break;
> +		if (cp[0] == TCPOPT_MAXSEG) {
> +			if (optlen != TCPOLEN_MAXSEG)
> +				break;
> +			ret = 0; /* report success */
> +			bcopy(cp + 2, &oldmss, sizeof(oldmss));
> +			/* Do not update lower MSS value */
> +			if (oldmss <= mss)
> +				break;
> +			bcopy(&mss, cp + 2, sizeof(mss));
> +			/* Update checksum if it is not delayed. */
> +			if ((m->m_pkthdr.csum_flags &
> +			    (CSUM_TCP | CSUM_TCP_IPV6)) == 0) {
> +				bcopy(&tcp->th_sum, &csum, sizeof(csum));
> +				csum = cksum_adjust(csum, oldmss, mss);
> +				bcopy(&csum, &tcp->th_sum, sizeof(csum));
> +			}
> +			break;
> +		}
> +	}
> +
> +	return (ret);
> +}
> +
> +#ifdef INET6
> +static int
> +tcpmod_ipv6_setmss(struct mbuf **mp, uint16_t mss)
> +{
> +	struct ip6_hdr *ip6;
> +	struct ip6_hbh *hbh;
> +	struct tcphdr *tcp;
> +	int hlen, plen, proto;
> +
> +	ip6 = mtod(*mp, struct ip6_hdr *);
> +	hlen = sizeof(*ip6);
> +	proto = ip6->ip6_nxt;
> +	/*
> +	 * Skip IPv6 extension headers and get the TCP header.
> +	 * ipfw_chk() has already done this work. So we are sure that
> +	 * we will not do an access to the out of bounds. For this
> +	 * reason we skip some checks here.
> +	 */
> +	while (proto == IPPROTO_HOPOPTS || proto == IPPROTO_ROUTING ||
> +	    proto == IPPROTO_DSTOPTS) {
> +		hbh = mtodo(*mp, hlen);
> +		proto = hbh->ip6h_nxt;
> +		hlen += hbh->ip6h_len << 3;
> +	}
> +	tcp = mtodo(*mp, hlen);
> +	plen = (*mp)->m_pkthdr.len - hlen;
> +	hlen = tcp->th_off << 2;
> +	/* We must have TCP options and enough data in a packet. */
> +	if (hlen <= sizeof(struct tcphdr) || hlen > plen)
> +		return (IP_FW_DENY);
> +	return (tcpmod_setmss(mp, tcp, hlen, mss));
> +}
> +#endif /* INET6 */
> +
> +#ifdef INET
> +static int
> +tcpmod_ipv4_setmss(struct mbuf **mp, uint16_t mss)
> +{
> +	struct tcphdr *tcp;
> +	struct ip *ip;
> +	int hlen, plen;
> +
> +	ip = mtod(*mp, struct ip *);
> +	hlen = ip->ip_hl << 2;
> +	tcp = mtodo(*mp, hlen);
> +	plen = (*mp)->m_pkthdr.len - hlen;
> +	hlen = tcp->th_off << 2;
> +	/* We must have TCP options and enough data in a packet. */
> +	if (hlen <= sizeof(struct tcphdr) || hlen > plen)
> +		return (IP_FW_DENY);
> +	return (tcpmod_setmss(mp, tcp, hlen, mss));
> +}
> +#endif /* INET */
> +
> +/*
> + * ipfw external action handler.
> + */
> +static int
> +ipfw_tcpmod(struct ip_fw_chain *chain, struct ip_fw_args *args,
> +    ipfw_insn *cmd, int *done)
> +{
> +	ipfw_insn *icmd;
> +	int ret;
> +
> +	*done = 0; /* try next rule if not matched */
> +	ret = IP_FW_DENY;
> +	icmd = cmd + 1;
> +	if (cmd->opcode != O_EXTERNAL_ACTION ||
> +	    cmd->arg1 != V_tcpmod_setmss_eid ||
> +	    icmd->opcode != O_EXTERNAL_DATA ||
> +	    icmd->len != F_INSN_SIZE(ipfw_insn))
> +		return (ret);
> +
> +	/*
> +	 * NOTE: ipfw_chk() can set f_id.proto from IPv6 fragment header,
> +	 * but f_id._flags can be filled only from real TCP header.
> +	 *
> +	 * NOTE: ipfw_chk() drops very short packets in the PULLUP_TO()
> +	 * macro. But we need to check that mbuf is contiguous more than
> +	 * IP+IP_options/IP_extensions+tcphdr length, because TCP header
> +	 * must have TCP options, and ipfw_chk() does PULLUP_TO() size of
> +	 * struct tcphdr.
> +	 *
> +	 * NOTE: we require only the presence of SYN flag. User should
> +	 * properly configure the rule to select the direction of packets,
> +	 * that should be modified.
> +	 */
> +	if (args->f_id.proto != IPPROTO_TCP ||
> +	    (args->f_id._flags & TH_SYN) == 0)
> +		return (ret);
> +
> +	switch (args->f_id.addr_type) {
> +#ifdef INET
> +		case 4:
> +			ret = tcpmod_ipv4_setmss(&args->m, htons(icmd->arg1));
> +			break;
> +#endif
> +#ifdef INET6
> +		case 6:
> +			ret = tcpmod_ipv6_setmss(&args->m, htons(icmd->arg1));
> +			break;
> +#endif
> +	}
> +	/*
> +	 * We return zero in both @ret and @done on success, and ipfw_chk()
> +	 * will update rule counters. Otherwise a packet will not be matched
> +	 * by rule.
> +	 */
> +	return (ret);
> +}
> +
> +int
> +tcpmod_init(struct ip_fw_chain *ch, int first)
> +{
> +
> +	V_tcpmod_setmss_eid = ipfw_add_eaction(ch, ipfw_tcpmod, "tcp-setmss");
> +	if (V_tcpmod_setmss_eid == 0)
> +		return (ENXIO);
> +	return (0);
> +}
> +
> +void
> +tcpmod_uninit(struct ip_fw_chain *ch, int last)
> +{
> +
> +	ipfw_del_eaction(ch, V_tcpmod_setmss_eid);
> +	V_tcpmod_setmss_eid = 0;
> +}
> +
>
>




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