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+/*
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+ * This is a module which is used for setting the MSS option in TCP packets.
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+ *
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+ * Copyright (C) 2000 Marc Boucher <marc@mbsi.ca>
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+ *
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+ * This program is free software; you can redistribute it and/or modify
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+ * it under the terms of the GNU General Public License version 2 as
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+ * published by the Free Software Foundation.
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+ */
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+
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+#include <linux/module.h>
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+#include <linux/skbuff.h>
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+#include <linux/ip.h>
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+#include <linux/ipv6.h>
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+#include <linux/tcp.h>
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+#include <net/ipv6.h>
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+#include <net/tcp.h>
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+
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+#include <linux/netfilter_ipv4/ip_tables.h>
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+#include <linux/netfilter_ipv6/ip6_tables.h>
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+#include <linux/netfilter/x_tables.h>
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+#include <linux/netfilter/xt_tcpudp.h>
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+#include <linux/netfilter/xt_TCPMSS.h>
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+
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+MODULE_LICENSE("GPL");
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+MODULE_AUTHOR("Marc Boucher <marc@mbsi.ca>");
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+MODULE_DESCRIPTION("x_tables TCP MSS modification module");
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+MODULE_ALIAS("ipt_TCPMSS");
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+MODULE_ALIAS("ip6t_TCPMSS");
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+
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+static inline unsigned int
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+optlen(const u_int8_t *opt, unsigned int offset)
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+{
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+ /* Beware zero-length options: make finite progress */
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+ if (opt[offset] <= TCPOPT_NOP || opt[offset+1] == 0)
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+ return 1;
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+ else
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+ return opt[offset+1];
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+}
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+
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+static int
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+tcpmss_mangle_packet(struct sk_buff **pskb,
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+ const struct xt_tcpmss_info *info,
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+ unsigned int tcphoff,
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+ unsigned int minlen)
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+{
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+ struct tcphdr *tcph;
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+ unsigned int tcplen, i;
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+ __be16 oldval;
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+ u16 newmss;
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+ u8 *opt;
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+
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+ if (!skb_make_writable(pskb, (*pskb)->len))
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+ return -1;
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+
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+ tcplen = (*pskb)->len - tcphoff;
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+ tcph = (struct tcphdr *)((*pskb)->nh.raw + tcphoff);
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+
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+ /* Since it passed flags test in tcp match, we know it is is
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+ not a fragment, and has data >= tcp header length. SYN
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+ packets should not contain data: if they did, then we risk
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+ running over MTU, sending Frag Needed and breaking things
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+ badly. --RR */
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+ if (tcplen != tcph->doff*4) {
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+ if (net_ratelimit())
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+ printk(KERN_ERR "xt_TCPMSS: bad length (%u bytes)\n",
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+ (*pskb)->len);
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+ return -1;
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+ }
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+
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+ if (info->mss == XT_TCPMSS_CLAMP_PMTU) {
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+ if (dst_mtu((*pskb)->dst) <= minlen) {
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+ if (net_ratelimit())
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+ printk(KERN_ERR "xt_TCPMSS: "
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+ "unknown or invalid path-MTU (%u)\n",
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+ dst_mtu((*pskb)->dst));
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+ return -1;
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+ }
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+ newmss = dst_mtu((*pskb)->dst) - minlen;
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+ } else
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+ newmss = info->mss;
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+
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+ opt = (u_int8_t *)tcph;
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+ for (i = sizeof(struct tcphdr); i < tcph->doff*4; i += optlen(opt, i)) {
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+ if (opt[i] == TCPOPT_MSS && tcph->doff*4 - i >= TCPOLEN_MSS &&
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+ opt[i+1] == TCPOLEN_MSS) {
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+ u_int16_t oldmss;
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+
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+ oldmss = (opt[i+2] << 8) | opt[i+3];
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+
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+ if (info->mss == XT_TCPMSS_CLAMP_PMTU &&
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+ oldmss <= newmss)
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+ return 0;
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+
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+ opt[i+2] = (newmss & 0xff00) >> 8;
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+ opt[i+3] = (newmss & 0x00ff);
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+
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+ nf_proto_csum_replace2(&tcph->check, *pskb,
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+ htons(oldmss), htons(newmss), 0);
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+ return 0;
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+ }
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+ }
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+
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+ /*
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+ * MSS Option not found ?! add it..
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+ */
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+ if (skb_tailroom((*pskb)) < TCPOLEN_MSS) {
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+ struct sk_buff *newskb;
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+
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+ newskb = skb_copy_expand(*pskb, skb_headroom(*pskb),
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+ TCPOLEN_MSS, GFP_ATOMIC);
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+ if (!newskb)
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+ return -1;
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+ kfree_skb(*pskb);
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+ *pskb = newskb;
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+ tcph = (struct tcphdr *)((*pskb)->nh.raw + tcphoff);
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+ }
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+
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+ skb_put((*pskb), TCPOLEN_MSS);
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+
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+ opt = (u_int8_t *)tcph + sizeof(struct tcphdr);
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+ memmove(opt + TCPOLEN_MSS, opt, tcplen - sizeof(struct tcphdr));
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+
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+ nf_proto_csum_replace2(&tcph->check, *pskb,
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+ htons(tcplen), htons(tcplen + TCPOLEN_MSS), 1);
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+ opt[0] = TCPOPT_MSS;
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+ opt[1] = TCPOLEN_MSS;
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+ opt[2] = (newmss & 0xff00) >> 8;
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+ opt[3] = (newmss & 0x00ff);
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+
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+ nf_proto_csum_replace4(&tcph->check, *pskb, 0, *((__be32 *)opt), 0);
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+
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+ oldval = ((__be16 *)tcph)[6];
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+ tcph->doff += TCPOLEN_MSS/4;
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+ nf_proto_csum_replace2(&tcph->check, *pskb,
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+ oldval, ((__be16 *)tcph)[6], 0);
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+ return TCPOLEN_MSS;
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+}
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+
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+static unsigned int
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+xt_tcpmss_target4(struct sk_buff **pskb,
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+ const struct net_device *in,
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+ const struct net_device *out,
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+ unsigned int hooknum,
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+ const struct xt_target *target,
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+ const void *targinfo)
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+{
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+ struct iphdr *iph = (*pskb)->nh.iph;
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+ __be16 newlen;
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+ int ret;
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+
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+ ret = tcpmss_mangle_packet(pskb, targinfo, iph->ihl * 4,
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+ sizeof(*iph) + sizeof(struct tcphdr));
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+ if (ret < 0)
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+ return NF_DROP;
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+ if (ret > 0) {
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+ iph = (*pskb)->nh.iph;
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+ newlen = htons(ntohs(iph->tot_len) + ret);
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+ nf_csum_replace2(&iph->check, iph->tot_len, newlen);
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+ iph->tot_len = newlen;
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+ }
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+ return XT_CONTINUE;
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+}
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+
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+#if defined(CONFIG_IP6_NF_IPTABLES) || defined(CONFIG_IP6_NF_IPTABLES_MODULE)
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+static unsigned int
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+xt_tcpmss_target6(struct sk_buff **pskb,
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+ const struct net_device *in,
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+ const struct net_device *out,
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+ unsigned int hooknum,
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+ const struct xt_target *target,
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+ const void *targinfo)
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+{
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+ struct ipv6hdr *ipv6h = (*pskb)->nh.ipv6h;
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+ u8 nexthdr;
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+ int tcphoff;
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+ int ret;
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+
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+ nexthdr = ipv6h->nexthdr;
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+ tcphoff = ipv6_skip_exthdr(*pskb, sizeof(*ipv6h), &nexthdr);
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+ if (tcphoff < 0) {
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+ WARN_ON(1);
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+ return NF_DROP;
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+ }
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+ ret = tcpmss_mangle_packet(pskb, targinfo, tcphoff,
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+ sizeof(*ipv6h) + sizeof(struct tcphdr));
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+ if (ret < 0)
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+ return NF_DROP;
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+ if (ret > 0) {
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+ ipv6h = (*pskb)->nh.ipv6h;
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+ ipv6h->payload_len = htons(ntohs(ipv6h->payload_len) + ret);
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+ }
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+ return XT_CONTINUE;
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+}
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+#endif
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+
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+#define TH_SYN 0x02
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+
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+/* Must specify -p tcp --syn */
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+static inline int find_syn_match(const struct xt_entry_match *m)
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+{
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+ const struct xt_tcp *tcpinfo = (const struct xt_tcp *)m->data;
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+
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+ if (strcmp(m->u.kernel.match->name, "tcp") == 0 &&
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+ tcpinfo->flg_cmp & TH_SYN &&
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+ !(tcpinfo->invflags & XT_TCP_INV_FLAGS))
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+ return 1;
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+
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+ return 0;
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+}
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+
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+static int
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+xt_tcpmss_checkentry4(const char *tablename,
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+ const void *entry,
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+ const struct xt_target *target,
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+ void *targinfo,
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+ unsigned int hook_mask)
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+{
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+ const struct xt_tcpmss_info *info = targinfo;
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+ const struct ipt_entry *e = entry;
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+
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+ if (info->mss == XT_TCPMSS_CLAMP_PMTU &&
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+ (hook_mask & ~((1 << NF_IP_FORWARD) |
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+ (1 << NF_IP_LOCAL_OUT) |
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+ (1 << NF_IP_POST_ROUTING))) != 0) {
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+ printk("xt_TCPMSS: path-MTU clamping only supported in "
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+ "FORWARD, OUTPUT and POSTROUTING hooks\n");
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+ return 0;
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+ }
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+ if (IPT_MATCH_ITERATE(e, find_syn_match))
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+ return 1;
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+ printk("xt_TCPMSS: Only works on TCP SYN packets\n");
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+ return 0;
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+}
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+
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+#if defined(CONFIG_IP6_NF_IPTABLES) || defined(CONFIG_IP6_NF_IPTABLES_MODULE)
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+static int
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+xt_tcpmss_checkentry6(const char *tablename,
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+ const void *entry,
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+ const struct xt_target *target,
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+ void *targinfo,
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+ unsigned int hook_mask)
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+{
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+ const struct xt_tcpmss_info *info = targinfo;
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+ const struct ip6t_entry *e = entry;
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+
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+ if (info->mss == XT_TCPMSS_CLAMP_PMTU &&
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+ (hook_mask & ~((1 << NF_IP6_FORWARD) |
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+ (1 << NF_IP6_LOCAL_OUT) |
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+ (1 << NF_IP6_POST_ROUTING))) != 0) {
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+ printk("xt_TCPMSS: path-MTU clamping only supported in "
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+ "FORWARD, OUTPUT and POSTROUTING hooks\n");
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+ return 0;
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+ }
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+ if (IP6T_MATCH_ITERATE(e, find_syn_match))
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+ return 1;
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+ printk("xt_TCPMSS: Only works on TCP SYN packets\n");
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+ return 0;
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+}
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+#endif
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+
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+static struct xt_target xt_tcpmss_reg[] = {
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+ {
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+ .family = AF_INET,
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+ .name = "TCPMSS",
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+ .checkentry = xt_tcpmss_checkentry4,
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+ .target = xt_tcpmss_target4,
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+ .targetsize = sizeof(struct xt_tcpmss_info),
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+ .proto = IPPROTO_TCP,
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+ .me = THIS_MODULE,
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+ },
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+#if defined(CONFIG_IP6_NF_IPTABLES) || defined(CONFIG_IP6_NF_IPTABLES_MODULE)
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+ {
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+ .family = AF_INET6,
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+ .name = "TCPMSS",
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+ .checkentry = xt_tcpmss_checkentry6,
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+ .target = xt_tcpmss_target6,
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+ .targetsize = sizeof(struct xt_tcpmss_info),
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+ .proto = IPPROTO_TCP,
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+ .me = THIS_MODULE,
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+ },
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+#endif
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+};
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+
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+static int __init xt_tcpmss_init(void)
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+{
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+ return xt_register_targets(xt_tcpmss_reg, ARRAY_SIZE(xt_tcpmss_reg));
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+}
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+
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+static void __exit xt_tcpmss_fini(void)
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+{
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+ xt_unregister_targets(xt_tcpmss_reg, ARRAY_SIZE(xt_tcpmss_reg));
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+}
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+
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+module_init(xt_tcpmss_init);
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+module_exit(xt_tcpmss_fini);
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Patrick McHardy