// SPDX-License-Identifier: GPL-2.0-only
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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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* Copyright (C) 2007 Patrick McHardy <kaber@trash.net>
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*/
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#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
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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/gfp.h>
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#include <linux/ipv6.h>
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#include <linux/tcp.h>
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#include <net/dst.h>
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#include <net/flow.h>
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#include <net/ipv6.h>
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#include <net/route.h>
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#include <net/tcp.h>
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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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MODULE_LICENSE("GPL");
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MODULE_AUTHOR("Marc Boucher <marc@mbsi.ca>");
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MODULE_DESCRIPTION("Xtables: TCP Maximum Segment Size (MSS) adjustment");
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MODULE_ALIAS("ipt_TCPMSS");
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MODULE_ALIAS("ip6t_TCPMSS");
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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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static u_int32_t tcpmss_reverse_mtu(struct net *net,
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const struct sk_buff *skb,
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unsigned int family)
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{
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struct flowi fl;
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struct rtable *rt = NULL;
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u_int32_t mtu = ~0U;
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if (family == PF_INET) {
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struct flowi4 *fl4 = &fl.u.ip4;
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memset(fl4, 0, sizeof(*fl4));
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fl4->daddr = ip_hdr(skb)->saddr;
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} else {
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struct flowi6 *fl6 = &fl.u.ip6;
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memset(fl6, 0, sizeof(*fl6));
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fl6->daddr = ipv6_hdr(skb)->saddr;
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}
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nf_route(net, (struct dst_entry **)&rt, &fl, false, family);
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if (rt != NULL) {
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mtu = dst_mtu(&rt->dst);
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dst_release(&rt->dst);
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}
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return mtu;
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}
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static int
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tcpmss_mangle_packet(struct sk_buff *skb,
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const struct xt_action_param *par,
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unsigned int family,
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unsigned int tcphoff,
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unsigned int minlen)
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{
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const struct xt_tcpmss_info *info = par->targinfo;
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struct tcphdr *tcph;
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int len, tcp_hdrlen;
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unsigned int i;
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__be16 oldval;
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u16 newmss;
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u8 *opt;
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/* This is a fragment, no TCP header is available */
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if (par->fragoff != 0)
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return 0;
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if (skb_ensure_writable(skb, skb->len))
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return -1;
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len = skb->len - tcphoff;
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if (len < (int)sizeof(struct tcphdr))
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return -1;
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tcph = (struct tcphdr *)(skb_network_header(skb) + tcphoff);
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tcp_hdrlen = tcph->doff * 4;
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if (len < tcp_hdrlen || tcp_hdrlen < sizeof(struct tcphdr))
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return -1;
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if (info->mss == XT_TCPMSS_CLAMP_PMTU) {
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struct net *net = xt_net(par);
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unsigned int in_mtu = tcpmss_reverse_mtu(net, skb, family);
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unsigned int min_mtu = min(dst_mtu(skb_dst(skb)), in_mtu);
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if (min_mtu <= minlen) {
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net_err_ratelimited("unknown or invalid path-MTU (%u)\n",
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min_mtu);
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return -1;
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}
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newmss = min_mtu - minlen;
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} else
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newmss = info->mss;
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opt = (u_int8_t *)tcph;
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for (i = sizeof(struct tcphdr); i <= tcp_hdrlen - TCPOLEN_MSS; i += optlen(opt, i)) {
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if (opt[i] == TCPOPT_MSS && opt[i+1] == TCPOLEN_MSS) {
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u_int16_t oldmss;
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oldmss = (opt[i+2] << 8) | opt[i+3];
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/* Never increase MSS, even when setting it, as
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* doing so results in problems for hosts that rely
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* on MSS being set correctly.
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*/
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if (oldmss <= newmss)
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return 0;
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opt[i+2] = (newmss & 0xff00) >> 8;
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opt[i+3] = newmss & 0x00ff;
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inet_proto_csum_replace2(&tcph->check, skb,
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htons(oldmss), htons(newmss),
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false);
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return 0;
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}
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}
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/* There is data after the header so the option can't be added
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* without moving it, and doing so may make the SYN packet
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* itself too large. Accept the packet unmodified instead.
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*/
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if (len > tcp_hdrlen)
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return 0;
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/* tcph->doff has 4 bits, do not wrap it to 0 */
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if (tcp_hdrlen >= 15 * 4)
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return 0;
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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(skb) < TCPOLEN_MSS) {
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if (pskb_expand_head(skb, 0,
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TCPOLEN_MSS - skb_tailroom(skb),
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GFP_ATOMIC))
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return -1;
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tcph = (struct tcphdr *)(skb_network_header(skb) + tcphoff);
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}
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skb_put(skb, TCPOLEN_MSS);
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/*
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* IPv4: RFC 1122 states "If an MSS option is not received at
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* connection setup, TCP MUST assume a default send MSS of 536".
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* IPv6: RFC 2460 states IPv6 has a minimum MTU of 1280 and a minimum
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* length IPv6 header of 60, ergo the default MSS value is 1220
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* Since no MSS was provided, we must use the default values
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*/
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if (xt_family(par) == NFPROTO_IPV4)
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newmss = min(newmss, (u16)536);
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else
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newmss = min(newmss, (u16)1220);
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opt = (u_int8_t *)tcph + sizeof(struct tcphdr);
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memmove(opt + TCPOLEN_MSS, opt, len - sizeof(struct tcphdr));
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inet_proto_csum_replace2(&tcph->check, skb,
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htons(len), htons(len + TCPOLEN_MSS), true);
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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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inet_proto_csum_replace4(&tcph->check, skb, 0, *((__be32 *)opt), false);
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oldval = ((__be16 *)tcph)[6];
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tcph->doff += TCPOLEN_MSS/4;
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inet_proto_csum_replace2(&tcph->check, skb,
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oldval, ((__be16 *)tcph)[6], false);
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return TCPOLEN_MSS;
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}
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static unsigned int
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tcpmss_tg4(struct sk_buff *skb, const struct xt_action_param *par)
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{
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struct iphdr *iph = ip_hdr(skb);
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__be16 newlen;
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int ret;
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ret = tcpmss_mangle_packet(skb, par,
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PF_INET,
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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 = ip_hdr(skb);
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newlen = htons(ntohs(iph->tot_len) + ret);
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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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#if IS_ENABLED(CONFIG_IP6_NF_IPTABLES)
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static unsigned int
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tcpmss_tg6(struct sk_buff *skb, const struct xt_action_param *par)
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{
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struct ipv6hdr *ipv6h = ipv6_hdr(skb);
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u8 nexthdr;
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__be16 frag_off, oldlen, newlen;
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int tcphoff;
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int ret;
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nexthdr = ipv6h->nexthdr;
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tcphoff = ipv6_skip_exthdr(skb, sizeof(*ipv6h), &nexthdr, &frag_off);
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if (tcphoff < 0)
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return NF_DROP;
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ret = tcpmss_mangle_packet(skb, par,
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PF_INET6,
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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 = ipv6_hdr(skb);
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oldlen = ipv6h->payload_len;
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newlen = htons(ntohs(oldlen) + ret);
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if (skb->ip_summed == CHECKSUM_COMPLETE)
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skb->csum = csum_add(csum_sub(skb->csum, oldlen),
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newlen);
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ipv6h->payload_len = newlen;
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}
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return XT_CONTINUE;
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}
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#endif
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/* Must specify -p tcp --syn */
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static inline bool 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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if (strcmp(m->u.kernel.match->name, "tcp") == 0 &&
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tcpinfo->flg_cmp & TCPHDR_SYN &&
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!(tcpinfo->invflags & XT_TCP_INV_FLAGS))
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return true;
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return false;
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}
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static int tcpmss_tg4_check(const struct xt_tgchk_param *par)
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{
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const struct xt_tcpmss_info *info = par->targinfo;
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const struct ipt_entry *e = par->entryinfo;
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const struct xt_entry_match *ematch;
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if (info->mss == XT_TCPMSS_CLAMP_PMTU &&
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(par->hook_mask & ~((1 << NF_INET_FORWARD) |
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(1 << NF_INET_LOCAL_OUT) |
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(1 << NF_INET_POST_ROUTING))) != 0) {
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pr_info_ratelimited("path-MTU clamping only supported in FORWARD, OUTPUT and POSTROUTING hooks\n");
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return -EINVAL;
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}
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if (par->nft_compat)
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return 0;
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xt_ematch_foreach(ematch, e)
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if (find_syn_match(ematch))
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return 0;
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pr_info_ratelimited("Only works on TCP SYN packets\n");
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return -EINVAL;
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}
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#if IS_ENABLED(CONFIG_IP6_NF_IPTABLES)
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static int tcpmss_tg6_check(const struct xt_tgchk_param *par)
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{
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const struct xt_tcpmss_info *info = par->targinfo;
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const struct ip6t_entry *e = par->entryinfo;
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const struct xt_entry_match *ematch;
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if (info->mss == XT_TCPMSS_CLAMP_PMTU &&
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(par->hook_mask & ~((1 << NF_INET_FORWARD) |
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(1 << NF_INET_LOCAL_OUT) |
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(1 << NF_INET_POST_ROUTING))) != 0) {
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pr_info_ratelimited("path-MTU clamping only supported in FORWARD, OUTPUT and POSTROUTING hooks\n");
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return -EINVAL;
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}
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if (par->nft_compat)
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return 0;
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xt_ematch_foreach(ematch, e)
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if (find_syn_match(ematch))
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return 0;
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pr_info_ratelimited("Only works on TCP SYN packets\n");
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return -EINVAL;
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}
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#endif
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static struct xt_target tcpmss_tg_reg[] __read_mostly = {
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{
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.family = NFPROTO_IPV4,
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.name = "TCPMSS",
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.checkentry = tcpmss_tg4_check,
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.target = tcpmss_tg4,
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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 IS_ENABLED(CONFIG_IP6_NF_IPTABLES)
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{
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.family = NFPROTO_IPV6,
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.name = "TCPMSS",
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.checkentry = tcpmss_tg6_check,
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.target = tcpmss_tg6,
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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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static int __init tcpmss_tg_init(void)
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{
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return xt_register_targets(tcpmss_tg_reg, ARRAY_SIZE(tcpmss_tg_reg));
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}
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static void __exit tcpmss_tg_exit(void)
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{
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xt_unregister_targets(tcpmss_tg_reg, ARRAY_SIZE(tcpmss_tg_reg));
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}
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module_init(tcpmss_tg_init);
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module_exit(tcpmss_tg_exit);
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