--- /dev/null
+/*
+ * This is a module which is used for setting the MSS option in TCP packets.
+ *
+ * Copyright (C) 2000 Marc Boucher <marc@mbsi.ca>
+ * Copyright (C) 2007 Patrick McHardy <kaber@trash.net>
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License version 2 as
+ * published by the Free Software Foundation.
+ */
+#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
+#include <linux/module.h>
+#include <linux/skbuff.h>
+#include <linux/ip.h>
+#include <linux/gfp.h>
+#include <linux/ipv6.h>
+#include <linux/tcp.h>
+#include <net/dst.h>
+#include <net/flow.h>
+#include <net/ipv6.h>
+#include <net/route.h>
+#include <net/tcp.h>
+
+#include <linux/netfilter_ipv4/ip_tables.h>
+#include <linux/netfilter_ipv6/ip6_tables.h>
+#include <linux/netfilter/x_tables.h>
+#include <linux/netfilter/xt_tcpudp.h>
+#include <linux/netfilter/xt_TCPMSS.h>
+
+MODULE_LICENSE("GPL");
+MODULE_AUTHOR("Marc Boucher <marc@mbsi.ca>");
+MODULE_DESCRIPTION("Xtables: TCP Maximum Segment Size (MSS) adjustment");
+MODULE_ALIAS("ipt_TCPMSS");
+MODULE_ALIAS("ip6t_TCPMSS");
+
+static inline unsigned int
+optlen(const u_int8_t *opt, unsigned int offset)
+{
+ /* Beware zero-length options: make finite progress */
+ if (opt[offset] <= TCPOPT_NOP || opt[offset+1] == 0)
+ return 1;
+ else
+ return opt[offset+1];
+}
+
+static u_int32_t tcpmss_reverse_mtu(struct net *net,
+ const struct sk_buff *skb,
+ unsigned int family)
+{
+ struct flowi fl;
+ const struct nf_afinfo *ai;
+ struct rtable *rt = NULL;
+ u_int32_t mtu = ~0U;
+
+ if (family == PF_INET) {
+ struct flowi4 *fl4 = &fl.u.ip4;
+ memset(fl4, 0, sizeof(*fl4));
+ fl4->daddr = ip_hdr(skb)->saddr;
+ } else {
+ struct flowi6 *fl6 = &fl.u.ip6;
+
+ memset(fl6, 0, sizeof(*fl6));
+ fl6->daddr = ipv6_hdr(skb)->saddr;
+ }
+ rcu_read_lock();
+ ai = nf_get_afinfo(family);
+ if (ai != NULL)
+ ai->route(net, (struct dst_entry **)&rt, &fl, false);
+ rcu_read_unlock();
+
+ if (rt != NULL) {
+ mtu = dst_mtu(&rt->dst);
+ dst_release(&rt->dst);
+ }
+ return mtu;
+}
+
+static int
+tcpmss_mangle_packet(struct sk_buff *skb,
+ const struct xt_action_param *par,
+ unsigned int family,
+ unsigned int tcphoff,
+ unsigned int minlen)
+{
+ const struct xt_tcpmss_info *info = par->targinfo;
+ struct tcphdr *tcph;
+ int len, tcp_hdrlen;
+ unsigned int i;
+ __be16 oldval;
+ u16 newmss;
+ u8 *opt;
+
+ /* This is a fragment, no TCP header is available */
+ if (par->fragoff != 0)
+ return 0;
+
+ if (!skb_make_writable(skb, skb->len))
+ return -1;
+
+ len = skb->len - tcphoff;
+ if (len < (int)sizeof(struct tcphdr))
+ return -1;
+
+ tcph = (struct tcphdr *)(skb_network_header(skb) + tcphoff);
+ tcp_hdrlen = tcph->doff * 4;
+
+ if (len < tcp_hdrlen)
+ return -1;
+
+ if (info->mss == XT_TCPMSS_CLAMP_PMTU) {
+ struct net *net = dev_net(par->in ? par->in : par->out);
+ unsigned int in_mtu = tcpmss_reverse_mtu(net, skb, family);
+
+ if (dst_mtu(skb_dst(skb)) <= minlen) {
+ net_err_ratelimited("unknown or invalid path-MTU (%u)\n",
+ dst_mtu(skb_dst(skb)));
+ return -1;
+ }
+ if (in_mtu <= minlen) {
+ net_err_ratelimited("unknown or invalid path-MTU (%u)\n",
+ in_mtu);
+ return -1;
+ }
+ newmss = min(dst_mtu(skb_dst(skb)), in_mtu) - minlen;
+ } else
+ newmss = info->mss;
+
+ opt = (u_int8_t *)tcph;
+ for (i = sizeof(struct tcphdr); i <= tcp_hdrlen - TCPOLEN_MSS; i += optlen(opt, i)) {
+ if (opt[i] == TCPOPT_MSS && opt[i+1] == TCPOLEN_MSS) {
+ u_int16_t oldmss;
+
+ oldmss = (opt[i+2] << 8) | opt[i+3];
+
+ /* Never increase MSS, even when setting it, as
+ * doing so results in problems for hosts that rely
+ * on MSS being set correctly.
+ */
+ if (oldmss <= newmss)
+ return 0;
+
+ opt[i+2] = (newmss & 0xff00) >> 8;
+ opt[i+3] = newmss & 0x00ff;
+
+ inet_proto_csum_replace2(&tcph->check, skb,
+ htons(oldmss), htons(newmss),
+ 0);
+ return 0;
+ }
+ }
+
+ /* There is data after the header so the option can't be added
+ * without moving it, and doing so may make the SYN packet
+ * itself too large. Accept the packet unmodified instead.
+ */
+ if (len > tcp_hdrlen)
+ return 0;
+
+ /*
+ * MSS Option not found ?! add it..
+ */
+ if (skb_tailroom(skb) < TCPOLEN_MSS) {
+ if (pskb_expand_head(skb, 0,
+ TCPOLEN_MSS - skb_tailroom(skb),
+ GFP_ATOMIC))
+ return -1;
+ tcph = (struct tcphdr *)(skb_network_header(skb) + tcphoff);
+ }
+
+ skb_put(skb, TCPOLEN_MSS);
+
+ /*
+ * IPv4: RFC 1122 states "If an MSS option is not received at
+ * connection setup, TCP MUST assume a default send MSS of 536".
+ * IPv6: RFC 2460 states IPv6 has a minimum MTU of 1280 and a minimum
+ * length IPv6 header of 60, ergo the default MSS value is 1220
+ * Since no MSS was provided, we must use the default values
+ */
+ if (par->family == NFPROTO_IPV4)
+ newmss = min(newmss, (u16)536);
+ else
+ newmss = min(newmss, (u16)1220);
+
+ opt = (u_int8_t *)tcph + sizeof(struct tcphdr);
+ memmove(opt + TCPOLEN_MSS, opt, len - sizeof(struct tcphdr));
+
+ inet_proto_csum_replace2(&tcph->check, skb,
+ htons(len), htons(len + TCPOLEN_MSS), 1);
+ opt[0] = TCPOPT_MSS;
+ opt[1] = TCPOLEN_MSS;
+ opt[2] = (newmss & 0xff00) >> 8;
+ opt[3] = newmss & 0x00ff;
+
+ inet_proto_csum_replace4(&tcph->check, skb, 0, *((__be32 *)opt), 0);
+
+ oldval = ((__be16 *)tcph)[6];
+ tcph->doff += TCPOLEN_MSS/4;
+ inet_proto_csum_replace2(&tcph->check, skb,
+ oldval, ((__be16 *)tcph)[6], 0);
+ return TCPOLEN_MSS;
+}
+
+static unsigned int
+tcpmss_tg4(struct sk_buff *skb, const struct xt_action_param *par)
+{
+ struct iphdr *iph = ip_hdr(skb);
+ __be16 newlen;
+ int ret;
+
+ ret = tcpmss_mangle_packet(skb, par,
+ PF_INET,
+ iph->ihl * 4,
+ sizeof(*iph) + sizeof(struct tcphdr));
+ if (ret < 0)
+ return NF_DROP;
+ if (ret > 0) {
+ iph = ip_hdr(skb);
+ newlen = htons(ntohs(iph->tot_len) + ret);
+ csum_replace2(&iph->check, iph->tot_len, newlen);
+ iph->tot_len = newlen;
+ }
+ return XT_CONTINUE;
+}
+
+#if IS_ENABLED(CONFIG_IP6_NF_IPTABLES)
+static unsigned int
+tcpmss_tg6(struct sk_buff *skb, const struct xt_action_param *par)
+{
+ struct ipv6hdr *ipv6h = ipv6_hdr(skb);
+ u8 nexthdr;
+ __be16 frag_off;
+ int tcphoff;
+ int ret;
+
+ nexthdr = ipv6h->nexthdr;
+ tcphoff = ipv6_skip_exthdr(skb, sizeof(*ipv6h), &nexthdr, &frag_off);
+ if (tcphoff < 0)
+ return NF_DROP;
+ ret = tcpmss_mangle_packet(skb, par,
+ PF_INET6,
+ tcphoff,
+ sizeof(*ipv6h) + sizeof(struct tcphdr));
+ if (ret < 0)
+ return NF_DROP;
+ if (ret > 0) {
+ ipv6h = ipv6_hdr(skb);
+ ipv6h->payload_len = htons(ntohs(ipv6h->payload_len) + ret);
+ }
+ return XT_CONTINUE;
+}
+#endif
+
+/* Must specify -p tcp --syn */
+static inline bool find_syn_match(const struct xt_entry_match *m)
+{
+ const struct xt_tcp *tcpinfo = (const struct xt_tcp *)m->data;
+
+ if (strcmp(m->u.kernel.match->name, "tcp") == 0 &&
+ tcpinfo->flg_cmp & TCPHDR_SYN &&
+ !(tcpinfo->invflags & XT_TCP_INV_FLAGS))
+ return true;
+
+ return false;
+}
+
+static int tcpmss_tg4_check(const struct xt_tgchk_param *par)
+{
+ const struct xt_tcpmss_info *info = par->targinfo;
+ const struct ipt_entry *e = par->entryinfo;
+ const struct xt_entry_match *ematch;
+
+ if (info->mss == XT_TCPMSS_CLAMP_PMTU &&
+ (par->hook_mask & ~((1 << NF_INET_FORWARD) |
+ (1 << NF_INET_LOCAL_OUT) |
+ (1 << NF_INET_POST_ROUTING))) != 0) {
+ pr_info("path-MTU clamping only supported in "
+ "FORWARD, OUTPUT and POSTROUTING hooks\n");
+ return -EINVAL;
+ }
+ xt_ematch_foreach(ematch, e)
+ if (find_syn_match(ematch))
+ return 0;
+ pr_info("Only works on TCP SYN packets\n");
+ return -EINVAL;
+}
+
+#if IS_ENABLED(CONFIG_IP6_NF_IPTABLES)
+static int tcpmss_tg6_check(const struct xt_tgchk_param *par)
+{
+ const struct xt_tcpmss_info *info = par->targinfo;
+ const struct ip6t_entry *e = par->entryinfo;
+ const struct xt_entry_match *ematch;
+
+ if (info->mss == XT_TCPMSS_CLAMP_PMTU &&
+ (par->hook_mask & ~((1 << NF_INET_FORWARD) |
+ (1 << NF_INET_LOCAL_OUT) |
+ (1 << NF_INET_POST_ROUTING))) != 0) {
+ pr_info("path-MTU clamping only supported in "
+ "FORWARD, OUTPUT and POSTROUTING hooks\n");
+ return -EINVAL;
+ }
+ xt_ematch_foreach(ematch, e)
+ if (find_syn_match(ematch))
+ return 0;
+ pr_info("Only works on TCP SYN packets\n");
+ return -EINVAL;
+}
+#endif
+
+static struct xt_target tcpmss_tg_reg[] __read_mostly = {
+ {
+ .family = NFPROTO_IPV4,
+ .name = "TCPMSS",
+ .checkentry = tcpmss_tg4_check,
+ .target = tcpmss_tg4,
+ .targetsize = sizeof(struct xt_tcpmss_info),
+ .proto = IPPROTO_TCP,
+ .me = THIS_MODULE,
+ },
+#if IS_ENABLED(CONFIG_IP6_NF_IPTABLES)
+ {
+ .family = NFPROTO_IPV6,
+ .name = "TCPMSS",
+ .checkentry = tcpmss_tg6_check,
+ .target = tcpmss_tg6,
+ .targetsize = sizeof(struct xt_tcpmss_info),
+ .proto = IPPROTO_TCP,
+ .me = THIS_MODULE,
+ },
+#endif
+};
+
+static int __init tcpmss_tg_init(void)
+{
+ return xt_register_targets(tcpmss_tg_reg, ARRAY_SIZE(tcpmss_tg_reg));
+}
+
+static void __exit tcpmss_tg_exit(void)
+{
+ xt_unregister_targets(tcpmss_tg_reg, ARRAY_SIZE(tcpmss_tg_reg));
+}
+
+module_init(tcpmss_tg_init);
+module_exit(tcpmss_tg_exit);
Code Review / kvmfornfv.git / blobdiff