| /* |
| * 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 = par->net; |
| unsigned int in_mtu = tcpmss_reverse_mtu(net, skb, family); |
| unsigned int min_mtu = min(dst_mtu(skb_dst(skb)), in_mtu); |
| |
| if (min_mtu <= minlen) { |
| net_err_ratelimited("unknown or invalid path-MTU (%u)\n", |
| min_mtu); |
| return -1; |
| } |
| newmss = min_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), |
| false); |
| 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), true); |
| 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), false); |
| |
| oldval = ((__be16 *)tcph)[6]; |
| tcph->doff += TCPOLEN_MSS/4; |
| inet_proto_csum_replace2(&tcph->check, skb, |
| oldval, ((__be16 *)tcph)[6], false); |
| 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, oldlen, newlen; |
| 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); |
| oldlen = ipv6h->payload_len; |
| newlen = htons(ntohs(oldlen) + ret); |
| if (skb->ip_summed == CHECKSUM_COMPLETE) |
| skb->csum = csum_add(csum_sub(skb->csum, oldlen), |
| newlen); |
| ipv6h->payload_len = newlen; |
| } |
| 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; |
| } |
| if (par->nft_compat) |
| return 0; |
| |
| 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; |
| } |
| if (par->nft_compat) |
| return 0; |
| |
| 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); |