diff options
Diffstat (limited to 'net/netfilter/xt_TCPMSS.c')
-rw-r--r-- | net/netfilter/xt_TCPMSS.c | 344 |
1 files changed, 344 insertions, 0 deletions
diff --git a/net/netfilter/xt_TCPMSS.c b/net/netfilter/xt_TCPMSS.c new file mode 100644 index 000000000..e762de5ee --- /dev/null +++ b/net/netfilter/xt_TCPMSS.c @@ -0,0 +1,344 @@ +/* + * 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); |