netfilter: add IPv6 SYNPROXY target

Add an IPv6 version of the SYNPROXY target. The main differences to the
IPv4 version is routing and IP header construction.

Signed-off-by: Patrick McHardy <kaber@trash.net>
Tested-by: Martin Topholm <mph@one.com>
Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

3 files changed, 509 insertions, 0 deletions

diff --git a/net/ipv6/netfilter/Kconfig b/net/ipv6/netfilter/Kconfig
index 4433ab40e7de..a7f842b29b67 100644
--- a/net/ipv6/netfilter/Kconfig
+++ b/net/ipv6/netfilter/Kconfig
@@ -153,6 +153,19 @@ config IP6_NF_TARGET_REJECT
 
           To compile it as a module, choose M here.  If unsure, say N.
 
+config IP6_NF_TARGET_SYNPROXY
+        tristate "SYNPROXY target support"
+        depends on NF_CONNTRACK && NETFILTER_ADVANCED
+        select NETFILTER_SYNPROXY
+        select SYN_COOKIES
+        help
+          The SYNPROXY target allows you to intercept TCP connections and
+          establish them using syncookies before they are passed on to the
+          server. This allows to avoid conntrack and server resource usage
+          during SYN-flood attacks.
+
+          To compile it as a module, choose M here. If unsure, say N.
+
 config IP6_NF_MANGLE
         tristate "Packet mangling"
         default m if NETFILTER_ADVANCED=n
diff --git a/net/ipv6/netfilter/Makefile b/net/ipv6/netfilter/Makefile
index ad13bf0d25f0..2b53738f798c 100644
--- a/net/ipv6/netfilter/Makefile
+++ b/net/ipv6/netfilter/Makefile
@@ -37,3 +37,4 @@ obj-$(CONFIG_IP6_NF_MATCH_RT) += ip6t_rt.o
 obj-$(CONFIG_IP6_NF_TARGET_MASQUERADE) += ip6t_MASQUERADE.o
 obj-$(CONFIG_IP6_NF_TARGET_NPT) += ip6t_NPT.o
 obj-$(CONFIG_IP6_NF_TARGET_REJECT) += ip6t_REJECT.o
+obj-$(CONFIG_IP6_NF_TARGET_SYNPROXY) += ip6t_SYNPROXY.o
diff --git a/net/ipv6/netfilter/ip6t_SYNPROXY.c b/net/ipv6/netfilter/ip6t_SYNPROXY.c
new file mode 100644
index 000000000000..4270a9b145e5
--- /dev/null
+++ b/net/ipv6/netfilter/ip6t_SYNPROXY.c
@@ -0,0 +1,495 @@
+/*
+ * Copyright (c) 2013 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.
+ */
+
+#include <linux/module.h>
+#include <linux/skbuff.h>
+#include <net/ip6_checksum.h>
+#include <net/ip6_route.h>
+#include <net/tcp.h>
+
+#include <linux/netfilter_ipv6/ip6_tables.h>
+#include <linux/netfilter/x_tables.h>
+#include <linux/netfilter/xt_SYNPROXY.h>
+#include <net/netfilter/nf_conntrack.h>
+#include <net/netfilter/nf_conntrack_seqadj.h>
+#include <net/netfilter/nf_conntrack_synproxy.h>
+
+static struct ipv6hdr *
+synproxy_build_ip(struct sk_buff *skb, const struct in6_addr *saddr,
+                                       const struct in6_addr *daddr)
+{
+        struct ipv6hdr *iph;
+
+        skb_reset_network_header(skb);
+        iph = (struct ipv6hdr *)skb_put(skb, sizeof(*iph));
+        ip6_flow_hdr(iph, 0, 0);
+        iph->hop_limit  = 64;   //XXX
+        iph->nexthdr    = IPPROTO_TCP;
+        iph->saddr      = *saddr;
+        iph->daddr      = *daddr;
+
+        return iph;
+}
+
+static void
+synproxy_send_tcp(const struct sk_buff *skb, struct sk_buff *nskb,
+                  struct nf_conntrack *nfct, enum ip_conntrack_info ctinfo,
+                  struct ipv6hdr *niph, struct tcphdr *nth,
+                  unsigned int tcp_hdr_size)
+{
+        struct net *net = nf_ct_net((struct nf_conn *)nfct);
+        struct dst_entry *dst;
+        struct flowi6 fl6;
+
+        nth->check = ~tcp_v6_check(tcp_hdr_size, &niph->saddr, &niph->daddr, 0);
+        nskb->ip_summed   = CHECKSUM_PARTIAL;
+        nskb->csum_start  = (unsigned char *)nth - nskb->head;
+        nskb->csum_offset = offsetof(struct tcphdr, check);
+
+        memset(&fl6, 0, sizeof(fl6));
+        fl6.flowi6_proto = IPPROTO_TCP;
+        fl6.saddr = niph->saddr;
+        fl6.daddr = niph->daddr;
+        fl6.fl6_sport = nth->source;
+        fl6.fl6_dport = nth->dest;
+        security_skb_classify_flow((struct sk_buff *)skb, flowi6_to_flowi(&fl6));
+        dst = ip6_route_output(net, NULL, &fl6);
+        if (dst == NULL || dst->error) {
+                dst_release(dst);
+                goto free_nskb;
+        }
+        dst = xfrm_lookup(net, dst, flowi6_to_flowi(&fl6), NULL, 0);
+        if (IS_ERR(dst))
+                goto free_nskb;
+
+        skb_dst_set(nskb, dst);
+
+        if (nfct) {
+                nskb->nfct = nfct;
+                nskb->nfctinfo = ctinfo;
+                nf_conntrack_get(nfct);
+        }
+
+        ip6_local_out(nskb);
+        return;
+
+free_nskb:
+        kfree_skb(nskb);
+}
+
+static void
+synproxy_send_client_synack(const struct sk_buff *skb, const struct tcphdr *th,
+                            const struct synproxy_options *opts)
+{
+        struct sk_buff *nskb;
+        struct ipv6hdr *iph, *niph;
+        struct tcphdr *nth;
+        unsigned int tcp_hdr_size;
+        u16 mss = opts->mss;
+
+        iph = ipv6_hdr(skb);
+
+        tcp_hdr_size = sizeof(*nth) + synproxy_options_size(opts);
+        nskb = alloc_skb(sizeof(*niph) + tcp_hdr_size + MAX_TCP_HEADER,
+                         GFP_ATOMIC);
+        if (nskb == NULL)
+                return;
+        skb_reserve(nskb, MAX_TCP_HEADER);
+
+        niph = synproxy_build_ip(nskb, &iph->daddr, &iph->saddr);
+
+        skb_reset_transport_header(nskb);
+        nth = (struct tcphdr *)skb_put(nskb, tcp_hdr_size);
+        nth->source     = th->dest;
+        nth->dest       = th->source;
+        nth->seq        = htonl(__cookie_v6_init_sequence(iph, th, &mss));
+        nth->ack_seq    = htonl(ntohl(th->seq) + 1);
+        tcp_flag_word(nth) = TCP_FLAG_SYN | TCP_FLAG_ACK;
+        if (opts->options & XT_SYNPROXY_OPT_ECN)
+                tcp_flag_word(nth) |= TCP_FLAG_ECE;
+        nth->doff       = tcp_hdr_size / 4;
+        nth->window     = 0;
+        nth->check      = 0;
+        nth->urg_ptr    = 0;
+
+        synproxy_build_options(nth, opts);
+
+        synproxy_send_tcp(skb, nskb, skb->nfct, IP_CT_ESTABLISHED_REPLY,
+                          niph, nth, tcp_hdr_size);
+}
+
+static void
+synproxy_send_server_syn(const struct synproxy_net *snet,
+                         const struct sk_buff *skb, const struct tcphdr *th,
+                         const struct synproxy_options *opts, u32 recv_seq)
+{
+        struct sk_buff *nskb;
+        struct ipv6hdr *iph, *niph;
+        struct tcphdr *nth;
+        unsigned int tcp_hdr_size;
+
+        iph = ipv6_hdr(skb);
+
+        tcp_hdr_size = sizeof(*nth) + synproxy_options_size(opts);
+        nskb = alloc_skb(sizeof(*niph) + tcp_hdr_size + MAX_TCP_HEADER,
+                         GFP_ATOMIC);
+        if (nskb == NULL)
+                return;
+        skb_reserve(nskb, MAX_TCP_HEADER);
+
+        niph = synproxy_build_ip(nskb, &iph->saddr, &iph->daddr);
+
+        skb_reset_transport_header(nskb);
+        nth = (struct tcphdr *)skb_put(nskb, tcp_hdr_size);
+        nth->source     = th->source;
+        nth->dest       = th->dest;
+        nth->seq        = htonl(recv_seq - 1);
+        /* ack_seq is used to relay our ISN to the synproxy hook to initialize
+         * sequence number translation once a connection tracking entry exists.
+         */
+        nth->ack_seq    = htonl(ntohl(th->ack_seq) - 1);
+        tcp_flag_word(nth) = TCP_FLAG_SYN;
+        if (opts->options & XT_SYNPROXY_OPT_ECN)
+                tcp_flag_word(nth) |= TCP_FLAG_ECE | TCP_FLAG_CWR;
+        nth->doff       = tcp_hdr_size / 4;
+        nth->window     = th->window;
+        nth->check      = 0;
+        nth->urg_ptr    = 0;
+
+        synproxy_build_options(nth, opts);
+
+        synproxy_send_tcp(skb, nskb, &snet->tmpl->ct_general, IP_CT_NEW,
+                          niph, nth, tcp_hdr_size);
+}
+
+static void
+synproxy_send_server_ack(const struct synproxy_net *snet,
+                         const struct ip_ct_tcp *state,
+                         const struct sk_buff *skb, const struct tcphdr *th,
+                         const struct synproxy_options *opts)
+{
+        struct sk_buff *nskb;
+        struct ipv6hdr *iph, *niph;
+        struct tcphdr *nth;
+        unsigned int tcp_hdr_size;
+
+        iph = ipv6_hdr(skb);
+
+        tcp_hdr_size = sizeof(*nth) + synproxy_options_size(opts);
+        nskb = alloc_skb(sizeof(*niph) + tcp_hdr_size + MAX_TCP_HEADER,
+                         GFP_ATOMIC);
+        if (nskb == NULL)
+                return;
+        skb_reserve(nskb, MAX_TCP_HEADER);
+
+        niph = synproxy_build_ip(nskb, &iph->daddr, &iph->saddr);
+
+        skb_reset_transport_header(nskb);
+        nth = (struct tcphdr *)skb_put(nskb, tcp_hdr_size);
+        nth->source     = th->dest;
+        nth->dest       = th->source;
+        nth->seq        = htonl(ntohl(th->ack_seq));
+        nth->ack_seq    = htonl(ntohl(th->seq) + 1);
+        tcp_flag_word(nth) = TCP_FLAG_ACK;
+        nth->doff       = tcp_hdr_size / 4;
+        nth->window     = htons(state->seen[IP_CT_DIR_ORIGINAL].td_maxwin);
+        nth->check      = 0;
+        nth->urg_ptr    = 0;
+
+        synproxy_build_options(nth, opts);
+
+        synproxy_send_tcp(skb, nskb, NULL, 0, niph, nth, tcp_hdr_size);
+}
+
+static void
+synproxy_send_client_ack(const struct synproxy_net *snet,
+                         const struct sk_buff *skb, const struct tcphdr *th,
+                         const struct synproxy_options *opts)
+{
+        struct sk_buff *nskb;
+        struct ipv6hdr *iph, *niph;
+        struct tcphdr *nth;
+        unsigned int tcp_hdr_size;
+
+        iph = ipv6_hdr(skb);
+
+        tcp_hdr_size = sizeof(*nth) + synproxy_options_size(opts);
+        nskb = alloc_skb(sizeof(*niph) + tcp_hdr_size + MAX_TCP_HEADER,
+                         GFP_ATOMIC);
+        if (nskb == NULL)
+                return;
+        skb_reserve(nskb, MAX_TCP_HEADER);
+
+        niph = synproxy_build_ip(nskb, &iph->saddr, &iph->daddr);
+
+        skb_reset_transport_header(nskb);
+        nth = (struct tcphdr *)skb_put(nskb, tcp_hdr_size);
+        nth->source     = th->source;
+        nth->dest       = th->dest;
+        nth->seq        = htonl(ntohl(th->seq) + 1);
+        nth->ack_seq    = th->ack_seq;
+        tcp_flag_word(nth) = TCP_FLAG_ACK;
+        nth->doff       = tcp_hdr_size / 4;
+        nth->window     = ntohs(htons(th->window) >> opts->wscale);
+        nth->check      = 0;
+        nth->urg_ptr    = 0;
+
+        synproxy_build_options(nth, opts);
+
+        synproxy_send_tcp(skb, nskb, NULL, 0, niph, nth, tcp_hdr_size);
+}
+
+static bool
+synproxy_recv_client_ack(const struct synproxy_net *snet,
+                         const struct sk_buff *skb, const struct tcphdr *th,
+                         struct synproxy_options *opts, u32 recv_seq)
+{
+        int mss;
+
+        mss = __cookie_v6_check(ipv6_hdr(skb), th, ntohl(th->ack_seq) - 1);
+        if (mss == 0) {
+                this_cpu_inc(snet->stats->cookie_invalid);
+                return false;
+        }
+
+        this_cpu_inc(snet->stats->cookie_valid);
+        opts->mss = mss;
+
+        if (opts->options & XT_SYNPROXY_OPT_TIMESTAMP)
+                synproxy_check_timestamp_cookie(opts);
+
+        synproxy_send_server_syn(snet, skb, th, opts, recv_seq);
+        return true;
+}
+
+static unsigned int
+synproxy_tg6(struct sk_buff *skb, const struct xt_action_param *par)
+{
+        const struct xt_synproxy_info *info = par->targinfo;
+        struct synproxy_net *snet = synproxy_pernet(dev_net(par->in));
+        struct synproxy_options opts = {};
+        struct tcphdr *th, _th;
+
+        if (nf_ip6_checksum(skb, par->hooknum, par->thoff, IPPROTO_TCP))
+                return NF_DROP;
+
+        th = skb_header_pointer(skb, par->thoff, sizeof(_th), &_th);
+        if (th == NULL)
+                return NF_DROP;
+
+        synproxy_parse_options(skb, par->thoff, th, &opts);
+
+        if (th->syn) {
+                /* Initial SYN from client */
+                this_cpu_inc(snet->stats->syn_received);
+
+                if (th->ece && th->cwr)
+                        opts.options |= XT_SYNPROXY_OPT_ECN;
+
+                opts.options &= info->options;
+                if (opts.options & XT_SYNPROXY_OPT_TIMESTAMP)
+                        synproxy_init_timestamp_cookie(info, &opts);
+                else
+                        opts.options &= ~(XT_SYNPROXY_OPT_WSCALE |
+                                          XT_SYNPROXY_OPT_SACK_PERM |
+                                          XT_SYNPROXY_OPT_ECN);
+
+                synproxy_send_client_synack(skb, th, &opts);
+        } else if (th->ack && !(th->fin || th->rst))
+                /* ACK from client */
+                synproxy_recv_client_ack(snet, skb, th, &opts, ntohl(th->seq));
+
+        return NF_DROP;
+}
+
+static unsigned int ipv6_synproxy_hook(unsigned int hooknum,
+                                       struct sk_buff *skb,
+                                       const struct net_device *in,
+                                       const struct net_device *out,
+                                       int (*okfn)(struct sk_buff *))
+{
+        struct synproxy_net *snet = synproxy_pernet(dev_net(in ? : out));
+        enum ip_conntrack_info ctinfo;
+        struct nf_conn *ct;
+        struct nf_conn_synproxy *synproxy;
+        struct synproxy_options opts = {};
+        const struct ip_ct_tcp *state;
+        struct tcphdr *th, _th;
+        __be16 frag_off;
+        u8 nexthdr;
+        int thoff;
+
+        ct = nf_ct_get(skb, &ctinfo);
+        if (ct == NULL)
+                return NF_ACCEPT;
+
+        synproxy = nfct_synproxy(ct);
+        if (synproxy == NULL)
+                return NF_ACCEPT;
+
+        if (nf_is_loopback_packet(skb))
+                return NF_ACCEPT;
+
+        nexthdr = ipv6_hdr(skb)->nexthdr;
+        thoff = ipv6_skip_exthdr(skb, sizeof(struct ipv6hdr), &nexthdr,
+                                 &frag_off);
+        if (thoff < 0)
+                return NF_ACCEPT;
+
+        th = skb_header_pointer(skb, thoff, sizeof(_th), &_th);
+        if (th == NULL)
+                return NF_DROP;
+
+        state = &ct->proto.tcp;
+        switch (state->state) {
+        case TCP_CONNTRACK_CLOSE:
+                if (th->rst && !test_bit(IPS_SEEN_REPLY_BIT, &ct->status)) {
+                        nf_ct_seqadj_init(ct, ctinfo, synproxy->isn -
+                                                      ntohl(th->seq) + 1);
+                        break;
+                }
+
+                if (!th->syn || th->ack ||
+                    CTINFO2DIR(ctinfo) != IP_CT_DIR_ORIGINAL)
+                        break;
+
+                /* Reopened connection - reset the sequence number and timestamp
+                 * adjustments, they will get initialized once the connection is
+                 * reestablished.
+                 */
+                nf_ct_seqadj_init(ct, ctinfo, 0);
+                synproxy->tsoff = 0;
+                this_cpu_inc(snet->stats->conn_reopened);
+
+                /* fall through */
+        case TCP_CONNTRACK_SYN_SENT:
+                synproxy_parse_options(skb, thoff, th, &opts);
+
+                if (!th->syn && th->ack &&
+                    CTINFO2DIR(ctinfo) == IP_CT_DIR_ORIGINAL) {
+                        /* Keep-Alives are sent with SEG.SEQ = SND.NXT-1,
+                         * therefore we need to add 1 to make the SYN sequence
+                         * number match the one of first SYN.
+                         */
+                        if (synproxy_recv_client_ack(snet, skb, th, &opts,
+                                                     ntohl(th->seq) + 1))
+                                this_cpu_inc(snet->stats->cookie_retrans);
+
+                        return NF_DROP;
+                }
+
+                synproxy->isn = ntohl(th->ack_seq);
+                if (opts.options & XT_SYNPROXY_OPT_TIMESTAMP)
+                        synproxy->its = opts.tsecr;
+                break;
+        case TCP_CONNTRACK_SYN_RECV:
+                if (!th->syn || !th->ack)
+                        break;
+
+                synproxy_parse_options(skb, thoff, th, &opts);
+                if (opts.options & XT_SYNPROXY_OPT_TIMESTAMP)
+                        synproxy->tsoff = opts.tsval - synproxy->its;
+
+                opts.options &= ~(XT_SYNPROXY_OPT_MSS |
+                                  XT_SYNPROXY_OPT_WSCALE |
+                                  XT_SYNPROXY_OPT_SACK_PERM);
+
+                swap(opts.tsval, opts.tsecr);
+                synproxy_send_server_ack(snet, state, skb, th, &opts);
+
+                nf_ct_seqadj_init(ct, ctinfo, synproxy->isn - ntohl(th->seq));
+
+                swap(opts.tsval, opts.tsecr);
+                synproxy_send_client_ack(snet, skb, th, &opts);
+
+                consume_skb(skb);
+                return NF_STOLEN;
+        default:
+                break;
+        }
+
+        synproxy_tstamp_adjust(skb, thoff, th, ct, ctinfo, synproxy);
+        return NF_ACCEPT;
+}
+
+static int synproxy_tg6_check(const struct xt_tgchk_param *par)
+{
+        const struct ip6t_entry *e = par->entryinfo;
+
+        if (!(e->ipv6.flags & IP6T_F_PROTO) ||
+            e->ipv6.proto != IPPROTO_TCP ||
+            e->ipv6.invflags & XT_INV_PROTO)
+                return -EINVAL;
+
+        return nf_ct_l3proto_try_module_get(par->family);
+}
+
+static void synproxy_tg6_destroy(const struct xt_tgdtor_param *par)
+{
+        nf_ct_l3proto_module_put(par->family);
+}
+
+static struct xt_target synproxy_tg6_reg __read_mostly = {
+        .name           = "SYNPROXY",
+        .family         = NFPROTO_IPV6,
+        .target         = synproxy_tg6,
+        .targetsize     = sizeof(struct xt_synproxy_info),
+        .checkentry     = synproxy_tg6_check,
+        .destroy        = synproxy_tg6_destroy,
+        .me             = THIS_MODULE,
+};
+
+static struct nf_hook_ops ipv6_synproxy_ops[] __read_mostly = {
+        {
+                .hook           = ipv6_synproxy_hook,
+                .owner          = THIS_MODULE,
+                .pf             = NFPROTO_IPV6,
+                .hooknum        = NF_INET_LOCAL_IN,
+                .priority       = NF_IP_PRI_CONNTRACK_CONFIRM - 1,
+        },
+        {
+                .hook           = ipv6_synproxy_hook,
+                .owner          = THIS_MODULE,
+                .pf             = NFPROTO_IPV6,
+                .hooknum        = NF_INET_POST_ROUTING,
+                .priority       = NF_IP_PRI_CONNTRACK_CONFIRM - 1,
+        },
+};
+
+static int __init synproxy_tg6_init(void)
+{
+        int err;
+
+        err = nf_register_hooks(ipv6_synproxy_ops,
+                                ARRAY_SIZE(ipv6_synproxy_ops));
+        if (err < 0)
+                goto err1;
+
+        err = xt_register_target(&synproxy_tg6_reg);
+        if (err < 0)
+                goto err2;
+
+        return 0;
+
+err2:
+        nf_unregister_hooks(ipv6_synproxy_ops, ARRAY_SIZE(ipv6_synproxy_ops));
+err1:
+        return err;
+}
+
+static void __exit synproxy_tg6_exit(void)
+{
+        xt_unregister_target(&synproxy_tg6_reg);
+        nf_unregister_hooks(ipv6_synproxy_ops, ARRAY_SIZE(ipv6_synproxy_ops));
+}
+
+module_init(synproxy_tg6_init);
+module_exit(synproxy_tg6_exit);
+
+MODULE_LICENSE("GPL");
+MODULE_AUTHOR("Patrick McHardy <kaber@trash.net>");