syncookies.c
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上传日期:2013-04-10
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- /*
- * Syncookies implementation for the Linux kernel
- *
- * Copyright (C) 1997 Andi Kleen
- * Based on ideas by D.J.Bernstein and Eric Schenk.
- *
- * This program is free software; you can redistribute it and/or
- * modify it under the terms of the GNU General Public License
- * as published by the Free Software Foundation; either version
- * 2 of the License, or (at your option) any later version.
- *
- * $Id: syncookies.c,v 1.17 2001/10/26 14:55:41 davem Exp $
- *
- * Missing: IPv6 support.
- */
- #include <linux/tcp.h>
- #include <linux/slab.h>
- #include <linux/random.h>
- #include <net/tcp.h>
- extern int sysctl_tcp_syncookies;
- /*
- * This table has to be sorted and terminated with (__u16)-1.
- * XXX generate a better table.
- * Unresolved Issues: HIPPI with a 64k MSS is not well supported.
- */
- static __u16 const msstab[] = {
- 64 - 1,
- 256 - 1,
- 512 - 1,
- 536 - 1,
- 1024 - 1,
- 1440 - 1,
- 1460 - 1,
- 4312 - 1,
- (__u16)-1
- };
- /* The number doesn't include the -1 terminator */
- #define NUM_MSS (sizeof(msstab)/sizeof(msstab[0]) - 1)
- /*
- * Generate a syncookie. mssp points to the mss, which is returned
- * rounded down to the value encoded in the cookie.
- */
- __u32 cookie_v4_init_sequence(struct sock *sk, struct sk_buff *skb, __u16 *mssp)
- {
- int mssind;
- const __u16 mss = *mssp;
-
- sk->tp_pinfo.af_tcp.last_synq_overflow = jiffies;
- /* XXX sort msstab[] by probability? Binary search? */
- for (mssind = 0; mss > msstab[mssind + 1]; mssind++)
- ;
- *mssp = msstab[mssind] + 1;
- NET_INC_STATS_BH(SyncookiesSent);
- return secure_tcp_syn_cookie(skb->nh.iph->saddr, skb->nh.iph->daddr,
- skb->h.th->source, skb->h.th->dest,
- ntohl(skb->h.th->seq),
- jiffies / (HZ * 60), mssind);
- }
- /*
- * This (misnamed) value is the age of syncookie which is permitted.
- * Its ideal value should be dependent on TCP_TIMEOUT_INIT and
- * sysctl_tcp_retries1. It's a rather complicated formula (exponential
- * backoff) to compute at runtime so it's currently hardcoded here.
- */
- #define COUNTER_TRIES 4
- /*
- * Check if a ack sequence number is a valid syncookie.
- * Return the decoded mss if it is, or 0 if not.
- */
- static inline int cookie_check(struct sk_buff *skb, __u32 cookie)
- {
- __u32 seq;
- __u32 mssind;
- seq = ntohl(skb->h.th->seq)-1;
- mssind = check_tcp_syn_cookie(cookie,
- skb->nh.iph->saddr, skb->nh.iph->daddr,
- skb->h.th->source, skb->h.th->dest,
- seq, jiffies / (HZ * 60), COUNTER_TRIES);
- return mssind < NUM_MSS ? msstab[mssind] + 1 : 0;
- }
- extern struct or_calltable or_ipv4;
- static inline struct sock *get_cookie_sock(struct sock *sk, struct sk_buff *skb,
- struct open_request *req,
- struct dst_entry *dst)
- {
- struct tcp_opt *tp = &(sk->tp_pinfo.af_tcp);
- struct sock *child;
- child = tp->af_specific->syn_recv_sock(sk, skb, req, dst);
- if (child)
- tcp_acceptq_queue(sk, req, child);
- else
- tcp_openreq_free(req);
- return child;
- }
- struct sock *cookie_v4_check(struct sock *sk, struct sk_buff *skb,
- struct ip_options *opt)
- {
- __u32 cookie = ntohl(skb->h.th->ack_seq) - 1;
- struct sock *ret = sk;
- struct open_request *req;
- int mss;
- struct rtable *rt;
- __u8 rcv_wscale;
- if (!sysctl_tcp_syncookies || !skb->h.th->ack)
- goto out;
- if (time_after(jiffies, sk->tp_pinfo.af_tcp.last_synq_overflow + TCP_TIMEOUT_INIT) ||
- (mss = cookie_check(skb, cookie)) == 0) {
- NET_INC_STATS_BH(SyncookiesFailed);
- goto out;
- }
- NET_INC_STATS_BH(SyncookiesRecv);
- req = tcp_openreq_alloc();
- ret = NULL;
- if (!req)
- goto out;
- req->rcv_isn = htonl(skb->h.th->seq) - 1;
- req->snt_isn = cookie;
- req->mss = mss;
- req->rmt_port = skb->h.th->source;
- req->af.v4_req.loc_addr = skb->nh.iph->daddr;
- req->af.v4_req.rmt_addr = skb->nh.iph->saddr;
- req->class = &or_ipv4; /* for savety */
- req->af.v4_req.opt = NULL;
- /* We throwed the options of the initial SYN away, so we hope
- * the ACK carries the same options again (see RFC1122 4.2.3.8)
- */
- if (opt && opt->optlen) {
- int opt_size = sizeof(struct ip_options) + opt->optlen;
- req->af.v4_req.opt = kmalloc(opt_size, GFP_ATOMIC);
- if (req->af.v4_req.opt) {
- if (ip_options_echo(req->af.v4_req.opt, skb)) {
- kfree(req->af.v4_req.opt);
- req->af.v4_req.opt = NULL;
- }
- }
- }
- req->snd_wscale = req->rcv_wscale = req->tstamp_ok = 0;
- req->wscale_ok = req->sack_ok = 0;
- req->expires = 0UL;
- req->retrans = 0;
-
- /*
- * We need to lookup the route here to get at the correct
- * window size. We should better make sure that the window size
- * hasn't changed since we received the original syn, but I see
- * no easy way to do this.
- */
- if (ip_route_output(&rt,
- opt &&
- opt->srr ? opt->faddr : req->af.v4_req.rmt_addr,
- req->af.v4_req.loc_addr,
- RT_CONN_FLAGS(sk),
- 0)) {
- tcp_openreq_free(req);
- goto out;
- }
- /* Try to redo what tcp_v4_send_synack did. */
- req->window_clamp = rt->u.dst.window;
- tcp_select_initial_window(tcp_full_space(sk), req->mss,
- &req->rcv_wnd, &req->window_clamp,
- 0, &rcv_wscale);
- /* BTW win scale with syncookies is 0 by definition */
- req->rcv_wscale = rcv_wscale;
- ret = get_cookie_sock(sk, skb, req, &rt->u.dst);
- out: return ret;
- }