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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/config.h>
#if defined(CONFIG_SYN_COOKIES)
#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;
}
#endif
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