![Florian Westphal](/assets/img/avatar_default.png)
This patch allows to set ECN on a per-route basis in case the sysctl
tcp_ecn is not set to 1. In other words, when ECN is set for specific
routes, it provides a tcp_ecn=1 behaviour for that route while the rest
of the stack acts according to the global settings.
One can use 'ip route change dev $dev $net features ecn' to toggle this.
Having a more fine-grained per-route setting can be beneficial for various
reasons, for example, 1) within data centers, or 2) local ISPs may deploy
ECN support for their own video/streaming services [1], etc.
There was a recent measurement study/paper [2] which scanned the Alexa's
publicly available top million websites list from a vantage point in US,
Europe and Asia:
Half of the Alexa list will now happily use ECN (tcp_ecn=2, most likely
blamed to commit 255cac91c3
("tcp: extend ECN sysctl to allow server-side
only ECN") ;)); the break in connectivity on-path was found is about
1 in 10,000 cases. Timeouts rather than receiving back RSTs were much
more common in the negotiation phase (and mostly seen in the Alexa
middle band, ranks around 50k-150k): from 12-thousand hosts on which
there _may_ be ECN-linked connection failures, only 79 failed with RST
when _not_ failing with RST when ECN is not requested.
It's unclear though, how much equipment in the wild actually marks CE
when buffers start to fill up.
We thought about a fallback to non-ECN for retransmitted SYNs as another
global option (which could perhaps one day be made default), but as Eric
points out, there's much more work needed to detect broken middleboxes.
Two examples Eric mentioned are buggy firewalls that accept only a single
SYN per flow, and middleboxes that successfully let an ECN flow establish,
but later mark CE for all packets (so cwnd converges to 1).
[1] http://www.ietf.org/proceedings/89/slides/slides-89-tsvarea-1.pdf, p.15
[2] http://ecn.ethz.ch/
Joint work with Daniel Borkmann.
Reference: http://thread.gmane.org/gmane.linux.network/335797
Suggested-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Acked-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Daniel Borkmann <dborkman@redhat.com>
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: David S. Miller <davem@davemloft.net>
400 lines
11 KiB
C
400 lines
11 KiB
C
/*
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* Syncookies implementation for the Linux kernel
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*
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* Copyright (C) 1997 Andi Kleen
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* Based on ideas by D.J.Bernstein and Eric Schenk.
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*
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* This program is free software; you can redistribute it and/or
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* modify it under the terms of the GNU General Public License
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* as published by the Free Software Foundation; either version
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* 2 of the License, or (at your option) any later version.
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*/
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#include <linux/tcp.h>
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#include <linux/slab.h>
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#include <linux/random.h>
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#include <linux/cryptohash.h>
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#include <linux/kernel.h>
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#include <linux/export.h>
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#include <net/tcp.h>
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#include <net/route.h>
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extern int sysctl_tcp_syncookies;
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static u32 syncookie_secret[2][16-4+SHA_DIGEST_WORDS] __read_mostly;
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#define COOKIEBITS 24 /* Upper bits store count */
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#define COOKIEMASK (((__u32)1 << COOKIEBITS) - 1)
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/* TCP Timestamp: 6 lowest bits of timestamp sent in the cookie SYN-ACK
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* stores TCP options:
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*
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* MSB LSB
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* | 31 ... 6 | 5 | 4 | 3 2 1 0 |
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* | Timestamp | ECN | SACK | WScale |
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*
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* When we receive a valid cookie-ACK, we look at the echoed tsval (if
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* any) to figure out which TCP options we should use for the rebuilt
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* connection.
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*
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* A WScale setting of '0xf' (which is an invalid scaling value)
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* means that original syn did not include the TCP window scaling option.
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*/
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#define TS_OPT_WSCALE_MASK 0xf
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#define TS_OPT_SACK BIT(4)
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#define TS_OPT_ECN BIT(5)
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/* There is no TS_OPT_TIMESTAMP:
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* if ACK contains timestamp option, we already know it was
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* requested/supported by the syn/synack exchange.
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*/
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#define TSBITS 6
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#define TSMASK (((__u32)1 << TSBITS) - 1)
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static DEFINE_PER_CPU(__u32 [16 + 5 + SHA_WORKSPACE_WORDS],
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ipv4_cookie_scratch);
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static u32 cookie_hash(__be32 saddr, __be32 daddr, __be16 sport, __be16 dport,
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u32 count, int c)
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{
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__u32 *tmp;
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net_get_random_once(syncookie_secret, sizeof(syncookie_secret));
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tmp = this_cpu_ptr(ipv4_cookie_scratch);
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memcpy(tmp + 4, syncookie_secret[c], sizeof(syncookie_secret[c]));
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tmp[0] = (__force u32)saddr;
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tmp[1] = (__force u32)daddr;
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tmp[2] = ((__force u32)sport << 16) + (__force u32)dport;
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tmp[3] = count;
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sha_transform(tmp + 16, (__u8 *)tmp, tmp + 16 + 5);
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return tmp[17];
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}
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/*
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* when syncookies are in effect and tcp timestamps are enabled we encode
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* tcp options in the lower bits of the timestamp value that will be
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* sent in the syn-ack.
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* Since subsequent timestamps use the normal tcp_time_stamp value, we
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* must make sure that the resulting initial timestamp is <= tcp_time_stamp.
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*/
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__u32 cookie_init_timestamp(struct request_sock *req)
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{
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struct inet_request_sock *ireq;
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u32 ts, ts_now = tcp_time_stamp;
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u32 options = 0;
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ireq = inet_rsk(req);
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options = ireq->wscale_ok ? ireq->snd_wscale : TS_OPT_WSCALE_MASK;
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if (ireq->sack_ok)
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options |= TS_OPT_SACK;
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if (ireq->ecn_ok)
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options |= TS_OPT_ECN;
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ts = ts_now & ~TSMASK;
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ts |= options;
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if (ts > ts_now) {
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ts >>= TSBITS;
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ts--;
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ts <<= TSBITS;
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ts |= options;
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}
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return ts;
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}
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static __u32 secure_tcp_syn_cookie(__be32 saddr, __be32 daddr, __be16 sport,
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__be16 dport, __u32 sseq, __u32 data)
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{
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/*
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* Compute the secure sequence number.
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* The output should be:
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* HASH(sec1,saddr,sport,daddr,dport,sec1) + sseq + (count * 2^24)
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* + (HASH(sec2,saddr,sport,daddr,dport,count,sec2) % 2^24).
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* Where sseq is their sequence number and count increases every
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* minute by 1.
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* As an extra hack, we add a small "data" value that encodes the
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* MSS into the second hash value.
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*/
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u32 count = tcp_cookie_time();
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return (cookie_hash(saddr, daddr, sport, dport, 0, 0) +
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sseq + (count << COOKIEBITS) +
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((cookie_hash(saddr, daddr, sport, dport, count, 1) + data)
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& COOKIEMASK));
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}
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/*
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* This retrieves the small "data" value from the syncookie.
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* If the syncookie is bad, the data returned will be out of
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* range. This must be checked by the caller.
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*
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* The count value used to generate the cookie must be less than
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* MAX_SYNCOOKIE_AGE minutes in the past.
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* The return value (__u32)-1 if this test fails.
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*/
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static __u32 check_tcp_syn_cookie(__u32 cookie, __be32 saddr, __be32 daddr,
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__be16 sport, __be16 dport, __u32 sseq)
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{
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u32 diff, count = tcp_cookie_time();
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/* Strip away the layers from the cookie */
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cookie -= cookie_hash(saddr, daddr, sport, dport, 0, 0) + sseq;
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/* Cookie is now reduced to (count * 2^24) ^ (hash % 2^24) */
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diff = (count - (cookie >> COOKIEBITS)) & ((__u32) -1 >> COOKIEBITS);
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if (diff >= MAX_SYNCOOKIE_AGE)
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return (__u32)-1;
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return (cookie -
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cookie_hash(saddr, daddr, sport, dport, count - diff, 1))
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& COOKIEMASK; /* Leaving the data behind */
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}
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/*
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* MSS Values are chosen based on the 2011 paper
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* 'An Analysis of TCP Maximum Segement Sizes' by S. Alcock and R. Nelson.
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* Values ..
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* .. lower than 536 are rare (< 0.2%)
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* .. between 537 and 1299 account for less than < 1.5% of observed values
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* .. in the 1300-1349 range account for about 15 to 20% of observed mss values
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* .. exceeding 1460 are very rare (< 0.04%)
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*
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* 1460 is the single most frequently announced mss value (30 to 46% depending
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* on monitor location). Table must be sorted.
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*/
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static __u16 const msstab[] = {
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536,
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1300,
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1440, /* 1440, 1452: PPPoE */
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1460,
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};
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/*
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* Generate a syncookie. mssp points to the mss, which is returned
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* rounded down to the value encoded in the cookie.
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*/
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u32 __cookie_v4_init_sequence(const struct iphdr *iph, const struct tcphdr *th,
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u16 *mssp)
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{
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int mssind;
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const __u16 mss = *mssp;
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for (mssind = ARRAY_SIZE(msstab) - 1; mssind ; mssind--)
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if (mss >= msstab[mssind])
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break;
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*mssp = msstab[mssind];
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return secure_tcp_syn_cookie(iph->saddr, iph->daddr,
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th->source, th->dest, ntohl(th->seq),
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mssind);
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}
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EXPORT_SYMBOL_GPL(__cookie_v4_init_sequence);
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__u32 cookie_v4_init_sequence(struct sock *sk, const struct sk_buff *skb,
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__u16 *mssp)
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{
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const struct iphdr *iph = ip_hdr(skb);
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const struct tcphdr *th = tcp_hdr(skb);
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tcp_synq_overflow(sk);
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NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_SYNCOOKIESSENT);
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return __cookie_v4_init_sequence(iph, th, mssp);
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}
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/*
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* Check if a ack sequence number is a valid syncookie.
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* Return the decoded mss if it is, or 0 if not.
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*/
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int __cookie_v4_check(const struct iphdr *iph, const struct tcphdr *th,
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u32 cookie)
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{
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__u32 seq = ntohl(th->seq) - 1;
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__u32 mssind = check_tcp_syn_cookie(cookie, iph->saddr, iph->daddr,
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th->source, th->dest, seq);
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return mssind < ARRAY_SIZE(msstab) ? msstab[mssind] : 0;
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}
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EXPORT_SYMBOL_GPL(__cookie_v4_check);
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static inline struct sock *get_cookie_sock(struct sock *sk, struct sk_buff *skb,
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struct request_sock *req,
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struct dst_entry *dst)
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{
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struct inet_connection_sock *icsk = inet_csk(sk);
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struct sock *child;
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child = icsk->icsk_af_ops->syn_recv_sock(sk, skb, req, dst);
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if (child)
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inet_csk_reqsk_queue_add(sk, req, child);
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else
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reqsk_free(req);
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return child;
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}
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/*
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* when syncookies are in effect and tcp timestamps are enabled we stored
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* additional tcp options in the timestamp.
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* This extracts these options from the timestamp echo.
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*
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* return false if we decode a tcp option that is disabled
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* on the host.
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*/
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bool cookie_timestamp_decode(struct tcp_options_received *tcp_opt)
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{
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/* echoed timestamp, lowest bits contain options */
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u32 options = tcp_opt->rcv_tsecr;
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if (!tcp_opt->saw_tstamp) {
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tcp_clear_options(tcp_opt);
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return true;
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}
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if (!sysctl_tcp_timestamps)
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return false;
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tcp_opt->sack_ok = (options & TS_OPT_SACK) ? TCP_SACK_SEEN : 0;
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if (tcp_opt->sack_ok && !sysctl_tcp_sack)
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return false;
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if ((options & TS_OPT_WSCALE_MASK) == TS_OPT_WSCALE_MASK)
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return true; /* no window scaling */
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tcp_opt->wscale_ok = 1;
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tcp_opt->snd_wscale = options & TS_OPT_WSCALE_MASK;
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return sysctl_tcp_window_scaling != 0;
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}
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EXPORT_SYMBOL(cookie_timestamp_decode);
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bool cookie_ecn_ok(const struct tcp_options_received *tcp_opt,
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const struct net *net, const struct dst_entry *dst)
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{
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bool ecn_ok = tcp_opt->rcv_tsecr & TS_OPT_ECN;
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if (!ecn_ok)
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return false;
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if (net->ipv4.sysctl_tcp_ecn)
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return true;
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return dst_feature(dst, RTAX_FEATURE_ECN);
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}
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EXPORT_SYMBOL(cookie_ecn_ok);
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struct sock *cookie_v4_check(struct sock *sk, struct sk_buff *skb)
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{
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struct ip_options *opt = &TCP_SKB_CB(skb)->header.h4.opt;
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struct tcp_options_received tcp_opt;
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struct inet_request_sock *ireq;
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struct tcp_request_sock *treq;
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struct tcp_sock *tp = tcp_sk(sk);
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const struct tcphdr *th = tcp_hdr(skb);
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__u32 cookie = ntohl(th->ack_seq) - 1;
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struct sock *ret = sk;
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struct request_sock *req;
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int mss;
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struct rtable *rt;
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__u8 rcv_wscale;
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struct flowi4 fl4;
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if (!sysctl_tcp_syncookies || !th->ack || th->rst)
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goto out;
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if (tcp_synq_no_recent_overflow(sk))
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goto out;
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mss = __cookie_v4_check(ip_hdr(skb), th, cookie);
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if (mss == 0) {
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NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_SYNCOOKIESFAILED);
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goto out;
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}
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NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_SYNCOOKIESRECV);
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/* check for timestamp cookie support */
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memset(&tcp_opt, 0, sizeof(tcp_opt));
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tcp_parse_options(skb, &tcp_opt, 0, NULL);
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if (!cookie_timestamp_decode(&tcp_opt))
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goto out;
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ret = NULL;
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req = inet_reqsk_alloc(&tcp_request_sock_ops); /* for safety */
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if (!req)
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goto out;
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ireq = inet_rsk(req);
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treq = tcp_rsk(req);
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treq->rcv_isn = ntohl(th->seq) - 1;
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treq->snt_isn = cookie;
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req->mss = mss;
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ireq->ir_num = ntohs(th->dest);
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ireq->ir_rmt_port = th->source;
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ireq->ir_loc_addr = ip_hdr(skb)->daddr;
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ireq->ir_rmt_addr = ip_hdr(skb)->saddr;
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ireq->ir_mark = inet_request_mark(sk, skb);
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ireq->snd_wscale = tcp_opt.snd_wscale;
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ireq->sack_ok = tcp_opt.sack_ok;
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ireq->wscale_ok = tcp_opt.wscale_ok;
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ireq->tstamp_ok = tcp_opt.saw_tstamp;
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req->ts_recent = tcp_opt.saw_tstamp ? tcp_opt.rcv_tsval : 0;
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treq->snt_synack = tcp_opt.saw_tstamp ? tcp_opt.rcv_tsecr : 0;
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treq->listener = NULL;
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/* We throwed the options of the initial SYN away, so we hope
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* the ACK carries the same options again (see RFC1122 4.2.3.8)
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*/
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ireq->opt = tcp_v4_save_options(skb);
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if (security_inet_conn_request(sk, skb, req)) {
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reqsk_free(req);
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goto out;
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}
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req->expires = 0UL;
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req->num_retrans = 0;
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/*
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* We need to lookup the route here to get at the correct
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* window size. We should better make sure that the window size
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* hasn't changed since we received the original syn, but I see
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* no easy way to do this.
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*/
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flowi4_init_output(&fl4, sk->sk_bound_dev_if, ireq->ir_mark,
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RT_CONN_FLAGS(sk), RT_SCOPE_UNIVERSE, IPPROTO_TCP,
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inet_sk_flowi_flags(sk),
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opt->srr ? opt->faddr : ireq->ir_rmt_addr,
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ireq->ir_loc_addr, th->source, th->dest);
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security_req_classify_flow(req, flowi4_to_flowi(&fl4));
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rt = ip_route_output_key(sock_net(sk), &fl4);
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if (IS_ERR(rt)) {
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reqsk_free(req);
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goto out;
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}
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/* Try to redo what tcp_v4_send_synack did. */
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req->window_clamp = tp->window_clamp ? :dst_metric(&rt->dst, RTAX_WINDOW);
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tcp_select_initial_window(tcp_full_space(sk), req->mss,
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&req->rcv_wnd, &req->window_clamp,
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ireq->wscale_ok, &rcv_wscale,
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dst_metric(&rt->dst, RTAX_INITRWND));
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ireq->rcv_wscale = rcv_wscale;
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ireq->ecn_ok = cookie_ecn_ok(&tcp_opt, sock_net(sk), &rt->dst);
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ret = get_cookie_sock(sk, skb, req, &rt->dst);
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/* ip_queue_xmit() depends on our flow being setup
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* Normal sockets get it right from inet_csk_route_child_sock()
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*/
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if (ret)
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inet_sk(ret)->cork.fl.u.ip4 = fl4;
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out: return ret;
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}
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