lib/dns/rrl.c

/*
 * Copyright (C) 2012-2017  Internet Systems Consortium, Inc. ("ISC")
 *
 * This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this
 * file, You can obtain one at http://mozilla.org/MPL/2.0/.
 */

/*! \file */

/*
 * Rate limit DNS responses.
 */

/* #define ISC_LIST_CHECKINIT */

#include <config.h>
#include <isc/mem.h>
#include <isc/net.h>
#include <isc/netaddr.h>
#include <isc/print.h>
#include <isc/util.h>

#include <dns/result.h>
#include <dns/rcode.h>
#include <dns/rdatatype.h>
#include <dns/rdataclass.h>
#include <dns/log.h>
#include <dns/rrl.h>
#include <dns/view.h>

static void
log_end(dns_rrl_t *rrl, dns_rrl_entry_t *e, isc_boolean_t early,
    char *log_buf, unsigned int log_buf_len);

/*
 * Get a modulus for a hash function that is tolerably likely to be
 * relatively prime to most inputs.  Of course, we get a prime for for initial
 * values not larger than the square of the last prime.  We often get a prime
 * after that.
 * This works well in practice for hash tables up to at least 100
 * times the square of the last prime and better than a multiplicative hash.
 */
static int
hash_divisor(unsigned int initial) {
    static isc_uint16_t primes[] = {
          3,   5,   7,  11,  13,  17,  19,  23,  29,  31,  37,  41,
         43,  47,  53,  59,  61,  67,  71,  73,  79,  83,  89,  97,
#if 0
        101, 103, 107, 109, 113, 127, 131, 137, 139, 149, 151, 157,
        163, 167, 173, 179, 181, 191, 193, 197, 199, 211, 223, 227,
        229, 233, 239, 241, 251, 257, 263, 269, 271, 277, 281, 283,
        293, 307, 311, 313, 317, 331, 337, 347, 349, 353, 359, 367,
        373, 379, 383, 389, 397, 401, 409, 419, 421, 431, 433, 439,
        443, 449, 457, 461, 463, 467, 479, 487, 491, 499, 503, 509,
        521, 523, 541, 547, 557, 563, 569, 571, 577, 587, 593, 599,
        601, 607, 613, 617, 619, 631, 641, 643, 647, 653, 659, 661,
        673, 677, 683, 691, 701, 709, 719, 727, 733, 739, 743, 751,
        757, 761, 769, 773, 787, 797, 809, 811, 821, 823, 827, 829,
        839, 853, 857, 859, 863, 877, 881, 883, 887, 907, 911, 919,
        929, 937, 941, 947, 953, 967, 971, 977, 983, 991, 997,1009,
#endif
    };
    int divisions, tries;
    unsigned int result;
    isc_uint16_t *pp, p;

    result = initial;

    if (primes[sizeof(primes)/sizeof(primes[0])-1] >= result) {
        pp = primes;
        while (*pp < result)
            ++pp;
        return (*pp);
    }

    if ((result & 1) == 0)
        ++result;

    divisions = 0;
    tries = 1;
    pp = primes;
    do {
        p = *pp++;
        ++divisions;
        if ((result % p) == 0) {
            ++tries;
            result += 2;
            pp = primes;
        }
    } while (pp < &primes[sizeof(primes) / sizeof(primes[0])]);

    if (isc_log_wouldlog(dns_lctx, DNS_RRL_LOG_DEBUG3))
        isc_log_write(dns_lctx, DNS_LOGCATEGORY_RRL,
                  DNS_LOGMODULE_REQUEST, DNS_RRL_LOG_DEBUG3,
                  "%d hash_divisor() divisions in %d tries"
                  " to get %d from %d",
                  divisions, tries, result, initial);

    return (result);
}

/*
 * Convert a timestamp to a number of seconds in the past.
 */
static inline int
delta_rrl_time(isc_stdtime_t ts, isc_stdtime_t now) {
    int delta;

    delta = now - ts;
    if (delta >= 0)
        return (delta);

    /*
     * The timestamp is in the future.  That future might result from
     * re-ordered requests, because we use timestamps on requests
     * instead of consulting a clock.  Timestamps in the distant future are
     * assumed to result from clock changes.  When the clock changes to
     * the past, make existing timestamps appear to be in the past.
     */
    if (delta < -DNS_RRL_MAX_TIME_TRAVEL)
        return (DNS_RRL_FOREVER);
    return (0);
}

static inline int
get_age(const dns_rrl_t *rrl, const dns_rrl_entry_t *e, isc_stdtime_t now) {
    if (!e->ts_valid)
        return (DNS_RRL_FOREVER);
    return (delta_rrl_time(e->ts + rrl->ts_bases[e->ts_gen], now));
}

static inline void
set_age(dns_rrl_t *rrl, dns_rrl_entry_t *e, isc_stdtime_t now) {
    dns_rrl_entry_t *e_old;
    unsigned int ts_gen;
    int i, ts;

    ts_gen = rrl->ts_gen;
    ts = now - rrl->ts_bases[ts_gen];
    if (ts < 0) {
        if (ts < -DNS_RRL_MAX_TIME_TRAVEL)
            ts = DNS_RRL_FOREVER;
        else
            ts = 0;
    }

    /*
     * Make a new timestamp base if the current base is too old.
     * All entries older than DNS_RRL_MAX_WINDOW seconds are ancient,
     * useless history.  Their timestamps can be treated as if they are
     * all the same.
     * We only do arithmetic on more recent timestamps, so bases for
     * older timestamps can be recycled provided the old timestamps are
     * marked as ancient history.
     * This loop is almost always very short because most entries are
     * recycled after one second and any entries that need to be marked
     * are older than (DNS_RRL_TS_BASES)*DNS_RRL_MAX_TS seconds.
     */
    if (ts >= DNS_RRL_MAX_TS) {
        ts_gen = (ts_gen + 1) % DNS_RRL_TS_BASES;
        for (e_old = ISC_LIST_TAIL(rrl->lru), i = 0;
             e_old != NULL && (e_old->ts_gen == ts_gen ||
                       !ISC_LINK_LINKED(e_old, hlink));
             e_old = ISC_LIST_PREV(e_old, lru), ++i)
        {
            e_old->ts_valid = ISC_FALSE;
        }
        if (i != 0)
            isc_log_write(dns_lctx, DNS_LOGCATEGORY_RRL,
                      DNS_LOGMODULE_REQUEST, DNS_RRL_LOG_DEBUG1,
                      "rrl new time base scanned %d entries"
                      " at %d for %d %d %d %d",
                      i, now, rrl->ts_bases[ts_gen],
                      rrl->ts_bases[(ts_gen + 1) %
                    DNS_RRL_TS_BASES],
                      rrl->ts_bases[(ts_gen + 2) %
                    DNS_RRL_TS_BASES],
                      rrl->ts_bases[(ts_gen + 3) %
                    DNS_RRL_TS_BASES]);
        rrl->ts_gen = ts_gen;
        rrl->ts_bases[ts_gen] = now;
        ts = 0;
    }

    e->ts_gen = ts_gen;
    e->ts = ts;
    e->ts_valid = ISC_TRUE;
}

static isc_result_t
expand_entries(dns_rrl_t *rrl, int newsize) {
    unsigned int bsize;
    dns_rrl_block_t *b;
    dns_rrl_entry_t *e;
    double rate;
    int i;

    if (rrl->num_entries + newsize >= rrl->max_entries &&
        rrl->max_entries != 0)
    {
        newsize = rrl->max_entries - rrl->num_entries;
        if (newsize <= 0)
            return (ISC_R_SUCCESS);
    }

    /*
     * Log expansions so that the user can tune max-table-size
     * and min-table-size.
     */
    if (isc_log_wouldlog(dns_lctx, DNS_RRL_LOG_DROP) &&
        rrl->hash != NULL) {
        rate = rrl->probes;
        if (rrl->searches != 0)
            rate /= rrl->searches;
        isc_log_write(dns_lctx, DNS_LOGCATEGORY_RRL,
                  DNS_LOGMODULE_REQUEST, DNS_RRL_LOG_DROP,
                  "increase from %d to %d RRL entries with"
                  " %d bins; average search length %.1f",
                  rrl->num_entries, rrl->num_entries+newsize,
                  rrl->hash->length, rate);
    }

    bsize = sizeof(dns_rrl_block_t) + (newsize-1)*sizeof(dns_rrl_entry_t);
    b = isc_mem_get(rrl->mctx, bsize);
    if (b == NULL) {
        isc_log_write(dns_lctx, DNS_LOGCATEGORY_RRL,
                  DNS_LOGMODULE_REQUEST, DNS_RRL_LOG_FAIL,
                  "isc_mem_get(%d) failed for RRL entries",
                  bsize);
        return (ISC_R_NOMEMORY);
    }
    memset(b, 0, bsize);
    b->size = bsize;

    e = b->entries;
    for (i = 0; i < newsize; ++i, ++e) {
        ISC_LINK_INIT(e, hlink);
        ISC_LIST_INITANDAPPEND(rrl->lru, e, lru);
    }
    rrl->num_entries += newsize;
    ISC_LIST_INITANDAPPEND(rrl->blocks, b, link);

    return (ISC_R_SUCCESS);
}

static inline dns_rrl_bin_t *
get_bin(dns_rrl_hash_t *hash, unsigned int hval) {
    INSIST(hash != NULL);
    return (&hash->bins[hval % hash->length]);
}

static void
free_old_hash(dns_rrl_t *rrl) {
    dns_rrl_hash_t *old_hash;
    dns_rrl_bin_t *old_bin;
    dns_rrl_entry_t *e, *e_next;

    old_hash = rrl->old_hash;
    for (old_bin = &old_hash->bins[0];
         old_bin < &old_hash->bins[old_hash->length];
         ++old_bin)
    {
        for (e = ISC_LIST_HEAD(*old_bin); e != NULL; e = e_next) {
            e_next = ISC_LIST_NEXT(e, hlink);
            ISC_LINK_INIT(e, hlink);
        }
    }

    isc_mem_put(rrl->mctx, old_hash,
            sizeof(*old_hash)
              + (old_hash->length - 1) * sizeof(old_hash->bins[0]));
    rrl->old_hash = NULL;
}

static isc_result_t
expand_rrl_hash(dns_rrl_t *rrl, isc_stdtime_t now) {
    dns_rrl_hash_t *hash;
    int old_bins, new_bins, hsize;
    double rate;

    if (rrl->old_hash != NULL)
        free_old_hash(rrl);

    /*
     * Most searches fail and so go to the end of the chain.
     * Use a small hash table load factor.
     */
    old_bins = (rrl->hash == NULL) ? 0 : rrl->hash->length;
    new_bins = old_bins/8 + old_bins;
    if (new_bins < rrl->num_entries)
        new_bins = rrl->num_entries;
    new_bins = hash_divisor(new_bins);

    hsize = sizeof(dns_rrl_hash_t) + (new_bins-1)*sizeof(hash->bins[0]);
    hash = isc_mem_get(rrl->mctx, hsize);
    if (hash == NULL) {
        isc_log_write(dns_lctx, DNS_LOGCATEGORY_RRL,
                  DNS_LOGMODULE_REQUEST, DNS_RRL_LOG_FAIL,
                  "isc_mem_get(%d) failed for"
                  " RRL hash table",
                  hsize);
        return (ISC_R_NOMEMORY);
    }
    memset(hash, 0, hsize);
    hash->length = new_bins;
    rrl->hash_gen ^= 1;
    hash->gen = rrl->hash_gen;

    if (isc_log_wouldlog(dns_lctx, DNS_RRL_LOG_DROP) && old_bins != 0) {
        rate = rrl->probes;
        if (rrl->searches != 0)
            rate /= rrl->searches;
        isc_log_write(dns_lctx, DNS_LOGCATEGORY_RRL,
                  DNS_LOGMODULE_REQUEST, DNS_RRL_LOG_DROP,
                  "increase from %d to %d RRL bins for"
                  " %d entries; average search length %.1f",
                  old_bins, new_bins, rrl->num_entries, rate);
    }

    rrl->old_hash = rrl->hash;
    if (rrl->old_hash != NULL)
        rrl->old_hash->check_time = now;
    rrl->hash = hash;

    return (ISC_R_SUCCESS);
}

static void
ref_entry(dns_rrl_t *rrl, dns_rrl_entry_t *e, int probes, isc_stdtime_t now) {
    /*
     * Make the entry most recently used.
     */
    if (ISC_LIST_HEAD(rrl->lru) != e) {
        if (e == rrl->last_logged)
            rrl->last_logged = ISC_LIST_PREV(e, lru);
        ISC_LIST_UNLINK(rrl->lru, e, lru);
        ISC_LIST_PREPEND(rrl->lru, e, lru);
    }

    /*
     * Expand the hash table if it is time and necessary.
     * This will leave the newly referenced entry in a chain in the
     * old hash table.  It will migrate to the new hash table the next
     * time it is used or be cut loose when the old hash table is destroyed.
     */
    rrl->probes += probes;
    ++rrl->searches;
    if (rrl->searches > 100 &&
        delta_rrl_time(rrl->hash->check_time, now) > 1) {
        if (rrl->probes/rrl->searches > 2)
            expand_rrl_hash(rrl, now);
        rrl->hash->check_time = now;
        rrl->probes = 0;
        rrl->searches = 0;
    }
}

static inline isc_boolean_t
key_cmp(const dns_rrl_key_t *a, const dns_rrl_key_t *b) {
    if (memcmp(a, b, sizeof(dns_rrl_key_t)) == 0)
        return (ISC_TRUE);
    return (ISC_FALSE);
}

static inline isc_uint32_t
hash_key(const dns_rrl_key_t *key) {
    isc_uint32_t hval;
    int i;

    hval = key->w[0];
    for (i = sizeof(key->w) / sizeof(key->w[0]) - 1; i >= 0; --i) {
        hval = key->w[i] + (hval<<1);
    }
    return (hval);
}

/*
 * Construct the hash table key.
 * Use a hash of the DNS query name to save space in the database.
 * Collisions result in legitimate rate limiting responses for one
 * query name also limiting responses for other names to the
 * same client.  This is rare and benign enough given the large
 * space costs compared to keeping the entire name in the database
 * entry or the time costs of dynamic allocation.
 */
static void
make_key(const dns_rrl_t *rrl, dns_rrl_key_t *key,
     const isc_sockaddr_t *client_addr,
     dns_rdatatype_t qtype, dns_name_t *qname, dns_rdataclass_t qclass,
     dns_rrl_rtype_t rtype)
{
    dns_name_t base;
    dns_offsets_t base_offsets;
    int labels, i;

    memset(key, 0, sizeof(*key));

    key->s.rtype = rtype;
    if (rtype == DNS_RRL_RTYPE_QUERY) {
        key->s.qtype = qtype;
        key->s.qclass = qclass & 0xff;
    } else if (rtype == DNS_RRL_RTYPE_REFERRAL ||
           rtype == DNS_RRL_RTYPE_NODATA) {
        /*
         * Because there is no qtype in the empty answer sections of
         * referral and NODATA responses, count them as the same.
         */
        key->s.qclass = qclass & 0xff;
    }

    if (qname != NULL && qname->labels != 0) {
        /*
         * Ignore the first label of wildcards.
         */
        if ((qname->attributes & DNS_NAMEATTR_WILDCARD) != 0 &&
            (labels = dns_name_countlabels(qname)) > 1)
        {
            dns_name_init(&base, base_offsets);
            dns_name_getlabelsequence(qname, 1, labels-1, &base);
            key->s.qname_hash =
                dns_name_fullhash(&base, ISC_FALSE);
        } else {
            key->s.qname_hash =
                dns_name_fullhash(qname, ISC_FALSE);
        }
    }

    switch (client_addr->type.sa.sa_family) {
    case AF_INET:
        key->s.ip[0] = (client_addr->type.sin.sin_addr.s_addr &
                  rrl->ipv4_mask);
        break;
    case AF_INET6:
        key->s.ipv6 = ISC_TRUE;
        memmove(key->s.ip, &client_addr->type.sin6.sin6_addr,
            sizeof(key->s.ip));
        for (i = 0; i < DNS_RRL_MAX_PREFIX/32; ++i)
            key->s.ip[i] &= rrl->ipv6_mask[i];
        break;
    }
}

static inline dns_rrl_rate_t *
get_rate(dns_rrl_t *rrl, dns_rrl_rtype_t rtype) {
    switch (rtype) {
    case DNS_RRL_RTYPE_QUERY:
        return (&rrl->responses_per_second);
    case DNS_RRL_RTYPE_REFERRAL:
        return (&rrl->referrals_per_second);
    case DNS_RRL_RTYPE_NODATA:
        return (&rrl->nodata_per_second);
    case DNS_RRL_RTYPE_NXDOMAIN:
        return (&rrl->nxdomains_per_second);
    case DNS_RRL_RTYPE_ERROR:
        return (&rrl->errors_per_second);
    case DNS_RRL_RTYPE_ALL:
        return (&rrl->all_per_second);
    default:
        INSIST(0);
    }
    return (NULL);
}

static int
response_balance(dns_rrl_t *rrl, const dns_rrl_entry_t *e, int age) {
    dns_rrl_rate_t *ratep;
    int balance, rate;

    if (e->key.s.rtype == DNS_RRL_RTYPE_TCP) {
        rate = 1;
    } else {
        ratep = get_rate(rrl, e->key.s.rtype);
        rate = ratep->scaled;
    }

    balance = e->responses + age * rate;
    if (balance > rate)
        balance = rate;
    return (balance);
}

/*
 * Search for an entry for a response and optionally create it.
 */
static dns_rrl_entry_t *
get_entry(dns_rrl_t *rrl, const isc_sockaddr_t *client_addr,
      dns_rdataclass_t qclass, dns_rdatatype_t qtype, dns_name_t *qname,
      dns_rrl_rtype_t rtype, isc_stdtime_t now, isc_boolean_t create,
      char *log_buf, unsigned int log_buf_len)
{
    dns_rrl_key_t key;
    isc_uint32_t hval;
    dns_rrl_entry_t *e;
    dns_rrl_hash_t *hash;
    dns_rrl_bin_t *new_bin, *old_bin;
    int probes, age;

    make_key(rrl, &key, client_addr, qtype, qname, qclass, rtype);
    hval = hash_key(&key);

    /*
     * Look for the entry in the current hash table.
     */
    new_bin = get_bin(rrl->hash, hval);
    probes = 1;
    e = ISC_LIST_HEAD(*new_bin);
    while (e != NULL) {
        if (key_cmp(&e->key, &key)) {
            ref_entry(rrl, e, probes, now);
            return (e);
        }
        ++probes;
        e = ISC_LIST_NEXT(e, hlink);
    }

    /*
     * Look in the old hash table.
     */
    if (rrl->old_hash != NULL) {
        old_bin = get_bin(rrl->old_hash, hval);
        e = ISC_LIST_HEAD(*old_bin);
        while (e != NULL) {
            if (key_cmp(&e->key, &key)) {
                ISC_LIST_UNLINK(*old_bin, e, hlink);
                ISC_LIST_PREPEND(*new_bin, e, hlink);
                e->hash_gen = rrl->hash_gen;
                ref_entry(rrl, e, probes, now);
                return (e);
            }
            e = ISC_LIST_NEXT(e, hlink);
        }

        /*
         * Discard prevous hash table when all of its entries are old.
         */
        age = delta_rrl_time(rrl->old_hash->check_time, now);
        if (age > rrl->window)
            free_old_hash(rrl);
    }

    if (!create)
        return (NULL);

    /*
     * The entry does not exist, so create it by finding a free entry.
     * Keep currently penalized and logged entries.
     * Try to make more entries if none are idle.
     * Steal the oldest entry if we cannot create more.
     */
    for (e = ISC_LIST_TAIL(rrl->lru);
         e != NULL;
         e = ISC_LIST_PREV(e, lru))
    {
        if (!ISC_LINK_LINKED(e, hlink))
            break;
        age = get_age(rrl, e, now);
        if (age <= 1) {
            e = NULL;
            break;
        }
        if (!e->logged && response_balance(rrl, e, age) > 0)
            break;
    }
    if (e == NULL) {
        expand_entries(rrl, ISC_MIN((rrl->num_entries+1)/2, 1000));
        e = ISC_LIST_TAIL(rrl->lru);
    }
    if (e->logged)
        log_end(rrl, e, ISC_TRUE, log_buf, log_buf_len);
    if (ISC_LINK_LINKED(e, hlink)) {
        if (e->hash_gen == rrl->hash_gen)
            hash = rrl->hash;
        else
            hash = rrl->old_hash;
        old_bin = get_bin(hash, hash_key(&e->key));
        ISC_LIST_UNLINK(*old_bin, e, hlink);
    }
    ISC_LIST_PREPEND(*new_bin, e, hlink);
    e->hash_gen = rrl->hash_gen;
    e->key = key;
    e->ts_valid = ISC_FALSE;
    ref_entry(rrl, e, probes, now);
    return (e);
}

static void
debit_log(const dns_rrl_entry_t *e, int age, const char *action) {
    char buf[sizeof("age=12345678")];
    const char *age_str;

    if (age == DNS_RRL_FOREVER) {
        age_str = "";
    } else {
        snprintf(buf, sizeof(buf), "age=%d", age);
        age_str = buf;
    }
    isc_log_write(dns_lctx, DNS_LOGCATEGORY_RRL,
              DNS_LOGMODULE_REQUEST, DNS_RRL_LOG_DEBUG3,
              "rrl %08x %6s  responses=%-3d %s",
              hash_key(&e->key), age_str, e->responses, action);
}

static inline dns_rrl_result_t
debit_rrl_entry(dns_rrl_t *rrl, dns_rrl_entry_t *e, double qps, double scale,
        const isc_sockaddr_t *client_addr, isc_stdtime_t now,
        char *log_buf, unsigned int log_buf_len)
{
    int rate, new_rate, slip, new_slip, age, log_secs, min;
    dns_rrl_rate_t *ratep;
    dns_rrl_entry_t const *credit_e;

    /*
     * Pick the rate counter.
     * Optionally adjust the rate by the estimated query/second rate.
     */
    ratep = get_rate(rrl, e->key.s.rtype);
    rate = ratep->r;
    if (rate == 0)
        return (DNS_RRL_RESULT_OK);

    if (scale < 1.0) {
        /*
         * The limit for clients that have used TCP is not scaled.
         */
        credit_e = get_entry(rrl, client_addr,
                     0, dns_rdatatype_none, NULL,
                     DNS_RRL_RTYPE_TCP, now, ISC_FALSE,
                     log_buf, log_buf_len);
        if (credit_e != NULL) {
            age = get_age(rrl, e, now);
            if (age < rrl->window)
                scale = 1.0;
        }
    }
    if (scale < 1.0) {
        new_rate = (int) (rate * scale);
        if (new_rate < 1)
            new_rate = 1;
        if (ratep->scaled != new_rate) {
            isc_log_write(dns_lctx, DNS_LOGCATEGORY_RRL,
                      DNS_LOGMODULE_REQUEST,
                      DNS_RRL_LOG_DEBUG1,
                      "%d qps scaled %s by %.2f"
                      " from %d to %d",
                      (int)qps, ratep->str, scale,
                      rate, new_rate);
            rate = new_rate;
            ratep->scaled = rate;
        }
    }

    min = -rrl->window * rate;

    /*
     * Treat time jumps into the recent past as no time.
     * Treat entries older than the window as if they were just created
     * Credit other entries.
     */
    age = get_age(rrl, e, now);
    if (age > 0) {
        /*
         * Credit tokens earned during elapsed time.
         */
        if (age > rrl->window) {
            e->responses = rate;
            e->slip_cnt = 0;
        } else {
            e->responses += rate*age;
            if (e->responses > rate) {
                e->responses = rate;
                e->slip_cnt = 0;
            }
        }
        /*
         * Find the seconds since last log message without overflowing
         * small counter.  This counter is reset when an entry is
         * created.  It is not necessarily reset when some requests
         * are answered provided other requests continue to be dropped
         * or slipped.  This can happen when the request rate is just
         * at the limit.
         */
        if (e->logged) {
            log_secs = e->log_secs;
            log_secs += age;
            if (log_secs > DNS_RRL_MAX_LOG_SECS || log_secs < 0)
                log_secs = DNS_RRL_MAX_LOG_SECS;
            e->log_secs = log_secs;
        }
    }
    set_age(rrl, e, now);

    /*
     * Debit the entry for this response.
     */
    if (--e->responses >= 0) {
        if (isc_log_wouldlog(dns_lctx, DNS_RRL_LOG_DEBUG3))
            debit_log(e, age, "");
        return (DNS_RRL_RESULT_OK);
    }

    if (e->responses < min)
        e->responses = min;

    /*
     * Drop this response unless it should slip or leak.
     */
    slip = rrl->slip.r;
    if (slip > 2 && scale < 1.0) {
        new_slip = (int) (slip * scale);
        if (new_slip < 2)
            new_slip = 2;
        if (rrl->slip.scaled != new_slip) {
            isc_log_write(dns_lctx, DNS_LOGCATEGORY_RRL,
                      DNS_LOGMODULE_REQUEST,
                      DNS_RRL_LOG_DEBUG1,
                      "%d qps scaled slip"
                      " by %.2f from %d to %d",
                      (int)qps, scale,
                      slip, new_slip);
            slip = new_slip;
            rrl->slip.scaled = slip;
        }
    }
    if (slip != 0 && e->key.s.rtype != DNS_RRL_RTYPE_ALL) {
        if (e->slip_cnt++ == 0) {
            if ((int) e->slip_cnt >= slip)
                e->slip_cnt = 0;
            if (isc_log_wouldlog(dns_lctx, DNS_RRL_LOG_DEBUG3))
                debit_log(e, age, "slip");
            return (DNS_RRL_RESULT_SLIP);
        } else if ((int) e->slip_cnt >= slip) {
            e->slip_cnt = 0;
        }
    }

    if (isc_log_wouldlog(dns_lctx, DNS_RRL_LOG_DEBUG3))
        debit_log(e, age, "drop");
    return (DNS_RRL_RESULT_DROP);
}

static inline dns_rrl_qname_buf_t *
get_qname(dns_rrl_t *rrl, const dns_rrl_entry_t *e) {
    dns_rrl_qname_buf_t *qbuf;

    qbuf = rrl->qnames[e->log_qname];
    if (qbuf == NULL || qbuf->e != e)
        return (NULL);
    return (qbuf);
}

static inline void
free_qname(dns_rrl_t *rrl, dns_rrl_entry_t *e) {
    dns_rrl_qname_buf_t *qbuf;

    qbuf = get_qname(rrl, e);
    if (qbuf != NULL) {
        qbuf->e = NULL;
        ISC_LIST_APPEND(rrl->qname_free, qbuf, link);
    }
}

static void
add_log_str(isc_buffer_t *lb, const char *str, unsigned int str_len) {
    isc_region_t region;

    isc_buffer_availableregion(lb, &region);
    if (str_len >= region.length) {
        if (region.length <= 0)
            return;
        str_len = region.length;
    }
    memmove(region.base, str, str_len);
    isc_buffer_add(lb, str_len);
}

#define ADD_LOG_CSTR(eb, s) add_log_str(eb, s, sizeof(s)-1)

/*
 * Build strings for the logs
 */
static void
make_log_buf(dns_rrl_t *rrl, dns_rrl_entry_t *e,
         const char *str1, const char *str2, isc_boolean_t plural,
         dns_name_t *qname, isc_boolean_t save_qname,
         dns_rrl_result_t rrl_result, isc_result_t resp_result,
         char *log_buf, unsigned int log_buf_len)
{
    isc_buffer_t lb;
    dns_rrl_qname_buf_t *qbuf;
    isc_netaddr_t cidr;
    char strbuf[ISC_MAX(sizeof("/123"), sizeof("  (12345678)"))];
    const char *rstr;
    isc_result_t msg_result;

    if (log_buf_len <= 1) {
        if (log_buf_len == 1)
            log_buf[0] = '\0';
        return;
    }
    isc_buffer_init(&lb, log_buf, log_buf_len-1);

    if (str1 != NULL)
        add_log_str(&lb, str1, strlen(str1));
    if (str2 != NULL)
        add_log_str(&lb, str2, strlen(str2));

    switch (rrl_result) {
    case DNS_RRL_RESULT_OK:
        break;
    case DNS_RRL_RESULT_DROP:
        ADD_LOG_CSTR(&lb, "drop ");
        break;
    case DNS_RRL_RESULT_SLIP:
        ADD_LOG_CSTR(&lb, "slip ");
        break;
    default:
        INSIST(0);
        break;
    }

    switch (e->key.s.rtype) {
    case DNS_RRL_RTYPE_QUERY:
        break;
    case DNS_RRL_RTYPE_REFERRAL:
        ADD_LOG_CSTR(&lb, "referral ");
        break;
    case DNS_RRL_RTYPE_NODATA:
        ADD_LOG_CSTR(&lb, "NODATA ");
        break;
    case DNS_RRL_RTYPE_NXDOMAIN:
        ADD_LOG_CSTR(&lb, "NXDOMAIN ");
        break;
    case DNS_RRL_RTYPE_ERROR:
        if (resp_result == ISC_R_SUCCESS) {
            ADD_LOG_CSTR(&lb, "error ");
        } else {
            rstr = isc_result_totext(resp_result);
            add_log_str(&lb, rstr, strlen(rstr));
            ADD_LOG_CSTR(&lb, " error ");
        }
        break;
    case DNS_RRL_RTYPE_ALL:
        ADD_LOG_CSTR(&lb, "all ");
        break;
    default:
        INSIST(0);
    }

    if (plural)
        ADD_LOG_CSTR(&lb, "responses to ");
    else
        ADD_LOG_CSTR(&lb, "response to ");

    memset(&cidr, 0, sizeof(cidr));
    if (e->key.s.ipv6) {
        snprintf(strbuf, sizeof(strbuf), "/%d", rrl->ipv6_prefixlen);
        cidr.family = AF_INET6;
        memset(&cidr.type.in6, 0,  sizeof(cidr.type.in6));
        memmove(&cidr.type.in6, e->key.s.ip, sizeof(e->key.s.ip));
    } else {
        snprintf(strbuf, sizeof(strbuf), "/%d", rrl->ipv4_prefixlen);
        cidr.family = AF_INET;
        cidr.type.in.s_addr = e->key.s.ip[0];
    }
    msg_result = isc_netaddr_totext(&cidr, &lb);
    if (msg_result != ISC_R_SUCCESS)
        ADD_LOG_CSTR(&lb, "?");
    add_log_str(&lb, strbuf, strlen(strbuf));

    if (e->key.s.rtype == DNS_RRL_RTYPE_QUERY ||
        e->key.s.rtype == DNS_RRL_RTYPE_REFERRAL ||
        e->key.s.rtype == DNS_RRL_RTYPE_NODATA ||
        e->key.s.rtype == DNS_RRL_RTYPE_NXDOMAIN) {
        qbuf = get_qname(rrl, e);
        if (save_qname && qbuf == NULL &&
            qname != NULL && dns_name_isabsolute(qname)) {
            /*
             * Capture the qname for the "stop limiting" message.
             */
            qbuf = ISC_LIST_TAIL(rrl->qname_free);
            if (qbuf != NULL) {
                ISC_LIST_UNLINK(rrl->qname_free, qbuf, link);
            } else if (rrl->num_qnames < DNS_RRL_QNAMES) {
                qbuf = isc_mem_get(rrl->mctx, sizeof(*qbuf));
                if (qbuf != NULL) {
                    memset(qbuf, 0, sizeof(*qbuf));
                    ISC_LINK_INIT(qbuf, link);
                    qbuf->index = rrl->num_qnames;
                    rrl->qnames[rrl->num_qnames++] = qbuf;
                } else {
                    isc_log_write(dns_lctx,
                              DNS_LOGCATEGORY_RRL,
                              DNS_LOGMODULE_REQUEST,
                              DNS_RRL_LOG_FAIL,
                              "isc_mem_get(%d)"
                              " failed for RRL qname",
                              (int)sizeof(*qbuf));
                }
            }
            if (qbuf != NULL) {
                e->log_qname = qbuf->index;
                qbuf->e = e;
                dns_fixedname_init(&qbuf->qname);
                dns_name_copy(qname,
                          dns_fixedname_name(&qbuf->qname),
                          NULL);
            }
        }
        if (qbuf != NULL)
            qname = dns_fixedname_name(&qbuf->qname);
        if (qname != NULL) {
            ADD_LOG_CSTR(&lb, " for ");
            (void)dns_name_totext(qname, ISC_TRUE, &lb);
        } else {
            ADD_LOG_CSTR(&lb, " for (?)");
        }
        if (e->key.s.rtype != DNS_RRL_RTYPE_NXDOMAIN) {
            ADD_LOG_CSTR(&lb, " ");
            (void)dns_rdataclass_totext(e->key.s.qclass, &lb);
            if (e->key.s.rtype == DNS_RRL_RTYPE_QUERY) {
                ADD_LOG_CSTR(&lb, " ");
                (void)dns_rdatatype_totext(e->key.s.qtype, &lb);
            }
        }
        snprintf(strbuf, sizeof(strbuf), "  (%08x)",
             e->key.s.qname_hash);
        add_log_str(&lb, strbuf, strlen(strbuf));
    }

    /*
     * We saved room for '\0'.
     */
    log_buf[isc_buffer_usedlength(&lb)] = '\0';
}

static void
log_end(dns_rrl_t *rrl, dns_rrl_entry_t *e, isc_boolean_t early,
    char *log_buf, unsigned int log_buf_len)
{
    if (e->logged) {
        make_log_buf(rrl, e,
                 early ? "*" : NULL,
                 rrl->log_only ? "would stop limiting "
                       : "stop limiting ",
                 ISC_TRUE, NULL, ISC_FALSE,
                 DNS_RRL_RESULT_OK, ISC_R_SUCCESS,
                 log_buf, log_buf_len);
        isc_log_write(dns_lctx, DNS_LOGCATEGORY_RRL,
                  DNS_LOGMODULE_REQUEST, DNS_RRL_LOG_DROP,
                  "%s", log_buf);
        free_qname(rrl, e);
        e->logged = ISC_FALSE;
        --rrl->num_logged;
    }
}

/*
 * Log messages for streams that have stopped being rate limited.
 */
static void
log_stops(dns_rrl_t *rrl, isc_stdtime_t now, int limit,
      char *log_buf, unsigned int log_buf_len)
{
    dns_rrl_entry_t *e;
    int age;

    for (e = rrl->last_logged; e != NULL; e = ISC_LIST_PREV(e, lru)) {
        if (!e->logged)
            continue;
        if (now != 0) {
            age = get_age(rrl, e, now);
            if (age < DNS_RRL_STOP_LOG_SECS ||
                response_balance(rrl, e, age) < 0)
                break;
        }

        log_end(rrl, e, now == 0, log_buf, log_buf_len);
        if (rrl->num_logged <= 0)
            break;

        /*
         * Too many messages could stall real work.
         */
        if (--limit < 0) {
            rrl->last_logged = ISC_LIST_PREV(e, lru);
            return;
        }
    }
    if (e == NULL) {
        INSIST(rrl->num_logged == 0);
        rrl->log_stops_time = now;
    }
    rrl->last_logged = e;
}

/*
 * Main rate limit interface.
 */
dns_rrl_result_t
dns_rrl(dns_view_t *view,
    const isc_sockaddr_t *client_addr, isc_boolean_t is_tcp,
    dns_rdataclass_t qclass, dns_rdatatype_t qtype,
    dns_name_t *qname, isc_result_t resp_result, isc_stdtime_t now,
    isc_boolean_t wouldlog, char *log_buf, unsigned int log_buf_len)
{
    dns_rrl_t *rrl;
    dns_rrl_rtype_t rtype;
    dns_rrl_entry_t *e;
    isc_netaddr_t netclient;
    int secs;
    double qps, scale;
    int exempt_match;
    isc_result_t result;
    dns_rrl_result_t rrl_result;

    INSIST(log_buf != NULL && log_buf_len > 0);

    rrl = view->rrl;
    if (rrl->exempt != NULL) {
        isc_netaddr_fromsockaddr(&netclient, client_addr);
        result = dns_acl_match(&netclient, NULL, rrl->exempt,
                       &view->aclenv, &exempt_match, NULL);
        if (result == ISC_R_SUCCESS && exempt_match > 0)
            return (DNS_RRL_RESULT_OK);
    }

    LOCK(&rrl->lock);

    /*
     * Estimate total query per second rate when scaling by qps.
     */
    if (rrl->qps_scale == 0) {
        qps = 0.0;
        scale = 1.0;
    } else {
        ++rrl->qps_responses;
        secs = delta_rrl_time(rrl->qps_time, now);
        if (secs <= 0) {
            qps = rrl->qps;
        } else {
            qps = (1.0*rrl->qps_responses) / secs;
            if (secs >= rrl->window) {
                if (isc_log_wouldlog(dns_lctx,
                             DNS_RRL_LOG_DEBUG3))
                    isc_log_write(dns_lctx,
                              DNS_LOGCATEGORY_RRL,
                              DNS_LOGMODULE_REQUEST,
                              DNS_RRL_LOG_DEBUG3,
                              "%d responses/%d seconds"
                              " = %d qps",
                              rrl->qps_responses, secs,
                              (int)qps);
                rrl->qps = qps;
                rrl->qps_responses = 0;
                rrl->qps_time = now;
            } else if (qps < rrl->qps) {
                qps = rrl->qps;
            }
        }
        scale = rrl->qps_scale / qps;
    }

    /*
     * Do maintenance once per second.
     */
    if (rrl->num_logged > 0 && rrl->log_stops_time != now)
        log_stops(rrl, now, 8, log_buf, log_buf_len);

    /*
     * Notice TCP responses when scaling limits by qps.
     * Do not try to rate limit TCP responses.
     */
    if (is_tcp) {
        if (scale < 1.0) {
            e = get_entry(rrl, client_addr,
                      0, dns_rdatatype_none, NULL,
                      DNS_RRL_RTYPE_TCP, now, ISC_TRUE,
                      log_buf, log_buf_len);
            if (e != NULL) {
                e->responses = -(rrl->window+1);
                set_age(rrl, e, now);
            }
        }
        UNLOCK(&rrl->lock);
        return (ISC_R_SUCCESS);
    }

    /*
     * Find the right kind of entry, creating it if necessary.
     * If that is impossible, then nothing more can be done
     */
    switch (resp_result) {
    case ISC_R_SUCCESS:
        rtype = DNS_RRL_RTYPE_QUERY;
        break;
    case DNS_R_DELEGATION:
        rtype = DNS_RRL_RTYPE_REFERRAL;
        break;
    case DNS_R_NXRRSET:
        rtype = DNS_RRL_RTYPE_NODATA;
        break;
    case DNS_R_NXDOMAIN:
        rtype = DNS_RRL_RTYPE_NXDOMAIN;
        break;
    default:
        rtype = DNS_RRL_RTYPE_ERROR;
        break;
    }
    e = get_entry(rrl, client_addr, qclass, qtype, qname, rtype,
              now, ISC_TRUE, log_buf, log_buf_len);
    if (e == NULL) {
        UNLOCK(&rrl->lock);
        return (DNS_RRL_RESULT_OK);
    }

    if (isc_log_wouldlog(dns_lctx, DNS_RRL_LOG_DEBUG1)) {
        /*
         * Do not worry about speed or releasing the lock.
         * This message appears before messages from debit_rrl_entry().
         */
        make_log_buf(rrl, e, "consider limiting ", NULL, ISC_FALSE,
                 qname, ISC_FALSE, DNS_RRL_RESULT_OK, resp_result,
                 log_buf, log_buf_len);
        isc_log_write(dns_lctx, DNS_LOGCATEGORY_RRL,
                  DNS_LOGMODULE_REQUEST, DNS_RRL_LOG_DEBUG1,
                  "%s", log_buf);
    }

    rrl_result = debit_rrl_entry(rrl, e, qps, scale, client_addr, now,
                     log_buf, log_buf_len);

    if (rrl->all_per_second.r != 0) {
        /*
         * We must debit the all-per-second token bucket if we have
         * an all-per-second limit for the IP address.
         * The all-per-second limit determines the log message
         * when both limits are hit.
         * The response limiting must continue if the
         * all-per-second limiting lapses.
         */
        dns_rrl_entry_t *e_all;
        dns_rrl_result_t rrl_all_result;

        e_all = get_entry(rrl, client_addr,
                  0, dns_rdatatype_none, NULL,
                  DNS_RRL_RTYPE_ALL, now, ISC_TRUE,
                  log_buf, log_buf_len);
        if (e_all == NULL) {
            UNLOCK(&rrl->lock);
            return (DNS_RRL_RESULT_OK);
        }
        rrl_all_result = debit_rrl_entry(rrl, e_all, qps, scale,
                         client_addr, now,
                         log_buf, log_buf_len);
        if (rrl_all_result != DNS_RRL_RESULT_OK) {
            e = e_all;
            rrl_result = rrl_all_result;
            if (isc_log_wouldlog(dns_lctx, DNS_RRL_LOG_DEBUG1)) {
                make_log_buf(rrl, e,
                         "prefer all-per-second limiting ",
                         NULL, ISC_TRUE, qname, ISC_FALSE,
                         DNS_RRL_RESULT_OK, resp_result,
                         log_buf, log_buf_len);
                isc_log_write(dns_lctx, DNS_LOGCATEGORY_RRL,
                          DNS_LOGMODULE_REQUEST,
                          DNS_RRL_LOG_DEBUG1,
                          "%s", log_buf);
            }
        }
    }

    if (rrl_result == DNS_RRL_RESULT_OK) {
        UNLOCK(&rrl->lock);
        return (DNS_RRL_RESULT_OK);
    }

    /*
     * Log occassionally in the rate-limit category.
     */
    if ((!e->logged || e->log_secs >= DNS_RRL_MAX_LOG_SECS) &&
        isc_log_wouldlog(dns_lctx, DNS_RRL_LOG_DROP)) {
        make_log_buf(rrl, e, rrl->log_only ? "would " : NULL,
                 e->logged ? "continue limiting " : "limit ",
                 ISC_TRUE, qname, ISC_TRUE,
                 DNS_RRL_RESULT_OK, resp_result,
                 log_buf, log_buf_len);
        if (!e->logged) {
            e->logged = ISC_TRUE;
            if (++rrl->num_logged <= 1)
                rrl->last_logged = e;
        }
        e->log_secs = 0;

        /*
         * Avoid holding the lock.
         */
        if (!wouldlog) {
            UNLOCK(&rrl->lock);
            e = NULL;
        }
        isc_log_write(dns_lctx, DNS_LOGCATEGORY_RRL,
                  DNS_LOGMODULE_REQUEST, DNS_RRL_LOG_DROP,
                  "%s", log_buf);
    }

    /*
     * Make a log message for the caller.
     */
    if (wouldlog)
        make_log_buf(rrl, e,
                 rrl->log_only ? "would rate limit " : "rate limit ",
                 NULL, ISC_FALSE, qname, ISC_FALSE,
                 rrl_result, resp_result, log_buf, log_buf_len);

    if (e != NULL) {
        /*
         * Do not save the qname unless we might need it for
         * the ending log message.
         */
        if (!e->logged)
            free_qname(rrl, e);
        UNLOCK(&rrl->lock);
    }

    return (rrl_result);
}

void
dns_rrl_view_destroy(dns_view_t *view) {
    dns_rrl_t *rrl;
    dns_rrl_block_t *b;
    dns_rrl_hash_t *h;
    char log_buf[DNS_RRL_LOG_BUF_LEN];
    int i;

    rrl = view->rrl;
    if (rrl == NULL)
        return;
    view->rrl = NULL;

    /*
     * Assume the caller takes care of locking the view and anything else.
     */

    if (rrl->num_logged > 0)
        log_stops(rrl, 0, ISC_INT32_MAX, log_buf, sizeof(log_buf));

    for (i = 0; i < DNS_RRL_QNAMES; ++i) {
        if (rrl->qnames[i] == NULL)
            break;
        isc_mem_put(rrl->mctx, rrl->qnames[i], sizeof(*rrl->qnames[i]));
    }

    if (rrl->exempt != NULL)
        dns_acl_detach(&rrl->exempt);

    DESTROYLOCK(&rrl->lock);

    while (!ISC_LIST_EMPTY(rrl->blocks)) {
        b = ISC_LIST_HEAD(rrl->blocks);
        ISC_LIST_UNLINK(rrl->blocks, b, link);
        isc_mem_put(rrl->mctx, b, b->size);
    }

    h = rrl->hash;
    if (h != NULL)
        isc_mem_put(rrl->mctx, h,
                sizeof(*h) + (h->length - 1) * sizeof(h->bins[0]));

    h = rrl->old_hash;
    if (h != NULL)
        isc_mem_put(rrl->mctx, h,
                sizeof(*h) + (h->length - 1) * sizeof(h->bins[0]));

    isc_mem_putanddetach(&rrl->mctx, rrl, sizeof(*rrl));
}

isc_result_t
dns_rrl_init(dns_rrl_t **rrlp, dns_view_t *view, int min_entries) {
    dns_rrl_t *rrl;
    isc_result_t result;

    *rrlp = NULL;

    rrl = isc_mem_get(view->mctx, sizeof(*rrl));
    if (rrl == NULL)
        return (ISC_R_NOMEMORY);
    memset(rrl, 0, sizeof(*rrl));
    isc_mem_attach(view->mctx, &rrl->mctx);
    result = isc_mutex_init(&rrl->lock);
    if (result != ISC_R_SUCCESS) {
        isc_mem_putanddetach(&rrl->mctx, rrl, sizeof(*rrl));
        return (result);
    }
    isc_stdtime_get(&rrl->ts_bases[0]);

    view->rrl = rrl;

    result = expand_entries(rrl, min_entries);
    if (result != ISC_R_SUCCESS) {
        dns_rrl_view_destroy(view);
        return (result);
    }
    result = expand_rrl_hash(rrl, 0);
    if (result != ISC_R_SUCCESS) {
        dns_rrl_view_destroy(view);
        return (result);
    }

    *rrlp = rrl;
    return (ISC_R_SUCCESS);
}