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Stop lookup_frec() returning old records.

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get_new_frec() garbage collects struct frec when they are
(by default) 40s old. This means that in an active dnsmasq
which is getting new queries, answers to old queries start to
be ignored soon after they hit 40s old. If there are no new
queries, get_new_frec() doesn't get called, and answers to old
queries will still be accepted for longer.

This patch stops lookup_frec() from returning old frecs which
are ready for GC. This helps avoid yet another variation
on the birthday attack, which is no particularly relevant,
given the need for an idle server.
This commit is contained in:
Simon Kelley
2025-08-21 14:17:01 +01:00
parent b52d1d2017
commit 06e2d479c9
1 changed files with 13 additions and 6 deletions
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19
src/forward.c
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@@ -17,7 +17,7 @@
#include "dnsmasq.h"
static struct frec *get_new_frec(time_t now, struct server *serv, int force);
static struct frec *lookup_frec(char *target, int class, int rrtype, int id, int flags, int flagmask);
static struct frec *lookup_frec(time_t now, char *target, int class, int rrtype, int id, int flags, int flagmask);
#ifdef HAVE_DNSSEC
static int tcp_key_recurse(time_t now, int status, struct dns_header *header, size_t n,
int class, char *name, char *keyname, struct server *server,
@@ -192,7 +192,7 @@ static void forward_query(int udpfd, union mysockaddr *udpaddr,
old_reply = 1;
fwd_flags = forward->flags;
}
else if (gotname && (forward = lookup_frec(daemon->namebuff, (int)rrclass, (int)rrtype, -1, fwd_flags,
else if (gotname && (forward = lookup_frec(now, daemon->namebuff, (int)rrclass, (int)rrtype, -1, fwd_flags,
FREC_CHECKING_DISABLED | FREC_AD_QUESTION | FREC_DO_QUESTION |
FREC_HAS_PHEADER | FREC_DNSKEY_QUERY | FREC_DS_QUERY | FREC_NO_CACHE)))
{
@@ -980,7 +980,7 @@ static void dnssec_validate(struct frec *forward, struct dns_header *header,
unsigned int flags = STAT_ISEQUAL(status, STAT_NEED_KEY) ? FREC_DNSKEY_QUERY : FREC_DS_QUERY;
struct frec *old;
if ((old = lookup_frec(daemon->keyname, forward->class, -1, -1, flags, flags)))
if ((old = lookup_frec(now, daemon->keyname, forward->class, -1, -1, flags, flags)))
{
/* This is tricky; it detects loops in the dependency
graph for DNSSEC validation, say validating A requires DS B
@@ -1170,7 +1170,7 @@ void reply_query(int fd, time_t now)
GETSHORT(rrtype, p);
GETSHORT(class, p);
if (!(forward = lookup_frec(daemon->namebuff, class, rrtype, ntohs(header->id), FREC_ANSWER, 0)))
if (!(forward = lookup_frec(now, daemon->namebuff, class, rrtype, ntohs(header->id), FREC_ANSWER, 0)))
return;
filter_servers(forward->sentto->arrayposn, F_SERVER, &first, &last);
@@ -3196,7 +3196,7 @@ static void query_full(time_t now, char *domain)
}
}
static struct frec *lookup_frec(char *target, int class, int rrtype, int id, int flags, int flagmask)
static struct frec *lookup_frec(time_t now, char *target, int class, int rrtype, int id, int flags, int flagmask)
{
struct frec *f;
struct dns_header *header;
@@ -3246,7 +3246,14 @@ static struct frec *lookup_frec(char *target, int class, int rrtype, int id, int
continue;
}
/* frecs older than this will get garbage-collected in
get_new_frec(), so don't return them here, so we have
consistent behaviour from an idle dnsmasq which
is not calling get_new_frec() often. */
if (difftime(now, f->time) >= 4*TIMEOUT)
return NULL;
return f;
}