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resolv: Implement strict-error stub resolver option (bug 27929)

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For now, do not enable this mode by default due to the potential
impact on compatibility with existing deployments.

Reviewed-by: DJ Delorie <dj@redhat.com>
This commit is contained in:
Florian Weimer 2024-07-24 12:06:47 +02:00
parent 95f61610f3
commit 765325951a
6 changed files with 84 additions and 36 deletions
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10
NEWS
View File

@ -15,6 +15,16 @@ Major new features:
the RES_OPTIONS=-no-aaaa environment variable performs AAAA DNS
queries when the glibc DNS stub resolver is used.
* The DNS stub resolver now supports the strict-error option. If
activated, getaddrinfo for the AF_UNSPEC address family (with dual
A/AAAA DNS lookups) attemps to obtain an A/AAAA response pair from
another DNS server if one of the responses indicates failure. Without
the strict-error option, getaddrinfo returns the A record data it has
obtained even if the AAAA query failed. The new strict error mode is
incompatible with some DNS environments which do not follow the RFCs,
which is why this mode is not enabled by default. A future version
of the library may turn it on by default, however.
Deprecated and removed features, and other changes affecting compatibility:
[Add deprecations, removals and changes affecting compatibility here]

1
resolv/res_init.c
View File

@ -695,6 +695,7 @@ res_setoptions (struct resolv_conf_parser *parser, const char *options)
{ STRnLEN ("use-vc"), RES_USEVC },
{ STRnLEN ("trust-ad"), RES_TRUSTAD },
{ STRnLEN ("no-aaaa"), RES_NOAAAA },
{ STRnLEN ("strict-error"), RES_STRICTERR },
};
#define noptions (sizeof (options) / sizeof (options[0]))
bool negate_option = *cp == '-';

21
resolv/res_send.c
View File

@ -1234,15 +1234,31 @@ send_dg(res_state statp,
if (thisansp_error) {
next_ns:
if (recvresp1 || (buf2 != NULL && recvresp2)) {
/* Outside of strict-error mode, use the first
response even if the second response is an
error. This allows parallel resolution to
succeed even if the recursive resolver
always answers with SERVFAIL for AAAA
queries (which still happens in practice
unfortunately).
In strict-error mode, always switch to the
next server and try to get a response from
there. */
if ((statp->options & RES_STRICTERR) == 0)
{
if (recvresp1 || (buf2 != NULL && recvresp2))
{
*resplen2 = 0;
return resplen;
}
if (buf2 != NULL && !single_request)
{
/* No data from the first reply. */
resplen = 0;
/* We are waiting for a possible second reply. */
/* We are waiting for a possible
second reply. */
if (matching_query == 1)
recvresp1 = 1;
else
@ -1250,6 +1266,7 @@ send_dg(res_state statp,
goto wait;
}
}
/* don't retry if called from dig */
if (!statp->pfcode)

1
resolv/resolv.h
View File

@ -133,6 +133,7 @@ struct res_sym {
#define RES_NORELOAD 0x02000000 /* No automatic configuration reload. */
#define RES_TRUSTAD 0x04000000 /* Request AD bit, keep it in responses. */
#define RES_NOAAAA 0x08000000 /* Suppress AAAA queries. */
#define RES_STRICTERR 0x10000000 /* Report more DNS errors as errors. */
#define RES_DEFAULT (RES_RECURSE|RES_DEFNAMES|RES_DNSRCH)

10
resolv/tst-resolv-res_init-skeleton.c
View File

@ -129,6 +129,7 @@ print_resp (FILE *fp, res_state resp)
print_option_flag (fp, &options, RES_NORELOAD, "no-reload");
print_option_flag (fp, &options, RES_TRUSTAD, "trust-ad");
print_option_flag (fp, &options, RES_NOAAAA, "no-aaaa");
print_option_flag (fp, &options, RES_STRICTERR, "strict-error");
fputc ('\n', fp);
if (options != 0)
fprintf (fp, "; error: unresolved option bits: 0x%x\n", options);
@ -741,6 +742,15 @@ struct test_case test_cases[] =
"nameserver 192.0.2.1\n"
"; nameserver[0]: [192.0.2.1]:53\n"
},
{.name = "strict-error flag",
.conf = "options strict-error\n"
"nameserver 192.0.2.1\n",
.expected = "options strict-error\n"
"search example.com\n"
"; search[0]: example.com\n"
"nameserver 192.0.2.1\n"
"; nameserver[0]: [192.0.2.1]:53\n"
},
{ NULL }
};

11
resolv/tst-resolv-semi-failure.c
View File

@ -67,6 +67,9 @@ response (const struct resolv_response_context *ctx,
resolv_response_close_record (b);
}
/* Set to 1 if strict error checking is enabled. */
static int do_strict_error;
static void
check_one (void)
{
@ -83,7 +86,10 @@ check_one (void)
struct addrinfo *ai;
int ret = getaddrinfo ("www.example", "80", &hints, &ai);
const char *expected;
if (ret == 0 && ai->ai_next != NULL)
/* In strict-error mode, a switch to the second name server
happens, and both responses are received, so a single
response is a bug. */
if (do_strict_error || (ret == 0 && ai->ai_next != NULL))
expected = ("address: STREAM/TCP 192.0.2.17 80\n"
"address: STREAM/TCP 2001:db8::1 80\n");
else
@ -99,6 +105,7 @@ check_one (void)
static int
do_test (void)
{
for (do_strict_error = 0; do_strict_error < 2; ++do_strict_error)
for (int do_single_lookup = 0; do_single_lookup < 2; ++do_single_lookup)
{
struct resolv_test *aux = resolv_test_start
@ -107,6 +114,8 @@ do_test (void)
.response_callback = response,
});
if (do_strict_error)
_res.options |= RES_STRICTERR;
if (do_single_lookup)
_res.options |= RES_SNGLKUP;