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resolved: properly signal transient errors back to NSS stack

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NSS mostly knows four error cases: SUCCESS, NOTFOUND, UNAVAIL, TRYAGAIN,
and they can all be used in nsswitch.conf to route requests.

So far nss-resolve would return SUCCESS + NOTFOUND + UNAVAIL. Let's also
return TRYAGAIN in some cases, specifically the ones where we are
currntly unable to resolve a request but likely could later. i.e.
errors caused by networking issues or such.

Fixes: #20786
This commit is contained in:
Lennart Poettering
2021-11-22 15:17:34 +01:00
parent 5a78106ad9
commit ae78529fc7
1 changed files with 46 additions and 9 deletions
Download Patch File Download Diff File Expand all files Collapse all files

55
src/nss-resolve/nss-resolve.c
View File

@ -41,6 +41,9 @@ NSS_GETHOSTBYNAME_PROTOTYPES(resolve);
NSS_GETHOSTBYADDR_PROTOTYPES(resolve);
static bool error_shall_fallback(const char *error_id) {
/* The Varlink errors where we shall signal "please fallback" back to the NSS stack, so that some
* fallback module can be loaded. (These are mostly all Varlink-internal errors, as apparently we
* then were unable to even do IPC with systemd-resolved.) */
return STR_IN_SET(error_id,
VARLINK_ERROR_DISCONNECTED,
VARLINK_ERROR_TIMEOUT,
@ -50,6 +53,16 @@ static bool error_shall_fallback(const char *error_id) {
VARLINK_ERROR_METHOD_NOT_IMPLEMENTED);
}
static bool error_shall_try_again(const char *error_id) {
/* The Varlink errors where we shall signal "can't answer now but might be able to later" back to the
* NSS stack. These are all errors that indicate lack of configuration or network problems. */
return STR_IN_SET(error_id,
"io.systemd.Resolve.NoNameServers",
"io.systemd.Resolve.QueryTimedOut",
"io.systemd.Resolve.MaxAttemptsReached",
"io.systemd.Resolve.NetworkDown");
}
static int connect_to_resolved(Varlink **ret) {
_cleanup_(varlink_unrefp) Varlink *link = NULL;
int r;
@ -242,9 +255,11 @@ enum nss_status _nss_resolve_gethostbyname4_r(
if (r < 0)
goto fail;
if (!isempty(error_id)) {
if (!error_shall_fallback(error_id))
goto not_found;
goto fail;
if (error_shall_try_again(error_id))
goto try_again;
if (error_shall_fallback(error_id))
goto fail;
goto not_found;
}
r = json_dispatch(rparams, resolve_hostname_reply_dispatch_table, NULL, json_dispatch_flags, &p);
@ -341,6 +356,12 @@ fail:
not_found:
*h_errnop = HOST_NOT_FOUND;
return NSS_STATUS_NOTFOUND;
try_again:
UNPROTECT_ERRNO;
*errnop = -r;
*h_errnop = TRY_AGAIN;
return NSS_STATUS_TRYAGAIN;
}
enum nss_status _nss_resolve_gethostbyname3_r(
@ -390,9 +411,11 @@ enum nss_status _nss_resolve_gethostbyname3_r(
if (r < 0)
goto fail;
if (!isempty(error_id)) {
if (!error_shall_fallback(error_id))
goto not_found;
goto fail;
if (error_shall_try_again(error_id))
goto try_again;
if (error_shall_fallback(error_id))
goto fail;
goto not_found;
}
r = json_dispatch(rparams, resolve_hostname_reply_dispatch_table, NULL, json_dispatch_flags, &p);
@ -508,6 +531,12 @@ fail:
not_found:
*h_errnop = HOST_NOT_FOUND;
return NSS_STATUS_NOTFOUND;
try_again:
UNPROTECT_ERRNO;
*errnop = -r;
*h_errnop = TRY_AGAIN;
return NSS_STATUS_TRYAGAIN;
}
typedef struct ResolveAddressReply {
@ -594,9 +623,11 @@ enum nss_status _nss_resolve_gethostbyaddr2_r(
if (r < 0)
goto fail;
if (!isempty(error_id)) {
if (!error_shall_fallback(error_id))
goto not_found;
goto fail;
if (error_shall_try_again(error_id))
goto try_again;
if (error_shall_fallback(error_id))
goto fail;
goto not_found;
}
r = json_dispatch(rparams, resolve_address_reply_dispatch_table, NULL, json_dispatch_flags, &p);
@ -694,6 +725,12 @@ fail:
not_found:
*h_errnop = HOST_NOT_FOUND;
return NSS_STATUS_NOTFOUND;
try_again:
UNPROTECT_ERRNO;
*errnop = -r;
*h_errnop = TRY_AGAIN;
return NSS_STATUS_TRYAGAIN;
}
NSS_GETHOSTBYNAME_FALLBACKS(resolve);