Commit a83c8cb0afd88d54b9cf67239f2495c9b0391e97 updated masterdump so
that stale records in "rndc dumpdb" output no longer shows 0 TTLs. In
this commit we change the name of the `rdataset->stale_ttl` field to
`rdataset->expired` to make its purpose clearer, and set it to zero in
cases where it's unused.
Add 'rbtdb->serve_stale_ttl' to various checks so that stale records
are not purged from the cache when they've been stale for RBTDB_VIRTUAL
(300) seconds.
Increment 'ns_statscounter_usedstale' when a stale answer is used.
Note: There was a question of whether 'overmem_purge' should be
purging ancient records, instead of stale ones. It is left as purging
stale records, since stale records could take up the majority of the
cache.
This submission is copyrighted Akamai Technologies, Inc. and provided
under an MPL 2.0 license.
This commit was originally authored by Kevin Chen, and was updated by
Matthijs Mekking to match recent serve-stale developments.
Once we resume a query, we should clear DNS_FETCHOPT_TRYSTALE_ONTIMEOUT
from the options to prevent triggering the stale-answer-client-timeout
on subsequent fetches.
If we don't this may cause a crash when for example when prefetch is
triggered after a query restart.
when a serve-stale answer has been sent, the client continues waiting
for a proper answer. if a final completion event for the client does
arrive, it can just be cleaned up without sending a response, similar
to a canceled fetch.
configuring with --enable-mutex-atomics flagged these incorrectly
initialised variables on systems where pthread_mutex_init doesn't
just zero out the structure.
The size of the array holding the pointers to clientmgr was created so
big it could hold the actual clientmgr objects, not just the pointer.
This commit fixes the size to be just the ncpus * sizeof(pointer).
The isc_nmiface_t type was holding just a single isc_sockaddr_t,
so we got rid of the datatype and use plain isc_sockaddr_t in place
where isc_nmiface_t was used before. This means less type-casting and
shorter path to access isc_sockaddr_t members.
At the same time, instead of keeping the reference to the isc_sockaddr_t
that was passed to us when we start listening, we will keep a local
copy. This prevents the data race on destruction of the ns_interface_t
objects where pending nmsockets could reference the sockaddr of already
destroyed ns_interface_t object.
Previously, as a way of reducing the contention between threads a
clientmgr object would be created for each interface/IP address.
We tasks being more strictly bound to netmgr workers, this is no longer
needed and we can just create clientmgr object per worker queue (ncpus).
Each clientmgr object than would have a single task and single memory
context.
Since a client object is bound to a netmgr handle, each client
will always be processed by the same netmgr worker, so we can
simplify the code by binding client->task to the same thread as
the client. Since ns__client_request() now runs in the same event
loop as client->task events, is no longer necessary to pause the
task manager before launching them.
Also removed some functions in isc_task that were not used.
The number of memory contexts created in the clientmgr was enormous. It
could easily create thousands of memory contexts because the formula was:
nprotocols * ncpus * ninterfaces * CLIENT_NMCTXS_PERCPU (8)
The original goal was to reduce the contention when allocating the
memory, but after a while nobody noticed that the amount of memory
context allocated would not reduce contention at all.
This commit removes the whole mctxpool and just uses the mctx from
clientmgr as the contention will be reduced directly in the allocator.
dns_message_gettempname() returns an initialized name with a dedicated
buffer, associated with a dns_fixedname object. Using dns_name_copynf()
to write a name into this object will actually copy the name data
from a source name. dns_name_clone() merely points target->ndata to
source->ndata, so it is faster, but it can lead to a use-after-free if
the source is freed before the target object is released via
dns_message_puttempname().
In a few places, clone was being used where copynf should have been;
this is now fixed.
As a side note, no memory was lost, because the ndata buffer used in
the dns_fixedname_t is internal to the structure, and is freed when
the dns_fixedname_t is freed regardless of the .ndata contents.
dns_message_gettempname() now returns a pointer to an initialized
name associated with a dns_fixedname_t object. it is no longer
necessary to allocate a buffer for temporary names associated with
the message object.
Network manager events that require interlock (pause, resume, listen)
are now always executed in the same worker thread, mgr->workers[0],
to prevent races.
"stoplistening" events no longer require interlock.
With taskmgr running on top of netmgr, the ordering of how the tasks and
netmgr shutdown interacts was wrong as previously isc_taskmgr_destroy()
was waiting until all tasks were properly shutdown and detached. This
responsibility was moved to netmgr, so we now need to do the following:
1. shutdown all the tasks - this schedules all shutdown events onto
the netmgr queue
2. shutdown the netmgr - this also makes sure all the tasks and
events are properly executed
3. Shutdown the taskmgr - this now waits for all the tasks to finish
running before returning
4. Shutdown the netmgr - this call waits for all the netmgr netievents
to finish before returning
This solves the race when the taskmgr object would be destroyed before
all the tasks were finished running in the netmgr loops.
Previously, netmgr, taskmgr, timermgr and socketmgr all had their own
isc_<*>mgr_create() and isc_<*>mgr_destroy() functions. The new
isc_managers_create() and isc_managers_destroy() fold all four into a
single function and makes sure the objects are created and destroy in
correct order.
Especially now, when taskmgr runs on top of netmgr, the correct order is
important and when the code was duplicated at many places it's easy to
make mistake.
The former isc_<*>mgr_create() and isc_<*>mgr_destroy() functions were
made private and a single call to isc_managers_create() and
isc_managers_destroy() is required at the program startup / shutdown.
When looking for key files, we could use isdigit rather than checking
if the character is within the range [0-9].
Use (unsigned char) cast to ensure the value is representable in the
unsigned char type (as suggested by the isdigit manpage).
Change " & 0xff" occurrences to the recommended (unsigned char) type
cast.
When answering a query, named should never attempt to add the same RRset
to the ANSWER section more than once. However, such a situation may
arise when chasing DNAME records: one of the DNAME records placed in the
ANSWER section may turn out to be the final answer to a client query,
but there is no way to know that in advance. Tweak the relevant INSIST
assertion in query_respond() so that it handles this case properly.
qctx->rdataset is freed later anyway, so there is no need to clean it up
in query_respond().
While working on the serve-stale backports, I noticed the following
oddities:
1. In the serve-stale system test, in one case we keep track of the
time how long it took for dig to complete. In commit
aaed7f9d8c2465790d769221dfe8378c7147f5eb, the code removed the
exception to check for result == ISC_R_SUCCESS on stale found
answers, and adjusted the test accordingly. This failed to update
the time tracking accordingly. Move the t1/t2 time track variables
back around the two dig commands to ensure the lookups resolved
faster than the resolver-query-timeout.
2. We can remove the setting of NS_QUERYATTR_STALEOK and
DNS_RDATASETATTR_STALE_ADDED on the "else if (stale_timeout)"
code path, because they are added later when we know we have
actually found a stale answer on a stale timeout lookup.
3. We should clear the NS_QUERYATTR_STALEOK flag from the client
query attributes instead of DNS_RDATASETATTR_STALE_ADDED (that
flag is set on the rdataset attributes).
4. In 'bin/named/config.c' we should set the configuration options
in alpabetical order.
5. In the ARM, in the backports we have added "(stale)" between
"cached" and "RRset" to make more clear a stale RRset may be
returned in this scenario.
This commit changes the taskmgr to run the individual tasks on the
netmgr internal workers. While an effort has been put into keeping the
taskmgr interface intact, couple of changes have been made:
* The taskmgr has no concept of universal privileged mode - rather the
tasks are either privileged or unprivileged (normal). The privileged
tasks are run as a first thing when the netmgr is unpaused. There
are now four different queues in in the netmgr:
1. priority queue - netievent on the priority queue are run even when
the taskmgr enter exclusive mode and netmgr is paused. This is
needed to properly start listening on the interfaces, free
resources and resume.
2. privileged task queue - only privileged tasks are queued here and
this is the first queue that gets processed when network manager
is unpaused using isc_nm_resume(). All netmgr workers need to
clean the privileged task queue before they all proceed normal
operation. Both task queues are processed when the workers are
finished.
3. task queue - only (traditional) task are scheduled here and this
queue along with privileged task queues are process when the
netmgr workers are finishing. This is needed to process the task
shutdown events.
4. normal queue - this is the queue with netmgr events, e.g. reading,
sending, callbacks and pretty much everything is processed here.
* The isc_taskmgr_create() now requires initialized netmgr (isc_nm_t)
object.
* The isc_nm_destroy() function now waits for indefinite time, but it
will print out the active objects when in tracing mode
(-DNETMGR_TRACE=1 and -DNETMGR_TRACE_VERBOSE=1), the netmgr has been
made a little bit more asynchronous and it might take longer time to
shutdown all the active networking connections.
* Previously, the isc_nm_stoplistening() was a synchronous operation.
This has been changed and the isc_nm_stoplistening() just schedules
the child sockets to stop listening and exits. This was needed to
prevent a deadlock as the the (traditional) tasks are now executed on
the netmgr threads.
* The socket selection logic in isc__nm_udp_send() was flawed, but
fortunatelly, it was broken, so we never hit the problem where we
created uvreq_t on a socket from nmhandle_t, but then a different
socket could be picked up and then we were trying to run the send
callback on a socket that had different threadid than currently
running.
When we are recursing, RPZ processing is not allowed. But when we are
performing a lookup due to "stale-answer-client-timeout", we are still
recursing. This effectively means that RPZ processing is disabled on
such a lookup.
In this case, bail the "stale-answer-client-timeout" lookup and wait
for recursion to complete, as we we can't perform the RPZ rewrite
rules reliably.
The dboption DNS_DBFIND_STALEONLY caused confusion because it implies
we are looking for stale data **only** and ignore any active RRsets in
the cache. Rename it to DNS_DBFIND_STALETIMEOUT as it is more clear
the option is related to a lookup due to "stale-answer-client-timeout".
Rename other usages of "staleonly", instead use "lookup due to...".
Also rename related function and variable names.
When doing a staleonly lookup we don't want to fallback to recursion.
After all, there are obviously problems with recursion, otherwise we
wouldn't do a staleonly lookup.
When resuming from recursion however, we should restore the
RECURSIONOK flag, allowing future required lookups for this client
to recurse.
When implementing "stale-answer-client-timeout", we decided that
we should only return positive answers prematurely to clients. A
negative response is not useful, and in that case it is better to
wait for the recursion to complete.
To do so, we check the result and if it is not ISC_R_SUCCESS, we
decide that it is not good enough. However, there are more return
codes that could lead to a positive answer (e.g. CNAME chains).
This commit removes the exception and now uses the same logic that
other stale lookups use to determine if we found a useful stale
answer (stale_found == true).
This means we can simplify two test cases in the serve-stale system
test: nodata.example is no longer treated differently than data.example.
The NS_QUERYATTR_ANSWERED attribute is to prevent sending a response
twice. Without the attribute, this may happen if a staleonly lookup
found a useful answer and sends a response to the client, and later
recursion ends and also tries to send a response.
The attribute was also used to mask adding a duplicate RRset. This is
considered harmful. When we created a response to the client with a
stale only lookup (regardless if we actually have send the response),
we should clear the rdatasets that were added during that lookup.
Mark such rdatasets with the a new attribute,
DNS_RDATASETATTR_STALE_ADDED. Set a query attribute
NS_QUERYATTR_STALEOK if we may have added rdatasets during a stale
only lookup. Before creating a response on a normal lookup, check if
we can expect rdatasets to have been added during a staleonly lookup.
If so, clear the rdatasets from the message with the attribute
DNS_RDATASETATTR_STALE_ADDED set.
With stale-answer-client-timeout, we may send a response to the client,
but we may want to hold on to the network manager handle, because
recursion is going on in the background, or we need to refresh a
stale RRset.
Simplify the setting of 'nodetach':
* During a staleonly lookup we should not detach the nmhandle, so just
set it prior to 'query_lookup()'.
* During a staleonly "stalefirst" lookup set the 'nodetach' to true
if we are going to refresh the RRset.
Now there is no longer the need to clear the 'nodetach' if we go
through the "dbfind_stale", "stale_refresh_window", or "stale_only"
paths.
When doing a staleonly lookup, ignore active RRsets from cache. If we
don't, we may add a duplicate RRset to the message, and hit an
assertion failure in query.c because adding the duplicate RRset to the
ANSWER section failed.
This can happen on a race condition. When a client query is received,
the recursion is started. When 'stale-answer-client-timeout' triggers
around the same time the recursion completes, the following sequence
of events may happen:
1. Queue the "try stale" fetch_callback() event to the client task.
2. Add the RRsets from the authoritative response to the cache.
3. Queue the "fetch complete" fetch_callback() event to the client task.
4. Execute the "try stale" fetch_callback(), which retrieves the
just-inserted RRset from the database.
5. In "ns_query_done()" we are still recursing, but the "staleonly"
query attribute has already been cleared. In other words, the
query will resume when recursion ends (it already has ended but is
still on the task queue).
6. Execute the "fetch complete" fetch_callback(). It finds the answer
from recursion in the cache again and tries to add the duplicate to
the answer section.
This commit changes the logic for finding stale answers in the cache,
such that on "stale_only" lookups actually only stale RRsets are
considered. It refactors the code so that code paths for "dbfind_stale",
"stale_refresh_window", and "stale_only" are more clear.
First we call some generic code that applies in all three cases,
formatting the domain name for logging purposes, increment the
trystale stats, and check if we actually found stale data that we can
use.
The "dbfind_stale" lookup will return SERVFAIL if we didn't found a
usable answer, otherwise we will continue with the lookup
(query_gotanswer()). This is no different as before the introduction of
"stale-answer-client-timeout" and "stale-refresh-time".
The "stale_refresh_window" lookup is similar to the "dbfind_stale"
lookup: return SERVFAIL if we didn't found a usable answer, otherwise
continue with the lookup (query_gotanswer()).
Finally the "stale_only" lookup.
If the "stale_only" lookup was triggered because of an actual client
timeout (stale-answer-client-timeout > 0), and if database lookup
returned a stale usable RRset, trigger a response to the client.
Otherwise return and wait until the recursion completes (or the
resolver query times out).
If the "stale_only" lookup is a "stale-anwer-client-timeout 0" lookup,
preferring stale data over a lookup. In this case if there was no stale
data, or the data was not a positive answer, retry the lookup with the
stale options cleared, a.k.a. a normal lookup. Otherwise, continue
with the lookup (query_gotanswer()) and refresh the stale RRset. This
will trigger a response to the client, but will not detach the handle
because a fetch will be created to refresh the RRset.
The stale-answer-client-timeout feature introduced a dependancy on
when a client may be detached from the handle. The dboption
DNS_DBFIND_STALEONLY was reused to track this attribute. This overloads
the meaning of this database option, and actually introduced a bug
because the option was checked in other places. In particular, in
'ns_query_done()' there is a check for 'RECURSING(qctx->client) &&
(!QUERY_STALEONLY(&qctx->client->query) || ...' and the condition is
satisfied because recursion has not completed yet and
DNS_DBFIND_STALEONLY is already cleared by that time (in
query_lookup()), because we found a useful answer and we should detach
the client from the handle after sending the response.
Add a new boolean to the client structure to keep track of client
detach from handle is allowed or not. It is only disallowed if we are
in a staleonly lookup and we didn't found a useful answer.
The RFC7828 specifies the keepalive interval to be 16-bit, specified in
units of 100 milliseconds and the configuration options tcp-*-timeouts
are following the suit. The units of 100 milliseconds are very
unintuitive and while we can't change the configuration and presentation
format, we should not follow this weird unit in the API.
This commit changes the isc_nm_(get|set)timeouts() functions to work
with milliseconds and convert the values to milliseconds before passing
them to the function, not just internally.
When we query the resolver for a domain name that is in the same zone
for which is already one or more fetches outstanding, we could
potentially hit the fetch limits. If so, recursion fails immediately
for the incoming query and if serve-stale is enabled, we may try to
return a stale answer.
If the resolver is also is authoritative for the parent zone (for
example the root zone), first a delegation is found, but we first
check the cache for a better response.
Nothing is found in the cache, so we try to recurse to find the
answer to the query.
Because of fetch-limits 'dns_resolver_createfetch()' returns an error,
which 'ns_query_recurse()' propagates to the caller,
'query_delegation_recurse()'.
Because serve-stale is enabled, 'query_usestale()' is called,
setting 'qctx->db' to the cache db, but leaving 'qctx->version'
untouched. Now 'query_lookup()' is called to search for stale data
in the cache database with a non-NULL 'qctx->version'
(which is set to a zone db version), and thus we hit an assertion
in rbtdb.
This crash was introduced in 'main' by commit
8bcd7fe69e5343071fc917738d6092a8b974ef3f.
- style, cleanup, and removal of unnecessary code.
- combined isc_nm_http_add_endpoint() and isc_nm_http_add_doh_endpoint()
into one function, renamed isc_http_endpoint().
- moved isc_nm_http_connect_send_request() into doh_test.c as a helper
function; remove it from the public API.
- renamed isc_http2 and isc_nm_http2 types and functions to just isc_http
and isc_nm_http, for consistency with other existing names.
- shortened a number of long names.
- the caller is now responsible for determining the peer address.
in isc_nm_httpconnect(); this eliminates the need to parse the URI
and the dependency on an external resolver.
- the caller is also now responsible for creating the SSL client context,
for consistency with isc_nm_tlsdnsconnect().
- added setter functions for HTTP/2 ALPN. instead of setting up ALPN in
isc_tlsctx_createclient(), we now have a function
isc_tlsctx_enable_http2client_alpn() that can be run from
isc_nm_httpconnect().
- refactored isc_nm_httprequest() into separate read and send functions.
isc_nm_send() or isc_nm_read() is called on an http socket, it will
be stored until a corresponding isc_nm_read() or _send() arrives; when
we have both halves of the pair the HTTP request will be initiated.
- isc_nm_httprequest() is renamed isc__nm_http_request() for use as an
internal helper function by the DoH unit test. (eventually doh_test
should be rewritten to use read and send, and this function should
be removed.)
- added implementations of isc__nm_tls_settimeout() and
isc__nm_http_settimeout().
- increased NGHTTP2 header block length for client connections to 128K.
- use isc_mem_t for internal memory allocations inside nghttp2, to
help track memory leaks.
- send "Cache-Control" header in requests and responses. (note:
currently we try to bypass HTTP caching proxies, but ideally we should
interact with them: https://tools.ietf.org/html/rfc8484#section-5.1)
The pthread_self(), thrd_current() or GetCurrentThreadId() could
actually be a pointer, so we should rather convert the value into
uintptr_t instead of unsigned long.
When a staleonly lookup doesn't find a satisfying answer, it should
not try to respond to the client.
This is not true when the initial lookup is staleonly (that is when
'stale-answer-client-timeout' is set to 0), because no resolver fetch
has been created at this point. In this case continue with the lookup
normally.
Fix a crash that can happen in the following scenario:
A client request is received. There is no data for it in the cache,
(not even stale data). A resolver fetch is created as part of
recursion.
Some time later, the fetch still hasn't completed, and
stale-answer-client-timeout is triggered. A staleonly lookup is
started. It will also find no data in the cache.
So 'query_lookup()' will call 'query_gotanswer()' with ISC_R_NOTFOUND,
so this will call 'query_notfound()' and this will start recursion.
We will eventually end up in 'ns_query_recurse()' and that requires
the client query fetch to be NULL:
REQUIRE(client->query.fetch == NULL);
If the previously started fetch is still running this assertion
fails.
The crash is easily prevented by not requiring recursion for
staleonly lookups.
Also remove a redundant setting of the staleonly flag at the end of
'query_lookup_staleonly()' before destroying the query context.
Add a system test to catch this case.
On 24-core machine, the tests would crash because we would run out of
the hazard pointers. We now adjust the number of hazard pointers to be
in the <128,256> interval based on the number of available cores.
Note: This is just a band-aid and needs a proper fix.
The 'query_usestale()' function was only called when in
'query_gotanswer()' and an unexpected error occurred. This may have
been "quota reached", and thus we were in some cases returning
stale data on fetch-limits (and if serve-stale enabled of course).
But we can also hit fetch-limits when recursing because we are
following a referral (in 'query_notfound()' and
'query_delegation_recurse()'). Here we should also check for using
stale data in case an error occurred.
Specifically don't check for using stale data when refetching a
zero TTL RRset from cache.
Move the setting of DNS_DBFIND_STALESTART into the 'query_usestale()'
function to avoid code duplication.
This commit completes the support for DNS-over-HTTP(S) built on top of
nghttp2 and plugs it into the BIND. Support for both GET and POST
requests is present, as required by RFC8484.
Both encrypted (via TLS) and unencrypted HTTP/2 connections are
supported. The latter are mostly there for debugging/troubleshooting
purposes and for the means of encryption offloading to third-party
software (as might be desirable in some environments to simplify TLS
certificates management).
If we did not attempt a fetch due to fetch-limits, we should not start
the stale-refresh-time window.
Introduce a new flag DNS_DBFIND_STALESTART to differentiate between
a resolver failure and unexpected error. If we are resuming, this
indicates a resolver failure, then start the stale-refresh-time window,
otherwise don't start the stale-refresh-time window, but still fall
back to using stale data.
(This commit also wraps some docstrings to 80 characters width)
Before this change, BIND will only fallback to using stale data if
there was an actual attempt to resolve the query. Then on a timeout,
the stale data from cache becomes eligible.
This commit changes this so that on any unexpected error stale data
becomes eligble (you would still have to have 'stale-answer-enable'
enabled of course).
If there is no stale data, this may return in an error again, so don't
loop on stale data lookup attempts. If the DNS_DBFIND_STALEOK flag is
set, this means we already tried to lookup stale data, so if that is
the case, don't use stale again.
First of all, there was a flaw in the code related to the
'stale-refresh-time' option. If stale answers are enabled, and we
returned stale data, then it was assumed that it was because we were
in the 'stale-refresh-time' window. But now we could also have returned
stale data because of a 'stale-answer-client-timeout'. To fix this,
introduce a rdataset attribute DNS_RDATASETATTR_STALE_WINDOW to
indicate whether the stale cache entry was returned because the
'stale-refresh-time' window is active.
Second, remove the special case handling when the result is
DNS_R_NCACHENXRRSET. This can be done more generic in the code block
when dealing with stale data.
Putting all stale case handling in the code block when dealing with
stale data makes the code more easy to follow.
Update documentation to be more verbose and to match then new code
flow.
Both functions employed the same code lines to allocate query context
buffers, which are used to store query results, so this shared portion
of code was extracted out to a new function, qctx_prepare_buffers.
Also, this commit uses qctx_init to initialize the query context whitin
query_refresh_rrset function.
This commit allows stale RRset to be used (if available) for responding
a query, before an attempt to refresh an expired, or otherwise resolve
an unavailable RRset in cache is made.
For that to work, a value of zero must be specified for
stale-answer-client-timeout statement.
To better understand the logic implemented, there are three flags being
used during database lookup and other parts of code that must be
understood:
. DNS_DBFIND_STALEOK: This flag is set when BIND fails to refresh a
RRset due to timeout (resolver-query-timeout), its intent is to
try to look for stale data in cache as a fallback, but only if
stale answers are enabled in configuration.
This flag is also used to activate stale-refresh-time window, since it
is the only way the database knows that a resolution has failed.
. DNS_DBFIND_STALEENABLED: This flag is used as a hint to the database
that it may use stale data. It is always set during query lookup if
stale answers are enabled, but only effectively used during
stale-refresh-time window. Also during this window, the resolver will
not try to resolve the query, in other words no attempt to refresh the
data in cache is made when the stale-refresh-time window is active.
. DNS_DBFIND_STALEONLY: This new introduced flag is used when we want
stale data from the database, but not due to a failure in resolution,
it also doesn't require stale-refresh-time window timer to be active.
As long as there is a stale RRset available, it should be returned.
It is mainly used in two situations:
1. When stale-answer-client-timeout timer is triggered: in that case
we want to know if there is stale data available to answer the
client.
2. When stale-answer-client-timeout value is set to zero: in that
case, we also want to know if there is some stale RRset available
to promptly answer the client.
We must also discern between three situations that may happen when
resolving a query after the addition of stale-answer-client-timeout
statement, and how to handle them:
1. Are we running query_lookup() due to stale-answer-client-timeout
timer being triggered?
In this case, we look for stale data, making use of
DNS_DBFIND_STALEONLY flag. If a stale RRset is available then
respond the client with the data found, mark this query as
answered (query attribute NS_QUERYATTR_ANSWERED), so when the
fetch completes the client won't be answered twice.
We must also take care of not detaching from the client, as a
fetch will still be running in background, this is handled by the
following snippet:
if (!QUERY_STALEONLY(&client->query)) {
isc_nmhandle_detach(&client->reqhandle);
}
Which basically tests if DNS_DBFIND_STALEONLY flag is set, which
means we are here due to a stale-answer-client-timeout timer
expiration.
2. Are we running query_lookup() due to resolver-query-timeout being
triggered?
In this case, DNS_DBFIND_STALEOK flag will be set and an attempt
to look for stale data will be made.
As already explained, this flag is algo used to activate
stale-refresh-time window, as it means that we failed to refresh
a RRset due to timeout.
It is ok in this situation to detach from the client, as the
fetch is already completed.
3. Are we running query_lookup() during the first time, looking for
a RRset in cache and stale-answer-client-timeout value is set to
zero?
In this case, if stale answers are enabled (probably), we must do
an initial database lookup with DNS_DBFIND_STALEONLY flag set, to
indicate to the database that we want stale data.
If we find an active RRset, proceed as normal, answer the client
and the query is done.
If we find a stale RRset we respond to the client and mark the
query as answered, but don't detach from the client yet as an
attempt in refreshing the RRset will still be made by means of
the new introduced function 'query_resolve'.
If no active or stale RRset is available, begin resolution as
usual.
The general logic behind the addition of this new feature works as
folows:
When a client query arrives, the basic path (query.c / ns_query_recurse)
was to create a fetch, waiting for completion in fetch_callback.
With the introduction of stale-answer-client-timeout, a new event of
type DNS_EVENT_TRYSTALE may invoke fetch_callback, whenever stale
answers are enabled and the fetch took longer than
stale-answer-client-timeout to complete.
When an event of type DNS_EVENT_TRYSTALE triggers fetch_callback, we
must ensure that the folowing happens:
1. Setup a new query context with the sole purpose of looking up for
stale RRset only data, for that matters a new flag was added
'DNS_DBFIND_STALEONLY' used in database lookups.
. If a stale RRset is found, mark the original client query as
answered (with a new query attribute named NS_QUERYATTR_ANSWERED),
so when the fetch completion event is received later, we avoid
answering the client twice.
. If a stale RRset is not found, cleanup and wait for the normal
fetch completion event.
2. In ns_query_done, we must change this part:
/*
* If we're recursing then just return; the query will
* resume when recursion ends.
*/
if (RECURSING(qctx->client)) {
return (qctx->result);
}
To this:
if (RECURSING(qctx->client) && !QUERY_STALEONLY(qctx->client)) {
return (qctx->result);
}
Otherwise we would not proceed to answer the client if it happened
that a stale answer was found when looking up for stale only data.
When an event of type DNS_EVENT_FETCHDONE triggers fetch_callback, we
proceed as before, resuming query, updating stats, etc, but a few
exceptions had to be added, most important of which are two:
1. Before answering the client (ns_client_send), check if the query
wasn't already answered before.
2. Before detaching a client, e.g.
isc_nmhandle_detach(&client->reqhandle), ensure that this is the
fetch completion event, and not the one triggered due to
stale-answer-client-timeout, so a correct call would be:
if (!QUERY_STALEONLY(client)) {
isc_nmhandle_detach(&client->reqhandle);
}
Other than these notes, comments were added in code in attempt to make
these updates easier to follow.
