dnsmasq separates forwarded cache entries from configured local names

Sat, 15 Apr 2028 00:00:00 +0000

dnsmasq can forward DNS questions upstream, retain supported answers in a bounded cache, and serve names configured locally. These are different sources of data. A repeated answer may be a cache hit or a local configuration result. Establishing which source supplied it is essential when a hostname disagrees with the authority or continues resolving after a cache flush.

When it breaks

Starting dnsmasq does not automatically make every application use it. Clients must direct their DNS queries through the intended listener, and some applications use other resolvers. Likewise, a correct local listener can forward to the wrong upstream or read unexpected host configuration. Diagnose the actual query path before enlarging the cache or restarting a daemon that never received the failing query.

A local name can deliberately override upstream data. Clearing the forwarding cache does not delete that configured name, so repeated flushing cannot make the upstream address win. Treat a local record as configuration with an owner and purpose. An answer with local TTL zero can still be returned repeatedly because dnsmasq generates it from configuration rather than retaining a forwarded response.

Cache capacity and freshness solve different problems. A larger cache can retain more eligible entries, but it does not extend their valid TTLs or make every possible query answerable during an upstream outage. New names and expired entries still need the appropriate resolution path. Cache-size tuning also does not directly specify the maximum number of simultaneous upstream queries or reserve application-level availability.

Check it quickly

Save the script as check-dnsmasq.py and run python3 check-dnsmasq.py with Docker. The pinned environment uses dnsmasq 2.90 and BIND 9.18.49. The test forwarders ignore ambient resolver and hosts files, bind explicit loopback listeners, and forward only to the disposable authority. They publish no host ports and use no DHCP configuration:

from pathlib import Path
from tempfile import TemporaryDirectory
import json,subprocess,uuid
recipe='FROM python@sha256:528257d48c1da0dcecc2e725d1ae34498d60c965f1241e39cd6a85a8859bdf84\nRUN apt-get update && apt-get install -y --no-install-recommends bind9=1:9.18.49-1~deb12u2 bind9-dnsutils=1:9.18.49-1~deb12u2 dnsmasq-base=2.90-4~deb12u2 python3-dnspython=2.3.0-1 && rm -rf /var/lib/apt/lists/*\nCMD ["/usr/bin/python3"]\n'
program=r"""
from contextlib import ExitStack,contextmanager
from pathlib import Path
from tempfile import TemporaryDirectory
import json,re,signal,subprocess,time
import dns.exception,dns.message,dns.query,dns.rcode,dns.rdatatype

def run(*args,check=True):
 result=subprocess.run(args,capture_output=True,text=True,timeout=15)
 if check:assert result.returncode==0,(args,result.returncode,result.stdout,result.stderr)
 return result
def query(name,kind='A',port=1053):
 return dns.query.udp(dns.message.make_query(name,kind),'127.0.0.1',port=port,timeout=1)
def values(response,kind='A'):
 wanted=dns.rdatatype.from_text(kind)
 return [item.to_text() for rrset in response.answer if rrset.rdtype==wanted for item in rrset]
def show(response):
 def section(records):return [{'name':str(rr.name),'type':dns.rdatatype.to_text(rr.rdtype),'ttl':rr.ttl,'data':[item.to_text() for item in rr]} for rr in records]
 return {'rcode':dns.rcode.to_text(response.rcode()),'answer':section(response.answer),'authority':section(response.authority)}
def wait(predicate,timeout=15):
 deadline=time.monotonic()+timeout
 while time.monotonic()<deadline:
 try:
 if predicate():return
 except (OSError,dns.exception.Timeout):pass
 time.sleep(.05)
 raise AssertionError('DNS fixture condition did not become true')
def stop(process):
 if process.poll() is None:
 process.terminate()
 try:process.wait(timeout=5)
 except subprocess.TimeoutExpired:process.kill();process.wait(timeout=5)

@contextmanager
def lab(records,soa_ttl=8,minimum=4,extra=''):
 with TemporaryDirectory(prefix='xbcatm-dns-') as directory,ExitStack() as stack:
 root=Path(directory);zone=root/'zone';conf=root/'named.conf';log=root/'named.log';serial=1
 def save(body):
 zone.write_text(f'$ORIGIN lab.test.\n$TTL 30\n@ {soa_ttl} IN SOA ns.lab.test. hostmaster.lab.test. ({serial} 60 60 3600 {minimum})\n@ IN NS ns.lab.test.\nns IN A 127.0.0.1\n'+body+'\n')
 run('named-checkzone','lab.test',str(zone))
 save(records)
 conf.write_text(f'''options {{ directory "{root}"; listen-on port 1053 {{127.0.0.1;}}; listen-on-v6 {{none;}};
recursion no; dnssec-validation no; notify no; querylog yes; max-cache-size 8m; pid-file "{root}/named.pid"; session-keyfile none;
{extra} }};
controls {{}};
zone "lab.test" {{ type primary; file "{zone}"; }};
''')
 run('named-checkconf',str(conf))
 output=stack.enter_context(log.open('w'))
 authority=subprocess.Popen(['named','-g','-c',str(conf),'-n','1','-U','1'],stdout=output,stderr=subprocess.STDOUT)
 stack.callback(stop,authority)
 def ready():
 assert authority.poll() is None,log.read_text()
 response=query('lab.test.','SOA');return response.rcode()==0 and response.answer[0][0].serial==serial
 wait(ready)
 def update(body):
 nonlocal serial
 serial+=1;save(body);authority.send_signal(signal.SIGHUP);wait(ready)
 def cache(port=1054,upstream=1053,size=150,options=()):
 logfile=root/('cache-'+str(port)+'.log');stream=stack.enter_context(logfile.open('w'))
 command=['dnsmasq','--no-daemon','--conf-file=/dev/null','--no-resolv','--no-hosts','--bind-interfaces',
 '--listen-address=127.0.0.1','--port='+str(port),'--server=127.0.0.1#'+str(upstream),
 '--cache-size='+str(size),'--log-queries','--log-facility=-',*options]
 process=subprocess.Popen(command,stdout=stream,stderr=subprocess.STDOUT);stack.callback(stop,process)
 def cache_ready():
 assert process.poll() is None,logfile.read_text()
 return query('lab.test.','SOA',port).rcode()==0
 wait(cache_ready);return process,logfile
 def queries(name,kind='A'):
 return sum(bool(re.search(r'query: '+re.escape(name.rstrip('.'))+r' IN '+kind+r'\b',line)) for line in log.read_text().splitlines())
 yield root,update,cache,queries
with lab('cached 20 IN A 127.0.0.2\nuncached 20 IN A 127.0.0.3\nlocal 20 IN A 127.0.0.4') as (_,update,cache,queries):
 daemon,log=cache(options=('--host-record=local.lab.test,127.0.0.9',))
 for _ in range(3):assert values(query('cached.lab.test.',port=1054))==['127.0.0.2']
 wait(lambda: queries('cached.lab.test')==1)
 cache(port=1055,size=0)
 for _ in range(3):assert values(query('uncached.lab.test.',port=1055))==['127.0.0.3']
 wait(lambda: queries('uncached.lab.test')==3)
 local=query('local.lab.test.',port=1054);assert values(local)==['127.0.0.9'] and local.answer[0].ttl==0
 assert queries('local.lab.test')==0
 authoritative=query('local.lab.test.');assert values(authoritative)==['127.0.0.4']
 wait(lambda: queries('local.lab.test')==1)
 update('cached 20 IN A 127.0.0.8\nuncached 20 IN A 127.0.0.3\nlocal 20 IN A 127.0.0.4')
 stale=query('cached.lab.test.',port=1054);assert values(stale)==['127.0.0.2'] and queries('cached.lab.test')==1
 previous=log.read_text().count('cleared cache');daemon.send_signal(signal.SIGHUP)
 wait(lambda: log.read_text().count('cleared cache')>previous)
 refreshed=query('cached.lab.test.',port=1054);assert values(refreshed)==['127.0.0.8']
 wait(lambda: queries('cached.lab.test')==2)
 retained=query('local.lab.test.',port=1054);assert values(retained)==['127.0.0.9']
 print(json.dumps({'three_lookups_upstream_counts':{'cache_enabled':1,'cache_disabled':3},'local_record':show(local),
 'same_name_authority':show(authoritative),'before_SIGHUP':show(stale),'after_SIGHUP':show(refreshed),
 'configured_local_record_retained':show(retained)},sort_keys=True))
"""
name='xbcatm-dns-'+uuid.uuid4().hex[:10]
def docker(*args,**kwargs):return subprocess.run(['docker',*args],capture_output=True,text=True,timeout=300,**kwargs)
try:
 with TemporaryDirectory(prefix=name+'-') as directory:
 root=Path(directory);(root/'Dockerfile').write_text(recipe)
 built=docker('build','-t',name,str(root));assert built.returncode==0,built.stderr
 result=docker('run','--rm','-i','--name',name,'--network','none','--memory','384m','--pids-limit','128',name,
 '/usr/bin/python3','-',input=program)
 assert result.returncode==0,result.stderr
 print(json.dumps(json.loads(result.stdout),sort_keys=True))
finally:
 docker('rm','-f',name);docker('image','rm',name)

Three queries for cached.lab.test through the cache-enabled listener all return 127.0.0.2, while the authoritative log records one query. A second listener uses cache-size=0. Three queries for a separate name through it generate three upstream queries. This directly verifies the cache’s effect on forwarding work; the experiment does not infer hits from response speed or assert a particular latency improvement.

The first listener also has host-record=local.lab.test,127.0.0.9. Its answer is 127.0.0.9 with TTL zero, and no upstream query is logged for that lookup. A direct authoritative query for the same name returns 127.0.0.4 and increments the authority’s counter. The differing results show an explicit local override, with a positive control confirming that the query counter detects real upstream traffic.

Next, the authority changes cached.lab.test to 127.0.0.8. The cache still returns its earlier 127.0.0.2 answer within the remaining TTL. The script sends SIGHUP to this specific dnsmasq process and waits for its cache-cleared log entry. The following query reaches the authority and returns 127.0.0.8, increasing the corresponding authoritative count from one to two.

After that same signal, local.lab.test continues returning the configured 127.0.0.9 value. The signal clears forwarded cache state while leaving the explicitly configured local record available. This is why “flush DNS” is incomplete troubleshooting advice: the expected result depends on which process owns which data, and whether that data is cached, configured, or stored somewhere else entirely.

Operate the listener and its data sources deliberately

Inspect the effective configuration, listener address, upstream destinations, and client settings together. Avoid creating a forwarding loop between two local resolvers. The example removes ambient inputs to isolate behavior; a real installation may intentionally use hosts files, DHCP leases, or resolver-management software. Record those dependencies so an automatic rewrite or a local override is visible during troubleshooting.

For dnsmasq, SIGHUP clears its cache and reloads certain data sources described by the manual, but it does not generally reread the main configuration file. Use the documented service procedure for the kind of change being made. A successful cache clear cannot be treated as proof that edited listener, forwarding, or other startup configuration has been activated.

Limit listener exposure to intended clients, especially when forwarding recursion on a reachable network. DHCP, DNSSEC validation, and encrypted upstream transport require their own configuration and verification; none is demonstrated by this cache experiment. Query logging is useful during diagnosis but can reveal client naming activity, so select its scope and retention according to the environment rather than leaving detailed logs indefinitely by habit.

Finally, compare cached answers with the authority when it is the expected source. For a configured local name, compare against its configuration. Track upstream failures, hits, and entry churn separately. Identify whether the discrepancy concerns capacity, lifetime, the upstream service, or a local naming rule.

Sources

dnsmasq manual, forwarding, cache-size, host-record, local TTL, and SIGHUP behavior, checked 2026-09-12.
RFC 1035 domain names, DNS resource records and cached lifetimes, checked 2026-09-12.

Dnsmasq · XB Field Notes

dnsmasq separates forwarded cache entries from configured local names

When it breaks

Check it quickly

Operate the listener and its data sources deliberately

Sources