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dns: preserve TTL values and enable systemd-resolved caching #14572

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dns: preserve TTL values and enable systemd-resolved caching #14572

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ab7c069
Fix DNS performance: preserve TTLs and enable systemd-resolved caching
syed-dawood Mar 28, 2026
d34f6f2
Merge branch 'master' into fix/dns-cache-ttl-preservation
syed-dawood Mar 31, 2026
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9 changes: 9 additions & 0 deletions src/linux/init/init.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -336,6 +336,15 @@ int GenerateSystemdUnits(int Argc, char** Argv)
// Mask NetworkManager-wait-online.service for the same reason, as it causes timeouts on distros using NetworkManager.
THROW_LAST_ERROR_IF(symlink("/dev/null", std::format("{}/NetworkManager-wait-online.service", installPath).c_str()) < 0);

// Enable DNS response caching in systemd-resolved. On some distros (notably Ubuntu) the default
// Cache=no-negative discards negative (NODATA/NXDOMAIN) responses, which causes repeated wire
// queries for unsupported record types (e.g. AAAA on IPv4-only networks). This override aligns
// all distros with the upstream systemd default of Cache=yes, ensuring systemd-resolved provides
// effective DNS caching inside the VM.
THROW_LAST_ERROR_IF(UtilMkdirPath("/run/systemd/resolved.conf.d", 0755) < 0);
constexpr auto* resolvedCacheConfig = "[Resolve]\nCache=yes\n";
THROW_LAST_ERROR_IF(WriteToFile("/run/systemd/resolved.conf.d/wsl-dns-cache.conf", resolvedCacheConfig) < 0);

// Only create the wslg unit if both enabled in wsl.conf, and if the wslg folder actually exists.
if (enableGuiApps && access("/mnt/wslg/runtime-dir", F_OK) == 0)
{
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8 changes: 7 additions & 1 deletion src/windows/common/DnsResolver.cpp
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Expand Up @@ -238,6 +238,11 @@ try
// Only unicast UDP & TCP queries are tunneled. Pass this flag to tell Windows DNS client to *not* resolve using multicast.
request.queryOptions |= DNS_QUERY_NO_MULTICAST;

// DnsQueryRaw bypasses the Windows DNS cache internally, which means all responses return with TTL values
// reset to their original values. Pass this flag so that the DNS client preserves the real (decremented) TTL
// values from cached upstream responses, enabling downstream caches (e.g. systemd-resolved) to use meaningful TTLs.
request.queryOptions |= DNS_QUERY_DONT_RESET_TTL_VALUES;

// In a DNS request from Linux there might be DNS records that Windows DNS client does not know how to parse.
// By default in this case Windows will fail the request. When the flag is enabled, Windows will extract the
// question from the DNS request and attempt to resolve it, ignoring the unknown records.
Expand All @@ -253,7 +258,8 @@ try
}

// Start the DNS request
// N.B. All DNS requests will bypass the Windows DNS cache
// N.B. DnsQueryRaw bypasses the Windows DNS cache; DNS_QUERY_DONT_RESET_TTL_VALUES is set above
// so that responses carry real TTL values for downstream caching (e.g. systemd-resolved).
const auto result = s_dnsQueryRaw.value()(&request, &localContext->m_cancelHandle);
if (result != DNS_REQUEST_PENDING)
{
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