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#1715 DNS cache: document dns_cache_negative_ttl #874

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1 change: 1 addition & 0 deletions content/momentum/4/config-options-summary.md
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Expand Up @@ -127,6 +127,7 @@ The `Version` column indicated the version(s) of Momentum that support the optio
| [disable_chunked](/momentum/mobile/mobile-reference/mobility-mm-7-listener) – Option used with aggregators who do not support chunked transfer-coding (Mobile Momentum) | both | false | 4.0 and later | http_listener, listen, pathway, pathway_group, peer |
| [disable_nagle_algorithm](/momentum/4/config/ref-disable-nagle-algorithm) – Disable nagle algorithm on sockets | both | false | 4.0 and later | control_listener, eccluster_listener, ecstream_listener, esmtp_listener, global, http_listener, listen, xmpp_listener |
| [disk_queue_drain_rate](/momentum/4/config/ref-disk-queue-drain-rate) – Control the rate at which messages are spooled in on start-up | both | 100 | 4.0 and later | global |
| [dns_cache_negative_ttl](/momentum/4/config/ref-dns-cache-negative-ttl) – How long the DNS response cache keeps a failed lookup | sending | 300 | 5.3.1 and later | global |
| [dns_cache_purge_interval](/momentum/4/config/ref-dns-cache-purge-interval) – How often the DNS response cache is scanned for stale entries | sending | 60 | 4.0 and later | global |
| [dns_expire_interval](/momentum/4/config/ref-dns-expire-interval) – How often to check for domains with expired DNS information | sending | 10 | 4.0 and later | global |
| [dns_failures_to_purge](/momentum/4/config/ref-dns-failures-to-purge) – Configure the maximum number of DNS lookups | sending | 10 | 4.0 and later | domain, global |
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1 change: 1 addition & 0 deletions content/momentum/4/config/index.md
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Expand Up @@ -62,6 +62,7 @@ description: "This chapter provides the definitions of the configuration options
| [delivery_response_timeout](/momentum/4/config/ref-delivery-response-timeout) | time to wait for a response to an outbound request |
| [disable_nagle_algorithm](/momentum/4/config/ref-disable-nagle-algorithm) | disable nagle algorithm on sockets |
| [disk_queue_drain_rate](/momentum/4/config/ref-disk-queue-drain-rate) | control the rate at which messages are spooled in on start-up |
| [dns_cache_negative_ttl](/momentum/4/config/ref-dns-cache-negative-ttl) | how long the DNS response cache keeps a failed lookup |
| [dns_cache_purge_interval](/momentum/4/config/ref-dns-cache-purge-interval) | how often the DNS response cache is scanned for stale entries |
| [dns_expire_interval](/momentum/4/config/ref-dns-expire-interval) | how often to check for domains with expired DNS information |
| [dns_failures_to_purge](/momentum/4/config/ref-dns-failures-to-purge) | configure the maximum number of DNS lookups |
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40 changes: 40 additions & 0 deletions content/momentum/4/config/ref-dns-cache-negative-ttl.md
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---
lastUpdated: "10/08/2026"
title: "dns_cache_negative_ttl"
description: "dns cache negative ttl how long the DNS response cache keeps a failed lookup dns cache negative ttl 300 How long in seconds the DNS response cache keeps a failed lookup whose answer has no records such as NXDOMAIN a server failure SERVFAIL or a timeout..."
---

<a name="conf.ref.dns_cache_negative_ttl"></a>
## Name

dns_cache_negative_ttl — how long the DNS response cache keeps a failed
lookup

## Synopsis

`dns_cache_negative_ttl = 300`

## Description

How long, in seconds, the DNS response cache keeps a failed lookup whose
answer has no records, such as NXDOMAIN, a server failure (SERVFAIL), or a
timeout. Lookups of that name and type get the cached failure until the next
purge after it expires (see
[dns_cache_purge_interval](/momentum/4/config/ref-dns-cache-purge-interval)).

A failed lookup is kept for `dns_cache_negative_ttl` or
[max_dns_ttl](/momentum/4/config/ref-max-dns-ttl) seconds, whichever is
lower. The value must not be negative.

The default value is `300` (seconds). RFC 2308 section 7.1 limits a cached
server failure to five minutes; a value above `300` keeps server failures
longer than that.

## Scope

`dns_cache_negative_ttl` is valid in the global scope.

## See Also

[max_dns_ttl](/momentum/4/config/ref-max-dns-ttl),
[dns_cache_purge_interval](/momentum/4/config/ref-dns-cache-purge-interval)
13 changes: 9 additions & 4 deletions content/momentum/4/config/ref-max-dns-ttl.md
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---
lastUpdated: "03/26/2020"
lastUpdated: "10/08/2026"
title: "max_dns_ttl"
description: "max dns ttl set the maximum DNS TTL max dns ttl 4294967295 max dns ttl and min dns ttl are optional overrides to the TTL values returned from DNS only as far as resolving SMTP destinations is concerned the DNS cache used by other parts of the system such as..."
description: "max dns ttl set the maximum DNS TTL max dns ttl 4294967295 max dns ttl and min dns ttl are optional overrides to the TTL values returned from DNS Setting a min dns ttl can be used to prevent excessive DNS lookups in the event that an SMTP destination you are delivering mail to has..."
---

<a name="conf.ref.max_dns_ttl"></a>
Expand All @@ -16,10 +16,14 @@ max_dns_ttl — set the maximum DNS TTL
<a name="idp25206016"></a>
## Description

`max_dns_ttl` and `min_dns_ttl` are optional overrides to the TTL values returned from DNS (only as far as resolving SMTP destinations is concerned; the DNS cache used by other parts of the system such as DKIM is not affected by these parameters). Setting a `min_dns_ttl` can be used to prevent excessive DNS lookups in the event that an SMTP destination you are delivering mail to has unreasonably short TTL values for its DNS records. Conversely, `max_dns_ttl` can be used to force lookups to happen more often if a destination domain has excessively long TTL values in its DNS records. Setting this option should only be necessary in exceptional circumstances.
`max_dns_ttl` and `min_dns_ttl` are optional overrides to the TTL values returned from DNS. Setting a `min_dns_ttl` can be used to prevent excessive DNS lookups in the event that an SMTP destination you are delivering mail to has unreasonably short TTL values for its DNS records. Conversely, `max_dns_ttl` can be used to force lookups to happen more often if a destination domain has excessively long TTL values in its DNS records. Setting this option should only be necessary in exceptional circumstances.

Default value is `4294967295`.

`max_dns_ttl` also limits how long the DNS response cache keeps a failed
lookup; see
[dns_cache_negative_ttl](/momentum/4/config/ref-dns-cache-negative-ttl).

<a name="idp25211152"></a>
## Scope

Expand All @@ -28,4 +32,5 @@ max_dns_ttl is valid in the global scope.
<a name="idp25212976"></a>
## See Also

[min_dns_ttl](/momentum/4/config/ref-min-dns-ttl)
[min_dns_ttl](/momentum/4/config/ref-min-dns-ttl),
[dns_cache_negative_ttl](/momentum/4/config/ref-dns-cache-negative-ttl)
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