When running a local caching DNS server (like BIND9), you might encounter situations where authoritative servers set extremely short TTLs (like Google's common 300-second TTL). This forces your cache to refresh frequently, defeating the purpose of local caching.

BIND9 provides a powerful feature called Response Policy Zones (RPZ) that lets you modify DNS responses, including TTL values:

options {
    response-policy {
        zone "ttloverride";
    };
};

zone "ttloverride" {
    type master;
    file "/etc/bind/ttloverride.db";
    allow-query { none; };
};

Create /etc/bind/ttloverride.db with content like:

$TTL 900 ; 15 minutes
www.l.google.com. CNAME .   ; Force 900s TTL for all CNAME records
*.l.google.com. A .         ; Force 900s TTL for all A records

For simpler setups, dnsmasq offers easier TTL overriding:

# In dnsmasq.conf
min-cache-ttl=900
max-cache-ttl=900

After implementing either solution, verify with dig:

dig @localhost www.google.com +ttlunits

You should now see your modified TTL values in the ANSWER SECTION.

While increasing TTL improves cache hit rates, be aware of:

• Potential stale records during IP changes
• Violating some services' terms (though TTL modification is generally acceptable)
• Memory usage with very large TTL values

When operating a caching DNS server, the Time-To-Live (TTL) values received from authoritative servers can sometimes conflict with your network optimization goals. In the example you provided, Google's servers return a TTL of 300 seconds (5 minutes) for www.l.google.com, but you might want to extend this to 900 seconds (15 minutes) to reduce query frequency.

The most elegant solution is using BIND9's response-policy zone (RPZ) capability, introduced in version 9.8. This allows you to modify responses before they're cached:

options {
    response-policy {
        zone "rpz.local";
    };
};

zone "rpz.local" {
    type primary;
    file "/etc/bind/rpz.local";
    allow-query { none; };
};

Then create /etc/bind/rpz.local with content like:

$TTL 1h
@ SOA localhost. root.localhost. (1 1h 15m 30d 2h)
  NS localhost.

; Override TTL for specific domains
www.l.google.com CNAME .   ; Force default TTL of 1 hour
*.google.com     CNAME .   ; Wildcard override

For a simpler approach affecting all cached records:

options {
    min-cache-ttl 900;  # 15 minutes minimum TTL
    max-cache-ttl 86400; # 1 day maximum TTL
};

If you're open to alternative DNS servers, dnsmasq provides straightforward TTL overriding:

# In dnsmasq.conf
local-ttl=900
neg-ttl=900

When overriding TTLs, be mindful of:

• Increased cache size requirements
• Potential staleness of records during IP changes
• Violation of domain owner's caching preferences

For critical infrastructure, consider implementing TTL overriding only for specific domains rather than applying it globally. Monitor your cache hit ratio before and after changes to validate the optimization.