How to know the default value of maximum TTL for Dnsmasq

I’m able to set a maximum TTL value with Dnsmasq by --max-ttl. But I wonder what the default value is if I don’t set --max-ttl. Does anyone know about it?

The default value is 0.

The --max-ttl option sets up the max_ttl attribute as witnessed there:

https://thekelleys.org.uk/gitweb/?p=dnsmasq.git;a=blob;f=src/option.c;h=3a870970b82335dcfa59b5cf1ad0ce765d4484a3;hb=HEAD#l3061

3060         else if (option == LOPT_MAXTTL)
3061           daemon->max_ttl = (unsigned long)ttl;

It is defined like that, in src/dnsmasq.h:

extern struct daemon {
  /* datastuctures representing the command-line and
     config file arguments. All set (including defaults)
     in option.c */

[..]

  unsigned long local_ttl, neg_ttl, max_ttl, min_cache_ttl, max_cache_ttl, auth_ttl, dhcp_ttl, use_dhcp_ttl;

So its default value is 0 per the C standard.

Later, you can find tests on that value to trigger specific behavior in src/rfc1035.c:

  /* Return the Max TTL value if it is lower than the actual TTL */
  if (daemon->max_ttl == 0 || ((unsigned)(crecp->ttd - now) < daemon->max_ttl))
    return crecp->ttd - now;
  else
    return daemon->max_ttl;