libunbound: setting a deadline/timeout for ub_resolve

Unbound
1

Hello,

I'm using libunbound to perform iterative DNS lookups for a diagnostic service: github.com/letsdebug/letsdebug .

One of the problems I have is when one or more of a domain's authoritative nameservers are timing out, ub_resolve takes an incredibly long time to give up, something like ~10 minutes when the ub_ctx is configured with these options:

    https://github.com/letsdebug/letsdebug/blob/e56f10a5741445645c44a6d6683b9f085f657999/dns_util.go#L84-L116

I've tried to grok the meaning of https://www.unbound.net/documentation/info_timeout.html but I'm not seeing the solution after some weeks of pondering the problem..

Ideally I would like to place a deadline of something like 180 seconds on the ub_resolve operation. I am using a fresh ub_ctx for each lookup, which is done to avoid sharing any resolver state between lookups.

Do you have any further guidance on the ability to restrict ub_resolve's runtime?

Thanks!
Alex

2

Hi Alex,

Hello,

I'm using libunbound to perform iterative DNS lookups for a diagnostic service: github.com/letsdebug/letsdebug .

One of the problems I have is when one or more of a domain's authoritative nameservers are timing out, ub_resolve takes an incredibly long time to give up, something like ~10 minutes when the ub_ctx is configured with these options:

If a timeout is needed, set a timeout. So the code calling libunbound
sets the timeout it wants. And then cancels or ignores the return value
of the lookup.

For example, use ub_resolve_async(), and then ub_fd(ctx) put that file
descriptor in select or poll, with a timeout that you want to select or
poll. So, sketching code that would look like this:
ub_resolve_async(ctx, name, type, class, 0, funcwhendone, &async_id);
struct pollfd fds[1];
fds[0].fd = ub_fd(ctx);
fds[0].events = POLLIN;
fds[0].revents = 0;
int r = poll(fds, 1, 180 * 1000);
if(r == 0) /* timeout */
else if(r == -1) /* poll has a system error */
else ub_process(ctx); /* calls funcwhendone */

At timeout you could call ub_cancel(ctx, async_id) or just ignore the
result when funcwhendone gets called (eventually). Or delete the
context, you say you only use it once, this also removes that callback.

Best regards, Wouter

3

Thanks Wouter, that very helpful.