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Posted to commits@nuttx.apache.org by "duduita (via GitHub)" <gi...@apache.org> on 2024/07/11 14:11:12 UTC
[I] DNS issues on NTP client [nuttx-apps]
duduita opened a new issue, #2440:
URL: https://github.com/apache/nuttx-apps/issues/2440
During NTP server querying through the `ntpclient.c`, different NTP server domain names (e.g., 0.uk.pool.ntp.org, 1.uk.pool.ntp.org) might resolve to the same set of IP addresses due to **DNS caching**. This can lead to **repeated queries to the same non-responsive IP addresses**, resulting in failures to obtain the correct time.
For example, in the following, there are some logs that I added to `ntpclient.c`, in order to understand why the NTP was failing:
```
[ 51.046000] [25] (Info) '0.pool.ntp.org' resolved to: 216.238.113.58
[ 51.046000] [25] (Info) ntpclient.c-447-gethostbyname for 0.pool.ntp.org OK
[ 51.046000] [25] (Info) ntpclient.c-480-Sending a NTP packet
[ 51.055000] [25] (Info) ntpclient.c-509-sendto ret: 68
[ 51.056000] [25] (Info) ntpclient.c-515-Recv a NTP packet
[ 56.055000] [25] (Info) ntpclient.c-521-recvfrom nbytes: -1
[ 56.056000] [25] (Info) '0.pool.ntp.org' resolved to: 216.238.113.58
[ 56.056000] [25] (Info) ntpclient.c-447-gethostbyname for 0.pool.ntp.org OK
[ 56.056000] [25] (Info) ntpclient.c-480-Sending a NTP packet
[ 56.063000] [25] (Info) ntpclient.c-509-sendto ret: 68
[ 56.063000] [25] (Info) ntpclient.c-515-Recv a NTP packet
[ 61.065000] [25] (Info) ntpclient.c-521-recvfrom nbytes: -1
[ 61.066000] [25] (Info) '0.pool.ntp.org' resolved to: 216.238.113.58
[ 61.066000] [25] (Info) ntpclient.c-447-gethostbyname for 0.pool.ntp.org OK
[ 61.066000] [25] (Info) ntpclient.c-480-Sending a NTP packet
[ 61.075000] [25] (Info) ntpclient.c-509-sendto ret: 68
[ 61.075000] [25] (Info) ntpclient.c-515-Recv a NTP packet
[ 66.075000] [25] (Info) ntpclient.c-521-recvfrom nbytes: -1
[ 66.076000] [25] (Info) '0.pool.ntp.org' resolved to: 216.238.113.58
[ 66.076000] [25] (Info) ntpclient.c-447-gethostbyname for 0.pool.ntp.org OK
[ 66.076000] [25] (Info) ntpclient.c-480-Sending a NTP packet
[ 66.085000] [25] (Info) ntpclient.c-509-sendto ret: 68
[ 66.085000] [25] (Info) ntpclient.c-515-Recv a NTP packet
[ 71.085000] [25] (Info) ntpclient.c-521-recvfrom nbytes: -1
[ 71.086000] [25] (Info) '0.pool.ntp.org' resolved to: 216.238.113.58
[ 71.086000] [25] (Info) ntpclient.c-447-gethostbyname for 0.pool.ntp.org OK
[ 71.086000] [25] (Info) ntpclient.c-480-Sending a NTP packet
[ 71.095000] [25] (Info) ntpclient.c-509-sendto ret: 68
[ 71.095000] [25] (Info) ntpclient.c-515-Recv a NTP packet
[ 76.095000] [25] (Info) ntpclient.c-521-recvfrom nbytes: -1
[ 76.095000] [25] (Info) ntpclient.c-563-ERROR: recvfrom() failed: 11
[ 76.095000] [25] (Info) ntpclient.c-589-The NTP client is terminating
```
To mitigate this issue, a possible option is to **flush the DNS cache after cycling through all configured NTP servers**, ensuring that subsequent DNS resolutions provide potentially new and responsive IP addresses, thereby increasing the likelihood of successful time synchronization. However, I cannot manipulate the DNS cache from the user space, unless I create an API for it.
Overall, do you have a workaround or a hack that I can use in order to solve this NTP issue? Or at least to force a new IP resolution for an NTP hostname after some failures?
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Re: [I] DNS issues on NTP client [nuttx-apps]
Posted by "acassis (via GitHub)" <gi...@apache.org>.
acassis commented on issue #2440:
URL: https://github.com/apache/nuttx-apps/issues/2440#issuecomment-2237471021
@duduita thank you for finding and reporting this issue!
@wengzhe did you see that?
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Re: [I] DNS issues on NTP client [nuttx-apps]
Posted by "wengzhe (via GitHub)" <gi...@apache.org>.
wengzhe commented on issue #2440:
URL: https://github.com/apache/nuttx-apps/issues/2440#issuecomment-2241975101
Hi @duduita , since 12.1.0, the DNS caching will become invalid after the TTL from the DNS server has expired, then we'll send a new query for the domain names. I found the TTL is always less than 100s (and normally ~20s) for `0.pool.ntp.org` in my local environment. Maybe your DNS server gives you non-responsive IP addresses with a longer TTL which causes this problem.
There is a hack that may force resolving the domains: set `CONFIG_NETDB_DNSCLIENT_LIFESEC` to a shorter value, e.g. 5 seconds, then after 5sec the cache will become invalid and the domain will be resolved again if you do the lookup.
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