NSCD DNS cache changes caused by hosts not effective in time

Add a record in the/etc/hosts file, found that the ping host name after the resolution does not take effect, that is, the result of the ping is not the same as the IP written in the hosts.


To view the ping command execution process through Strace:
Strace-f-f-o/tmp/ping.txt Ping bi.v.addev.com

You can see that the entire ping process is first connected to the local NSCD cache, which cleans up the NSCD cache (Nscd-i hosts) and discovers that the hosts bound IP immediately takes effect.

View NSCD profile:/etc/nscd.conf learned that NSCD will cache three services passwd the group hosts, so it will record three libraries, corresponding to the source/etc/passwd,/etc/hosts and/etc/, respectively Resolv.conf Each library holds two copies of the cache, one for the record, and one for the record. Each cache is saved with a time-to-Live (TTL). Its role is to increase the cache in this, speed up such as DNS resolution and so on.

Enable-cache hosts Yes
The local hosts cache is turned on

       #       logfile                  /var/log/nscd.log#        threads                  4#       max-threads              32         server-user             nscd#        stat-user                somebody        debug-level              0#        reload-count            5         paranoia                no#        restart-interval        3600        enable-cache             passwd          yes         positive-time-to-live   passwd           600        negative-time-to-live    passwd          20         suggested-size          passwd           211        check-files              passwd           yes        persistent               passwd           yes        shared                   passwd           yes        max-db-size              passwd           33554432        auto-propagate           passwd          yes         enable-cache            group            yes         positive-time-to-live   group            3600        negative-time-to-live   group            60         suggested-size          group            211        check-files              group            yes        persistent               group           yes         shared                   group            yes        max-db-size              group            33554432        auto-propagate           group           yes         enable-cache             hosts           yes         positive-time-to-live   hosts            3600        negative-time-to-live   hosts            20         suggested-size          hosts            211        check-files              hosts            yes        persistent               hosts            yes        shared                   hosts            yes        max-db-size              hosts            33554432        enable-cache             services         yes        positive-time-to-live   services         28800         negative-time-to-live   services        20         suggested-size          services         211        check-files              services         yes        persistent               services        yes         shared                   services        yes         max-db-size              services        33554432

Description

Server-user User

If this option is set, NSCD will run as the user, not as root. This option is ignored if each user uses a separate cache (-s parameter).

Enable-cache Service <yes|no>

Enables or disables the established service cache.

Positive-time-to-live Service Value

Sets the TTL (Time-to-live) for items in the specified cache for the service (successful requests). Value is in seconds. A larger value increases the cache hit ratio to reduce the average response time, but increases the consistency of the cache.

Negative-time-to-live Service Value

Sets the TTL (Time to live) of items (failed requests) that the service has negative in the specified cache. Value is in seconds. If there are files owned by a UID (user ID) that is not in the system database (for example, when the Linux kernel source code is unpacked as root), the performance will be significantly improved, and a smaller value should be maintained to reduce the cache consistency problem.

Suggested-size Service Value

This is the size of the internal hash table, and value should maintain a prime number to achieve the optimization effect.

Check-files Service <yes|no>

Enables or disables checking for changes to files belonging to the specified service. These files are/etc/passwd,/etc/group, and/etc/hosts.

Clear Cache

Nscd-i passwd

Nscd-i Group

Nscd-i hosts

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NSCD DNS cache changes caused by hosts not effective in time