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DNS Caching Configuration

Last updated: 2022-05-20 18:49:24


    When the client initiates an access request to an address, it will query whether the local DNS cache has relevant records, and if so, it will directly access the corresponding IP; otherwise, it will delegate global query to the recursive server.

    As DNS resolution uses the UDP protocol for communication, it is greatly affected by the network environment and may have a delay of several seconds in extreme conditions. In SCF use cases, the domain resolution delay may cause function execution failures due to timeout and affect the normal business logic. When a function is invoked frequently, the DNS server's resolution frequency may exceed the limit, which also will cause function execution failures.

    SCF offers DNS caching configuration to solve the above problems. DNS caching can improve the domain resolution efficiency and increase the domain resolution success rate by mitigating network jitters.

    Use Cases

    This feature is applicable to scenarios where an address is requested in the function code and the function is invoked frequently.


    As code deployment-based event-triggered functions, HTTP-triggered functions, and image deployment-based functions have different implementation mechanisms, you should enable DNS caching in the corresponding steps:

    Code deployment-based event-triggered function

    1. Log in to the SCF console and select the target function to enter the function details page.
    2. On the function configuration page, click Edit in the top-right corner and select Enable DNS Caching.
    3. Click Save.

    HTTP-triggered function

    1. Add the following command to the HTTP-triggered function's bootstrap file scf_bootstrap to start the nscd process and enable DNS caching:

      /var/lang/bin/nscd -f /var/lang/conf/nscd.conf
    2. Deploy the updated scf_bootstrap and the function code together in the cloud. Then, DNS caching will be enabled for new invocations.

    Image deployment-based function

    1. Install nscd during image creation. Taking CentOS as an example, you can run the following command to install nscd:

      yum install nscd -y
    2. Update the default /etc/nscd.conf file with the following content:

      \# /etc/nscd.conf
      \# An example Name Service Cache config file. This file is needed by nscd.
      \# WARNING: Running nscd with a secondary caching service like sssd may lead to
      \# unexpected behaviour, especially with how long entries are cached.
      \# Legal entries are:
      \# logfile <file>
      \# debug-level <level>
      \# threads <initial #threads to use>
      \# max-threads <maximum #threads to use>
      \# server-user <user to run server as instead of root>
      \# server-user is ignored if nscd is started with -S parameters
      \# stat-user <user who is allowed to request statistics>
      \# reload-count unlimited|<number>
      \# paranoia <yes|no>
      \# restart-interval <time in seconds>
      \# enable-cache <service> <yes|no>
      \# positive-time-to-live <service> <time in seconds>
      \# negative-time-to-live <service> <time in seconds>
      \# suggested-size <service> <prime number>
      \# check-files <service> <yes|no>
      \# persistent <service> <yes|no>
      \# shared <service> <yes|no>
      \# NOTE: Setting 'shared' to a value of 'yes' will accelerate the lookup,
      \# but those lookups will not be counted as cache hits
      \# i.e. 'nscd -g' may show '0%'.
      \# max-db-size <service> <number bytes>
      \# auto-propagate <service> <yes|no>
      \# Currently supported cache names (services): passwd, group, hosts, services
      \# logfile /var/log/nscd.log
      \# threads 4
      \# max-threads 32
      server-user root
      \# stat-user somebody
      debug-level 0
      reload-count 2
      paranoia no
      \# restart-interval 3600
      enable-cache passwd no
      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 no
      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 300
      negative-time-to-live hosts 0
      suggested-size hosts 211
      check-files hosts no
      persistent hosts no
      shared hosts yes
      max-db-size hosts 8388608
      enable-cache services no
      positive-time-to-live services 600
      negative-time-to-live services 3
      suggested-size services 211
      check-files services yes
      persistent services yes
      shared services yes
      max-db-size services 33554432
      enable-cache netgroup no
      positive-time-to-live n etgroup 28800
      negative-time-to-live netgroup 20
      suggested-size netgroup 211
      check-files netgroup yes
      persistent netgroup yes
      shared netgroup yes
      max-db-size netgroup 33554432
    3. Add the following command to the bootstrap file scf_bootstrap to start nscd process and enable DNS caching.
      Taking CentOS as an example, add the following command to the bootstrap file:

      ${PATH}/nscd -f /etc/nscd.conf 

    ${PATH} is the absolute path where nscd is installed.

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