How to make persistent changes to the /etc/resolv.conf

Solution Verified - Updated

Environment

  • Red Hat Enterprise Linux 5, 6, 7, 8, 9, 10
  • initscripts
  • NetworkManager
  • DNS

Issue

  • DNS servers in /etc/resolv.conf change after a reboot or network service restart
  • resolv.conf gets overwritten
  • Why do entries in /etc/resolv.conf get cleared after a system reboot?
  • How to make permanent changes to the /etc/resolv.conf
  • Reboot removes or changes entries in /etc/resolv.conf

Resolution

In general, NetworkManager is managing /etc/resolv.conf. On RHEL7, issues with initscripts prevented this. If you are on RHEL7, please ensure

  • To update initscripts to 9.49.41-1.el7 (released with RHBA-2018:0983) or later to ensure you are not affected. These fixes are part of RHEL7.5GA and later.
  • To update NetworkManager to 1.8.0-9.el7 (released with RHSA-2017:2299) or later to ensure you are not affected. These fixes are part of RHEL7.4GA and later.

Depending on the desired outcome, following is possible:

  • Goal: use DNS servers obtained from the DHCP server:
    • RHEL will by default (so unmodified RHEL deployments) overwrite /etc/resolv.confas soon as any network interfaces use DHCP. No further action required.
  • Goal: use DNS servers obtained from the DHCP server, and additional DNS servers:

    • For this, "BOOTPROTO=dhcp" should be used in the ifcfg-interface file, and additionally DNS1=<dns-server-ip> should be configured in the interfaces ifcfg-interface file. With this, both the DHCP-DNS servers, and the DNS1-server will be written to /etc/resolv.conf. The DOMAIN option can be used to configure search domains, for example:

      ```
DOMAIN="domain1 domain2 domain3"
```
  • Goal: do not overwrite DNS servers in /etc/resolv.conf:

    • If resolv.conf should not be overwritten with the DNS servers obtained via DHCP, then the DHCP interfaces should have PEERDNS=no set in their ifcfg file, for example:

      ```
$ cat /etc/sysconfig/network-scripts/ifcfg-eth1
TYPE=Ethernet
DEVICE=eth1
BOOTPROTO=dhcp
PEERDNS=no
```

    • On RHEL 9 and later, /etc/sysconfig/network-scripts/ifcfg-* file has been deprecated. Instead, you can use /etc/NetworkManager/system-connections/<Connection_Name>.nmconnection file, for example:

      ```
[ipv4]
dns=192.168.122.1;
dns-search=example.com;
ignore-auto-dns=true
method=auto
```

      Or from the command line, you can run:

      ```
# nmcli con modify <Connection_Name> ipv4.ignore-auto-dns true ipv4.dns-search "example.com" ipv4.dns "192.168.122.1"
```

      Note the combination of method=auto and ignore-auto-dns=true ignores automatically configured name servers and search domains, and only name servers and search domains specified in the "dns" and "dns-search" properties are used. Thus, this will not prevent overwriting of resolv.conf file itself. You still need to specify dns servers and search domains via nmcli command line or in <Connection_Name>.nmconnection file, not directly editing resolv.conf file.

    • As an alternative, if NetworkManager is used, it can be configured to not update /etc/resolv.conf in /etc/NetworkManager/NetworkManager.conf or /etc/NetworkManager/conf.d/90-dns-none.conf:

      ```
[main]
dns=none
```

      After the change, NetworkManager has to be restarted using systemctl restart NetworkManager.
      This is particularly important to avoid the issue described in Solution 6975705.

    • As a further alternative (or workaround), you could also modify /etc/resolv.conf as desired and then make the file immutable to changes (details are in the chattr(1) manpage). After making the file immutable "+i", further modifications will be impossible before reversing that operation with "-i". Please be aware that this is a common source for problems, for example when multiple persons administrate a system, a first person makes the file immutable and other persons are not aware of the chattr command.

      ```
# chattr +i /etc/resolv.conf
```

In environments using cloud-init where dhcp needs to update resolv.conf but custom DNS options are required

follow How to make /etc/resolv.conf changes persistent in a cloud environment

Root Cause

RHEL will by default (so without further configuration/modification) overwrite /etc/resolv.confas soon as any network interfaces use DHCP. Depending on the exact version if initscripts/NetworkManager, "RESOLV_MODS=no" or "PEERDNS=no" in the relevant /etc/sysconfig/network-scripts/ifcfg-* files can prevent this.

Diagnostic Steps

  • Messages from the /var/log/messages:
Oct 14 12:40:52 hostname NET[22961]: /etc/sysconfig/network-scripts/ifdown-post : updated /etc/resolv.conf
Oct 14 12:40:57 hostname NET[23256]: /etc/sysconfig/network-scripts/ifup-post : updated /etc/resolv.conf
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