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Ivan
Asked: 2009-09-18 09:26:32 +0800 CST

Find out the DNS server for a DHCP connection in Linux

  • 772

I know they're defined in /etc/resolv.conf, but what if it's not there? And more specifically, how do you find the DNS server returned by DHCP?

In GNOME you can use the NetworkManager applet to see the primary DNS for any connection, so how would you do the same from the command line?

linux

10 Answers

  1. Best Answer

    Usually dhclient.leases file is located at /var/lib/dhcp3/dhclient.leases, type the following command:

    less /var/lib/dhcp3/dhclient.leases

    OR

    cat /var/lib/dhcp3/dhclient.leases

    OR

    You can just use grep command to get DHCP server address, enter:

    grep dhcp-server-identifier /var/lib/dhcp3/dhclient.leases

    OR

    dhclient eth0

    • 23

  2. I recently had this problem where my dhcpcd was misconfigured as was not setting DNS servers. I found out that I can query which nameservers are available by DHCP with the following command:

    sudo dhcpcd -o domain_name_servers -T

    The command will output a bunch of network connection information. Look for the line beginning with new_domain_name_servers.

    From here I was able to manually set the nameservers.

    • 9

  3. One more solution to query the DHCP server:

    sudo nmap --script broadcast-dhcp-discover

    The output tells you the name server offered by the DHCP server:

    Starting Nmap 7.60 ( https://nmap.org ) at 2019-12-12 23:50 UTC
Pre-scan script results:
| broadcast-dhcp-discover: 
|   Response 1 of 1: 
|     IP Offered: 144.123.211.44
|     DHCP Message Type: DHCPOFFER
|     Server Identifier: 144.123.99.200
|     IP Address Lease Time: 5m00s
|     Subnet Mask: 255.255.0.0
|     Router: 144.123.99.200
|     Domain Name Server: 144.123.99.200
|_    Domain Name: localdomain
    • 7

  4. I found my DHCP lease info at /var/lib/dhclient/dhclient-eth0.leases in case anyone can't find it at /var/lib/dhcp3/dhclient.leases

    • 3

  5. well, why not simply dig that?

    marcus@marcus ~ $ dig google.de

; <<>> DiG 9.4.3-P3 <<>> google.de
;; global options:  printcmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 27579
;; flags: qr rd ra; QUERY: 1, ANSWER: 3, AUTHORITY: 6, ADDITIONAL: 0

;; QUESTION SECTION:
;google.de.         IN  A

;; ANSWER SECTION:
google.de.      1785    IN  A   209.85.229.104
google.de.      1785    IN  A   216.239.59.104
google.de.      1785    IN  A   74.125.77.104

;; AUTHORITY SECTION:
de.         23431   IN  NS  L.DE.NET.
de.         23431   IN  NS  Z.NIC.de.
de.         23431   IN  NS  F.NIC.de.
de.         23431   IN  NS  C.DE.NET.
de.         23431   IN  NS  S.DE.NET.
de.         23431   IN  NS  A.NIC.de.

;; Query time: 1 msec
;; SERVER: 192.168.1.15#53(192.168.1.15)
;; WHEN: Thu Sep 17 19:33:39 2009
;; MSG SIZE  rcvd: 181

    look at the last lines: ;; SERVER: 192.168.1.15#53(192.168.1.15)

    • 2

  6. If there is nothing in /etc/resolv.conf, DNS resolution (that is, resolution using dns as defined in /etc/nsswitch.conf) will not work. This is what dig, host and the libc routines use to do DNS resolution. If there is nothing specified in this file, DNS resolution will not work (though host resolution may work via other means, if another host resolution method is configured, such as NIS (this is unlikely)).

    • 1

  7. this command show the DNS server on your net

    dig | grep SERVER: | awk -F# '{ print $1 }' | awk -F: '{ print $2 }'

    172.17.0.1

    • 1

  8. If you are using NetworkManager, to get ipv4 DNS servers for YourConnectionName connection, you can use something like

    nmcli -g IP4.DNS connection show YourConnectionName

    Use nmcli connection show YourConnectionName to get all possible options and values.

    • 1

  9. Read /etc/resolv.conf. That's all.

    man resolv.conf is useful, too.

    • 0

  10. In modern distributions it is looks like

    ps aux | grep dhcli | grep -v grep                                                    
root       367  0.0  0.0  16128  3076 ?        S    Apr30   0:00 /sbin/dhclient -d -q -sf /usr/lib/NetworkManager/nm-dhcp-helper -pf /var/run/dhclient-enp3s0.pid -lf /var/lib/NetworkManager/dhclient-324caec3-58f9-3f00-bff4-2f8bd7d2aafa-enp3s0.lease -cf /var/lib/NetworkManager/dhclient-enp3s0.conf enp3s0


tail -16 /var/lib/NetworkManager/dhclient-324caec3-58f9-3f00-bff4-2f8bd7d2aafa-enp3s0.lease
lease {
  interface "enp3s0";
  fixed-address 10.191.104.3;
  option subnet-mask 255.255.255.0;
  option routers 10.191.104.254;
  option dhcp-lease-time 86400;
  option dhcp-message-type 5;
  option domain-name-servers 91.193.220.3,91.193.220.7;
  option dhcp-server-identifier 91.193.220.3;
  option ntp-servers 91.193.220.6;
  option broadcast-address 10.191.104.255;
  option domain-name "myISP.ru";
  renew 1 2017/05/01 12:56:02;
  rebind 2 2017/05/02 00:27:11;
  expire 2 2017/05/02 03:27:11;
}

    where 91.193.220.7 is my ISP's DNS server

    nslookup serverfault.com 91.193.220.7
Server:     91.193.220.7
Address:    91.193.220.7#53

Non-authoritative answer:
Name:   serverfault.com
Address: 151.101.1.69
Name:   serverfault.com
Address: 151.101.65.69
Name:   serverfault.com
Address: 151.101.193.69
Name:   serverfault.com
Address: 151.101.129.69
    • 0