Resolve domain name linux
NAME
systemd-resolve - Resolve domain names, IPV4 and IPv6 addresses, DNS resource records, and services
SYNOPSIS
systemd-resolve [OPTIONS. ] HOSTNAME. systemd-resolve [OPTIONS. ] ADDRESS. systemd-resolve [OPTIONS. ] --type=TYPE RRDOMAIN. systemd-resolve [OPTIONS. ] --service [[NAME] TYPE] DOMAIN systemd-resolve [OPTIONS. ] --statistics systemd-resolve [OPTIONS. ] --reset-statistics
DESCRIPTION
systemd-resolve may be used to resolve domain names, IPv4 and IPv6 addresses, DNS resource records and services with the systemd-resolved.service(8) resolver service. By default, the specified list of parameters will be resolved as hostnames, retrieving their IPv4 and IPv6 addresses. If the parameters specified are formatted as IPv4 or IPv6 operation the reverse operation is done, and a hostname is retrieved for the specified addresses. The --type= switch may be used to specify a DNS resource record type (A, AAAA, SOA, MX, . ) in order to request a specific DNS resource record, instead of the address or reverse address lookups. The special value "help" may be used to list known values. The --service switch may be used to resolve SRV[1] and DNS-SD[2] services (see below). In this mode, between one and three arguments are required. If three parameters are passed the first is assumed to be the DNS-SD service name, the second the SRV service type, and the third the domain to search in. In this case a full DNS-SD style SRV and TXT lookup is executed. If only two parameters are specified, the first is assumed to be the SRV service type, and the second the domain to look in. In this case no TXT RR is requested. Finally, if only one parameter is specified, it is assumed to be a domain name, that is already prefixed with an SRV type, and an SRV lookup is done (no TXT). The --statistics switch may be used to show resolver statistics, including information about the number of succesful and failed DNSSEC validations. The --reset-statistics may be used to reset various statistics counters maintained the resolver, including those shown in the --statistics output. This operation requires root privileges.
OPTIONS
-4, -6 By default, when resolving a hostname, both IPv4 and IPv6 addresses are acquired. By specifying -4 only IPv4 addresses are requested, by specifying -6 only IPv6 addresses are requested. -i INTERFACE, --interface=INTERFACE Specifies the network interface to execute the query on. This may either be specified as numeric interface index or as network interface string (e.g. "en0"). Note that this option has no effect if system-wide DNS configuration (as configured in /etc/resolv.conf or /etc/systemd/resolve.conf) in place of per-link configuration is used. -p PROTOCOL, --protocol=PROTOCOL Specifies the network protocol for the query. May be one of "dns" (i.e. classic unicast DNS), "llmnr" (Link-Local Multicast Name Resolution[3]), "llmr-ipv4", "llmnr-ipv6" (LLMNR via the indicated underlying IP protocols). By default the lookup is done via all protocols suitable for the lookup. If used, limits the set of protocols that may be used. Use this option multiple times to enable resolving via multiple protocols at the same time. The setting "llmnr" is identical to specifying this switch once with "llmnr-ipv4" and once via "llmnr-ipv6". Note that this option does not force the service to resolve the operation with the specified protocol, as that might require a suitable network interface and configuration. The special value "help" may be used to list known values. -t TYPE, --type=TYPE, -c CLASS, --class=CLASS Specifies the DNS resource record type (e.g. A, AAAA, MX, . ) and class (e.g. IN, ANY, . ) to look up. If these options are used a DNS resource record set matching the specified class and type is requested. The class defaults to IN if only a type is specified. The special value "help" may be used to list known values. --service Enables service resolution. This enables DNS-SD and simple SRV service resolution, depending on the specified list of parameters (see above). --service-address=BOOL Takes a boolean parameter. If true (the default), when doing a service lookup with --service the hostnames contained in the SRV resource records are resolved as well. --service-txt=BOOL Takes a boolean parameter. If true (the default), when doing a DNS-SD service lookup with --service the TXT service metadata record is resolved as well. --cname=BOOL Takes a boolean parameter. If true (the default), DNS CNAME or DNAME redirections are followed. Otherwise, if a CNAME or DNAME record is encountered while resolving, an error is returned. --search=BOOL Takes a boolean parameter. If true (the default), any specified single-label hostnames will be searched in the domains configured in the search domain list, if it is non-empty. Otherwise, the search domain logic is disabled. --legend=BOOL Takes a boolean parameter. If true (the default), column headers and meta information about the query response are shown. Otherwise, this output is suppressed. --statistics If specified general resolver statistics are shown, including information whether DNSSEC is enabled and available, as well as resolution and validation statistics. --reset-statistics Resets the statistics counters shown in --statistics to zero. -h, --help Print a short help text and exit. --version Print a short version string and exit.
EXAMPLES
Example 1. Retrieve the addresses of the "www.0pointer.net" domain $ systemd-resolve www.0pointer.net Example 2. Retrieve the domain of the "85.214.157.71" IP address $ systemd-resolve 85.214.157.71 Example 3. Retrieve the MX record of the "0pointer.net" domain $ systemd-resolve -t MX 0pointer.net Example 4. Resolve an SRV service $ systemd-resolve --service _xmpp-server._tcp gmail.com
SEE ALSO
systemd(1), systemd-resolved.service(8)
NOTES
1. SRV https://tools.ietf.org/html/rfc2782 2. DNS-SD https://tools.ietf.org/html/rfc6763 3. Link-Local Multicast Name Resolution https://tools.ietf.org/html/rfc4795
Configuring DNS in the resolv.conf File
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1. Overview
Domain Name System (DNS) is a vital service for the Intranet and Internet. It’s responsible for translating machine names into IP addresses. In this tutorial, we’ll explain multiple ways to configure DNS in the Linux operating system.
2. Domain Name System
DNS is a mechanism to make the internet human-friendly. Computers communicate with each other using their IP addresses. There are lots of IP addresses in the internet world, and it’s impossible to remember them all. To solve this issue and make it more human-friendly, DNS was invented. DNS servers map IP addresses to hostnames. When we enter a domain name like baeldung.com into our browser, the computer finds our nearest DNS server and asks what’s the correct IP address for baeldung.com.
Then, it returns the IP address to our system so that it can communicate with the baeldung.com server. For example, DNS translates the domain name baeldung.com to IP address 172.66.40.248. In Linux, there are DNS lookup tools like nslookup and dig, which are made to query DNS servers. For getting the IP address of a domain name, we can use the nslookup command:
$ nslookup baeldung.com Server: 8.8.8.8 Address: 8.8.8.8#53 Non-authoritative answer: Name: baeldung.com Address: 172.66.40.248 Name: baeldung.com Address: 172.66.43.8 3. DNS Configuration
There are two approaches to configuring a Linux system’s DNS service: Resolver Configuration File and Hosts.
3.1. Resolver Configuration File
We’re able to set the DNS configuration in network interface config files. However, this is not the only way. We can configure a DNS service using the /etc/resolv.conf file. If we want to change our DNS configuration, we can use the nameserver keyword:
$ cat /etc/resolv.conf nameserver 8.8.8.8 nameserver 4.2.2.4 In the above configuration, we set the DNS servers to public DNS servers like the Google server (8.8.8.8). Also, note that any changes made manually to the /etc/resolv.conf configuration file is bound to be overwritten upon changes in the network or upon system reboot.
3.2. Hosts
Another way to configure a Linux system’s DNS service is by manipulating the /etc/hosts file. The /etc/hosts file contains server names and their IP addresses statically saved:
$ cat /etc/hosts 127.0.0.1 localhostThis file can be changed by the root user and will map domain names to IP addresses. The /etc/hosts file has a higher priority than /etc/resolv.conf file.
4. DNS Priority
DNS priority tells the system about the priority of DNS lookup. Linux normally performs lookups in /etc/hosts before it uses DNS. We can modify this behavior by editing the /etc/nsswitch.conf file, and specifically, the hosts line. Let’s check the DNS lookup order in /etc/nsswitch.conf file:
$ cat /etc/nsswitch.conf | grep hosts hosts: files mdns4_minimal [NOTFOUND=return] dnsThe above configuration means that DNS lookup refers to files (/etc/hosts) first, and then DNS servers specified in DNS (/etc/resolv.conf). This means when the system wants to find the IP address of a domain name, it first reads the /etc/hosts file and then /etc/resolv.conf.
5. Conclusion
In this article, we explained the DNS service and various ways of configuring it in the Linux operating system. Additionally, we discussed how Linux determines the DNS lookup priority.