SRV Record Basics

SRV record is a category of data in DNS, designed to indicate what services are available like SIP, XMPP. SRV Record is defined in RFC 2782. Now, with wider usage of Cloud DNS, DNS with Geographical Redundancy and many new protocols for newer web services, newer entries are also checked even by the basic DNS checking tools. SRV records allow specific services, such as VOIP or IM, to be be directed to a separate location. Session Initiation Protocol or SIP, for example is related to IP Telephony. If any DNS testing tool is showing hazard symbol against your blank SRV Record – clearly know, this record, usually is not required for functioning of a plain website. This is an informative article on SRV Record, it actually offers no step for troubleshooting.

 

SRV Record Basics and Record Format

 

In addition to the newer Networking Protocols, customer implementations of older protocols such as LDAP , SMTP can also be adding support for SRV records. For example, these are normal, quite optimized result for reverse DNS :

 

 

No SRV Record is showing up there. You can see a result here :

 

SRV record contains the following standard information:

1. Service: the symbolic name (usually starting with the underscore symbol) of service (like  _sip).
2. Protocol: usually it is either “_tcp” for TCP or “_udp” for UDP.
3. Domain name: the domain of validity of the registration (fully qualified domain name or fully qualified local DNS zone being defined for the same original authority).
4. TTL : standard DNS entry indicating the duration of Time-To-Live of the response (in seconds).
5. Class: standard DNS address indicating the class (this is always IN for Internet) field.
6. Type: the identifier type of DNS record (SRV is always a service record)
7. Priority: The priority of the target server (non-negative integer value, the more higher the value is, the more the server will be available). If there are multiple records in different priorities for the same service and the same protocol, only one record for each priority will be returned to DNS clients (clients are supposed to then attempt to connect to the returned server priority with the lowest priority value among the returned records, but if that fails, they can use the next highest priority server in the list of servers that are returned). Different priorities help implement (possibly different in their content and more limited opportunities, for example one or more alternative servers running read-only support certain changes without the service) emergency services.
8. Weight: relative weight for records with the same priority (non-zero integer value allows the DNS server to return a alétoirement target servers with the same priority, with a distribution corresponding to the weight indicated on the total weight of the other records with the same priority ). The weight does not effect if there is only one target server, the same priority for the same service and the same protocol (This parameter allows a fairly basic load distribution across multiple servers for services very popular with many customers making separate DNS queries, but it has no effect if the guests make their DNS requests through a single proxy caching DNS server).
9. Port: the port number (TCP or UDP as the protocol above) where the service is available.
10. Target: the name of the server that provides the service (must be resolved in IPv4 or IPv6 address by other DNS queries on A or AAAA records the name of the target service) with the protocol on the specified port.

 

SRV Record : Load balancing and Standard Used Protocols

 

Customers start by using the SRV record with the lowest priority and fall back on other records only when the connection fails. Thus, a service may have a server designated as the backup server, which is only used in case of failure of the primary server: it suffices to insert a SRV record with a higher priority to the primary server. This is important for Social Networking Services which uses Chat service like Facebook.

SRV record is commonly used for :

1. Minecraft
2. Teamspeak
3. CalDAV
4. CardDAV
5. SMTP Authorization (Client)
6. DNS Service Discovery
7. IMPS
8. Kerberos
9. LDAP
10. Puppet
11. SIP
12. XMPP
13. Mail submission
14. Post Office Protocol
15. Internet Message Access Protocol

For example, you can add chat service (for Google Apps) :

 

Tagged With _kerberos _upd srv non-zero priority

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