SRV Record in DNS: A Straightforward Guide

In the world of DNS (Domain Name System), most people are familiar with A records that map domain names to IP addresses. However, when it comes to discovering services and handling advanced features like load balancing and failover, SRV records in DNS come into play. SRV (Service) records provide a flexible way to direct traffic to services based on criteria like priority and weight, offering greater control than traditional DNS records.

Jump to Solution: Learn how to configure SRV records in DNS

What Are SRV Records in DNS?

An SRV record (Service record) is a DNS record used to define the location (hostname and port) of servers for specific services. Unlike traditional DNS A records, which only map domain names to IP addresses, SRV records contain information about:

Service Type (e.g., SIP, FTP)
Protocol (e.g., TCP, UDP)
Priority and Weight (used for load balancing and failover)
Port Number (the port that the service is running on)
Target Host (the server that provides the service)

Why Use SRV Records Instead of A Records?

While A records are excellent for simple use cases (like directing traffic to a single web server), SRV records allow more detailed control, especially when services need to be accessed on specific ports, or when you need load balancing and redundancy.

For example, SRV records allow you to:

Distribute traffic between multiple servers using weights.
Ensure failover with priority-based routing.
Dynamically resolve services without changing the client’s configuration.

How SRV Records Work

When a client queries for an SRV record, it sends a query like _service._protocol.domain.com to the DNS server. The DNS server responds with a list of servers that can provide the requested service, along with their priority, weight, and port number. Here’s a basic example:

_sip._tcp.example.com 86400 IN SRV 10 60 5060 sipserver1.example.com
_sip._tcp.example.com 86400 IN SRV 20 40 5060 sipserver2.example.com

Understanding SRV Record Priority and Weight

Priority: Clients try to connect to the server with the lowest priority first. If that server is unavailable, they will try the next one.
Weight: When two or more servers have the same priority, weight is used to distribute traffic. A higher weight means the server will receive more traffic.

For example, if Server A has a weight of 60 and Server B has a weight of 40, 60% of the traffic will be directed to Server A, and 40% to Server B.

Failover and Load Balancing with SRV Records

SRV records are crucial for managing failover and load balancing. In the event that the primary server is down, the client will automatically try the secondary server based on the priority and weight set in the SRV records.

A hot standby server, for instance, might be configured to handle traffic only when the primary server fails, ensuring smooth failover without manual intervention. Alternatively, an active-active setup can distribute traffic between multiple servers for load balancing.

Potential Issues with SRV Record Weighting

While SRV records help distribute traffic using weights, there's always a theoretical chance that all clients could connect to one server. This is because the weighted random selection algorithm relies on randomness, and while it's designed to balance traffic proportionally over time, it may lead to uneven distribution in isolated cases. However, with enough connections, the traffic should balance according to the weight ratio (e.g., 60% to Server A and 40% to Server B).

Hot Standby vs. Active-Active Load Balancing

One of the key decisions when using SRV records is whether to set up servers in active-active or active-passive configurations:

Hot Standby (Active-Passive): In some setups, a hot standby server is kept ready to take over instantly in case the main server fails. While this ensures zero downtime for critical services, it may appear to waste resources because the standby server isn’t actively handling traffic. This approach is common in mission-critical environments where failover speed is paramount.
Active-Active (Load Balancing): In an active-active configuration, multiple servers share the load simultaneously. Traffic is distributed based on weights, and if one server fails, the other can handle the full load. This setup ensures resource optimization, as all servers are actively used, but requires careful handling of sessions and state between servers.

Both approaches have their use cases, and SRV records allow for both load balancing (active-active) and failover management (active-passive) depending on the business needs.

How to Configure SRV Records in DNS

To configure SRV records, follow these steps:

Log in to your DNS management system.
Select the domain you want to modify.
Add an SRV record by specifying the service name, protocol, priority, weight, port, and target host.
Save the configuration and allow DNS propagation.

Here’s an example configuration:

Service: _sip
Protocol: _tcp
Priority: 10
Weight: 60
Port: 5060
Target Host: sipserver1.example.com

Check SRV Record Using NSLOOKUP

nslookup -type=SRV _service._protocol.domain

Server:  dns.example.com
Address:  192.168.1.1

Non-authoritative answer:
_sip._tcp.example.com  SRV service location:
          priority = 10
          weight = 60
          port = 5060
          svr hostname = sipserver1.example.com
_sip._tcp.example.com  SRV service location:
          priority = 20
          weight = 40
          port = 5060
          svr hostname = sipserver2.example.com

Conclusion

Using SRV records in DNS gives you the flexibility to direct traffic to specific services, handle failover with ease, and distribute load between servers. Whether you're setting up a VoIP service, web application, or any other service that requires dynamic routing, SRV records offer powerful control over how your clients interact with your services.

If you have any questions or need help setting up SRV records, feel free to leave a comment!