Searching ALOHA 10.5
Service Discovery with SRV Records
Service Discovery with SRV Records
DNS for Service Discovery is a feature that can update an HAProxy configuration during runtime, such as IP addresses and ports, without making explicit changes to configuration files.
You can configure HAProxy to inspect DNS SRV records so it can set ports and IP addresses. An SRV record returns a hostname and port number. The hostname, in turn, maps to an A record that contains the IP address.
![[DNS SRV records]](/fr/documentation/aloha/10-5/assets/images/dns_srv_records-5f3179e049d5a3e8717927e440718ffb9af80afb283f245f5f5723557a313c99.png)
About DNS SRV records
DNS SRV records are resources used to identify computers that host specific services. They are contained in the ANSWER section of DNS responses and have the following structure:
_service._proto.name. TTL class SRV priority weight port target
where:
| Standard network service name (taken from /etc/services) or a port number |
| Standard protocol name ("tcp" or "udp") |
| Name of the service, i.e. the name used in the query |
| Validity period for the response (HAProxy ignores this field because it maintains its own expiry data defined in the configuration) |
| DNS class ("IN") |
| DNS record type ("SRV") |
| Priority of the target host. Lower value = higher preference (HAProxy ignores this field but may use it later to indicate active / backup state) |
| Relative weight in case of records with the same priority. Higher number = higher preference |
| Port where the service is configured |
| Hostname of the machine providing the service, ending in a dot Note Usually, the DNS server also returns the resolution for the targets mentioned, and it provides that information in the ADDITIONAL SECTION. |
Configure DNS A and SRV records
Update your DNS nameserver to resolve a service name, such as myservice.example.local, to one or more hostnames and ports via SRV records. Those hostnames should resolve to IP addresses using A records.
Add DNS A records that resolve multiple hostnames, such as host1, host2 and host3, to different IP addresses.
Add the same number of SRV records that resolve a service name, such as _myservice._tcp.example.local, to the hostnames you defined and the port on the host where the service listens.
Query the nameserver
-
Query the nameserver directly with the
dig
tool to ensure that it returns the correct records, as follows:$ dig @192.168.50.30 -p 53 SRV _myservice._tcp.example.local
;; QUESTION SECTION: ;_myservice._tcp.example.local. IN SRV ;; ANSWER SECTION: _myservice._tcp.example.local. 0 IN SRV 0 0 8080 host1. _myservice._tcp.example.local. 0 IN SRV 0 0 8081 host2. _myservice._tcp.example.local. 0 IN SRV 0 0 8082 host3. ;; ADDITIONAL SECTION: host1. 0 IN A 192.168.50.31 host2. 0 IN A 192.168.50.32 host3. 0 IN A 192.168.50.33
Configure a resolvers section
You add a resolvers
section in your HAProxy configuration file to set the DNS nameservers for HAProxy to watch for changes.
Edit HAProxy's configuration file: browse the LB Layer 7 tab from the GUI.
Add a
resolvers
section.Add one or more
nameserver
lines to specify the IP addresses and ports of your DNS nameservers.Set the
accepted_payload_size
to 8192 to allow larger DNS payloads, which is required to receive more server IPs within a single DNS result.
resolvers mydns
nameserver dns1 192.168.50.30:53
accepted_payload_size 8192
Add a server-template
-
Use a
server-template
in abackend
to set the template for theserver
lines when HAProxy queries your DNS servers, as follows:backend <name> server-template <prefix> <num | range> <fqdn>[:<port>] [params*]
where:
prefix
A string used as a prefix for each server name. (e.g. web)
num
The number of servers you want to generate automatically. This number will increment and append to the
prefix
of each server when forming its name.range
The number of servers you want to generate automatically, specified as starting and ending numbers to append to the
prefix
of each server when forming its name.fqdn
The fully-qualified domain name used to query the nameservers.
port
A hardcoded port number.
params*
Additional
server
parameters.backend webservers balance roundrobin server-template web 5 _myservice._tcp.example.local resolvers mydns check init-addr none
In this example, the
server-template
directive:Adds the specified number of servers (5) to the backend.
Appends "web" as a prefix to their names.
Queries the service name _myservice._tcp.example.local.
Have the SRV records fill in the ports.
Specifies the
resolvers
sectionmydns
The
init-addr none
argument means that HAProxy can initialize without having to resolve the IP addresses at startup. It can resolve them during runtime.
This is equivalent to adding a
backend
to HAProxy that looks like this:backend webservers balance roundrobin server web1 192.168.50.31:8080 check server web2 192.168.50.32:8081 check server web3 192.168.50.33:8082 check server web4 check disabled server web5 check disabled
When you add more records to your nameserver, they automatically go into the backend to fill the web4 and web5 slots.