One way to implement Anycast DNS Server throughout the internet is to use BGP protocol and to announce your DNS IP block from multiple locations so that redundancy and low-latency targets are achieved. There are many ways to design such a infrastructure; you could use hardware routers, you could use load-Balancers in front of your servers etc. I would like to share a script here for the sceanario where Quagga BGPD daemon is used as a software router either on the DNS server or any other server purely as a router:
Using python dnspython library, it is easy to perform checks of a DNS servers ability to respond to different type of DNS queries. Based on the DNS servers answer you could stop, start Quagga BGPD daemon.
You could check the examples on http://www.dnspython.org/examples.html to understand how dnspython is used. A simple A type request is as below:
test_domain = "www.test.com" server_to_test = ["127.0.0.1"] answers = anycast_server1.query(test_domain, "A") anycast_server1 = dns.resolver.Resolver() anycast_server1.timeout=1.0 anycast_server1.lifetime=1.0 anycast_server1.nameservers = server_to_test for data in answers: print data
If DNS and BGPD daemon is on the same server, script should forward the query to local server: 127.0.0.1. Below is the script i wrote to check local server with an A type DNS request, and based on the answer act either to start or stop the BGPD daemon. You could download the latest version of the script from https://github.com/ercintorun/dns-check-quagga-act.
# -*- coding: utf-8 -*-
import dns.resolver, psutil, commands, time, logging, datetime
###VARIABLES
script_run_time = 60
test_domain = "www.test.com"
server_to_test = ["127.0.0.1"]
#############
#############
#############
###logging file folder and logging level config
logging.basicConfig(filename='/var/log/dnsscript.log', filemode='a', level=logging.INFO,
format='%(asctime)s [%(name)s] %(levelname)s (%(threadName)-10s): %(message)s')
###define dns servers, parameters
anycast_server1 = dns.resolver.Resolver()
anycast_server1.timeout=1.0
anycast_server1.lifetime=1.0
anycast_server1.nameservers = server_to_test
###time to run the script
starttime = time.time()
timeout = time.time()+ script_run_time
###kill process function
def kill_process(PROCNAME):
for proc in psutil.process_iter():
if proc.name() == PROCNAME:
proc.kill()
###start a loop with an amount of script_run_time value
while True:
time.sleep(0.8)
if time.time()> timeout:
break
else:
###get all daemon names into list
daemon_list=[]
for proc in psutil.process_iter():
daemon_list.append(proc.name())
###start the control
if "bgpd" not in daemon_list:
try:
answers = anycast_server1.query(test_domain, "A")
commands.getoutput ("/etc/init.d/bgpd restart")
logging.warning("DNS successful, BGP daemon is down, bgpd restarted")
time.sleep(2) #give bgp 2 second to get up again
except dns.resolver.NXDOMAIN:
logging.warning("DNS exception: No such domain, BGP daemon is already down, no change done")
except dns.resolver.Timeout:
logging.warning("DNS exception: Timed out while resolving, BGP daemon is already down, no change done ")
except dns.exception.DNSException:
logging.warning("DNS exception: Unhandled exception, BGP daemon is already down, no change done")
else:
try:
answers = anycast_server1.query(test_domain, "A")
for data in answers:
resolved = data
logging.info("DNS successful, nothing has been changed, last resolved ip is: "+str(data))
except dns.resolver.NXDOMAIN:
kill_process("bgpd")
logging.warning("DNS exception: No such domain, BGP daemon terminated")
except dns.resolver.Timeout:
kill_process("bgpd")
logging.warning("DNS exception: Timed out while resolving, BGP daemon terminated")
except dns.exception.DNSException:
kill_process("bgpd")
logging.warning("DNS exception: Unhandled exception, BGP daemon terminated")
If you examine the script you could see that it works for 60 seconds. If you add this script to crontab with 1 minute interval it will check DNS server 60-70 times per minute (on local server) continiously. I've used psutil library to fetch active daemons list on Linux to check whether BGPD daemon is active or not. Script,
- If DNS successful and BGPD is active does nothing
- If DNS successful and BGPD is passive, restarts BGPD using "commands" libary
- IF DNS unsuccessful and BGPD is active, stops BGPD
- IF DNS unsuccessful and BGPD is passive, does nothing
Also you could see that the script logs its actions for each query to "/var/log/dnsscript.log". If you would like to decrease log size, you could change "level=logging.INFO" to "level=logging.WARNING" to not to log no-action-taken checks.
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