Consul / dnsmasq setup with DHCP

[Ameir Abdeldayem]

Hey everyone,

I'm setting up dnsmasq locally on my instances in order to forward requests to consul for the `.consul` zone.  I manually set nameserver 127.0.0.1 in my resolv.conf, and am able to query things (in Consul and not) just fine.

My question, which I haven't seen an answer to after a good bit of searching, is: what happens when DHCP options change to pass new resolvers?

Note that my question isn't: how do I keep nameserver 127.0.0.1 constant in resolv.conf?  I know that I can set that in dhclient options.  Instead, what if a network change is made to use a different set of resolvers?  Most posts seem to have something like resolv-file=/etc/resolv.dnsmasq.conf in the dnsmasq config, but that file is manually-updated.  Has anyone gotten dhclient to update that file instead?  That appears to be the most fool-proof way, but I'm not finding much on that.

Any tips would be well-appreciated.

Thanks!

-Ameir

[Brian Lalor]

On CentOS 7, I’m doing the following:

> /etc/NetworkManager/conf.d/000-hands-of-my-resolv.conf
# -*- ini -*-

[main]

## configure NetworkManager to not manage /etc/resolv.conf, as we'll bring our own
## man NetworkManager.conf
dns=none

> /etc/NetworkManager/dispatcher.d/50-update-dnsmasq-resolv-conf
#!/bin/bash

## script to be executed when dhcp changes are made; maintains /etc/resolv.conf
## man NetworkManager

interface="${1}"
action="${2}"

/bin/logger -t "${0}" \
"invoked for interface ${interface:-<not_provided>} and action ${action} with nameservers '${IP4_NAMESERVERS}', domains '${IP4_DOMAINS}'"

if [ -n "${IP4_NAMESERVERS}" ] && [ -n "${IP4_DOMAINS}" ]; then
echo "# generated by ${0}; do not edit" >| /etc/resolv.conf.dnsmasq

for ns in ${IP4_NAMESERVERS}; do
echo "nameserver ${ns}" >> /etc/resolv.conf.dnsmasq
done

## use IP of eth0, so we can use the same resolv.conf and mount it in
## containers
local_ip=$( ip addr show eth0 | awk '/inet / {print substr($2, 0, index($2, "/") - 1)}' )
echo "# generated by ${0}; do not edit" >| /etc/resolv.conf
echo "nameserver ${local_ip:-127.0.0.1}" >> /etc/resolv.conf

if [ -n "${IP4_DOMAINS}" ]; then
echo "search ${IP4_DOMAINS}" >> /etc/resolv.conf
fi
fi

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—
Brian Lalor
bla...@bravo5.org

[Ameir Abdeldayem]

Thanks for that snippet, Brian.  We're on CentOS 7 as well, but not using NetworkManager; I'll dig into this a bit and see if I can adapt this to our environment.

Thanks!

-Ameir

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—
Brian Lalor
bla...@bravo5.org

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[Brian Lalor]

I think I found the RHEL docs a little more complete when using NetworkManager than without.  I’ve attached what I’m using for CentOS 6 with dhclient.  It’s installed at /etc/dhcp/dhclient-enter-hooks.  There may be more required to make this work, and I seem to recall it being a really-big-hammer approach, but it Works For Me™. :-)

[Michael Fischer]

You're not using the dnsmasq plugin for NetworkManager? It seems to
make everything work for us.

> https://groups.google.com/d/msgid/consul-tool/34C6A4BB-79C5-4450-8706-A32A626C333F%40bravo5.org.