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DNS Forwarder does not reload hosts file when modified.
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Description

I have my DHCP server feeding into /etc/hosts. I have my DNS server forwarding dns, and also serving the contents of /etc/hosts.

The DNS server only gives out addresses that were in /etc/hosts when the dns forwarder was started, but doesn't pick up new ones.

Plug in wap-3c34. It shows in DHCP leases:

10.35.2.8        78:8a:20:0a:0e:c8  2018/05/24 14:58:59                            wap-3c34

But isn't available in DNS:

$ host wap-3c34
Host wap-3c34 not found: 3(NXDOMAIN)

If we do run restart dns forwarder on our firewall:

$ host wap-3c34
wap-3c34.admin.codethink.co.uk has address 10.35.2.8

Talking to @dmbaturin in IRC:

<dmbaturin> thinkl33t: Ok, now this is tricky. Since DHCPD doesn't execute anything when it gives a new lease (AFAIR), we'd have to teach pdns to re-read the hosts file for that.

Details

Difficulty level
Normal (likely a few hours)
Version
VyOS 1.2.0-rolling+201805210337
Why the issue appeared?
Will be filled on close

Event Timeline

thinkl33t created this task.May 23 2018, 3:10 PM
syncer assigned this task to dmbaturin.May 23 2018, 3:34 PM
syncer added a subscriber: syncer.

We can use something like
https://jpmens.net/2011/07/06/execute-a-script-when-isc-dhcp-hands-out-a-new-lease/
but reload pdns instead

thinkl33t added a comment.May 23 2018, 3:40 PM

pdns doesn't have a built-in reload (at least not available through systemd), but i've just tested and the following works:

sudo rec_control reload-zones
dmbaturin added a comment.May 23 2018, 3:53 PM

We can make a script that checks if service is configured and reload it if it's not then.

EwaldvanGeffen added a subscriber: EwaldvanGeffen.May 23 2018, 4:48 PM

Thinking ahead: what happens when you have a very active DHCP server? Shouldn't we rate-limit reload-zones to max once/n sec ?

dmbaturin added a comment.May 24 2018, 5:22 PM

@EwaldvanGeffen Good point. Do you have ideas how to implement it?

dmbaturin edited projects, added VyOS 1.2 Crux (VyOS 1.2.0-rc1); removed VyOS 1.2 Crux.May 24 2018, 5:24 PM
syncer triaged this task as Normal priority.May 27 2018, 9:37 AM
thinkl33t claimed this task.Aug 2 2018, 9:40 AM

This was solved in https://github.com/vyos/vyatta-cfg-dhcp-server/commit/7488d52ce2946a4eaa47b1d909970f24956529c7

thinkl33t closed this task as Resolved.Aug 2 2018, 9:40 AM
syncer added a project: VyOS-1.2.0-GA.Oct 15 2018, 10:42 PM
syncer moved this task from Needs Triage to Finished on the VyOS 1.2 Crux (VyOS 1.2.0-rc1) board.Oct 15 2018, 10:42 PM