Question

[Glenn Satchell]

Hi Leslie,

I know about capturing packets on a 10G interface :) many gigabytes in a 
few seconds...

So you need to use filters when capturing, eg with tcpdump

   tcpdump -i eth0 host <other dhcp server IP or name> and tcp port 647

will only capture the failover traffic on eth0 directed to or from the 
other server, and ignore the rest.

   tcpdump udp and port 68 or port 67

will capture dhcp packets.

You can add options like "-c 100" to stop after 100 packets are 
captured. "-w filename" will capture to a file and you can copy this 
file to your desktop and use wireshark to read it.

With failover, it's better to restart one dhcp server, wait for it to 
sync, then restart the other one. If you shut down both and then start 
them, then they come up in recover mode.

Also looking at failover connections:

   netstat -ant | grep 647

should show an established connection between the two servers.

regards,
Glenn

On 2022-06-03 15:39, Leslie Rhorer wrote:

> On 6/2/2022 11:30 PM, Gregory Sloop wrote:
> 
>> Are you seeing balance messages every hour as the two re-balance the 
>> available lease pool?
> No, I don't think so.  It has only been a couple of hours since I have 
> had both online, however.
> 
>> You say they are both handling leases properly, but how do you know 
>> this? (That a machine gets a lease from somewhere is not good 
>> evidence.)
> 
> Do you mean because some other machine / device could be issuing 
> leases?  No.  In that case,
> 
> 1. Killing both servers would not take down any DHCP clients.  If both 
> servers are shut down, DHCP clients start failing in about an hour, 
> until they are all dead.
> 
> 2. DHCP responses on the LAN stop completely the moment both servers 
> are taken down.
> 
> 3. No other machine would know anything about the list of dynamically 
> assigned fixed IP addresses in dhcpd.static.  None of the addresses of 
> any of the clients ever change.
> 
> 4. Whenever one server is shut down, the other responds with tons of 
> responses in  the log.
> 
>> A packet capture in front of the secondary might be helpful to see 
>> what traffic is passing - both to the peer and to clients.
> While not impossible, that is a bit easier said than done.  The links 
> between the servers are 10G.  I can look into it.
> 
>> (I hate making captures, at least as much as the next person, but dang 
>> if they don't, nearly always, show something that was different than I 
>> assumed. So, I've just gotten a lot less averse to getting captures. 
>> Yeah, they'll probably take me extra time to setup and get and paw 
>> through, [all when I could be fixin' stuff!] but they can save hours 
>> or days of fruitless searching for a fix, when I don't even really 
>> *know* what's wrong yet. Don't know about anyone else, but fixing 
>> problems gets a whole lot easier when I actually know what's wrong, or 
>> at least have a good idea what's going on. :)
> 
> Agreed, although when an interface is chunking away at over 10,000 
> packets per second...
> 
> If something doesn't break loose, I will see about loading Wireshark.