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Today's Topics:
1. Re: dhcpd.conf fails to push options to fixed-address clients
(Simon)
2. Re: dhcpd.conf fails to push options to fixed-address clients
(Kenneth Porter)
3. Re: dhcpd.conf fails to push options to fixed-address clients
(Glenn Satchell)
----------------------------------------------------------------------
Message: 1
Date: Sat, 19 Feb 2022 16:51:27 +0000
From: Simon <dh...@thehobsons.co.uk>
To: Users of ISC DHCP <dhcp-users@lists.isc.org>
Subject: Re: dhcpd.conf fails to push options to fixed-address clients
Message-ID: <6c508ecc-391c-437b-8f57-4176c6853...@thehobsons.co.uk>
Content-Type: text/plain; charset=utf-8
Kenneth Porter <sh...@sewingwitch.com> wrote:
> I use host declarations in dhcpd.conf to assigned fixed addresses to wireless
> access points (WAP), printers, and other IoT gear. I'm trying to assign a
> fixed address to a TP-Link Archer AX 20 being used as a WAP. The router sends
> a DHCP request for a renewed address about every 30 seconds, filling my
> dhcpd.log. I ran tcpdump to see what's going on and it looks like dhcpd isn't
> sending the requested parameter list which includes the netmask, lease time,
> and other useful items that are definitely configured in dhcpd.conf and which
> work for all my dynamic assignments. I probably never noticed it before with
> other devices because they don't default to a very short lease time when they
> don't get one in the DHCPACK.
>
> My dhcpd.conf (after expanding file includes) can be viewed here. I tried it
> with hosts at top-level and within a group with the desired options.
>
> <https://pastebin.com/yAjNziqm>
>
> Here's what tcpdump shows. Note how the client requests lease length,
> subnet-mask, etc. but the server doesn't send them.
>
> <https://pastebin.com/EbvphXh2>
>
> I'm using dhcp-4.2.5-83.el7.centos.1.x86_64 on CentOS 7.9.2009.
What is in the leases file for 10.96.0.250 ?
Without digging out the book to check, I?m guessing that the server is sending
a BOOTP response rather than a DHCP response. You have BOOTP enabled, so if
that device previously had a BOOTP lease, then it?s possible the server is
using the and ignoring what the client is asking for. You may have to remove
any exiting leases for that device/address.
I alway deny BOOTP, it?s a long time since I came across a device needing it,
and as BOOTP has no concept of lease length, all BOOTP leases are effectively
infinite which means network churn can leave you with no free leases. If you do
have a device that needs it, you can allow it on a per-device basis.
Simon
------------------------------
Message: 2
Date: Sat, 19 Feb 2022 22:52:16 -0800
From: Kenneth Porter <sh...@sewingwitch.com>
To: dhcp-users@lists.isc.org
Subject: Re: dhcpd.conf fails to push options to fixed-address clients
Message-ID: <c66829c7-2ecb-1edf-a264-622e8cfd2...@sewingwitch.com>
Content-Type: text/plain; charset=UTF-8; format=flowed
On 2/19/2022 8:51 AM, Simon wrote:
> What is in the leases file for 10.96.0.250 ?
Nothing whatsoever.
>
> Without digging out the book to check, I?m guessing that the server is
> sending a BOOTP response rather than a DHCP response. You have BOOTP enabled,
> so if that device previously had a BOOTP lease, then it?s possible the server
> is using the and ignoring what the client is asking for. You may have to
> remove any exiting leases for that device/address.
>
> I alway deny BOOTP, it?s a long time since I came across a device needing it,
> and as BOOTP has no concept of lease length, all BOOTP leases are effectively
> infinite which means network churn can leave you with no free leases. If you
> do have a device that needs it, you can allow it on a per-device basis.
Sounds logical. I changed "allow bootp" to "deny bootp" and the tcpdump
looks the same. Still nothing in the lease file. The TP-Link router
continues to issue a request about every 30 seconds.
Example exchange:
[root@orifice dhcpd]# tcpdump -i enp0s9 -vv -s 500 udp port bootps or
udp port bootpc
tcpdump: listening on enp0s9, link-type EN10MB (Ethernet), capture size
500 bytes
22:49:14.171212 IP (tos 0x0, ttl 64, id 22967, offset 0, flags [DF],
proto UDP (17), length 399)
??? 10.96.0.250.bootpc > orifice.lsi.lan.bootps: [udp sum ok]
BOOTP/DHCP, Request from 60:32:b1:a6:be:ac (oui Unknown), length 371,
????????? Client-IP 10.96.0.250
????????? Client-Ethernet-Address 60:32:b1:a6:be:ac (oui Unknown)
????????? Vendor-rfc1048 Extensions
??????????? Magic Cookie 0x63825363
??????????? DHCP-Message Option 53, length 1: Request
??????????? Client-ID Option 61, length 7: ether 60:32:b1:a6:be:ac
??????????? MSZ Option 57, length 2: 1024
??????????? Parameter-Request Option 55, length 10:
????????????? Subnet-Mask, Default-Gateway, Domain-Name-Server, Hostname
????????????? Domain-Name, BR, Static-Route, NTP
????????????? Classless-Static-Route, Classless-Static-Route-Microsoft
??????????? Vendor-Class Option 60, length 8: "MSFT 5.0"
??????????? Hostname Option 12, length 10: "ArcherAX20"
22:49:14.171673 IP (tos 0x0, ttl 64, id 12659, offset 0, flags [DF],
proto UDP (17), length 328)
??? orifice.lsi.lan.bootps > 10.96.0.250.bootpc: [bad udp cksum 0x1783
-> 0xa0ef!] BOOTP/DHCP, Reply, length 300, xid 0x23b1a93f, Fl
????????? Client-IP 10.96.0.250
????????? Your-IP 10.96.0.250
????????? Server-IP orifice.lsi.lan
????????? Client-Ethernet-Address 60:32:b1:a6:be:ac (oui Unknown)
????????? file "pxelinux.0"[|bootp]
------------------------------
Message: 3
Date: Sun, 20 Feb 2022 22:45:34 +1100
From: Glenn Satchell <glenn.satch...@uniq.com.au>
To: Users of ISC DHCP <dhcp-users@lists.isc.org>
Subject: Re: dhcpd.conf fails to push options to fixed-address clients
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