[Dnsmasq-discuss] bug:DHCP Relay not responding with DHCP OFFER.

Jason Kary jkary_98 at yahoo.com
Thu Apr 27 22:02:20 BST 2017


Hi Folks,

I have a basic setup for DHCP relay across VLANS in DNSMASQ.  

My configuration file looks like: 

bogus-priv
interface=ens160
log-dhcp
dhcp-range=10.168.102.100,10.168.102.150,255.255.255.0,12h

The client and server are running on a VMs in separate VLANS.  DHCP requests appear to be coming across:

root at DHCP-UBUNTU-SERVER:~# tcpdump -i ens160 port 67 or port 68 -n
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on ens160, link-type EN10MB (Ethernet), capture size 262144 bytes
03:58:40.966944 IP 10.168.102.1.67 > 10.168.101.20.67: BOOTP/DHCP, Request from 00:0c:29:65:e0:ea, length 322
03:58:46.487767 IP 10.168.102.1.67 > 10.168.101.20.67: BOOTP/DHCP, Request from 00:0c:29:65:e0:ea, length 322
03:58:54.424895 IP 10.168.102.1.67 > 10.168.101.20.67: BOOTP/DHCP, Request from 00:0c:29:65:e0:ea, length 322
03:59:07.795712 IP 10.168.102.1.67 > 10.168.101.20.67: BOOTP/DHCP, Request from 00:0c:29:65:e0:ea, length 322
03:59:19.196022 IP 10.168.102.1.67 > 10.168.101.20.67: BOOTP/DHCP, Request from 00:0c:29:65:e0:ea, length 322

root at DHCP-UBUNTU-SERVER:~# iptables -L
Chain INPUT (policy ACCEPT)
target     prot opt source               destination

Chain FORWARD (policy ACCEPT)
target     prot opt source               destination

Chain OUTPUT (policy ACCEPT)
target     prot opt source               destination
root at DHCP-UBUNTU-SERVER:~#

The syslog log indicates the DCHP OFFERS are ‘supposed’ to be going out however nothing is seen on the wire.

Apr 27 04:03:26 DHCP-UBUNTU-SERVER dnsmasq-dhcp[17767]: 1121794364 available DHCP range: 10.168.102.100 -- 10.168.102.150
Apr 27 04:03:29 DHCP-UBUNTU-SERVER dnsmasq-dhcp[17767]: 1121794364 DHCPDISCOVER(ens160) 00:0c:29:65:e0:ea
Apr 27 04:03:29 DHCP-UBUNTU-SERVER dnsmasq-dhcp[17767]: 1121794364 tags: ens160
Apr 27 04:03:29 DHCP-UBUNTU-SERVER dnsmasq-dhcp[17767]: 1121794364 DHCPOFFER(ens160) 10.168.102.128 00:0c:29:65:e0:ea
Apr 27 04:03:29 DHCP-UBUNTU-SERVER dnsmasq-dhcp[17767]: 1121794364 requested options: 1:netmask, 28:broadcast, 2:time-offset, 121:classless-static-route,
Apr 27 04:03:29 DHCP-UBUNTU-SERVER dnsmasq-dhcp[17767]: 1121794364 requested options: 15:domain-name, 6:dns-server, 12:hostname,
Apr 27 04:03:29 DHCP-UBUNTU-SERVER dnsmasq-dhcp[17767]: 1121794364 requested options: 40:nis-domain, 41:nis-server, 42:ntp-server,
Apr 27 04:03:29 DHCP-UBUNTU-SERVER dnsmasq-dhcp[17767]: 1121794364 requested options: 26:mtu, 119:domain-search, 3:router, 121:classless-static-route,
Apr 27 04:03:29 DHCP-UBUNTU-SERVER dnsmasq-dhcp[17767]: 1121794364 requested options: 249, 33:static-route, 252, 42:ntp-server
Apr 27 04:03:29 DHCP-UBUNTU-SERVER dnsmasq-dhcp[17767]: 1121794364 next server: 10.168.101.20
Apr 27 04:03:29 DHCP-UBUNTU-SERVER dnsmasq-dhcp[17767]: 1121794364 sent size:  1 option: 53 message-type  2
Apr 27 04:03:29 DHCP-UBUNTU-SERVER dnsmasq-dhcp[17767]: 1121794364 sent size:  4 option: 54 server-identifier  10.168.101.20
Apr 27 04:03:29 DHCP-UBUNTU-SERVER dnsmasq-dhcp[17767]: 1121794364 sent size:  4 option: 51 lease-time  12h
Apr 27 04:03:29 DHCP-UBUNTU-SERVER dnsmasq-dhcp[17767]: 1121794364 sent size:  4 option: 58 T1  6h
Apr 27 04:03:29 DHCP-UBUNTU-SERVER dnsmasq-dhcp[17767]: 1121794364 sent size:  4 option: 59 T2  10h30m
Apr 27 04:03:29 DHCP-UBUNTU-SERVER dnsmasq-dhcp[17767]: 1121794364 sent size:  4 option:  1 netmask  255.255.255.0
Apr 27 04:03:29 DHCP-UBUNTU-SERVER dnsmasq-dhcp[17767]: 1121794364 sent size:  4 option: 28 broadcast  10.168.102.255
Apr 27 04:03:29 DHCP-UBUNTU-SERVER dnsmasq-dhcp[17767]: 1121794364 sent size:  4 option:  3 router  10.168.102.1
Apr 27 04:03:29 DHCP-UBUNTU-SERVER dnsmasq-dhcp[17767]: 1121794364 sent size:  4 option:  6 dns-server  10.168.101.20
Apr 27 04:03:29 DHCP-UBUNTU-SERVER dnsmasq-dhcp[17767]: 1121794364 sent size: 20 option: 82 agent-id  01:0a:01:08:00:06:00:4c:4f:2a:00:2f:02:06…

I’ve been trying to trace this issue and it is like the sendmsg system call is not working properly.  I believe routing is setup properly on the DHCP server.

root at DHCP-UBUNTU-SERVER:~# netstat -nr
Kernel IP routing table
Destination     Gateway         Genmask         Flags   MSS Window  irtt Iface
0.0.0.0         172.31.13.1     0.0.0.0         UG        0 0          0 ens192
10.168.101.0    0.0.0.0         255.255.255.0   U         0 0          0 ens160
10.168.102.0    0.0.0.0         255.255.255.0   U         0 0          0 ens160
172.31.13.0     0.0.0.0         255.255.255.0   U         0 0          0 ens192
root at DHCP-UBUNTU-SERVER:~#

Can anyone seen something obvious that I am doing wrong?

Take Care
Jason
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