Hi All,
Im on an ubuntu 16.04 machine and was trying to setup a quick dhcp
server for a test with some wifi equipment that needs a custom dhcp
option (43) and ran into two problems I couldn't find my way around.
The first was serving dhcp in the first place. Had two ethernet
interfaces in the machine (both usb), one of which was on a network
192.168.121.0/24 (which im trying to be a dhcp server for) and the other
was on a completely different range (itself being provided dhcp). taking
the config from what libvirt created on its bridge interface and
adjusting appropriately I had a few issues and eventually just had this:
interface=enx00e04c681254
dhcp-range=192.168.121.10,192.168.121.100,12h
dhcp-option=43,15,03:0C:31:39:32:2E:31:36:38:2E:31:32:30:2E:31
But I ran into a very odd issue in that dnsmasq wouldnt serve DHCP on
the 192.168.121.1/24 interface until it saw a request on the other
interface (for which it had no range), as an example:
root@test:~/tmp# dnsmasq -C dnsmasq.conf --port=0 -d --no-ping
dnsmasq: started, version 2.75 DNS disabled
dnsmasq: compile time options: IPv6 GNU-getopt DBus i18n IDN DHCP DHCPv6
no-Lua TFTP conntrack ipset auth DNSSEC loop-detect inotify
dnsmasq-dhcp: DHCP, IP range 192.168.121.10 -- 192.168.121.100, lease
time 12h
dnsmasq-dhcp: no address range available for DHCP request via
enx00e04d6801e2
dnsmasq-dhcp: DHCPDISCOVER(enx00e04c681254) 4c:fa:ca:5e:ef:e0
dnsmasq-dhcp: DHCPOFFER(enx00e04c681254) 192.168.121.80 4c:fa:ca:5e:ef:e0
dnsmasq-dhcp: DHCPREQUEST(enx00e04c681254) 192.168.121.80 4c:fa:ca:5e:ef:e0
dnsmasq-dhcp: DHCPACK(enx00e04c681254) 192.168.121.80 4c:fa:ca:5e:ef:e0
it was receiving multiple requests on the enx00e04c681254 interface
fairly continuously but never responded until it saw a request on
enx00e04d6801e2 (for which it had no ability to provide a lease and
shouldn't have been bound to it anyway). Took quite a while to realize
this was occuring. Tried various incarnations of the listen, bind,
dynamic-bind, etc options, but couldnt find a reason why it might occur
(even watching it with strace wasnt very enlightening). There were at
this point no other dnsmasq's running on the machine and no firewall.
Second problem I couldnt quite figure out is how to send an custom
vendor option that has an ascii encoded ip adddress prefixed with 0x03
and 0x0c (bit of searching found a few people who were struggling with
similar) but could never get it to work quite right. initially i just
had dhcp-option:43,03:0C:31:39:32:2E:31:36:38:2E:31:32:30:2E:31 (should
be 0x3 0xc then ascii for 192.168.120.1), the end result (according to
tcpdump) was this:
Vendor-Option Option 43, length 30:
48.51.48.67.51.49.51.57.51.50.50.69.51.49.51.54.51.56.50.69.51.49.51.50.51.48.50.69.51.49
which is actually "03:0C:31" as ascii text.
A bit of searching/reading and i had this (added the length to the
option - though im not sure this is actually accurate though it did
work): dhcp-option=43,15,03:0C:31:39:32:2E:31:36:38:2E:31:32:30:2E:31
and that resulting in this:
Vendor-Option Option 43, length 15:
3.12.49.57.50.46.49.54.56.46.49.50.48.46.49
which looks correct, but i actually made a mistake here and wanted
192.168.120.2 so i changed it to this:
dhcp-option=43,15,03:0C:31:39:32:2E:31:36:38:2E:31:32:30:2E:32
(incremented last digit) and that resulted in this (according to tcpdump):
Vendor-Option Option 43, length 47:
49.53.44.48.51.58.48.67.58.51.49.58.51.57.58.51.50.58.50.69.58.51.49.58.51.54.58.51.56.58.50.69.58.51.49.58.51.50.58.51.48.58.50.69.58.51.50
which is actually "15,03:0C..." it appeared to interpret the option as
plain text at that point again?
The way I read the man page, dhcp-option=XX,AA:BB:CC:DD:EE should result
in it sending option XX with a length of 5 and each colon-seperated
chunk in the data field being a hex value for each of the bytes (AA =
hex value of byte 1, etc). Also found a post that suggested
dhcp-option=XX,YY,AA:BB:CC... sets the length to be YY (which i dont
think is actually true?)
Im not sure if I am misinterpreting the man page or the ubuntu package
version of dnsmasq is a little broken (though the ones it uses for
things like libvirt work just fine)?
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