On 18/10/2023 08:58, Luigi Baldoni via Dnsmasq-discuss wrote:
Hello,
I'm having a hard time making dnsmasq run together with kea-dhcp4-server on the
same machine.
Even though they listen on different interfaces, the first one prevents the
other from starting.
With the old isc-dhcp-server, "bind-interfaces" was enough. But now strace shows
'bind(4, {sa_family=AF_INET, sin_port=htons(67),
sin_addr=inet_addr("0.0.0.0")}, 16) = -1 EADDRINUSE (Address already in use)'
no matter how much I tinker with the configuration.
Any ideas?
This is tricky. Making DHCP work for IPv4 really requires binding the
wildcard address, 0.0.0.0 and this makes running multiple servers on
different interface of the same machine problematic. Dnsmasq does its
best, and pretty much works for multiple dnsmasq instances. There are
two different actions needed: 1) Set the socket option SO_REUSEPORT on
the DHCP socket: this allows multiple processes to bind the same port
number. Dnsmasq always does this when bind-interfaces is set. 2) Bind
the socket to a physical interface, so that packets send to
255.255.255.255 get send to the correct dnsmasq instance based on which
interface they arrive on. Dnsmasq does this when bind-interfaces is set,
and it's configured using --interface to listen on exactly one interface.
The problem you have, I think, is that Kea is not sharing nicely in the
same way. The Kea code on github doesn't set SO_REUSEPORT for DHCPv4 (it
does for DHCPv6) The old ISC server does raw packet IO to avoid the
problems with the kernel IP stack for DHCPv4, and that's probably why it
works. I've not looked at Kea in detail, but it's likely that it uses
the same approach to making DHCPv4 work using the kernel IP stack that
dnsmasq does, but it looks like it's not had the time that dnsmasq has
to accrete the workarounds needed to run multiple DHCP servers in one
kernel.
I'd suggest that this is a Kea problem, not a dnsmasq one.
Cheers,
Simon.
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