How to let built-in DHCP assign a static IP to LXC container based on name, not MAC address

Question

I know, that I can assign static IP manually, using /etc/network/interfaces.

I also know, that I can read the MAC address of the LXC container (e.g. by looking for lxc.network.hwaddr entry in /var/lib/lxc/<container-name>/config and assign the IP based using entries dhcp-host=<mac-addr>,10.0.3.3 in /etc/dnsmasq.d/<some file>.

In the file /etc/default/lxc-net I read

# Uncomment the next line if you'd like to use a conf-file for the lxcbr0
# dnsmasq.  For instance, you can use 'dhcp-host=mail1,10.0.3.100' to have
# container 'mail1' always get ip address 10.0.3.100.
#LXC_DHCP_CONFILE=/etc/lxc/dnsmasq.conf

That would suit my needs; unfortunately doing so has no effect.

score 20 · Answer 1 · edited Mar 09 '15 at 11:13

I ran into this recently and I think I found an easy solution. I (only) tested it on Ubuntu 14.04.

First, uncomment this line /etc/default/lxc-net:

LXC_DHCP_CONFILE=/etc/lxc/dnsmasq.conf

In /etc/lxc/dnsmasq.conf, define a dhcp-hostsfile:

dhcp-hostsfile=/etc/lxc/dnsmasq-hosts.conf

Then add entries in /etc/lxc/dnsmasq-hosts.conf like this:

mail,10.0.3.16
web,10.0.3.17

Beware: changes will become effective after you have restarted lxc-net (which restarts dnsmasq):

service lxc-net restart

Afterwards you can modify /etc/lxc/dnsmasq-hosts.conf and send the SIGHUP signal to dnsmasq:

killall -s SIGHUP dnsmasq

So yes, you need to restart lxc-net, but only once. Hope this helps.

score 4 · Answer 2 · answered Aug 25 '14 at 07:48

It works fine in Ubuntu 14.04.1

Uncomment this line /etc/default/lxc-net

#LXC_DHCP_CONFILE=/etc/lxc/dnsmasq.conf

stop all containers, restart lxc-net:

service lxc-net restart

Configure ip addresses in /etc/lxc/dnsmasq.conf

dhcp-host={NAME},10.0.3.2

where {NAME} is the name of your LXC container:

/var/lib/lxc/{NAME}

Adam Ryczkowski · Answer 3 · 2014-11-05T09:38:02.790

The Tombart's answer works if you are patient enough to wait for the DNS refresh AND you are willing to restart the container (the guest) afterwards.

What follows is the recipe that requires that all other possibly running lxc containers are shut down. If you cannot afford that, then I see no way of forcing new dnsmasq configuration. (For some reasone signaling HUP to dnsmasq's pid found in /run/lxc/dnsmasq.pid doesn't work either.)

So if you want to have something that works instantly and there is no other lxc containers running, follow my answer. $name is the name of the node for which we want to reset the assignment, and $internalif is the name of the LXC's bridged adapter. You can get the value of the $internalif with e.g. augtool -L -A --transform "Shellvars incl /etc/default/lxc-net" get "/files/etc/default/lxc-net/LXC_BRIDGE" | sed -En 's/\/.* = (.*)/\1/p' if you install augeas-tools but usually it is just lxcbr0.

sudo lxc-stop -n $name >/dev/null
sudo service lxc-net stop >/dev/null
if [ -d /sys/class/net/$internalif ]; then
   sudo brctl delbr $internalif >/dev/null #Why? See below.
fi
sudo rm /var/lib/misc/dnsmasq.$internalif.leases
sudo service lxc-net start >/dev/null
sudo lxc-start -d -n $name >/dev/null
sleep 5

Unfortunately, there is a bug (feature?) in the /etc/init/lxc-net.conf in Ubuntu 14.04 that prevents reloading the dnsmasq unless the bridge device is down for the host.

score 0 · Accepted Answer · answered Nov 05 '14 at 15:51

This solution works by patching the lxc upstart scripts. It does split the complex task of bringing the lxcbr0 bridge up and starting a dnsmasq into two separate jobs. Now you don't need to restart the whole lxc-net bridge to just reload the dnsmasq - reloading the sudo service restart lxc-dnsmasq is sufficient and does not require shutting down the bridge.

Stop the lxc-net service sudo service lxc-net stop and make sure there is no lxcbr0 (or equivalent) bridge up.
Replace the contents of the /etc/init/lxc-net.conf with the following contents:

.

description "lxc network"
author "Serge Hallyn <serge.hallyn@canonical.com>"

start on starting lxc
stop on stopped lxc

env USE_LXC_BRIDGE="true"
env LXC_BRIDGE="lxcbr0"
env LXC_ADDR="10.0.3.1"
env LXC_NETMASK="255.255.255.0"
env LXC_NETWORK="10.0.3.0/24"
env varrun="/run/lxc"
env LXC_DOMAIN=""

pre-start script
    [ -f /etc/default/lxc ] && . /etc/default/lxc

    [ "x$USE_LXC_BRIDGE" = "xtrue" ] || { stop; exit 0; }

    use_iptables_lock="-w"
    iptables -w -L -n > /dev/null 2>&1 || use_iptables_lock=""
    cleanup() {
        # dnsmasq failed to start, clean up the bridge
        iptables $use_iptables_lock -D INPUT -i ${LXC_BRIDGE} -p udp --dport 67 -j ACCEPT
        iptables $use_iptables_lock -D INPUT -i ${LXC_BRIDGE} -p tcp --dport 67 -j ACCEPT
        iptables $use_iptables_lock -D INPUT -i ${LXC_BRIDGE} -p udp --dport 53 -j ACCEPT
        iptables $use_iptables_lock -D INPUT -i ${LXC_BRIDGE} -p tcp --dport 53 -j ACCEPT
        iptables $use_iptables_lock -D FORWARD -i ${LXC_BRIDGE} -j ACCEPT
        iptables $use_iptables_lock -D FORWARD -o ${LXC_BRIDGE} -j ACCEPT
        iptables $use_iptables_lock -t nat -D POSTROUTING -s ${LXC_NETWORK} ! -d ${LXC_NETWORK} -j MASQUERADE || true
        iptables $use_iptables_lock -t mangle -D POSTROUTING -o ${LXC_BRIDGE} -p udp -m udp --dport 68 -j CHECKSUM --checksum-fill
        ifconfig ${LXC_BRIDGE} down || true
        brctl delbr ${LXC_BRIDGE} || true
    }
    if [ -d /sys/class/net/${LXC_BRIDGE} ]; then
        if [ ! -f ${varrun}/network_up ]; then
            # bridge exists, but we didn't start it
            stop;
        fi
        exit 0;
    fi

    # set up the lxc network
    brctl addbr ${LXC_BRIDGE} || { echo "Missing bridge support in kernel"; stop; exit 0; }
    echo 1 > /proc/sys/net/ipv4/ip_forward
    mkdir -p ${varrun}
    ifconfig ${LXC_BRIDGE} ${LXC_ADDR} netmask ${LXC_NETMASK} up
    iptables $use_iptables_lock -I INPUT -i ${LXC_BRIDGE} -p udp --dport 67 -j ACCEPT
    iptables $use_iptables_lock -I INPUT -i ${LXC_BRIDGE} -p tcp --dport 67 -j ACCEPT
    iptables $use_iptables_lock -I INPUT -i ${LXC_BRIDGE} -p udp --dport 53 -j ACCEPT
    iptables $use_iptables_lock -I INPUT -i ${LXC_BRIDGE} -p tcp --dport 53 -j ACCEPT
    iptables $use_iptables_lock -I FORWARD -i ${LXC_BRIDGE} -j ACCEPT
    iptables $use_iptables_lock -I FORWARD -o ${LXC_BRIDGE} -j ACCEPT
    iptables $use_iptables_lock -t nat -A POSTROUTING -s ${LXC_NETWORK} ! -d ${LXC_NETWORK} -j MASQUERADE
    iptables $use_iptables_lock -t mangle -A POSTROUTING -o ${LXC_BRIDGE} -p udp -m udp --dport 68 -j CHECKSUM --checksum-fill

    touch ${varrun}/network_up
end script

post-stop script
    [ -f /etc/default/lxc ] && . /etc/default/lxc
    [ -f "${varrun}/network_up" ] || exit 0;
    # if $LXC_BRIDGE has attached interfaces, don't shut it down
    ls /sys/class/net/${LXC_BRIDGE}/brif/* > /dev/null 2>&1 && exit 0;

    if [ -d /sys/class/net/${LXC_BRIDGE} ]; then
        use_iptables_lock="-w"
        iptables -w -L -n > /dev/null 2>&1 || use_iptables_lock=""
        ifconfig ${LXC_BRIDGE} down
        iptables $use_iptables_lock -D INPUT -i ${LXC_BRIDGE} -p udp --dport 67 -j ACCEPT
        iptables $use_iptables_lock -D INPUT -i ${LXC_BRIDGE} -p tcp --dport 67 -j ACCEPT
        iptables $use_iptables_lock -D INPUT -i ${LXC_BRIDGE} -p udp --dport 53 -j ACCEPT
        iptables $use_iptables_lock -D INPUT -i ${LXC_BRIDGE} -p tcp --dport 53 -j ACCEPT
        iptables $use_iptables_lock -D FORWARD -i ${LXC_BRIDGE} -j ACCEPT
        iptables $use_iptables_lock -D FORWARD -o ${LXC_BRIDGE} -j ACCEPT
        iptables $use_iptables_lock -t nat -D POSTROUTING -s ${LXC_NETWORK} ! -d ${LXC_NETWORK} -j MASQUERADE || true
        iptables $use_iptables_lock -t mangle -D POSTROUTING -o ${LXC_BRIDGE} -p udp -m udp --dport 68 -j CHECKSUM --checksum-fill
        pid=`cat ${varrun}/dnsmasq.pid 2>/dev/null` && kill -9 $pid || true
        rm -f ${varrun}/dnsmasq.pid
        brctl delbr ${LXC_BRIDGE}
    fi
    rm -f ${varrun}/network_up
end script

Add another file, /etc/init/lxc-dnsmasq with the following contents:

.

description "lxc dnsmasq service"
author "Adam Ryczkowski, ispired by Serge Hallyn <serge.hallyn@canonical.com>"

expect fork

start on started lxc-net
stop on stopped lxc-net

env USE_LXC_BRIDGE="true"
env LXC_BRIDGE="lxcbr0"
env LXC_ADDR="10.0.3.1"
env LXC_NETMASK="255.255.255.0"
env LXC_NETWORK="10.0.3.0/24"
env LXC_DHCP_RANGE="10.0.3.2,10.0.3.254"
env LXC_DHCP_MAX="253"
env LXC_DHCP_CONFILE=""
env varrun="/run/lxc-dnsmasq"
env LXC_DOMAIN=""

pre-start script
    [ -f /etc/default/lxc ] && . /etc/default/lxc

    [ "x$USE_LXC_BRIDGE" = "xtrue" ] || { stop; exit 0; }

    if [ ! -d ${varrun} ]; then
        mkdir -p ${varrun}
    fi
    opts="$LXC_DOMAIN_ARG -u lxc-dnsmasq --strict-order --bind-interfaces --pid-file=${varrun}/dnsmasq.pid --conf-file=${LXC_DHCP_CONFILE} --listen-address ${LXC_ADDR} --dhcp-range ${LXC_DHCP_RANGE} --dhcp-lease-max=${LXC_DHCP_MAX} --dhcp-no-override --except-interface=lo --interface=${LXC_BRIDGE} --dhcp-leasefile=/var/lib/misc/dnsmasq2.${LXC_BRIDGE}.leases --dhcp-authoritative --keep-in-foreground"

    /usr/sbin/dnsmasq $opts &

end script

post-stop script
    if [ -f ${varrun}/dnsmasq.pid ]; then
        PID=`cat ${varrun}/dnsmasq.pid`
        kill $PID
    fi
end script

score 0 · Answer 5 · answered Dec 15 '16 at 20:59

Here is simple python script which releases LXC dnsmasq lease. You can run it from host machine or forge it from another container - yes that works!:

#!/usr/bin/env python
from scapy.all import *
conf.checkIPaddr=False
leaseMAC = '00:16:3e:11:71:b0' #container MAC here
releaseIP='10.0.3.33' #container IP here
serverIP='10.0.3.1'
hostname='container-name-here'
rawMAC = leaseMAC.replace(':','').decode('hex')
send(IP(dst=serverIP) / \
     UDP(sport=68,dport=67) / \
     BOOTP(chaddr=rawMAC, ciaddr=releaseIP, xid=RandInt()) / \
     DHCP(options=[('message-type','release'),('server_id',serverIP),('hostname',hostname), ('end')]))

The perequisite for above is scapy python library:

pip install scapy

Once run you should see in system log something like:

dnsmasq-dhcp[3242]: DHCPRELEASE(lxcbr0) 10.0.3.33 00:16:3e:11:71:b0 container-name-here

To confirm just check if entry is removed from /var/lib/misc/dnsmasq.lxcbr0.leases. The container itself will keep the IP so it should be stopped before starting any new container which should reuse the IP.

score 0 · Answer 6 · answered Oct 27 '17 at 06:04

I realize my answer is years late, but maybe it helps someone else. The issue was that you edited code specific to the LXC Ubuntu package (write_lxc_net function) that was meant to be written to another destination as a string, not processed within the lxc-net script itself!

As a result, the dnsmasq process didn't receive the config file you tried to tried to pass it, leaving you with "no effect", as you say.

Instead, you'd want to set this variable near the top of the script, among the rest:

#!/bin/sh -

distrosysconfdir="/etc/default"
varrun="/run/lxc"
varlib="/var/lib"

# These can be overridden in /etc/default/lxc
#   or in /etc/default/lxc-net

USE_LXC_BRIDGE="true"
LXC_BRIDGE="lxcbr0"
LXC_BRIDGE_MAC="00:16:3e:00:00:00"
LXC_ADDR="10.0.3.1"
LXC_NETMASK="255.255.255.0"
LXC_NETWORK="10.0.3.0/24"
LXC_DHCP_RANGE="10.0.3.2,10.0.3.254"
LXC_DHCP_MAX="253"
LXC_DHCP_CONFILE="/etc/lxc/dnsmasq.conf"   <-- Here for instance
LXC_DOMAIN=""

How to let built-in DHCP assign a static IP to LXC container based on name, not MAC address

6 Answers6

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