dnsmasq server conf:

domain=test,1.2.3.4/24
dhcp-range=test,1.2.3.100,1.2.3.199,1m
dhcp-option=42,1.2.3.1
#dhcp-option=121,0.0.0.0/0,1.2.3.1

test system conf:

ubuntu@lp1835581-e:~$ cat /etc/systemd/network/10-ens8.network 
[Match]
Name=ens8

[Network]
DHCP=ipv4
LinkLocalAddressing=ipv6
Address=1.2.3.99/24




ubuntu@lp1835581-e:~$ dpkg -l systemd|grep ii
ii  systemd        242-7ubuntu3 amd64        system and service manager
ubuntu@lp1835581-e:~$ ip -4 a show ens8
3: ens8: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc fq_codel state UP group default qlen 1000
    inet 1.2.3.99/24 brd 1.2.3.255 scope global ens8
       valid_lft forever preferred_lft forever
    inet 1.2.3.170/24 brd 1.2.3.255 scope global secondary dynamic ens8
       valid_lft 98sec preferred_lft 98sec
ubuntu@lp1835581-e:~$ ip -4 r
default via 1.2.3.1 dev ens8 proto dhcp src 1.2.3.170 metric 1024 
1.2.3.0/24 dev ens8 proto kernel scope link src 1.2.3.99 
1.2.3.1 dev ens8 proto dhcp scope link src 1.2.3.170 metric 1024 


route src is correctly set to dhcp addr, when dnsmasq not configured with classless route



uncomment dnsmasq server conf classless route:

domain=test,1.2.3.4/24
dhcp-range=test,1.2.3.100,1.2.3.199,1m
dhcp-option=42,1.2.3.1
dhcp-option=121,0.0.0.0/0,1.2.3.1

reboot test system:

ubuntu@lp1835581-e:~$ ip -4 a show ens8
3: ens8: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc fq_codel state UP group default qlen 1000
    inet 1.2.3.99/24 brd 1.2.3.255 scope global ens8
       valid_lft forever preferred_lft forever
    inet 1.2.3.170/24 brd 1.2.3.255 scope global secondary dynamic ens8
       valid_lft 110sec preferred_lft 110sec
ubuntu@lp1835581-e:~$ ip -4 r
default via 1.2.3.1 dev ens8 proto dhcp metric 1024 
1.2.3.0/24 dev ens8 proto kernel scope link src 1.2.3.99 


route src incorrectly not set, when dnsmasq set to classless route




change dnsmasq server again to comment out classless route and restart it, then reboot test system:

ubuntu@lp1835581-e:~$ dpkg -l systemd|grep ii
ii  systemd        242-7ubuntu3.2 amd64        system and service manager
ubuntu@lp1835581-e:~$ ip -4 a show ens8
3: ens8: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc fq_codel state UP group default qlen 1000
    inet 1.2.3.99/24 brd 1.2.3.255 scope global ens8
       valid_lft forever preferred_lft forever
    inet 1.2.3.170/24 brd 1.2.3.255 scope global secondary dynamic ens8
       valid_lft 97sec preferred_lft 97sec
ubuntu@lp1835581-e:~$ ip -4 r
default via 1.2.3.1 dev ens8 proto dhcp src 1.2.3.170 metric 1024 
1.2.3.0/24 dev ens8 proto kernel scope link src 1.2.3.99 
1.2.3.1 dev ens8 proto dhcp scope link src 1.2.3.170 metric 1024 


re-enable classless route on server and reboot test system:

ubuntu@lp1835581-e:~$ ip -4 a show ens8
3: ens8: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc fq_codel state UP group default qlen 1000
    inet 1.2.3.99/24 brd 1.2.3.255 scope global ens8
       valid_lft forever preferred_lft forever
    inet 1.2.3.170/24 brd 1.2.3.255 scope global secondary dynamic ens8
       valid_lft 104sec preferred_lft 104sec
ubuntu@lp1835581-e:~$ ip -4 r
default via 1.2.3.1 dev ens8 proto dhcp src 1.2.3.170 metric 1024 
1.2.3.0/24 dev ens8 proto kernel scope link src 1.2.3.99 


route is correctly sourced from dhcp address