Bsrgvty

I want to set up a bridge so that guest VMs on my headless server use the LAN DHCP and not dnsmasq. I followed these instructions: https://jamielinux.com/docs/libvirt-networking-handbook/bridged-network.html#bridge-debian. Looking at the ifconfig results, it seems to give me exactly what I was looking for:

br0: flags=4163<UP,BROADCAST,RUNNING,MULTICAST> mtu 1500
 inet 192.168.1.210 netmask 255.255.255.0 broadcast 192.168.1.255
 inet6 fe80::4216:7eff:fe63:7516 prefixlen 64 scopeid 0x20<link>
 ether 40:16:7e:63:75:16 txqueuelen 1000 (Ethernet)
 RX packets 8255 bytes 653898 (653.8 KB)
 RX errors 0 dropped 0 overruns 0 frame 0
 TX packets 633 bytes 60185 (60.1 KB)
 TX errors 0 dropped 0 overruns 0 carrier 0 collisions 0

enp3s0: flags=4163<UP,BROADCAST,RUNNING,MULTICAST> mtu 1500
 inet 192.168.1.195 netmask 255.255.255.0 broadcast 192.168.1.255
 ether 40:16:7e:63:75:16 txqueuelen 1000 (Ethernet)
 RX packets 42190 bytes 48646124 (48.6 MB)
 RX errors 0 dropped 3 overruns 0 frame 0
 TX packets 9808 bytes 889965 (889.9 KB)
 TX errors 0 dropped 0 overruns 0 carrier 0 collisions 0

lo: flags=73<UP,LOOPBACK,RUNNING> mtu 65536
 inet 127.0.0.1 netmask 255.0.0.0
 inet6 ::1 prefixlen 128 scopeid 0x10<host>
 loop txqueuelen 1000 (Local Loopback)
 RX packets 2215 bytes 510180 (510.1 KB)
 RX errors 0 dropped 0 overruns 0 frame 0
 TX packets 2215 bytes 510180 (510.1 KB)
 TX errors 0 dropped 0 overruns 0 carrier 0 collisions 0

virbr0: flags=4099<UP,BROADCAST,MULTICAST> mtu 1500
 inet 192.168.122.1 netmask 255.255.255.0 broadcast 192.168.122.255
 ether 52:54:00:7a:ca:5e txqueuelen 1000 (Ethernet)
 RX packets 0 bytes 0 (0.0 B)
 RX errors 0 dropped 0 overruns 0 frame 0
 TX packets 0 bytes 0 (0.0 B)
 TX errors 0 dropped 0 overruns 0 carrier 0 collisions 0

However, if I try to SSH into my server (192.168.1.195) from another machine (192.168.1.196), I can't connect. Pings from the server can't seem to reach other devices on my LAN, but pings from the server to 8.8.8.8 seem to work OK. My /etc/network/interfaces file looks like the below (the MAC is my server's NIC):

iface enp3s0 inet manual

auto br0
iface br0 inet static
 # Use the MAC address identified above.
 hwaddress ether 40:16:7e:63:75:16
 address 192.168.1.210
 netmask 255.255.255.0
 gateway 192.168.1.1

 bridge_ports enp3s0
 # If you want to turn on Spanning Tree Protocol, ask your hosting
 # provider first as it may conflict with their network.
 bridge_stp on
 # If STP is off, set to 0. If STP is on, set to 2 (or greater).
 bridge_fd 0

I've spent the last two days googling, and re-imaged my server a couple of times as well when things got too broken, and I think I'm just losing the plot... My server is Ubuntu 18.04.2, completely stock other than the libvirt and ssh packages installed. How can I get the server to be reachable by my other network machines?

Sometimes it pays to just leave things alone for a day or two and not look at them. Then come back to it fresh. In the end, setting up a bridge seemed to be fairly straightforward, with the following two steps:

1. Update /etc/sysctl.conf

Uncomment the line that says ".net.ipv4.ip_forward=1"

2. Update /etc/network/interfaces

For me, this was the entirety of the file. If you have more interfaces, you may want to adjust accordingly. enp3s0 was the ID of my physical interface.

# ifupdown has been replaced by netplan(5) on this system. See
# /etc/netplan for current configuration.
# To re-enable ifupdown on this system, you can run:
# sudo apt install ifupdown
iface enp3s0 inet manual

auto lo
iface lo inet loopback

auto br0
iface br0 inet dhcp
 bridge_ports enp3s0
 bridge_stp off
 bridge_fd 0
 bridge_maxwait 0

I used an IP address reservation in my router to make sure my VM always got the same IP address. Using DHCP made configuration overall a bit easier for me.

Thanks again to RobertRSeattle and Doug Smythies for putting up with me.