Libvirt network bridging issues

Question

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?

score 4 · Answer 1 · answered Apr 16 '19 at 16:47

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:

Update /etc/sysctl.conf

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

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.

score 0 · Answer 2 · answered Jun 20 '20 at 01:58

I had a working bridge that went down. I tried everything. Went on a witch hunt looking for docker files thinking docker had destroyed my networking. It turned out that the network interface name within the guest had a change of heart and enp1s0 became enp7s0. fixed the /etc/netplan yaml and everything is back.

Libvirt network bridging issues

2 Answers2