[SOLVED] No DNS access on LCX container

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BuildIT Consulting

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Apr 12, 2022
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Hi guys,

I got an issue when trying to setup a local network for my containers and vms.

I setup the host network to the outside internet, during install. That worked fine.
Then an added local network according to this article: Proxmox Wiki

But my test container has no DNS available.

Pinging any IP works fine, but as soon as I try to resolve a domain it says:
Code: 
$ ping google.com
ping: google.com: Temporary failure in name resolution

Details:

Interfaces Host:
Code: 
$ cat /etc/network/interfaces
# network interface settings; autogenerated
# Please do NOT modify this file directly, unless you know what
# you're doing.
#
# If you want to manage parts of the network configuration manually,
# please utilize the 'source' or 'source-directory' directives to do
# so.
# PVE will preserve these directives, but will NOT read its network
# configuration from sourced files, so do not attempt to move any of
# the PVE managed interfaces into external files!

auto lo
iface lo inet loopback

auto eno1
iface eno1 inet static
        address 185.94.252.29/24
        gateway 185.94.252.1

iface eno2 inet manual

auto vmbr0
iface vmbr0 inet static
        address 10.10.10.1/24
        bridge-ports none
        bridge-stp off
        bridge-fd 0

        post-up   echo 1 > /proc/sys/net/ipv4/ip_forward
        post-up   iptables -t nat -A POSTROUTING -s '10.10.10.0/24' -o eno1 -j MASQUERADE
        post-down iptables -t nat -D POSTROUTING -s '10.10.10.0/24' -o eno1 -j MASQUERADE
        post-up   iptables -t raw -I PREROUTING -i fwbr+ -j CT --zone 1
        post-down iptables -t raw -D PREROUTING -i fwbr+ -j CT --zone 1

Interface Container:
Code: 
$ cat /etc/network/interfaces
auto lo
iface lo inet loopback

auto eth0
iface eth0 inet static
        address 10.10.10.2/24
        gateway 10.10.10.1

Resolve Container:
Code: 
$ cat /etc/resolv.conf
# --- BEGIN PVE ---
search domain.internal
nameserver 8.8.8.8
# --- END PVE ---

Proxmox verison:
Code: 
proxmox-ve: 7.1-1 (running kernel: 5.13.19-2-pve)
pve-manager: 7.1-12 (running version: 7.1-12/b3c09de3)
pve-kernel-helper: 7.1-14
pve-kernel-5.13: 7.1-9
pve-kernel-5.13.19-6-pve: 5.13.19-15
pve-kernel-5.13.19-2-pve: 5.13.19-4
ceph-fuse: 15.2.15-pve1
corosync: 3.1.5-pve2
criu: 3.15-1+pve-1
glusterfs-client: 9.2-1
ifupdown2: 3.1.0-1+pmx3
ksm-control-daemon: 1.4-1
libjs-extjs: 7.0.0-1
libknet1: 1.22-pve2
libproxmox-acme-perl: 1.4.1
libproxmox-backup-qemu0: 1.2.0-1
libpve-access-control: 7.1-7
libpve-apiclient-perl: 3.2-1
libpve-common-perl: 7.1-5
libpve-guest-common-perl: 4.1-1
libpve-http-server-perl: 4.1-1
libpve-storage-perl: 7.1-1
libspice-server1: 0.14.3-2.1
lvm2: 2.03.11-2.1
lxc-pve: 4.0.11-1
lxcfs: 4.0.11-pve1
novnc-pve: 1.3.0-2
proxmox-backup-client: 2.1.5-1
proxmox-backup-file-restore: 2.1.5-1
proxmox-mini-journalreader: 1.3-1
proxmox-widget-toolkit: 3.4-7
pve-cluster: 7.1-3
pve-container: 4.1-4
pve-docs: 7.1-2
pve-edk2-firmware: 3.20210831-2
pve-firewall: 4.2-5
pve-firmware: 3.3-6
pve-ha-manager: 3.3-3
pve-i18n: 2.6-2
pve-qemu-kvm: 6.1.1-2
pve-xtermjs: 4.16.0-1
qemu-server: 7.1-4
smartmontools: 7.2-1
spiceterm: 3.2-2
swtpm: 0.7.1~bpo11+1
vncterm: 1.7-1
zfsutils-linux: 2.1.4-pve1
 
Last edited:
If I understand your post correctly, then
* pinging the DNS server from the host works
* DNS resolution on the host works
* DNS resolution in the container does NOT work
* but pinging the DNS server from the container works (?)

Can you "manually" resolve domain names e.g. using dig -t A proxmox.com 8.8.8.8 ?

If so, this sounds more like a configuration / software problem than a network problem.
 
Hi,
yes, that is exactly how to unterstand my issue.

No manually resolving domain names does not work.

Code: 
$ dig -t A proxmox.com 1.1.1.1

; <<>> DiG 9.16.27-Debian <<>> -t A proxmox.com 1.1.1.1
;; global options: +cmd
;; connection timed out; no servers could be reached

;; connection timed out; no servers could be reached

I also updated my configuration since I had no internet at all meanwhile:

Host:
Code: 
$ cat /etc/network/interfaces
auto lo
iface lo inet loopback

auto eno1
iface eno1 inet manual
#used externally

iface eno2 inet manual

auto vmbr0
iface vmbr0 inet static
        address PUBLIC.IP/24
        gateway PUBLIC.GATEWAY
        bridge-ports eno1
        bridge-stp off
        bridge-fd 0
        bridge-vlan-aware yes
        bridge-vids 2-4094
#enternal

auto vmbr1
iface vmbr1 inet static
        address 10.0.0.1/24
        bridge-ports none
        bridge-stp off
        bridge-fd 0
        bridge-vlan-aware yes
        bridge-vids 2-4094
#internal

post-up   echo 1 > /proc/sys/net/ipv4/ip_forward
post-up   iptables -t nat -A POSTROUTING -s '10.0.0.1/24' -o vmbr0 -j MASQUERADE
post-down iptables -t nat -D POSTROUTING -s '10.0.0.1/24' -o vmbr0 -j MASQUERADE
post-up   iptables -t raw -I PREROUTING -i fwbr+ -j CT --zone 1
post-down iptables -t raw -D PREROUTING -i fwbr+ -j CT --zone 1

1649860293976.pngd

CT:
Code: 
$ cat /etc/network/interfaces
auto lo
iface lo inet loopback

auto eth0
iface eth0 inet static
        address 10.0.0.2/24
        gateway 10.0.0.1
        hwaddress fa:1f:4f:4d:45:3f

1649860260701.png
 
Solved, solution below.

Solution
After switching to SNAT my issues were solved.

Adding the following line to the config of my interfaces resulted in a working system:

Code: 
post-up iptables -t nat -A POSTROUTING -s 10.0.0.0/24 -o vmbr0 -j SNAT --to-source My.Public.IP
post-down iptables -t nat -D POSTROUTING -s 10.0.0.0/24 -o vmbr0 -j SNAT --to-source My.Public.IP
 
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