Adding netfilter and vrf experts. On Wed, 28 Sep 2022 16:02:43 +0200 Maximilien Cuony wrote: > Hello, > > We're using VRF with a machine used as a router and have a specific > issue where the router doesn't handle his own packets correctly during > NATing if the packet is coming from a different VRF. > > We had the issue with debian buster (4.19), but the issue solved itself > when we updated to debian bullseye (5.10.92). > > However, during an upgrade of debian bullseye to the latest kernel, the > issue appeared again (5.10.140). > > We did a bisection and this leaded us to > "b0d67ef5b43aedbb558b9def2da5b4fffeb19966 net: allow unbound socket for > packets in VRF when tcp_l3mdev_accept set [ Upstream commit > 944fd1aeacb627fa617f85f8e5a34f7ae8ea4d8e ]". > > Simplified case setup: > > There is two machines in the setup. They both forward packets > (net.ipv4.ip_forward = 1) and there is two interface between them. > > The main machine has two VRF. The default VRF is using the second > machine as the default route, on a specific interface. > The second machine has as default route to main machine, on the other > VRF using the second pair of interfaces. > > On the main machine, the second interface is in a specific VRF. In that > VRF, packets are NATed to the internet on a third interface. > > A visual schema with the normal flow is available there: > https://etinacra.ch/kernel.png > > Configuration command: > > Main machine: > sysctl -w net.ipv4.tcp_l3mdev_accept = 1 > sysctl -w systnet.ipv4.ip_forward = 1 > iptables -t raw -A PREROUTING -i eth0 -j CT --zone 5 > iptables -t raw -A OUTPUT -o eth0 -j CT --zone 5 > iptables -t nat -A POSTROUTING -o eth2 -j SNAT --to 192.168.1.1 > cat /etc/network/interfaces > > auto firewall > iface firewall > vrf-table 1200 > > auto eth0 > iface eth0 > address 192.168.5.1/24 > gateway 192.168.5.2 > > auto eth1 > iface eth1 > address 192.168.10.1/24 > vrf firewall > up ip route add 192.168.5.0/24 via 192.168.10.2 vrf firewall > > auto eth2 > iface eth2 > address 192.168.1.1/24 > gateway 192.168.1.250 > vrf firewall > > == > > Second machine: > > sysctl -w net.ipv4.ip_forward = 1 > > cat /etc/network/interfaces > > auto eth0 > iface eth0 > address 192.168.5.2/24 > > auto eth1 > iface eth1 > address 192.168.10.2/24 > gateway 192.168.10.1 > > == > > Without issue, if we look at a tcpdump on all interface on the main > machine, everything is fine (output truncated): > > 10:28:32.811283 eth0 Out IP 192.168.5.1.55750 > 99.99.99.99.80: Flags > [S], seq 2216112145 > 10:28:32.811666 eth1 In IP 192.168.5.1.55750 > 99.99.99.99.80: Flags > [S], seq 2216112145 > 10:28:32.811679 eth2 Out IP 192.168.1.1.55750 > 99.99.99.99.80: Flags > [S], seq 2216112145 > 10:28:32.835138 eth2 In IP 99.99.99.99.80 > 192.168.1.1.55750: Flags > [S.], seq 383992840, ack 2216112146 > 10:28:32.835152 eth1 Out IP 99.99.99.99.80 > 192.168.5.1.55750: Flags > [S.], seq 383992840, ack 2216112146 > 10:28:32.835457 eth0 In IP 99.99.99.99.80 > 192.168.5.1.55750: Flags > [S.], seq 383992840, ack 2216112146 > 10:28:32.835511 eth0 Out IP 192.168.5.1.55750 > 99.99.99.99.80: Flags > [.], ack 1, win 502 > > However when the issue is present, the SYNACK does arrives on eth2, but > is never "unNATed" back to eth1: > > 10:25:07.644433 eth0 Out IP 192.168.5.1.48684 > 99.99.99.99.80: Flags > [S], seq 3207393154 > 10:25:07.644782 eth1 In IP 192.168.5.1.48684 > 99.99.99.99.80: Flags > [S], seq 3207393154 > 10:25:07.644793 eth2 Out IP 192.168.1.1.48684 > 99.99.99.99.80: Flags > [S], seq 3207393154 > 10:25:07.668551 eth2 In IP 54.36.61.42.80 > 192.168.1.1.48684: Flags > [S.], seq 823335485, ack 3207393155 > > The issue is only with TCP connections. UDP or ICMP works fine. > > Turing off net.ipv4.tcp_l3mdev_accept back to 0 also fix the issue, but > we need this flag since we use some sockets that does not understand VRFs. > > We did have a look at the diff and the code of inet_bound_dev_eq, but we > didn't understand much the real problem - but it does seem now that > bound_dev_if if now checked not to be False before the bound_dev_if == > dif || bound_dev_if == sdif comparison, something that was not the case > before (especially since it's dependent on l3mdev_accept). > > Maybe our setup is wrong and we should not be able to route packets like > that? > > Thanks a lot and have a nice day! > > Maximilien Cuony > >
