POSTROUTING not accepted by iptables

Edit2: As the issue is not what I thought it is, I will ask the moderators to close the thread and will post another one if I can't figure out the issue, which could be misconfiguration on the VPN server or client side

Edit: based on rds100's comment (iptables -t nat -L -n), it seems that the rule is there, but I may be missing some modules for this to work or something may be misconfigured either on the VPN server or client side.

Post was:
As the title says, for some reason POSTROUTING is not accepted by iptables. I am trying to set up a VPN, it wasn't working and I tracked it down to this. If I do:
sudo iptables -t nat -A POSTROUTING -s 10.8.0.0/24 -o eth0 -j MASQUERADE
and then
iptables -L
there is no mention of the POSTROUTING chain at all.

I know that on some platforms, for this to work something it has to be enabled by the provider on the node (in the kernel?), but this is Xen. Any ideas what may be causing it?