As a heavy user of
iptables NAT rules, (advanced home networking, various VPNs on EC2 and other IaaS providers), I feel it’s time for me to better understand how NAT works under the hood with the help of
That’s why I started reading
iptables tutorial for the second time.
NOTE: I personally think it is the best
iptablestutorial freely available (even better than some published books). Look at no others ;-D
The first time I read
iptablestutorial back in 2012, I was frustrated because my networking knowledge and skills were not up to the level the tutorial requires. So I ended up cherry-picking the useful parts only ;-(
Now it’s totally different, many thanks to XenServer and continuously learning.
I’d like to share some gotchas after reading
iptables tutorial for the 2nd time ;-D
Both targets do source NAT (or SNAT) in the
POSTROUTING chain in the
MASQUERADEdoes NOT require
--to-sourceas it was made to work with dynamically assigned IP addresses.
SNATworks with static IP addresses.
MASQUERADEhas extra overhead and is slower than
SNATbecause each time the
MASQUERADEtarget gets hit by a packet, it has to check for the IP address to use.
IMPORTANT: It is still possible to use
MASQUERADE target with static IP. Be aware that it will add extra overhead (to compute).
For example, network
192.168.100.0/24 is used for IPsec (strongSwan) clients, so-called right subnet which is the private subnet behind the left participant (IPsec Server). VPN Clients will be assigned IP addresses within the
rightsubnet as defined in its connection.
For IPsec VPN clients to be able to access the Internet, NAT MUST be done by using
iptables. Outgoing packets need to have their source IP address rewritten to the router (gateway) IP address in their IP header.
Match packets from the
192.168.100.0/24 network and are leaving from
eth0 (interface name can vary based on different hypervisors or Linux distributions - systemd VS sysv init).
NOTE: Don’t have to do DNAT for incoming packets within the
PREROUTINGchain in the
nattable if we turn on ip forwarding in the kernel by
echo 1 | sudo tee /proc/sys/net/ipv4/ip_forwardso as to forward packets between interfaces, cool!
BTW: DO NOT use
sudo echo 1 >to make changes to
procfs, it won’t work, why? RTFM ;-D
Recently I’ve been looking into CloudStack / CloudPlatform Networking, especially the Virtual Private Cloud (VPC) implementation compared with AWS VPC.
The Hypervisor Configuration Guide it explicitly requires Linux Bridge as network backend plus the following kernel parameters:
# switch from ovs to bridge
Above indicates that basic zone networking uses Linux Bridge, the Security Group implementation is based on Linux Bridge and
NOTE: I reckon AWS EC2 / VPC Security Group is the same.
Use to match packets based on the identity of the process that created them.
The owner can be specified as the process ID either of the user who issued the command in question, that of the group, the process, the session, or that of the command itself.
NOTE: The owner match only works within the OUTPUT chain, for obvious reasons: It is pretty much impossible to find out any information about the identity of the instance that sent a packet from the other end, or where there is an intermediate hop to the real destination.
Even within the OUTPUT chain it is not very reliable, since certain packets may not have an owner. Notorious packets of that sort are (among other things) the different ICMP responses. ICMP responses will never match.
iptables -A OUTPUT -m owner --cmd-owner nginx
I started learning
iptables systematically in 2012, soon addicted to it and kept sucking everything that I didn’t know or understand. Countless hours spent…
Plus over 2 years field experience with XenServer (Cloud Infrastructure), I can claim that my Linux networking skill has leaped to a new level ;-D
The leap all started from in-depth learning of
iptables, it’s worth it ;-D
Below is the Traversing of tables and chains diagram, just to pay tribute to the author of iptables tutorial - Oskar Andreasson ;-D
NOTE: Oskar wrote the excellent in-depth tutorial 10 years ago, imagine how good he (his networking) is now ;-D