Contents
What is difference between Prerouting and Postrouting?
PREROUTING: Immediately after being received by an interface. POSTROUTING: Right before leaving an interface. INPUT: Right before being handed to a local process. OUTPUT: Right after being created by a local process.
Which chain is the first thing a packet hits when entering the system?
When a packet first enters the firewall, it hits the hardware and then gets passed on to the proper device driver in the kernel. Then the packet starts to go through a series of steps in the kernel, before it is either sent to the correct application (locally), or forwarded to another host – or whatever happens to it.
When to use the POSTROUTING chain in mangle?
The POSTROUTING chain in the mangle table is mainly used when we want to do mangling on packets before they leave our host, but after the actual routing decisions. This chain will be hit by both packets just traversing the firewall, as well as packets created by the firewall itself.
What does a POSTROUTING chain do in iptables?
IPTables contains several predefined and/or user-defined tables. Each table contains chains and chain contain packet rules. IPTables uses NAT table to forward packets to another node. What is POSTROUTING? A Postrouting chain in NAT table means altering the IP packet after the routing is completed.
When do we hit the mangle INPUT chain?
Routing decision, i.e., is the packet destined for our local host or to be forwarded and where. At this point, the mangle INPUT chain is hit. We use this chain to mangle packets, after they have been routed, but before they are actually sent to the process on the machine.
How does a POSTROUTING chain in Nat table work?
A Postrouting chain in NAT table means altering the IP packet after the routing is completed. Logically, a postrouting can be used to change the Source Address. As the routing is completed and destination has his own address, the only unknown address that can be masked is the Source.