Where do the Linux kernel router code replace the MAC address

The router will replace the source MAC address of the package it received with the address of the previous hop and the destination MAC address with the address of next hop.

The linux provides a functionality to worked as a router. My question is how the kernel code implement the function for mac address update during its package forwarding process? And where is this part of code

I try to find the codes in /net/ipv4, but can not found anything开发者_StackOverflow社区...

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That is not what actually happens.

IP is not dependent on ethernet, so what happens is dependent upon the underlying protocol of the lower layer.

The same thing happens if it is a locally-originated IP packet, or if it is one which has been routed for another host.

Linux's IPv4 stack is not ethernet-dependent in any way, in fact lots of other link-layer protocols are supported by the kernel. IP being a WAN protocol, you can route between different underlying protocols. Some examples are

• ppp, slip (serial lines)
• PPTP, GRE (for tunnels, mostly VPNs)
• IP over ATM
• Token ring (mostly legacy, I think)
• Loopback and dummy (for local communication only)
• Wifi (although this is actually mostly identical to ethernet)

So what actually happens when routing IP frames from one ethernet interface to another is that the link-layer is stripped off completely, then a new link-layer is formed after routing. If the protocol were not ethernet, an appropriate link-layer packet for that protocol would be used instead.

So nobody "changes the MAC address", but rather, the link-layer packet is just completely rebuilt.