Supose a simple home router gateway and the following:

lan0 - Network interface responsible for LAN-side of the router wan0 - Network interface responsible for WAN-side of the router

A default route exists like the following:

Destination Interface Mask wan0

Also lan0 and wan0 have IP's assigned referencing local subnet and ISP subnet, respectively.


If a packet is sent from a connected host on the LAN-side of the router to an external host (e.g. Google DNS host) which one is true?

A) lan0's RX counter will be incremented; lan0's TX counter will be incremented; wan0's RX counter will be incremented; wan0's TX counter will be incremented;

B) lan0's RX counter will be incremented; wan0's TX counter will be incremented;

C) lan0's RX counter will be incremented;

D) wan0's TX counter will be incremented;

E) None of the above

If possible, a good explanation will be appreciated. Thank you.

Edit: Simplifying even more

When forwarding packets from one interface to another does RX counter on the receiving interface gets incremented?

  • 1
    This smells like a homework assignment. – psusi Jul 15 '15 at 1:05
  • It's not. I just formatted as a clear question. I could not find any documentation saying about forwarded packets between interfaces could increase counters internally. – Guilherme da Silva Sengès Jul 15 '15 at 2:49

Ok, I did a practical test with a home router gateway using OpenWRT, a local DHCP server and a laptop with the following connections:

           WAN                     LAN

DHCP Server <------> OpenWRT Router <------> Laptop

Sending single UDP packets from laptop through Google DNS server ( I could confirm that only RX (LAN) and TX (WAN) were incremented. Also if no external connectivity were available, only RX (LAN) gets incremented.

Conclusion: Forwarding packets between interfaces will not increment counters between them. RX and TX only counts received and transmitted bytes to some endpoint.

Correct option: B)

  • Except for the fact that the DNS server sends a reply, and the reply increments the other two counters. – psusi Jul 16 '15 at 1:40
  • That's not correct for my scenario. I sent an UDP packet directly to IP No name resolution was done and also UDP is connectionless so no replies are given by the host. – Guilherme da Silva Sengès Jul 17 '15 at 3:13

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