News for hackers. Stuff that matters.

A comment on AT&T Fiber in the SF Bay Area is flipping bits. Parent comment

Re:AT&T Fiber in the SF Bay Area is flipping bits by tyu2 on Monday December 07, 2020 @09:31PM (#25340948)
Hardware that forwards packets usually forwards IP packets, it doesn't care about TCP checksums and doesn't recalculate them. TCP stack in your OS is the one that does that.

It has to be a special kind of hardware that does much deeper packet inspection (DPI) to recalculate TCP checksums, usually used for spying, throttling, censorship, injecting ads, injecting exploits, etc., but not merely routing/forwarding packets.

[ Parent | on HN ]
Re:AT&T Fiber in the SF Bay Area is flipping bits by pclmulqdq on Monday December 07, 2020 @11:03PM (#25341620)
Switches (L2 devices) recalculate Ethernet CRCs (L2 error detection codes), and routers tend to calculate TCP/UDP checksums (L3 error detection codes) and everything below. I've seen exactly this issue with switches that have bad RAM before, and I assume that they just have a router with bad RAM (and without ECC RAM, which looks embarrassing).
[ Parent | on HN ]
Re:AT&T Fiber in the SF Bay Area is flipping bits by noncoml on Monday December 07, 2020 @11:49PM (#25341973)
To my knowledge, from working on an actual software router, a router will only touch the TTL and recalculate the IP header checksum.

There is no reason for it to touch the TCP header.

[ Parent | on HN ]
Re:AT&T Fiber in the SF Bay Area is flipping bits by garmaine on Tuesday December 08, 2020 @12:21AM (#25342196)
Agreed there is no logical reason for it to touch the TCP header.

And yet, an unfortunate number of L2 switches do exactly that. :(

[ Parent | on HN ]
Re:AT&T Fiber in the SF Bay Area is flipping bits by Thorrez on Tuesday December 08, 2020 @01:10AM (#25342420)
Doesn't NAT (specifically carrier-grade NAT in this case) often modify the port? Although I don't know if AT&T does carrier-grade NAT.

https://en.wikipedia.org/wiki/Carrier-grade_NAT

[ Parent | on HN ]
Re:AT&T Fiber in the SF Bay Area is flipping bits by noncoml on Tuesday December 08, 2020 @01:26AM (#25342490)
> an unfortunate number of L2 switches do exactly that

Can you name any? Just curious

[ Parent | on HN ]