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Re: Android client SYN packets dropped



On 2021-05-15 15:33, Arto Huusko wrote:
Hello, I need help resolving a problem in my LAN.

My Android 10 phone is unable to connect to any TCP services hosted
on a NetBSD -current 9.99.73 server. Connections to the services
from other devices on the same LAN work.

My setup:

- NetBSD server connected to an Asus router via 1Gb ethernet
- Various wifi devices connected to the same Asus router
- Asus router bridges the wifi and ethernet networks

Here is what I have found:

The NetBSD server receives SYN packet from the Android device. And then
nothing happens.

Here is a working SYN, SYN-ACK pair from a wireless Linux laptop:

16:23:07.941446 IP 192.168.1.137.52091 > 192.168.1.2.http: Flags [S], seq 2002477302, win 64106, options [mss 9158,nop,wscale 8,nop,nop,sackOK], length 0 16:23:07.941470 IP 192.168.1.2.http > 192.168.1.137.52091: Flags [S.], seq 1642340388, ack 2002477303, win 65535, options [mss 1460,nop,wscale 3,sackOK,eol], length 0


And this is a SYN packet from the Android device:

16:05:08.452102 IP 192.168.1.133.45086 > 192.168.1.2.http: Flags [S], seq 3821589786, win 65535, options [mss 1460,sackOK,TS val 1473889759 ecr 0,nop,wscale 12], length 0

The NetBSD server does not send any response.

Looking at netstat -s output, the dropped SYN packets are recorded here:

         90 SYNs dropped (no route or no space)

that increases every time I try to connect from the Android device.


How can I fix this?

Is your phone getting into the ARP cache? If not, then the packets would indeed be dropped like that. Check that ARP requests go out. If they do, check if you are getting any responses...

Also check that you don't have anything else with the same MAC address, or else your switch might also be directing ARP traffic wrong.

  Johnny

--
Johnny Billquist                  || "I'm on a bus
                                  ||  on a psychedelic trip
email: bqt%softjar.se@localhost             ||  Reading murder books
pdp is alive!                     ||  tryin' to stay hip" - B. Idol


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