[Solved] The connection is established for a second and then dropped

Installing and Using OpenWrt
1

I had some problems with my openwrt router but finally I managed to unbrick it.

When I booted the router in recovery mode, I had no problem with establishing a connection. I easily sent openwrt image to the router through TFTP with one command without any problems.

I installed the latest version of OpenWrt on it. From the beginning I had a problem to connect with it. Port 1 LED on the router was blinking about once every 2.5 seconds. When I tried to ping 192.168.1.1 I got an error:

ping: connect: Network is unreachable

I don't remember how, but one time I managed to connect. Then again there was a problem with connection. I decided to reflash openwrt image. This did not help. Since then I have not been able to connect to the router at all.

Here's the output of 'sudo tcpdump -Ani enp0s25 udp -v' command:

tcpdump: listening on enp0s25, link-type EN10MB (Ethernet), snapshot length 262144 bytes
14:11:05.675451 IP (tos 0xc0, ttl 64, id 0, offset 0, flags [DF], proto UDP (17), length 318)
    0.0.0.0.68 > 255.255.255.255.67: BOOTP/DHCP, Request from 50:7b:9d:32:53:3d, length 290, xid 0xcce3aeb7, secs 1, Flags [none]
	  Client-Ethernet-Address 50:7b:9d:32:53:3d
	  Vendor-rfc1048 Extensions
	    Magic Cookie 0x63825363
	    DHCP-Message (53), length 1: Request
	    Client-ID (61), length 7: ether 50:7b:9d:32:53:3d
	    Parameter-Request (55), length 17: 
	      Subnet-Mask (1), Time-Zone (2), Domain-Name-Server (6), Hostname (12)
	      Domain-Name (15), MTU (26), BR (28), Classless-Static-Route (121)
	      Default-Gateway (3), Static-Route (33), YD (40), YS (41)
	      NTP (42), Unknown (119), Classless-Static-Route-Microsoft (249), Unknown (252)
	      RP (17)
	    MSZ (57), length 2: 576
	    Requested-IP (50), length 4: 192.168.1.113
	    Hostname (12), length 6: "antoni"
E..>..@.@.8..........D.C.*..............................P{.2S=..........................................................................................................................................................................................................c.Sc5..=..P{.2S=7........y.!()*w...9..@2....q..antoni.
14:11:09.524667 IP6 (flowlabel 0x28cfd, hlim 255, next-header UDP (17) payload length: 146) fe80::dd:4422:2a7e:8542.5353 > ff02::fb.5353: [bad udp cksum 0xf3e1 -> 0xace7!] 0*- [0q] 2/0/0 2.4.5.8.e.7.a.2.2.2.4.4.d.d.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.8.e.f.ip6.arpa. (Cache flush) PTR antoni.local., antoni.local. (Cache flush) AAAA fe80::dd:4422:2a7e:8542 (138)
`.................D"*~.B.....................................2.4.5.8.e.7.a.2.2.2.4.4.d.d.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.8.e.f.ip6.arpa........x...antoni.local..`.......x............D"*~.B

50:7b:9d:32:53:3d is my computer's ENP0S25 interface MAC.

2

It appears to have been a cable problem. I connected to this router with 0.5m cat7 cable and everything worked great.

Previously I connected to the router with 5-7m long cat5e cable, which I also checked on another router and it worked fine, without any problem and I got transfer speeds ~1000 Mbps. I don't know why this cable works with the other router and doesn't work with this one....

3

If your problem is solved, please consider marking this topic as [Solved]. See How to mark a topic as [Solved] for a short how-to.

4

It’s not solved, I still don’t know what was the problem. This cat 5e cable works perfectly for me except for this one situation.

5

The RJ45 plugs can be damaged / deformed so they only make good contact in some sockets. It would be best to not use the suspect cable.

6

I made it myself... Maybe I bought RJ45 plugs too cheap?

7

Could be the pieces themselves, or could be that you didn’t make it quite right. There is likely an intermittent connection - maybe the wire isn’t inserted all the way into the rj45 or maybe the crimp didn’t go deep enough. Or maybe a wire was nicked or broken when you stripped the outer sheath.

A proper cable tester can help diagnose non-intermittent issues, but for now, either scrap the cable or re-terminate the ends.

8

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