[SOLVED] Ethernet only works when routed through another computer

jb1223 · 2021-02-12 09:04:00

Until very recently, I've been using my macbook to convert a wifi connection into an ethernet connection for my desktop, which dual boots Windows 10 and Arch. In both OSs, this required no extra setup that I can recall - it's been a while, but I'm 99% sure it just worked as soon as I plugged it in. This is generally how all of my ethernet connections have gone.

In the past few days, I finally got a direct wire set up from my router to my desktop, eliminating my macbook as the middleman. Strangely enough, it works just fine for Windows 10, but I can't connect to anything when I'm running Arch. The macbook setup still works fine, though, so at least the computer is still usable.

I have a rudimentary understanding of this stuff, but it's not nearly enough to diagnose my problems - I'm not sure if the issue is with my router or with my setup. I'd usually suspect my setup, but given that previous connections have been fine, I'd put the odds at 50/50. Anyway, here's what I've explored:

First, I pinged around and got the following results:

www.google.com

ping: www.google.com: Temporary failure in name resolution

8.8.8.8
```
ping: connect: Network is unreachable
```
192.168.1.254 (my default gateway)
```
ping: connect: Network is unreachable
```

With the macbook connection, these all work as expected. Not even being able to connect to the default gateway leads me to believe that the problem is with the router, but again I'm not sure.

Next, I checked out the output of ip addr. For both connection types, the loopback device was the same:

1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN group default qlen 1000
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
    inet 127.0.0.1/8 scope host lo
       valid_lft forever preferred_lft forever
    inet6 ::1/128 scope host 
       valid_lft forever preferred_lft forever

The first and last two lines of the ethernet device are the same for both connections:

2: enp0s31f6: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc fq_codel state UP group default qlen 1000
    link/ether 70:85:c2:6d:10:fa brd ff:ff:ff:ff:ff:ff
    ...
    inet6 fe80::df49:da6b:3da5:b4a4/64 scope link 
       valid_lft forever preferred_lft forever

However, the macbook connection has the following lines:

    inet 192.168.2.2/24 brd 192.168.2.255 scope global dynamic noprefixroute enp0s31f6
       valid_lft 85146sec preferred_lft 74454sec

while the direct connection instead has nothing.

This is where my basic understanding starts to break down. It seems like with the macbook, there are ipv4 and ipv6 connections, but with the direct connection, there's only ipv6, which doesn't work for some reason.

Trying to go off of that, I attempted to disable ipv6 in my dhcpcd.conf (pastebin, relevant lines at bottom). Unfortunately, that didn't do much other than change some of the bytes in the last address of the ip addr output.

I also checked out the routing tables. For both the macbook connection and the direct connection, ip -6 route gives:

::1 dev lo proto kernel metric 256 pref medium
fe80::/64 dev enp0s31f6 proto kernel metric 256 pref medium

On the other hand, ip route show with the macbook connection returns

default via 192.168.2.1 dev enp0s31f6 proto dhcp src 192.168.2.2 metric 1002 
192.168.2.0/24 dev enp0s31f6 proto dhcp scope link src 192.168.2.2 metric 1002

but it is empty for the direct connection. So again, it seems like I'm not getting an ipv4 address. But I don't fully understand routing tables, so that could be wrong.

I have checked multiple times that dhcpcd.service is enabled, and explicitly starting/restarting hasn't fixed any issues.

My NIC (intel) is detected, and the module (e1000e) is loaded.

Another thing to note is that the direct connection doesn't work even in a clean live USB environment.

Does anyone have any suggestions? This isn't super pressing, but I'd be immensely grateful for any advice. Thanks in advance!

On a side note, if anyone has any resources to help with understanding this stuff, I'd really appreciate links. I want to know what I'm looking at, but finding explanations has been rough.

Last edited by jb1223 (2021-02-13 17:42:49)

respiranto · 2021-02-13 09:27:35

What happens when you manually run `dhcpcd enp0s31f6' (as root)?

jb1223 · 2021-02-13 17:40:35

Hi respiranto! Thanks for the suggestion - I did try running that if I remember correctly, but it wasn't doing anything.

Fortunately, a power outage last night seems to have fixed my problem as of 10 minutes ago. The direct ethernet connection now seems to work no matter which OS I boot into, whether I fully shut down first or just restart from the other OS.

What seems most likely is that the router just needed to be restarted - usually I only do that when it's not working at all, but I guess this is another restart scenario.

I'll go ahead and mark this as solved, given that everything seems to be in working order. Moral of the story - restarting the router isn't just for when it's totally broken.

Thanks again!

ewaller · 2021-02-13 17:54:59

Hey neighbor, I know you marked this as 'solved', but I did not see what you are using to control your network. I am wondering if you may have a nondeterministic set up with more than one thing trying to control your network. If you do, it may be hit and miss whether things work.

Are you, by chance, at that school down on California Blvd?

jb1223 · 2021-02-14 03:25:14

Hi there ewaller! That's a good point, I should've mentioned it, but I'm just using dhcpcd as far as I can tell. I don't have a wifi card, so even though I have NetworkManager downloaded, it's disabled.

And good catch! I was at that school, just graduated this past December, on the job search now

Are you associated, or just local? I'm on the other coast now, but I do miss the area

Last edited by jb1223 (2021-02-14 03:38:11)

ewaller · 2021-02-14 15:43:47

Not associated, unfortunately; I am UCLA engineering some 36 years ago. Just a local, but had been thinking of buying you lunch. Oh well.

Arch Linux

#1 2021-02-12 09:04:00

[SOLVED] Ethernet only works when routed through another computer

#2 2021-02-13 09:27:35

Re: [SOLVED] Ethernet only works when routed through another computer

#3 2021-02-13 17:40:35

Re: [SOLVED] Ethernet only works when routed through another computer

#4 2021-02-13 17:54:59

Re: [SOLVED] Ethernet only works when routed through another computer

#5 2021-02-14 03:25:14

Re: [SOLVED] Ethernet only works when routed through another computer

#6 2021-02-14 15:43:47

Re: [SOLVED] Ethernet only works when routed through another computer

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