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#1 2013-03-15 16:44:03

poseidon
Member
Registered: 2012-06-07
Posts: 60

[SOLVED] Can't reach DNS 10.* from w/i a 172-net on a systemd archlinu

... I even can't reach the gateway.

What I have:

1 arch linux machine (tower PC, inst'ed arch 1 to 2 years ago) updgraded from rc.conf to systemd, still has an rc.conf.
1 archlinuxarm machine (RPi), latest release, i.e. original systemd config, an rc.conf not even exists.

The PC 172.16.121.77 works fine, i.e. it can reach all machines in the LAN and it can reach the Internet. RPI can ping the PC, the PC can ping the RPi, but the RPi 172.16.121.84 cannot ping the gateway 172.16.121.65, let alone the DNS, which (in both cases) is 10.10.40.21.

Both machines are in this net (output of ipcalc for the RPi's address):

Address:   172.16.121.84        10101100.00010000.01111001.010 10100
Netmask:   255.255.255.224 = 27 11111111.11111111.11111111.111 00000
Wildcard:  0.0.0.31             00000000.00000000.00000000.000 11111
=>
Network:   172.16.121.64/27     10101100.00010000.01111001.010 00000
HostMin:   172.16.121.65        10101100.00010000.01111001.010 00001
HostMax:   172.16.121.94        10101100.00010000.01111001.010 11110
Broadcast: 172.16.121.95        10101100.00010000.01111001.010 11111
Hosts/Net: 30                    Class B, Private Internet

172.16.121.65 is the gateway.

ip addr show on the PC (lines omitted except the inet line) yields:
inet 172.16.121.77/27 brd 172.16.121.255 scope global eth0

ip addr show on the RPi (lines omitted except the inet line) yields:
inet 172.16.121.84/27 brd 172.16.121.95 scope global eth0

ip route show on the PC yields:
default via 172.16.121.65 dev eth0
172.16.121.64/27 dev eth0  proto kernel  scope link  src 172.16.121.77

ip route show on the RPi yields:
default via 172.16.121.65 dev eth0
172.16.121.64/27 dev eth0  proto kernel  scope link  src 172.16.121.84

I can't find any difference that could explain, why the RPi hasn't full network access.

Any idea?

Last edited by poseidon (2013-03-20 11:33:06)

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#2 2013-03-16 23:16:36

p0x8
Member
Registered: 2012-09-20
Posts: 67

Re: [SOLVED] Can't reach DNS 10.* from w/i a 172-net on a systemd archlinu

The broadcast addresses of the RPi and the PC are different. This might be preventing the RPi from resolving the router's MAC address, and would explain your issues. Oddly, the RPi broadcast address is the correct one for the /27 network mask you're using, and the PC configuration should not even be valid.

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#3 2013-03-17 08:43:33

poseidon
Member
Registered: 2012-06-07
Posts: 60

Re: [SOLVED] Can't reach DNS 10.* from w/i a 172-net on a systemd archlinu

Yes, I had seen that too. Forgot to mention, that I tried the wrong broadcast with the RPi also. I couldn't see any effect, i.e. the DNS and theGW still not even ping'able.

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#4 2013-03-17 14:07:18

gsgleason
Member
Registered: 2012-10-08
Posts: 60

Re: [SOLVED] Can't reach DNS 10.* from w/i a 172-net on a systemd archlinu

Are you manually setting that incorrect broadcast address on the PC?  If not, something is very wrong.

Anyway, fist see if arp is working.  From RPi:

ping -c 1 172.16.121.65; arp -an|grep 172.16.121.65

See if it is able to resolve the MAC of the router.

Also 172.16.121.95 is the correct broadcast address for that subnet.

Can the RPi ping/arp the PC?

Remember that ping in itself is not a conslusive test.  The gateway or something else could be denying icmp echo.

What kind of router is the gateway?

Last edited by gsgleason (2013-03-17 14:25:08)

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#5 2013-03-19 09:04:51

poseidon
Member
Registered: 2012-06-07
Posts: 60

Re: [SOLVED] Can't reach DNS 10.* from w/i a 172-net on a systemd archlinu

gsgleason wrote:

Are you manually setting that incorrect broadcast address on the PC?  If not, something is very wrong.

Anyway, fist see if arp is working.  From RPi:

ping -c 1 172.16.121.65; arp -an|grep 172.16.121.65

See if it is able to resolve the MAC of the router.

PING 172.16.121.65 (172.16.121.65) 56(84) bytes of data.
From 172.16.121.84 icmp_seq=1 Destination Host Unreachable

--- 172.16.121.65 ping statistics ---
1 packets transmitted, 0 received, +1 errors, 100% packet loss, time 0ms

? (172.16.121.65) at <incomplete> on eth0


gsgleason wrote:

Also 172.16.121.95 is the correct broadcast address for that subnet.

Can the RPi ping/arp the PC?

Yes, that works.


gsgleason wrote:

Remember that ping in itself is not a conslusive test.  The gateway or something else could be denying icmp echo.

What kind of router is the gateway?

Have to check that (I'm in a lab).

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#6 2013-03-20 11:32:25

poseidon
Member
Registered: 2012-06-07
Posts: 60

Re: [SOLVED] Can't reach DNS 10.* from w/i a 172-net on a systemd archlinu

Man, I tell you... It has turned out that there was a limit set of how many different MAC addresses are allowed to connect to the lab network. It had been set to twenty and my RasPi was the the 21st. Incredible. The limit has been raised now and I'm online.  Sorry for bothering you.

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