One IP in subnet not reachable from outside, but others work fine

thatleviathan

5/31/23, 6:09 PM

We had virtual host crash the other day, which caused a few VMs to reset and come back online. One of them, after coming back up, is not reachable on the network from outside the subnet it lives on. However, it is reachable from things in the same subnet. Nothing was changed on the VM other than it rebooted and migrated to a different host; other VMs on the same host are not having any network issues. We have tried migrating it to other host in the cluster but no change.

Oddly, if I apply a different IP address in the same subnet to the VM, it becomes reachable on that IP. It's as if just the one IP address is somehow broken or blocked. There are no firewall rules on the VM, and since it's in the same subnet, the gateway and netmask remain the same.

Normal configuration, not working:

IP 10.20.30.40/26; Gateway 10.20.30.1 - only reachable from other VMs in 10.20.30.0/26; it can ping gateway but nothing past it

Test configuration, same VM, reachable from outside:

IP 10.20.30.41/26; Gateway 10.20.30.1 - Reachable from outside 10.20.30.0/26 with no issues, and can reach out as well

I'm struggling to understand what could be wrong with the "original" IP that could be causing this. If it's something outside the VM I can have the Network team check, but I have no idea what to even ask them. What should I be checking?

0 + 0

networking

linux

virtual-machines

NiKiZe

5/31/23, 6:34 PM

double check subnet masks, trace it, tcpdump on each router, remember routing in both directions, also remember firewalls and also things like fail2ban and other rate limiting rules.

Brandon Xavier

5/31/23, 6:44 PM

Sounds like a bad entry in an ARP cache - try clearing the ARP cache on the gateway device.

shearn89

6/1/23, 9:25 AM

Is it using DHCP or static IP? Has it come up with a lease that has the wrong subnet mask? What routes does it say it has?

Elon Musk

I sit in a Tesla and translated this thread with Ai:

EN: One IP in subnet not reachable from outside, but others work fine

TH: IP หนึ่งรายการในเครือข่ายย่อยไม่สามารถเข้าถึงได้จากภายนอก แต่รายการอื่นๆ ทำงานได้ดี

RO: Un IP din subrețea nu este accesibil din exterior, dar altele funcționează bine

RU: Один IP в подсети недоступен извне, но другие работают нормально

VI: Một IP trong mạng con không thể truy cập được từ bên ngoài, nhưng những IP khác hoạt động tốt

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