Linux dropping packet on first hop

JustAGuy

6/24/23, 12:37 PM

I have two sites that are connected through an MPLS tunnel. On-Site A, I have a Linux machine and on Site B, we have multiple servers.

Now the problem is, when I am trying to ping from Site B to Site A(from server-->linux machine), it is working properly.

But, when I try pinging from Site A to B (from Linux to servers), it's dropping.

I used traceroute and the packet is dropping on first-hop only.

[root@localhost ~]# traceroute 10.100.10.1
traceroute to 10.100.10.1 (10.100.10.1), 30 hops max, 60 byte packets
 1  * * *
 2  * * *
 3  * * *
 4  * * *
 5  * * *
 6  * * *
 7  * * *
 8  * * *
 9  * * *
10  * * *
11  * * *
12  * * *
13  * * *
14  * * *
15  * * *
16  * * *^C

0 + 0

networking

Thomas Ward

6/24/23, 3:26 PM

Did you verify from packet dumps on the other side of the tunnel to make sure data is actually being *received* on the other side of the MPLS? Have you checked firewalls on either side of the tunnel to make sure traffic from either side is permitted in the direction you're trying to do? A lot of times it's firewalls or such at play, not the Linux systems themselves.

JustAGuy

6/24/23, 3:43 PM

Thanks for replying. The problem is we cannot ping to the Linux machine default gateway(Firewall) also. It should go till its local gateway, then only we can assume that the firewall is dropping the packets.

Thomas Ward

6/24/23, 3:46 PM

Depending on the configuration of the gateway itself it *might not be configured* to send data back for the tracert. Use `mtr` instead of default traceroute if you can as it lets us see exactly how far traffic goes. Try and use a destination on the other side of the tunnel or to the Internet first to make sure that traffic still goes outside and if you see the first hop not reply. If that's the case, then it's likely whatever gateway there *is* blocking the ICMP reply, but that's not always indicative of a problem there, moreso network route rules or such tend to be a problem (firewalls)

Elon Musk

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

EN: Linux dropping packet on first hop

TH: Linux ทิ้งแพ็กเก็ตในการกระโดดครั้งแรก

RO: Linux aruncă pachetul la primul hop

RU: Linux отбрасывает пакет на первом прыжке

VI: Linux bỏ gói trên bước nhảy đầu tiên

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