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Knowledge Base

Verifying gateway IP connection using the tcpdump command (1008017)

Last Updated: 3/20/2024Categories: TroubleshootingTotal Views: 26719Language:
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Purpose

This article provides information about the tcpdump command and how to use the command to verify gateway IP connection.

Resolution

Note: Before you perform the steps in this article, configure the host ESX for network sniffing. For more information, see Troubleshooting network issues by capturing and sniffing network traffic (1004090) .

The tcpdump command is used to:

Verify connectivity issues with various network nodes (for example, latency, session, and packet dropping)

Verify TCP/IP communication between network applications

Intermediate TCP/IP stack issues

Ping is an easy way to tell if an interface is functioning. While the ping command executes, run tcpdump to sniff ICMP messages. There are two parts to an ICMP sequence: echo and reply. For every ICMP request, there is a reply from the destination host to the target host. Failure to see the ICMP messages when there is active ping between two nodes is indicative of IP configuration issues. IP implementations are required to support ICMP (which provides error reporting, flow control and first-hop gateway redirection).

Note: Many firewalls block IP ICMP. Use the telnet command to see if a firewall is blocking the connection.

To use the tcpdump command to verify gateway IP connectivity:

Ping the desired host. Use a command similar to:

ping -I vswif1 <10.10.10.1>

where <10.10.10.1> is the IP address of the gateway

If the ping is successful, the output looks similar to:

PING 10.10.10.1 (10.10.10.1) from 10.10.10.20 vswif1: 56(84) bytes of data.
64 bytes from 10.10.10.1: icmp_seq=0 ttl=255 time=0.611 ms
64 bytes from 10.10.10.1: icmp_seq=1 ttl=255 time=0.374 ms
64 bytes from 10.10.10.1: icmp_seq=2 ttl=255 time=0.334 ms
64 bytes from 10.10.10.1: icmp_seq=3 ttl=255 time=0.311 ms

If the ping is unsuccessful, the output looks similar to:

ping -I vswif1 10.10.10.1
From 10.10.10.20 icmp_seq=91 Destination Host Unreachable
From 10.10.10.20 icmp_seq=92 Destination Host Unreachable
From 10.10.10.20 icmp_seq=93 Destination Host Unreachable
From 10.10.10.20 icmp_seq=94 Destination Host Unreachable
From 10.10.10.20 icmp_seq=95 Destination Host Unreachable

While the ping command executes, run the tcpdump command:

tcpdump -i vswif1 -n -vvv ether proto <IP>

If the command is successful, the output looks similar to:

tcpdump: listening on vswif1
03:34:45.742359 10.10.10.20 > 10.10.10.1: icmp: echo request (DF) (ttl 64, id 0, len 84)
03:34:45.742649 10.10.10.1 > 10.10.10.20: icmp: echo reply (DF) (ttl 255, id 0, len 84)
03:34:46.587244 10.10.10.51.32768 > 10.10.10.220.902: udp 66 (DF) (ttl 64, id 0, len 94)
03:34:46.742345 10.10.10.20 > 10.10.10.1: icmp: echo request (DF) (ttl 64, id 0, len 84)
03:34:46.742668 10.10.10.1 > 10.10.10.20: icmp: echo reply (DF) (ttl 255, id 0, len 84)

If the command is unsuccessful, the output looks similar to:

06:13:19.852336 10.10.10.20 > 10.10.10.1: icmp: echo request (DF) (ttl 64, id 0, len 84)
06:13:20.852339 10.10.10.20 > 10.10.10.1: icmp: echo request (DF) (ttl 64, id 0, len 84)
06:13:21.852337 10.10.10.20 > 10.10.10.1: icmp: echo request (DF) (ttl 64, id 0, len 84)
06:13:22.852368 10.10.10.20 > 10.10.10.1: icmp: echo request (DF) (ttl 64, id 0, len 84)
06:13:23.852375 10.10.10.20 > 10.10.10.1: icmp: echo request (DF) (ttl 64, id 0, len 84)
06:13:24.882561 10.10.10.20 > 10.10.10.1: icmp: echo request (DF) (ttl 64, id 0, len 84)

Note: The output for the unsuccessful tcpdump command indicates that gateway interface 10.10.10.1 is not replying to the echo request.

Repeat steps 1 and 2 with the arping commands:

arping -I vswif0(eth0) <10.10.10.1<SPAN style="COLOR: #000000">>

where <10.10.10.1> is the IP address of the gateway

arp -a (to view arp list)

While the arping command executes, run the tcpdump command and look for an echo reply.

If there is no echo reply, check gateway interface configuration and reset by disabling and re-enabling.

For more information, see Resolving virtual machine IP address conflicts issues (1008177).

Related Information

For more information, see http://www.tcpdump.org/ or the TCPDUMP man pages at http://www.tcpdump.org/tcpdump_man.html .

To access the man pages from the command line, run the command:

man tcpdump

Note: The preceding links were correct as of April 23, 2009. If you find that the link is broken, please provide feedback and a VMware employee will update the link.

For more information about troubleshooting TCP/IP connection issues, see Troubleshooting virtual machine TCP/IP connection issues (1007842).

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