Server Fault Stack Exchange
Server Fault Stack Exchange

Questions tagged [icmp]

ICMP stands for Internet Control Message Protocol. It is primarily used by the operating systems of networked computers to send error messages indicating service status.

ICMP is not normally used by end-users or to send messages. It's main use is in diagnostics and checking that the network is functioning correctly.

ICMP is just one communications protocol on the web. Others include:

  • Transmission Control Protocol (TCP)
  • User Datagram Protocol (UDP)
  • Hypertext Transfer Protocol (HTTP)
  • Post Office Protocol (POP3)
  • File Transfer Protocol (FTP)
  • Internet Message Access Protocol (IMAP)

ICMP on Wikipedia

226 questions
1
vote
1 answer

Cisco ASA 5500 - Internet connection dies when adding ICMP exception to firewall

When I add ICMP to be allowed both on inside / outside interfaces the internet connection dies. Does anyone know why this happens? I kind of observed that this behavior takes place not just for ICMP.
Bogdan Sorlea
  • 163
  • 2
  • 5
1
vote
0 answers

Why is my CentOS box rejecting some (but not all) ICMP ping requests from the same remote host?

CentOS 4.x I've got several old CentOS 4.x systems and have configured iptables to allow ICMP traffic. Originally, I had an iptables rule that looked like: -A RH-Firewall-1-INPUT -p icmp --icmp-type echo-request -m limit --limit 1/s -j…
Mike B
  • 11,871
  • 42
  • 107
  • 168
1
vote
1 answer

Nagios dropping ICMP packets

I am running a Nagios server on vmware 4.0 and every now and again during the day it alerts that some of servers cannot be reached via ICMP, clearly staging that a certain percentage of packets send are lost. This does not happen to all servers. I…
Ankh2054
  • 1,414
  • 13
  • 23
1
vote
2 answers

What is the difference between flow control and congestion control at layer 2 and 3?

Is there a difference between flow control and congestion control at the following layers? layer 2(data link layer) layer 3(network layer) If they are not same, then how is each handled at the following layers? layer 2(data link layer) layer…
gsinha
  • 323
  • 1
  • 4
  • 15
1
vote
1 answer

What is the effect of stale ARP cache entry on traffic to that DIP till the entry ages out of the ARP table?

If the host, for which an ARP table entry is present on the connected router, has changed its NIC(so that its MAC address has changed), what will happen when a packet is sent out on the link with the old MAC address by the router? Will any kind of…
gsinha
  • 323
  • 1
  • 4
  • 15
1
vote
0 answers

Where is HP Procurve route type "ICMP" coming from?

I've got the following test network: Architecture: edge_a: 192.168.e.1 [edge vlan] (edge switch, no acl) [admin vlan] 192.168.m.1/24 core: 192.168.m.2/24 [admin vlan] (core router, yes acl) [admin vlan] edge_b: 192.168.m.3/24 [admin vlan]…
Shaun
  • 31
  • 2
1
vote
1 answer

ICMP Data Field Modified - What does it Mean?

Normal ICMP Data fields are composed of a pretty standard 32 byte string of alphabet characters. abcdefghijklmnopqrstuvwabcdefghi I have captured a series of ICMP echo requests using WireShark with a modified Data field and I have no idea what it…
Lucretius
  • 459
  • 1
  • 4
  • 14
1
vote
1 answer

Linux router - how to send icmp unreachable messages to LAN clients

I have a Linux home router with eth0 (192.168.0.2/24) and bridge br0 for LAN clients (192.168.1.1/24). Bridge br0 contains eth1 (wired) and eth2 (wireless). The router routes all LAN traffic via eth0 (192.168.0.2) to an ADSL router (192.168.0.1) via…
drumfire
  • 1,699
  • 3
  • 14
  • 21
1
vote
1 answer

iptables owner match drops icmp for GIDs but not for UIDs

I have a strange behaviour of iptables that I cannot explain or understand. I have a user test which has its primary group test. I want to block any network access from this user. Now, if I say iptables -A OUTPUT -o eth0 -m owner --uid-owner test -j…
bitmask
  • 123
  • 1
  • 1
  • 8
1
vote
2 answers

Pinging same address multiple times

I am just wondering if it is possible to ping the same ip address from the same source multiple times (ie fast enough so they get pipelined) and still get a valid result. By a valid result, I mean that you record a valid round trip time for all ping…
user623879
1
vote
2 answers

Windows Server 2008 machine only responds to PING from some machines

I have a colocated Windows Server 2008 machine that replies to PING from some machines but not others (not most machines). I'm not sure where to start to diagnose what the problem is. The server is a slightly odd setup: The server itself (S1) is…
Rory
  • 482
  • 5
  • 12
  • 22
1
vote
0 answers

Linux confusing MAC addresses for its IP addresses to Cisco switch?

I have multiple CentOS boxes (both Centos 7) with two different servers. These servers have 4 10GB interfaces each. They are connected to two Cisco Nexus switches. Each NIC has a different IP address, all in the same domain. In my example, my server…
user1955162
  • 296
  • 3
  • 16
1
vote
0 answers

ICMP TTL exceeded replies with destination IP from NAT router

I'm playing with nftables and observe strange behaviour which I cannot explain. I have three VMs, source, router and destination. All run latest Oracle EL 8.5 and are configured via nft. source has single network interface enp0s8 with IP…
Reisse
  • 11
  • 1
1
vote
0 answers

PMTUD/ICMP black hole problems across a VXLAN tunnel

I'm running a Proxmox PVE host and am trying to use VXLAN to connect the machines running on it to various networks in our Lab. However, I'm running into weird MTU-related problems that I don't understand. First my setup. The basic layout is that…
Christopher Engelhard
  • 11
  • 1
1
vote
1 answer

Ping across IPSec tunnel sends both ICMP and ESP packets

I've been using Strongswan to setup an IPSec tunnel between two units. The tunnels SAs get setup without any issues and traffic can pass across the connection. Whenever I ping across the tunnel, the ping request is sent as BOTH an ESP and ICMP…
jcnoe
  • 21
  • 2
1 2 3
…
15 16