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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
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icmp ping reply not received by ping process, seen by tcpdump/wireshark

I'm having an issue with getting the actual ping process to receive the reply even though I can clearly see the response in the tcpdump output. I'm running on: Mac OS X 10.11.4 El Capitan Bridge interface with tap interface as a member qemu is…
McBirdy
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Linux PCs sending "ICMP udp port unreachable" to DNS server

So digging around in my syslog I have been noticing a lot of ICMP packets being flagged by our ASAs: %ASA-4-313005: No matching connection for ICMP error message: icmp src Internet:x.x.21.122 dst MGMT:x.x.36.55 (type 3, code 3) on Internet…
JustNobody30
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ICMP replies - ingress or egress interface (e.g. from a traceroute)

When a traceroute is initiated and receives an ICMP reply from the nodes, which interface should the be replying be from according to RFC 1812. they are actually replying from ingress (where they receive the packet) or egress (where the packet…
leviu
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Why woud ping fail but not traceroute

For a few days now I started making the interesting observation of a machine intermittently not responding to an ICMP ping, while it still responds to an ICMP based traceroute (specifically set via the -I parameter). As far as I know both would use…
user297763
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What should happen when receiving an ICMP Fragmentation Needed message?

In general, and in particular on a Solaris 10 machine ... See, we are having an issue in our network. I handle some reverse proxy instances on a Solaris 10 server that is behind a load balancer, and some users access this via a NAT firewall and ...…
Jesús Couto Fandiño
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Outgoing packet has NATed src address

Configuration/Topology: There are 3 machines hadoop2 | hadoop | driver eth0 10.10.15.3 | eth0 10.10.15.2 | tap0 192.168.0.199 route default to 10.10.15.1 | tap0…
Dawid Pura
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Linux is sending ARP requests to hosts in other subnets?

Setup Host B <--> Router <--> Host A Host A: IP = 192.168.1.10, Net = 192.168.1.0/24, VLAN = 1, Default GW = 192.168.1.1 (Router) Host B: IP = 192.168.2.10, Net = 192.168.2.0/24, VLAN = 20, Default GW = 192.168.2.1 (Router) Router: IP =…
Deputy Rock
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with fully permissive iptables, tcpdump shows icmp echo requests arriving, but no reply's are sent

I'm a bit stumped on where to look next. When using SoftEther IPsec VPN into my network, I can access every server in the main subnet but one. When behind the main firewall, the server responds to all network traffic as expected. Please note, that…
Nate Byrnes
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What is the "right way" to monitor network?

My production servers are kept at US east coast and some of the supporting apps are kept at Amsterdam in Europe. There is a Nagios instance running at US east coast as well which does a few port checks and a few checks via ssh. Problem is that…
Aditya Patawari
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Test a server using MTR through TCP

Is it possible to use MTR over TCP? If not is there an alternative? I have done some research and I am not finding any way to do such a thing, so I'm wondering if anyone has had any experience with doing such a thing. If there really is no such…
Matthew Salsamendi
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Generating ICMP packets when TTL=2?

By inspecting the payload of ICMP time-exceeded packets, I noticed that sometimes it is the last but one router (when ttl=2 in the returned packet) or even a previous one(up to 5 hops before, ttl=5) that drops the packet and generates an ICMP…
Ricky Robinson
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Understanding how Tracert works

From what I gathered so far, Tracert works by sending 3 ICMP echo messages. Starting with a TTL value of 1. For each router the packet encounters, the TTL value will be decremented. For the 1st router, 1-1 = 0, so an ICMP "time exceeded" message…
iridescent
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Can i stop tcp handshake by send icmp of unreachable host

This is what happened: client - send syn server - send icmp (unreachable host [i also try to add code =10] ) wait for 100 ms send syn-ack client - send ack in response to the syn-ack. Dosnt the client should reset the tcp handshake…
Avihai Marchiano
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Can fragments of a packet be refragmented again?

In IPv4, fragmentation is done by routers on way to the destination if DF(do not fragment) flag is not set in the IP packet. Once a packet is fragmented, its fragments may take different paths (due to various reasons like topology changes) to the…
gsinha
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Can i change the default AWS EC2 Public DNS?

I'm using Route 53 and created A record to the Elastic IP that associated with an instance. In Security Group, i activated the "All ICMP" and make it accessible from anywhere (0.0.0.0/0) so i can ping it. When i pinged my domain this is the…
panggi
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