3 ms·
Check out the online ICMP / Fragmentation checker: IPv4 version: http://icmpcheck.popcount.org http://icmpcheck.popcount.org IPv6 version: http://icmpcheckv6.
by majke 9y ago
Check out the online ICMP / Fragmentation checker:
IPv4 version: http://icmpcheck.popcount.org http://icmpcheck.popcount.org
IPv6 version: http://icmpcheckv6.popcount.org http://icmpcheckv6.popcount.org
If you see red - this means your ISP / cable modem is not really internet happy.
Funny thing, the ISP I use fails on the ICMP PMTU message delivery test on the first refresh, but succeeds on the second. What happens? The ICMP is delivered to middle box, which then FRAGMENTS MY TCP PACKETS. In other word - the ICMP is stopped at a middle box, who will ignore my packets DF flag and still, fragment them without my system even knowing. On second refresh the middle box remembers the Path MTU will be saved so the middle box will reduce the MSS on SYN packet. This is remembered for about 15 minutes. Middle boxes suck.
- user5994461 9y agoOr it means that ICMP traffic is not working. Misconfigured networks are incredibly common. There is a massive misconception that ICMP is not needed or should be blocked for security. For example, you can google "should i open/block ICPM" and none of the stack overflow answers will tell you what's the right thing to do.
- jlgaddis 9y agoIndeed, and those who like to blindly drop all ICMP are gonna be hurting when it comes time to deploy IPv6. As noted in RFC4890 [0], "ICMPv6 is essential to the functioning of IPv6 ... Filtering strategies designed for the corresponding protocol, ICMP, in IPv4 networks are not directly applicable." So, if you currently just drop all ICMP at the edge, you might as well get used to doing it the right way now and save yourself some trouble in the future. [0]: https://www.ietf.org/rfc/rfc4890.txt https://www.ietf.org/rfc/rfc4890.txt