Tag Archives: Fragmentation

It was MTU! Zscaler over GRE behind Palo, blocking ICMP Frag Needed

ICMP/ICMPv6, Palo Alto Networks, , , , , , , ,

The other day, I was troubleshooting an issue where users reported that “some websites are working while some are not“. Uh. This is almost the worst scenario to face from a networker’s perspective. It’s way easier if things do or don’t work at all, but not this “some don’t” situation.

The scenario: Using Zscaler for outbound Internet connections, connected via a GRE tunnel from a Palo Alto Networks firewall. TL;DR: If it’s not DNS, it’s MTU. 😂 The “Suppress ICMP Frag Needed” option within the ICMP Drop section of the Zone Protection Profile did what it is meant to do: block “ICMP fragmentation needed” messages. Unfortunately, this killed *some* sessions which had the “Don’t fragment” bit set but exceeded the (lower) MTU of the GRE tunnel.

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Path MTU Discovery

ICMP/ICMPv6, Internet Access, Routing, , , , , , ,

One of the mysteries for me in IP networks was the Path MTU Discovery (PMTUD) process. I’ve seldom seen any problems with the MTU at all. Fortunately, while troubleshooting some router issues, I captured several ICMP “packet too big” errors along with the original packets. 👍🏻

Let’s have a look at those PMTUD processes for IPv6 and legacy IP with Wireshark. Of course, these captured connections are part of the Ultimate PCAP as well, hence, you can download the most current version of it and analyze it by yourself.

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DNS Capture: UDP, TCP, IP-Fragmentation, EDNS, ECS, Cookie

DNS/DNSSEC, , , , , , , , , , , ,

It’s not always this simple DNS thing such as “single query – single answer, both via UDP”. Sometimes you have some more options or bigger messages that look and behave differently on the network. For example: IP fragmentation for larger DNS answers that do not fit into a single UDP datagram (hopefully not after the DNS flag day 2020 anymore), or DNS via TCP, or some newer options within the EDNS space such as “EDNS Client Subnet” (ECS) or DNS cookies.

I won’t explain any details about those options, but I am publishing a pcap with that kind of packets along with some Wireshark screenshots. Feel free to dig into it.

Continue reading DNS Capture: UDP, TCP, IP-Fragmentation, EDNS, ECS, Cookie →