pmtu-diag.py

Bidirectional Path MTU diagnostics via raw ICMP, built on scapy.

pmtu-diag.py measures the Path MTU separately for each direction of a link and tells you whether PMTU Discovery actually works on the path — something a plain ping cannot do. It crafts and parses packets as structured objects (pkt[IP].flags.MF, pkt[ICMP].type, pkt[IP].len) rather than scraping ping/tcpdump text, so behaviour is deterministic and identical on Linux and macOS.

Two run modes: interactive CLI use (colored, full header) and a quiet mode for embedding behind a web frontend (--quiet-header) — see below.

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Install/Update

/bin/bash -c "$(curl -fsSL https://raw.githubusercontent.com/ewaldj/pmtu-diag/main/e-install.sh)"

e-install.sh downloads pmtu-diag.py from this repository, installs it into a target directory (default /usr/local/bin, prompted interactively), and creates an extensionless symlink pmtu-diag alongside it. It uses sudo automatically if the target directory isn't writable by the current user. Re-running the command updates an existing install in place.

What it measures

Direction	Method
Forward (us → target)	DF echo probes + binary search, cross-checked against the next-hop MTU reported in ICMP Fragmentation Needed (Type 3 / Code 4, RFC 1191).
Reply (target → us)	A non-DF request whose echo reply is large enough to force the target/return path to fragment; inbound fragments are captured at link level before kernel reassembly, and their IP total length is read directly.
PMTUD health	Detects whether the ICMP Fragmentation Needed signal is actually returned, distinguishing a working path from an ICMP-filtered black hole.

This separation matters: a path can carry a large forward MTU while the return path silently fragments (asymmetric MTU), and a NAT/VPN gateway can swallow the ICMP signal entirely, causing TCP to stall instead of failing cleanly.

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Why link-level capture

The kernel reassembles inbound IP fragments before a normal socket (scapy's sr()) ever sees them, so sr() reports one clean packet and hides the fragmentation. pmtu-diag.py runs an AsyncSniffer (BPF / AF_PACKET, non-promiscuous) to capture the raw fragments on the wire. The IP total length of the first fragment (MF set, offset 0) is the return-path MTU at the narrowest hop.

Note on precision: a fragment's first part is rounded down to an 8-byte boundary, so the captured value is a lower bound — the true MTU lies in reply_mtu … reply_mtu + 7.

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Requirements

• Python 3.8+
• scapy
• Raw socket access: root, or a process with effective CAP_NET_RAW (Linux only). The script checks for this itself (have_cap_net_raw(), via /proc/self/status) and auto-elevates via sudo only if neither is present.

Install scapy

# Debian / Ubuntu
sudo apt install python3-scapy        # or: pip install scapy

# macOS (Homebrew Python)
pip3 install scapy

# generic / PEP 668 environments
pip install scapy --break-system-packages

Running without root (CAP_NET_RAW), e.g. for a web backend

For non-interactive use (cron, a web backend calling the script) you don't want a sudo password prompt. Grant CAP_NET_RAW to a dedicated copy of the Python interpreter instead — have_cap_net_raw() picks this up automatically and skips the sudo re-exec.

Capabilities only work on filesystems supporting the security.capability xattr (ext4/xfs/btrfs — not tmpfs/overlayfs/NFS/9p).

# 1. Locate the real interpreter binary (not a venv shim/symlink)
readlink -f "$(which python3)"

# 2. Dedicated copy — do NOT grant the capability to the system-wide python3,
#    or every script run by it could open raw sockets.
sudo cp /usr/bin/python3.11 /usr/local/bin/python3-pmtudiag

# 3. Grant the capability
sudo setcap cap_net_raw+eip /usr/local/bin/python3-pmtudiag

# 4. Verify
getcap /usr/local/bin/python3-pmtudiag
# -> /usr/local/bin/python3-pmtudiag cap_net_raw=eip

# 5. Restrict who can use it
sudo groupadd pmtudiag
sudo chown root:pmtudiag /usr/local/bin/python3-pmtudiag
sudo chmod 750 /usr/local/bin/python3-pmtudiag
sudo usermod -aG pmtudiag <user>       # re-login (or `newgrp pmtudiag`) to apply

Run with the dedicated interpreter instead of python3/sudo:

/usr/local/bin/python3-pmtudiag pmtu-diag.py <target-IP>

Troubleshooting:

• setcap silently not taking effect → confirm the filesystem (df -T <path>) supports xattrs; check echo $? after setcap; confirm libcap2-bin is installed.
• Module not found only through the copy → check sys.path/sys.prefix (/usr/local/bin/python3-pmtudiag -c "import sys; print(sys.prefix, sys.path)"); fix via PYTHONPATH/PYTHONHOME.
• Root but still failing → in a container, root can lack CAP_NET_RAW in the effective set even though euid == 0; the script detects and reports this case explicitly rather than silently falling back to sudo.
• The capability is tied to that exact file; re-copy and re-setcap after any interpreter upgrade/reinstall.

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Usage

./pmtu-diag.py <target-IP> [-i IFACE] [-c COUNT] [--max MTU] [--timeout SEC] [--quiet-header]

Option	Default	Description
target	—	Destination IP address or hostname (required).
-i, --iface	scapy route lookup	Network interface to probe from.
-c, --count	3	Probes per test; a majority (count//2 + 1) must succeed to confirm a size.
--max	local MTU	Upper MTU bound for the forward binary search.
--timeout	2.0	Per-probe reply timeout in seconds.
--quiet-header	off	Suppress the banner/timestamp/interface/local-MTU/source-IP header lines. For embedding behind a web frontend (e.g. pmtu.php): those server-internal details aren't meaningful to a visitor, and the target is already shown on the page. The 1/5 … 5/5 section headers still print.

The script auto-elevates with sudo when neither root nor CAP_NET_RAW is available, so a leading sudo is optional interactively.

Examples

# Probe a remote gateway over the default route
./pmtu-diag.py 192.0.2.1

# Force a specific interface and a wider search ceiling
./pmtu-diag.py 198.51.100.10 -i en0 --max 9000

# Higher confidence on a lossy/mobile link
./pmtu-diag.py 203.0.113.5 -c 5 --timeout 3

# Web-embedded call (no banner/interface/local-IP details)
./pmtu-diag.py 192.0.2.1 --quiet-header

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Output

The run is structured in five stages:

1. Reachability — basic ICMP echo sanity check.
2. PMTUD function test — small DF must pass; oversized DF should return Fragmentation Needed.
3. Forward path MTU — verified by DF binary search.
4. Reply path MTU — measured from captured inbound fragments.
5. Result — effective bidirectional PMTU, path symmetry, PMTUD verdict, and an MSS suggestion.

The final report includes a ready-to-use MSS clamp, e.g.:

Cisco IOS:   ip tcp adjust-mss 1452   (on the interface)

PMTUD verdicts

Verdict	Meaning
WORKING	ICMP Fragmentation Needed was returned — PMTUD functions.
ICMP_FILTERED	DF packets dropped silently — black-hole risk, clamp MSS.
NOT_TRIGGERED	No bottleneck below the search ceiling — raise --max to test.
OFFLOAD_BYPASS	DF appears bypassed (NIC offload / path fragmenting).

A suspected PMTUD black hole / NAT issue is flagged separately when DF packets vanish without any ICMP signal — typical of NAT, CGNAT, or VPN gateways that fail to relay ICMP back to the host.

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Notes & limitations

• IPv4 only. ICMPv6 Packet Too Big is not yet handled.
• Target must be a remote IP. A loopback / local address is detected and rejected — local traffic never crosses the NIC, so raw probes see no reply.
• On macOS/BPF the OS rejects DF packets larger than the egress NIC MTU locally (Message too long). The script caps probes at the local MTU and reports such cases as a local send limit, not a path result.
• The reply-path MTU is a lower bound (8-byte fragment rounding, see above).
• CAP_NET_RAW auto-detection (have_cap_net_raw()) is Linux-only; on macOS the script always relies on euid == 0 / sudo.

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Versioning

Current version: VERSION at the top of pmtu-diag.py. Each change bumps it by 0.01; the previous version is archived to Archive/pmtu-diag-v<old-version>.py.

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License / attribution

by ewald@jeitler.cc — https://www.jeitler.guru

When I wrote this code, only God and I knew how it worked. Now only God and the AI know it. And since the AI helped write it… good luck to all of us.