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Add MTU and TCP-MSS discovery tool
Open, Requires assessmentPublicFEATURE REQUEST

Description

Create "MTU discovery tool"
This is discovery tool will propose you the best values for MTU and TCP-MSS for a specific interface.

For example nmap

vyos@r2-roll# sudo nmap -e eth0 --script path-mtu -F 1.1.1.1 
Starting Nmap 7.70 ( https://nmap.org ) at 2020-09-10 15:32 UTC
Nmap scan report for one.one.one.one (1.1.1.1)
Host is up (0.0063s latency).
Not shown: 97 filtered ports
PORT    STATE SERVICE
53/tcp  open  domain
80/tcp  open  http
443/tcp open  https

Host script results:
|_path-mtu: PMTU == 1500

Change mtu

set interfaces ethernet eth0 mtu 1300

Nmap

vyos@r2-roll# sudo nmap -e eth0 --script path-mtu -F 1.1.1.1 
Starting Nmap 7.70 ( https://nmap.org ) at 2020-09-10 15:34 UTC
Nmap scan report for one.one.one.one (1.1.1.1)
Host is up (0.0063s latency).
Not shown: 97 filtered ports
PORT    STATE SERVICE
53/tcp  open  domain
80/tcp  open  http
443/tcp open  https

Host script results:
|_path-mtu: PMTU == 1300

MTU size - 40 bytes = tcp-mss

Details

Difficulty level
Unknown (require assessment)
Version
-
Why the issue appeared?
Will be filled on close
Is it a breaking change?
Unspecified (possibly destroys the router)

Event Timeline

jack9603301 added a subscriber: jack9603301.
pasik added a subscriber: pasik.Thu, Sep 10, 5:26 PM
This comment was removed by Viacheslav.

PR https://github.com/vyos/vyos-1x/pull/545

vyos@r1-roll:~$ force mtu host 192.168.122.1

Recommended maximum values (or less) for target 192.168.122.1:
---
MTU:     1500
TCP-MSS: 1460
TCP-MSS_IPv6: 1440