Tech Note: SSH timeout failures between linux hosts
PROBLEM: could not ssh between Synology nas and Qnap nas using commandline ssh. Putty from windows worked fine.
Symptom: SSH timeout occurs at "expecting SSH2_MSG_KEX_ECDH_REPLY" stage.
SSHd config file must be fine because I am able to putty from a windows client. I noticed that I connected the Synology directly to the port of my Verizion FIOS Actiontec router. This router has four 1GbaseT lan ports. However it only allows an MTU size of 1500 on the LAN network configuration.
The apparant cause was the max MTU mismatch between the 9000 set on all LAN devices and the Actiontec router.
Solution was to connect the Synology to my HP switch which supports Jumboframes. SSH between Synology and Qnap devices succeeded.