Lo scenario:
- Alice PC (TelecomItalia ADSL2+ 8M)
- Bob PC (Fastweb UltraFibra 100M)
Alice vuole connettersi verso Bob. Alice riesce a vedere le l’Hello World dell’Apache di Bob, ma non a scaricare file. Alice riesce anche ad iniziare una connessione ssh, ma resta bloccata su SSH2_MSG_KEXINIT:
alice@pc $ ssh bob.pc -vvv OpenSSH_6.6.1, OpenSSL 1.0.1f 6 Jan 2014 debug1: Reading configuration data /home/alice/.ssh/config debug1: /home/alice/.ssh/config line 1: Applying options for bob.pc debug3: ciphers ok: [arcfour] debug1: Reading configuration data /etc/ssh/ssh_config debug1: /etc/ssh/ssh_config line 19: Applying options for * debug2: ssh_connect: needpriv 0 debug1: Connecting to bob.pc [aaa.bbb.ccc.ddd] port 22. debug1: Connection established. debug3: Incorrect RSA1 identifier debug3: Could not load "/home/alice/.ssh/id_rsa" as a RSA1 public key debug1: identity file /home/alice/.ssh/id_rsa type 1 debug1: identity file /home/alice/.ssh/id_rsa-cert type -1 debug1: identity file /home/alice/.ssh/id_dsa type -1 debug1: identity file /home/alice/.ssh/id_dsa-cert type -1 debug1: identity file /home/alice/.ssh/id_ecdsa type -1 debug1: identity file /home/alice/.ssh/id_ecdsa-cert type -1 debug1: identity file /home/alice/.ssh/id_ed25519 type -1 debug1: identity file /home/alice/.ssh/id_ed25519-cert type -1 debug1: Enabling compatibility mode for protocol 2.0 debug1: Local version string SSH-2.0-OpenSSH_6.6.1p1 Ubuntu-8 debug1: Remote protocol version 2.0, remote software version OpenSSH_6.6.1p1 Ubuntu-8 debug1: match: OpenSSH_6.6.1p1 Ubuntu-8 pat OpenSSH_6.6.1* compat 0x04000000 debug2: fd 3 setting O_NONBLOCK debug3: put_host_port: [bob.pc]:22 debug3: load_hostkeys: loading entries for host "[bob.pc]:22" from file "/home/alice/.ssh/known_hosts" debug3: load_hostkeys: found key type ECDSA in file /home/alice/.ssh/known_hosts:98 debug3: load_hostkeys: loaded 1 keys debug3: order_hostkeyalgs: prefer hostkeyalgs: ecdsa-sha2-nistp256-cert-v01@openssh.com,ecdsa-sha2-nistp384-cert-v01@openssh.com,ecdsa-sha2-nistp521-cert-v01@openssh.com,ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521 debug1: SSH2_MSG_KEXINIT sent
La connessione qui è partita, ma sembra che durante l’invio di pacchetti IP più grandi il tutto si blocca.
Alice ha un modem TelecomItalia Alice Gate Wifi, non configurabile, e usa connessione PPPoE 8/35. Dando un’occhiata all’RFC 2516 (A Method for Transmitting PPP Over Ethernet) si legge:
The Maximum-Receive-Unit (MRU) option MUST NOT be negotiated to a larger size than 1492. Since Ethernet has a maximum payload size of 1500 octets, the PPPoE header is 6 octets and the PPP Protocol ID is 2 octets, the PPP MTU MUST NOT be greater than 1492
Visto che Fastweb UltraFibra (sembra) usi di default un MTU superiore (1500) è necessario che Bob riconfiguri l’interfaccia di rete affinché Alice possa connettersi impostando un Maximum Transmission Unit inferiore:
bob@pc $ sudo ifconfig eth0 mtu 1492
Con questa impostazione Alice può finalmente comunicare con il PC di Bob:
alice@pc $ ssh bob.pc alice@bob.pc $