Troubleshooting – connettere PPPoE verso Fastweb Ultra Fibra

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

mtu2

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 $

 

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