Hi,

when I log in from an Octane to an Indigo2 using fw_openssh-3.7.1p2 and
RSA key authentication on both machines, I always get a "Warning: No
xauth data; using fake authentication data for X11 forwarding."

[be@octane be]$ ssh indigo
Warning: No xauth data; using fake authentication data for X11 forwarding.
[be@indigo be]$

Client's ssh_config:
$ egrep -v '^$|#' /etc/ssh/ssh_config
ForwardX11 yes

Server's sshd_config:
$ egrep -v '^$|#' /etc/ssh/sshd_config
Protocol 2
X11Forwarding yes
Subsystem sftp /usr/freeware/libexec/sftp-server

In 2003, there was a thread about it on comp.security.ssh (SGI
fw_openssh 3.5p1 and "Warning: No xauth data; using fake authentication
data for X11 forwarding.") but no solution was found:
http://www.derkeiler.com/Newsgroups/...3-01/0298.html

I can suppress the warning being printed by using the ssh option -q but
I don't know if it's wise to do so:

[be@octane be]$ ssh -q indigo
[be@indigo be]$

Debug output using -vv:

[be@octane be]$ ssh -vv indigo
OpenSSH_3.7.1p2, SSH protocols 1.5/2.0, OpenSSL 0.9.6j 10 Apr 2003
debug1: Reading configuration data /home/be/.ssh/config
debug1: Reading configuration data /etc/ssh/ssh_config
debug2: ssh_connect: needpriv 0
debug1: Connecting to indigo [192.168.1.13] port 22.
debug1: Connection established.
debug1: identity file /home/be/.ssh/identity type 0
debug1: identity file /home/be/.ssh/id_rsa type -1
debug2: key_type_from_name: unknown key type '-----BEGIN'
debug2: key_type_from_name: unknown key type '-----END'
debug1: identity file /home/be/.ssh/id_dsa type 2
debug1: Remote protocol version 2.0, remote software version OpenSSH_3.7.1p2
debug1: match: OpenSSH_3.7.1p2 pat OpenSSH*
debug1: Enabling compatibility mode for protocol 2.0
debug1: Local version string SSH-2.0-OpenSSH_3.7.1p2
debug1: SSH2_MSG_KEXINIT sent
debug1: SSH2_MSG_KEXINIT received
debug2: kex_parse_kexinit:
diffie-hellman-group-exchange-sha1,diffie-hellman-group1-sha1
debug2: kex_parse_kexinit: ssh-rsa,ssh-dss
debug2: kex_parse_kexinit:
aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour,aes192-cbc,aes256-cbc,rijndael-cbc@lysator.liu.se,aes128-ctr,aes192-ctr,aes256-ctr
debug2: kex_parse_kexinit:
aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour,aes192-cbc,aes256-cbc,rijndael-cbc@lysator.liu.se,aes128-ctr,aes192-ctr,aes256-ctr
debug2: kex_parse_kexinit:
hmac-md5,hmac-sha1,hmac-ripemd160,hmac-ripemd160@openssh.com,hmac-sha1-96,hmac-md5-96
debug2: kex_parse_kexinit:
hmac-md5,hmac-sha1,hmac-ripemd160,hmac-ripemd160@openssh.com,hmac-sha1-96,hmac-md5-96
debug2: kex_parse_kexinit: none,zlib
debug2: kex_parse_kexinit: none,zlib
debug2: kex_parse_kexinit:
debug2: kex_parse_kexinit:
debug2: kex_parse_kexinit: first_kex_follows 0
debug2: kex_parse_kexinit: reserved 0
debug2: kex_parse_kexinit:
diffie-hellman-group-exchange-sha1,diffie-hellman-group1-sha1
debug2: kex_parse_kexinit: ssh-rsa,ssh-dss
debug2: kex_parse_kexinit:
aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour,aes192-cbc,aes256-cbc,rijndael-cbc@lysator.liu.se,aes128-ctr,aes192-ctr,aes256-ctr
debug2: kex_parse_kexinit:
aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour,aes192-cbc,aes256-cbc,rijndael-cbc@lysator.liu.se,aes128-ctr,aes192-ctr,aes256-ctr
debug2: kex_parse_kexinit:
hmac-md5,hmac-sha1,hmac-ripemd160,hmac-ripemd160@openssh.com,hmac-sha1-96,hmac-md5-96
debug2: kex_parse_kexinit:
hmac-md5,hmac-sha1,hmac-ripemd160,hmac-ripemd160@openssh.com,hmac-sha1-96,hmac-md5-96
debug2: kex_parse_kexinit: none,zlib
debug2: kex_parse_kexinit: none,zlib
debug2: kex_parse_kexinit:
debug2: kex_parse_kexinit:
debug2: kex_parse_kexinit: first_kex_follows 0
debug2: kex_parse_kexinit: reserved 0
debug2: mac_init: found hmac-md5
debug1: kex: server->client aes128-cbc hmac-md5 none
debug2: mac_init: found hmac-md5
debug1: kex: client->server aes128-cbc hmac-md5 none
debug1: SSH2_MSG_KEX_DH_GEX_REQUEST sent
debug1: expecting SSH2_MSG_KEX_DH_GEX_GROUP
debug2: dh_gen_key: priv key bits set: 146/256
debug2: bits set: 1654/3191
debug1: SSH2_MSG_KEX_DH_GEX_INIT sent
debug1: expecting SSH2_MSG_KEX_DH_GEX_REPLY
debug1: Host 'indigo' is known and matches the RSA host key.
debug1: Found key in /home/be/.ssh/known_hosts:7
debug2: bits set: 1565/3191
debug1: ssh_rsa_verify: signature correct
debug2: kex_derive_keys
debug2: set_newkeys: mode 1
debug1: SSH2_MSG_NEWKEYS sent
debug1: expecting SSH2_MSG_NEWKEYS
debug2: set_newkeys: mode 0
debug1: SSH2_MSG_NEWKEYS received
debug1: SSH2_MSG_SERVICE_REQUEST sent
debug2: service_accept: ssh-userauth
debug1: SSH2_MSG_SERVICE_ACCEPT received
debug2: key: /home/be/.ssh/id_rsa (0)
debug2: key: /home/be/.ssh/id_dsa (1006a8b8)
debug1: Authentications that can continue:
publickey,password,keyboard-interactive
debug1: Next authentication method: publickey
debug1: Trying private key: /home/be/.ssh/id_rsa
debug1: Offering public key: /home/be/.ssh/id_dsa
debug2: we sent a publickey packet, wait for reply
debug1: Server accepts key: pkalg ssh-dss blen 435
debug2: input_userauth_pk_ok: fp
46:fa:7c:ae:35:cb:ee:2c:40:46:90:78:71:1c:78:ba
debug1: read PEM private key done: type DSA
debug1: Authentication succeeded (publickey).
debug1: channel 0: new [client-session]
debug2: channel 0: send open
debug1: Entering interactive session.
debug2: callback start
debug2: ssh_session2_setup: id 0
debug2: channel 0: request pty-req
debug2: x11_get_proto: /usr/bin/X11/xauth list :0.0 2>/dev/null
Warning: No xauth data; using fake authentication data for X11 forwarding.
debug1: Requesting X11 forwarding with authentication spoofing.
debug2: channel 0: request x11-req
debug2: channel 0: request shell
debug2: fd 3 setting TCP_NODELAY
debug2: callback done
debug2: channel 0: open confirm rwindow 0 rmax 32768
debug2: channel 0: rcvd adjust 131072
[be@indigo be]$