Hello all, long time no see...
I've been using ssh for quite a while to connect to a specific machine using
public key authentication. Now I want to connect to the same machine but a
different username. I did the usual copy of the local .ssh/id_dsa.pub into
the remote .ssh/authorized_keys but still the public key authentication
doesn't work. It reverts to password login which works fine.

Any idea what may be wrong but the public key authentication ?

Here's a login debug:
dargaud@penguin ~/home 0 $ ssh -vv otheruser@somewhere.remote.com
OpenSSH_4.4p1, OpenSSL 0.9.8d 28 Sep 2006
debug2: ssh_connect: needpriv 0
debug1: Connecting to somewhere.remote.com [IP.IP.IP.IP] port 22.
debug1: Connection established.
debug1: identity file /home/dargaud/.ssh/identity type -1
debug1: identity file /home/dargaud/.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/dargaud/.ssh/id_dsa type 2
debug1: Remote protocol version 1.99, remote software version
OpenSSH_3.6.1p2
debug1: match: OpenSSH_3.6.1p2 pat OpenSSH_3.*
debug1: Enabling compatibility mode for protocol 2.0
debug1: Local version string SSH-2.0-OpenSSH_4.4
debug2: fd 3 setting O_NONBLOCK
debug1: SSH2_MSG_KEXINIT sent
debug1: SSH2_MSG_KEXINIT received
debug2: kex_parse_kexinit:
diffie-hellman-group-exchange-sha256,diffie-hellman-g
roup-exchange-sha1,diffie-hellman-group14-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,arcfour1
28,arcfour256,arcfour,aes192-cbc,aes256-cbc,rijndael-cbc@lysator.liu.se,aes128-c
tr,aes192-ctr,aes256-ctr
debug2: kex_parse_kexinit:
aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour1
28,arcfour256,arcfour,aes192-cbc,aes256-cbc,rijndael-cbc@lysator.liu.se,aes128-c
tr,aes192-ctr,aes256-ctr
debug2: kex_parse_kexinit:
hmac-md5,hmac-sha1,hmac-ripemd160,hmac-ripemd160@open
ssh.com,hmac-sha1-96,hmac-md5-96
debug2: kex_parse_kexinit:
hmac-md5,hmac-sha1,hmac-ripemd160,hmac-ripemd160@open
ssh.com,hmac-sha1-96,hmac-md5-96
debug2: kex_parse_kexinit: none,zlib@openssh.com,zlib
debug2: kex_parse_kexinit: none,zlib@openssh.com,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-gro
up1-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
debug2: kex_parse_kexinit:
aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour,
aes192-cbc,aes256-cbc,rijndael-cbc@lysator.liu.se
debug2: kex_parse_kexinit:
hmac-md5,hmac-sha1,hmac-ripemd160,hmac-ripemd160@open
ssh.com,hmac-sha1-96,hmac-md5-96
debug2: kex_parse_kexinit:
hmac-md5,hmac-sha1,hmac-ripemd160,hmac-ripemd160@open
ssh.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(1024<1024<8192) sent
debug1: expecting SSH2_MSG_KEX_DH_GEX_GROUP
debug2: dh_gen_key: priv key bits set: 124/256
debug2: bits set: 494/1024
debug1: SSH2_MSG_KEX_DH_GEX_INIT sent
debug1: expecting SSH2_MSG_KEX_DH_GEX_REPLY
debug1: Host 'somewhere.remote.com' is known and matches the RSA host key.
debug1: Found key in /home/dargaud/.ssh/known_hosts:10
debug2: bits set: 507/1024
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/dargaud/.ssh/identity (0x0)
debug2: key: /home/dargaud/.ssh/id_rsa (0x0)
debug2: key: /home/dargaud/.ssh/id_dsa (0x6aa7a8)
debug1: Authentications that can continue:
publickey,password,keyboard-interacti
ve
debug1: Next authentication method: publickey
debug1: Trying private key: /home/dargaud/.ssh/identity
debug1: Trying private key: /home/dargaud/.ssh/id_rsa
debug1: Offering public key: /home/dargaud/.ssh/id_dsa
debug2: we sent a publickey packet, wait for reply
debug1: Authentications that can continue:
publickey,password,keyboard-interacti
ve
debug2: we did not send a packet, disable method
debug1: Next authentication method: keyboard-interactive
debug2: userauth_kbdint
debug2: we sent a keyboard-interactive packet, wait for reply
debug1: Authentications that can continue:
publickey,password,keyboard-interacti
ve
debug2: we did not send a packet, disable method
debug1: Next authentication method: password
otheruser@somewhere.remote.com's password:
debug2: we sent a password packet, wait for reply
debug1: Authentication succeeded (password).
debug1: channel 0: new [client-session]
debug2: channel 0: send open
debug1: Entering interactive session.
debug2: callback start
debug2: client_session2_setup: id 0
debug2: channel 0: request pty-req confirm 0
debug2: channel 0: request shell confirm 0
debug2: fd 3 setting TCP_NODELAY
debug2: callback done
debug2: channel 0: open confirm rwindow 0 rmax 32768
debug2: channel 0: rcvd adjust 131072
Last login: Thu Nov 2 13:47:51 2006 from IP.IP.IP.IP
[otheruser@somewhere otheruser]$


--
Guillaume Dargaud
http://www.gdargaud.net/Antarctica/

A few more precisions:

Origin system: WinXP with cygwin
$ uname -a
CYGWIN_NT-5.1 penguin 1.5.21(0.156/4/2) 2006-07-30 14:21 i686 Cygwin
$ ssh -V
OpenSSH_4.4p1, OpenSSL 0.9.8d 28 Sep 2006

Remote system with working public key (same username as above):
$ uname -a
Linux somewhere1.remote.com 2.4.20-29.7.progeny.9 #1 Fri Jan 7 17:11:38 EST
2005 i686 unknown
$ ssh -V
OpenSSH_3.1p1, SSH protocols 1.5/2.0, OpenSSL 0x0090602f


Remote system with failing public key (different username):
$ uname -a
Linux somewhere2.remote.com 2.4.21-47.ELsmp #1 SMP Wed Jul 5 20:38:41 EDT
2006 i686 i686 i386 GNU/Linux
$ ssh -V
OpenSSH_3.6.1p2, SSH protocols 1.5/2.0, OpenSSL 0x0090701f

I though the target systems where clones of each other, but it's not the
case.
I've checked the chmod of .ssh/* for all 3 machines.
--
Guillaume Dargaud
http://www.gdargaud.net/
"Even if you can hit a glass on the table four out of five times when
you're clipping your toenails, women will never truly appreciate your
skill." — Clynch Varnadore.

Sorry, solved it after much googling.
..ssh was chmod 700 instead of 755 on remote machine.
--
Guillaume Dargaud
http://www.gdargaud.net/
"They say that 50% of all marriages end in divorce. That's not as bad as
it sounds, considering that the other 50% end in death."