ssh is not aware of client hostname?

Question

I have two servers. Say A and B.

I ssh-copy-ided on both servers to each other.

And I can connect from A to B without password. But I failed to connect from B to A.

I found that audit log is different

B's log when connecting from A to B (success)
    ... (hostname=192.168.0.1, addr=192.168.0.1, terminal=sshd ref=success

A's log when connecting from B to A (failed)
    ... (hostname=?, addr=192.168.0.1, terminal=sshd ref=failed

Where should I look into?

Here comes what ssh -v showed.

$ ssh -v user@<a's ip address>
OpenSSH_4.3p2, OpenSSL 0.9.8e-fips-rhel5 01 Jul 2008
debug1: Reading configuration data /etc/ssh/ssh_config
debug1: Applying options for *
debug1: Connecting to <a_ip_address> [a_ip_address] port 22.
debug1: Connection established.
debug1: identity file /home/user/.ssh/identity type -1
debug1: identity file /home/user/.ssh/id_rsa type 1
debug1: identity file /home/user/.ssh/id_dsa type -1
debug1: loaded 3 keys
debug1: Remote protocol version 2.0, remote software version OpenSSH_4.3
debug1: match: OpenSSH_4.3 pat OpenSSH*
debug1: Enabling compatibility mode for protocol 2.0
debug1: Local version string SSH-2.0-OpenSSH_4.3
debug1: SSH2_MSG_KEXINIT sent
debug1: SSH2_MSG_KEXINIT received
debug1: kex: server->client aes128-cbc hmac-md5 none
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
debug1: SSH2_MSG_KEX_DH_GEX_INIT sent
debug1: expecting SSH2_MSG_KEX_DH_GEX_REPLY
debug1: Host '<a_ip_address>' is known and matches the RSA host key.
debug1: Found key in /home/<user>/.ssh/known_hosts:8
debug1: ssh_rsa_verify: signature correct
debug1: SSH2_MSG_NEWKEYS sent
debug1: expecting SSH2_MSG_NEWKEYS
debug1: SSH2_MSG_NEWKEYS received
debug1: SSH2_MSG_SERVICE_REQUEST sent
debug1: SSH2_MSG_SERVICE_ACCEPT received
debug1: Authentications that can continue: publickey,gssapi-with-mic,password
debug1: Next authentication method: gssapi-with-mic
debug1: Unspecified GSS failure.  Minor code may provide more information
Unknown code krb5 195

debug1: Unspecified GSS failure.  Minor code may provide more information
Unknown code krb5 195

debug1: Unspecified GSS failure.  Minor code may provide more information
Unknown code krb5 195

debug1: Next authentication method: publickey
debug1: Trying private key: /home/<user>/.ssh/identity
debug1: Offering public key: /home/<user>/.ssh/id_rsa
debug1: Authentications that can continue: publickey,gssapi-with-mic,password
debug1: Trying private key: /home/<user>/.ssh/id_dsa
debug1: Next authentication method: password
user@<a_ip_address>'s password:

Answer 1

The root cause of my problem is that the home directory, the .ssh directory, and its child files on the target server have a wrong permissions.

I discovered this issue by looking at messages in log files in /var/log such as secure, and audit/*.

Answer 2

To answer the questions asked, even though they turned out to not be the OP's actual problem, in case searchers are looking for that actual question:

• sshd tries to determine the client's hostname if configuration option UseDNS is on. The upstream default for this was 'yes' until 6.8 in 2015-03 (just after the Q was posted), but distros or packages may change the default, and of course a sysadmin can always override the default. But the availability and/or correctness of the client hostname has no effect on public-key authentication, only some info in the log entries.

• ssh-copy-id is so named because it copies a publickey file which implicitly identifies a user -- not a host -- and these files by default are named like id_rsa.pub id_dsa.pub id_ecdsa.pub id_ed25519.pub although this can be changed. A publickey file may (or may not) contain a comment that gives the user name, and by default ssh-keygen creates this comment as userid@hostname where the key is generated (which may differ from where the key is used for a connection), but this can be overridden or changed or removed.