Linux – Public key authentication fails on port 22 but succeeds on alternate port

I am able to log into my server with a password, but not with my public key. I'm running CentOS release 6.3 (Final) on a Rackspace.com server

I added my local ~/.ssh/id_rsa.pub to the remote server's ~/.ssh/authorized_keys and remote permissions seem okay.

$ ll -d .
drwxr-x---. 15 fort apache 4096 Feb 22 16:07 .
$ ll -d .ssh
drwx------. 2 fort apache 4096 Feb 17 19:40 .ssh
$ ll -d .ssh/authorized_keys
-rw-------. 1 fort fort 2034 Feb 18 06:06 .ssh/authorized_keys

I checked that the server is accepting public key authentication:

$ ssh -o PreferredAuthentications=none fort@fort
Permission denied (publickey,gssapi-keyex,gssapi-with-mic,password,keyboard-interactive).

Connecting with -vvv shows the failure

$ ssh -vvv -o PreferredAuthentications=publickey fort@fort
OpenSSH_7.3p1, LibreSSL 2.4.1
debug1: Reading configuration data /Users/kim/.ssh/config
...
debug1: Connecting to cedar.greencitypartnerships.org [108.166.125.240] port 22.
debug1: Connection established.
... 
debug1: Authentications that can continue: publickey,gssapi-keyex,gssapi-with-mic,password,keyboard-interactive
debug3: start over, passed a different list publickey,gssapi-keyex,gssapi-with-mic,password,keyboard-interactive
debug3: preferred publickey
debug3: authmethod_lookup publickey
debug3: remaining preferred: 
debug3: authmethod_is_enabled publickey
debug1: Next authentication method: publickey
debug1: Offering RSA public key: /Users/kim/.ssh/id_rsa
debug3: send_pubkey_test
debug3: send packet: type 50
debug2: we sent a publickey packet, wait for reply
debug3: receive packet: type 51
...
debug1: No more authentication methods to try.
Permission denied (publickey,gssapi-keyex,gssapi-with-mic,password,keyboard-interactive).
$

This is all I see when tailing /var/log/secure on the server

# tail -f /var/log/secure
...
Feb 24 06:12:16 fort sshd[3064]: Connection closed by 97.113.252.17

Here is what I get tailing another server I –can– log into

# tail -f /var/log/secure
...
Feb 23 22:14:12 cedar sshd[2187]: Accepted publickey for cedar from 192.168.56.1 port 53004 ssh2
Feb 23 22:14:12 cedar sshd[2187]: pam_unix(sshd:session): session opened for user cedar by (uid=0)

I then tried creating a public key on the server itself, adding it to the server's ~/.ssh/authorized_keys. I got the same failure

$ ssh -o PreferredAuthentications=publickey localhost
Permission denied (publickey,gssapi-keyex,gssapi-with-mic,password,keyboard-interactive).

and

# tail -f /var/log/secure
...
Feb 24 06:30:37 fort sshd[3841]: Connection closed by ::1

Next, I tried running sshd in debug mode on another port. I –was– able to log in this time.

 # /usr/sbin/sshd -d -p 27

and on the same server …

$ ssh -p 27 -o PreferredAuthentications=publickey localhost
Last login: Fri Feb 24 06:34:37 2017 from 97-113-252-17.tukw.qwest.net
Environment:
  LANG=en_US.UTF-8
  USER=fort
  ...
 $

To confirm that sshd did not start working because I was running in debug mode, I started it up normally. Again, I was able to successfully log in.

# /usr/sbin/sshd -p 27
# 

and

$ ssh -p 27 -o PreferredAuthentications=publickey localhost
Last login: Fri Feb 24 06:39:30 2017 from localhost
[fort@fort ~]$

and

# tail -f /var/log/secure
...
Feb 24 06:46:58 fort sshd[4595]: Server listening on 0.0.0.0 port 27.
Feb 24 06:46:58 fort sshd[4595]: Server listening on :: port 27.
Feb 24 06:48:13 fort sshd[4629]: Accepted publickey for fort from ::1 port 51302 ssh2
Feb 24 06:48:13 fort sshd[4629]: pam_unix(sshd:session): session opened for user fort by (uid=0)

I changed the LogLevel to DEBUG, and restarted sshd on both port 22 and 27.

Here's where the port 22 connection fails:

debug1: trying public key file /home/fort/.ssh/authorized_keys
debug1: restore_uid: 0/0
debug1: temporarily_use_uid: 502/502 (e=0/0)
debug1: trying public key file /home/fort/.ssh/authorized_keys2
debug1: restore_uid: 0/0
Failed publickey for fort from ::1 port 44994 ssh2

Here's where the port 27 connection succeeds

debug1: trying public key file /home/fort/.ssh/authorized_keys
debug1: fd 4 clearing O_NONBLOCK
debug1: matching key found: file /home/fort/.ssh/authorized_keys, line 6
Found matching RSA key: 8f:87:0e:15:b5:88:49:04:b7:34:79:9d:7e:c2:8d:fa

What might be allowing me to use public key authentication on port 27, but not port 22? Could there be alternate settings used by /etc/init.d/sshd? What should I try next to get public key authentication working on port 22?