Linux: ssh equivalence and SELinux

This is quick post summarizing issues encountered while setting up ssh equivalence on EC2 instance. I was setting up  RHEL7 EC2 instances and followed below procedure to setup ssh equivalence

  • Generate rsa key-pair using ssh-keygen -t rsa on both hosts
  • Copy the public keys to the remote server in authorized_keys file
  • Modify file permission to 600

But when I tried to perform ssh to remote host , it failed with following error.

Permission denied (publickey,gssapi-keyex,gssapi-with-mic).

I verified directory (.ssh) and file permissions were correct. Then, I checked for SELinux context using ls - Z option.

-bash-4.2$ ls -lZ *
-rw-r--r--. postgres postgres unconfined_u:object_r:postgresql_db_t:s0 authorized_keys
-rw-------. postgres postgres unconfined_u:object_r:postgresql_db_t:s0 id_rsa
-rw-r--r--. postgres postgres unconfined_u:object_r:postgresql_db_t:s0 id_rsa.pub
-rw-r--r--. postgres postgres unconfined_u:object_r:postgresql_db_t:s0 known_hosts

As per above output, these files are running with postgresql_db_t type context. I used getenforce to verify that SELinux was in enforcing mode on this host. It can also be verified by viewing contents of /etc/selinux/config .

# cat /etc/selinux/config

# This file controls the state of SELinux on the system.
# SELINUX= can take one of these three values:
# enforcing - SELinux security policy is enforced.
# permissive - SELinux prints warnings instead of enforcing.
# disabled - No SELinux policy is loaded.
SELINUX=enforcing
# SELINUXTYPE= can take one of three two values:
# targeted - Targeted processes are protected,
# minimum - Modification of targeted policy. Only selected processes are protected.
# mls - Multi Level Security protection.
SELINUXTYPE=targeted

I decided to use restorecon command to restores SELinux security context for files and directories to their default values .

-bash-4.2$ restorecon -Rv /var/lib/pgsql/.ssh/
restorecon reset /var/lib/pgsql/.ssh context unconfined_u:object_r:postgresql_db_t:s0->unconfined_u:object_r:ssh_home_t:s0
restorecon reset /var/lib/pgsql/.ssh/id_rsa context unconfined_u:object_r:postgresql_db_t:s0->unconfined_u:object_r:ssh_home_t:s0
restorecon reset /var/lib/pgsql/.ssh/id_rsa.pub context unconfined_u:object_r:postgresql_db_t:s0->unconfined_u:object_r:ssh_home_t:s0
restorecon reset /var/lib/pgsql/.ssh/authorized_keys context unconfined_u:object_r:postgresql_db_t:s0->unconfined_u:object_r:ssh_home_t:s0
restorecon reset /var/lib/pgsql/.ssh/known_hosts context unconfined_u:object_r:postgresql_db_t:s0->unconfined_u:object_r:ssh_home_t:s0

-bash-4.2$ ls -lZ *
-rw-r--r--. postgres postgres unconfined_u:object_r:ssh_home_t:s0 authorized_keys
-rw-------. postgres postgres unconfined_u:object_r:ssh_home_t:s0 id_rsa
-rw-r--r--. postgres postgres unconfined_u:object_r:ssh_home_t:s0 id_rsa.pub
-rw-r--r--. postgres postgres unconfined_u:object_r:ssh_home_t:s0 known_hosts

As you can see, restorecon restored permission by changing type from postgresql_db_t to ssh_home_t. I performed ssh again and it worked !

Referencehttps://access.redhat.com/documentation/en-us/red_hat_enterprise_linux/7/html/selinux_users_and_administrators_guide/sect-security-enhanced_linux-working_with_selinux-selinux_contexts_labeling_files

 

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