SSM Fleet Manager shows `localhost` instead of actual hostname in "Computer Name" column

Question

I have a number of EC2 and on-premises managed instances (Raspberry Pi with Ubuntu 23.10). SSM Agent is pre-installed on the EC2 hosts and installed according to the docs on the on-premises instances, and Advanced Instances is enabled in SSM. Each host has a hostname defined according to the steps documented [here](https://repost.aws/knowledge-center/linux-static-hostname). Running `hostname` on the host returns the correct hostname, and the hostname survives a reboot, so cloud-init is also configured correctly with `preserve_hostname: true` specified in `/etc/cloud/cloud.cfg`. Example from one of the EC2 hosts:

```
ubuntu@dev-backend:~$ hostname
dev-backend
ubuntu@dev-backend:~$ cat /etc/hosts
127.0.0.1 localhost dev-backend

# The following lines are desirable for IPv6 capable hosts
::1 ip6-localhost ip6-loopback
fe00::0 ip6-localnet
ff00::0 ip6-mcastprefix
ff02::1 ip6-allnodes
ff02::2 ip6-allrouters
ff02::3 ip6-allhosts
```

When I enable the "Computer Name" column in Fleet Manager, all hostnames (except one) appear as "localhost":
![Enter image description here](/media/postImages/original/IMNAKj6SloRkikjhXZSj_UiA)

Why doesn't SSM Agent correctly report the hostname to Fleet Manager?

Accepted Answer

I was able to resolve this issue with help from ChatGPT. The issue was the way I was using Ansible to template the hostname as a second entry for the loopback interface, so even though the correct hostname was specified in both `/etc/hosts` and `/etc/hostname`, `hostname -f` (used internally by SSM Agent) was still returning `localhost` as the first matching entry. The solution was to put the hostname on the entry for `127.0.1.1` instead, which causes `hostname -f` to return the correct output. This works because IETF defines the loopback address as `127.0.0.0/8`. Working `/etc/hosts` file:
```
127.0.0.1 localhost
127.0.1.1 dev-backend

# The following lines are desirable for IPv6 capable hosts
::1 ip6-localhost ip6-loopback
fe00::0 ip6-localnet
ff00::0 ip6-mcastprefix
ff02::1 ip6-allnodes
ff02::2 ip6-allrouters
ff02::3 ip6-allhosts
```

Answer

A few more interesting differences in the `/etc/hosts` files on these nodes:
The single on-premises host that is reporting the hostname correctly:
```
pi@nucleus-v3-7:~$ cat /etc/hosts
# Your system has configured 'manage_etc_hosts' as True.
# As a result, if you wish for changes to this file to persist
# then you will need to either
# a.) make changes to the master file in /etc/cloud/templates/hosts.debian.tmpl
# b.) change or remove the value of 'manage_etc_hosts' in
#     /etc/cloud/cloud.cfg or cloud-config from user-data
#
127.0.1.1 nucleus-v3-7 pi
127.0.0.1 localhost

# The following lines are desirable for IPv6 capable hosts
::1 localhost ip6-localhost ip6-loopback
ff02::1 ip6-allnodes
ff02::2 ip6-allrouters
```
The other on-premises host that reports `localhost`:
```
pi@pi-1:~$ cat /etc/hosts
# Your system has configured 'manage_etc_hosts' as True.
# As a result, if you wish for changes to this file to persist
# then you will need to either
# a.) make changes to the master file in /etc/cloud/templates/hosts.debian.tmpl
# b.) change or remove the value of 'manage_etc_hosts' in
#     /etc/cloud/cloud.cfg or cloud-config from user-data
#
127.0.1.1 pi pi
127.0.0.1 localhost pi-1

# The following lines are desirable for IPv6 capable hosts
::1 localhost ip6-localhost ip6-loopback
ff02::1 ip6-allnodes
ff02::2 ip6-allrouters
```
And an EC2 host:
```
ubuntu@dev-backend:~$ cat /etc/hosts
127.0.0.1 localhost dev-backend

# The following lines are desirable for IPv6 capable hosts
::1 ip6-localhost ip6-loopback
fe00::0 ip6-localnet
ff00::0 ip6-mcastprefix
ff02::1 ip6-allnodes
ff02::2 ip6-allrouters
ff02::3 ip6-allhosts
```

SSM Fleet Manager shows `localhost` instead of actual hostname in "Computer Name" column

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