When working with SSH connections in a networked environment, you might encounter a puzzling scenario where host or nslookup successfully resolves a hostname while SSH fails with:

$ ssh storage
ssh: Could not resolve hostname storage: Name or service not known

$ host storage
storage has address 192.168.20.103

This discrepancy occurs because different tools use different resolution methods:

• SSH: Uses the system's resolver libraries (getaddrinfo()) which check multiple sources
• host/nslookup: Directly query the configured DNS servers

1. Missing Search Domain Configuration

Check your /etc/resolv.conf:

$ cat /etc/resolv.conf
search example.com
nameserver 8.8.8.8

If the search domain is missing, try:

$ ssh storage.example.com

2. Improper /etc/hosts Configuration

Verify your local hosts file:

$ cat /etc/hosts
127.0.0.1   localhost
192.168.20.103 storage

If the entry exists but SSH still fails, check for:

$ getent hosts storage

3. IPv6 vs IPv4 Preference

Modern systems prefer IPv6. Force IPv4 with:

$ ssh -4 storage

Or disable IPv6 in SSH config:

AddressFamily inet

Using strace to Trace System Calls

Run SSH with strace to see resolution attempts:

$ strace -e trace=network ssh storage

Checking nsswitch.conf

Inspect the name service switch configuration:

$ cat /etc/nsswitch.conf
hosts: files dns

The order matters - 'files' before 'dns' means /etc/hosts is checked first.

Option 1: Update SSH Config

Add to ~/.ssh/config:

Host storage
    HostName 192.168.20.103
    User yourusername

Option 2: System-wide DNS Configuration

For systemd-resolved systems:

$ sudo systemd-resolve --interface=eth0 --set-dns=192.168.1.1
$ sudo systemd-resolve --set-search=example.com

Verify resolution works with:

$ getent hosts storage
$ ping -c 1 storage
$ ssh -v storage

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I recently encountered a puzzling scenario where basic DNS tools (host, dig, nslookup) could resolve a hostname perfectly, while SSH stubbornly refused to cooperate:

$ ssh storage
ssh: Could not resolve hostname storage: Name or service not known

$ host storage
storage has address 192.168.20.103

This discrepancy occurs because different tools use different resolution methods:

# Traditional tools use /etc/nsswitch.conf order (usually files -> dns):
host/nslookup -> Pure DNS query
ssh -> System resolver (glibc) following nsswitch.conf

• Missing search domain: Check /etc/resolv.conf for missing search directives
• nsswitch misconfiguration: Verify /etc/nsswitch.conf has "dns" in hosts line
• SSH config overrides: Look for VerifyHostKeyDNS or CanonicalizeHostname in ~/.ssh/config

To pinpoint the issue:

# Check system resolver behavior
$ getent hosts storage

# Verify nsswitch configuration
$ grep hosts /etc/nsswitch.conf

# Test with full qualification
$ ssh storage.example.com

# Check SSH debug output
$ ssh -v storage

Quick fix: Add to /etc/hosts:

192.168.20.103  storage

Permanent solution (for Debian/Ubuntu):

# Edit nsswitch.conf
$ sudo sed -i 's/^hosts:.*/hosts: files dns/' /etc/nsswitch.conf

# Ensure resolv.conf has search domain
$ echo "search example.com" | sudo tee -a /etc/resolv.conf

For complex environments using systemd-resolved:

# Check resolved status
$ systemd-resolve --status

# Add DNS-override
$ sudo resolvectl dns eth0 192.168.1.1
$ sudo resolvectl domain eth0 "example.com"