Script to resolve a target host via secondary SSH host, then ssh to the target IP directly

ssh-via-dns-proxy.sh

#!/bin/bash

# Utility script to first resolve the IP address of the target host to SSH into
# by SSHing into a secondary proxy host. For example, this can be used when
# the origin host does not support resolving names via mDNS, but the jump host
# does, while avoiding the overhead of routing the SSH connection through the
# jump host when the origin and target are already both in the same network.

set -e
set -o pipefail

function show_usage() {
    cat <<-END
Resolve a hostname via a proxy host, then SSH to it.
Usage: $0 [OPTION]... HOST [-- TARGET SSH ARGS]
  -P, --proxy-host [user@]host[:port] SSH to this host to resolve the hostname
  -i identify_file                    Provide an identity file to connect to the
                                        proxy host
  -d, --debug                         Print debugging information

The proxy host can also be set via the SSH_DNS_PROXY_HOST env var
END
}

# Parse arguments
NUM_PARAMS=0
MAX_PARAMS=1
while (( "$#" )); do
  case "$1" in
      -P|--proxy-host)
          SSH_DNS_PROXY_HOST=$2
          shift 2
          ;;
      -i)
	  IDENTITY_FILE=$2
	  shift 2
	  ;;
      -d|--debug)
          DEBUG=true
          shift 1
          ;;
      -h|--help)
          show_usage
          exit 0
          ;;
      --) # end argument parsing
          shift
          break
          ;;
      -*|--*=) # unsupported flags
          echo "Error: Unsupported flag $1" >&2
          show_usage
          exit 1
          ;;
      *) # preserve positional arguments
          if (($NUM_PARAMS < $MAX_PARAMS)); then
              if (($NUM_PARAMS == 0)); then
                  TARGET_HOST=$1
              fi
              NUM_PARAMS=$NUM_PARAMS+1
              shift
          else
              echo "Error: too many parameters ($NUM_PARAMS)" >&2
              show_usage
              exit 1
          fi
          ;;
    esac
done

if [[ -z "$TARGET_HOST" ]]; then
    echo "Error: missing required parameter: HOST" >&2
    show_usage
    exit 1
fi

if [[ -z "$SSH_DNS_PROXY_HOST" ]]; then
    echo "Error: missing required option: --proxy-host" >&2
    show_usage
    exit 1
fi

if [[ "$DEBUG" = true ]]; then
    echo "Resolving $TARGET_HOST via $SSH_DNS_PROXY_HOST" >&2
fi

PROXY_SSH_ARGS=
if [[ ! -z "$IDENTITY_FILE" ]]; then
    PROXY_SSH_ARGS="-i $IDENTITY_FILE"
fi

TARGET_IP=`ssh $PROXY_SSH_ARGS $SSH_DNS_PROXY_HOST "getent hosts $TARGET_HOST | cut -d ' ' -f 1"`

if [[ -z "$TARGET_IP" ]]; then
    echo "Failed to resolve hostname for $TARGET_HOST on $SSH_DNS_PROXY_HOST" >&2
    exit 2
fi

if [[ "$DEBUG" = true ]]; then
    echo "Resolved  $TARGET_HOST to $TARGET_IP" >&2
fi

if [[ "$DEBUG" = true ]]; then
    echo "Executing SSH command: ssh $TARGET_IP $*" >&2
fi

exec ssh $TARGET_IP $*