Dachshunds make a great watchdogs because they have a definite sense of “my yard,” “my home,” and “my car.” Part of the reason that dachshunds will sound the alert when they feel a threat to these areas is because they tend to be territorial, or protective of what they have defined as their property. When a dog is willing to bite to protect his property, however, he crosses the line from being a good watchdog to a potentially dangerous, territorially aggressive dog.
An indication that your dachshund is headed down this path is when he won't let anyone come in the front door, even when you have told him it is okay. Having to put your dachshund in a crate when someone comes to visit is not only a pain in the neck, it also is not helping your dog to be more accepting of visitors to his domain.
One way to help desensitize your dachshund is to take the dog out of the home as much as possible every single day, rain or shine. Take your dachshund on long walks, to parks, on car rides, even on trips if you can. It is unclear why this seems to work for territorial dogs, but it does seem to lessen their desire to protect and defend their property.
Also, as you might have guessed, if you have an intact male, neuter him. This may have a profound effect on the territorial dog.
Never put your guests at risk of being bitten by your dog. Have your dachsie greet guests on leash, and outdoors if the dog feels less territorial in front of the home. If your dachshund is so territorial that you cannot trust him in the home with guests, find a professional trainer or animal behaviorist to work on this problem before it escalates into a legal liability.
Unsupervised dachshunds can also develop into territorial canine monsters. As explained, if you allow your dog access to the front door while you are gone, he will eventually perceive that he is capable of chasing strangers away by barking. Keep your dachshund crated while you are gone, and inside when you can't supervise his backyard activities. Continue with his obedience training classes, too!