A Jump Server or Jump Host is a special purpose computer on a network typically used to manage devices in a separate security zone. The most common example is managing a host in a DMZ from trusted networks or computers.
In the 1990's when co-location facilities became more common there was a need to provide access between dissimilar security zones. The Jump Server concept emerged to meet this need. The Jump Server would span the two networks and typically be used in conjunction with a proxy service such as SOCKS to provide access from an administrative desktop to the managed device. As SSH tunnelling became common it became the de-facto method of access.
A Jump Server is a hardened and monitored device that spans two dissimilar security zones and provides a controlled means of access between them. User access is tightly controlled and monitored.
Jump Servers are typically placed between a secure zone and a DMZ to provide transparent management of devices on the DMZ once a management session has been established.
A typical configuration is a hardened Unix machine configured with SSH and a local firewall. A SSH connection is then made from the administrator's desktop to the Jump Server and SSH forwarding is used to access the target machine in the DMZ.
Using an SSH tunnel to the target host allows the use of insecure protocols to manage servers without creating special firewall rules or exposing the traffic on the inside network.
The Jump Server acts as a single audit point for traffic and also a single place where user accounts can be managed. A prospective administrator must log into the jump server in order to gain access to the DMZ assets and all access can be logged for later audit.
A Jump Server can be a potential risk in a network's design. There are several ways on improving the security (it security); - reducing the subnet size (increasing the amount of subnets), and secure those vlans by means of a firewall or decent router - use secure passwords on all stations - do not give away full management excelsheets to everybody, only give people the ip adresses they need - etc.