Category Archives: Ping/Traceroute

Nping aka Layer 4 Ping

I was missing a generic layer 4 ping in my toolbox. Initially searching for a mere TCP ping, I have found Nping which completely satisfies my needs and gives so much more. ;)

What’s a layer 4 ping, and why? –> A normal ping (= ICMP echo-request) reveals whether the destination IP address, that is: the mere server/VM, is up and running. That’s great for a layer 3 networker since routing to and from the destination is already working. However, it does NOT reveal whether or not a service at layer 4 (TCP or UDP) is up and running as well. That’s what a layer 4 ping is about: sending TCP SYNs to the port in question, waiting for a “SYN ACK” (port is listening) or “RST”/no reply (port is not available). Common use cases: Waiting for a service to start again after an upgrade, or waiting for new firewall policies (to allow or deny) a certain port.

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Discovering Policy-Based Routes with Layer 4 Traceroutes (LFT)

I already published a few examples how you can use layer four traceroutes in order to pass firewall policies that block ping but allow some well-known ports such as 80 or 443. Long story short: Using TCP SYN packets on an opened firewall port with the TTL trick will probably succeed compared to a classical traceroute based on ICMP echo-requests.

Another nice use case for layer 4 traceroutes is the recognition of policy based routes within your own network (or even beyond). That is: Depending on the TCP/UDP port used for the traceroute you can reveal which paths your packets take over the network. This is quite useful compared to classical traceroutes that only reveal the straightforward routing tables but not the policy based ones.

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Detect DNS Spoofing: dnstraceroute

Another great tool from Babak Farrokhi is dnstraceroute. It is part of the DNSDiag toolkit from which I already showed the dnsping feature. With dnstraceroute you can verify whether a DNS request is indeed answered by the correct DNS server destination or whether a man-in-the-middle has spoofed/hijacked the DNS reply. It works by using the traceroute trick by incrementing the TTL value within the IP header from 1 to 30.

Beside detecting malicious DNS spoofing attacks, it can also be used to verify security features such as DNS sinkholing. I am showing the usage as well as a test case for verifying a sinkhole feature.

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Advanced Ping: httping, dnsping, smtpping

I really love ping! It is easy to use and directly reveals whether the network works or not. Refer to Why Ping is no Security Flaw! (But your Friend) and Advanced Tracerouting. At least outgoing pings (from trust to untrust) should be allowed without any security concerns. However, many companies are denying these ICMP echo-requests from untrust into the DMZ which makes it difficult to test whether all servers are up and running.

I was sitting at the customer’s site replacing the DMZ firewall. Of course I wanted to know (from the outside) whether all servers are connected correctly (NAT) and whether the firewall permits the connections (policy). However, ping was not allowed. Therefore I used several layer 7 ping tools that generate HTTP, DNS, or SMTP sessions (instead of ICMP echo-requests) and revealed whether the services (and not only the servers) were running. Great!

This post shows the installation and usage of httping, dnsping, and smtpping on a Linux machine, in my case a Ubuntu server 14.04.4 LTS, as well as some Wireshark screenshots from captured sessions. Finally, a pcap file can be downloaded that shows the sample runs of all three tools.

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Advanced Tracerouting

A commonly misunderstanding of traceroute is that it fully relies on ping. “If I block ping at my firewall, no one can use traceroute to reveal my internal routing path”. Unfortunately this is not true. If traceroute is used with TCP SYN packets on permitted TCP/UDP ports, all intermediary firewalls will handle the IP packets with TTL = 0 corresponding to the RFCs and will reply with an ICMP time exceeded packet to the source.

In this post I am listing an example that uses traceroute with TCP port 25 (SMTP) to traverse a firewall. A sample pcap file can be downloaded while some Wireshark screenshots show a few details.

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Why Ping is no Security Flaw! (But your Friend)

One core topic when designing firewall policies is the following question: Is ping a security attack? Should ICMP echo-request messages be blocked in almost any directions?

My short answer: Ping is your friend. :) You won’t block hackers if you block ping. Instead, ping is quite useful for network administrators checking basic network connectivity. That is: I suggest to allow ping anywhere around, accept incoming connections from the Internet to the trusted networks.

Here comes a discussion:

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MRTG/Routers2: Measuring Ping Times

MRTG can also evaluate values from external scripts such as the “mrtg-ping-probe” program which returns the round-trip time from the initiated ping command to the specified destination host. With an additional GraphStyle called “range” from Routers2, these ping times can be displayed in the monitoring system. This graph style shows the “min” and “max” RTT in one vertical line instead of two independent lines.

Since there is not much to say about this process, I will only paste my MRTG/Routers2 config for mrtg-ping-probe and will show a few example graphs here.

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