Tag Archives: TTL

Compare & Troubleshoot DNS Servers: dnseval

The third tool out of the DNSDiag toolkit from Babak is dnseval. “dnseval is a bulk ping utility that sends an arbitrary DNS query to a given list of DNS servers. This script is meant for comparing response times of multiple DNS servers at once”. It is not only listing the response times but also further information about the DNS responses such as the TTL and the flags. Really great for comparison and troubleshooting different DNS forwarders as well as own authoritative DNS server responses as seen by others.

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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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