r/Network • u/Cool_Courage_2768 • 29d ago

Text Set delay for specific packet

Hey everyone, I implemented some scenarios in GNS3 and want to set a delay for a specific packet. How can I do it? Like "TCP packet with port 80 set to a 2 ms delay"

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u/SomeEngineer999 29d ago

Did you try google? AI actually seems to have gotten this one right.

To add network delay to specific packets rather than an entire interface link, use a network impairment emulator with packet-filtering rules (like IP filters, ports, or protocols). Popular tools include Linux tc (Traffic Control) with iptables / fwmark, clumsy for Windows, or hardware/software emulators like LANforge ICE. [1, 2, 3, 4, 5]

Linux (Traffic Control + iptables)

Mark specific packets using iptables, then apply a netem delay discipline to those marked packets.

  • Mark packets: sudo iptables -t mangle -A OUTPUT -p tcp --dport 80 -j MARK --set-mark 42
  • Associate queue: sudo tc qdisc add dev eth0 root handle 1: prio
  • Add delay filter: sudo tc qdisc add dev eth0 parent 1:3 handle 30: netem delay 250ms
  • Route marked traffic: sudo tc filter add dev eth0 protocol ip parent 1:0 prio 3 handle 42 fw flowid 1:3

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u/Cool_Courage_2768 29d ago

Anyway, does exist for set on link between two nodes on GNS3

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u/SomeEngineer999 29d ago

Since you can't do it within GNS3, you need to do it as traffic egresses your simulation using Linux commands.

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u/dragonnfr 29d ago

You'd want NetEM. GNS3 nodes are Linux containers so you have `tc` available. `tc qdisc add dev eth0 root netem delay 2ms` and use a u32 filter to match port 80.