Getting incoming TTL in GO

Question

I'm struggling with a small project I am working on. I want to implement functionality in GO that allows me to set the IP header TTL on an outgoing UDP packet that I then send and view the received TTL on the other end. I have a tried using a number of connections provided by the 'net' library and have so far had no success (I can set the TTL but I can't read it). Can anyone suggest a way of sending and receiving UDP packets that provide access to the packet's TTL field?

UDP doesn't have a TTL, that is part of the IP protocol. Have you tried to use lower level packages like godoc.org/golang.org/x/net/ipv4 or github.com/google/gopacket? — JimB
Yes, I've used both, I've tried using a packetConn from ipv4 but the ip header is still stripped away before I receive a message. I cannot figure out how to use the RawConn correctly, if you could show me how to use a RawConn to send/receive UDP messages that would be very helpful. — Brash Man

Sky Sky · Accepted Answer · 2020-04-20T21:20:27

Set TTL:

var conn *icmp.PacketConn

// for IPv4
conn.IPv4PacketConn().SetTTL(p.ttl) 
conn.IPv4PacketConn().SetControlMessage(ipv4.FlagTTL, true)

// for IPv6
conn.IPv6PacketConn().SetHopLimit(p.ttl) 
conn.IPv6PacketConn().SetControlMessage(ipv6.FlagHopLimit, true)

Recieve TTL:

    var ttl, n int
    bytes := make([]byte, 512)
    if IPv4 {
        var cm *ipv4.ControlMessage
        n, cm, _, err = conn.IPv4PacketConn().ReadFrom(bytes)
        if cm != nil {
            ttl = cm.TTL
        }
    } else {
        var cm *ipv6.ControlMessage
        n, cm, _, err = conn.IPv6PacketConn().ReadFrom(bytes)
        if cm != nil {
            ttl = cm.HopLimit
        }
    }

Getting incoming TTL in GO

1 Answers