├── README.md
└── gre-keepalive.pl


/README.md:
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 1 | # linux-gre-keepalive
 2 | Userspace daemon in perl to handle Cisco GRE keepalives. Works in Linux, should work in any *nix derivative
 3 | 
 4 | Requires Net::Pcap, NetPacket::IP, and Proc::Daemon
 5 | 
 6 | (all 3 have stable debian perl packages in the standard repositories)
 7 | 
 8 | Usage:
 9 | 
10 | sysctl -w net.ipv4.ip_forward=1
11 | 
12 | sysctl -w net.ipv6.conf.all.forwarding=1
13 | 
14 | ip tunnel add mytunnel mode gre remote x.x.x.x local y.y.y.y ttl 255 pmtudisc
15 | 
16 | ip link set mytunnel up
17 | 
18 | ./gre-keepalive.pl mytunnel
19 | 
20 | 
21 | This daemon does not initiate keepalive packets, but does look for ones sent by the originating system and redirects them as a standard Cisco router would, thus causing the GRE tunnel to go up/up, and causing it to go up/down if connectivity is lost. 
22 | 


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/gre-keepalive.pl:
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 1 | #!/usr/bin/perl
 2 | 
 3 | use strict;
 4 | use warnings;
 5 | 
 6 | use Net::Pcap;
 7 | use Proc::Daemon;
 8 | use Socket;
 9 | 
10 | Proc::Daemon::Init;
11 | 
12 | socket(my $socket, AF_INET, SOCK_RAW, 255) || die $!;
13 | setsockopt($socket, 0, 1, 1);
14 | 
15 | my $dev = $ARGV[0];
16 | my $err;
17 | my $pcap = Net::Pcap::open_live($dev, 1024, 0, 0, \$err);
18 | 
19 | my $filter = "proto gre";
20 | my $filter_t;
21 | if (Net::Pcap::compile($pcap, \$filter_t, $filter, 1, 0) == -1) {
22 |     die "Unable to compile filter string '$filter'\n";
23 | }
24 | Net::Pcap::setfilter($pcap, $filter_t);
25 | 
26 | Net::Pcap::loop($pcap, -1, \&process_packet, $socket);
27 | 
28 | Net::Pcap::close($pcap);
29 | 
30 | sub process_packet {
31 |     my ($socket, $header, $packet) = @_;
32 | 
33 |     # Strip the "cooked capture" header.
34 |     $packet = unpack("x16a*", $packet);
35 | 
36 |     my $dest_ip = unpack("x16a4", $packet);
37 |     if (!send($socket, $packet, 0, pack_sockaddr_in(0, $dest_ip))) {
38 |         die "Couldn't send packet: $!";
39 |     }
40 | }
41 | 
42 | 


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