[root@mailserver /]# ip a
1: lo: <LOOPBACK,UP,10000> mtu 16436 qdisc noqueue
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
    inet 127.0.0.1/8 brd 127.255.255.255 scope host lo
3: venet0: <BROADCAST,POINTOPOINT,NOARP,UP,10000> mtu 1500 qdisc noqueue
    link/void
    inet 127.0.0.1/24 scope host venet0
    inet 192.168.99.1/32 scope global venet0:0

[root@mailserver /]# tcpdump -i venet0 port 25
tcpdump: WARNING: arptype 65535 not supported by libpcap - falling back 
to cooked socket
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on venet0, link-type LINUX_SLL (Linux cooked), capture size 96 
bytes
13:30:38.972506 IP 192.168.100.10.47080 > 192.168.99.1.smtp: S 
2596062421:2596062421(0) win 5840 <mss 1460,nop,nop,sackOK,nop,wscale 4>
13:30:38.972527 IP 192.168.99.1.smtp > 192.168.100.10.47080: S 
1133420120:1133420120(0) ack 2596062422 win 5840 <mss 
1460,nop,nop,sackOK,nop,wscale 2>
13:30:39.001294 IP 192.168.100.10.47080 > 192.168.99.1.smtp: . ack 1 win 365
13:30:39.001764 IP 192.168.99.1.smtp > 192.168.100.10.47080: P 1:39(38) 
ack 1 win 1460
13:30:39.028856 IP 192.168.100.10.47080 > 192.168.99.1.smtp: . ack 39 
win 365
13:30:43.855556 IP 192.168.100.10.47080 > 192.168.99.1.smtp: P 1:7(6) 
ack 39 win 365
13:30:43.855578 IP 192.168.99.1.smtp > 192.168.100.10.47080: . ack 7 win 
1460
13:30:43.855631 IP 192.168.99.1.smtp > 192.168.100.10.47080: P 39:74(35) 
ack 7 win 1460
13:30:43.855651 IP 192.168.99.1.smtp > 192.168.100.10.47080: F 74:74(0) 
ack 7 win 1460
13:30:43.882783 IP 192.168.100.10.47080 > 192.168.99.1.smtp: . ack 74 
win 365

[root@gate sysconfig]# ifconfig
eth0      Link encap:Ethernet  HWaddr 00:15:
          inet addr:192.168.100.10  Bcast:192.168.100.255  
Mask:255.255.255.0
..
eth1      Link encap:Ethernet  HWaddr 00:15:
          inet addr:192.168.23.2  Bcast:192.168.23.255  Mask:255.255.255.0
..
eth2      Link encap:Ethernet  HWaddr 00:11:
  ..ppp999 интерфейс
eth3      Link encap:Ethernet  HWaddr 00:17:
          inet addr:192.168.101.1  Bcast:192.168.101.255  Mask:255.255.255.0
 ..
lo        Link encap:Local Loopback
          inet addr:127.0.0.1  Mask:255.0.0.0
..
ppp999    Link encap:Point-to-Point Protocol
          inet addr:ххххххххх  P-t-P:хххххххх  Mask:255.255.255.255
..
tun1      Link encap:UNSPEC  HWaddr 
00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00
          inet addr:192.168.202.1  P-t-P:192.168.202.2  Mask:255.255.255.255
..
venet0    Link encap:UNSPEC  HWaddr 
00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00
...