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SubjectSync interfaces and tcpdump

Hello,

I'm having a very interesting problem with a synchronous card and tcpdump.
Everything works fine as far as communications (i.e. packets can flow
through the link with no problems), but when I run tcpdump to look at the
interface traffic, I only get garbage at tcpdump's output!

With a ping traffic, for instance, this is what I get:

# ifconfig hdlc1
hdlc1 Link encap:Point-to-Point Protocol
inet addr:10.0.0.2 P-t-P:10.0.0.1 Mask:255.255.255.252
UP POINTOPOINT RUNNING NOARP MULTICAST MTU:1500 Metric:1
RX packets:7 errors:0 dropped:0 overruns:0 frame:0
TX packets:7 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:10
Interrupt:12 Memory:e0300000-e033ffff

# tcpdump -i hdlc1 -n &
[1] 1522
tcpdump: listening on hdlc1

# ping 10.0.0.1
14:48:43.139306 [|ip]
14:48:43.139852 [|ip]
PING 10.0.0.1 (10.0.0.1): 56 data bytes
14:48:43.496968 64.1.96.160 > 10.0.0.2: (frag 17664:-27@672) [tos 0x3]
14:48:43.498130 30.1.136.72 > 10.0.0.1: (frag 17664:-27@672) [tos 0x3]
[ttl 0]
14:48:43.498130 30.1.136.72 > 10.0.0.1: (frag 17664:-27@672) [tos 0x3]
[ttl 0]
64 bytes from 10.0.0.1: icmp_seq=0 ttl=30 time=2.0 ms
14:48:44.489368 64.1.96.158 > 10.0.0.2: (frag 17664:-27@672) [tos 0x3]
14:48:44.490527 30.1.136.71 > 10.0.0.1: (frag 17664:-27@672) [tos 0x3]
[ttl 0]
14:48:44.490527 30.1.136.71 > 10.0.0.1: (frag 17664:-27@672) [tos 0x3]
[ttl 0]
64 bytes from 10.0.0.1: icmp_seq=1 ttl=30 time=2.0 ms
14:48:45.489366 64.1.96.155 > 10.0.0.2: (frag 17664:-27@672) [tos 0x3]
14:48:45.490535 30.1.136.70 > 10.0.0.1: (frag 17664:-27@672) [tos 0x3]
[ttl 0]
14:48:45.490535 30.1.136.70 > 10.0.0.1: (frag 17664:-27@672) [tos 0x3]
[ttl 0]
64 bytes from 10.0.0.1: icmp_seq=2 ttl=30 time=2.0 ms
--- 10.0.0.1 ping statistics ---
3 packets transmitted, 3 packets received, 0% packet loss
round-trip min/avg/max = 2.0/2.0/2.0 ms

Just for comparison, what should show up is as follows (ping on the
Ethernet interface):

PING AA.BB.CC.EE (AA.BB.CC.EE): 56 data bytes
14:54:47.740150 AA.BB.CC.DD > AA.BB.CC.EE: icmp: echo request
14:54:47.741007 AA.BB.CC.EE > AA.BB.CC.DD: icmp: echo reply
64 bytes from AA.BB.CC.EE: icmp_seq=0 ttl=128 time=1.8 ms
14:54:48.739373 AA.BB.CC.DD > AA.BB.CC.EE: icmp: echo request
14:54:48.740193 AA.BB.CC.EE > AA.BB.CC.DD: icmp: echo reply
64 bytes from AA.BB.CC.EE: icmp_seq=1 ttl=128 time=1.3 ms
14:54:49.739369 AA.BB.CC.DD > AA.BB.CC.EE: icmp: echo request
14:54:49.740270 AA.BB.CC.EE > AA.BB.CC.DD: icmp: echo reply
64 bytes from AA.BB.CC.EE: icmp_seq=2 ttl=128 time=1.4 ms

--- AA.BB.CC.EE ping statistics ---
3 packets transmitted, 3 packets received, 0% packet loss
round-trip min/avg/max = 1.3/1.5/1.8 ms

Does anyone here knows the possible cause(s) for this?? Does an interface
have to meet some special requirements in order to be "listenable" by
tcpdump?? :)

TIA and Regards,
Ivan


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